Study on the origin of the open-circuit photovoltage enhancement in TiO2 nanotube-based dye sensitised solar cells

A larger value (0.86 V) of open-circuit photovoltage (Voc) of the TiO2 nanotube (NT)-based dye sensitised solar cells (DSSCs) than that of DSSCs constructed from TiO2 nanoparticles (NPs) has been observed. Work function of TiO2 NTs is about 0.3 eV less than that of TiO2 NPs, which indicates that the conduction band in TiO2 NTs may has a more negative shift in comparison with that of TiO2 NPs after TiO2 contacts with oxidative electrolyte. This may be one origin of Voc enhancement. Moreover, comparative analyses on surface photovoltage and transient photovoltage of TiO2 NPs and NTs show that the larger built-in field in the interface between TiO2 NTs and Ti foil can further improve charge separation and transport in the TiO2 NT film, which will result in recombination decrement and charge lifetime increment. This may be the other origin of Voc enhancement.