Giant Optical Absorption and Ferroelectric Polarization of BiCoO2S Perovskite Oxysulfide by First Principles Prediction

Obtaining an ideal ferroelectric photovaoltaic (FE-PV) material with the narrow bandgap and the large ferroelectric polarization can enable us achieve practical FE-PV performance greatly. By the introduction of sulfur in the perovskite tetragonal BiCoO3 with C-type antiferromagnetic ordering, it is found that the bandgap of BiCoO2S decreases significantly (about 1.2 eV) while maintaining a large polarization (about 1.86 C/m2) that are similar to the value of 1.793 C/m2 of BiCoO3. The most noteworthy is that the optical absorption of BiCoO2S is remarkable higher than those of BiCoO3 and others FE-PV materials. The decrease of 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 smaller electronegativity of sulfur is introduced into the BiCoO3 to substitute oxygen. The narrow bandgap and the high optical absorption of BiCoO2S films grown on different substrates is 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 the multilayer FE-PV devices based on the different bandgaps from different layers.