Giant optical absorption and ferroelectric polarization of BiCoO2S perovskite oxysulfide by first principles prediction

Abstract

Obtaining an ideal ferroelectric photovoltaic (FE-PV) material with a narrow bandgap and a large ferroelectric polarization value can enable us to achieve great practical FE-PV performance. By the introduction of sulfur into the tetragonal BiCoO₃ perovskite with a C-type antiferromagnetic ordering, it is found that the bandgap of BiCoO₂S decreases significantly (about 1.2 eV) while maintaining a large polarization value (about 1.86 C m⁻²) that is similar to the value of 1.793 C m⁻² of BiCoO₃. Most noteworthy is that the optical absorption of BiCoO₂S is remarkably higher than those of BiCoO₃ and other FE-PV materials. The decrease of the BiCoO₂S bandgap originates from the movement of Co 3d states to a low-energy position due to the reduction of the Co ionicity when the less electronegative sulfur is introduced into BiCoO₃ to substitute oxygen. The narrow bandgap and the high optical absorption of the BiCoO₂S films grown on different substrates are favorable for FE-PV applications. In addition, the bandgap of BiCoO₂S can be modulated by the doping amount of sulfur, which can help us fabricate multilayer FE-PV devices based on different bandgaps from different layers.