Semiconducting ferroelectric photovoltaics through Zn2+ doping into KNbO3 and polarization rotation

We demonstrate a new band engineering strategy for the design of semiconductor perovskite ferroelectrics for photovoltaic and other applications from first principles. We study six ferroelectric solid solutions created by partially substituting Zn2+ for Nb5+ into the parent KNbO3 material, combined with charge compensation at the A sites with different combinations of higher valence cations. Our first-principles calculations with the HSE06 functional yield a low band gap of only 2.1 eV for the 75%KNbO3-25%(Sr1/2La1/2)(Zn1/2Nb1/2)O3 solid solution, and this can be lowered further by 0.6 eV under strain through polarization rotation. The large polarization, especially under strain, of these materials provides a charge separation route by the bulk photovoltaic effect that could potentially allow power conversion efficiency beyond the Shockley-Queisser limit. This band engineering strategy is applicable to other perovskites and should be realizable by standard solid-state synthesis and thin film growth methods.