Bulk Photovoltaic Effects in Halide Perovskites

The bulk photovoltaic effect allows generating photocurrents without the need for semiconductor interfaces, provided the material has an asymmetric crystal structure. Piezoelectrics have intrinsically asymmetric lattices and therefore display bulk photovoltaic effects, but asymmetry can also be generated in non-piezoelectric materials by bending them, thanks to the so-called flexo-photovoltaic coupling. Yet the actual magnitude and relevance of this effect for photovoltaic materials is not known. In this paper we report the flexo-photovoltaic effect of two archetypal halide perovskites, MAPbBr₃ and MAPbI₃, and find it to be many orders of magnitude higher than for the reference flexoelectric SrTiO₃, whose coefficient we also measure. In addition, we find that MAPbI₃ single crystals display hysteresis in open-circuit photovoltage as a function of poling voltage so that, from a photovoltaic point of view, this material is functionally ferroelectric. Last but not least, we find that bulk photovoltaic effects can substantially modulate the photovoltaic output of junction-based solar cells made with perovskite single crystals.