2, National Renewable Energy Laboratory, Golden, Colorado, United States
Recent developments in photoferroics have led to increased power efficiencies greater than 8%. However, many of the polar materials that have been used in photoferroic devices have bandgaps of 3 eV or more, making the effectiveness of these materials as solar absorbers limited. This has spurred interest in theoretical modeling and experimental realization of new polar materials, with a focus in band gap tuning. In efforts to explore Bi-based perovskite oxides, electronic structure calculations were performed on BiMO3 perovskite structures to determine band gaps and the electronic dielectric functions (GW approximation), as well as the ionic dielectric constant and the piezoelectric tensors (density functional perturbation theory). BiCoO3 showed promise as a low band gap semiconductor with a predicted gap of 2.08 eV, a polar P4mm structure, and a predicted relative permittivity of 44.6. To further investigate this material’s potential as the solar absorber in a photoferroic device, BiCoO3 films were grown via combinatorial pulsed laser deposition. Utilizing a multi-target deposition with Bi2O3 and CoO, the substrate, temperature, chemistry and partial pressure of oxygen were varied to achieve a tetragonal P4mm structure. X-ray diffraction was used to track the phase formation was tracked as function of composition and temperature. Additionally, x-ray fluorescence and UV-vis absorption were used to characterize the compostion and band gap. Upon achievement of proper composition, epitaxial strain was used to create high quality films with an aligned internal polarization. From these films metal-semiconductor-metal structure was used for electronic and photoferroic testing.