Charge Transport Limitations in Self-Assembled TiO2

Solid-state dye-sensitized solar cells offer the possibility of high power conversion efficiencies due to few fundamental losses in dye regeneration. Despite continuous progress, limitations in charge diffusion through the mesoporous photoanode are still partially responsible for constraints in device thickness and hence result in reduced light absorption with the most common sensitizers. Here, we examine block copolymer assembled photoanodes with similar surface area and morphology but a large variation in crystal size. We observe that the crystal size has a profound effect on the electron transport, which is not explicable by considering solely the ratio between free and trapped electrons. Our results are consistent with the long range mobility of conduction band electrons being strongly influenced by grain boundaries. Therefore, maximizing the crystal size while maintaining high enough surface area will be an important route forward.