Origin of the switchable photocurrent direction in BiFeO thin films.

We report external bias driven switchable photocurrent (anodic and cathodic) in 2.3 eV indirect band gap perovskite (BiFeO) photoactive thin films. Depending on the applied bias our BiFeO 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 BiFeO. We found that the Fermi level () is at -4.96 eV (. vacuum) with a mid-gap at -4.93 eV (. vacuum). Our photochemically determined flat band potential () was found to be 0.3 V . NHE (-4.8 V . vacuum). These band positions indicate that is close to mid-gap, and 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 BiFeO 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.