Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Fig. 1. (A) Schematic representation of a dye-sensitised solar cell and
(B) the chain of reactions (transmission of an electron) leading to
energy conversion (see text for details) [9]
being. Eventually, various dyes can be tried out in the setup to
improve the efficiency of the photon absorption and charge separation
steps.
The Concept of Bulk Heterojunction
Chromophores in DSSCs have much greater absorption coefficient
than crystalline silicon. However, a single dye layer on the
semiconductor surface has proven not to be sufficient. This is because
full coverage of the surface is difficult to achieve and, what
is more, the optically-active area of the molecule is smaller than
the molecule itself. Thus a thicker layer of the chromophores is
needed. As a junction in planar configuration is not able to meet
this requirement, the concept of a “bulk (hetero)junction” [1] has
been introduced (also used in all-organic solar cells [10]) (Fig. 2).
As the molecules need to have contact with both the semiconductor
and the electrolyte in order to be able to play their role,
the bulk junction effect is achieved by using mesoscopic titanium
dioxide nanocrystals. This leads to a tremendous increase in the
interface surface (approximately by a factor of 1000). Because of
this, light rays propagating perpendicularly to the surface cross
many molecules increase the chance of incident photons being
absorbed. Experimental results show that 97.5% of the 500 nm
light is absorbed by a 12 μm dyed mesoporous titania film [11].
However, too thick a layer forces electrons to make a longer way
to be withdrawn from the cell, increasing the probability of recombination.
Moreover, it slows down diffusion inside the pores
– experimental results show that the diffusion coefficient of the I − 3
ion in acetonitrile in a 55% porous TiO 2
membrane decreases by
an order of magnitude with respect to free diffusion in the same
solvent [5]. Because of this trade-off, precise optimisation of the
thickness of the titania layer is necessary.
Fig. 2. Schematic representation of the concept of a bulk (hetero‐)
junction [13]
Processes at the Semiconductor-Chromophore-
Electrolyte Interface
The previously described working cycle of a DSSC is the ideal
mechanism of energy generation. There are also many sources
of losses that have to be taken into account. One of them is associated
with the semiconductor-dye-electrolyte interface. There
are some more “parasitic” pathways that electrons can take, thus
decreasing the cell performance (Fig. 3) [12]. After having excited
the chromophore, electrons are injected to the conduction band of
the semiconductor (Fig. 3, τ inject
). The process is typically very competitive
with the other paths of electron relaxation. Afterwards the
electron delocalises into the bulk (τ bulk
), relaxing simultaneously via
vibration (τ relax
). If the electron thermalises faster than it delocalises
in the bulk, it may remain on the surface, which often supports
surface states due to unsaturated surface bonds or lattice defects.
The electron can also return from the bulk as the surface area is of
the semiconductor is large by design (see Fig. 2) and because it
is attracted by the positively charged chromophore. There are two
additional pathways of relaxation from the surface states: the electron
can either regenerate a chromophore (τ chrom.
) or be accepted
by a triiodide molecule in the electrolyte (τ electr.
). In both cases the
cell is short-circuited and electron energy is lost. The focus of this
work is highly related to the subtle interplay between the abovementioned
elements because this is the key to the kinetics of the
cell operation.
Fig. 3. (A) Processes taking place at the semiconductor-dye-electrolyte interface (numbers are the continuation after Fig. 1) and the names of
corresponding time constants τ (see text) along with (B) simulated values of the time constants [12]
Elektronika 6/2012 127