Photodriven heterogeneous charge transfer with transition-metal ...

This article was published as part of the 

2009 Renewable Energy issue 

Reviewing the latest developments in renewable 

energy research 

Guest Editors Professor Daniel Nocera and Professor Dirk Guldi 

Please take a look at the issue 1 table of contents to access 

the other reviews.

CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews 

Photodriven heterogeneous charge transfer with transition-metal 

compounds anchored to TiO 2 semiconductor surfacesw 

Shane Ardo and Gerald J. Meyer* 

Received 24th October 2008 

First published as an Advance Article on the web 1st December 2008 

DOI: 10.1039/b804321n 

A critical review of light-driven interfacial charge-transfer reactions of transition-metal 

compounds anchored to mesoporous, nanocrystalline TiO 2 (anatase) thin films is described. The 

review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, 

mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and 

interfacial charge-recombination processes that have been garnered through various spectroscopic 

and electrochemical techniques. The relevance of these processes to optimization of solar-energyconversion 

efficiencies is discussed (483 references). 

1. Introduction 

A Rationale 

Hoffert has elegantly documented recent power needs on the 

terawatt (TW = 10 12 W) scale. 1,2 As the worldwide rate of energy 

expenditure is directly related to the number of people on Earth, 

the population growth experienced over the last quarter-century 

is staggering: a 45% increase which equates to roughly two 

billion people and 6 TW of additional power (B63% increase). 3 

This coupled with the urbanisation of third-world and industrialized 

nations and cities has led to an increase in the demand for 

fuel that has subsequently driven gas and oil prices to record 

highs. 3 Regardless of their price, the continued use of fossil fuels 

cannot be a long-term solution as they come from a limited stock 

and the deleterious environmental consequences of their combustion 

have become self-evident. Thus, the numbers alone, i.e. 

Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 

21218, USA 

w Part of the renewable energy theme issue. 

Shane Ardo was born in San 

Francisco, California in 1977. 

He obtained a BS in mathematics 

from Towson University 

in 1999 and an MS in 

nutrition and food science 

from UM-College Park in 

2005. Since joining Johns 

Hopkins University for a 

Shane Ardo 

PhD program in chemistry, 

Shane has been awarded an 

MS degree and was recently 

selected to describe renewable 

energy sources at the inaugural 

Eaton E. Lattman lecture 

series. He currently studies 

molecular, photo-induced processes at nanocrystalline TiO2 

interfaces for application in dye-sensitized solar cells and photoelectrosynthetic 

hydrogen formation. Shane also enjoys soccer, 

hiking, and camping with his fiance´e and friends. 

population, energy demand, and fuel prices, do not convey the 

severity of the problem. Concern should be elicited as the ice-core 

data over the past three-quarters-of-a-million years correlates 

temperature with greenhouse gas concentration 5,6 and current 

atmospheric CO2 levels of 4380 ppm 4 exceed any values attained 

over this same time period. 5 Further, outside of natural photosynthesis, 

there exist no means by which our society could 

significantly lower the concentration of CO2. The increased 

average global temperature and rates of glacial melting measured 

over the last few decades are telling signs. 7 There is real reason for 

concern. Regardless of one’s opinion on the causes of global 

climate change, it is very difficult to argue with two key points: 

(1) humans need to conserve energy and (2) commercially viable 

and sustainable energy conversion processes need to be discovered, 

designed, and developed. 

The motivation for this review stems from the urgent need 

for inexpensive and sustainable materials that can be used for 

solar energy conversion. Hoffert and co-workers concluded 

that in order to avoid catastrophic planetary changes Earth 

will require at least 10 terawatts of carbon-neutral energy by 

Gerald (Jerry) J. Meyer was 

born in Oconomowoc Wisconsin 

in 1962. He received a BS in 

chemistry and mathematics from 

SUNY-Albany and a PhD in 

chemistry from UW-Madison. 

After a post-doctoral appointment 

at UNC-Chapel Hill, he 

joined the faculty at Johns 

Hopkins University in 1991 

where he is now the 

Bernard N. Baker Professor of 

Chemistry with a joint appointment 

in the Materials Science & 

Jerry Meyer Engineering Department. In 

addition to his interests in environmental 

chemistry and solar energy conversion, Jerry enjoys long 

distance running, tennis, cooking, gardening, hiking, and spending 

time with his wife, Lisa, and daughters, Caroline and Jillian. 

This journal is c The Royal Society of Chemistry 2009 Chem.Soc.Rev., 2009, 38, 115–164 | 115

Fig. 1 A schematic depicting a champion dye-sensitized solar cell (DSSC) illustrating the approximate relative energetics of individual electrontransfer 

reactions along with their corresponding rate constants or current densities. The steps highlighted in this review are shown as blue Roman 

numerals and subcategorized by capitalized letters: (I) sensitizer light absorption; (II) excited-state electron injection; (III) regeneration of the 

oxidized sensitizer by an electron donor in the electrolyte; (IV) charge recombination of TiO 2 electrons, TiO 2(e )s, to (A) oxidized sensitizers or 

(B) oxidized donors. Adapted from Fig. 9 of ref. 11. 

the year 2050, which was approximately equal to the worldwide 

energy requirement in the year 1998. 1,2 They also described 

the pitfalls of a ‘wait-and-see’ approach and 

recommended immediate action. It has now been dubbed the 

Terawatt Challenge. 8 The sun is the one source that on its own 

could supply the world’s projected energy demand and in a 

sustainable fashion. 4 To put it in perspective, the amount of 

solar energy reaching the earth in one day could power the 

planet for an entire year. 8,9 Remaining is the challenge of 

harvesting and storing this energy in a cost-effective way. It is 

our assumption that molecular approaches to this challenge 

will ultimately be most successful. The relative ease by which the 

spectroscopic and electrochemical properties of molecules can be 

tuned through synthetic manipulation allows for many minute 

variations on solar-energy-conversion schemes to be rapidly 

studied. Additionally, chemical bonds afford large energy storage 

capacities, i.e. energy densities, and power densities that exceed 

those obtainable from most other storage methods. 

B Background 

When we started our research program at Johns Hopkins 

University in 1991, molecular approaches to solar-energy conversion 

were solely of academic interest. The hard fact was that 

the most efficient ‘‘molecular solar cells’’ were comprised of cold 

water running over illuminated black paint. In the same year 

much progress in the field was realized when Grätzel and 

O’Regan reported an order-of-magnitude increase in solar 

light-to-electrical power conversion efficiency from dye-sensitized 

solar cells (DSSCs). 10 Their significant advance was to replace 

the planar electrode materials of the past with high surface area, 

mesoporous, nanocrystalline semiconductor thin films. The 

actual surface area for sensitizerz binding was up to three 

z Since a photocurrent is generated with light of lower energy than the 

bandgap of TiO 2, the chromophoric dyes are referred to as sensitizers, 

a term that we will use throughout this review. 

orders-of-magnitude larger than the geometric surface area, 

which is critical for solar harvesting with molecular compounds. 

11 Today, confirmed efficiencies in excess of 10% have 

been established. 12 

The general mechanisms for light-to-electrical power conversion 

in DSSCs were developed in early sensitization studies 

of planar and single-crystal semiconductors. Mechanisms like 

that shown in Fig. 1 can be found in many excellent reviews on 

this area. 11,13 In short: (I) light is absorbed by a sensitizer to 

form a molecular excited state; (II) the excited state may inject 

an electron into the semiconductor thus causing charge separation; 

(III) the oxidized sensitizer is ‘‘regenerated’’ by an 

external electron donor. Once the electron has performed 

useful work in the external circuit, it returns to a counter 

electrode where it reduces the oxidized electron donor. Hence 

the solar cell is termed ‘regenerative’ as all oxidation chemistry 

at the dye-sensitized electrode is reversed at a dark counter 

electrode such that no net chemistry occurs. It is now possible 

to include rate constants and/or current densities for many of 

these processes as well as for (IV) the unwanted charge 

recombination of TiO 2 electrons to: (A) oxidized sensitizers; 

or (B) oxidized donors in the electrolyte. There are a tremendous 

number of details in an operational Gra¨tzel-type cell and 

the values in Fig. 1 represent a good starting point for their 

general comparison. However, the time scales and current 

densities are often misleading as they may be specific to a 

certain class of sensitizers or electrolytes and/or may be 

abstracted from experimental data obtained in the absence 

of some components of the operational Gra¨tzel-type cell.y 

y The term Grätzel-type cell was chosen so as to highlight the distinction 

between the DSSCs employed in the seminal Nature paper—the use of 

mesoporous, nanocrystalline TiO2 (anatase) thin-film electrodes— 

and those in previous DSSC fabrications. Although historically accurate, 

this distinction will only be noted in the Introduction section and not 

throughout this review as preferred by Professor Gra¨tzel. 14 

116 | Chem.Soc.Rev., 2009, 38, 115–164 This journal is c The Royal Society of Chemistry 2009

In this review we provide some of the details that have 

arisen from recent research of heterogeneous, charge-transfer 

processes involved in the transduction of energy in TiO 2-based 

DSSCs. They predominantly deal with actions occurring at 

mesoporous, nanocrystalline TiO 2 (anatase) electrodes sensitized 

to visible light with transition-metal coordination compounds. 

Space limitations prevented us from including results 

obtained in the active areas of research with organic and 

quantum dot sensitizers. The review highlights molecular insights 

into the interfacial, charge-transfer processes I–IV 

that have been garnered through various spectroscopic and 

electrochemical measurements. As this review illustrates, many 

of the details remain poorly understood. Notwithstanding, 

numerous interesting and informative studies have been 

performed in order to probe electrolyte- or counter electrodebased 

phenomena 15 as well as charge transport through 

mesoporous films. 16–20 These will be discussed only as they 

are relevant to processes I–IV. 

Understanding the operation of a Gra¨tzel cell is not the only 

reason to study excited states and interfacial electron transfer at 

semiconductor interfaces on the molecular level, a point often 

missed by reviewers in this area. Indeed the spirit of using 

inexpensive processing and non-toxic, abundant, high surfacearea 

materials for solar-energy conversion is exactly on track. 13 It 

is a sound approach toward practical solutions to the Terawatt 

Challenge that could ultimately provide future generations the 

relatively low-cost power that we enjoy today, but in a sustainable 

fashion. 8 To build on the success of the Gra¨tzel cell and 

develop low-cost materials capable of solar-energy conversion 

and storage is just one of many motivations for understanding 

interfacial charge transfer in precise molecular detail. 

2. Solar harvesting with metal-polypyridyl 

compounds 

One sun of solar irradiance at an Air Mass of 1.5 (AM1.5) and 

under standard, U.S. atmospheric conditions (1000 W m 2 )is 

often taken as an average irradiance and spectral distribution of 

sunlight in the United States. The spectrum is available in 

downloadable format form the National Renewable Energy 

Laboratory (NREL) website. 21 The fraction of light that is 

absorbed by a DSSC is wavelength dependent and is often called 

the light harvesting efficiency (LHE) or the more generally 

preferred IUPAC name, absorptance (a(l)). 22 The absorptance 

of a monolayer of sensitizers anchored to a flat surface is related 

to (a) the molar extinction coefficient of the sensitizer, e(l), and 

(b) the surface area occupied by the dye, ADye, i.e. the footprint: 

aðlÞ ¼1 

IðlÞ 

IoðlÞ ¼ 1 10 AbsorbanceðlÞ ; where ð1Þ 

Absorbance ðlÞ ¼log IoðlÞ 

IðlÞ 

¼ 1000 eðlÞ 

1 

ADye 

where Io(l) is the intensity of the incoming incident light and I(l) 

is the intensity of the light transmitted through the sample. 

Calculations show that even monolayers of phthalocyanines 

and porphyrins, which have among the highest extinction coefficients 

known, absorb far less than 1% of the 1 sun, AM1.5 

spectrum on planar surfaces. 23 This underscores the need for 

ð2Þ 

high surface-area materials to increase the LHE of a molecular 

monolayer of sensitizers. 

The effectiveness of a solar cell is measured by its power 

output. This value is the product of its current and voltage. Thus, 

determination of the solar cell’s current–voltage relationship 

often aids in assessing its performance, Fig. 2. The lightto-electrical 

power conversion efficiency of a solar cell (Z) isthe 

product of the open-circuit photovoltage (Voc), short-circuit 

photocurrent (isc), and fill factor (FF) divided by the product of 

the incident irradiance (Po)andtheareaofthesolarcell(Acell). 24 

Z ¼ iscVocFF 

PoAcell 

Very often P o is set to be 1 sun of AM1.5 solar irradiation, 

i.e. 1000 W m 2 . V oc is the maximum Gibbs free energy that one 

can abstract from a regenerative solar cell, while i sc is the 

maximum rate that charge can flow through the external circuit. 

The long-wavelength absorption edge sets a thermodynamic limit 

to the Voc and the LHE can be used to calculate the maximum 

possible isc; thus, assuming FF = 1, the largest possible Z can 

be estimated all from a simple absorption measurement of the 

solar cell. The optimal isc assumption is, within experimental 

uncertainty, realized in champion DSSCs. 25 However, the 

spectroscopically estimated maximum V oc values are much 

greater than those that have been observed experimentally. 

A subtlety is that the i sc of a solar cell is directly related to its 

absorptance (a), but not its absorbance. The absorbances and 

absorptances are approximately equal at low sensitizer concentrations 

but differ significantly at the high sensitizer concentrations 

used in champion DSSCs. Therefore, the normalized 

photocurrent action spectrum, i.e. a plot of the incident 

photon-to-current efficiency (IPCE), or external quantum efficiency, 

as a function of excitation wavelength, should coincide 

with the normalized sensitizer absorptance spectrum. One is 

Fig. 2 Typical current–voltage curve for a champion DSSC under 

approximately 1 sun, AM1.5 illumination (0.998 suns). Labeled are the 

short-circuit photocurrent (isc), open-circuit photovoltage (Voc), and 

power point (PP) along with its corresponding photovoltage (V PP) 

and photocurrent (iPP). The fill factor (FF) is the area of the shaded 

region, which is bounded by the V PP and i PP, divided by the area of the 

region outlined by the dashed line, which is bounded by the Voc and isc. 

The curve in magenta represents the power as a function of voltage in 

arbitrary units further illustrating that the PP coincides with the 

condition of maximum power output. Adapted from Fig. 6 of ref. 26. 

This journal is c The Royal Society of Chemistry 2009 Chem.Soc.Rev., 2009, 38, 115–164 | 117 

ð3Þ

often interested in not only the monochromatic absorptance but 

the integrated, a(l)-weighted solar flux divided by the total 1 

sun, AM1.5 photon flux as well. The latter represents the 

overall percentage of solar light absorbed where the numerator 

serves as an upper limit to the i sc of the DSSC. 

The FF can be related to i sc and V oc through the corresponding 

values at the power point (PP): 

FF ¼ iPPVPP 

ð4Þ 

iscVoc 

where the PP occurs at the maximum product of the cell 

output photovoltage and photocurrent obtained along the 

current–voltage curve, Fig. 2. While FF = 1 is ideal, such a 

value cannot be achieved due to various loss mechanisms such 

as charge recombination. 

A Orbitals and electronic transitions 

The metal-to-ligand charge transfer (MLCT) excited states of dp 6 

coordination compounds have emerged as the most efficient for 

solar harvesting and sensitization of wide-bandgap semiconductor 

materials. As the name implies, light absorption promotes an 

electron from the Metal d orbitals to the Ligand p* orbitals, 

d(p) - p*. 27–29 A number of electric-dipole-allowed Charge- 

Transfer transitions are observed which give rise to intense 

absorption bands in the visible region with moderate extinction 

coefficients. There is no formal spin for each excited state due to 

heavy-atom spin–orbit coupling from the transition-metal 

center (especially for 4d and 5d metals). 30,31 Crosby et al. have 

proposed that the excited state is accurately described by solely 

the symmetry label of the molecular point group to which it 

belongs, corresponding to an irreducible representation, and not 

the spin and an orbital individually. 30 The effects of spin–orbit 

coupling must be introduced in order to rationalize the 

relative oscillator strengths and absorption spectra of 

M(bpy) 3 2+ (M = Fe II ,Ru II and Os II ) compounds, where bpy 

is 2,2 0 -bipyridine. 

The classical example of a compound with such transitions is 

Ru(bpy)3 2+ which is arguably the most well-studied, coordination 

compound. Its lowest-energy state is three-fold symmetric 

and is best described by the symmetry label D3,Fig.3.Basedon 

the Franck–Condon principle, immediately following excitation 

the initial excited state ought to possess the same structural 

symmetry as the ground state. 32–34 Thus, in the absence of 

Jahn–Teller distortions or solvent-induced fluctuations, the 

initial, Franck–Condon excited state formed via an MLCT 

transition in Ru(bpy)3 2+ could consist of a delocalized electronic 

wavefunction on all three bpy ligands each formally possessing 

1/3 of an electronic charge. Monitoring the conversion of 

this excited-state from D3 to C2 symmetry is a non-trivial task, 

although some evidence supports the notion that conversion 

occurs by T2 dephasing. 35,36 Based on the absence of an electric 

dipole for D3 symmetry molecules and minor, but clearly 

observable, solvent-dependent, ground-state MLCT absorption 

features, the initial excited-state electron is thought to localize 

on a single bpy. 37 Time-resolved resonance Raman spectroscopy 

of Ru(bpy) 3 2+ shows clear evidence for localization on 

nanosecond and longer time scales. 38 This localized excited state 

has the reduced-symmetry designation C2 and an estimated 

dipole moment of B10 Debye. 37,39 

Demas and colleagues have shown that intersystem crossing 

to a manifold of relaxed, MLCT excited states occurs with a 

quantum yield near unity in fluid solution, Fig. 4(a). 41–43 

Although not formally triplet or singlet in nature, the predominantly 

triplet character of the lowest-energy excited state, 

1E 0 , 44,45 and singlet character of the initial Franck–Condon 

state rationalizes why the transition between them is often 

termed intersystem crossing. It is for this reason that these states 

will be labeled as 3 MLCT and 1 MLCT, respectively, throughout 

this review. Crosby, Hager, and colleagues have shown that 

photoluminescence (PL) arises from three closely spaced electronic 

states. 46–50 Rapid thermal equilibrium between this manifold 

of states, okT in energy apart, happens such that PL occurs 

from what appears to be a single thermally equilibrated excited 

state, or thexi state. 51,52 Yersin et al. discovered evidence for two 

more highest-energy states by temperature-dependent emission 

polarization experiments and labeled them per the D 3 0 double 

symmetry group, which takes into account the spin–orbit 

Fig. 4(b). 44,45 These transitions are generally supported by those 

Fig. 3 Molecular-orbital diagrams for Ru(L)6 2+ -type compounds in their ground state with: GS-Oh) octahedral, Oh, symmetry; or GS-D3) 

reduced D 3 symmetry, like for Ru(bpy) 3 2+ . Also shown are excited-state molecular-orbital diagrams for: 3 MLCT-D3) the initial, Franck–Condon 

excited state formed under the ground-state D3 symmetry, where the excited electron is delocalized equally over each ligand; and 3 MLCT-C2) the 

excited state possessing reduced C 2 symmetry where the excited electron is localized on one ligand. Taken from Fig. 2 of ref. 40. 


Fig. 4 (A) A Jablonski-type energy diagram for Ru(bpy)3 2+ illustrating its manifold of thermally equilibrated excited states, i.e. the thexi state. 

The quantum yield for intersystem crossing, f ISC, is approximately unity. Taken from Scheme 1 of ref. 54. (B) The relative energy levels for the 

excited states of Ru(bpy)3 2+ under the D3 0 double group, which takes spin–orbit coupling into consideration. Taken from Fig. 3 of ref. 44. 

obtained from computational Density Functional Theory (DFT) 

calculations. 53 

As many of the sensitizers employed in champion DSSCs are 

of the form cis-Ru(LL) 2X 2, where LL is a bpy-like ligand and X 

is a non-chromophoric ligand, their spectral differences and 

similarities to Ru(bpy) 3 2+ are discussed. When LL = bpy and 

X=NC the compound’s spectrum is solvatochromic and the 

Ru III/II reduction potential, E o (Ru III/II ), is more negative as 

compared to Ru(bpy)3 2+ . 55 This coupled with the relatively 

insensitive energetics of the p* orbitals of the bpy ligand leads 

to red-shifted absorption and emission maxima. These lowestenergy, 

actinic transitions are MLCTinnatureandresultinan 

electron on the bpy-based chromophoric ligand and a hole that is 

partially delocalized on the cyano ligands, thus greatly decreasing 

the Lewis basicity of the cyano ligands. The most efficient 

sensitizerforDSSCsiscalledN3,whereLL=4,4 0 -dicarboxylic 

acid-bpy (dcb) and X = SCN . 25 Although less solvatochromic 

than the cyano derivative, its visible absorption spectrum, and 

that of its ‘LL = bpy’ derivative, exhibit two well-resolved 

bands. It has been postulated that this occurs due to a shift in 

the electron density of the highest occupied molecular orbital 

(HOMO) from the Ru II -metal center to the isothiocyanate 

ligands. 56–58 By DFT it was calculated that B75% of the 

HOMO density resides on the isothiocyanate ligands and that 

B75% of this density resides on the sulfur atom. 

B Tuning of the absorption spectrum and redox properties 

An important aspect of dp 6 coordination compounds is that 

their colors can be widely tuned using synthetic chemistry. The 

MLCT absorption bands can be tuned in energy by altering 

the substituents on the bpy ligands or by controlling the extent 

of d(p)-p* back-bonding donation to nonchromophoric 

ligands. How these changes affect the photophysical properties 

of the compounds have been the subject of many investigations 

affording further insights into the factors that govern 

radiative and nonradiative excited-state decay. As just 

mentioned, the compound that has emerged as the most 

efficient sensitizer for DSSC application is N3. 25 N3 gains 

red absorption over Ru(dcb)3 2+ however at the expense of the 

E o (Ru III/II ). Although not generally vital to DSSCs, this loss in 

driving force for regeneration of the oxidized sensitizer would 

further limit the sensitizer’s ability to perform a ‘holy grail’ of 

chemistry: water oxidation. 59,60 However, in terms of DSSC 

light-to-electrical power conversion efficiency, N3 and closely 

related analogues remain unsurpassed, Fig. 5.z A similarly 

successful Ru II -based sensitizer, which is based on terpyridine 

rather than bpy, is the so called ‘black dye’: [Ru(tct)(NCS)3] , 

where tct is 4 0 ,4 00 ,4 000 -tricarboxylic acid-tpy and tpy is 

2,2 0 :6 0 ,2 00 -terpyridine. It extends the spectral sensitivity of the 

solar cell significantly towards the red as compared to N3. 68 

However, a lower extinction coefficient throughout the visible 

region results in an overall less-efficient DSSC. 

The reduction potentials of the thexi state of the sensitizers, 

E o (Ru III/II* ) and E o (Ru II*/+ ), can be estimated using thermochemical 

cycles. 69,70 In many cases the spectroscopic and 

electrochemical data needed for such calculations can be 

measured in situ, i.e. for the sensitizer anchored to the semiconductor 

film. Previous studies have shown that molecules 

anchored to mesoporous, nanocrystalline TiO2, ZrO2, or 

Al2O3 thin films can be reversibly oxidized in standard electrochemical 

cells provided that the surface coverage exceeds a 

percolation threshold. 71–73 Cyclic voltammetry and spectroelectrochemistry 

are thus powerful in situ tools for determining 

formal reduction potentials and absorption spectra of relevant 

redox states. The excited-state reduction potential for 

the oxidation of the thermally equilibrated excited state, 

E o (Ru III/II* ), is calculated by the following equation: 

E o (Ru III/II* )=E o (Ru III/II ) DG ES (5) 

z Analogues where one or more of the carboxylic acid groups have 

been deprotonated, e.g. the dianion salt of N3 with tetrabutylammonium 

counterions (TBA + )—N719, 61 or one of the dcb ligands has 

been replaced by a more hydrophobic bipyridine ligand, e.g. 

Z907—with replacement by 4,4 0 -dinonyl-bpy 62–64 —and K19—with 

replacement by 4,4 0 -bis(p-hexyloxystyryl)-bpy. 65–67 


Fig. 5 The chemical structures of the most successful Ru II -based sensitizers employed in champion DSSCs. 

where DG ES is the free energy stored in the thexi state. 69,70,74,75 

This energy can be estimated by the PL onset or through a 

Franck–Condon lineshape analysis of the corrected PL spectrum. 

The reduction potential of the initially formed, 

Franck–Condon excited state can be calculated by substituting 

the excitation energy for DGES. 

As previously mentioned, Ru(bpy)3 2+ and most other trisheteroleptic 

Ru II compounds have redox and optical properties 

that are fairly insensitive to their environments. 37,76 However, 

this is not the case for ammine and cyano compounds of the 

type [M(bpy 0 )(X) 4] 2 ,2+ or [cis-M(bpy 0 ) 2(X) 2] 0,2+ ,M=Fe, 

Ru, or Os and X = CN or NH 3. 76 Outer-sphere interactions 

with the cyano ligands have a profound influence on E o (M III/II ) 

and hence the color of the compound. [Ru(dcb)(CN)4] 2 

is 

highly solvatochromic; 78 the maximum of the lower-energy 

MLCT band of Ru(dcb)(CN)4/TiO2 was observed at 450 

10 nm in tetrahydrofuran and at 500 20 nm in dimethylformamide. 

78 The color change was due to a shift of E o (Ru III/II ) 

with solvent. The complex maintained this solvatochromism 

upon attachment to mesoporous, nanocrystalline TiO2 

(anatase) thin films although the magnitude of the effect 

decreased. Solvent tuning altered the spectral responses of 

DSSCs based on these materials in a predictable way. For 

[Fe(bpy)(CN) 4] 2 

compounds, the excited-state reorganization 

energy in acetonitrile was found to be significantly larger on 

TiO2 than in fluid solution (l = 0.32 eV versus 0.10 eV, 

respectively). 77 This increased reorganization energy may 

be due to the restricted translational mobility of the semiconductor-bound 

iron compounds and the ambidentate 

Fe II –CN–Ti IV linkages. Interestingly, a recent Raman study 

has shown that when anchored to TiO2, the solvent reorganization 

energy of N3 decreased by a factor of six. 79 Further studies 

are needed to provide fundamental information on the solvation 

environment of similar semiconductor-bound molecules. 

A shortcoming of actinic sensitization by MLCT transitions 

is their relatively low extinction coefficients as compared to 

p - p* transitions often found in organic sensitizers. Thus 

6–10 mm thick films of nanocrystalline TiO2 are required for 

efficient solar harvesting and increased LHE with Ru II -based 

coordination compounds. This precludes the use of many 

classes of semiconductor materials that have inherently low 

surface areas. Ru(bpy)3 2+ has a molar extinction coefficient of 

about 15 000 M 1 cm 1 for its MLCT-based electronic transitions. 

80 In contrast, natural and synthetic organic pigments 

also absorb solar photons but with extinction coefficients that 

are often in excess of 200 000 M 1 cm 1 . 23 It has long been 

known that addition of substituents to bpy with low lying 

p orbitals (such as aromatics, esters, carboxylic acids, or 

unsaturated organics) can enhance MLCT extinction coefficients 

relative to unsubstituted bpy. 65–67,81–85 Interestingly, 

4 and 4 0 disubstitution of bpy has been found to increase 

these extinction coefficients more effectively than does disubstitution 

in the 5 and 5 0 positions. 86 The preparation of high 

extinction coefficient, heteroleptic N3 derivatives, where one 

of the dcb ligands is replaced by a 4,4 0 -disubstituted bpy is an 

extremely active area of research. 65–67,82,84,85,87 

We recently found that employing bpy ligands bridged in the 

3and3 0 positions by dithiolene is a viable alternative to the 

more traditional and widely pursued approach of introducing 

conjugated groups in the 4 and 4 0 positions. 81 Substituent 

effects in this position are not as well documented as they 

sterically force the two pyridyl rings out of planarity, behavior 

that can decrease the stability of the compound. This issue is 

circumvented with bridging ligands but at the expense of 

opening up the N–Ru–N bite angle thereby stabilizing 

ligand-field states and decreasing the excited-state lifetime. 

Nevertheless, it was notable that these first-derivative, MLCTdithiolene 

compounds have extinction coefficients for their lowest-energy 

transitions that are comparable to the highest ever 

reported based on Ru II (4,4 0 -disubstituted-bpy) compounds, 

4.4 10 4 M 1 cm 1 . In a similar absorption region the largest 

value for the dyes often employed in champion DSSCs, Fig. 5, is 

1.8 10 4 M 1 cm 1 for K19 66 and to the best of our knowledge 

no compounds exceed 3.9 10 4 M 1 cm 1 beyond 450 nm. 83,85 

Part of this success was that the dithiolene-bpy ligands 

have intraligand absorption bands, in addition to the MLCT 

absorption bands, in the visible region. 

An alternative strategy for increasing the LHE is to use 

nature’s antenna effect, Fig. 6. 88–94 Multiple pigments that are 

suitably arranged can absorb light and vectorally transfer their 

energy to a central pigment that can then inject an electron 

into the semiconductor. If the additional pigments do not 

increase the footprint of the sensitizer on the semiconductor 

surface, this is a method for enhancing the LHE. Indeed, the 

trinuclear Ru II sensitizer utilized in the celebrated 1991 Nature 

paper had been previously designed in Italy to function as an 

antennae. 91 An issue with the Ru(dcb)2(CN)2 group used as 

the energy transfer acceptor and surface anchor was the cis 


Fig. 6 A scheme depicting an array of sensitizers bound to a planar 

TiO2 surface consisting of cis- and trans-[(Ru(bpy)2(pz))4(ina)] 8+ on 

the left and right, respectively, where pz is ambidentate pyrazine and 

ina is isonicotinic acid. The trans orientation may allow for increased 

absorptance, a, without increasing the projected footprint of the 

sensitizer. Taken from Fig. 1 of ref. 95. 

geometry of the ambidentate cyano ligands, which resulted in 

a larger footprint as the number of Ru II pigments was 

increased. In this regard, a trans geometry is more preferred. 95 

The synthesis of molecules that function as antennae and their 

use in DSSCs continues to be an active area of research that 

may one day enable the efficient sensitization of planar 

semiconductor materials. 93 

C Excited-state time scales 

The excited-state lifetime of [Ru III (bpy)2(bpy )] 2+ * is B1 

microsecond in water. 96 The radiative rate constant is typically 

about two orders-of-magnitude smaller than the non-radiative 

rate constant and hence the excited-state lifetime is controlled 

by the latter. 96 Ru II - and Os II -polypyridyl excited states have 

been shown to follow Jortner’s Energy Gap Law, where the 

non-radiative rate constant increases exponentially with decreasing 

energy gap. 97–101 For this reason, it has proven to be 

difficult to prepare compounds that emit in the infrared region 

and have long-lived excited states. A large ligand-field splitting 

parameter is required for the observation of long lifetimes in 

this class of excited states. The presence of low-lying, ligandfield 

states can rapidly deactivate MLCT excited states and 

decrease excited-state lifetimes. A classical example of this is 

Fe(bpy)3 2+ which, until recently, was thought to be completely 

non-emissive due to rapid and quantitative internal conversion/intersystem 

crossing through ligand-field states. 

As described further below, one fascinating aspect of DSSCs 

is the ultrafast excited-state injection into the semiconductor 

which has been observed under many experimental conditions. 

102–119 It is therefore useful to describe the time scales 

on which Ru II -based coordination compounds undergo equilibration 

to their MLCT thexi states. Using transient absorption 

anisotropy measurements on Ru(bpy)3 2+ in acetonitrile, 

McCusker and colleagues have identified charge-localizing 

decoherence of the initial, Franck–Condon, D3-symmetrical 

excited state occurring with a lifetime of 59 fs. 36 The kinetics 

were proposed to be coupled to inertial solvent dynamics as 

the lifetimes were solvent dependent in nitrile solvents and 

ranged from 59 to 173 fs in an order expected based on such a 

hypothesis. The contradictory conclusion that formation of 

such a C2-symmetrical excited state occurs immediately upon 

light excitation can be disregarded assuming a decoherent 

mechanism for the randomization of the initially formed, 

D 3-symmetrical excited state. 39,120 The reason for this was 

that the techniques previously employed, i.e. resonance 

Raman and Stark effect spectroscopy, solely report on coherent 

states, like that of the localized 1 MLCT excited state, and not 

on delocalized states, like the initial, Franck–Condon excited 

state. Speculation of longer-lived charge hopping as a means 

of randomizing the ligand radical excited state is also not 

possible based on these observations, although the anisotropic 

results are still not fully understood. 35,121,122 

Subsequently, by femtosecond fluorescence upconversion it 

was shown that the lifetime of the 1 MLCT excited state of 

Ru(bpy) 3 2+ was 45 15 fs. As this measurement directly 

probes the spin of the electrons, this lifetime is that of the true 

singlet-to-triplet intersystem crossing to the vibrationally ‘hot’ 

triplet manifold of states, Fig. 7–3. 123 This value agrees well 

with those obtained in water using time-resolved, femtosecond 

stimulated Raman spectroscopy and polychromatic, femtosecond 

fluorescence upconversion. 124,125 Spectral features 

lasting B300 fs and observed by femtosecond, magic-angle 

transient absorption spectroscopy were also assigned to relaxation 

of the charge-localized, 1 MLCT excited state of 

Ru(bpy) 3 2+ to the triplet-character thexi state. 126 As this 

method probes the absorption of states and not spin directly, 

the reported half-time (t 1/2 = B100 fs) provided an upper 

limit to the true intersystem-crossing lifetime. Additionally, it 

could be reporting on both intersystem crossing and vibrational 

cooling within the manifold of triplet-character states, 

Fig. 7–3 and 7–4, respectively. Further evidence for such a 

Fig. 7 Lennard-Jones potential energy wells illustrating the relative 

electronic and vibrational energies and lifetimes for Ru(bpy)3 2+ . Both 

internal-conversion thermal relaxation (2) and intersystem crossing (3) 

occur in the sub-picosecond time scale while the lifetime of the thexi 

state (5) is up to a microsecond. Taken from Fig. 9 of ref. 129. 


process was obtained by employing similar measurements, 

however in addition to the sub-picosecond component, a 

higher energy (360 nm), longer-lifetime (B5 ps) transient 

feature was also present. 35 As the ligand radical has a rather 

high extinction coefficient here, this component was assigned 

to vibrational relaxation to form the thexi state. This vibrational–relaxation 

lifetime within the manifold of states was 

shown to vary from B0.6 to 5.0 ps and be rather solvent 

dependent. 35,123,127,128 Using picosecond Kerr-gated, timeresolved 

resonance Raman spectroscopy the lifetime of this 

relaxation was shown to be B20 ps for homoleptic and heteroleptic 

Ru(bpy)3 2+ -based molecules of varying charges and isotopic 

compositions and in a variety of solvents. 129 For 

comparison, N3 0 s 1 MLCT excited-state t 1/2 wasreportedtobe 

B30 fs and thermal relaxation within its triplet-character manifold 

was found to occur with a B80 fs half-time when bound to 

mesoporous, nanocrystalline TiO 2 thin films. 107 

D Dye sensitization 

Some early dye sensitization studies employed Ru(bpy)3 2+ 

dissolved in the external electrolyte. 130,131 However, it was 

soon found that anchoring the sensitizer to the semiconductor 

surface was a more practical approach. 132 Anchoring transition-metal 

compounds to the TiO2 surface requires functional 

groups that can form strong bonds with the metal-oxide 

surface. Functional groups based on carboxylic acids, phosphonates, 

alcohols, amides, siloxanes, acetyl acetonates, and 

cyanides have all been tested. 72 The aforementioned dcb 

ligand with two carboxylic acid groups remains the most 

successful in terms of absolute efficiency in DSSCs. In 1979, 

Goodenough and co-workers first proposed that dehydrative 

coupling of carboxylic acid groups with surface titanols would 

result in the formation of ester-type linkages. 132 He suggested 

that the p* orbitals of the dcb ligand would promote rapid 

excited-state electron injection into the conduction band of 

TiO2 but not that of SnO2 or ZnO. The difference being one 

of symmetry as the TiO2 conduction band is comprised mainly 

of unfilled d orbitals where that of SnO2 and ZnO possess 

predominantly s-orbital character, Fig. 8(a)/(c). This latter 

suggestion now has some experimental verification. 104 Interestingly, 

the proposed coupling is optimal when the ester and 

bpy p-systems are co-planar, yet such a geometry is not found 

in the ground state due to unfavorable steric interactions. In 

crystal structures of the corresponding ethyl ester compound, 

the plane defined by the C–CQO of the ester group is skewed 

by 10–151 from the plane of the pyridine ring, Fig. 8(b). 133 

Furthermore, there is no measurable resonance enhancement 

of the symmetric COO stretching mode in Raman experiments 

further indicating that these groups are unconjugated in solution 

and when bound to TiO2. 134,135 However, upon MLCT excitation, 

the bpy ring is formally reduced by one electron and the 

ester group may twist. Persson et al. have shown computationally 

that the planar geometry enhances excited-state injection. 136 

Only under very acidic, non-aqueous conditions has 

evidence for an ester-type linkage been observed. 137 Physisorption 

through a solvation layer has been proposed by 

Hester and colleagues. 138 Under most conditions relevant to 

DSSCs, the predominant binding mode elucidated through IR 

Fig. 8 Orbital diagrams for ester-type binding to the surface of metal 

oxides. (A) For TiO2, the overlap of the extended p system and the Ti 

3d orbitals are thought to aid in electron injection. (B) When carboxylates 

are rotated in such a way as to minimize orbital overlap, the 

injection yields are thought to suffer. (C) Similar effects are proposed 

for SnO2 as the Sn s orbitals have less efficient orbital mixing with the 

carboxylate p system. Adapted from Fig. 4 of ref. 132. 

studies is a carboxylate-type linkage; 137 unfortunately, 

the data does not allow for direct identification of the 

surface site(s) involved in the sensitizer–semiconductor 

bond. 132,134,137,139–141 Deacon and Phillips have tabulated 

vibrational data for metal-carboxylate compounds whose 

structures were determined crystallographically. 142 An empirical 

relation between the energy separation of the COO asymmetric 

and symmetric stretches and the carboxylate–metal 

coordination mode was found. This same approach has been 

used to predict the carboxylate binding mode on the anatase 

TiO2 surface, presumably to Ti IV sites. 134,140,141 In agreement 

with theoretical studies, the analysis is most consistent with the 

carboxylate oxygens binding to separate Ti IV -metal centers. 

134,137,140,143 Such carboxylate linkages were observed 

even when the binding group was originally an ester, e.g. with 

the deeb ligand which is 4,4 0 -(C2H5CO2)2-bpy. Therefore, we 

make no distinction between deeb and dcb throughout this 

review. Similarly, as the extent of deprotonation of sensitizers 

on the TiO2 surface is often unknown, the overall formal 

charge of semiconductor-bound sensitizers is often omitted. 

While transition-metal compounds based on dcb ligands 

remain the most successful for DSSCs, an important limitation 

is their poor stability in water. 144 Moderate stability has been 

reported in acidic electrolytes, but the sensitizers rapidly 


desorb when the pH is raised above pH B3.5. 145 In aqueous 

solutions, the most stable linkages appear to be those based on 

phosphonate groups. 144 

There now exists a large body of literature on the sensitization 

of TiO 2 by Fe II -, Ru II -, Os II - and Re I -polypyridyl compounds. 

146 There have also been some reports of sensitization 

by d 8 compounds based on Pt II , that also have MLCT-like 

excited states, and d 10 Cu I compounds. 147–149 Some of these 

results are highlighted in this review as alteration of the metal 

center has, in some cases, provided insights into mechanistic 

details of dye sensitization. 

It is often tacitly assumed that the manifold of MLCT 

excited states observed in dilute solution or frozen glasses is 

unperturbed by the semiconductor surface. This assumption is 

often necessary as ultrafast injection precludes characterization 

of the excited state. However, as described in more detail 

below, the acceptor states in TiO 2 can be widely tuned in 

energy by controlling the concentration of potential-determining 

ions at the interface. With this approach and by utilizing 

sensitizers that are weak photoreductants, data on MLCT 

excited states anchored to TiO2 are now becoming available. 

One interesting finding is that the proximity of the sensitizers 

to one another on the surface affords efficient lateral energy 

transfer across the semiconductor surface. 150,151 Monte-Carlo 

simulations indicate a (30 ns) 1 energy transfer hopping rate 

constant at saturation surface coverage. 152 There is also evidence 

that the ligand-field states are destabilized upon surface binding. 

For example, compounds of the type [cis-Ru(bpy) 2(ina) 2] 2+ , 

where ina is isonicotinic acid, are non-emissive in fluid solution 

with high quantum yields for photo-induced ligand loss, behavior 

that is expected for compounds with low-lying, ligand-field 

excited states. However, upon binding to MO2 (M = Ti or Zr) 

thin films, the compounds were found to be photoluminescent 

with temperature-dependent, excited-state lifetimes that were 

B50 ns at room temperature. 54 Both static and dynamic 

excited-state quenching were observed as the temperature was 

raised providing direct evidence that the intersystem-crossing 

quantum yield was temperature dependent and less than unity. 

Interestingly, when bound to TiO 2 thin films there was an 

inverse relation between the temperature and the quantum yield 

for photo-induced, interfacial electron injection, herein referred 

to as the ‘injection yield.’ 

3. Photo-induced, interfacial charge separation 

The excited-state, interfacial-charge-separation mechanism 

shown in Fig. 1 is in fact only one of three mechanisms 

identified for electron injection. Said mechanisms differ 

by the state of the sensitizer and location of the electron 

that is transferred to the semiconductor: (1) the excited 

state, i.e. [Ru III (bpy)2(dcb )] 2+ *; (2) the reduced state, i.e. 

[Ru II (bpy)2(dcb )] + ;or(3)via a molecule-to-particle charge 

transfer event, i.e. Ru II –CN–Ti IV . An important variable 

for all of these sensitization mechanisms is the overlap of 

the molecular donor levels with the acceptor states of the 

semiconductor. 

Gerischer formulated a theory for excited-state injection 

into wide-bandgap semiconductors. 153–155 The rate of interfacial 

electron transfer at an electrode surface is proportional 

to the overlap of occupied donor excited states with unoccupied 

acceptor states: 

k inj B R k(E)D(E)W don(E) dE (6) 

where E is the energy, k(E) is the transfer frequency, D(E) is 

the density of unoccupied acceptor states (DOS) in the semiconductor, 

and W don(E) is the sensitizer donor distribution 

function. Fluctuations in the solvation of the sensitizer give 

rise to a distribution of excited-state energies. Gerischer 

defined the Gaussian donor and acceptor excited-state distri- 

bution functions, W(E): 

1 

WðEÞ ¼pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 

exp 

4plkBT 

ðE EÞ 2 

! 

4lkBT 

where l is the reorganization energy of interfacial electron 

transfer, kB is Boltzmann’s constant, T is the temperature, and 

1E is the energy of the most probable solvation state. Thus, the 

rate constant of, and often the efficiency for, injection from the 

sensitizer are expected to depend critically on the overlap of 

the sensitizer excited-state distribution function with the 

semiconductor DOS. 

A Density of states in nanocrystalline TiO2 thin films 

used in DSSCs 

What are the density of unoccupied acceptor states, i.e. DOS, 

in nanocrystalline, anatase TiO2 thin films? This question 

remains somewhat unresolved. The classical method for determining 

these in the solid state is via photoelectron spectroscopy. 

Hagfeldt and co-workers have reported such data for a 

nanocrystalline TiO2 thin film sensitized with N3 in the 

presence and absence of Li + salts, Fig. 9(a). 156 This data 

shows a broad distribution of trap states centered at B1 eV 

below the energy of the conduction band edge (Ecb). However, 

it is well known that the flatband potentials of the semiconductors 

are very sensitive to environment. Therefore, the 

absolute and relative energies in vacuum may not be as 

relevant to a DSSC. In the field of photoelectrochemistry, 

the standard approach for determining the flatband potentials 

of semiconductor electrodes is Mott–Schottky analysis of 

capacitance data. 157 The analysis is based on the potentialdependent 

capacitance of a depletion layer at the semiconductor 

surface, behavior that is not likely observed for B20 nm 

anatase crystals that are expected to be fully depleted near 

kT. 18,158–165 Rothenberger and co-workers have proposed an 

accumulation-layer model to describe the potential distribution 

within the TiO 2 particles at negative applied potentials. 166 

This model assumes that the band-edge positions remain fixed 

as the Fermi-energy is raised into accumulation conditions, 

behavior that has little literature precedence in electrolyte 

solutions. Nevertheless, the model provides the only literature 

estimates of Ecb available for these materials in organic and 

aqueous solvents with common electrolytes. 166–170 The literature 

values give the impression that the nanocrystalline TiO2 

thin films have a well-defined Ecb. Even if this is the case, there 

is a tremendous compilation of data supporting the notion 

that the acceptor states relevant to interfacial charge separation 

and recombination are more localized and are reduced 

more easily than literature E cb values indicate. 171–174 


ð7Þ

Fig. 9 (A) The density of acceptor states, DOS, for TiO2 thin film electrodes as measured by photoelectron spectroscopy and electrochemical methods 

(smallerplot).Thefigureswerescaledsoastoaligntheenergiesofthe(surface) deep trap states, exponential DOS near the conduction band, and 

conduction band edge; however, the energy differences among these states are dissimilar. Adapted from Fig. 1(b) of ref. 156 and Fig. 3 of ref. 171. 

(B) A diagram depicting the proposed energetic and spatial location of these same states as a function of their depth in a nanoparticle relative to the 

energy of the conduction band edge, Ecb, and the energy of the solution redox electrolyte, EF,redox. Adapted from Fig. 2(a) of ref. 171. 

Many electrochemical, photochemical, and spectroscopic 

studies have supported the suggestion that mesoporous, nanocrystalline 

TiO2 thin films possess a tailing of the DOS rather 

than an abrupt onset from an ideal Ecb. Determination of the 

precise form of these tailing states is non-trivial, however a 

novel computational method for determination of the absolute 

DOS distribution at zero Kelvin was recently reported by 

Bisquert and Zaban, and colleagues. 172,174 Although fundamentally 

important, the room temperature apparent DOS 

distribution is more relevant to the functioning DSSC. At room 

temperature, this distribution is thought to have an exponential 

dependence on the applied voltage as determined from electrochemical 

techniques where Fermi-level pinning was deduced 

to be negligible, Fig. 9(a), inset, 171,173–175 and recently by 

a spectroelectrochemical procedure. 176 Additionally, nonexponential 

kinetics for excited-state electron injection can be 

rationalized by invoking an exponential DOS at the TiO 2 

surface. 177–180 And by assuming said distribution is composed 

of bulk, intra-bandgap states, Fig. 9(b), diffusion of TiO 2 

electrons, TiO2(e )s,8 and dispersive recombination kinetics 

can be modeled satisfactorily by employing a multiple-trapping, 

continuous-time random walk model. 178,181–186 In addition, via 

these same techniques, Kavan et al., and many others since, 

have reported that TiO2 thin-film electrodes contain a relatively 

large population of deep, surface trap states at an energy 

located within the bandgap and prior to a significant portion 

of the exponential distribution. 171,187–193 These states are believed 

to be unsaturated Ti IV surface states where oxygen 

vacancies reside. The energetics of such states were shown to 

be affected by surface chelation from various molecules due to 

the Lewis acidic and basic characteristics of the unsaturated 

Ti IV and surface-bound molecules, respectively. 188,189,192,194 

8 In much of the literature on nanocrystalline, anatase TiO 2, there is 

contention as to whether electrons in TiO2 are located in the diffuse 

conduction band, bulk exponential trap states, or deep surface trap 

states. For simplicity and clarity, collectively they will be denoted as 

TiO 2(e )s throughout this review. Although it is often implied that 

TiO 2(e )s are those located within the exponential DOS, no distinction 

will be made unless it aids in understanding the desired point. 

The TiO2(e )s inferred from electrochemical measurements 

have spectroscopic signatures as well. As the indirect bandgap 

of anatase TiO2 is 3.2 eV, its ground-state UV-Vis absorption 

spectrum consists of a fundamental absorption edge at 

B385 nm and, in some cases, an Urbach tail at longer 

wavelengths. 195–198 The features observed for TiO 2(e )s in 

mesoporous, nanocrystalline TiO 2 (anatase) thin-film electrodes 

consist of a minor Burnstein–Moss shift, i.e. a blue shift in the 

fundamental absorption edge, 199,200 and a gradual rise in 

absorbance that tails to the near-IR, 201,202 and peaks at 

B1350 nm. 203 The extinction coefficient for the broad, 

featureless, visible-near-IR absorbance ranged from 640 to 

1300 M 1 cm 1 at 700 to 800 nm based on choice of solvent 

and electrolyte. 166,187,204,205 Similar features were present upon 

electrochemical bias of a single-crystal TiO2 (rutile) electrode 

to form TiO 2(e )s: a broad near-IR spectroscopic feature that 

peaked at 1500 nm. 206 As evidenced by spectroelectrochemical 

measurements, in addition to the current required to generate 

the ‘‘typical’’ TiO 2(e ) absorption features, an additional 

current pre-peak has been observed that is often largest in 

aqueous electrolyte, Fig. 10(a). 187,189,190 This has been 

ascribed to filling deep, surface trap states. It was determined 

that B12 of these surface states existed per 12 nm nanocrystallite 

and that they exhibited an absorption peak centered at 

B400 nm (e400 nm = B1900 M 1 cm 1 ), Fig. 10(b). 187 

Additionally, a new, broad absorption peak centered at 

B750 nm was observed (e 700nm = B2200–2800 M 1 cm 1 ), 

after passing 440 mC cm 2 (B100 TiO 2(e )/particle) in the 

presence of cations with large charge-to-radius ratios, i.e. Li + , 

Na + , in acetonitrile or strongly basic aqueous electrolytes, 

Fig. 10(c). 205,207,208 This feature is indicative of small cation 

intercalation into the anatase lattice to form new 

phases. 156,190,209–215 Li + intercalation into highly reduced 

anatase TiO2 is known to form Li0.5TiO2 phases, 216,217 however 

such phases are not expected to be relevant to 

operational DSSCs. 

The TiO2(e ) states above are often described as shallow 

trap states and not entirely free conduction band electrons as 

their absorption would tail much farther into the IR, 218,219 


Fig. 10 (A) A cyclic voltammogram of a TiO2 thin-film electrode in aqueous electrolyte. The large, reversible peak was indicative of filling and 

emptying the TiO2 DOS whereas the smaller pre-peak, present during the cathodic scan only, was assigned to the filling of deep trap states. Taken 

from Fig. 3(a) of ref. 187. (B) The absorption spectra of these biased electrodes illustrated the spectroscopic features associated with occupation of 

deep trap states, at 0.30 V and in bold, and formation of TiO2(e )s, at 0.70 V. Taken from Fig. 2 of ref. 187. (C) The absorption spectra of a 

thin film electrode in LiClO4 acetonitrile electrolyte biased to 1.50 V where formation of a new species, i.e. Li0.5TiO2 phases, was clearly evident 

near 750 nm. Taken from Fig. 3(a) of ref 205. 

they exhibit a sharp electron paramagnetic resonance (EPR) 

spectrum at 77 K, 220–222 and their apparent DOS follows an 

exponential distribution 156,171–175,177–180 with a non-ideality 

factor often greater than one. 171,178,183–185,223,224 An apparent 

exponential DOS distribution is also expected from theory for 

an ideal intrinsic semiconductor even though the actual underlying 

DOS distribution follows a power-law relationship with 

energy. 225 However, the presence of a non-ideality factor 

unequal to unity is often attributed to a large concentration 

of trap states 218,219 Notwithstanding, using time-resolved 

infrared (TRIR) spectroscopy it was shown that the transient 

absorption features of TiO 2(e )s in TiO 2 and TiO 2–Pt colloids 

can be modeled as a function of the wavenumber to the 

1.5 power, indicative of free conduction band electrons. 

203,226 As electrons are thought to trap in TiO2 at 

coordinatively unsaturated Ti IV atoms within a picosecond it 

was proposed that trapped electron thermalization to the 

conduction band may be possible at room temperature. 

A final comment with regard to the semiconductor DOS is 

that they are not singular material parameters. The most wellknown 

example is the nearly Nernstian shift, i.e. 59 mV/pH 

unit, in aqueous solution over the pH range H 0 = 8 to 

H = +23 due to protonation/deprotonation of surface titanol 

groups on TiO2. 166,169,227,228 It has also been known for quite 

some time that the flatband (and conduction band edge) 

potential of mesoporous, nanocrystalline TiO2 (anatase) can 

be widely tuned by the presence of cations in non-aqueous 

supporting electrolyte. This affect is greatest with cations 

possessing a large charge-to-radius ratio in the order Mg 2+ 

4 Li + 4 Na + 4 K + 4 TBA + . 167,168 For example, Ecb has 

been reported to be 1.0 V vs. SCE ( 0.76 V vs. NHE 229 )in 

0.1 M LiClO 4 acetonitrile electrolyte and B 2.0 V ( 1.76 V) 

when Li + was replaced by TBA + . The direction of the bandedge 

shifts has been confirmed by excited-state quenching data 

described below. Interestingly, this same order has been 

observed for the equilibrium constants for cation adsorption 

onto TiO2 in aqueous solutions 230–233 and an electrolyte’s 

‘‘drying effect,’’ ionic association constant in aprotic solvents, 

and hydroxide association constant. 234 Although this shift 

is non-Nernstian, the behavior has been shown to be logarithmic 

in LiClO4 activity in acetonitrile and other aprotic 

mixed solvent systems. 167,168 Similar behavior was not observed 

in protic solvents hypothesized to be due to selective solvation 

of Li + by the protic solvent molecules. 167,168 In TBA + salts the 

flatband potential has been shown to depend logarithmically on 

the auto-ionization/autoprotolysis constant of the solvent. 167,168 

Thus, most likely, the large variations in Ecb (41V)inducedby 

the above ‘potential determining’ ions can be wholly explained 

by cation-coupled reduction potentials for TiO2 acceptor states, 

due to surface adsorption and/or intercalation into the anatase 

lattice. This same cation-dependent shift in E cb can be used to 

promote photo-induced electron injection from surface-bound 

sensitizers. 

B Ultrafast, excited-state electron injection 

After light absorption, the MLCT excited state of the sensitizer 

may inject an electron into the anatase nanocrystallite, a 

process also referred to as interfacial charge separation. For 

sensitizers like N3, light absorption formally promotes an 

electron from the metal center to a dcb ligand that is directly 

bound to the semiconductor surface. Therefore, excited-state 

charge separation occurs from the p* orbitals of the organic 

ligand to the acceptor states in TiO2, Fig. 8(b). There is now an 

overwhelming body of data that indicates that such charge 

separation occurs on a femto- to pico-second time scale. 

Experimentally, ultrafast spectroscopists have all found that 

excited-state electron injection into TiO2 is non-exponential, 

behavior attributed to the surface heterogeneity of TiO2 and its 

DOS, distributions of sensitizer binding modes, strengths, and 

interactions, and multiple ultrafast injection processes occurring 

from various states in the thermal relaxation pathway, i.e. 

Franck–Condon singlet injection, internally converted singlet 

injection, intersystem crossing to the triplet state(s) followed by 

injection. This has been thoroughly reviewed for both organic 

and transition-metal coordination compounds bound to semiconductor 

metal oxides. 102,115,119 While the explanations given 

to rationalize the complex kinetics observed for excited-state 

injection for Ru II sensitizers are often reasonable, satisfactory 

mechanistic models are still lacking. 

It has been suggested that ultrafast, interfacial charge separation, 

following light absorption, does not always occur from the 

thexi state but rather often from the initial, Franck–Condon 


excited state. Evidence for room-temperature injection occurring 

with a lifetime faster than a molecular vibration, i.e. kBT/h = 

B1.6 10 13 s = 160 fs, 235,236 eludes to this phenomenon. 102–119 

This would imply that injection is occurring before thermal 

relaxation of the molecular excited state. 

i Coherent, singlet injection. Willig and co-workers found 

that excited-state electron injection from N3* into TiO 2 

occurred in o25 fs under ultrahigh-vacuum conditions. 109 

The process therefore did not involve redistribution of vibrational 

excitation energy by exchange with phonons in the solid 

and thus was entirely different from the weak-electroniccoupling 

case of Marcus–Levich–Jortner–Gerischer-type electron 

transfer. 154,155,237–240 The finite reaction time for injection 

ruled out direct excitation of an electron from the Ru II -metal 

center to the semiconductor, yet the sub-100 fs rise-time implied 

vibrational wave packet motion-induced electron transfer. A 

detailed analysis of theoretical and empirical results supporting 

these conclusions using a perylene sensitizer can be found 

elsewhere. 241–244 Briefly, clear, resolvable periodic beats, 

Fig. 11(a), were observable in the ultrafast transient signals 

consistent with coherent, singlet injection from the perylene 

singlet excited state. Fourier transform of these beats adequately 

reproduced the normal Raman modes of perylene, 

Fig. 11(b),(c). Thus, the oscillatory behavior was ascribed to 

pulsed electron transfer due to periodic surface crossing to the 

non-linear DOS in the semiconductor realized by vibrational 

wave-packet motion in the excited perylene, Fig. 11(d). 

The quantitative, ultrafast excited-state electron injection 

reported for N3/TiO 2 under ultrahigh-vacuum conditions was 

not always observed when the sensitized thin films were placed 

in organic solvents or electrolytes. Under such conditions, 

injection was non-exponential and occurred on the femtosecond 

to hundreds-of-picoseconds time scale. For N3 and 

porphyrin-based sensitizers, Durrant and co-workers found 

that a sum of three exponentials was required to fit the 

injection data, including an ultrafast o100 fs component. 

117,118 Interestingly, the rate constants for bpy- and 

porphyrin-based dyes were similar. These same authors later 

found that N719—the dianion salt of N3 with TBA + counterions—had 

a 30-fold slower rate of injection as compared to 

N3. 245 After performing multiple washings of the N3/TiO2 

films in neat ethanol the injection rates were similar to that of 

N719/TiO2 films. It was suggested that the labile protons from 

the carboxylic acid binding groups of N3 had lowered the Ecb 

and promoted more favorable energetics for injection. To 

control this variable Lian and co-workers pre-treated 

N3/TiO 2 thin films for one day in aqueous buffer solutions 

at pH 2, 4, 6, or 8. 103 After removing weakly bound and 

desorbed sensitizers, the biphasic kinetics and injection yields 

were found to be pH dependent. As the pH was raised from 

2 to 8, there was a decrease in the rate of the slower component 

to injection, the ratio of the faster-to-slower components to 

injection, and the injection yield. Such behavior is consistent 

with the expected Nernstian shift of Ecb towards the vacuum 

level as the pH is raised. 

Gra¨tzel and co-workers reported that the slower picosecond 

components for excited-state electron injection could be 

by employing a low concentration or sonicated dying solution 

or a lower surface-coverage thin film. 246,247 Under such conditions, 

only an ultrafast component (o20 fs) for injection 

remained. In support of this, Piotrowiak and co-workers found 

that dialysis of sensitized TiO2 colloids resulted in much shorter 

excited-state lifetimes as measured by time-correlated single 

photon counting. 248 However, in this case multi-exponential 

kinetics were required to adequately fit the observed data. 

Lian and co-workers found that excited-state electron injection 

into TiO2 was biphasic for three [cis-Ru(dcb)2(X)2] 0,0,2+ 

Fig. 11 (A) An ultrafast, time-resolved, single-wavelength absorption difference spectrum for perylene/TiO2 displaying periodic beats. The 

Fourier transform, (B), of the periodic beats, (inset), effectively reproduced the normal modes of perylene, (C). (D) A schematic depicting 

the model used to rationalize the empirical data; periodic crossing of the molecular vibrational wavepacket with the TiO2 DOS. Taken from 

Fig. 5 and 10, respectively of ref. 244. 


compounds (X = SCN ,X=NC , or (X)2 = dcb) and fit a 

two-state model. 103 The rate of the slower component was 

directly related to the sensitizer excited-state reduction potential 

while the relative magnitude showed the opposite trend. 

No noticeable changes were apparent for the fast component 

within the time resolution of the measurement, i.e. B200 fs. 

With Re(dcb)CO3Cl/TiO2 it was suggested that ultrafast 

injection (o50 fs) was from a vibrationally ‘hot’ state. 103–106 

As measured by femtosecond TRIR spectroscopy, the CQO 

stretching band in the excited state red-shifted by 10 cm 1 over 

10 ps. The difference in rate constants implied that injection 

occurred before thermal electron relaxation and reorganization 

of the inner-sphere ligand environment. This same group 

reported the injection dependence for carboxylic acid versus 

phosphonic acid linkers with Re(dmb-X 2)CO 3Cl sensitizers, 

where dmb is 4,4 0 -dimethyl-bpy (X = COOH or PO 3H 2). 116 

The sensitizer with X = PO 3H 2 resulted in faster injection which 

was in conflict with previous findings employing organic sensitizers. 

108 However, the experimental data was supported by DFT 

calculations on the anionic versions of the sensitizers showing 

that there was a stronger electronic coupling between the dmb-X2 

and Ti IV -metal centers when bound through phosphonate 

linkages. 116 Additionally, solvent-dependent injection rates 

were studied using Re(dcb)CO3Cl sensitizers. 249 It was found 

that the rate of the slow, picosecond component for injection 

decreased in the order water (pH 2) 4 MeOH E EtOH 4 water 

(pH 8) 4 DMF which could be expected based on the proposed 

E cb for TiO 2 and electron transfer in the Marcus normal region. 

However, changes were not as large as expected due to trace 

water adsorbate whose presence was verified by FTIR. 

McCusker and co-workers found excitation wavelengthdependent, 

tri-exponential kinetics for N3/TiO2, cis- 

Ru(dcb)2(CN)2/TiO2, and their osmium analogues. 112 For 

the Ru II -based sensitizers, excitation at shorter wavelengths 

resulted in a larger amplitude femtosecond component, assigned 

to electron injection from the 1 MLCT excited state, and 

thus a smaller picosecond amplitude, assigned to injection 

from the 3 MLCT excited state. On the picosecond-time scale, 

3 MLCT components were much more dominant for osmium 

analogues where the spin–orbit coupling was larger. For all 

sensitizers studied, the rate of the picosecond component was 

found to be directly related to the sensitizer excited-state 

reduction potential, E o (Ru III/II* ), consistent with electron injection 

from the thexi state. 

At about the same time, Sundstro¨m and co-workers reported 

stimulated emission from the initially formed, singlet 

excited state of N3 with a B70 and B30 fs half rise-time 

in solution and on TiO2, respectively. 107,111 These time 

scales were similar to those measured by femtosecond transient 

absorption spectroscopy for intersystem crossing, 

Fig. 12(b). 107,111,114 The branching ratio for electron injection 

from the 1 MLCT state and intersystem crossing to the 3 MLCT 

state resulted in time constants of B50 and B75 fs for each 

process, respectively. Excitation into the low-energy shoulder 

of N3’s absorption spectrum was shown to directly populate 

N3’s 3 MLCT manifold both in solution (t = B70 fs) and on 

TiO 2. It was also shown that injection became slower and less 

efficient, i.e. from B100% to B50%, as the excitation light 

was shifted towards longer wavelengths, Fig. 12(b). This was 

postulated to be due to injection from a manifold of 3 MLCT 

excited states. Similar findings have been observed for 

[Ru(bpy)2(dcb)] 2+ on SnO2 but resulting in slightly larger 

half-times, 250 behavior that is consistent with Goodenough’s 

hypothesis. 132 These same authors found that by varying the 

method of TiO2 film preparation, both rate constants for the 

biphasic injection kinetics for N3* into TiO 2 were directly 

related to the degree of TiO 2 crystallinity. 110 Similar effects 

have been observed with organic sensitizers. 251,252 

As mentioned previously, spin arguments with Ru II and 

Os II sensitizers are complicated by spin–orbit coupling that 

effectively mixes the spin states, no longer making spin a good 

quantum number. With organic sensitizers this is not the case 

and well-defined singlet and triplet states have been shown to 

sensitize TiO2. With a Ti IV -phthalocyanine sensitizer anchored 

to TiO2 via an axial 3,4,-dihydroxybenzoic acid ligand, wavelength-dependent 

injection yields were apparent. 253 Although 

such behavior could have been attributed to ‘hot’ injection, 

this was not thought to be the case here. The rate constants 

for excited-state injection from the equilibrated singlet and 

triplet excited states were proposed to be different. This stateselective 

injection was assigned to efficient kinetic competition 

between injection from the S2 excited state (from excitation 

Fig. 12 (A) Picosecond transient absorption difference spectra for N3/TiO 2 where changes due to 3 MLCT excited-state injection are noted. Taken 

from Fig. 2 of ref. 107. (B) Time-resolved, single-wavelength absorption difference spectra for N3 and N3/TiO2 demonstrating relaxation within 

the triplet-character manifold of states on the picosecond time scale. Excitation wavelengths are indicated on the figure. Taken from Fig. 4 of 

ref. 114. (C) A schematic depicting the possible interfacial, excited-state processes: (a) ultrafast, ‘hot’ injection; (b) intersystem crossing; 

(c) vibrational relaxation; and (d) slower thexi-state injection. Taken from Fig. 9 of ref. 111. 


into the Soret band) and internal conversion/vibrational relaxation 

of this state to the lower lying S1 state (that can be 

directly populated with excitation into the Q bands). 

ii TiO 2 DOS and quasi-Fermi-level dependence. Another 

school of thought is that the multiphasic, excited-state electroninjection 

kinetics do not result solely from heterogeneity of the 

sensitizer but also reflect differences in the TiO2 DOS, Fig. 13. 

There is growing evidence that a well-defined Ecb is not 

relevant to excited-state injection for these sensitized nanocrystalline 

thin films. The previously mentioned slower injection 

for N719 over N3, is thought to result from proton 

adsorption-induced shifts in the DOS. 245 Interestingly, even 

though injection was observed to be slower after excitation of 

N719/TiO2, the energy conversion efficiency was found to be 

higher. 177 This occurs because proton-induced shifting of the 

DOS is positive on an electrochemical scale and can thus lower 

the V oc. Keep in mind that due to the long-lived nature of the 

MLCT excited states, a quantitative injection yield could 

occur even if injection dynamics were slowed to the few 

nanosecond time scale. Indeed, it was shown that multiphasic 

injection half-times for N719 and N3 were 12 and 0.4 ps, 

respectively, with over an order-of-magnitude slower chargeseparation 

dynamics for N719. The injection kinetic data fit a 

model employing an exponentially increasing DOS, which is 

apparent elsewhere in the literature, 171,173–175,178–180 and activationless 

excited-stated electron transfer from a Gaussian 

distribution of energy offsets. Monte Carlo numerical simulations 

178,179 were in excellent agreement with the empirical 

reaction dynamics. It was also found that the excited-state 

injection kinetics for N719 were over 20 times slower in a full 

DSSC versus an inert electrolyte. However, the complete 

DSSC still showed excellent photovoltaic performance, due 

to the sluggish charge recombination kinetics and minimal 

Fig. 13 A Gerischer Diagram illustrating excited-state electron injection 

from surface-bound sensitizers into the DOS of the TiO 2 

nanocrystallites. E is the electrochemical potential of the conduction 

band edge (E cb), of the deep trap states (E T), and of the sensitizer at 

standard-state conditions (E 0 (A/D) and E 0 (A/D*), for the ground and 

thexi states, respectively). D(E) is the TiO 2 DOS, W don(E) and 

Wdon*(E) are the sensitizer donor distribution functions of the ground 

and thexi states, W acc(E) and W acc*(E) are the sensitizer acceptor 

distribution functions, and l is the reorganization energy. Adapted 

from Fig. 3 of ref. 171 and Fig. 3(a) of ref. 119. 

‘kinetic redundancy,’ where the time scale for injection was 

sufficiently less than the excited-state lifetime but not by an 

excessive amount. 

We have shown that the excited state of Ru(bpy) 2(dcb)/TiO2 thin films immersed in acetonitrile exhibit both static and 

dynamic quenching when Li + is introduced into solution. 254 

This was ascribed to photo-induced electron injection into 

TiO2 acceptor states where said states become thermodynamically 

accessible due to the positive cation-induced shift of 

the DOS. This was further supported by the monotonic and 

somewhat linear increase in both PLI/PLIo and injection yield 

with the logarithmic concentration of Li + (PLI is photoluminescence 

intensity). The trend for such behavior was linear in 

the charge-to-radius ratio of the 2 mM cation employed in the 

order Ca 2+ 4 Ba 2+ E Sr 2+ 4 Li + 4 Na + 4 K + 4 Rb + 

E Cs + E TBA + , and smallest for neat acetonitrile. 

As mentioned previously, in the absence of such external 

cations the flatband potential has been reported to scale 

logarithmically with the solution auto-ionization constant. 167,168 

This implies that proton activity determines the potential of the 

TiO2 DOS in these neat solvents. Thus, a strategy to introduce 

cations with a large charge-to-radius ratio into non-aqueous 

electrolytes was employed: acid- and base-pretreatment of TiO2 

thin films using H2SO4, HCl,orHClO4and NaOH, respectively. 

137 It was shown that when [Ru(bpy) 2(deeb)] 2+ sensitizers 

were bound to acid pre-treated TiO2 films they bound as the acid 

form, i.e. –COOH, and injected electrons much better than base 

pre-treated films, which bound as the carboxylate form, i.e. 

–COO . In fact, injection yields in neat acetonitrile and IPCEs 

in TBAI/I2 electrolyte were o10% for pH 43 pre-treatment 

while for pH o2.5 pre-treatment injection yields were 480%. 

(It is of note that the point-of-zero charge of TiO2 is B5–6 255–258 

while the pKa of the sensitizer carboxylic acid groups are 1.75 and 

2.80.) 259 Upon addition of LiClO4 to base pre-treated films, 

injection yields increased significantly; for acid pre-treated 

films, most of the dyes desorbed. 

All of the previously described photo-induced electron 

injection studies were performed on equilibrated systems 

under open-circuit conditions. It is important to quantify 

interfacial charge separation under short-circuit conditions—when 

the system is initially at a steady state—and to 

specifically quantify the effect(s) TiO2(e )s have on excitedstate 

injection. The first such report studied the biasdependence 

on the injection yield from a photoexcited 

Ru(dcb)3/TiO2 thin-film electrode in a pH 3, 0.2 M LiClO4 

aqueous electrolyte. 158 Upon reverse bias or near open-circuit 

conditions, the injection yield was essentially unity. However, 

as the electrode was biased in the forward direction, closer to 

the operational power point of the electrode, the injection 

yield dropped to B0.5, Fig. 14. This was ascribed to the filling 

of the DOS in TiO2 leading to decreased injection and an 

increase in PLI. Similar behavior was observed on sensitized 

SnO2 electrodes. 260 A complication in these studies is that 

forward bias can result in desorption of the sensitizers from 

the semiconductor surface, which will by itself lower injection 

yields and increase the PLI. 261 

A seven-fold increase in the half-time for excited-state 

electron injection from fully deprotonated N3/TiO2 in acetonitrile 

was obtained by omission of Li + from the solution. 262 


Fig. 14 The quantum yield of excited-state electron injection from 

surface-bound Ru(dcb) 3 2+ into TiO2 as a function of electrochemical 

applied bias. Inset: The photoluminescence spectra of Ru(dcb)3/TiO2 

thin film electrodes at the indicated potentials. Taken from Fig. 8 of 

ref. 158. 

Biasing the sensitized electrode to 700 mV vs. Ag/AgCl, the 

most negative bias where desorption/degradation did not occur, 

resulted in the same injection yield on the longest time scales 

studied, 600 ps, but with significant attenuation of the 

fast component to injection. The half-times for injection were 

25-fold slower at this applied bias and could be modeled by 

non-adiabatic electron transfer theory where, prior to injection, 

thermal equilibrium of the excited state was assumed. Since up to 

40% of the injection occurred on the sub-molecular vibration 

time scale, i.e. B160 fs, the injection kinetics were most likely 

modeled under conditions where the assumption was valid. 

iii Distance dependence. The multiphasic character and 

picosecond dynamics of excited-state electron injection into 

TiO 2 alludes to the idea that at least some injection is occurring 

from a thexi state. This state, which can be described by a 

Boltzmann population, may exhibit behavior typical of thermal 

electron transfer and/or electron tunneling. The latter is clearly 

evident by temperature- and distance-dependent studies. At low 

temperatures a constant, nonzero rate for injection may persist 

while the room-temperature injection rate constants ought to 

exhibit an exponential dependence on distance: 

k = koexp[ bx] (8) 

where b is the dampening factor. A dampening factor, 

b = B1.0 A˚ 1 , is often indicative of saturated-hydrocarbon, 

through-bond superexchange tunneling behavior; 263–266 generally, 

larger values imply at least partial through-space 

character while smaller ones are associated with tunneling 

through conjugated p systems. 266 

An early study demonstrated that efficient excited-state 

electron injection did occur from sensitizers of the general 

type Ru(dmb)2(L) 2+ , where L contained unconjugated 

–(CH2)x– linkers between the Ru-chelating bpy moiety and 

one carboxylic acid group. 72 A more systematic study was 

later reported using three Re(bpy(CH2)2n(COOH)2)CO3Cl 

(n = 0, 1, 3) sensitizers where it was shown that ultrafast 

injection into TiO 2 did not occur when electronic coupling 

between the surface-bound ligand and the TiO 2 surface was 

removed by unconjugated methylene spacers, i.e. when n =1 

or 3. 104,105 For the same two sensitizers, the slower picosecond 

injection process could be successfully fit to a stretched 

exponential and the distance dependence of the injection rate 

could be qualitatively modeled by eqn (8) using b =1.2foreach 

C–C bond, indicative of nonadiabatic electron transfer. The 

4200-fold increase in injection rate from n =1ton =0could 

not be fit to such a model and was explained as adiabatic electron 

transfer due to a greatly increased strong electronic coupling 

from the lack of an unconjugated spacer moiety, Fig. 15. 

Detailed comparison of the n = 0 with the n = 1 or 3 compounds 

were complicated by the fact that the n = 0 compound had 

significantly different photophysical and redox properties. 

The distance dependence of excited-state electron injection 

was also explored using Ru II sensitizers that contained a bpy 

ligand derivatized with a conjugated oligo(xylylene) linker and 

bound to TiO2 via an ethynylcarboxyphenyl group. 267 A series 

of three compounds, with zero, one, or two linkers, was 

employed. A mere two-fold difference in injection rate constant 

was inferred by the difference in integrated PL spectra of 

the dyes in solution and on TiO2. The lack of the expected 

large differences was proposed to result from the flexibility of 

the one-carboxyl sensitizer attachment, that allowed proximity 

of the Ru II -metal center and the TiO2 surface in all three cases. 

In a related study, sub-picosecond injection was observed for 

2+ 

tripodal, Ru(bpy) 3 -based sensitizers with an oligo(phenyleneethynylene) 

linker covalently bound to a tricarboxyphenyladamantane 

base calculated to have Ru–TiO2 distances over 

24 A˚ . 268–270 This study did not systematically show that rates 

vary with distance but did provide strong evidence that the 

distance dependence on injection rate was not large. 

With three phosphonated, ‘black dye’-like compounds of 

the form [Ru(40-PO3(Ph)n-tpy)(NCS)3] 3 

(n = 0, 1, 2) the 

distance-dependence of excited-state electron injection 

through conjugated linkers was studied. 271 Femtosecond 

pump–probe transient absorption measurements revealed that 

Fig. 15 Time-resolved, single-wavelength absorption difference spectra 

for Re(bpy(CH2)2n(COOH)2)CO3Cl/TiO2 (n = 0, 1, 3) illustrating 

that the rate of injection was inversely related to n. Taken from Fig. 9 

of ref. 104. 


Fig. 16 (A) A diagram of a TiO 2/Al 2O 3 core-shell nanoparticle. (B) Time-resolved, single-wavelength absorption difference spectra for 

Ru(4 0 -PO3 2 -tpy)(NCS)3/TiO2 thin films illustrating that the rate of injection was inversely related to the size of the Al2O3 overlayer. Al2O3 

overlayer thickness in nanometers are shown. Taken from Fig. 5 and 6, respectively, of ref. 271. 

the rate of each phase of an observed biphasic injection 

process was dependent on distance. The fast picosecond 

component fit nicely to an exponential distance-dependent 

model, eqn (8), with dampening factor, b = 0.19 A ˚ 1 , while 

the slower component for injection was assumed to be due to 

injection from loosely bound or aggregated dyes. As this 

dampening factor was much smaller than typical values obtained 

for donor–bridge–acceptor systems in solution, it was 

proposed that nuclear reorganization played a negligible role 

in injection, a hypothesis supported by DFT calculations. 

The distance-dependence was investigated by yet another 

means using the sensitizer lacking phenylene bridges, i.e. n =0; 

a core-shell architecture 272–276 was employed with Al 2O 3 shells 

varying from 0.6–6 nm in thickness conformally deposited on 

TiO2 prior to thin film preparation, Fig. 16(a). 271 The insulating 

shell required tunneling for almost all excited-state injection. 

As tunneling is not only a factor of distance but barrier 

height as well, this architecture allowed solely the distance to 

be altered. Neglecting ultrafast injection, which was assumed 

to be from dyes adsorbed directly onto TiO2 from small holes 

in the Al 2O 3, it was shown that the picosecond biphasic nature 

of injection resulted in b = 0.11 A˚ 1 and 0.04 A˚ 1 for the fast 

and slow components, respectively, Fig. 16(b). As the barrier 

to the conduction band of bulk, crystalline Al 2O 3 is very large, 

dampening factors over an order-of-magnitude larger were 

expected. It was proposed that the electronic structure of thin 

alumina layers differed from that of bulk Al2O3. 277 

Using three rigid-rod, Ru(bpy)3 2+ -based compounds containing 

a conjugated bpy ligand derivatized with an oligo- 

(phenyleneethynylene) linker and anchored to TiO2 via a 

dicarboxyphenyl group the distance dependence of excitedstate 

electron injection was studied, Fig. 17(a). 278 It was found 

that a monotonic decrease in injection rate occurred as the 

number of linkers was increased. However, this dependence 

only resulted in a dampening factor, b = 0.04 A˚ 1 , for both 

the slow and fast picosecond components, whereas a similar 

study on SnO2 resulted in a value of B0.8 A ˚ 1 , 279 and 

theoretical values were 40.4 A ˚ 1 . Although this small distance 

dependence agrees rather well with the conclusions from 

the phosphonated, ‘black dye’-like compounds, these results 

were further complicated by the lack of an expected similar 

trend in injection yields, where the middle-length spacer was 

found to inject best, Fig. 17(b). 

Fig. 17 (A) A diagram of a rigid-rod, Ru(bpy) 3 2+ -based sensitizer 

bound to a TiO2 nanocrystallite. (B) Time-resolved, single-wavelength 

absorption difference spectra of these TiO 2-bound sensitizers containing 

rods of oligo(phenyleneethynylene) linkers (n = 1, 2, 3). Although 

the injection yields were not distance-dependent, the rates were 

inversely related to n. Taken from cover artwork and Fig. 3A, 

respectively, of ref. 278. 


The observation of efficient and rapid excited-state electron 

injection through saturated and unsaturated spacers raises the 

question of whether the MLCT excited state need be localized 

on a ligand that is directly attached to the semiconductor 

surface. In other words, could the surface linker be on a nonchromophoric 

ligand? An early test of this was performed with 

a bimetallic Re I (dcb)CO3–L–Ru II (bpy)2(CN) (L = CN or 

NC ) compound. 280 Long-wavelength excitation selectively 

promoted an electron from the Ru II -metal center to a bpy 

ligand that was not anchored to the semiconductor surface, yet 

still resulted in a large photocurrent in regenerative DSSCs. 

The dcb ligand is structurally the same as two ina ligands 

connected in the 2 and 2 0 positions. The extra covalent bond in 

the dcb ligand increases the overall conjugation and thus 

lowers its LUMO energy. Using a comparative study of two 

heteroleptic Ru II compounds, one with a dcb ligand and the 

other with two ina ligands, the effect of remote versus adjacent 

excited-state electron injection was directly studied, Fig. 18. 281 

Both compounds exhibited a similar pH-dependent injection 

at pH 42 even though the thexi state of the latter compound 

contained an electron localized on a ligand that was not bound 

to the TiO2 surface. The observations of efficient injection 

from sensitizers with an ina ligand has been observed for 

Re(bpy)CO3(ina) + as well. 282 

The ina ligand, and substituted analogues, can also be 

coordinated to axial sites in porphyrinic macrocycles. A Ru II - 

phthalocyanine sensitizer with axial 3,4-dicarboxylic acidpyridine 

was employed. 283 Thepyridinederivativeallowedfor 

surface binding of the sensitizer to TiO2 however the major near 

IR–visible light absorption features were due to intraligand 

p - p* transitions that were localized on the phthalocyaninato 

ligand. Quasi-monochromatic light excitation resulted in excitedstate 

electron injection into TiO2 with a maximum IPCE 4 60%, 

due entirely to remote injection. Similar remote injection results 

have been obtained for similar p - p* transition molecules: 

a Ti IV phthalocyanine with a 3,4-dihydroxybenzoic acid 

surface-binding ligand and other Ru II phthalocyanines with a 

4-carboxylic acid-pyridine surface-binding ligand. 253,284,285 

iv ‘Hot’ injection. It is somewhat surprising that the photoinduced 

ultrafast electron injection observed by so many 

Fig. 18 A schematic illustrating two different injection schemes 

depending on the surface-bound ligands: (A) Remote excitedstate 

injection pathway for cis-Ru(dpp) 2(ina) 2/TiO 2, where dpp is 

4,7-diphenylphenanthroline, due to excited-state localization on a 

dpp ligand. (B) Adjacent excited-state injection pathway for 

Ru(dpp)2(dcb)/TiO2 as the excited state is localized on the surfacebound 

dcb ligand. Taken from cover artwork of ref. 281. 

authors is not manifest in operational DSSCs. One might 

anticipate that blue photons would give rise to higher photocurrents 

than would red ones due to the stronger reducing 

power of the Franck–Condon excited states in the former case. 

In other words, the absorptance and photocurrent action 

spectra would deviate significantly from each other with much 

less photocurrent at long wavelengths than would be expected 

based on the fraction of light absorbed. A possible reason why 

such behavior is not commonly observed is that the presence 

of the redox mediator (typically 0.5 M LiI/0.05 M I2), slows 

injection. Keep in mind that most of the interfacial 

charge-separation data described above was obtained in inert 

electrolytes, solvent, or vacuum. The little data available for 

excited-state injection in the presence of the I3 /I redox 

mediator indicates, in fact, that charge separation is still 

quantitative but that the kinetics are significantly altered. 177 

Another possibility is that there is quantitative injection from 

the sum of upper—i.e. Franck–Condon, vibrationally ‘hot,’ 

etc.—and thermally relaxed excited states so that the photocurrent 

action spectrum simply does not report on the ultrafast 

processes. 

In addition to the ultrafast, ‘hot’ injection observed in high 

vacuum, solvents, and electrolytes, there are in fact a few 

literature observations that indicate that photo-induced ‘hot’electron 

injection occurs in DSSCs. The first example was with 

cis-Fe(dcb) 2(CN) 2/TiO2 that exhibited a band-selective photocurrent 

action spectrum. 286 Although the photocurrent efficiency 

was poor at all excitation wavelengths (o10%), there 

was over a five-fold increase in the absorbed-photon-tocurrent 

efficiency (APCE), or internal quantum efficiency, when 

the higher-energy absorption band was excited. This behavior 

was attributed to band-selective injection yields occurring 

when injection was not from the thexi state. Thus, this further 

supported the interfacial, ‘hot’-injection mechanism. Similar 

behavior was reported for cis-Ru(dcbq)2(NCS)2/TiO2, where 

dcbq is 4,40-dicarboxylic acid-2,20-biquinoline. 287 

It was 

shown that the APCE was wavelength-dependent with the 

lower-energy band exhibiting smaller APCEs and rationalized 

as injection from this band being thermodynamically unfavorable. 

This is indicative of ‘hot’-injection processes from both 

bands with additional thexi-state injection from the higher 

energy band only. The hypothesis was tested with SnO2 thin 

films, whose Ecb is B500 mV more positive than that of TiO2. 

The photocurrent action spectrum better traced the absorptance 

spectrum as the APCE was no longer wavelength 

dependent. The third example we are aware of was reported 

by Heimer and co-workers who also observed APCEs that 

were wavelength dependent on TiO 2, but not on SnO 2, with 

N3-like derivatives with the carboxylic acid groups present in 

the 5 and 5 0 positions. 288 

An alternative approach for quantifying ‘hot’-electron injection 

is to measure the injection yields as a function of the 

excitation wavelength. Moser and Gra¨tzel first reported 

studies of this type. 289 For N3/TiO2, the nanosecond injection 

yields were quantitative and wavelength independent. 

However, when N3 was anchored to Nb2O5, who’s Ecb 

is more difficult to reduce, wavelength-dependent injection 

yields were observed that decreased as the excitation 

wavelength increased. When the sensitizer was changed to 


cis-Ru(2,6-bis(1 0 -methylbenzimidazol-2 0 -yl)pyridine)(dcbq)- 

(NCS)2, the injection yields were found to be strongly 

wavelength dependent. The dcbq ligands have low-lying p* 

LUMO energies, too low for the thexi state of the sensitizer 

to inject an electron into TiO 2. Thus, ‘hot’ injection from 

an upper vibrational excited state was, without-a-doubt, 

occurring. Therefore, wavelength-dependent injection yields 

measured on nanosecond time scales are a signature of 

injection from upper, or ‘hot’, excited states. 

We have recently observed similar behavior with Ru II -ammine 

sensitizers. 290 These compounds have low-lying, ligandfield 

states and, in some regards, have excited-state properties 

more similar to Fe II compounds than Ru(bpy)3 2+ . Wavelength-dependent 

injection yields were observed for 

Ru(NH 3) 5(ina)/TiO 2 with blue-light excitation giving almost 

twice the injection yield as with green light, Fig. 19. Interestingly, 

with the tetra-ammine compounds, Ru(NH 3) 4(dcb)/ 

TiO2, the injection yields were also wavelength dependent 

and were sensitive to isotopic substitution of the ammines. 

The injection yields were about 30% larger for ND3 than NH3 

at all excitation wavelengths studied, data that is consistent 

with ‘hot’-electron injection. 

There is now compelling evidence that charge separation 

from an MLCT excited state into TiO2 can occur faster than 

vibrational relaxation. The question of whether this is necessary 

or useful has arisen. The injected electrons are expected to 

trap at coordinatively unsaturated Ti IV sites within a picosecond. 

291–295 If the electron could be collected in an external 

circuit prior to trapping, energy that would otherwise be lost 

to phonon creation could be harvested. This would allow for 

1 sun, AM1.5 light-to-electrical power conversion efficiencies 

over the single-junction Shockley–Queisser limit of 31% 296 

and up to the ‘hot’-injection-limited efficiency of 66%. 297 

Thus, ‘hot’ injection is the first step towards achieving single 

junction, solar light-to-electrical power conversion efficiencies 

greater than 31%. However, should wave-packet dephasing 

108,298 accompany photo-induced electron transfer from 

the sensitizer to TiO 2 acceptor states, ‘hot’-electron capture 

may be impossible. Willig et al. have time-resolved such 

thermal relaxation in TiO2 to picoseconds. 243 The dephasing 

theory implies that electron transfer from the electronic wave 

packet in TiO2, representing the initially formed ‘hot’ electron, 

to a point contact is virtually impossible. 109,243,244 It is thought 

that over time the wave packet expands throughout the film 

and that elastic and inelastic scattering events alter the 

momentum as well as the electronic and vibrational energy 

Fig. 19 A schematic illustrating excitation wavelength-dependent 

injection yields for Ru(NH3)5(ina)/TiO2, i.e. 15% with 532 nm and 

30% with 416 nm. Taken from cover artwork of ref. 290. 

Fig. 20 A schematic illustrating that excited-state electron injection 

and subsequent reduction of a co-bound molecular acceptor, A, was 

only thermodynamically possible if injection occurred from the initial, 

Franck–Condon excited state and not the thexi state. Formation of A 

after pulsed-laser light excitation of Ru(bpy)2(dcbq)/TiO2 was shown 

to be due to ‘hot’ electron injection followed by TiO 2-mediated 

electron transport to the acceptor. Taken from cover artwork of 

ref. 135. 

of the electron. Thus, most often this cooled state would reach 

the interface for collection or reactivity. The realization of 

ultrafast electron transfer from/through the semiconductor was 

proposed to be feasible only under a specific and unlikely 

distribution of ‘hot’ electrons. Therefore, even if the excess 

energy is not lost through vibrational relaxation of the excited 

state of the sensitizers, it will most likely be lost to phonon 

creation on the other side of the interface. 

If the TiO2 DOS lie above the excited-state reduction potential 

of the sensitizer it could be possible to drive an electrontransfer 

reaction that would be thermodynamically unfavorable 

from the thexi state, Fig. 20. Such a process was recently 

realized with heteroleptic sensitizers possessing the previously 

mentioned dcbq ligands, such as [Ru(bpy)2(dcbq)] 2+ . 135,299 In 

this case, the acceptor was a ground-state sensitizer. The 

interfacial energetics were such that Ecb/DOS were more 

negative than E o (Ru II/+ ) and E o (Ru III/II* ). Therefore after 

‘hot’-electron injection, resulting in a TiO2(e ) and Ru III ,the 

TiO2(e ) could recombine with the oxidized sensitizer or reduce 

another surface-bound sensitizer to form Ru III /TiO2/Ru + . 

Using nanosecond transient absorption spectroscopy, spectral 

evidence for the allowed processes in Fig. 20 and wavelengthdependent 

injection yields lead to the conclusion that ‘hot’ 

injection was occurring with this compound. The conclusion 

that formation of Ru III /TiO2/Ru + is mediated by the TiO2 

DOS was supported by the fact that there was insufficient free 

energy stored in the thexi state of the sensitizer to reduce the 

acceptor. Thus, the proposed mechanism was that ‘hot’ injection 

was followed by reduction of a TiO2-bound acceptor. This 

serves as a proof-of-principle example and suggests that ‘hot’ 

injection can be used to drive ‘uphill’ redox reactions of 

relevance to exceeding the Shockley–Queisser limit. 

C Metal-to-particle charge transfer 

There is another mechanism of photo-induced electron injection 

that has been observed for metal-cyano compounds 

anchored to TiO 2. This mechanism has been termed metalto-particle 

charge transfer (MPCT). This is apparent based on 


the observations that: (a) when said compounds are anchored 

to TiO2 a new absorption band is formed that was not present 

in fluid solution and (b) light excitation into said absorption 

band results in immediate formation of TiO 2(e )/S + . An 

interesting feature of such sensitizers is that, by definition, 

the injection yield is unity as light absorption and electron 

transfer to TiO2 are one in the same process. This is in contrast 

to injection from MLCT excited states described above whose 

injection efficiency has been shown to be a function of the pH, 

ionic strength, excitation wavelength, and temperature. 300 

MPCT absorption bands were observed for the first time 

upon binding M(CN)x n+ complexes to TiO2 nanocrystallites 

(M = Fe, Ru, Os, Re, Mo, W). 301,302 Some of these adducts 

extended the visible light photoresponse of TiO 2 beyond 700 nm. 

Hupp et al. discovered that the resonance Raman spectrum of 

Fe(CN) 6/TiO 2 colloids exhibited the coupling of ten vibrational 

modes to MPCT, three of which were surface modes. 303,304 

Jortner and colleagues have previously described an applicable 

theoretical model for describing such multimode electron transfer, 

305–310 however the coupling of multiple surface modes to 

interfacial electron transfer was unprecedented experimentally. 

Fe II -based coordination compounds containing both MPCT 

and MLCT bands of the type [Fe(LL)(CN)4] 2 were studied, 

whereLL=bpy,dmb,or4,4 0 -diphenyl-bpy. It was shown that 

the MLCT absorption bands were solvatochromic whereas the 

MPCT bands were not. 77,311 In light of this and with electrochemical 

results indicating that E o (Fe III/II ) did shift with solvent, 

it was suggested that the TiO 2 DOS shifted as well and in a 

concerted fashion. There exists a precedence for molecular and 

TiO2 reduction potentials shifting in concert when the molecule is 

poised within the ionic double layer, i.e. Helmholtz and diffuse 

layers. 312,313 Using an [Fe(bpy)(CN)4] 2 

sensitizer, the possibility 

for a dual-mechanism of sensitization was explored, Fig. 21. 311 

Excitation directly into the MPCT band would inherently result 

in an injection yield of unity and be independent of the experimental 

conditions. In acetonitrile solutions it was shown that the 

injection yield could be reversibly tuned with the addition of 

LiClO4; clearly this was not a result of a MPCT transition as the 

UV-Vis absorption spectrum was largely independent of electrolyte. 

Thus, not only was direct MPCT present in this system, but 

less-efficient electron injection from a proximal MLCT excited 

state was apparent as well. 

Fig. 21 Ball-and-stick models for Fe(bpy)(CN)4/TiO2 portraying two 

possible mechanisms for photo-induced electron injection into TiO 2: 

(A) direct, metal-to-particle charge transfer (MPCT) sensitization; (B) 

sensitization by means of a metal-to-ligand charge-transfer (MLCT) 

excitation followed by excited-state electron injection. Taken from 

cover artwork of ref. 311. 

Intervalence charge-transfer (IVCT) bands exist for mixedvalence, 

polymeric Fe II –CN–Ti IV cyano complexes and are 

speculated to be related to MPCT bands in Fe(CN) 6/TiO 2. 314 

The MPCT absorption bands were similar in energy and spectral 

width to those previously described for outer-sphere charge 

transfer with iron-cyano anions. From this, a question arises: 

does light absorption on TiO2 promote and electron from Fe II to 

an adjacent Ti IV site or to a Ti IV site within the interior of a 

nanocrystallite? By electroabsorption (Stark) spectroscopy it was 

determined that the MPCT distance was 5.3 A ˚ basedonthe 

dipole moment change and the Liptay treatment. 314 This was 

within error of the distance from Fe to Ti using molecular 

modeling on [(CN)5Fe II –CN–Ti IV (H2O)4O] 2 , although it was 

slightly larger than empirical values measured for related 

Fe II –CN–M compounds. Similar distances were found for 

Mo-, Ru- and W-cyano complexes on TiO 2 and all support the 

hypothesis that MPCT bands represent electronic transitions to 

an orbital on a Ti atom that is in proximity to the bound cyano 

nitrogen atom. 315 Additionally, based on the above calculated 

distance and the fact that the free NC ligands are even further 

from the surface than the metal center, identification of the 

process as MPCT and not ligand-to-metal charge transfer 

(LMCT), i.e. from NC to Ti, was substantiated. 

Some organic bases are also known to display direct, chargetransfer 

absorption bands when anchored to TiO 2, the most 

well-known being catechol. 188,300 Two Os II -polypyridyl compounds 

with bpy-catechol derivatives for surface attachment 

were recently reported. 316 Absorption features assigned as 

direct catechol-to-particle charge transfer and MLCT on TiO2 

and ZrO2 (a wide-bandgap semiconductor with EBG = 4.5 eV) 

were observed and PL assigned to radiative charge recombination 

was reported. Similar PL behavior was previously reported 

for organic compounds anchored to TiO2, but the assignment 

of this to radiative recombination was later questioned. 300,317 

D Reduced-sensitizer injection 

An alternative mechanism exists for photo-induced electron 

injection wherein the excited-state is reduced prior to interfacial 

charge separation, Fig. 22(b). This results in injection 

from a non-electronically excited sensitizer. For this reason 

DSSCs operating under this mechanism would appropriately 

be termed regenerative galvanic cells as injection would be a 

dark, thermodynamically favorable process. 318 This alternative 

sensitization method has been called supersensitization; 153 

the donor is termed the supersensitizer due to its requirement 

in achieving effective overall sensitization. 319 A unique aspect 

of this mechanism is that the oxidized form of the sensitizer is 

never generated. Therefore, it may be particularly well-suited 

for sensitizers that are unstable in their oxidized forms. An 

advantage with MLCT excited states is that the reduced form 

of the sensitizer is a stronger reductant than the MLCT excited 

state, typically by 200 to 400 mV. 

Kirsch-De Mesmaeker and co-workers first reported compelling 

evidence for reduced ruthenium sensitizers transferring 

electrons to SnO2 electrodes. 319 The coincidence of Stern– 

Volmer constants measured by analysis of the photocurrent 

enhancement and PL quenching with hydroquinone donors left 

little doubt as to the sensitization mechanism. Additional 


Fig. 22 Ball-and-stick models for Ru(bpy)2(dcb)/TiO2 depicting the 

two possible mechanisms for photo-induced electron injection into 

TiO2 and regeneration of the sensitizer: (A) MLCT excited-state 

electron injection followed by regeneration of the oxidized sensitizer 

by the solution donor, D; (B) Reduced sensitizer injection that results 

from reductive quenching of the excited state by D followed by dark 

electron injection from the reduced sensitizer. Taken from Scheme 1 of 

ref. 320. 

spectroscopic evidence for photo-induced electron injection by 

reduced sensitizers was reported for Ru(bpy)2(dcb)/TiO2 in 

0.1 M TBAClO4 acetonitrile electrolyte containing neutral, 

organic phenothiazine (PTZ) electron donors. 321 Nanosecond 

transient absorption data demonstrated the rapid formation of 

TiO 2(e )/PTZ + , while in the absence of PTZ there was littleto-no 

evidence for injection. Injection was rate limited by 

diffusional quenching of the MLCT excited state so the 

Ru II (bpy) 2(dcb )/TiO 2 intermediate was not directly observed. 

An interesting case of photo-induced, reduced-sensitizer 

electron injection was reported with the bimetallic sensitizer 

(bpy)2 Ru II –BL–Rh III (dcb)2/TiO2, BL = 1,2-bis[4-(4 0 -methylbpy)]ethane, 

Fig. 23(a). 322 About two-thirds of the MLCT 

excited states of the ruthenium chromophore were quenched 

by electron transfer to the Rh(dcb)2 group to form a 

(bpy)2Ru III –BL–Rh II (dcb)2/TiO2 charge-separated state while 

the remaining directly injected an electron into TiO2.** This 

observed branching ratio was proposed to result from different 

surface orientations. Approximately 40% of the intramolecular, 

charge-separated state, (bpy) 2Ru III –BL–Rh II (dcb) 2/TiO 2, 

injected electrons into TiO2 to form (bpy)2Ru III –BL– 

Rh III (dcb)2/TiO2(e ), while the remaining underwent backelectron 

transfer to form ground-state products, Fig. 23(b). 

The Ru II* -based injection occurred within the time resolution 

of the instrument, i.e. o10 ns, while the Rh II -based injection 

occurred in o100 ns following light excitation. 

In order to realize efficient DSSCs that operate by this 

mechanism, sensitizers that are potent photo-oxidants must 

be utilized. This stems from that fact that the I 3 /I redox 

mediator is the only redox mediator that yields high light-toelectrical 

power conversion efficiencies and the I /I reduction 

potential is rather positive. Ru II sensitizers that are strong 

excited-state oxidants can be prepared with ligands such as 

2,2 0 -bipyrazine (bpz). For example, the E o (Ru 2+*/+ ) of 

[Ru(bpz)2(deeb)] 2+ was found to be greater than +1.0 V vs. 

SCE 69,70,323 (+1.24 V vs. NHE 229 ). While the excited state of 

this and related sensitizers were found to be efficiently 

quenched by iodide or phenothiazine donors, the reduced form 

** We emphasize that while the scheme and this abbreviation imply 

that the reduction is metal based, it may in fact be ligand localized, i.e. 

on a dcb. 

Fig. 23 (A) A diagram of a [(bpy) 2Ru II –BL–Rh III (dcb) 2] 5+ sensitizer 

bound to a TiO2 nanocrystallite. (B) A schematic depicting the relative 

redox energies, lifetimes, and quantum yields for each step in the 

photo-induced injection process. Taken from Fig. 10 and 12, respectively, 

of ref. 322. 

of the compound that resulted, Ru II (bpz)(bpz )(deeb)/TiO 2, 

did not inject electrons into TiO 2. 320 In fact, very similar 

transient absorption features were observed in solution, 

on TiO2, and on ZrO2, while extremely small photocurrents 

(IPCE o 10 4 ) were observed in DSSCs. Some improvement 

was observed when the semiconductor was changed to SnO2, 

but the injection yields remained poor. 323 

Another interesting case of reductive quenching of an 

excited state that did not result in electron injection was 

reported for the mono-anion of Z907/TiO 2 in the presence 

of a high concentration of 1-propyl-3-methylimidazolium 

iodide. 324 A new transient spectroscopic feature was discovered 

that was attributed to the reduced sensitizer, which 

presumably formed by reductive quenching of the excited state 

by iodide. The decay of this species was attributed to a backreaction 

with I3 (t1/2 = B1 ms) yet it is not clear why this 

state did not inject electrons into TiO2. 

4. Sensitizer regeneration 

A Intramolecular regeneration 

Considerable effort has been set forth to regenerate the 

oxidized sensitizer by intramolecular electron transfer. This 

could be considered a ‘‘hole’’ transfer reaction that translates 

the oxidizing equivalent away from the Ru III -metal center and, 

ideally, the TiO2 surface. Very similar mechanisms are wellknown 

in the field of supramolecular photochemistry where 

compounds of the type (D)n–C–(A)m are often employed ((D)n 

are donor molecules, C is a chromophore/sensitizer, and (A)m 

are acceptor molecules). When solely two components are 

present the compounds are termed dyads. 88,94,325,326 At TiO 2 

interfaces a variety of C*–D dyads have been characterized; 

to our knowledge, Wrighton and co-workers were the first to 


attach a dyad to a semiconductor electrode. 327 The ability to 

control hole-transfer reactions at the molecular level is important 

for many classes of solar cells. One can envision 

future-generation DSSCs where multiple hole-transfer steps 

translate the oxidizing equivalent from the sensitized interface 

directly to a counter electrode thereby eliminating the need for 

the solution-based redox mediators that are required today, 

e.g. I3 /I . 

In practice there are at least two ways in which D–C*/TiO2 

- D + –C/TiO2(e ) reactions can occur. They correspond to 

charge-separation mechanisms from the excited or reduced 

states that were described in sections 3/B and 3/D, respectively. 

Since C* is a weaker oxidant than C + ,itispossibletodesign 

dyads covalently bound to weak donors that only react by the 

first mechanism. When strong electron donors are used, the 

mechanistic pathway is dependent on the relative rate constants 

for excited-state electron injection and intramolecular charge 

separation. Since excited-state injection is often found to be 

ultrafast, the first mechanism probably predominates even 

though it cannot always be unambiguously identified. 

It should be pointed out that in some regards N3/TiO2 is 

thought to undergo a similar intramolecular, charge-transfer 

process. DFT calculations for N3 + predict considerable hole 

density on the isothiocyanate ligands. 56–58 It is also known 

from electrochemical measurements that there are two closely 

spaced oxidations for N3, the first is predominantly metal 

based while the second is mainly isothiocyanate based. Therefore, 

in the charge-separated state, N3 + /TiO 2(e ), there is 

likely some partial ‘‘hole transfer’’ from the Ru III -metal center 

to the isothiocyanate ligands. In most of the examples discussed 

below, the electronic coupling between the electron 

donor and the Ru-metal center is much weaker, giving rise to 

complete hole hopping rather than partial charge transfer. 

i Organic donors. In collaboration with Bignozzi and his 

research group in Ferrara, Italy, we reported the first timeresolved 

spectroscopic studies of intramolecular sensitizer 

regeneration with the dyad [Ru(4-CH3,4 0 -CH2-PTZbpy)(dcb)2] 

2+ . 328,329 Comparative studies in fluid methanol 

solution, visible-light excitation of this dyad resulted in the 

creation of the MLCT excited state that was quickly quenched 

by electron transfer from the PTZ group. The reductive 

excited-state quenching was moderately exergonic (o0.25 eV) 

and had an approximate rate constant of B2.5 10 8 s 1 in 

methanol. The corresponding charge-recombination step was 

faster than the quenching by PTZ and thus there was little 

appreciable spectroscopic observation of the electron-transfer 

product. 

When the dyad was anchored to TiO 2 thin films and 

immersed in acetonitrile, MLCT excitation resulted in a new 

charge-separated state with an electron in TiO2 and an oxidized 

PTZ group, abbreviated PTZ + -Ru II /TiO2(e ). It was 

not possible to determine the mechanism of charge separation 

yet the authors speculated that after excited-state electron 

injection, electron transfer from PTZ to the Ru III -metal center 

( DG B 0.36 eV) produced PTZ + -Ru II /TiO2(e ). Recombination 

of TiO 2(e )s with PTZ + to yield ground-state products 

occurred with a rate constant of 3.6 10 3 s 1 . Excitation of a 

model compound that did not contain the PTZ donor under 

otherwise identical conditions gave rise to the immediate 

formation of a charge-separated state, Ru III /TiO2(e ), whose 

recombination kinetics were complex and analyzed by a distribution 

model with an average rate constant of 3.9 10 6 s 1 . 

Therefore, translating the ‘‘hole’’ from the Ru III -metal center 

to the pendant PTZ moiety slowed TiO 2(e ) recombination by 

about three orders of magnitude. This work provided an 

example of how the principles of stepwise charge separation, 

originally developed in the field of supramolecular photochemistry, 

can be applied to solid-state materials. 

Shortly thereafter, Grätzel and co-workers reported dyads that 

could undergo intramolecular ‘‘hole’’ transfer after excited-state 

electron injection. 330 These authors emphasized the significant 

color changes that accompanied electron transfer and potential 

applications in photochromic devices. Interestingly, they observed 

long-lived charge-separation, like the PTZ + -Ru II / 

TiO 2(e ) described above, in some cases while not in others. 

Since that time a number of dyads have been attached to 

TiO2 and are discussed further below. A commonly utilized 

electron donor is a triarylamine moiety, NAr3. Three 

Ru II -NAr3-type sensitizers with tpy-based ligands were 

studied in order to determine the optimal spatial location of 

the amine donor in relation to the excited-state electron. 330 

One compound had a 4-(N,N-di-p-anisylamino)phenyl group 

conjugated to a second tpy ligand, the second contained a 

benzyl ether interlocking group between the same amine donor 

and the tpy ligand, and the third compound contained only a 

4,4 0 ,4 00 -trimethyl-tpy ligand, Fig. 24. The latter compound was 

also co-adsorbed with a donor moiety bound to a phosphonated 

ether. Using long-wavelength resonance Raman spectroscopy 

it was deduced that the excited electron in the excited 

state of the first compound was located on the donor-containing 

tpy ligand whereas for the other two compounds it was located 

on the surface-bound phosphonated-tpy ligand. For all three 

compounds, photo-induced electron injection occurred 

quantitatively in o1 ns in air. In propylene carbonate, the 

quantum yield for formation of NAr 3 + -Ru II /TiO2(e ) was 

only 0.60 and occurred within 20 ns for the first compound 

while for the second compound it was unity with biphasic 

kinetics, a 10 ns component and a 100 ns component. 

The third compound with the co-bound donor experienced 

practically unity conversion to the NAr3 + /TiO2(e ) chargeseparated 

state. This study illustrated that remote injection 

was less efficient than injection from a surface-bound ligand. 

Some remarkably long-lived charge-separated states 

were observed after pulsed-light excitation of similar compounds. 

By covalently attaching the ether-NAr 3 donor 

group just described above to a dmb ligand in a 

cis-Ru(dmb-X)(dcb)(NCS) 2/TiO 2 system, the half-life of the 

charge-separated state was found to be over half a second, as 

shown schematically, Fig. 25(a). 331 Haque, Durrant, and 

colleagues increased this lifetime further by employing 

Ru(4,4 0 -(R)2-bpy)(dcb)2/TiO2 systems, where R contained 

one triphenylamine group (NPh3), two NPh3 groups, or a 

poly(vinyl-NPh3) group of about 100 units, Fig. 25(b). 332 The 

introduction of about 100 amines increased the half-life of 

the charge-separated state to over 4 seconds, as compared to 

350 ms and 5 ms for the other two compounds, respectively. 

The kinetics for excited-state electron injection and subsequent 


Fig. 24 Chemical structures of sensitizers containing intramolecular, 

or nearby, organic donors so as to increase the charge-separation 

distance. Taken from Fig. 5 of ref. 330. 

hole transfer from the Ru III -metal center to the covalently 

bound NPh3 moiety occurred within the instrument response 

time, i.e. B10 ns. 

N3 derivatives, cis-Ru(4,4 0 -(R)2-bpy)(dcb)(NCS)2, R = 

NPh3 or CH3, bound to TiO2 thin films were examined in order 

to study the effects of Ru III -hole transfer to a triphenylamine 

moiety. 82 Unexpectedly, both sensitizers exhibited similar transient 

features providing no evidence for hole transfer in the 

former sensitizer. Notwithstanding, the photoelectrochemical 

properties of the two sensitized thin-film electrodes differed 

Fig. 25 (A) A schematic depicting the cis-Ru(dmb-ether-NAr 3)- 

(dcb)(NCS)2 sensitizer bound to a TiO2 nanocrystallite and the overall 

mechanism for photoinduced charge separation and recombination 

with corresponding time scales. Taken from cover artwork of ref. 331. 

(B) The chemical structure of the sensitizer employed to increase 

the half-time of the S + /TiO2(e ) charge-separated state to over 4 s 

(n = 100). Taken from Scheme 1 of ref. 332. 

Fig. 26 The chemical structure of the sensitizer employed to study 

intramolecular charge separation on TiO2 thin films. The hole was 

successfully transferred away from the Ru III -metal center and TiO 2 

surface to the carotenoid moiety. Taken from Scheme 3 of ref. 333. 

significantly and a much larger Voc was measured for the 

NPh 3-containing sensitizer. The authors speculated that the 

enhanced V oc resulted from a larger dipole that was nascently 

formed on the sensitizer bearing the NPh 3 moiety. Based on the 

enhanced extinction coefficient of this dye and the results 

obtained when the small-perturbation Voc-decay technique 

was employed, it was proposed that photo-induced electron 

injection into the TiO2 acceptor states and partial hole delocalization 

from the Ru III -metal center to the NPh3 moiety 

occurred in one concerted step. Thus, increased charge separation 

could be achieved concomitant with electron injection by 

partial delocalization of the hole on the ligand. 

A series of novel sensitizers each containing a 4 0 -X-tpy 

ligand (X = Ph–PO 3(C 2H 5) 2, Ph–PO 3H 2, PO 3(C 2H 5) 2, 

PO 3H 2, or COOH) bound to TiO 2 were studied with hopes 

of increasing the charge-separation distance between the 

TiO2(e ) and oxidized sensitizer. 333 Only the sensitizer shown 

in Fig. 26, containing a phenyl-amide-carotenoid bound to a 

second tpy, provided unequivocal evidence for intramolecular 

sensitizer regeneration and thus increased charge separation, 

which was complete in o10 ns. However, even though DFT 

calculations indicated that the LUMO and the MLCT excited 

state were located on the 4 0 -phenylphosphonate-tpy ligand 

which was bound to TiO2, injection yields were poor. It was 

postulated that the out-of-plane phenyl spacer gave poor 

electronic coupling to TiO 2. 

The compound [Ru(BTL)(deeb) 2] 2+ , where BTL is 9 0 -[4,5bis(cyanoethylthio)]-1,3-dithiol-2-ylidene]-4 

0 ,5 0 -diazafluorene, 

was found to have an extinction coefficient almost three times 

as large as Ru(bpy)3 2+ in the visible region. 81 Interestingly, the 

transient absorption features in solution and on TiO2 differed 

greatly. In solution, a transient state was observed with 

spectroscopic properties characteristic of an MLCT excited 

state, with t =25nsat 40 1C, whereas when bound to TiO2 

a large positive absorption feature near 520 nm was observed 

and assigned to the oxidized dithiolene ligand. In fluid solution 

the driving force for reductive quenching of the MLCT excited 

state was unfavorable. However, when anchored to TiO 2,an 

electron was injected and the hole had translated from the 

Ru III -metal center to the dithiolene-containing ligand, within 

10 ns after light excitation, Fig. 27. 

ii Transition-metal donors. Although an organic donor is 

more optimal for practical applications, an advantage of using a 

transition metal as the donor is that its redox potential can be 

more easily tuned over wide energies by utilizing different ligands. 

The bimetallic sensitizer [Cl(bpy)2Os II –bpa–Ru II (dcb)2Cl] 2+ , 

abbreviated Os-bpa-Ru, where bpa is 1,2-bis(4-pyridyl)ethane, 

was anchored to TiO 2. 334 Pulsed 532 nm or 416 nm light 

excitation of a Os-bpa-Ru/TiO 2 thinfilmimmersedin1.0M 


Fig. 27 A schematic depicting a novel, high extinction coefficient 

sensitizer bound to a TiO 2 nanocrystallite and photo-induced electronand 

hole-transfer mechanisms. This sensitizer is unique in that the 

extended conjugation on the dithiolene-containing ligand is in the 3 

and 3 0 positions. Taken from cover artwork of ref. 81. 

LiClO4 acetonitrile electrolyte resulted in rapid excited-state 

electron injection (Ru II* +TiO2 - Ru III +TiO2(e )) and 

intramolecular electron transfer (Os II –bpa–Ru III - Os III – 

bpa–Ru II ) to ultimately form an interfacial charge-separated 

state with a TiO 2(e ) and an oxidized Os III -metal center, Os III – 

bpa–Ru/TiO 2(e ). This same state was also generated after 

selective 3 MLCT excitation of the Os II moiety with 683 nm light. 

The rates of intramolecular and interfacial electron transfer were 

fast, k 4 10 8 s 1 , while interfacial charge recombination, 

Os III –bpa–Ru/TiO2(e ) - Os II –bpa–Ru/TiO2, required milliseconds 

for completion. The results show a general strategy for 

promoting rapid intramolecular hole transfer (Os II –bpa–Ru III - 

Os III –bpa–Ru II ) after excited-state electron injection and a ‘remote,’ 

excited-state electron-injection process that occurs after 

direct excitation of the Os II chromophore, whose thexi state 

possesses far too little energy to transfer energy to the ruthenium 

moiety. 

Related studies with (bpy)2M II –bpt–Ru II (dcb)2/TiO2 thin 

films (M = Ru or Os), abbreviated M–bpt–Ru, where bpt-H 

= 3,5-bis(pyridin-2-yl)-1,2,4-triazole, showed evidence for 

two different electron-injection mechanisms depending on 

M. 335 For the all ruthenium compound, excited-state energy 

transfer to the TiO2-bound, dcb-containing, ruthenium moiety 

followed by excited-state injection was deduced based on 

transient PL and absorbance measurements. Although not 

directly observed, hole transfer to the proximal ruthenium 

moiety was thermodynamically favorable after excited-state 

injection. For the M = Os compound, excitation into the 

Ru II -based MLCT band resulted in excited-state energy transfer 

to the proximal osmium moiety prior to injection, and after 

remote injection the hole was proposed to remain on the 

Os-metal center. The expected Os III –bpt–Ru II /TiO 2(e ) 

product formed within the laser pulse (B10 ns). It was 

proposed that since some of the exciting light was absorbed 

by the Os II moiety, and energy transfer to the ruthenium 

moiety was energetically unfavorable, some remote injection 

from the Os-localized excited state also occurred in this case. 

Studies with a solution and surface-bound trinuclear ruthenium 

complex, (Ru III –Ru II )(L)–amide–(bpy)Ru II (dcb)2/TiO2, 

revealed that MLCT excitation of the mononuclear Ru II -metal 

center resulted in a transient absorption spectrum indicative of 

(Ru III –Ru II )(L)–amide–(bpy)Ru III (dcb) 2/TiO 2(e ), Fig. 28. 336 

This intramolecular charge-separated compound was completely 

formed by 200 ps, at which time the injection yield was 

deemed to be o10%. However, by 300 ns a spectrum consistent 

with (Ru III –Ru III )(L)–amide–(bpy)Ru II (dcb)2/TiO2(e ) 

was observed and was shown to have a half-life, t1/2 = B1 ms. 

This illustrates that slow hole transfer can occur over large 

distances under the appropriate conditions. 

Coordination compounds of the form [(LL)(L 0 L 0 )Ru II - 

(BL 0 )Ru II (LL)(L 0 L 0 )] n+ (n = 2, 3 depending on the number 

of deprotonated carboxylic acid functional groups) were investigated 

on TiO 2, where LL and L 0 L 0 are bpy and/or dcb 

and BL 0 is a bridging ligand: either tetrapyrido[3,2-a:2 0 ,3 0 - 

c:3 00 ,2 00 -h:2 000 ,3 000 -j]phenazine (tpphz) or 1,4-bis(phen-[5,6-d]imidazol-2-yl)benzene 

(bfimbz), where phen is 1,10-phenanthroline. 

337 As the BL 0 ligands are rigid and linear heteroaromatic 

entities, remote, excited-state electron injection could 

be examined with little fear of unexpected outer-sphere 

ligand–surface interactions due to ligand flexibility. It was 

shown that when BL 0 was tpphz—a ligand possessing p* 

energetics below the p* levels of the surface-bound dcb 

ligand—injection could be time resolved due to the thexi state 

being localized on tpphz, away from a surface-bound dcb 

ligand and with less reducing power for injection. However, 

this slow injection was found to be not only distance- and/or 

driving force-dependent but orientation-dependent as well. 

When [(bpy)(dcb)Ru II (tpphz)Ru II (bpy)(dcb)] n+ (n = 2 or 3) 

was employed as the sensitizer injection could be time-resolved 

Fig. 28 A schematic depicting a sensitizer employed to study intramolecular 

charge separation on TiO2 thin films. Interestingly, slow 

intramolecular charge separation between the mononuclear Ru III and 

dinuclear Ru II –Ru III could be observed on the hundreds of nanoseconds 

time scale. Taken from cover artwork of ref. 336. 


Fig. 29 Density Functional Theory (DFT) optimized geometry for 

two bimetallic Ru II compounds. When a distal ligand possessed 

carboxylic acid functional groups capable of binding to the TiO2 

surface, the geometry of the minimized energy configuration had it 

binding to the surface as well (b). Although the LUMO of the doubly 

bound form was spatially closer to the TiO2 surface, the singly bound 

sensitizer had a faster injection rate due to better electronic coupling 

with the TiO2 DOS. Taken from Fig. 7 of ref. 337. 

using nanosecond transient absorption spectroscopy, whereas 

with [ (bpy)2Ru II (tpphz)Ru II (bpy)(dcb)] n+ (n = 2 or 3) it 

could not, kinj 410 8 s 1 . Using DFT geometry optimization 

software it was hypothesized that electronic coupling, and not 

distance from the TiO 2 surface, could explain the differences, 

Fig. 29. The location of the p* orbital of the heterobinuclear 

complex in relation to the TiO 2 surface allowed for better 

electronic coupling between the sensitizer and the TiO2 DOS 

even though the Nphenazine–Ti distance was increased by over a 

factor of two. Photoelectrochemical measurements supported 

this and indicated that by increasing the distance for backelectron 

transfer the photocurrent efficiency could be enhanced. 

B Intermolecular regeneration 

In DSSCs, redox mediators are added to the external electrolyte. 

The reduced form of the mediator must regenerate the 

oxidized sensitizer by electron transfer prior to recombination 

with the injected electron. The oxidized form of the redox 

mediator is then reduced at the platinum counter electrode, a 

process not described herein. Ideally, all redox states of the 

redox mediator would not competitively absorb light. 

Although ion-pairing or surface adsorption with such mediators 

may occur, for the organization of this review we consider 

these to be intermolecular electron-transfer reactions. 

i Regeneration by iodide 

a Sensitizers in solution. By far the most effective donor in 

DSSCs is iodide. 12 All confirmed reports of light-to-electrical 

power conversion efficiencies over 10% utilize iodide and 

state-of-the-art DSSCs require iodide. 12 While many of the 

details of iodide oxidation at sensitized electrodes are now 

becoming available, it is important to point out that the 

aqueous redox chemistry of iodide and homogeneous reactions 

with transition-metal compounds have long been 

known. 338–342 

Shown in Scheme 1 is a Latimer-type diagram for the 

aqueous redox chemistry of iodide. Additional values and 

details are available in the review by Stanbury. 338 The formal 

one-electron reduction potential of the iodine atom is very 

positive, E o (I /I ) = +1.33 V vs. NHE. 338 Therefore, a potent 

oxidant is required to generate iodine atoms. However, another 

pathway exists in which two iodides can be oxidized 

directly to I2 , E o (I2 ) = +1.03 V vs. NHE. 338 Based on 

potentials alone, it is tempting to conclude that this latter 

pathway is the only mechanism available to oxidized sensitizers 

like N3 + , since generation of iodine atoms would be 

thermodynamically unfavorable by close to 250 mV. 25 However, 

it should be kept in mind that the potentials listed are for 

standard-state conditions in aqueous electrolytes and that 

adsorption to the TiO 2 surface may have a significant effect. 

Walter and Elliott have provided evidence that interactions 

between iodide and the bpy ring may also activate iodide. 343 

Furthermore, the values given in Scheme 1 are for aqueous 

solutions. Since there is good reason to believe that the 

reduction potentials will vary significantly with solvent, it 

would be tremendously helpful to this field if a corresponding 

Latimer-type diagram in acetonitrile was available since there 

is good reason to believe that the reduction potentials will vary 

significantly with solvent. For example, the equilibrium constant 

for reaction (9) is reported to be 410 6 M 1 in 

CH 3CN 344–349 but is only 700–800 M 1 in water. 350 

I +I2 " I3 (9) 

It is not trivial to obtain the one-electron reduction potentials 

experimentally. We and others before us have found that only 

two-electron redox processes are observed by voltammetry 

measurements at metal electrodes. 344,351,352 Stanbury has examined 

iodide oxidation by a series of Fe III compounds in 

acetonitrile and from this, the sole one-electron transfer 

reduction potential available in acetonitrile that we are aware 

of was established through kinetic inhibition measurements, 

E o (I /I ) = +0.60 0.01 V vs. the ferrocenium/ferrocene 

redox couple (FeCp2 +/0 ) 353 (+1.15 vs. NHE 229 ). 

Scheme 1 


The transition-metal redox chemistry of iodide has previously 

been reviewed. 341,342 Two mechanisms have been 

observed, based on reactions (10) and (11): 

Mox +I - Mred +I (10) 

Mox +2I - Mred +I2 (11) 

Both are first order in transition-metal compound, M ox, while 

(10) is first order in iodide and (11) is second order in iodide. 

Proposed mechanisms for (11), the overall third-order reaction, 

include I reacting with an [M ox,I ] ion-pair or M ox with 

an [I ,I ] ion-pair. A wide variety of transition-metal compounds 

have been studied and linear free-energy relations for 

both reactions now exist. In some cases, with mild oxidants 

such as Mox = Os(bpy)3 3+ , the reverse reactions became 

significant. 339–341 

Much less is known about MLCT excited-state oxidation of 

iodide. The alternative, reduced-sensitizer electron-injection 

process requires interactions of the excited state and iodide. 

Early studies with [Ru III (bpy) 2(bpy )] 2+ * revealed very inefficient 

electron transfer, i.e. 1 10 6 M 1 s 1 . 354,355 Interestingly, 

excited-state quenching of [Ru III (bpy) 2(dcb )] 2+ * 

anchored to SiO2 appears to be somewhat more efficient, i.e. 

1 10 8 M 1 s 1 . 356 

We recently found that excited-state electron-transfer reactions 

with iodide were significant when ion-paired with 

the ground-state sensitizer. 133,357 Addition of iodide to a 

dichloromethane solution of [Ru(bpy)2(deeb)] 2+ resulted in 

significant changes to the ground-state absorption spectrum. 

A decrease in PL and excited-state lifetime accompanied the 

absorption changes consistent with both static- and dynamicquenching 

mechanisms, respectively. A Benesi–Hildebrandtype 

analysis of these absorption changes yielded equilibrium 

constants for ion-pairing that were within experimental error 

the same as those abstracted from PL quenching data, 

Keq = 59 700 M 1 . Similar behavior was observed in 

acetonitrile and/or with Ru(bpy)3 2+ , however an iodide 

concentration that was two orders of magnitude larger was 

required. Transient absorption measurements clearly showed 

an electron-transfer mechanism with the appearance of I2 

and no evidence for intermediate iodine atom formation; thus 

the mechanism appeared to follow reaction (11). The cage 

escape yields were low, f = 0.25, but increased to 0.50 with 

Ru(bpy) 3 2+ . Remarkably, the solid-state crystal structure of 

Ru(bpy) 2(deeb)I 2 had both iodides associated with the carbonyl 

oxygens of the ester groups, Fig. 30. One might have 

anticipated that Coulombic repulsion would have resulted in a 

larger inter-ionic distance then the B6 A ˚ observed. If a similar 

structure exists in solution the iodides would be well-positioned 

for a concerted reduction of [Ru III (bpy)2(deeb )] 2+ * and 

formation of I2 . This is an intriguing possibility as excitedstate 

reactions that form chemical bonds are rare in all of 

photochemistry. Although, evidence for intermediate I formation 

by reaction (10) has recently been observed in our labs. 358 

b Sensitizer/TiO2 systems. The first heterogeneous reduction 

of Ru III -polypyridyl compounds by iodide was reported 

by Fitzmaurice and Frei. 359 Photo-induced electron injection 

into colloidal TiO 2 from [Ru III (dcb) 2(dcb )] 2+ * was followed 

Fig. 30 Space-filling representation of the crystal structure of a single 

sensitizer determined by X-ray diffraction showing two iodides associated 

with the deeb ligand in [Ru(bpy) 2(deeb)] 2+ . This geometry 

would allow for facile reductive quenching of the excited or oxidized 

forms of the molecule and the proximity of a second iodide could favor 

I2 generation as per eqn (11). Taken from cover artwork of ref. 133. 

by oxidation of iodide in acidic aqueous solution. From the 

pseudo-first-order transient kinetics in 0.5 to 100 mM KI, 

a second-order rate constant for iodide oxidation of 

B2.5 10 9 M 1 s 1 was abstracted. The data were ascribed 

to be most consistent with formation of ion-pairs. 

Since that time there have been a number of studies aimed at 

abstracting the rate at which the Ru II form of the sensitizer is 

regenerated. These experiments were usually performed by 

monitoring the recovery of the MLCT absorption bleach after 

pulsed-laser excitation at wavelengths where the iodide oxidation 

products did not appreciably absorb light. While this has 

proven to be a reasonable way of quantifying rate constants 

for regeneration of the Ru II state, little information regarding 

the mechanism(s) of iodide oxidation is obtained. For this 

reason, we briefly summarize the key observations. 

Most studies of this type were performed with N3/TiO2. At 

low iodide concentrations, the regeneration rate was found to 

be first order in iodide. At higher iodide concentrations, a 

static component was often observed. Under the 0.5 M iodide 

concentration of a DSSC, regeneration is often stated to be 

complete within 10 ns. 11,13 The rate constant for regeneration 

of the oxidized dye, Ru III (bpy) 2(dcb)/SnO 2, by iodide was 

determined to be 1.2 10 10 M 1 s 1 . 356 Durrant and coworkers 

have recently provided evidence that the regeneration 

rate is dependent on the E o (Ru III/II ) of the sensitizer. 360 With 

Ru(dcb)2(CN)2/TiO2 thin films an intermediate was observed 

and assigned to a [Ru III ,I ] ion-pair. Reaction of this with a 

second iodide was proposed to yield I2 . 

The rate of reactivity of iodide with N719 + /TiO2(e ) increased 

in the presence of Li + and other cations with large 

charge-to-radius ratios. 361 It was also noted that the half-time 

for sensitizer regeneration abruptly shortened when the concentration 

of Li + was increased to between 10 and 50 mM, 

Fig. 31(a). Using electrophoretic measurements, the point of 

zero z-potential (PZZP) was determined to occur at 3 mM 

Li + , a concentration slightly less than that required for the 

abrupt change in half-time, Fig. 31(b). Also, by titration of 

iodide to positively charged TiO2 particles in the presence of 

Mg 2+ , experimental data suggested that iodide adsorbed 


Fig. 31 (A) Plot of the inverse half-lives for the regeneration of N719 + /TiO2(e ) by iodide versus the logarithm of the concentration of Li + . 

(B) A similar plot depicting the point of zero z-potential (PZZP) for TiO2 nanoparticles as a function of the logarithm of the Li + concentration for 

(b) unsensitized TiO 2 and (d) N719/TiO 2. Both plots for N719/TiO 2 noticeably change behavior near 2 10 2 –2 10 3 M. Taken from Fig. 4a 

and 3, respectively, of ref. 361. 

within the Helmholtz layer of the particles even in the presence 

of bulky dyes. It was concluded that the abrupt change to 

faster sensitizer regeneration occurred due to ion-pairing of 

iodide anions with the TiO2 surface or sensitizer resulting in an 

increased occurrence of the faster termolecular reaction (11). 

ii Regeneration by donors other than iodide. An examination 

of the Latimer-type diagram in Scheme 1 reveals the 

significant problem with the I3 /I redox shuttle required for 

champion DSSCs. Iodide is oxidized at +1.33 V vs. NHE 

(or +1.03 V if reaction (11) is operative) at the sensitized 

electrode and I 3 is ideally reduced at +0.04 V (or I 2 at +0.21 V 

given the I 3 - I 2 + I equilibrium) at the Pt counter 

electrode. Thus at least a half of a volt of free energy is lost 

with this redox mediator under standard conditions. While the 

Latimer-type diagram depicts aqueous values, there is reason 

to believe that the losses are almost as large under nonstandard 

conditions in acetonitrile electrolytes, thus accounting 

for the non-optimal Vocs that are typically measured in 

DSSCs. Another issue with the I3 /I redox mediator is that a 

facile reduction of I3 at the counter electrode in DSSCs is 

required so as to minimize voltage losses. Platinum has a large 

exchange current density and transfer coefficient for this 

reaction but is expensive. 362 Electrode materials like graphite 

do not perform as well and the corrosive nature of the 

electrolyte towards less expensive metals like silver or copper 

precludes their use. 362 Similarly, I2 has an appreciable vapor 

pressure at room temperature and thus extra care must be 

taken to ensure a thoroughly and tightly sealed solar cell. 363 

Therefore, there is ample reason to identify alternative redox 

mediators for DSSCs. 

a Organic donors. With a few notable exceptions there has 

been very little progress in using one-electron transfer, outersphere 

redox couples as mediators. The published literature 

does not accurately reflect the experimental efforts that have 

been put forth in this area. This stems from the fact that it is 

neither rewarding nor easy to publish data on solar light-toelectrical 

power conversion efficiencies of o0.1%. Some time 

ago we showed that phenothiazine donors were able to 

efficiently regenerate the oxidized sensitizer. 329 However, one 

needed a pico-ammeter to measure any photocurrent due to 

quantitative recombination of TiO 2(e )s with PTZ + . In other 

words, PTZ + molecules were unable to escape the mesoporous 

film before recombination. This result appears to be very 

general. Gregg and co-workers found similar behavior with 

FeCp2 donors. 363 By coating the sensitized electrode with 

silanes, a large increase in the photoelectrochemical response 

was observed that was reasonably attributed to attenuation of 

the recombination reaction of TiO 2(e )s with FeCp 2 + , Fig. 32. 

In early aqueous DSSC studies, Gra¨tzel showed that hydroquinone 

was a satisfactory donor. 364 A polycrystalline TiO2 

(anatase) electrode sensitized with Ru(dcb)3 2+ in 10 mM 

aqueous NaCl (pH 2.6 with HCl) solution with 1 mM hydroquinone 

gave maximum IPCE values of 44%. Three years 

later, a DSSC containing 1 mM aqueous HClO4 and either 

10 mM hydroquinone/100 mM LiClO4 or 1 M KI electrolyte 

resulted in similar maximum IPCE values, Fig. 33. 365 

A comparison of halide redox mediators in acidic aqueous 

electrolyte, i.e. 1 mM HClO 4, illustrated that Ru(dcb) 3/TiO 2 

thin-film electrodes in electrolyte solution containing 1 M 

LiClO 4/1 mM Br 2 resulted in a monochromatic light-toelectrical 

power conversion efficiency of 12%. 365 However, 

the redox mediator was outperformed by the I3 /I 

redox mediator under short-circuit conditions. The more 

negative photocurrent onset observed for iodide, relative to 

Fig. 32 Current–voltage curves under simulated solar irradiance conditions 

and in the dark showing that silanization of a Ru(bpy) 2(dcb)/TiO 2 

thin film electrode dramatically improved the current–voltage characteristic 

of regenerative solar cells employing FeCp 2 +/0 as the redox couple. 

This data is consistent with the silanes attenuating TiO2(e )+FeCp2 + 

recombination. Taken from Fig. 7a of ref. 363. 


Fig. 33 Current–voltage curves for Ru(dcb)3/TiO2 in aqueous electrolyte 

illustrating that bromide and dihydroquinone function nearly 

as well as iodide in DSSCs under short-circuit conditions. Taken from 

Fig. 3 of ref. 365. 

hydroquinone and bromide, suggested a surface adsorptioninduced 

shift in the flatband potential in the iodide-containing 

electrolyte. 

A comparative study of the pseudohalide redox mediators 

(SeCN) 2/SeCN and (SCN) 2/SCN with the standard I3 /I 

redox mediator in N3/TiO2 regenerative DSSCs was reported. 

366 As the reduction potentials of the pseudohalides 

were 190 and 430 mV more positive than I3 /I , respectively, 

it was postulated that an increased Voc would result. Interestingly, 

the Voc for the DSSC containing the (SeCN)2/KSeCN 

redox mediator was about the same as for the I3 /I redox 

mediator while that for (SCN)2/NaSCN was considerably 

smaller. The iscs differed by a factor of four under monochromatic 

(500 nm) light excitation. The injection yields were 

independent of the redox mediator in 250 mM LiClO4 acetonitrile 

electrolyte containing 100 mM of the sodium or potassium 

salt of the reduced redox mediator, but the rate of 

regeneration followed the order I 4 SeCN 4 SCN . While 

SCN and SeCN oxidation by N3 + /TiO2 was thermodynamically 

favored, the oxidation kinetics were sluggish which 

allowed a larger fraction of the injected electrons to recombine 

with N3 + . 

Interestingly, Wang and Grätzel observed more promising 

behavior for Z907/TiO2 thin films with the (SeCN)2/SeCN 

pseudohalide redox mediator in the 1-ethyl-3-methylimidazolium 

(EMI) selenocyanate ionic liquid with added K(SeCN)3. 367 

Although this ionic liquid was found to be 35 times less viscous 

than the traditional 1-propyl-3-methylimidazolium (PMI) 

iodide ionic liquid, it was over 28 times more conductive at 

room temperature and could solubilize approximately eight 

times more (SeCN)2/SeCN than PMI could with I3 /I . 

By transient absorption spectroscopy, it was shown that 

Z907 + /TiO2(e ) could be regenerated fastest in EMI-SeCN 

as compared to PMI-I and the analogous EMI-SCN ionic 

liquid. This was contrary to the findings of Oskam et al. in 

acetonitrile electrolytes. 366 

It was also shown that the 

maximum IPCE was close to unity and the overall lightto-electrical 

power conversion efficiency under 1 sun, 

AM1.5-simulated irradiation was 7.5%. 

Recent studies have investigated 2,2,6,6-tetramethyl-1-piperidinyloxy 

radical (TEMPO) as a possible redox mediator. 

368,369 Nitrosyl tetrafluoroborate (NOBF 4) was added to 

TEMPO in order to generate a TEMPO + /TEMPO redox 

couple in a 1 : 9 stoichiometry, similar to the ratio of I 3 /I 

employed in champion DSSCs. When 1 M TEMPO was 

compared to the same concentration of iodide both the isc 

and Voc were slightly increased. The light-to-electrical power 

conversion efficiency for an organic sensitizer bound to TiO2 

under 1 sun, AM1.5-simulated irradiation was 5.4%. When 

the TEMPO concentration was decreased to 0.1 M, the Voc 

actually increased to B910 mV. 

b Transition-metal donors. Octahedral Co II diimine compounds 

have proven to be effective donors for sensitizer 

regeneration and Co III/II redox mediators have led to promising 

light-to-electrical power conversion efficiencies in DSSCs. 

The Co III/II self-exchange rate constants are known to be 

particularly sluggish, behavior that is reasonably understood 

by the d 6 /d 7 electronic configurations that give rise to large 

inner-sphere reorganization energies. 370 It is possible that 

these same electronic factors are responsible for the slow rate 

constants for TiO2(e )+Co III recombination reactions and 

the reasonable photocurrent efficiencies that have been reported 

when Co III/II redox couples have been used. 

The first studies of cobalt mediators were by Grätzel and coworkers. 

371 A DSSC based on the [Co III/II (dbbip)2] 3+/2+ 

redox couple, where dbbip is 2,6-bis(1 0 -butylbenzimidazol-2 0 - 

yl)pyridine), resulted in photovoltaic performance that rivalled 

the traditional I3 /I redox mediator when a B150 nm thick 

spray-pyrolyzed titania underlayer was deposited on the electrode 

substrate. It was shown that the exchange current 

density for the Co III/II couple at fluorine-doped tin oxide 

(FTO) was 7 10 6 A cm 2 in an acetonitrile–ethylene 

carbonate (40 : 60, v/v) electrolyte. 362 As this value was at 

least two orders-of-magnitude lower than that measured at 

platinum but more than two orders of magnitude higher than 

that of the I3 /I redox couple measured at FTO in the same 

electrolyte, a titania blocking layer was employed to slow this 

undesirable reaction. Using the cis-Ru(4-methyl-4 0 -hexadecylbpy)(dcb)(NCS)2 

sensitizer and a 1 : 9 stoichiometric ratio of 

Co III :Co II in the same electrolyte mixture, a maximum IPCE 

of 465% was realized and, under 0.094 suns, AM1.5simulated 

irradiation, a light-to-electrical power conversion 

efficiency of 5.2% was measured. 371 The use of a neutral 

sensitizer was found to be necessary in order to attenuate 

the adsorption of cationic redox species onto TiO 2. When 

Co II (dbbip) 2 2+ was added above a threshold of 10 mM, the 

second-order rate constant for regeneration—first order in 

N3 + /TiO2 and first order in Co II (dbbip)2 2+ —was 2.9 

10 6 M 1 s 1 , approximately an order-of-magnitude smaller 

than values reported for NaI. However, at 100 mM the 

pseudo-first-order rate constants were similar to those found 

with the same concentration of TBAI. 361 The change in 

apparent second-order rate constant was thought to be due 

to surface adsorption of the cationic redox couple at low 

concentrations. Adding to previous work, it was shown that 

the i sc for a DSSC employing Z907/TiO 2 was dependent on 

the counterion of the solution Co III/II redox mediator; the 


perchlorate salt worked the best. 372 As expected, for the 

perchlorate-based redox mediators whose E o (Co III/II ) varied 

over 190 mV, a similar 180 mV variation in V oc was realized. 

The largest V oc recorded was 660 mV accompanied by a 7.9% 

light-to-electrical power conversion efficiency under 0.1 suns, 

AM1.5-simulated irradiation. 

Upon introduction of LiClO4 to DSSCs the lifetime of the 

TiO2(e )s 373,374 increased for cobalt-based redox couples 

whereas for the I3 /I redox mediator it decreased. This was 

rationalized as being due to a decrease in the local concentration 

of the cationic or neutral cobalt-based redox couples near 

the TiO2 surface when cationic Li + was present. 373,374 

A family of cobalt redox couples employing derivatives of 

bpy, phen, and tpy ligands were studied by Bignozzi, Elliott, 

and colleagues. 375 The best cobalt-based mediator, based on 

[Co III/II (DTB) 3] 3+/2+ perchlorate (DTB = 4,4 0 -di-tert-butylbpy), 

resulted in DSSCs exhibiting light-to-electrical power 

conversion efficiencies within 80% of that of a comparable 

I3 /I -mediated DSSC under 1 sun, AM1.5-simulated conditions. 

Also, in contrast to the I3 /I redox mediator, addition 

of Li + to the cells increased not only the isc but the Voc as well! 

This was proposed to be due to a decrease in the rate of 

recombination of TiO2(e )s and Co III most likely from an 

increase in overpotential for reduction of the Co III species at 

the back FTO contact. In addition, cyclic voltammograms 

with platinum electrodes revealed sluggish interfacial, Co III/II 

electron-transfer kinetics relative to carbon and gold. 

Although gold was optimal, FTO electrodes coated with 

graphite nanoparticles initially outperformed platinum in 

DSSCs; however, the carbon-coated FTO electrodes degraded 

with time. Nevertheless, the initial response was encouraging 

and shows promise for replacing platinum with a less-expensive, 

carbon-based material for use as a counter electrode. 

Although large reorganization energies and slow-electron 

transfer kinetics for cobalt-based redox couples are advantageous 

as they attenuate the unwanted, recombination reaction, 

TiO 2(e ) + Co III - TiO 2 + Co II , these characteristics 

are undesirable with respect to sensitizer regeneration, 

S + /TiO 2(e )+Co II - S/TiO 2(e )+Co III . Rapid sensitizer 

regeneration and sluggish recombination kinetics are traits 

that make the I3 /I redox mediator optimal. By using comediators 

in conjunction with [Co III/II (DTB)3] 3+/2+ it was 

proposed that these traits could be realized in non-iodidebased 

systems. 124 Both PTZ and FeCp2 were employed in 

Z907/TiO2 DSSCs, due to their small reorganization energies, 

rapid electron transfer kinetics, and reduction potentials intermediate 

between that of [Co III/II (DTB) 3] 3+/2+ and Z907 +/0 . 

By transient absorption spectroscopy the bleach due to 

Z907 + /TiO 2(e ) was found to recover in the presence of 0.1 M 

donor in the order FeCp 2 4 PTZ 4 Co(DTB) 3 2+ 4 no 

donor, whereas by chronocoulometry at FTO, PTZ/Co II 1:2 

molar mixtures were found to turnover 45% faster than 

FeCp2/Co II mixtures. Thus, the maximum IPCE, i.e. 480%, 

was achieved for 0.075/0.15 M PTZ/Co II (1:2 molar ratio) in 

acetonitrile after generating steady currents by photolysis for 

10–15 min in order to generate some Co III . The Voc and FF, 

650 mV and 0.63, respectively, under 0.1 suns, AM1.5simulated 

conditions, were both larger than for an equivalent 

DSSC employing the LiI/I 2 redox system (0.3/0.03 M). 

However, the light-to-electrical power conversion efficiency 

was less due to mass-transport limitations of the bulky 

cobalt redox mediator. By binding Os(dcb) 2Cl 2 to FTO the 

exchange current for [Co III (DTB) 3] 3+ reduction was greatly 

enhanced. 376 When employed in an N3/TiO 2 DSSC, the i sc and 

V oc were only slightly attenuated as compared to a gold 

counter electrode and, using a three electrode measurement, 

the potential of the Os(dcb)2Cl2/FTO counter electrode was 

only slightly perturbed near open-circuit conditions. 

Co III/II redox couples based on triazine ligands have been 

synthesized and characterized. 377 The heteroleptic Co(triazine-R)- 

Cl2 compounds were shown to have E o (Co III/II )=B+0.75 V 

vs. SCE (+0.99 V vs. NHE 229 ), considerably more positive 

than previously reported Co-based redox mediators. However, 

this redox couple has yet to be employed in a functioning 

DSSC. It will be interesting to see if regeneration by this redox 

mediator can compete kinetically with charge recombination. 

Cu I has a d 10 electronic configuration and compounds like 

Cu(bpy)2 + often adopt a tetrahedral geometry in solution and 

in the solid state. The Cu II form is subject to a Jahn–Teller 

distortion that often manifests itself in a geometry with more 

co-planar diimine ligands, i.e. a flattening, and a fifth ligand 

from solvent or a counterion axially ligated. It is possible to 

photoinduce these structural changes and they have been 

characterized by time-resolved X-ray techniques. 378 Like the 

Co III/II redox mediators, Cu II/I couples have large reorganization 

energies, slow self-exchange rate constants and show some 

modest success as mediators in DSSCs. For example, Cu I - 

pyridyl and Cu I -pyridyl-quinoline compounds have been studied 

in DSSCs. 379 The best-performing mediators produced a 

maximum IPCE of B40% and yielded higher Vocs and FFs 

than the I3 /I redox couple under the same experimental 

conditions. This was attributed to a decreased dark current 

due to the large reorganization energy of the Cu II/I redox 

couple. 

Unfortunately the large reorganization energies for Cu II/I 

redox mediators suffer the same pitfalls as their Co III/II 

counterparts, i.e. slow sensitizer regeneration. Thus, a Cu I 

compound with a distorted tetrahedral geometry was employed 

in order to help reduce the large reorganization 

energy. 380 When [Cu II/I (dmp)2] 2+/+ was employed as the 

redox mediator, where dmp is 2,9-dimethyl-phen, a light-toelectrical 

power conversion efficiency of 2.2% under 0.2 suns, 

AM1.5-simulated irradiation was obtained with N719/TiO2based 

DSSCs. The methyl groups were proposed to prevent 

planarization of the dmp ligands which manifests itself in a 

positive shift in E o (Cu II/I ). Significantly, a higher V oc was 

realized with the copper mediator as compared with I 3 /I 

under the same experimental conditions. 

iii Time scale for regeneration. Rates of regeneration of the 

oxidized sensitizer that is produced after excited-state injection 

have now been quantified with organic and inorganic donors. 

By covalently binding the donor to the sensitizer, intramolecular 

regeneration is observed. Iodide oxidation can be complicated 

by the presence of multiple reaction mechanisms and 

by ion-pairing with the sensitizer or semiconductor surface, 

but nonetheless is now reasonably well understood. 


Fig. 34 (A) Absorption and photoluminescence spectra of Ru(dtb)2(dcb)/TiO2 in 0.1 M LiClO4 acetonitrile electrolyte (red spectra) and in neat 

acetonitrile after removal of the LiClO 4 by ten neat acetonitrile washings (black spectra). (B) Transient absorption difference spectra for three 

Ru(dtb)2(dcb)/TiO2 thin films at the indicated surface coverages and delay times measured after pulsed 532 nm excitation in 0.1/0.5 M LiClO4/ 

TBAI acetonitrile electrolyte. Overlaid are simulations of the data represented by dashed lines. Inset: Time-resolved, single wavelength absorption 

difference spectra measured at 510 nm for each surface coverage, corresponding to cation transfer, and a single difference spectrum measured at 

433 nm (black spectrum with orange fit), corresponding to I 3 loss due to TiO 2(e )+I 3 recombination. Taken from Fig. 1 and 2, respectively, of 

ref. 381. 

Recent results from our laboratories suggest new directions 

for fundamental research and raise the question of what the 

term ‘regeneration’ actually means. These new results are best 

understood with an example, [Ru(dtb)2(dcb)] 2+ , where dtb is 

4,4 0 -di-tert-butyl-bpy. Fig. 34(a) shows the absorption and PL 

spectra of a Ru(dtb)2(dcb)/TiO2 thin film immersed in 0.1 M 

LiClO4 acetonitrile and in neat acetonitrile. In the presence of 

Li + both maxima red-shifted and their intensity decreased 

relative to neat acetonitrile. The significant quenching of the 

PL results from enhanced excited-state electron injection into 

TiO 2 as described previously herein. 254 

Pulsed 532 nm excitation of Ru(dtb) 2(dcb)/TiO 2 in 0.1/0.5 M 

LiClO 4/TBAI acetonitrile electrolyte resulted in the microsecond 

absorption difference spectrum, Fig. 34(b). Under such 

conditions, one would expect to observe a TiO2(e ) and 

oxidized iodide products, e.g. I3 . Regardless of the mechanism 

for iodide oxidation, most of the I2 should have 

disproportionated by this sufficiently long time delay. The 

absorption features characteristic of I3 (l o 420 nm) and 

TiO2(e )s (l 4 560 nm) were indeed observed. However, the 

absorption band centered at 460 nm and the bleach at 510 nm 

could not be assigned to any conceivable electron-transfer 

products. 

Spectral modeling indicated that the absorption features at 

460 and 510 nm resulted from [Ru(dtb)2(dcb)] 2+ sensitizers 

that were regenerated in a Li + -deficient milieu. In other 

words, the sensitizers that were initially photo-excited had 

an absorption spectrum shown in red while immediately after 

regeneration their spectrum was that shown in black, 

Fig. 34(a). Overlaid on the data in Fig. 34(b) are simulations 

based on the weighted addition of (1) the absorption spectrum 

of I3 , (2) the TiO2(e ) absorption spectrum, and (3) the 

difference in the absorption spectra of Ru(dtb) 2(dcb)/TiO2 in 

the absence minus the presence of Li + . Similar sensitizer 

absorption features were observed when PTZ was used in 

place of iodide. The excellent agreement between observed and 

simulated spectra provided compelling evidence that these 

sensitizers were regenerated in an environment that lacked 

outer-sphere Li + interaction(s). This behavior was also observed 

after pulsed-light excitation of Ru(bpy)2(dcb)/TiO2 and 

Ru(bpy)2(dcbq)/TiO2 in 0.5 M iodide electrolyte as well as 

previously in the published literature for N3/TiO2 382 and 

Ru(bpy) 2(dcb)/SnO2. 356 In all cases, absorption features were 

observed that were not due to oxidized iodide products, 

TiO2(e )s, or other redox states of the sensitizers. They were, 

however, reasonably described as sensitizers regenerated in an 

environment depleted of Li + . 

The absorption changes that correspond to cation transfer 

were well described by the Kohlrausch–Williams–Watts 

(KWW) function for a distribution of rate constants (distributions 

are shown in Fig. 35(a)): 

" # 

b 

t 

C ¼ Co exp 

ð12Þ 

where t o is the most representative lifetime, i.e. the mode, and 

b is inversely related to the width of the underlying Levy 

distribution, 0 o b o 1. 383–385 Kohlrausch first proposed the 

function empirically and it was later popularized by Williams 

and Watts. The inverse Laplace transform is known analytically 

for discrete values of b and can be approximated for 

others allowing the distribution of rate constants to be directly 

recovered. 386 Values for to = 4.1 ( 2.5) 10 5 s and b = 0.16 

0.01 were found. The low b values corresponded to a broad 

Levy distribution of rate constants. Tens of microseconds to 

even milliseconds were required for completion of the cation 


to

Fig. 35 (A) Normalized Levy distributions of lifetimes calculated using the indicated values of b from the KWW model. As b approaches 1, the 

distribution approaches a Dirac d function in shape and thus the kinetics would begin to follow a simple first-order model. Taken from Fig. 2 of 

ref. 402. (B) Simulated time-resolved spectra based on the random-flight multiple-trapping model of Tachiya and colleagues on the left and the 

multiple-trapping, nearest-neighbor CTRW model of Nelson et al. on the right. Although similar in shape, the trapping–detrapping rate is four 

orders-of-magnitude slower in the former. Taken from Fig. 18 of ref. 401. 

transfer when the Ru(dtb)2(dcb)/TiO2 thin films contained a 

high surface coverage of sensitizers. While the large tert-butyl 

groups may inhibit cation motion, similar time scales for 

cation transfer were observed for Ru(bpy) 2(dcb)/TiO 2 and 

N3/TiO 2. The time scale for cation transfer was in itself 

surprising considering that the sensitized film was immersed 

in 0.1 M Li + -containing electrolyte, thus highlighting the 

locality of said effect. Even though the sensitizer surface 

coverage was high, only a small concentration of sensitizers, 

approximately equal to that of TiO2(e )s, were found to be in 

this Li + -deficient environment. 

The spectral data indicated that Li + transfer away from the 

sensitizer occurred in o10 ns. Furube et al. reported timeresolved 

infrared data consistent with picosecond Li + transfer 

after excited-state injection by coumarin sensitizers, behavior 

attributed to Coulombic repulsion between the oxidized coumarin 

and Li + . 387 As cations are required for charge compensation 

of the injected electron, that could also induce Li + 

to migrate away from the oxidized sensitizer. As mentioned 

previously, intercalation of Li + is known to accompany 

reduction of anatase TiO2. In this regard, we found that the 

same sensitizer spectral changes could be observed by partial 

electrochemical reduction of the TiO2, i.e. when no oxidized 

sensitizer was present, indicating that charge compensation 

plays a role. 

After fast excited-state electron injection into TiO 2 and 

regeneration by iodide, sensitizers were present in an environment 

distinctly different from that prior to light absorption. 

Significantly, the newly generated sensitizers were in an environment 

that is known to be less favorable for excited-state 

electron injection. 254 Under 1 sun, AM1.5 irradiation, the slow 

(ms—ms) cation transfer is not expected to limit the efficiency 

of DSSCs as a typical Ru II sensitizer absorbs light approximately 

twice every second. 388 However, at higher irradiances 

or at planar TiO2 surfaces this effect may limit light-toelectrical 

power conversion efficiencies. In all cases, the sensitization 

rate constants shown in Fig. 1 need to be modified. 

The oxidized Ru III sensitizer may be reduced to Ru II on a 

nanosecond time scale, however it is not brought back to the 

environment prior to light absorption until slow (ms—ms) 

cation transfer has taken place. 

5. Charge recombination 

Charge-recombination processes at sensitized semiconductor 

interfaces have been studied in considerable detail. Reactions 

of TiO2(e )s with: (A) the oxidized sensitizer; and (B) acceptors 

in the electrolyte, Step IV in Fig. 1, have received much 

attention. As described in section 4/B/i/b, regeneration of the 

oxidized sensitizer by iodide is rapid and quantitative in 

champion DSSCs. Therefore, reaction IV–A is generally not 

relevant to these cells. It is, however, important in DSSCs 

employing alternative redox mediators or sensitizers whose 

ground-state reduction potentials are less favorable for regeneration 

than that of N3, E o (Ru III/II ) o +0.85 V vs. SCE 25 

(+1.09 V vs. NHE 229 ). It may also become relevant at high 

irradiances or in viscous electrolytes. The transparent nature 

of the mesoporous, nanocrystalline TiO2 (anatase) thin films 

allows this process to be quantified in a transmission mode 

with signal-to-noise ratios comparable to what can be 

achieved in fluid solution. Mechanistic insights have been 

gained from transient absorption measurements made under 

open-circuit conditions in the absence of an external electron 

donor such that each injected electron recombines with oxidized 

sensitizers. 

In champion DSSCs, charge recombination to acceptors 

within the I3 /I electrolyte is a very inefficient process. The 

fraction of TiO2(e )s that recombine by this pathway is 

usually so small (o0.01) that it does not significantly influence 

the isc. However, TiO2(e )s that recombine by this pathway 

are thought to have a significant influence on the quasi-Fermi 

level of the semiconductor and hence a large effect on the Voc. 

Intensity-modulated photovoltage/photocurrent spectroscopy 

(IMVS/IMPS) and time-domain transient photovoltage/ 

photocurrent decays with appropriate modeling, have provided 

some insights into the mechanisms of this unwanted 

reaction. 

A Electron-transport-limited charge recombination 

In the late 1990s, our group at Johns Hopkins University and 

the groups at Imperial College in London provided evidence 

that charge recombination was not slow because of inherently 

small rate constants but because efficient separation of the 


injected electron and the oxidized sensitizer resulted in nongeminate 

recombination. Our own kinetic data showed that 

charge recombination was modeled by an equal-concentration, 

second-order process much like the analogous process in fluid 

solution. 254,389 The concentration of charge-separated states 

was controlled by modulating the excitation irradiance or the 

Li + concentration. When the concentration of chargeseparated 

states, i.e. Ru III /TiO2(e ), was varied by over a 

factor of ten, the same second-order rate constant was 

abstracted from the data. A single second-order rate constant 

could be used to fit the first few microseconds of recombination, 

whereas a sum of two rate constants, i.e. a bi-second 

order kinetic model, was required to fit the entire transient. 

The possibility that a distribution of second-order rate constants 

had underlain the observed kinetic behavior could not 

be ruled out. The observed second-order rate constant, which 

was obtained directly from the transient absorption data, had 

the unconventional units of s 1 . Accurate conversion to the 

more common units of M 1 s 1 was complicated by the 

heterogeneity of the sample which resulted in an ill-defined 

homogeneous optical path length. Assuming adherence to 

Beer’s Law and an optical path length equivalent to the film 

thickness, the back-electron transfer rate constants for 

Ru III (bpy)2(dcb)/TiO2(e ) - Ru II (bpy)2(dcb)/TiO2 were 

found to be B9 10 11 and 3 10 10 M 1 s 1 in 1.0 M LiClO 4 

acetonitrile electrolyte. Other researchers have employed the 

same methodology to abstract a weighted-average of the 

equal-concentration, second-order rate constants in units of 

M 1 s 1 . 390 The key advance was that efficient separation of 

the TiO2(e ) from S + occurred thereby giving rise to somewhat-isolated 

TiO2(e )s and oxidized sensitizers. Recombination 

was second order in nature not first order as had been 

previously assumed in the kinetic modeling. In fact, actinometry 

measurements showed no evidence for first-order, 

geminate recombination; every injected electron recombined 

by the second-order mechanism. 

Shortly thereafter, Nelson and the group from Imperial 

College proposed a model for charge recombination that 

involved transport of the injected electron to the oxidized 

sensitizer. 183,185 The central idea was that charge carriers 

become trapped in localized states and that the kinetics for 

charge transport are dominated by the time constants for 

release from those states. Transient phenomena of this kind 

are termed ‘dispersive’ 391–393 when they are rate-limited by this 

step. A numerical model initially derived by Scher and Montroll 

based on a ‘continuous-time’ random walk (CTRW) 

where species move by diffusion on a lattice was utilized. 394,395 

The dispersive nature of such kinetics was introduced by 

applying a power-law, waiting-time distribution time step, 

c p t 1 b ,0o b o 1. For ‘normal’ diffusion the time step 

would be drawn from a Poisson distribution, c p e t/t . The 

result of such a model are kinetics that follow the KWW 

function, which represents a distribution of rate constants as 

shown in Fig. 35(a), 383–385 with b being equal to the inverse of 

the DOS non-ideality factor. 185 Also, with this model, the t1/2 

for the recombination ought to vary with the number of 

TiO2(e )s per particle, n, as 

t 1/2 = Cn 1/b 

(13) 

where t1/2 = to(ln 2) 1/b and C is a constant. 184,185 This has 

been confirmed experimentally. A mean lifetime for the kinetic 

process can also be calculated by the first moment of the 

KWW function: 

tKWW ¼ to 

b 

G 1 

b 

ð14Þ 

where G(x) is the Gamma function. 396,397 This model is often 

applicable in fractal systems or regarding relaxation in solids. 

398,399 When such systems are applied to fast charge 

recombination from TiO2(e )s to oxidized sensitizers or acceptors 

in solution the model fits rather well. 178,182,184,186 

Two models for trap-limited diffusion in disorder media 

were proposed by Nelson et al. 185 The first was based on 

multiple-trapping-limited recombination as derived from the 

CTRW model where steps to all nearest neighbors were 

equally likely. The other was based on tunneling-limited 

recombination, where quantum-mechanical tunneling can 

result in long-range interactions, i.e. farther than nearestneighbor. 

Fits to experimental charge-recombination data 

under external bias and plots of the half-lives versus the 

concentration of TiO2(e )s strongly ruled out the latter model 

and supported the former given an exponential DOS. 

Later, an extension to the multiple-trapping, nearestneighbor 

CTRW model proposed by Nelson et al. was reported 

by Tachiya and colleagues. 400,401 This new randomflight 

multiple-trapping model included the possibility of many 

neighbor interactions, where the probability that the detrapped 

electron will be captured by any empty, surface trap 

state within the nanoparticle is equal. Although similar in 

shape to the model proposed by Nelson et al., the calculated 

trapping–detrapping rate as a function of time was many 

orders-of-magnitude slower, Fig. 35(b). 

A hopping model that differs from the multiple-trapping 

model has also been proposed by Bisquert. 403 Instead of 

activated detrapping to the conduction band, electron transport 

occurs by direct transfer via localized states located at 

energies just below Ecb. However, a very high carrier density is 

needed to validate the model as all of the above models predict 

similar behavior at lower carrier densities. 

There is now a wide body of charge-recombination data that 

is well modeled by the multiple-trapping, nearest-neighbor 

CTRW model and the KWW function. It allows a great deal 

of experimental data to be modeled with only two independent 

variables. However, the derived parameters (to and b) do not 

always provide insights into the underlying dynamics. The 

KWW function has been ‘‘derived’’ by at least three different 

groups: (1) the already discussed CTRW model of Scher and 

Montroll; 394,395 (2) a distribution of serially linked first-order 

rate constants by Anderson; 404 and (3) fractal time concepts by 

Shlesinger. 405 Although not as rigorous, Plonka has also 

shown that dispersive, second-order kinetics can lead to 

behavior that is well modeled by the KWW function. 406 

Anderson’s model is based on a distribution of first-order rate 

constants whose magnitudes decrease with time. Such behavior 

can be very difficult to distinguish from a second-order 

process where the rate decreases with time but the rate 

constant does not. We have in fact shown that it is often 

impossible to conclude whether a distribution or a sum of two 


discrete rate constants underlie complex kinetic behavior 

based simply on the quality of the fit. 407 The point here is 

that while modeling charge recombination based on Scher and 

Montroll’s CTRW model makes good physical sense, it is not 

a unique fit and other models may ultimately provide more 

insights. 

i Comparisons to single-crystal anatase. Experimentally a 

great deal is known about the transport of injected electrons 

through mesoporous, nanocrystalline TiO2 (anatase) thin 

films. The mobility of and diffusion coefficient for free conduction-band 

electrons in single-crystal anatase and rutile 

TiO2 in the absence of solvent were found to be on the order 

of 1–10 cm 2 V 1 s 1 and 10 1 cm 2 s 1 , respectively, and were 

inversely related to temperature due to optical phonon scattering. 

408–410 Similar, but often slightly lower, values for 

TiO 2(e )s in mesoporous, nanocrystalline TiO 2 are obtained 

when trapping–detrapping events are removed either from 

the experiment—based on the technique—or from the 

results—based on modeling. 411–413 Using a trap-filling model 

Ko¨nenkamp was able to estimate the TiO2(e ) free carrier 

mobility for air- or N2-filled, mesoporous, nanocrystalline 

TiO2 (anatase) thin film, Schottky barrier electrodes to be 

B2.4 cm 2 V 1 s 1 . 412 O’Regan and colleagues were able to 

measure TiO2(e ) mobilities for air-filled, mesoporous, nanocrystalline 

TiO2 (Degussa P25) thin films using terahertz 

spectroscopy. 411 This technique is unique as it measures the 

average mobility of charges due to intraparticle transport on 

the B10 ps time domain, prior to electron trapping. Although 

the calculated mobilities were two orders-of-magnitude slower 

than the values calculated for single-crystal rutile electrodes, 

this could be explained by employing the Drude model and the 

appropriate Maxwell–Garnet effective medium theory. It was 

concluded that the reduced terahertz mobility observed in the 

porous sample was due to screening of the applied field by the 

polar TiO2 matrix. Additionally, Bisquert and colleagues 

determined that the trapping–detrapping-limited TiO2(e ) 

diffusion reported on an average, effective diffusion coefficient 

whereas, for comparison to single-crystal values, the more 

appropriate tracer, or jump, diffusion coefficient should be 

obtained. 180,413–416 By employing the analytical solutions to 

this novel model, Peter determined tracer diffusion coefficients 

near Voc conditions that were comparable to those obtained 

for single-crystal anatase, i.e. about one order-of-magnitude 

smaller. 413 

ii Ambipolar diffusion. An interesting aspect of diffusion 

that is rather unique to nanocrystalline TiO2 (anatase) thin 

films in DSSCs results from the high ionic concentrations and 

mesoporosity of the thin film electrodes. Diffusion of charged 

particles in solution and in highly conductive media are 

shielded by counterions as required in order to maintain 

‘quasi-neutrality.’ Thus, over large volumes neutrality is preserved, 

however on the scale of the Debye length charge 

imbalances can exist. For nanocrystalline, anatase TiO2 the 

situation is often different as the pores of anatase TiO2 are 

large enough to accommodate cations with large charge-toradius 

ratios 156,190,205,207–210,212–215,417–420 and thus the Debye 

length is on the order of 1 A˚ . 410 In champion DSSCs each 

injected electron is thought to be immediately shielded by a sea 

of oppositely charged Li + and the long-range, macroscopic 

electric field across the film is negligible, Fig. 36. 18,159–162 

It is for this reason that TiO 2(e ) diffusion is governed by 

the concerted motion of the electronic and electrolyte charges 

per the formula: 17,410,421 

Damb ¼ DnDpðn þ pÞ 

ð15Þ 

nDn þ pDp 

where Damb is this ambipolar diffusion coefficient and n, Dn, p 

and Dp are the anionic- and cationic-charge densities and 

diffusion coefficients, respectively. 219 As champion DSSCs 

employ 0.5 M electrolyte solutions and are often evaluated 

under 1 sun, AM1.5 conditions where the concentration of 

TiO2(e )s is far less than the concentration of electrolyte in 

solution, Damb is approximately the diffusion coefficient for 

the less dense carrier, i.e. the TiO2(e )s, even though the 

apparent diffusion coefficient for TiO2(e )s, Dn, is larger. This 

large concentration of oppositely charged and mobile Li + 

results in the injected electron being the minority carrier. 410 

The net outcome is an attenuation in the rate of TiO2(e ) 

diffusion and an increase in the rate of Li + diffusion relative to 

their rates of diffusion in each other’s absence. 17 

This ambipolar diffusion model was described in the early 

1950s and solved computationally using a non-linear model in 

the late 1960s. 422,423 Searson and co-workers first reported 

that the TiO2(e ) diffusion coefficient, in thin-film electrodes, 

was dependent on the light intensity and thus also on the 

concentration of TiO2(e )s. 424 Hagfeldt and colleagues later 

reported that TiO2(e ) diffusion in unsensitized mesoporous, 

nanocrystalline TiO2 (anatase) thin-film electrodes followed a 

cation-dependent mechanism. 425 They termed the solution 

Fig. 36 (A) A diagram illustrating the space-charge potential drop across a TiO 2 nanocrystallite that is B20 nm in diameter before and after 

contact with a solution electrolyte. (B) Color-coded topographical model illustrating the relative potential distribution for an ordered mesoporous 

network of such nanocrystallites. Taken from Fig. 5 and 6, respectively, of ref. 18. 


electrolyte an image cloud care of classical physics terminology. 

426 Reduction of the electrolyte concentration from 500 to 

20 mM resulted in a five-fold decline in the diffusion coefficient, 

indicative of an ambipolar diffusion mechanism. With 

laser pulses at 2 Hz in 500 mM LiClO 4 acetonitrile electrolyte, 

the diffusion coefficients were shown to significantly slow, 

possibly due to insufficient time for Li + deintercalation from 

within the anatase lattice. These same researchers later showed 

similar behavior for sensitized N3/TiO2 thin-film electrodes. 427 

Although not fit to an ambipolar diffusion model it was 

apparent that the cation concentration limited the TiO2(e ) 

collection time. 

Frank and colleagues were the first to quantify ambipolar 

diffusion coefficients for mesoporous, nanocrystalline TiO 2 (anatase) 

thin-film electrodes. 410 Using laser-pulsed photocurrent 

transients, both in the absence and presence of a constant 

background illumination, ambipolar diffusion coefficients for 

N719/TiO2 electrodes were obtained over a large range of 

excitation energies. These diffusion coefficients ranged greatly 

from 3 10 8 to 10 4 cm 2 s 1 for low to high irradiances, 

respectively. The difference in the calculated ambipolar diffusion 

coefficient and TiO2(e ) diffusion coefficient was greatest under 

the highest illumination intensities studied; however the difference 

in the values was only B15%.Thus,TiO2(e ) diffusion 

coefficients were satisfactory estimates for the more accurate 

ambipolar diffusion coefficients. Further support for the ambipolar 

diffusion model was later established by these same 

researchers based on the arrival-time detection of current from 

TiO2(e )s at the back FTO contact, rather than displacement 

current, under counter-electrode side illumination. 428 This implied 

that the electric field of the TiO2(e )s was shielded by the 

electrolyte and that only TiO2(e )s that physically arrived at the 

FTO–TiO2 junction registered a photocurrent. It was originally 

thought that the large concentration of electrolyte in functioning 

DSSCs would effectively shield the TiO2(e )s and have little 

effect on their transport. However, this is only the case under 

steady-state conditions. At early times after a perturbation, the 

diffusion coefficient for TiO 2(e )s is substantial and an ‘ionic 

drag’ on the free electron mobility is present. 16 It was shown that 

when the sea of counter-charged species was rather dilute the 

TiO2(e ) transport became less dispersive at early times. In 

contrast, the concentration of counter-charged species had little 

influence on the steady-state limit of the TiO2(e )s diffusion 

coefficient. Thus, on short time scales the ambipolar effect 

hindered fast electron transport through the TiO2 film while 

under steady-state conditions, where transport was trap limited, 

ionic drag was generally absent. 

Yanagida and co-workers found evidence that further supported 

the ambipolar diffusion model using Li + -concentrationdependent 

transient photocurrent studies with unsensitized 

TiO2 thin-film electrodes in ethanol electrolyte. It was shown 

that the ambipolar diffusion model adequately described the 

data and that the TiO2(e ) diffusion coefficient was over 

two orders-of-magnitude larger than that of Li + . 429 Additional 

studies performed by varying the irradiance in either 

700 mM or 5 mM LiClO4-electrolyte solutions resulted 

in the expected ambipolar diffusion trend. In 700 mM electrolyte, 

a direct relationship between the ambipolar diffusion 

coefficient and irradiance was observed while in 5 mM electrolyte, 

an inverse relationship existed as expected by the diffusion 

of Li + now being rate limiting, Fig. 37(a). Using similar 

experimental procedures, these same researchers showed that 

the diffusion coefficients for various cations, i.e. Li + ,Na + , 

Mg 2+ , TBA + , dimethylhexylimidazolium cation (DMHI + ), 

could accurately be extracted from the low-concentrationelectrolyte 

data fit to the ambipolar diffusion model. 430 

However, at higher electrolyte concentrations significant and 

unexpected increases in the ambipolar diffusion constant were 

obtained. Only for the TBA + data did the ambipolar diffusion 

model result in a satisfactory fit for all concentrations studied, 

Fig. 37(b). The empirically determined diffusion coefficients 

for the other cations were well above expected values in the 

order DMHI + 4 Li + 4 Na + , assumed to be due to specific 

adsorption. By employing UV-Vis spectroscopy of the soaking 

solutions, it was indeed shown that there was multilayer 

absorption of DMHI + on TiO2 whereas practically no 

absorption of TBA + occurred. Additionally, when plots 

the ambipolar diffusion coefficient versus the concentration 

of cation were obtained for Li + and TBA + , a noticeable 

hysteresis was present for Li + assigned to Li + adsorption 

onto TiO 2. 

iii Activation energy. As stated above, the diffusion coefficient 

for TiO2(e )s is dependent on their concentration. 424 

Thus, it would seem likely that the activation energy for 

electron transport within this network of nanocrystallites 

Fig. 37 (A) A log–log plot of the empirical diffusion coefficients as a function of TiO2(e ) density. The monotonic increasing trend for the data in 

700 mM LiClO 4 and the inflection in the 5 mM data can be satisfactorily modeled by the ambipolar diffusion model. Taken from Fig. 5 of ref. 429. 

(B) A nearly perfect fit of the ambipolar diffusion constant versus the logarithm of the concentration of TBA + data to the ambipolar diffusion 

model for a large concentration of TiO 2(e )s (circles); also shown is data at low TiO 2(e ) density (squares). Taken from Fig. 3 of ref. 430. 


would also be TiO2(e )-concentration dependent. By employing 

conductivity measurements, activation energies for electron 

transport of B0.3 eV were obtained for mesoporous, 

nanocrystalline TiO 2 thin-film electrodes under typical DSSC 

working conditions. 431 

Most measurements of transport times in mesoporous, 

nanocrystalline TiO2 thin-film electrodes are determined under 

short-circuit conditions, as elsewhere transport is often RC 

limited. 432 However it is often desirable/necessary to determine 

these transport times near DSSC working conditions, i.e. 

near VPP/Voc. 413 O’Regan and colleagues have developed a 

novel method for doing so by measuring photovoltage transients 

at various preset voltages and entering the results in a 

zero-free-parameter model. 432 Using this novel method it was 

shown that TiO 2(e ) transport though N3/TiO 2 thin-film 

electrodes fits the multiple-trapping model, whereby detrapping 

limits TiO 2(e ) transport and recombination. 433 The 

method also allowed for a more accurate calculation of 

activation energies for TiO2(e ) transport, as prior methods 

did not allow for the facile correction of the temperature 

dependence of Ecb. The results seemed to indicate that the 

activation energy was not as large as the energy difference 

between the trap states and the conduction band, consistent 

with other reports. 434–436 Thus the results are in agreement 

with the aforementioned hopping model proposed by Bisquert 

where electrons need not fully thermalize to the conduction 

band in order to hop between trap states within TiO 2. 403 

Following the work of Bisquert and Vikhrenko, 415,416 a 

model employing a quasi-static approximation was proposed 

that accounted for previously measured activation energies 

without invoking the temperature dependence of Ecb. 437 In 

contrast, this model predicted concerted and equal shifts in Ecb 

and the energy of the quasi-Fermi level with temperature. 

B Recombination to the oxidized sensitizer 

Early studies of TiO 2 colloidal solutions and thin films sensitized 

to visible light with Ru II -based and organic sensitizers 

established that charge recombination to the oxidized sensitizer 

occurred on a tens- to hundreds-of-microseconds time 

scale. 158,438 The observation of efficient sensitization from 

compounds with very short excited-state lifetimes, such as 

[cis-Ru(dcb)2(H2O)2] 2+ , indicated that excited-state electron 

injection was a sub-nanosecond process. 439 The question 

then naturally arose: why does such a fortuitous difference 

in interfacial charge-separation and charge-recombination 

rate constants exist at the TiO 2 interface? For MLCT excited 

states part of the explanation was that injection occurred 

from the p* orbitals of a surface-bound, dcb ligand while 

recombination was to the t2g orbitals of the Ru III -metal 

center. In other words, there is a built-in type of rectification 

in these sensitizers whose orbitals provide strong electronic 

coupling for charge separation but inhibit recombination. 

218,219 It was also known that charge recombination 

was in the Marcus inverted region whereas excited-state injection 

was nearly activationless. 438 Such orbital participation 

and thermodynamics could explain the large difference in 

interfacial charge-separation and charge-recombination rate 

constants. 

i Lateral electron transfer via surface-bound adsorbates. 

While it is often tacitly assumed that transport of the injected 

electron is most relevant to charge recombination, it is important 

to emphasize that the oxidized sensitizer also has some 

mobility. Lateral charge transfer across semiconductor surfaces 

is often initiated by charge-transfer reactions at the 

transparent conductive electrode (TCE) that supports the 

TiO2 thin film. Such hole transfer can almost entirely be 

eliminated with the addition of a blocking layer on the back 

TCE support prior to thin-film deposition. Thus the major 

means for hole transfer is by lateral sensitizer-mediated hopping 

of charge or physical movement/diffusion of the bound 

sensitizers. Should the diffusion coefficient for such a process 

be independent of sensitizer concentration, the latter mechanism 

is assumed. However, a sharp surface-coverage onset to 

the diffusion is consistent with an underlying self-exchange, 

charge-transfer reaction and a percolation threshold. 71 A 

percolation threshold is formally defined for the conductivity 

inside a composite material as ‘‘the critical concentration 

above which an infinite cluster of conductive sites spans the 

network’’. 71 It is often assessed by measuring diffusion coefficients 

over a range of surface coverages via chronoamperometry/chronocoulometry 

and Cottrell/Anson plots (i vs.t 0.5 /Q 

vs. t 0.5 ). However a novel technique utilizing chronoabsorption 

measurements and spectrophotometric Anson plots 

(DA vs. t 0.5 ) has also been utilized. 71 

Electrochemical investigation of the E o (Ru III/II ) for sensitizers 

bound to mesoporous, nanocrystalline TiO 2 (anatase) 

thin-film electrodes revealed that by integration of the area 

under the cyclic voltammogram, B10% of the concentration 

of spectroscopically quantified sensitizers had been oxidized/ 

reduced. 72 Although some dyes could directly adsorb to the 

FTO electrode, this could not wholly explain the B10% that 

were electro-active as this corresponded to over an order-ofmagnitude 

larger surface coverage than was physically possible. 

It was proposed, for the first time, that self-exchange 

charge transfer processes across the TiO 2 surface could be 

occurring. 

In the first study of lateral hole transfer across the surface of 

mesoporous, nanocrystalline metal-oxide thin films, a percolation 

threshold was found to exist. 71 This threshold was found 

to be 50% of saturation surface coverage for phosphonated 

triarylamines adsorbed onto TiO2, ZrO2, orAl2O3 as determined 

by chronoabsorption measurements and spectrophotometric 

Anson plots. The mechanism for this hole transfer was 

deduced to be via self-exchange hole transfer with eventual 

mediation by the back TCE support. This process was not 

limited by the ion motion in solution for the systems studied. 

Employing Z907 bound to metal-oxide, thin-film electrodes, 

it was found that a chemically reversible anodic wave was 

present on TiO2 and highly insulating Al2O3. 440 The percolation 

threshold was found to be B50% of saturation surface 

coverage and faster lateral hole diffusion coefficients were 

observed in acetonitrile-based electrolytes versus purely ionic 

liquids. This was proposed to be due to the two orders-ofmagnitude 

higher viscosity of the ionic liquid that resulted in 

slower effective ambipolar-diffusion rates. It was also clearly 

deduced that the increased hole diffusion coefficients for 

Z907 + and [cis-Ru(dmb)(dcb)(NCS) 2] + under saturation 


surface coverages were due to efficient hole transfer between 

isothiocyanate ligands. This was evident by comparison to the 

slower diffusional, hole-hopping rates for (NC ) 2-containing 

dyes, Ru(bpy) 3 2+ -type dyes, dyes with trans isothiocyanate 

ligands, cis-Ru(dmb)(dcb)(NCS) 2 with mercury-poisoned isothiocyanate 

ligands, and N3—whose intermolecular-isothiocyanate 

ligands are further separated on the surface. The 

fastest diffusion coefficient for hole transfer was achieved 

with [cis-Ru(dmb)(dcb)(NCS)2] + and was determined to be 

1.1 10 8 cm 2 s 1 . 

While studying the percolation threshold for three TiO2bound 

sensitizers containing donor moieties shown in Fig. 24, 

it was discovered that one of them exhibited a long-lived 

photochroism. 330 Photochroism occurs when oxidized-donor 

spectral features are still present even when oxidation of the 

sensitizer is followed by application of an appropriate bias that 

could thermodynamically reduce the oxidized donors but not 

the TiO2 DOS. This was attributed to the extended conjugation 

of the sensitizer that inhibited its free rotation and the 

formation of a surface-conducting monolayer. Thus, facile 

percolation of charge from the back FTO contact to every 

surface-bound donor could not be realized. 

The percolation threshold for Os(bpy)2(dcb)/TiO2 thin-film 

electrodes was found to be 60% of saturation surface coverage. 

73 However, at coverages less than the percolation threshold, 

but with the addition of Ru(bpy) 3 2+ in solution, 

mediation of oxidative hole transfer occurred upon stepping 

the potential positive of E o (Ru III/II ). Similarly, addition of 

Os(bpy)3 2+ in solution increased the rate of both reductive 

electron transfer and oxidative hole transfer upon biasing the 

film in the appropriate direction due to solution-based, dyemediated 

charge transfer. 

By employing a sensitizer with a lower E o (Ru II/+ ) than Ecb 

and relying on ‘hot’-electron injection, laser-flash photolysis of 

Ru II (bpy)2(dcbq)/TiO2 thin films in acetonitrile resulted in 

immediate, i.e. o10 ns, formation of Ru III (bpy) 2(dcbq)/TiO 2 

and Ru II (bpy) 2(dcbq )/TiO 2 charge-separated states. 135 Fits 

to stretched exponentials via the KWW function resulted in 

average rate constants, i.e. t o 1 , for [Ru II (bpy)2(dcbq )] + + 

[Ru III (bpy)2(dcbq)] 3+ recombination of 8 5 10 5 s 1 . But a 

question remained: was recombination predominantly due to 

electron or hole transfer reactions? By employing chronoabsorption 

measurements and spectrophotometric Anson plots, 

the diffusion coefficient for the Ru II/+ self-exchange reaction 

on TiO2 was found to be over an order-of-magnitude larger 

than that for the Ru III/II reaction. Solution self-exchange rate 

constants for Ru(bpy) 3 2+ were within a factor of two the same 

for Ru III/II and Ru II/+ in acetonitrile. 441–444 It was proposed 

that the TiO 2 DOS mediated electron, but not hole, transfer. 

Using Ru II (bpy) 2(dcbq)/TiO 2, Fe III (PPIX)Cl/TiO 2 and 

Fe III (PPIX)(py)2/TiO2 thin-film electrodes, in acetonitrile, 

methanol, and methanol, respectively, no percolation threshold 

for electron transfer was observed in TBA + electrolytes 

(PPIX is protoporphyrin IX and py is pyridine). 445 Interestingly, 

after being reduced the diffusion coefficients for their 

oxidation were over two orders-of-magnitude slower for the 

Fe II -based, PPIX coordination compounds. The diffusion 

coefficient for electron-transfer reduction for each molecular 

acceptor was at an intermediate value between these two 

re-oxidation extremes but were within experimental error of 

one another. This was rationalized based on a Gerischer-type 

model where the fluctuating energy levels for Fe II had a much 

poorer overlap with the TiO 2 DOS as compared to those of 

dcbq . 

ii Final TiO2(e )–sensitizer + charge recombination 

a E o (M +/0 ) and TiO2 DOS and quasi-Fermi-level dependence. 

Semiclassical, non-adiabatic Marcus Theory predicts a 

parabolic-dependence on the logarithm of the electron-transfer 

rate constant with the standard-state driving force for the 

reaction. 240,446,447 The maximum rate constant occurs at the 

vertex of this parabola and represents activationless electron 

transfer and thus should be temperature independent. 

Electron-transfer processes occurring at larger driving forces 

are actually slower and are located in an energetic/kinetic 

region termed the inverted region. Moser and Gra¨tzel reported 

practically temperature-independent rate constants for charge 

recombination from colloidal TiO2 to surface-bound, oxidized, 

organic sensitizers over a 4200 degree temperature 

window. 438 Based on numerical simulations employing a 

quantum-mechanical model for non-adiabatic electron transfer, 

including an average high-frequency vibrational mode 

from the sensitizer, 305–310 it was shown that under conditions 

of moderate solvent reorganization energy, practically activationless 

electron-transfer behavior could be observed well into 

the Marcus inverted region. Thus, this highly exergonic recombination 

reaction was concluded to fall deeply within the 

Marcus kinetic inverted region even though the roughly 

temperature-independent rate constants eluded to activationless 

behavior. 

Driving-force-dependent electron transfer can be quantified 

at dye-sensitized TiO2 interfaces where excited-state electron 

injection into TiO2 leaves an electron at a particular standardstate 

potential and a ‘‘hole’’ on the sensitizer. The E cb, and 

subsequently the free energy of the TiO 2(e )s, can be varied by 

altering the pH or the concentration of cations, as was employed 

in excited-state injection studies in section 3/B/ii, while 

the free energy of the ‘‘hole,’’ i.e. E o (Ru III/II ), can be controlled 

through synthetic manipulation of the coordinated ligands. 

Lever has an empirical model that allows these potentials to 

be accurately determined before the sensitizer is synthesized. 448 

However, in some cases the environment and the proximity of 

the sensitizer to the TiO2 surface can in itself result in different 

measured E o (Ru III/II )s. Zaban et al. have previously shown that 

the E o (Ru III/II )oftheRu II (LL)(mpt)CN sensitizer, where LL is 

1,2-bis(4 0 -methyl-bpy-4-yl)ethane and mpt is 4 0 -phosphonic 

acid-tpy, became pH-dependent when bound to mesoporous, 

nanocrystalline TiO 2 (anatase) thin films and shifted in a nearly 

Nernstian fashion in concert with Ecb. 313 Similar behavior was 

also shown for eight other sensitizers who possessed pHindependent 

E o (M +/0 )s in fluid solution but whose E o (M +/0 ) 

shifted 21 to 53 mV/pH unit when bound to TiO2. 312 These 

sensitizers were either organic or inorganic with metal center 

being Ru II ,Fe III or Mg II and ligands being tetracarboxyphthalocyaninato-, 

dcb- or dpb-based, where dpb is 4,4 0 -diphosphonic 

acid-bpy. It was proposed that the position of the adsorbed 

sensitizer within the ionic double layer could explain the 

differences in shifts of E o (M +/0 ) per pH unit. 


Fig. 38 Plot of the logarithm of the inverse of the half-times/lifetimes versus the driving force for two different studies of TiO 2(e )+S + 

recombination. The trend in (A) implies that the electron transfer was in the Marcus inverted kinetic region while the trend in (B) seems to follow 

activationless electron transfer. Taken from Fig. 5 of ref. 390 and Fig. 4 of ref. 450, respectively. 

Employing a family of MLCT sensitizers based on osmium, 

ruthenium and rhenium whose ground state reduction potential 

varied by about B960 mV, it was shown that recombination 

kinetics from a TiO2(e ) to an oxidized sensitizer were independent 

of sensitizer employed. 389 The transient data was 

successfully fit to an equal-concentration, bi-second-order 

kinetic model on a 100 ns and longer time scale. The data 

could also be successfully fit to four first-order rate constants 

but the rationale for this fit was less apparent. Of note was that 

the raw transient data was insensitive to the sensitizer reduction 

potential, the molecular geometry, the nature of the metal 

center employed—i.e. Re, Ru, or Os—and the number of 

carboxylic acid groups present—i.e. two or four. The insensitivity 

of the second-order rate constants to these parameters 

was attributed to the reaction being rate limited by TiO2(e )oxidized 

sensitizer (h + ) encounters or lack of change in the 

apparent driving force for the recombination reaction due to 

concerted E cb and ground-state E o shifts, as was shown previously. 

312,313 Charge transport within the sensitized film may 

also have been a second-order process. 

In a separate study by Lewis and co-workers, charge 

recombination to five Ru III or Os III compounds were fit to 

the same equal-concentration, bi-second-order recombination 

model. 390 By using the weighted average of the second-order 

rate constants in conjunction with semiclassical Marcus theory, 

charge recombination to oxidized sensitizers, like N3 + /TiO2, 

was found to fall in the Marcus inverted kinetic region with a 

total reorganization energy, l = B1.0 eV, Fig. 38(a). The 

temperature-dependent electron-transfer kinetics were similar 

to those observed by Dang and Hupp with Ru-phen based 

coordination compounds electrostatically bound to colloidal 

SnO 2 nanoparticles. 449 The kinetics suggested that while 

nuclear tunneling was negligible, solvent reorganization and 

low-frequency, metal–ligand vibrational modes assisted the 

recombination reaction, as opposed to high-frequency, 

igand-based vibrational modes. Lewis and co-workers also 

reported that the activation energy for charge recombination 

was slightly larger for Os II - versus Ru II -based sensitizers. 

More recently, eight different sensitizers whose E o (Ru III/II ) 

spanned B500 mV were employed to examine the drivingforce 

dependence on charge recombination. 450 The transient 

spectroscopic data was fit to a multiple-trapping, nearestneighbor 

CTRW kinetic model by using the KWW function 

and the driving-force dependence was quantified based on t1/2, 

the time it took for half of the injected electrons to recombine. 

Using the inverse of these half-lives the data was shown to be 

relatively insensitive to variations in E o (Ru III/II ). This was 

interpreted as being indicative of reactions lying near the peak 

of the Marcus free energy curve, DG o = Bl, and with l = 

B0.8 eV, Fig. 38(b). Therefore, these authors concluded that 

charge recombination to N3 + , and other similar sensitizers, 

was nearly activationless. It is interesting to note that while the 

E o (Ru III/II ) values were generally in good agreement with 

Lewis and colleagues, 390 the magnitude of the driving force 

differed significantly due to discrepancies in the reducing 

power of the TiO2(e )s, Fig. 38. 

Employing [Ru II (depb)3] 2+ or [Ru II (dpb)3] 10 , where depb 

is 4,4 0 -diethylphosphonate-bpy bound to mesoporous, nanocrystalline 

TiO 2 (anatase) thin films, the pH dependence of 

recombination in aqueous solution was studied by Hupp and 

co-workers. 451 The fast exponential component to the biphasic 

recovery was shown to be invariant of pH (or H 0) over a 

19 pH-unit range even though the Ecb of TiO2 is known to shift 

in a nearly Nernstian fashion with pH, Fig. 39(a). One might 

expect the E o (Ru III/II ) to shift in a concerted fashion as 

observed by Zaban et al., 312,313 however this was not the case 

as the E o (Ru III/II ) of the surface-bound sensitizer was shown to 

have only a minor pH-dependence (5 mV/pH unit) over a 

47.5 pH unit range, Fig. 39(b). 452 This less than Nernstianorder-of-magnitude 

shift in E o (Ru III/II ) should have been 

largely overcome by the potential shift in E cb. As a continuation 

of this study, Ru(dpb) 2(LL)/TiO 2 thin films were shown 

to exhibit minor, but apparent, Marcus normal region behavior, 

where LL were bpy and phen derivatives. 453 This unexpected 

result, given the large variations in driving force, was 

explained as sequential electron- and proton-transfer reactions. 

It was proposed that the rate-limiting step was backelectron 

transfer, however this step did not release all of the 

free energy in the overall reaction, and thus the variation in 

driving force for this step was solely dependent on changes in 

E o (Ru III/II ), Fig. 39(c). As the E o (Ru III/II ) differed only slightly 

with pH, and was actually found to vary with the z-potential 


Fig. 39 (A) Plot of the logarithm of recombination rate constant versus pH for TiO 2 thin films. As the energy of the conduction band edge, E cb, 

shifts in a nearly Nernstian fashion with pH (dashed line), this plot illustrates driving-force independent recombination rates (solid line with 

points). Taken from Fig. 4 of ref. 451. (B) Plot of the standard-state reduction potential of the surface-bound compounds (E o (Ru III/II )) versus the 

pH, confirming that the E 0 (Ru III/II ) did not change with the pH of the solution and thus further supporting the driving-force independent 

mechanism. Taken from Fig. 5 of ref. 452. (C) Pourbaix-type diagram of the TiO 2(e )+S + recombination reaction. The driving-force 

independent recombination rates were rationalized as being due to rate-limiting electron transfer followed by exergonic proton transfer. Taken 

from Fig. 5 of ref. 453. 

of the TiO2, only a small deviation in driving force was 

actually realized. 

The described charge-recombination studies with altered 

thermodynamic driving force provide a somewhat conflicting 

series of conclusions: rate constants independent of the 

ground-state reduction potential of the sensitizer, in the 

inverted region, and near the top of the Marcus curve. 

Undoubtedly, these differences arise from details of the preparation 

of the TiO2 thin films, surface chemistry such as TiCl4 

pretreatments, 454 and species present in the electrolyte. Additional 

research is required to identify the critical variables, 

especially in non-aqueous electrolytes. 

While the driving-force dependence for charge recombination 

to the oxidized sensitizer in mesoporous, nanocrystalline 

thin films is not completely established, the behavior in 

aqueous colloidal solutions appears to be more clear. Hupp 

and co-workers monitored charge recombination to six different 

Fe(CN) 5(L) n 

sensitizers bound to colloidal TiO2 in 

aqueous pH 2 solution on a nanosecond time scale. 304 The 

recombination kinetics were found to be biphasic with a 

fast first-order component followed by longer non-exponential 

kinetics. The first-order component varied with E o (Fe III/II ) 

as expected if the process were occurring in the Marcus 

inverted region. Likewise, recombination reactions from 

electrostatically bound Ru II - and Os II -polypyridyl coordination 

compounds to colloidal SnO2 nanoparticles exhibited 

a pH-sensitive kinetic behavior. 455 This is in stark contrast 

to the pH-independent behavior observed for recombination 

from similar compounds covalently anchored to mesoporous, 

nanocrystalline TiO2 (anatase) thin films. It was 

suggested that this difference was due to the significant trap 

density in TiO2 and that ionic attachment introduced fewer 

surface traps. 

As with studies on charge separation of section 3/B/ii, it is 

also useful to study charge recombination under steady-state, 

working conditions at short circuit. The first account of such a 

study was performed by Gra¨tzel and co-workers in 1990. 158 By 

exciting at least a quarter of the sensitizers on Ru(dcb) 3/TiO2 thin-film electrodes in LiClO4 aqueous electrolytes it was 

shown that the half-times for the non-exponential recombination 

kinetics decreased by almost three orders of magnitude 

under forward-bias versus reverse-bias conditions. This same 

increase in rate was found for N3/TiO 2 thin-film electrodes in 

ethylene carbonate–propylene carbonate (1 : 1) solution as the 

irradiance was increased to generate B1 toB50 TiO 2(e )s/ 

particle. 456 The kinetics were fit to the multiple-trapping, 

nearest-neighbor CTRW model and KWW function. The 

half-times were invariant on irradiance when o1 TiO2(e )/ 

particle was generated. 

Durrant and co-workers showed the half-time for 

N3 + +TiO2(e ) recombination exhibited an exponential dependence 

on applied voltage in 0.1 M TBA + trifluoromethanesulfonate 

ethanol electrolyte, Fig. 40(a). 457 When immersed 

in ethanol electrolytes and on applying an electrochemical bias 

from +100 to 600 V vs. Ag/AgCl, the half-time for the 

recombination reaction decreased by more than seven orders 

of magnitude while the injection yield remained unchanged. 456 

By changing the electrolyte from ethanol/TBAT (Electrolyte 

A), where TBAT is tetrabutylammonium triflate, to CH3CN/ 

TBAClO4/LiClO4 (Electrolyte B) and then adding 4-tertbutylpyridine 

(Electrolyte C), similar bias-dependent, 

TiO2(e ) recombination rates were observable but, again, only 

under conditions of 41 TiO2(e )/particle, Fig. 40(b). However, 

when comparing Electrolyte A to B at different biases, 

their half-times for recombination differed to varying degrees, 

but with the half-time in Electrolyte B always being larger. As 

the driving force for recombination would decrease in the 

presence of Li + due to the positive shift in Ecb, this data does 

not fit that of Marcus inverted behavior. Additionally, the 

multiple-trapping, nearest-neighbor CTRW model for recombination 

was further supported over a bi-second-order model. 

254,389,390 The greater than seven orders-of-magnitude 

decrease in half-time would result in approximately the same 

increase in initial rate. Given the proposed second-order 

process, the differential rate law v = k2[TiO2(e )][S + ] would 

require the generation of 10 7 TiO 2(e )s/particle which was 


Fig. 40 (A) Bias-dependent, time-resolved, single-wavelength absorption difference spectra for N3/TiO2 thin film electrodes indicating that as the 

density of acceptor states in the TiO2 electrode were filled, the rate of TiO2(e )+N3 + recombination increased. Applied biases are indicated on the 

figure. Taken from Fig. 2 of ref. 457. (B) Plot of the logarithm of the recombination half-life versus the applied bias in the indicated electrolytes. See 

text for electrolyte compositions. Taken from Fig. 6 of ref. 456. 

highly improbable based on the magnitude of spectroscopic 

signals and approximate extinction coefficients. 

Although the rate of charge recombination of TiO2(e )s and 

N3 + was dependent on the concentration of electrochemically 

generated TiO2(e )s, this was not the case for regeneration by 

the solution redox mediator. 382 At moderate LiI concentration, 

i.e. 30 mM, and at an applied bias approximately equal to the 

Voc under 1 sun, AM1.5-simulated conditions, the kinetic 

partitioning between TiO 2(e )s + N3 + recombination and 

regeneration of N3 + by the reduced solution redox mediator 

were similar. But under the typical DSSC conditions of 0.5 M 

LiI and 0.05 M I2 and 1 sun, AM1.5-simulated irradiance, the 

rate of sensitizer regeneration would be greatly favored and 

TiO2(e )s + N3 + recombination would not limit performance. 

b Distance dependence. Distance-dependent electron tunneling 

behavior can be studied for recombination from a 

TiO2(e ) to an oxidized surface-bound acceptor. Rate 

constants due to tunneling should exhibit an exponential 

dependence on distance, eqn (8), as was the case for 

electron-injection tunneling behavior in section 3/B/iii. 

Three novel sensitizers bound to TiO2 via an mpt ligand and 

containing a nearby triarylamine donor moiety were studied in 

order to determine the distance dependence for recombination, 

Fig. 24. 330 The donor nitrogen atoms were calculated to 

be 12 A˚ ,18A˚ , and 24 A˚ from the surface. Assuming the 

typical dampening factor, b = 1.2 A ˚ 1 , for ‘through space’ 

electronic coupling between the TiO2(e )s and NAr3 + , the 

expected 6 A ˚ -distance dependence was not observed and was 

off by a factor of three. It was thought that this was due to the 

fact that the dyes may not bind exactly in a perpendicular 

orientation to the surface. 

Employing three Ru II sensitizers containing a bpy ligand 

derivatized with zero, one or two oligo(xylylene) linkers, 

the distance dependence of TiO 2(e ) recombination was 

studied. 267 By comparing the weighted average of the 

bi-second-order rate constants 390 differences in backelectron-transfer 

rate constants were found to be within 

experimental error of one another. As was the case with 

electron injection, the lack of expected large differences in rate 

constant were proposed to result from variable Ru–TiO2 

distances due to the flexibility of the linker groups. 

The distance dependence of back-electron transfer was 

examined in acetonitrile for three rigid-rod Ru(bpy)3 2+ -based 

sensitizers containing zero, one, two, or three rigid phenyleneethynylene 

linkers covalently attached to TiO 2 via two methyl 

ester anchoring groups. 268,458 It was shown that recombination 

kinetics from a TiO2(e )toRu III were successfully fit by 

an equal-concentration, bi-second-order kinetic model 

254,389,390 and that the average of the rate constants was 

essentially independent of sensitizer employed. Thus, the expected 

distance dependence for the rate of back-electron 

transfer was not observed. This was partially supported by 

the fact that sensitizers lacking binding groups were still found 

to bind to TiO2 indicating that expected Ru–TiO2 distances 

may be incorrect due to alternative binding orientations. 

Durrant and colleagues studied Ru(4,4 0 -(R) 2-bpy)(dcb) 2/TiO 2 

thin films, where R contained oligo(NPh 3) groups at 

varying distances from the Ru II -metal center, to determine 

the distance dependence for back-electron transfer. 332 After 

combining this data with other data from their laboratories, 

253,331,450,459 it was clearly evident that the TiO2(e ) 

back-electron transfer rate constants displayed an exponential 

dependence on spatial separation with dampening factor, 

b = 0.95 0.2 A˚ 1 . Additionally, cis-Ru(4,4 0 -vinyl(NPh3)nbpy)(dcb)(NCS)2 

(n = 1, 2) were synthesized and DFT 

calculations suggested that their HOMOs resided predominantly 

on the NPh 3 moieties at a distance of 10.5 and 11.6 A˚ , 

respectively. 460 When compared to Z907/TiO 2, these dyes 

exhibited larger t 1/2, i.e. 0.22, 1.8, 3 ms for Z907, n =1, 

and n = 2, respectively, that followed the expected 

exponential trend. 


Fig. 41 (A) Time-resolved, single-wavelength absorption difference spectra for Ru(4 0 -PO3 2 -tpy)(NCS)3/TiO2 thin films illustrating that the rate 

of recombination was inversely related to the size of the Al 2O 3 overlayer illustrated in Fig. 16(a). Al 2O 3 overlayer thickness in nanometers are 

shown. (B) Plot of the natural logarithm of the recombination half-life versus the Al2O3 coating thickness illustrating tunneling behavior. Taken 

from Fig. 8 and 9, respectively, of ref. 271. 

Using the mono-phosphonated version of the ‘black dye,’ 

[Ru(mpt)(NCS)3] 3 , bound to the same TiO2/Al2O3 core–shell 

particles mentioned above for electron injection, Fig. 16(a), 

the distance dependence of TiO2(e ) recombination was 

studied. 271 Tunneling was required for electron recombination 

and the kinetics were non-exponential with half-times ranging 

from 6 ms to 60 ms for 0 to 6 nm thick Al 2O 3 overlayers, 

Fig. 41(a). Using signal half-times it was shown that charge 

recombination of TiO 2(e )s/Al 2O 3 and [Ru(mpt)(NCS) 3] + 

resulted in a dampening factor, b = 0.15 A˚ 1 , Fig. 41(b). 

This result, combined with the predominant dampening factor 

for injection, b = 0.11 A ˚ 1 , illustrates that recombination is 

attenuated to a greater degree than injection when TiO2/Al2O3 

core-shell designs are employed. The approximately six-timessmaller 

dampening factor when compared to the results from 

the molecular approach used by Durrant and co-workers 

suggests that intra-bandgap states within the Al2O3 coating 

could be present and mediating this back-electron transfer. 

As mentioned above, Haque, Durrant, and colleagues significantly 

increased the charge-separated lifetime to over 4 s by 

employing a Ru(4,4 0 -(R) 2-bpy)(dcb) 2/TiO 2 system, where R 

contained a poly(vinyl-NPh3) group of about 100 units, 

Fig. 25(b). 332 The half-times for the charge-separated state 

increased from 350 ms to 5 ms to 4 s as the number of vinyl- 

NPh3 subunits increased from 1 to 2 to 100, respectively. Also, 

when fit to the multiple-trapping, nearest-neighbor CTRW 

model and the KWW function, the dampening factor ranged 

from 0.4 to 0.9 to 1, respectively; the latter indicating a monoexponential, 

first-order recombination mechanism, Fig. 42. 

Clifford et al. analyzed two free-base porphyrin sensitizers, 

meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin and 

meso-5-(4-carboxyphenyl )-10,15,20-tris(4-diphenylaminophenyl)porphyrin, 

by transient absorption spectroscopy and 

found that TiO2(e ) recombination with the oxidized porphyrin 

ring was sufficiently slowed for the latter sensitizer. 461 The 

former sensitizer’s kinetics were satisfactorily fit to the multiple-trapping, 

nearest-neighbor CTRW model and the KWW 

function, b = 0.31, whereas the kinetics for the latter sensitizer 

were perfectly first order in nature, i.e. b = 1, Fig. 43(a). This 

was believed to be due to a different rate-limiting step for 


for Ru(4,4 0 -(R)2-bpy)(dcb)2/TiO2 thin films, where R contained 

one triphenylamine (NPh 3) group (1), two NPh 3 groups (2), or a 

poly(vinyl-NPh3) group of about 100 units (3), as depicted in 

Fig. 25(b). Taken from Fig. 2 of ref. 332. 

recombination between each oxidized sensitizer and the 

TiO 2(e )s. While detrapping was rate limiting for the former 

dye, the final TiO 2(e )+S + electron-transfer step limited the 

latter. The explanation for a first-order rate constant, and not 

an equal-concentration, second-order rate constant, was that 

the intrinsic concentration of TiO2(e )s was much larger than 

the additional concentration resulting from sensitizer, excitedstate 

injection. 182 Thus, as the concentration of oxidized 

sensitizers decreased due to TiO2(e )+S + recombination, 

the TiO2(e ) concentration changed little resulting in the 

observed pseudo-first-order kinetic behavior. 

The back-electron transfer kinetics for two Ru II -based 

sensitizers were compared: N719 and a novel sensitizer with 

a covalently bound triarylamine donor where the hole was 

efficiently transferred away from TiO2 within the instrument 

response time. 331 As was seen by Clifford et al. the kinetics of 

the faster back-electron transfer from N719 were dispersive 

while the slower kinetics for the novel sensitizer were first 

order in nature. 


Fig. 43 Time-resolved, single-wavelength absorption difference spectra for two free-base porphyrin sensitizers, meso-5,10,15,20-tetrakis- 

(4-carboxyphenyl)porphyrin (i, dye 1) and meso-5-(4-carboxyphenyl)-10,15,20-tris(4-diphenylaminophenyl)porphyrin (ii, dye 2), bound to TiO 2 

thin films illustrating that the recombination kinetics were dispersive, and could be fit to the multiple-trapping, nearest-neighbor CTRW model of 

Nelson et al., and a first-order kinetic mechanism, respectively. (A) The data is displayed as the logarithm of the DAbsorbance versus the time to 

illustrate the first-order nature of the latter sensitizer’s kinetics, i.e. ii, dye 2. Taken from Fig. 1(b) of ref. 461. (B) The same data displayed as the 

DAbsorbance versus the logarithm of the time to illustrate the dispersive nature of the former sensitizer’s kinetics, i.e. i, dye 1. Taken from Fig. 1(a) 

of ref. 461. (C) The same data and plot as in B but fit equally as well to the new Coulomb-trap, random-flight multiple-trapping model of Tachiya 

and colleagues. Taken from Fig. 4 of ref. 462. 

Tachiya and colleagues proposed the Coulomb trap model 

for back-electron transfer to oxidized sensitizers which employs 

the random-flight multiple-trapping model for TiO2(e ) 

transport. 462 The model is based on the idea that a TiO2(e ) 

only feels the Coulombic attraction to a dye cation when it is 

trapped near the cation. This stabilization interaction effectively 

increases the activation energy for the detrapping not 

only of said TiO2(e ) but of neighboring TiO2(e )s as well. By 

employing an exponential TiO2 DOS that are pre-filled to a 

reasonable dark concentration of TiO2(e )s, i.e. 0.1 per particle, 

166 and assuming that the effective dielectric constant at 

the site of this Coulombic attraction is low, i.e. o5, the 

experimental data of Clifford et al. can be modeled extremely 

well. Fig. 43(b)/(c) illustrates the comparison of the two 

models: the multiple-trapping, nearest-neighbor CTRW model 

by Nelson et al. in the middle, b, and the new Coulomb-trap, 

random-flight multiple-trapping model by Tachiya and colleagues 

on the right, c. 

C Recombination to acceptors in solution 

i Proposed mechanisms for recombination in I3 /I - 

containing electrolytes. Given the rather high iodide concentration 

and fast regeneration kinetics observed in champion 

DSSCs, the recombination reaction of TiO 2(e )s with surfacebound 

oxidized sensitizers, TiO 2(e )+S + , is considerably 

slower than sensitizer regeneration, S + +D- S+D + . 382 

This implies that the primary TiO 2(e ) acceptors in such cells 

are oxidized forms of the redox mediator in solution, D + . 

Molecular identification of the preferred solution acceptor 

species is of great importance. Possible candidates include I , 

I2 ,I3 or I2, but unambiguous identification has not been 

obtained. Durrant, Nelson, and co-workers have reported 

evidence that the reaction of unsensitized TiO2(e )s with I2 

is kinetically faster than the reaction with I3 . 463 

Measurements of V oc provide an indirect, but powerful, tool 

for characterizing charge recombination processes. Although 

the V oc is the maximum Gibbs free energy that one can obtain 

from a regenerative solar cell, it can often be kinetically 

derived under steady-state, illumination conditions. The modified 

diode equation is often found to be relevant for DSSCs: 

Voc ¼ mkBT 

e 

¼ mkBT 

e 

ln Ratecharge 

in 

Rate charge 

out 

0 

! 

Ioaf inj 

B 

ln@P 

ni Q 

ki½TiO2ðe ÞŠ 

i 

j 

½AijŠ nij 

1 

C 

A ð16Þ 

where m is the DOS non-ideality factor, kB is Boltzmann’s 

constant, T is the temperature, e is the elementary charge, Io is 

the incoming light flux, a is the absorptance, f inj is the 

injection yield, k i are the rate constants for recombination of 

TiO 2(e )s with acceptor species, A ij, of order v ij. 464–468 At 

room temperature, this equation predicts a 59 mV increase 

in Voc per 10-fold increase in the ratio of the rate of electron 

injection into TiO2 and the rate of recombination. Each 

decade of change in Io may also result in a 59 mV change in 

Voc. Decadic deviations are attributed to m a 1. Changes in 

the Voc measured under steady-state time and frequency 

(IMVS) domains are often observed by tuning the p* levels 

of the sensitizer, 469,470 chemisorption (for example, by 4-tertbutylpyridine) 

or surface functionalization, I 2-sensitizer coordination, 

altering the concentration or nature of cations in the 

electrolyte, or laterally transferring the hole further from the 

TiO 2 surface. These have been linked, directly, to changes in 

the recombination rate constant. 

An early application of this to DSSCs was observed with the 

intramolecular charge separation as previously described, 328 

where the Voc was measured for Ru(dmb)(dcb)2/TiO2 

and Ru(4-CH3, 4 0 -CH2-PTZ-bpy)(dcb)2/TiO2 thin film electrodes 

as a function of irradiance in 0.1 M LiClO4 or NaI/I2 

(0.5/0.05 M) propylene carbonate electrolytes. The potentials 

measured versus aAg + /Ag pseudo-reference electrode indicated 

that the V oc was 175 10 mV larger for Ru(4-CH 3, 

4 0 -CH 2-PTZ-bpy)(dcb) 2/TiO 2 versus Ru(dmb)(dcb) 2/TiO 2. 


Fig. 44 Plot of Voc versus the logarithm of the irradiance for three 

TiO 2-bound, Ru(bpy) 3 2+ -based sensitizers containing one (circles) or 

two (triangles) rigid phenyleneethynylene linkers covalently attached 

to a 1,3,5,7-tetraphenyladamantane core or simply a deeb ligand 

(squares) in 0.1/0.005 M TBAI/I2 dichloromethane electrolyte. The 

farther the Ru II -metal center, and thus proposed ion-paired I and/or 

I3 , from the TiO2 surface, the longer the lifetime of the chargeseparated 

state and thus increased V oc. Taken from Fig. 3 of ref. 471. 

The photocurrents were approximately the same for both 

sensitizers over four orders-of-magnitude irradiance indicating 

that the numerator of eqn (16) was sensitizer independent 

while the denominator was not. The charge recombination rate 

constants were measured spectroscopically, k =3.9 10 6 s 1 

and k = 3.6 10 3 s 1 , respectively. With these rate constants 

a DVoc of B200 mV was calculated which is close to the value 

175 10 mV that was measured experimentally. Remarkably, 

these molecular interfaces behaved as ideal diodes, m =1, 

over this four orders-of-magnitude change in irradiance. Interestingly 

in the presence of the redox mediator, i.e. I 3 /I , 

the non-ideality factor was 2. The V oc remained larger for 

Ru(4-CH 3, 4 0 -CH 2-PTZ-bpy)(dcb) 2/TiO 2 suggesting that 

charge recombination to oxidized iodide products was also 

inhibited for this compound. More recently, we have obtained 

experimental evidence that there may indeed be an advantage 

in oxidizing iodide farther from the TiO2 surface. In dichloromethane 

electrolytes containing 0.1/0.005 M TBAI/I2, the 

Voc was found to be directly related to the distance the hole 

was from the surface. 471 Using [Ru(bpy)2(deeb)] 2+ or tripodal, 

Ru(bpy) 3 2+ -based sensitizers containing one or two rigid 

phenyleneethynylene linkers covalently attached to a 1,3,5,7tetraphenyladamantane 

core and attached to the TiO 2 surface 

in a tripodal-shaped binding configuration, the distance between 

the Ru III -metal center and the TiO2 surface was 

varied. 268,269,471 As ion-pairing was previously shown to occur 

in dichloromethane with [Ru(bpy)2(deeb)] 2+ and I or 

I3 , 133,357,472 it was proposed that after ion-paired I photooxidation, 

TiO2(e ) recombination may occur to ion-paired 

I3 at a fixed distance from the surface. Assuming the injection 

yield was invariant on the length of the spacer, the Voc data 

highly supported this distance-dependent recombination mechanism, 

Fig. 44. 

In early studies with a series of cis-Ru(dcb)2X2/TiO2 

thin film electrodes (M = Ru, Os), the intensity-dependent 

photocurrent and V oc values could generally be rationalized 

based on the redox properties of the compound. The V oc 

was never found to be sensitizer dependent. However, 

recently there is some evidence that the recombination rate 

constants can be tuned through the p* levels of the sensitizer 

and halide coordination sites on macrocyclic compounds. 

285,470 Indeed any time large photocurrents and small 

Voc values are measured, it is of interest and suggests that there 

remains some charge-recombination pathway to the sensitized 

interface. Arakawa and co-workers, and more recently 

Bignozzi and colleagues, have identified such conditions with 

Ru II and Os II compounds that have diimine ligands with 

low-lying p* levels. 469,470 Recall, too, that if E cb is raised, 

the photogenerated TiO 2(e )s can be fully trapped on 

dcbq ligands, 299 behavior that is consistent with that 

reported here. 

However, in order to calculate the Voc, the absorbed photon 

flux, Ioa, the DOS non-ideality factor, m, the injection yield, 

finj, and the TiO2(e ) recombination rate based on the overall 

recombination mechanism—the denominator within the 

Napierian logarithm—need be determined. 464–468 For the 

reasons previously discussed, there are no accepted and 

straightforward means of measuring the denominator as even 

the chemical identity of the acceptor is unknown, much less 

the mechanism by which it reacts. Doing this is no simple task 

given the numerous possible reaction intermediates, as depicted 

in Scheme 1 in section 4/B/i/a. The elementary reaction 

steps that have been proposed thus far are: 

I +I2 " I3 (9, restated) 

I 2 + TiO 2(e ) - I 2 

(17)ww 

2I2 - I3 +I (18a)zz 

I2 + TiO2(e ) - 2I (18b)yy 

I 2 " 2I ads 

(19) 

Iads + TiO2(e ) - I (20) 

TiO 2(e ) trapped " TiO 2(e ) free 

TiO2(e )free - TiO2(e )reactive 

(21) 

(22) 

X + TiO2(e )reactive - X (23) 

Many possible differential rate laws can be obtained by 

assuming that various steps are in pre-equilibrium, others are 

kinetically rapid, and one is rate determining. The integrated 

ww An equivalent reaction would be I2 + TiO2(e ) - Iads +I . 

zz Based on the products of (17)ww, an equivalent reaction would be 

2(I ads +I ) - I 2 +2I - I 3 +I . 

yy Based on the products of (17)ww, an equivalent reaction would be 

(I ads +I ) + TiO 2(e ) - 2I . 


ate laws based on a range of hypothesized mechanisms are as 

follows (see Table 1): 

Rate / ½I3 Š2½TiO2ðe ÞŠ 2 

½I Š 2 

ð24aÞ 

Rate / ½I 3 Š½TiO2ðe ÞŠ 2 

½I Š 

Rate / ½I 3 Š½TiO2ðe ÞŠ 

½I Š 

Rate / ½I 3 Š0:5 ½TiO2ðe ÞŠ 

½I Š 0:5 

ð24bÞ 

ð24cÞ 

ð24dÞ 

Rate /½TiO2ðe ÞŠ ð24eÞ 

As is apparent, unambiguous determination of the integrated 

rate law and overall reaction mechanism requires 

knowledge of the reaction order for not just one, but two 

species. Thus, certain experiments alone cannot irrefutably 

establish the overall mechanism. Unfortunately, the current 

body of literature paints a somewhat conflicting story and thus 

results from the literature will be explained according to which 

of the five integrated rate laws and reaction mechanisms above 

could apply, i.e. eqn (24a–e). 

a Electrochemical techniques. Peter and colleagues studied 

N3/TiO2 DSSCs in acetonitrile electrolyte by IMVS/IMPS, a 

novel potentiostatic–galvanostatic–potentiostatic (PGP) 

method, and transient photovoltage/photocurrent measurements 

while under background illumination. Based on an 

inverse-square root relationship of the TiO2(e ) lifetime, tn, 

0.51 

and background light intensity, i.e. tn p Io , it was 

deduced that the recombination reaction of TiO2(e )s with 

the I3 /I redox mediator was second order in TiO2(e ) 

density, i.e. 1/0.51, and thus an I2 intermediate was proposed. 

223,473 This is indicative of mechanism (24a) or (24b). 

The same conclusion was drawn from the PGP method where 

the DSSCs were illuminated with a blue-light-emitting diode 

under open-circuit conditions and then were rapidly short 

circuited for chronocoulometric measurements. 474–476 The 

pseudo-second-order rate constant from the IMVS and PGP 

measurements was determined to be 0.6 and 1.1 10 4 M 1 s 1 

at 50 mM I3 , respectively. Unfortunately, a non-inversesquare 

root relationship between tn and the isc at various 

0.62 

green-light laser irradiances, i.e. tn p isc , was also determined 

by transient photovoltage/photocurrent measurements. 

477 Explanations of the results were that either 

recombination is not first order in TiO2(e )s or the interfacial 

electron-transfer rate constant depends on trap occupancy 

and/or the rate of TiO2(e ) diffusion. An explanation for the 

apparent discrepancy among experimental results may be the 

variations in the background/initial photon fluxes employed: 

effectively o0.2, 1 and o0.01 suns, AM1.5-simulated conditions, 

respectively. In none of these studies was the order of the 

reaction with respect to I2 , I3 , I or I2 concentration 

explored. 

Frank and co-workers studied N3/TiO2 DSSCs in acetonitrile– 

NMO (50 : 50 wt%) electrolyte, where NMO is 3-methyl- 

2-oxazolidinone, and by two procedures found a second-order 

dependence on I 3 : plots of V oc versus the concentration of I 3 

and IMVS in the presence of two different I 3 /I concentrations. 

464,478,479 The light-intensity, power-law dependence of 

the lifetime, as determined by IMVS and IMPS, was modeled 

to be the order of the reaction in TiO2(e )s and was approximated 

to be second order based on the calculated powers of 

B2.2 and B2.7, respectively. The results are consistent with 

mechanism (24a). These same authors studied N719/TiO2 

DSSCs in 3-methoxypropionitrile electrolyte and deduced that 

recombination was TiO2(e )-diffusion limited. 480 These conclusions 

were based on a model of TiO 2(e )-diffusion-limited 

recombination and fits to transient photovoltage and photocurrent 

data as a function of background white-light intensity 

ranging approximately three orders-of-magnitude in irradiance 

up to B1 sun. This data was more consistent with 

mechanism (24c), (24d) or (24e). 

By measuring the photovoltage under conditions of up to 

0.82 suns illumination and in the presence of varied concentrations 

of I3 for N3/TiO2 DSSCs in nitrile electrolyte, 

Hagfeldt and co-workers determined the recombination reaction 

to be first order in I3 . 481 They proposed the alternative 

mechanism (24b) with elementary steps (17)ww and (18b).yy 

Although the reaction order with respect to the density of 

TiO 2(e )s was not determined, mechanism (24c) was unlikely 

due to the high irradiances employed. The difference between 

these results and those of Frank and co-workers was rationalized 

by the solvent composition: 0 versus 50 wt% NMO, 

respectively, where it was proposed that NMO prevented 

reaction (17)ww from occurring, in favor of reaction (17). 

The diode-equation non-ideality factor was calculated to be 

very close to one, i.e. 1.08, but could be in error based on 

somewhat condition-dependent values often cited in the literature, 

i.e. B1.7–3.8. 171,178,183–185,223,224 Such error could be 

due to the fact that the non-ideality factor was obtained from 

V oc versus light intensity plots and was calculated based on the 

unlikely assumptions that the injection yield and TiO 2(e ) 

recombination rate were independent of the irradiance. 254 Due 

to the inverse relationship between non-ideality factor and 

reaction order in I3 for this model, a non-ideality factor of 2 

would have resulted in a reaction order in I3 of 0.5, consistent 

with mechanism (24d). 

b Spectroscopic techniques. Hagfeldt and co-workers employed 

nanosecond transient absorption spectroscopy to study 

iodide turnover and regeneration from ‘black dye’/TiO 2 thin 

films in 3-methoxypropionitrile containing 0.5 M LiI and 

50 mM I2. 482 The reduction of ‘black dye’ + /TiO2 by iodide 

resulted in the formation of I2 , which occurred in o20 ns. 

At higher irradiances, the transient spectroscopic signal for 

I2 and TiO2(e )s at 760 nm decayed with a half-time of 

B100 attributed to TiO2(e ) + I2 recombination 

(eqn (18b)). This suggested mechanism (24b) was highly likely. 

The first phase of the overall biphasic kinetics were fit to an 

equal-concentration, second-order integrated rate law with a 

non-zero baseline, but with much error in the extracted rate 

constant. The equal-concentration, second-order kinetics were 


Table 1 Reaction orders for the proposed rate laws 

Order in: 

Eqn [I3 ] TiO2(e ) [I ] Comments 

(24a) 2 2 2 Reaction (9) is in pre-equilibrium, reaction (17) is fast, and reaction (18a) is the rate-determining step. 

(24b) 1 2 1 Reaction (9) is in pre-equilibrium, reaction (17) is fast, and reaction (18b) is the rate-determining step. 

(24c) 1 1 1 Reaction (9) is in pre-equilibrium and reaction (17) is the rate-determining step. 

(24d) 0.5 1 0.5 Reaction (9) is in pre-equilibrium, reaction (19) is fast, and reaction (20) is the rate-determining step. 

(24e) 0 1 0 Reaction (21) is rate determining and reactions (22) and (23) follow. 

believed to be present due to equal concentrations of I2 and 

TiO2(e )s formed after pulsed-light excitation via reaction (11) 

or equal concentrations of I and TiO2(e )s formed via reaction 

(10) followed by rapid and quantitative I2 formation via 

reaction (25): 

I +I - I2 (25) 

Based on the absorption at 760 nm at the end of the first phase 

of the kinetics and the extinction coefficients of TiO2(e )s and 

I2 , it was calculated that B1 TiO2(e ), and thus 1 I2 , was 

present per particle regardless of the initial equal concentrations 

of the species. Under these conditions or at low irra- 

was found to be rather slow, i.e. on 

diances, the decay of I 2 

the microsecond time scale, and likely occurred via the dismutation 

reaction (18a). The fact that the difference in these 

mechanisms occurred with irradiance and TiO2(e ) concentration 

did not seem coincidental as other researchers had seen 

similar behavior at 41 TiO2(e )/particle. 483 This implied that 

at the local concentration of I2 produced by the higher laser 

irradiances, I2 is the favorable TiO2(e ) acceptor, whereas at 

lower irradiances, I2 or I3 are the favorable electron acceptors. 

It was proposed that the loss of TiO2(e )s at this low 

concentration should follow a single-exponential kinetic model 

as now the rate-limiting step would be reaction (17), and 

thus mechanism (24c) would be operative. Although, 

one could envision that under these conditions TiO 2(e ) 

detrapping could limit transport whereby mechanism (24e) 

would then explain the kinetics. 

Nelson et al. predicted a sublinear power-law variation of 

TiO2(e ) density with light intensity, i.e. n = CIo b , and 

deduced that the recombination reaction would be first order 

in the density of TiO2(e )s at low light intensities, indicative of 

mechanism (24c), (24d) or (24e). 182,185 The former result has 

been previously observed 479 and the latter behavior was seen 

above with recombination of TiO 2(e )s and oxidized sensitizers 

in section 5/B/ii/b. 331,461 Using transient absorption spectroscopy 

and N3/TiO 2 thin-film electrodes in propylene 

carbonate electrolyte containing iodide, Durrant, Nelson, 

and co-workers monitored the kinetics for the loss of 

I2 . 382 After photo-excitation of B1% of the sensitizers, 

electron injection into TiO2, and hole transfer to iodide, I2 

was proposed to be formed by (11) or (10) followed by 

reaction (25). As evidenced by studies performed under variable-applied 

bias conditions, the rate of I2 loss was deter- 

mined to be second order in the concentration of I2 and 0th 

order in TiO2(e ) density consistent with the I2 dismutation 

reaction (18a). It was noted that the TiO2(e )s most likely 

recombined with I3 or I2 in a latter step but that neither 

species could be unambiguously identified in the spectra. Thus, 

this experiment modeled the fate of I2 but did not detail the 

reaction of the TiO2(e )s, which may be more relevant to the 

functioning DSSC. However, a mechanism consistent with 

(24a) could be deduced. 

Durrant, Nelson, and co-workers studied the recombination 

kinetics for TiO 2(e )s with I 2 by transient absorption spectroscopy 

on unsensitized TiO 2 thin films in acetonitrile electrolytes 

by monitoring the loss of TiO 2(e )s after their formation 

by bandgap excitation and subsequent sacrificial hole scavenging 

with MeOH or Fe(CN)6 4 . 463 By transiently generating 

o13 TiO2(e )s/particle, it was shown that in the limit of a high 

concentration of I2, the TiO2(e ) kinetics followed the multiple-trapping, 

nearest-neighbor CTRW model and KWW function 

observed for trap-limited recombination. In the limit of a 

low concentration of I2 the kinetics became mono-exponential, 

as predicted by the dispersive, electron-transfer theory when 

heterogeneous electron transfer is limited by the concentration 

of acceptors, Fig. 45. Based on Monte Carlo simulations 

performed to mimic this behavior and a plot of half-lives 

versus the concentration of I2, second-order processes were 

ruled out. The kinetics for the loss of TiO2(e )s in the presence 

of 50 mM I2 was found to be over two orders-of-magnitude 

faster in the absence of 0.7 M LiI, and more dispersive. Given 

the strongly favorable equilibrium of I2 + I - I3 in 

acetonitrile, Keq 4 10 6 M 1 (eqn (9)), 344–349 this implies that 

I2 is a better acceptor than I3 for TiO2(e )s in acetonitrile. 


for unsensitized TiO2 thin films as a function of I2 concentration. 

Inset: A log–log plot of the half-life versus the concentration of I 2 

illustrating the proposed first-order recombination behavior in the 

concentration of I 2. Taken from Fig. 5 of ref. 463. 


The lack of any signal for I2 during recombination implied 

that the reaction mechanism for the loss of TiO2(e )s to the 

I 3 /I redox mediator on unsensitized TiO 2 thin films is 

analogous to the same reaction at the platinum counter 

electrode, i.e. dissociative adsorption followed by electron 

transfer and mechanism (24d). 

6. Conclusions 

It has been eighteen years since the celebrated Grätzel and 

O’Regan paper first appeared in Nature. This review demonstrates 

the tremendous progress that has been made towards 

developing a molecular-level understanding of charge-transfer 

processes at sensitized TiO2 interfaces. The time scales and 

dynamics for excited-state electron injection into TiO2 have 

been quantified precisely under many experimental conditions. 

Regeneration of the photo-oxidized sensitizer by a variety of 

outer-sphere electron donors, including iodide, has also been 

quantified in some detail. Much less progress has been made 

towards our understanding of the unwanted charge recombination 

to oxidized iodide species, i.e. TiO 2(e ) + A. This is at least 

in part due to the inefficiency of these processes which makes 

characterization difficult. Fundamental data on the identity of 

the Acceptor(s) as well as the reduction mechanism(s) are still 

lacking. Nevertheless, studies have shown that the sensitizer p* 

orbitals and charged ions in the electrolytes can play specific 

roles. Given the recent breakthroughs and the keen interest in 

these reactions, rapid progress is expected. A molecular-level 

understanding of the mechanisms for charge separation and 

recombination at sensitized semiconductor interfaces may ultimately 

enable optimal light-to-electrical power conversion in 

DSSCs and in future-generation photovoltaics. 

Acknowledgements 

The Division of Chemical Sciences, Office of Basic Energy 

Sciences, Office of Energy Research, U.S. Department of 

Energy, the National Science Foundation, and the donors of 

the Petroleum Research Fund, administered by the ACS, are 

gratefully acknowledged for research support. 

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