Switchable photovoltaic effect in anatase TiO₂ films via donor-acceptor co-doping

V⁵⁺-Bi³⁺ co-doped polycrystalline TiO₂ films ((V_0.5Bi_0.5)_xTi_1-xO₂) prepared on Pt/Ti/SiO₂/Si substrates by sol-gel method have been found to exhibit switchable photovoltaic properties. The significant photovoltaic effect of as-fabricated films indicates existence of self-polarization within the films, which results in the formation of a built-in electric field. And as-fabricated (V_0.5Bi_0.5)_0.02Ti_0.98O₂ film has a photovoltaic effect under 365 nm light illumination (8 mW/cm²), with open circuit voltage and short circuit current density of 0.217 V and 1.24 μA/cm². It has been also found that the photovoltaic effects of the co-doped films can be switched via an external electric field. After negative electric field poling, the open circuit voltage of the (V_0.5Bi_0.5)_0.02Ti_0.98O₂ film increases to 0.258 V. However, the open circuit voltage and short circuit current of the film can be reversed after positive electric field poling, and the open circuit voltage and short circuit current increase with the increase in poling electric field, reaching maximum values of 0.635 V and 4.91 μA/cm², respectively. Based on experiment and calculation results, this study clearly reveals the relationship between crystal structure, ferroelectricity and photovoltaic effect. The donor-acceptor co-doped TiO₂ films can therefore be applied in ferroelectric photovoltaic devices.