Materials design of visible-light ferroelectric photovoltaics from first principles

Further improvement of the power conversion efficiencies of conventional perovskite ferroelectric oxides has been strongly impeded by their wide band gaps. Here, we use several band gap engineering strategies to design low band gap ferroelectric materials from first principles. We show that polarization rotation is useful for reducing the band gaps of strongly distorted perovskites. A variety of visible-light ferroelectric solid solutions are designed by combining Zn substitution into KNbO₃ with polarization rotation. Alternatively, the band gaps can be reduced by the introduction of low-lying intermediate bands through Bi⁵⁺ substitution. With this strategy, two Bi⁵⁺-containing visible-light ferroelectric solid solutions are designed, which exhibit comparable bulk photocurrent to that of prototypical ferroelectric oxides, but with lower photon energies, as evidenced by the shift current calculations.