Highly efficient SiO2/TiO2 composite photoelectrodes for dye-sensitized solar cells

A composite SiO2/TiO2 photoelectrode (PE) architecture for incorporating in dye-sensitized solar cells (DSC) was developed aiming to increase the electron mobility, the specific surface area and the transparency. The developed PE is made of a mesoporous SiO2 scaffold layer covered by a conformal TiO2 film deposited by atomic layer deposition (ALD). The shrinking core model was used to predict the minimum pulse time required for the complete coverage of the inner SiO2 scaffold surface area (ca. 160 m(2) g(-1)). The TiO2 film thickness proved to be related to two DSC operation regimes: transport limited regime (D-eff proportional to TiO2 thickness, where D-eff. is the electron effective diffusivity) and a recombination limited regime (D-eff constant); in the latter regime, the minimum recombination rate constant, k(r), resulted in maximum DSC performance. DSCs based on the new PE coated with an optimized TiO2 film thickness (3.3 nm, 150 ALD cycles) exhibited an average of 11% higher energy conversion efficiency than the standard devices, eta = 9.20% vs. eta = 8.25% and 10% more transmittance in the visible range. To the best of the authors' knowledge this is the best reported DSC efficiency using a SiO2/TiO2 architecture. The superior performance exhibited by the new PE was ascribed to: higher surface area and lower electron recombination. The increased recombination resistance was attributed to lower morphological defects and better dye coverage of the TiO2 layer. This work shows that an ALD deposited TiO2 film ranging from 3 to 4 nm in thickness is suitable for the dye adsorption and electron transport; when applied over a large surface area scaffold, the TiO2 nanolayer displayed the same transport abilities as reference TiO2 mesoporous films. The proposed PE opens the doors to new architectures and the use of new semiconductors with better properties concerning electronic mobility, conduction band energy level, transparency and recombination,