The Correlation between Structure, Electronic Structure, and Shift Current in Visible-Light Ferroelectrics from First Principles

Shift current, as a dominant mechanism of the bulk photovoltaic effect (BPVE), remains not well understood, especially in terms of its connection to material's structure and electronic structure. This holds especially for the recently designed and demonstrated visible-light ferroelectric photovoltaics, Pb(Ni,Ti)O$_{3-\delta}$ and (K,Ba)(Ni,Nb)O$_{3-\delta}$, that have great structural and electronic tunabilities. Here, we study the BPVE of the visible-light-absorbing ferroelectrics by calculating shift current from first principles. The effects of phase, lattice distortion, oxygen vacancy, cation arrangement, composition, and strain on BPVE are systematically studied. The wavefunction nature of the contributing electronic states dictates the eventual shift current yield, which can be significantly affected by the change of the O vacancy location, cation arrangement, and strain. Consequently, under broad spectrum illumination, the total current can be greatly enhanced by reducing the cancellation of counter propagating currents and by increasing the shift vector magnitude. This not only is helpful for understanding other photovoltaic mechanisms that relate to the motion of the photocurrent carriers, but also provides guidelines for the design of the photovoltaic converters.