Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 116

Fig. 18 Effect of counter
electrode material on the
QDSSC performance. The
effect is especially evident in
the FF (Reprinted permission
with Giménez et al. [ 24 ])
Hodes et al. [ 155 ] suggesting PbS, CoS, and Cu 2 S as alternative counter electrodes
for a good charge transfer with aqueous polysulfide. All three counter electrode
materials have been investigated in QDSSCs. PbS counter electrodes increases
significantly the cell performance of QDSSCs with polysulfide electrolyte, espe-
cially by the increase of FF and photocurrent [ 57 ]. PbS has been also studied in
combination with carbon black [ 156 ]. Carbon foam was also checked [ 157 ]. Cu 2 S,
CoS, and combination of both materials have been also employed in the develop-
ment of QDSSCs [ 158 ]. But probably Cu 2 S has been the material most used in the
preparation of efficient counter electrodes for polysulfide electrolyte and the record
cells with polysulfide, 6.36 % efficiency with CdSe x Te 1-x colloidal quantum dots,
[ 38 ] has been prepared using this material as the base of the counter electrode. This
material presents higher performance that platinized and Au counter electrodes, see
Fig. 18 . Generally, it is prepared from a thin foil [ 24 ]. But in this format the sealing
of active electrode and counter electrode becomes difficult. To solve this incon-
venience Cu 2 S pastes that can be deposited on transparent conductive oxide
substrates has been developed [ 159 ]. An interesting alternative is the use of Cu 2 S-
reduced graphene oxide composite [ 160 ]. Briefly, graphene oxide is reduced by
adding Cu ions in its structure, see Fig. 19 .Cu 2 S is finally formed with the help of
sulfur atoms (just by using a polysulfide solution). Finally, a powder is obtained and
properly treated can be deposited by doctor blade in the conductive substrate. This
counter electrode has been used in the preparation of record cell in that moment
with 5.4 % efficiency [ 161 ] (Fig. 20 ).
7 Recombination and Surface States
In this sense, Sb 2 S 3 exhibits an interesting potentiality for the development of
efficient QDSSCs, but in order to improve the obtained results, it is needed to
reduce the recombination in photovoltaic devices using this material as light