Bulk- and Flexo-Photovoltaic effects

Two years after the invention of modern prototype solar cells, it was found that BaTiO₃exhibits a photovoltaic effect distinct from that of p-n junctions, later called the bulk photovoltaic (BPV) effect. Under uniform illumination, a homogeneous ferroelectric material gives rise to a short-circuit current and produces an anomalously large photo-voltage well exceeding the bandgap energy. The microscopic origins of this effect supposed to originate from the asymmetric distribution of photoexcited non-equilibrium carriers in k-space, caused by absence of centrosymmetry. The talk will present a short history and the basics of the bulk photovoltaic effect, tip enhancement, as well as the electronic origin of the anomalous BPV in some materials such as BiFeO₃. Potential applications such as energy harvesting or light-induced reversible switching of ferroelectric polarization at room temperature will be adressed. I will show how the BPV effect may be used in optical switching tunnel junctions or other similar devices. Finally, I will discuss a new photovoltaic effect which turns the BPV effect into a universal effect allowed in all semiconductors by mediation of the flexoelectric effect. [M.-M. Yang D. J. Kim, & M. Alexe, Flexo-Photovoltaic Effect, Science 360, 904 (2018)]