Semiconducting ferroelectrics for photovoltaics through Zn2+ doping into KNbO3

Using first-principles calculations, we design and predict six low band gap ferroelectric solid solutions by partially substituting Zn$^{2+}$ for Nb$^{5+}$ into the parent KNbO$_{3}$ material, combined with charge compensations at the $A$-sites by different combinations of higher valence cations (Ba$^{2+}$, Sr$^{2+}$, Pb$^{2+}$ and La$^{3+}$, Bi$^{3+}$). In particular, our HSE06 calculations yield a low band gap of only 2.11 eV for the 75\%KNbO$_{3}-25\%$(Sr$_{1/2}$La$_{1/2}$)(Zn$_{1/2}$Nb$_{1/2}$)O$_{3}$ (KN-SLZN) solid solution, and this can be lowered further by 0.6-0.7 eV upon in-plane compressive strains, allowing for more efficient visible light photovoltaic energy harvesting. The maintaining or enhancing of the polarizations of KN-SLZN provides an efficient charge separation route by the bulk photovoltaic effect that could make the power conversion efficiency (PCE) go beyond the Shockley$-$Queisser limit. We argue that these newly designed low band gap ferroelectric solid solutions can be experimentally synthesized and are promising for photovoltaics. In addition, we demonstrate a new strategy to engineer the band gap while maintaining the polarization of the ferroelectric perovskites, which can be well applied to other systems.