Tip-enhanced photovoltaic effects in bismuth ferrite

Photoelectric properties of multiferroic BiFeO3, such as large photovoltages exceeding several times the bandgap or switchable photocurrents, have recently generated much interest. This is mostly because of potential applications in photovoltaic or photocatalytic devices. Although the fundamentally different (from classical semiconductors) polarization-related charge separation mechanisms allow unlimited photovoltages, these materials have never been considered a viable alternative to classical semiconductor-based photovoltaics because of a very low light conversion efficiency of less than 10−4. Here we investigate the anomalous photovoltaic effect in bismuth ferrite BiFeO3 single crystals, using photoelectric atomic force microscopy and piezoresponse force microscopy. We show that using a nanoscale top electrode, the photoexcited carriers are efficiently collected and the external quantum efficiency is enhanced by up to seven orders of magnitude. This enhancement might provide a viable alternative for the effective use of the anomalous photovoltaic effect in photovoltaic applications.