Dye-sensitized Solar Cells Using Mesoporous TiO2-based Electrodes

Dye-sensitized Solar Cells Using Mesoporous
TiO2-based Electrodes
Supachai Ngamsinlapasathian, Takuya Fujieda,
Yoshikazu Suzuki, Susumu Yoshikawa
Kyoto University
Institute of Advanced Energy

Introduction
Energy and Environmental Crisis
Pollutant emission factors for
electrical generation (g/kWh)
1
Energy Source CO 2
Coal 322.8
Oil 258.5
Natural Gas 178
Photovoltaic 5.3
Solar cell is one of the most promising
ways to solve these problems !!!
The strong points of solar cells are:
direct conversion of solar radiation into electricity,
no mechanical moving parts, no noise,
no high temperatures, no pollution,
solar cell module have a very long lifetime,
the energy source, the sun, is free and inexhaustible,
solar cell is a very flexible energy source,
its power ranging from microwatts to megawatts.

Introduction
2
Several works have been directed toward improving the efficiency of
dye-sensitized solar cells.
Developing new dyes Suppressing electron
recombination
Improving
photoelectrode
Energy level
E/V vs NHE
-0.5
E c (-0.5 V)
Electrons
Injection
Dye
Load
LUMO
(-0.7 V)
Fermi level
Counter electrode (Pt)
0
TiO 2 electrode
ΔE 1 ΔE 2
Excitation
Max voltage
0.5
E g
I - /I 3- redox mediator
(0.4 V)
1.0
HOMO
(1 V)
Electrolyte
TCO Glass
TCO Glass

Introduction
3
Light harvesting efficiency is very important factor for solar cell performance.
Synthesized by Hydrothermal process
Absorbance
A (small size)
A
N719
Wavelength (nm)
M (A+B)
(small size + large size)
M
TiO 2 particles BET surface area (m 2 /g)
IPCE (%)
A (23 nm) 66.3
B (100 nm) 15.4
Type of plate
Nanoparticles
A (23 nm) B (100 nm)
A 100 0
M (A+B) 60 40
23 nm
100 nm
A
M
Schematic film morphologies of TiO 2 photoelectrodes
Arakawa et. al., Coordination Chemistry Reviews, 248 (2004) 1381-1389.
Wavelength (nm)

Introduction
4
Current density (mAcm -2 )
DSC made from multilayer TiO 2
Some researchers added ZrO 2 or SiO 2 in TiO 2
η 10.23%,
Jsc 18.17 mAcm -2 ,
Voc 764 mA, and
FF 0.737
Ref 1
Current density (mAcm -2 )
η 9.04%,
Jsc 18.8 mAcm -2 ,
Voc 740 mA, and
FF 0.640
Ref 2
Photovoltage (V)
Photovoltage (V)
To balance the surface area and light
scattering, multilayer was introduces
as the electrode for DSC.
for scattering the light.
1) Arakawa et. al., Coordination Chemistry Reviews, 248 (2004) 1381-1389.
2) Lee et. al., Solar Energy Materials & Solar Cells, 90 (2006) 2398-2404.

Aim of this work 5
To propose the new approach to prepare mesoporous
titania by surfactant-assisted templating method
To optimize sintering condition for preparing the
electrode of dye-sensitized solar cells
To prepare electrode for obtaining high solar energy
conversion efficiency by using double-layered structure

Experimental 6
Preparation method of nanocrystalline mesoporous titania
Step A
Step B
Tetra-isopropylorthotitanate(TIPT) + Acetylacetone(ACA)
Ti(OCH(CH 3
) 2
) 4
+ CH 3
COCH 2
COCH 3
(TIPT)
(ACA)
0.1M LAHC aq. Solution (pH 4.5)
H
H 3
C C CH 3
C C
O
Ti O
Surfactant : Laurylamine hydrochloride(LAHC)
CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 NH 3+ Cl -
Pri
[TIPT-ACA] / [LAHC] = 4
After mixing After 1day After 3h After 5h After 5 days
O
O
OPri
Pri
+
HOCH(CH 3
) 2
Wash with IPA
40℃
80℃ 80℃ 80℃
stirring
reaction

TEM images
7
(101)
(004)
(200)
(215)
The size of MP-TiO 2
was estimated to be 7 to
15 nm.
Electron diffraction of MP-TiO 2
displays
the Debye-Scherrer rings of anatase.

XRD & BET surface area
8
Intensity/ a.u.
A
R
A A
AA AA T45
MPTiO 2
R R R
P25
20 30 40 50 60 70 80
2theta/ degree
commercial
synthesized
commercial
Adsorbed amount / ml(STP) g -1
200
150
100
50
0
MPTiO 2
Type IV
0 0.5 1
Relative pressure P/P 0
Synthesized TiO 2 had a prevalent
mesoporous structure
MPTiO 2
and T45 film had mainly
consisted of an anatase phase.
P25 film was composed of
anatase and rutile phase.
BET surface area
Code Surface area (m 2 g -1 )
MP-TiO 2 110
P25 47
T45 13.5

Preparation of the photoelectrode
9
Preparation of titania electrode
Repetitive coating
TiO 2
powder Titania gel Sintered at 300 o C for
10 min
Coat with the doctor blading
method
SCN
NCS
Sintered at 350 o C to
550 o C
Titanium plate
TBAOOC
HOO C
N
Ru
N
N
N
COOH
COOTBA
Pt
HOO C
Prepared solar cells (0.25cm 2 )
COOH
Titania film is coated with a
monolayer of N719 dye.
N719 dye
cis-dithiocyanate-N,N’-bis (4-carboxylate -4-
tetrabutylammoniumcarboxylate-2,2’-
bipyridine)ruthenium(II)

Evaluation of DSC
10
Solar simulator
Standard solar cell for the calibration
Bunko-Keiki CEP-2000
・AM 1.5
・100 mW/cm 2
・Calibrated by standard cell (BS-
520)
Tencor Alpha-step Profiler for
measuring film thickness
・Area 0.0534 cm 2
・J SC
= 11.6 mA/cm 2
・V OC
= 0.540 V
・FF = 0.702
・η = 4.40 %

Photovoltaic properties
11
J sc/ mAcm -2
20
15
10
MP-TiO 2
P25
33% increase
Cracked MP-TiO 2 films
A high J sc of the cell
made from MP-TiO 2
could be due to:
high anatase phase
content
high surface area
5
MP-TiO 2 [anatase]
P25 [anatase+rutile]
0
0 5 10 15 20
Thickness/ μm
Jsc of MPTiO 2 cells was higher than that of P25 cell by 33%
at film thickness of 3.5 μm.

Photovoltaic properties of TiO 2 cells
12
Code Thickness (μm) Jsc (mAcm -2 ) Voc (mV) FF η(%)
MP-TiO 2 6.1 11.9 750 0.690 6.0
P25 12.0 10.4 700 0.678 4.9
T45 10.9 5.3 730 0.680 2.6
BET surface area
The cell made from T45 TiO 2 is
very low efficiency due to the
small amount of adsorbed dye.
The efficiency of MPTiO 2
obtained 6% with using film
thickness only 6 μm.
Code Surface area (m 2 g -1 )
MP-TiO 2 110
P25 47
T45 13.5
Appearance of electrode
MPTiO 2 Transparent
P25 White
T45 White

How to increase the light absorption in DSCs
13
Using the dye with absorb the light in the wide wavelength region
Improving the structure of the electrode
IPCE = Incident photon to current conversion efficiency
100
TiO 2 films were sensitized by N719 dye
90
80
70
Transparent
MPTiO 2 (2.5 μm)
Blended MPTiO 2 +P25 (17.5 μm)
White
Thick P25 cell absorbed
more spectra in the red
region than MPTiO 2 cell.
% IPCE
60
50
40
30
P25 (17 μm)
White
IPCE spectra of MPTiO 2
can be improved by adding
P25 (MPTiO 2 +P25).
20
10
0
400 450 500 550 600 650 700 750 800
Wavelength/ nm

SEM image
14
Morphologies of blended MPTiO 2 +P25
Low magnification
High magnification
MPTiO 2
450 o C
450 o C
450 o C
100 nm
P25 (Scattering center)
500 o C
50 nm
500 o C
500 o C
100 nm
50 nm

Surface area & Film porosity of MPTiO 2
+P25
15
Temp Surface area (m 2 g -1 ) Porosity (%)
350 o C 110 53
400 o C 84 51
450 o C 62 50
500 o C 51 47
Loss in surface area
Pore damage
Particle growth
Amount of adsorbed dye
Increasing in sintering temperature
rapidly decreased the amount of
adsorbed dye on the electrode.
Amount of chemisorbed dye/ 10 -8 molcm -2
20
15
10
5
0
300 350 400 450 500 550
Sintering temperature/ o C

Photocurrent-voltage characteristics of blended MPTiO 2 +P25
15
Sintering temperature
Photocurrent
The reasons for improved J sc
are considered to be due to
Crystallinity
Light Sintering scattering for 1h
Amount of chemisorbed dye/ x10 -8 molcm -2
20
15
10
5
Jsc
Dye
20
15
10
5
Short-circuit current density/ mAcm -2
0
Sintering time: 1 h
300 350 400 450 500 550
Sintering temperature/ o C
The electrode sintered at 500 o C possessed not only the enough surface area
to absorb dye but also decrease the interconnection between particles
0

Double layered electrode for DSC
16
Single layered electrode
Back scattering
Light
Blended MP-TiO 2 with
P25 or T45 TiO 2 layer
FTO
T45
T45
T45
T45
T45
T45
T45
T45
T45
The back-scattering of light due to the large particles is unavoidably in light loss
Double layered electrode
Light
Double layered electrode was fabricated
to improve the cell performance by
FTO
1) MP-TiO 2 layer
2) Blended MP-TiO 2 with
T45 TiO 2 layer
T45
T45
T45
T45
T45
Increasing the amount of
adsorbed dye
Improving the light scattering
Decreasing the back scattering
of light

Double layered electrode for DSC
17
Photovoltaic properties
Single layer
Single layer
Code Thickness Amount of Jsc
Voc FF η
adsorbed dye
(μm) (molcm -2 ) (mAcm -2 ) (mV) (%)
MP-TiO 2 +P25 12.3 11.2x10 -8 14.2 759 0.743 8.00
MP-TiO 2 +T45 11.9 8.8x10 -8 16.2 753 0.703 8.60
Double layered cell 11.9 12.3x10 -8 17.1 751 0.711 9.15
The efficiency of the cell made from MP-TiO 2
+T45 is higher than that
of the cell made from MP-TiO 2
-P25.
The cell performance of double layer is improved by increasing the
current density (Jsc).

Double layered electrode for DSC
18
IPCE spectra
IV curve of double layer cell
%IPCE
100
90
80
70
60
50
40
30
20
Double layer cell
Single layer cell
A
MP-TiO 2 +T45 cell
Double layer cell
A
Photocurrent density/ mAcm -2
20
15
10
5
Sintering condition: 500 o C for 1 h
Double layer C-01 cell
Efficiency 9.15%,
Jsc 17.12 mAcm -2 ,
Voc 751 mV and
FF 0.711
Efficiency 9.15%, Jsc 17.12 mAcm -2 ,
Voc 0.751 V, and FF 0.711
(Film thickness 11.9 mm)
(film thickness 11.9 μm)
10
FTO 15 Ohm/sq
0
400 500 600 700 800
Wavelength/ nm
The higher IPCE of double layered cell
is attributed to the increasing in amount
of adsorbed dye and decreasing in back
scattering.
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Photovoltage/ V
The efficiency of 9.15% was obtained
by using double layered structure.

Conclusions
19
TiO 2 prepared by surfactant-assisted templating method
possessed a prevalent mesoporous structure and had a potential
for using as an electrode in DSC.
The addition of TiO 2 powder in MPTiO 2 improved the cell
performance.
The electrode sintered at 500 o C possessed not only the
enough surface area to absorb dye but also decrease the
interconnection between particles
The efficiency of double layered cell was better than that
of only single layer of MP-TiO 2 +P25 and MP-TiO 2 +T45.

Thank you for your attention

TiO 2 Electrode : Structure Type
UnitCell Rutile Anatase Brookite
a(Å) 4.5845 3.7842 9.184
b(Å) 4.5845 3.7842 5.447
c(Å) 2.9533 9.5146 5.145
Rutile
Brookite
Ruthenium
complex
N3 Dye
Anatase
~ 1.05 nm
TiO 2
Anatase sturcture
http://ruby.colorado.edu/~smyth/min/tio2.html
M. Grätzel, Pure Appl. Chem., 73 (2001) 459

Effect of chenodeoxycholic acid on IV properties
CH 3
CH 3
HO
OH
O
Chenodeoxycholic acid
OH
= Additive
e -
Self-quenching
e - e - e -
is not occurred
TiO 2
e -
Dye
No aggregation
Chenodeoxycholic acid Thickness (μm) Jsc (mAcm -2 ) Voc (mV) FF η(%)
0 mM 11.9 14.3 719 0. 699 7.18
0.3 mM 11.7 14.9 720 0.720 7.71
3 mM 12.3 14.2 759 0.743 8.00
10 mM 12.1 14.0 756 0.728 7.73
30 mM 11.8 14.4 751 0.711 7.70
The Jsc and Voc can be appreciably improved by addition of chenodeoxycholic
acid to the dye solution used for dye-uptake.

The Air Mass 1.5 Global (AM1.5) spectrum. Click on the graph for a scalable version.