Analysis of Electron Transfer Properties of ZnO and TiO2 Photoanodes for Dye-Sensitized Solar Cells

Mesoporous TiO2 nanoparticle films are used as photoanodes for high efficiency dye-sensitized solar cells (DSC). In spite of excellent photovoltaic power conversion efficiencies (PCE) displayed by titanium dioxide nanoparticle structures, the transport rate of electrons is known to be low due to low electron mobility. So the alternate oxides including ZnO, that possesses high electron mobility, are being investigated as a potential candidate for photoanodes. However, the PCE with ZnO is still lower than TiO2 and this is typically attributed to the low internal surface area. In this work, we attempt to make a one-to-one comparison of the photovoltaic performance and the electron transfer dynamics involved in DSC, with ZnO and TiO2 as photoanodes. Previously such comparative investigations were hampered due to the morphological differences (internal surface area, pore diameter, porosity) that exist between zinc oxide and titanium dioxide films. We circumvent this issue by depositing different thickness of these oxides, by atomic layer deposition (ALD), on an arbitrary mesoporous insulating template and subsequently used them as photoanodes. Our results reveal that at an optimal thickness, ZnO exhibits photovoltaic performances similar to TiO2, but the internal electron transfer properties differ. The higher photogenerated electron transport rate contributed to the performances of ZnO but in the case of TiO2, it’s the low recombination rate, dye loading and fast electron injection.