Uni-directional electric current flow, such as that found in a diode, is essential for modern electronics. It usually occurs at asymmetric interfaces such as p-n junctions or metal-semiconductor interfaces with Schottky barriers. We have recently discovered a diode effect associated with the direction of bulk electric polarization in BiFeO 3 , which is a ferroelectric with a relatively small optical gap edge of ~2.2 eV. We found that bulk electric conduction in ferroelectric monodomain BiFeO 3 crystals is highly non-linear and uni-directional. This diode effect switches its direction when the electric polarization is flipped by an external voltage. Associated with the diode effect, large directional photocurrent at zero bias can be induced by visible light in ferroelectric monodomain BiFeO 3 ; i.e., a significant photovoltaic effect is observed. Our discovery  of substantial diode-like and photovoltaic effects in BiFeO 3 is of significant importance for fundamental understanding of charge dynamics in leaky multiferroics, and may lead to new design approaches for multifunctional devices combining magnetic, electronic and optical functionalities.
 T. Choi et al. , Science 324 , 63 (2009).