Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 98

designed for the most efficient conversion of the solar spectrum. A large number of
semiconductors CdS, [ 19 , 88 ] PbS, [ 89 - 91 ], and CdSe, [ 85 , 92 , 93 ] etc., have been
electrodeposited with varying coating parameters such as electrolyte concentra-
tion, pH, and applied potential, etc. The ED method is restricted to electrically
conductive materials. In the electrochemical deposition, the substrate (mesoporous
TiO 2 -coated TCO) is submerged in a liquid solution (electrolyte). When an
electrical potential is applied between a conducting area of the substrate and a
counter electrode (usually platinum) in the liquid, a chemical redox process takes
place resulting in the formation of a semiconductor QDs layer on the substrate.
The schematic of hybrid electrochemical/chemical deposition of CdS QDs are
presented in Fig. 5 [ 94 ].
Recently, Wang group, [ 13 ] and X-Y Yu et al., [ 19 ] demonstrated the elec-
trodeposition of CdSe QDs on SnO 2 and ZnO nanostructures, respectively. The
completely covered CdSe QDs by electrodeposition on mesoporous SnO 2
photoanodes showed 17.4 mA cm -2 with 3.68 % PCE, which is relatively a high
performance compared to the previous reports on SnO 2 photoanodes-based
QDSSCs. The growth methodology of electrodeposited CdSe QDs at different
cycles is explained in Fig. 6 . The size of QDs was influenced by number of coating
cycles. However, more number of coating cycles may block the pores.
In this section, we have summarized the most important sensitization methods
for QDSSCs. We observe the relative simplicity of the methods, that they do not
require vacuum or high temperature conditions. Therefore, chemical approach of
QDs sensitization method is inherently low cost and remarkable efficiency higher
than 6 % has been reported. This fact advocates that QDSSCs have tremendous
potential for the future development of low cost photovoltaic devices.
4 Photoanodes in QDSCs
The photoanode of a QDSSC functions as selective contact for electrons [ 4 ]. In
addition, it also works as an electron ''vehicle'' to transport the injected electrons
from the excited QDs sensitizers to the outer circuit. In general, the photoanode
materials need to satisfy several properties: first, the energy gap of the semicon-
ductor could match with that of the QDs-sensitizer to ensure an effective injection
of the photo-induced electrons from the QDs to the semiconductor, ensuring in
addition electron selectivity blocking holes. Second, the semiconductor electrode
must have a high surface area to accommodate more QDs, so as to harvest as much
photon as possible. The photoanode material is the indispensible component of
QDSSCs which plays crucial role in sensitizer loading, electron injection, trans-
portation, and collection, and therefore exhibits significant influence on the pho-
tocurrent, photovoltage, and the power conversion efficiency. Recently,
remarkable efforts have been paid to the design of the chemical composition,
structure, and morphology of the semiconductor photoanode.