Quantum-Dot-Sensitized Solar Cell Using a Photoanode Prepared by in Situ Photodeposition of CdS on Nanocrystalline TiO2 Films

CdS quantum dots (QDs) have been incorporated into mesoporous TiO2 nanocrystalline films by a photodeposition (PD) technique we have recently developed [CdS(PD)/mp-TiO2], and for comparison, the conventional successive ionic layer adsorption and reaction (SILAR) and self-assembled monolayer (SAM) methods have also been used for preparing the coupling system. The most important characterstic of the PD technique is that the efficicent interfacial charge transfer between the semiconductors is guaranteed because the photocatalytic redox property of TiO2 is taken advatage of to form the heteronanojunction. The N-2 adsorption-desorption data analysis by the Barret-Joyner-Halenda method and the elemental depth profile by electron probe microanalysis showed that CdS QDs are distributed in the mesopores of the film without pore-blocking in the PD sample and with partial pore-blocking in the SILAR sample, whereas only the upper part of the film is covered with CdS QDs in the SAM sample. The PD technique enables one to control the loading amount and particle size of CdS QDs by UV-light irradiation time (lambda > 320 nm) with excellent reproducibility. Owing to these unique features, sandwich-type solar cells using the CdS(PD)/mp-TiO2(photoanode showed a power conversion efficiency (eta) under simulated sunlight (AM 1.5, 100 mW cm(-2)) of up to 2.51% more than those for the cells employing CdS(SILAR)/mp-TiO2 (eta = 1.21%) and CdS(SAM)/mp-TiO2 (eta = 0.14%).