Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 118

recombination (slight increase in R rec ) is observed when CdS is used as sensitizer.
But, in the case of Sb 2 S 3 recombination is even enhanced in comparison with bare
electrode. This fact indicates that the inorganic sensitizer itself has an active role in
recombination process, in other word the sensitizer introduces active recombina-
tion site where photoinjected electron in the TiO 2 find a pathway for recombi-
nation. Here a clear difference with DSSC is highlighted, while molecular dyes
only presents the HOMO and LUMO levels for shelter electrons, in the case of
QDs or inorganic semiconductors, surface states could play an important role in
the QDSSCs performance [ 7 ]. In fact the main effect of the well-known ZnS
treatment that enhances significantly QDSSCs in many cases, [ 10 , 24 , 34 ]isto
passivate the surface states of the inorganic sensitizer [ 162 ]. Thus, the recombi-
nation though the sensitizer in QDSSCs has to be reduced in order to improve the
photovoltaic conversion efficiency, passivation treatments of TiO 2 surface [ 163 ],
or of the light absorbing material itself [ 164 , 165 ] have shown a tremendous effect
in the cell performance, but further analysis in this direction is needed to continue
the optimization of QDSSCs. On the other hand, each kind of inorganic semi-
conductor could require a different passivation process in terms of stability and
efficiency of the semiconductor layer.
Finally as introduction for the next section, it has to be highlighted that in
addition one of the effective approaches to control the recombination rate in
QD-sensitized solar cells (QDSSCs) is to densely cover the QDs on mesoporous-
nanostructured electrodes [ 10 ] without naked sites, that directly block the physical
contact between electrolyte and TiO 2 [ 166 , 167 ] In this configuration, semicon-
ductor sensitizer layer is acting as a barrier between electrons in the TiO 2 and
accepting species in the electrolyte hindering this recombination path way, but
recombination though sensitizer semiconductor is still possible as it has been
previously commented. Taking into account this consideration, the procedure in
with electrodes sensitized also plays an important role in the recombination of
QDSSCs [ 71 ].
8 The Perovskite Revolution
Recently, a new type of photovoltaic cells with organometallic halide perovskites
as light harvester materials have burst onto the scene gaining great attention in the
scientific community. It is due mainly because a rapid succession of occurrences of
record devices in a short-time period, which it is now reaching above of 14 % of
certified efficiency, [ 168 ] and even further increase has been announced. Here, we
briefly review the most striking results in the current hottest topic on photovoltaics.