Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 93

maintain the pH of the chemical bath, which directly control the reaction rate and
hence overcome the bulk precipitation in the chemical bath [ 61 ]. For example, the
reaction mechanism of in situ coating of CdSe QDs onto mesoporous TiO 2 is
explained as follows: [ 62 ]

Cd 2 þ þ HSe þ OH $ CdSe ð s Þþ H 2 O ð 1 Þ

ð CdSe Þ m þ Cd 2 þ þ HSe þ OH $ð CdSe Þ m þ H 2 O

ð 2 Þ

ð CdSe Þ m þð CdSe Þ n $ð CdSe Þ m þ n ð 3 Þ

ð Cd Þ 2 þ þ 2OH $ Cd ð OH Þ 2 ð s Þ ð 4 Þ

Cd ð OH Þ 2 ð s Þþ HSe ! CdSe þ OH þ H 2 O ð 5 Þ

Reactions ( 1 )-( 2 ) describe the formation of CdSe without the formation of
Cd(OH) 2 seed layer. Subsequently, Cd 2+ and Se 2- species are forming larger
clusters by coalescence or aggregation with other clusters ( 3 ). Coalescence refers
to a combination of two clusters to form one single crystal, while aggregation
means the formation of two or more separate but contacting crystals. The latter
was suggested to be the terminating step in crystal growth for CBD CdSe films. As
for CdSe, the reverse direction of reaction ( 5 ) will lead to redissolution of the solid
phase and will control the size of the Cd(OH) 2 crystallites (i.e., colloidal particles)
in the solution. Thus, lower temperatures result in a higher concentration of small
Cd(OH) 2 nucleii and results in the formation of CdSe crystal with smaller size.
The effect of Se 2- ions at a Cd(OH) 2 surface will be replaced by a selenide due to
much lower solubility of CdSe compared with Cd(OH) 2 . The Cd(OH)* nucleus
will eventually be converted to CdSe (reaction 1 ). Thus, the reaction will continue
by formation of hydroxide and its subsequent conversion into CdSe (4 and 5).
Therefore, the nucleation process on coating surface takes vital role in determining
QDs formation mechanism, which is often controlled by precursor concentration
and chemical bath temperature and is explained elsewhere by Gorer et al. [ 62 ]
(Fig. 2 ). Different QD size, as can be deduced from the different bandgap, can be
obtained by varying temperature and concentration in the CBD process, see Fig. 2 .
The typical chemical bath deposition of CdSe QDs is as follows [ 63 , 64 ]: a
chemical bath solution was prepared by mixing 80 mM CdSO 4 and 80 mM
sodium selenosulfite (Na 2 SeSO 3 ) solution with 120 mM nitriloacetic acid. The
mesoporous TiO 2 electrodes were immersed in the chemical bath at 10 C for 12
hours. Finally, the films were soaked in deionized water and dried with nitrogen.
Recently, antimony sulfide (Sb 2 S 3 ) QDs sensitizers prepared by CBD method
showed efficient PCE performance 4-5 % in both liquid-type and solid-state
hybrid solar cells [ 65 - 68 ]. Deposition of Sb 2 S 3 coating by CBD is cheaper than the
other available techniques. Typical coating of Sb 2 S 3 QDs is explained elsewhere
[ 67 , 68 ]. Briefly, the mixture of 1 M solution of SbCl 3 in acetone and 1 M
Na 2 S 2 O 3 cold aqueous solution addition with cold water is adjusted to have a final