Optical properties of Bi(Fe,Mn)O3: Polar oxides for solar energy applications

Polar oxide heterostructures such as bismuth ferrite provide a novel approach to solar energy conversion. The photovoltaic efficiency of BiFeO3 is, however, limited by the 2.67 eV bandgap. Like many other perovkites, bismuth ferrite offers bandgap control via cationic substitution. In this work, we use manganese substitution to control the oxidation state of iron and enhance the photoferrroelectric properties of the material. While BiFeO3 has a 2.67 eV direct bandgap, our optical properties work demonstrates that BiMnO3 displays a greatly reduced 0.75 eV bandgap. Alloys show values that are tunable in this range, providing an excellent match with the 1.5 eV maximum in the solar spectrum. Based upon the overall intensity of the lowest energy electronic excitation and its temperature dependence, comparison with chemically similar materials with Mn3+ centers in a locally distorted octahedral environment, and existing electronic structure calculations, we assign the near infrared excitation that develops in the Mn-substituted materials as charge transfer in nature.