First-principles calculation of the bulk photovoltaic effect in KNbO3 and (K,Ba)(Ni,Nb)O3-δ

The connection between noncentrosymmetric materials' structure, electronic structure, and bulk photovoltaic performance remains not well understood. In particular, it is still unclear which photovoltaic (PV) mechanisms are relevant for the recently demonstrated visible-light ferroelectric photovoltaic (K,Ba)(Ni,Nb)O3-delta. In this paper, we study the bulk photovoltaic effect (BPVE) of (K,Ba)(Ni,Nb)O3-delta and KNbO3 by calculating the shift current from first principles. The effects of structural phase, lattice distortion, oxygen vacancies, cation arrangement, composition, and strain on BPVE are systematically studied. We find that (K,Ba)(Ni,Nb)O3-delta has a comparable shift current with that of the broadly explored BiFeO3, but for a much lower photon energy. In particular, the Glass coefficient of (K,Ba)(Ni,Nb)O-5 in a simple layered structure can be as large as 12 times that of BiFeO3. Furthermore, the nature of the wavefunctions dictates the eventual shift current yield, which can be significantly affected and engineered by changing the O vacancy location, cation arrangement, and strain. This is not only helpful for understanding other PV mechanisms that relate to the motion of the photocurrent carriers, but also provides guidelines for the design and optimization of PV materials.