First-Principles Design and Analysis of an Efficient, Pb-Free Ferroelectric Photovoltaic Absorber Derived from ZnSnO₃

Ferroelectric oxides, which exhibit a spontaneous and reversible electric polarization, have recently gained interest for photovoltaic applications, because this polarization can potentially facilitate exciton separation and carrier extraction while also generating open-circuit voltages orders-of-magnitude larger than the band gap. However, photovoltaic efficiencies in these materials are often limited by large band gaps and high hole effective masses. Developing a means to simultaneously reduce both without destroying the ferroelectric polarization could revolutionize photovoltaic technologies. In this work, we use first-principles computations to describe how chemical substitution of oxygen in the recently characterized ferroelectric ZnSnO 3 with sulfur to form ZnSnS 3 reduces the band gap to a near-optimal 1.3 eV while leaving the polarization virtually unchanged. Furthermore, we show that other key photovoltaic materials characteristics, such as hole effective mass, dielectric constant, and absorption coefficient, are also dramatically improved by sulfur substitution. Finally, we demonstrate that the well-known semiconductor GaN provides an excellent substrate on which to grow ZnSnS 3, forming a strongly bound interface that is free of intrinsic midgap states and exhibits an ideal band alignment for efficient carrier extraction. This work advances the search for an Earth-abundant, nontoxic, ferroelectric material exhibiting strong solar absorption, and may enable the development of solar cells with both high photovoltage and photocurrent.