Integrating Band Engineering and the Flexoelectric Effect Induced by a Composition Gradient for High Photocurrent Density in Bismuth Ferrite Films

Photovoltaic energy as one of the important alternatives to traditional fossil fuels has always been a research hot spot in the field of renewable and clean solar energy. Very recently, the anomalous ferroelectric photovoltaic effect in multiferroic bismuth ferrite (BiFeO3) has attracted much attention due to the above-bandgap photovoltage and switchable photocurrent. However, its photocurrent density mostly in the magnitudes of μA/cm2 resulted in a poor power conversion efficiency, which severely hampered its practical application as a photovoltaic device. In this case, a novel approach was designed to improve the photocurrent density of BiFeO3 through the cooperative effect of the gradient distribution of oxygen vacancies and consequently induced the flexoelectric effect realized in the (La, Co) gradient-doped BiFeO3 multilayers. Subsequent results and analysis indicated that the photocurrent density of the gradient-doped multilayer BiFeO3 sample was nearly 3 times as much as that of the conventional doped single-layer sample. Furthermore, a possible mechanism was proposed herein to demonstrate roles of band engineering and the flexoelectric effect on the photovoltaic performance of the gradient-doped BiFeO3 film.