The bulk photovoltaic effect (BPVE) is the generation of photocurrents in the bulk of a single-phase material. It holds advantages over traditional photovoltaics based on p-n junctions, such as above-band gap photovoltages, and current generation in the bulk without the need for interface engineering. Despite numerous theoretical and experimental research efforts into the BPVE, there has been no systematic investigation into its maximum magnitude attainable in solid-state materials. In this talk, we present an upper bound on the dominant microscopic mechanism of BPVE: the shift current response. We show that this bound depends on the band gap, band width, and geometrical properties of the material in question. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large BPVE materials will likely be discovered by future materials research guided by the factors mentioned in this work.
These results imply that small band gap materials can potentially host large BPVE. As examples, we propose materials which are tuned across a band-gap-closing phase transition from a normal semiconductor into a topological insulating phase. This class includes some inorganic layered semiconductors, such as BiTeI, and inorganic halide perovskites, such as CsPbI3. We show that this results in a dramatic enhancement of photocurrent as well as an abrupt reversal in its direction. Using first-principles calculations, we show that that this effect is robust across different materials systems as long as such a transition into a topologically insulating phase is present.