Environmental Engineering Reference
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the electrolyte increases artificially the photocurrents, but this effect disappear
when the methanol is exhausted. Obviously, this electrolyte cannot be used in a
solar cell that demands a long device lifetime.
Related with this aspect, it is also important to work in the long-term stability of
QDSSCs. There are very few report in this issue, probably as the first aspects that
the authors wants to optimize is the efficiency as for most of the QDSSCs studied it
lays significantly below than the efficiency reported for DSSCs. The main problem
of liquid devices is the volatility of the electrolyte. It forces to use very sophis-
ticated (and sometimes expensive) sealing processes in order to avoid electrolyte
leakage. A common procedure systematically used in DSSCs is to employ ionic
liquid, with significantly lower volatility that the standard solvents, as electrolytes.
The reduced volatility eases the process of sealing that cheapens the cell at the
same time that extend the device lifetime. The price of this improvement is a
reduction in efficiency due to the increase in the diffusion of species in the elec-
trolyte. A similar approximation has been developed for QDSSCs but using ionic
liquids based on sulfide/polysulfide instead of the ionic liquids based on iodine
employed on DSSCs [
150
]. Despite, further work is required to optimize the
results sulfide/polysulfide ionic liquids could present promising results in terms of
cell stability.
Nevertheless at this moment there is no optimum electrolyte for QDSSCs. Proof
of this is the fact that the efficiencies obtained in liquid QDSSCs are no higher than
those obtained in solid cells, while in the case of DSSCs the efficiencies of liquid
cells is sensibly higher than solid ones. At least until the apparition of the cells
using lead halide perovskite as light absorbing material, we will discuss this
especial case in the
Sect. 8
. With the exception of perovskites the semiconductor
providing the highest efficiency in all-solid sensitized devices is Sb
2
S
3
, with an
efficiency of 6.3 % [
67
]. The study of this material in solid configuration was
approached for the first time by Larramona and co-workers [
151
] and indepen-
dently by Messina et al. [
152
]. In the case of Larramona CuSCN, a wide band gap
p-type semiconductor, was used as HTM, while in Messina's work they contacted
directly Sb
2
S
3
for hole extraction. Very encouraging results were reported later by
the groups of Hodes, [
49
] and Hodes and Grätzel, [
153
] in this last one using spiro
OMeTAD as HTM. But the higher results have been reported by the groups of
Seok and Grätzel [
68
,
154
] using conductive polymers as HTM that boosted the
efficiency to unprecedented values, at that moment, values of 5-6 %, even higher
than liquid QDSSCs.
6 Counter Electrode
The last consideration concerning polysulfide electrolyte is related with the counter
electrode. We have already discussed that platinized counter electrodes are one of
the cause of poor FF obtained for QDSSCs with polysulfide electrolyte [
21
,
146
].
This problematic cause was already discussed at the beginning of the 1980s by
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