Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 114

nanoparticulate-based QDSSC (2.4 %) due to the higher V oc , although the amount
of CdSe QDs in the inverse opal case might be a half of that in the nanoparticulate
case due to the difference of surface area. The higher V oc in the IO QDSSCs was
ascribed to the larger fraction of electron injection to the TiO 2 resulting in a higher
quasi Fermi level, detected with ultrafast optical measurements, Fig. 17 d. In
addition to the above comparative results, Samadpour et al. also compared the
performance of IO-based QDSSCs along with variety of nanostructured electrodes
(nanotube, nanofiber, and nanoparticulate) [ 71 ]. They suggested that choosing the
semiconductor deposition strategy (CBD or SILAR) is more important where
the pore blockage takes a key role which influences the efficiency of QDSSCs.
5 Hole Transporting Material
Concerning the hole transporting material, it is a major difference with liquid
DSSCs as many of the most employed semiconductors for light harvesting are not
stable in solutions with the conventional I - /I 3 - redox couple [ 7 ]. Stability problems
could be solved by the use of Co redox couples, but the photocurrents obtained were
relatively low [ 40 , 60 , 76 ]. The other approximation used to solve this issue was the
coating of the semiconductor QDs with a protecting layer of amorphous TiO 2 [ 147 ].
However, obtained photocurrent was not much appreciable. In order to solve these
issues, the most common redox couple employed for QDSSCs is the polysulfide
(S 2- /S 2- ), generally in a aqueous solution [ 148 ]. Polysulfide redox helps in the
stability of semiconductor light absorber in the liquid devices, it also allows high
photocurrents (photocurrents as high as 22 mA/cm 2 has been recently reported for
PbS/CdS QDSSCs [ 149 ]), but it introduces an additional problem of a bad charge
transfer with the platinized counter electrodes [ 21 ] commonly used in DSSCs. As a
consequence very bad fill factors, FF, are commonly reported with platinized
electrodes and polysulfide redox, see for example, refs. [ 21 , 146 ]. In order to
replace this inconvenient Pt catalytic electrodes, the fourth aspect commented
previously have been developed for polysulfide electrolyte. We will come back
with this issue later.
There are three interesting aspects that is worthy to comment related with
polysulfide redox electrolytes. The first one is related with a deceptive practice that
unfortunately is reported in a relatively significant number of papers. One of the
requirements that an electrolyte for sensitized solar cells has to fulfil is the stability
during a complete charge extraction process. Reduced species of the redox couple
are oxidized during the hole regeneration of the sensitizer. Oxidized species dif-
fuse to the counter electrode where they are reduced again in a fully regenerative
process. This is the case of aqueous polysulfide electrolyte, but it is not estrange to
find a paper where methanol is added as solvent. Methanol acts as a hole scavenger
regenerating oxidized sensitizers, but it is not regenerated itself at the counter
electrode, as the redox couples. As a consequence cells prepared with methanol in