In situ preparation of QDs

The in situ preparation method, where the QDs are directly generated on the surface of metal oxide film electrode, mainly includes chemical bath deposition (CBD) (Diguna et al., 2007) and successive ionic layer adsorption and reaction (SILAR) (Lee et al., 2009a). There are many of advantages of the in situ deposition approaches. Firstly, the in situ deposition approaches are easy to process since it does not need any expensive equipment and multiple complicated steps. Secondly, the QDs are in direct electronic contact with metal oxide, which not only makes the QDs good anchoring to the electrodes but also shortens the electron diffusion length. Thirdly, they can easily produce metal oxide films with high surface coverage of the sensitizing QDs, which increases the absorption of light. However, this method still has several intrinsic limitations. For example, it is difficult to control chemical composition, crystallinity, size distribution and surface properties of QDs, which may hamper the effective exploitation of QDs advantages. .

For CBD method, QDs are deposited in situ by immersing the wide-bandgap nanostructured electrode (usually metal oxide) into a solution that contains the cationic and anionic precursors, which react slowly in one bath under different temperature. Most of the sulfides and selenides can be prepared by this method. Lee et al developed a method coupling self – assembled monolayer and CBD, as well as a modified CBD process performed in an alcohol system to assemble CdS into a TiO2 film (Lin et al., 2007). These modified processes have been proved to be efficient for CdS QD-SC, and CdS QD-SCs exhibit a power conversion efficiencies of 1.84% and 1.15% respectively for iodide/triiodide and polysulfide electrolytes (Lin et al., 2007).

In the SILAR approach, the cationic and anionic precursor solution is placed in two vessels respectively. Firstly, the nanostructured electrode is immersed into the solution containing the metal cation, and then the nanostructured electrode absorbing the metal cation on the surface is taken out from the metal cation solution and dip into the second precursor solution containing the anion. After the second rinsing step, the deposition cycle completes. The average QD size can be controlled by the number of deposition cycles. This method has been used in particular to prepare metal sulfides, but recently it has been expanded to metal selenides and tellurides. For example, Gratzel et al deposited the CdSe and CdTe QDs in situ onto mesoporous TiO2 films using SILAR approach (Lee et al., 2009b). After some
optimization of these QD-sensitized TiO2 films in solar cells, over 4% overall efficiency was achieved at 100 W/m2 with about 50% IPCE at its maximum (Lee et al., 2009b).