Origin of the switchable photocurrent direction in BiFeO₃ thin films

We report external bias driven switchable photocurrent (anodic and cathodic) in 2.3 eV indirect band gap perovskite (BiFeO₃) photoactive thin films. Depending on the applied bias our BiFeO₃ films exhibit photocurrents more usually found in p- or n-type semiconductor photoelectrodes. In order to understand the anomalous behaviour ambient photoemission spectroscopy and Kelvin-probe techniques have been used to determine the band structure of the BiFeO₃. We found that the Fermi level (E_f) is at -4.96 eV (vs. vacuum) with a mid-gap at -4.93 eV (vs. vacuum). Our photochemically determined flat band potential (E_fb) was found to be 0.3 V vs. NHE (-4.8 V vs. vacuum). These band positions indicate that E_f is close to mid-gap, and E_fb is close to the equilibrium with the electrolyte enabling either cathodic or anodic band bending. We show an ability to control switching from n- to p-type behaviour through the application of external bias to the BiFeO₃ thin film. This ability to control majority carrier dynamics at low applied bias opens a number of applications in novel optoelectronic switches, logic and energy conversion devices.