Enhanced photovoltaic properties of gradient calcium-doped BiFeO3 films

The photovoltaic properties of ferroelectric films have been extensively studied due to their potential applications in the fields of photodetection, energy conversion harvesting and storage. However, the effect of the gradient distribution of oxygen vacancies on the photovoltaic properties remains unclear. Herein, we prepared BiFeO3 (BFO) and two types of gradient calcium-doped BiFeO3 (BiFeO3/Bi0.95Ca0.05FeO2.975/Bi0.90Ca0.15FeO2.950/Bi0.85Ca0.15FeO2.925: BCFO-1 and Bi0.85Ca0.15FeO2925/Bi0.90Ca0.0FeO2.950/Bi0.95Ca0.05FeO2.975/BiFeO3: BCFO-2) films deposited on fluorine-doped tin oxide glass substrates. Piezoresponse force microscopy studies indicate the upward self-polarization phenomenon in BFO and BCFO-1 films, while the downward self-polarization phenomenon in BCFO-2 films. The J-V characteristic curves show rectifier behaviors for these films due to the configuration of oxygen vacancies. For photovoltaic response, the open circuit voltage of BCFO-1 films is more than 2 times higher than that of BFO films, and the short-circuit photocurrent densities of BCFO-1 and BCFO-2 films are increased by nearly 32 and 6 times, respectively. The direction and magnitude of photovoltaic response are possibly associated with the energy band modulation governed by self-polarization and the gradient distribution of oxygen vacancies. The work demonstrates the merits of gradient doping with lower valence cation towards enhanced photovoltaic properties with high stability for ferroelectric oxides.