Preparation of multilayered cdse quantum dot sensitizers by electrostatic layer-by-layer assembly and a series of post-treatments toward efficient quantum dot-sensitized mesoporous TiO ₂ solar cells

A multilayer of CdSe quantum dots (QDs) was prepared on the mesoporous surface of a nanoparticulate TiO ₂ film by a layer-by-layer (LBL) assembly using the electrostatic interaction of the oppositely charged QD surface for application as a sensitizer in QD-sensitized TiO ₂ solar cells. To maximize the absorption of incident light and the generation of excitons by CdSe QDs within a fixed thickness of TiO ₂ film, the experimental conditions of QD deposition were optimized by controlling the concentration of salt added into the QD-dissolved solutions and repeating the LBL deposition a few times. A proper concentration of salt was found to be critical in providing a deep penetration of QDs into the mesopore, thus leading to a dense and uniform distribution throughout the whole TiO ₂ matrix while anchoring the oppositely charged QDs alternately in a controllable way. A series of post-treatments with (1) CdCl ₂, (2) thermal annealing, and (3) ZnS-coating was found to be very critical in improving the overall photovoltaic properties, presumably through a better connection between QDs, effective passivation of QD's surface, and a high impedance of recombination, which were proved by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) experiments. With a proper post-treatment of multilayered QDs as a sensitizer, the overall power conversion efficiency in the CdSe QD-sensitized TiO ₂ solar cells could reach 1.9% under standard illumination condition of simulated AM 1.5G (100 mW/cm ²).