Environmental Engineering Reference
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avoid agglomeration [ 39 ]. It has been shown that the ligand exchange helps
anchoring QDs to mesoporous electrode, for example, TOPO molecules coating
CdSe QDs were substituted by pyridine in order to enhance the QD loading of
sensitized electrode [ 40 ]. This strategy has been employed with significant success
by the group of Zhong, where oleic acid is substituted by mercaptopropionic acid
(MPA) [ 41 ]. Then aqueous solution with MPA-capped QDs is pipetted directly on
the electrode surface, where it stayed for 2 h before rinsing sequentially with water
and ethanol and then drying with nitrogen. This procedure has produced QDSSCs
with efficiencies higher than 5 % [ 36 , 37 ] or even 6 %, [ 38 ] as it has been already
commented.
The methods presented so far have the drawbacks that need long duration for
sensitization in most of the cases, as the dipping process to attach the linker
molecule and/or the QDs requires several hours for an optimum loading [ 8 ].
Sensitization time can be significantly reduced by employing the electrophoretic
technique. Basically, in this technique an electrical field is applied between two
electrodes dipped in a QD solution. Ionized QDs are attracted to the electrodes and
attached in one or both electrodes [ 42 ]. CQD deposition rate on the mesoporous
electrode depends on the applied voltage, high CQD loading can be obtained in
few hours [ 28 , 29 ] or even in few minutes [ 27 ]. Even, electrophoresis has been
used to attach rod shape sensitizers (nanorods) to TiO 2 electrodes, [ 29 ] to prepare
electrodes with QDs of different sizes [ 30 ] or to assemble CdSe QDs and fullerene
for an innovative solar cell [ 43 ]. Nevertheless the efficiencies reported for
QDSSCs using electrophoresis are shown below the performance reported for
other attaching modes with CQDs.
3.2 Chemical Bath Deposition
The second main approximation for the sensitization with inorganic semiconductor
is direct growth of light absorbing material on the surface of the wide bandgap
semiconductor electrode. The remaining section will be dedicated to this last
approximation with three low cost techniques that will be overviewed: (a) Chemical
bath deposition (b) successive ionic layer adsorption and reaction (SILAR), and (c)
electrodeposition. Chemical bath deposition (CBD) is a convenient method to
assemble the QDs on a variety of substrates (conducting and nonconducting) at
elevated temperatures compared with most other semiconductor QDs deposition
methods [ 44 ]. The simplicity of CBD process together with the inherent low
temperature operations results in superior control over the QDs particle size [ 17 ].
Several articles explain the variety of QDs assembly by CBD method, i.e., CdS, [ 18 ,
45 , 46 ] CdSe, [ 34 , 47 , 48 ]Sb 2 S 3 [ 49 - 52 ], CdTe, [ 53 - 56 ]PbS[ 27 , 57 - 60 ], etc. The
two main mechanisms for the CBD process are (1) ion-by-ion deposition onto a
coating surface without bulk precipitation in the deposition solution and (2) bulk
precipitation (or colloid formation) with diffusion of the bulk semiconductor
clusters to the coating surface [ 44 ]. The complex agents were usually utilized to
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