Enhancement of phase stability and optoelectronic performance of BiFeO3 thin films via cation co-substitution

Abstract

Compositional engineering of BiFeO₃ can significantly boost its photovoltaic performance. Therefore, controlling site substitution and understanding how it affects the optical and electronic properties while achieving robust and stable phases is essential to continue progressing in this field. Here the influence of cation co-substitution in BiFeO₃ on phase purity, optical and electronic properties is investigated by means of X-ray diffraction, spectroscopic ellipsometry and X-ray absorption spectroscopy, respectively. Piezoelectric force microscopy and ferroelectric characterization at room temperature has been carried out in co-doped BiFeO₃ films. First-principles calculations are also performed and compared to the experimental observations. It is shown that the incorporation of La³⁺ in Bi(Fe,Co)O₃ films improves phase purity and stability while preserving the reduced band gap achieved in metastable Bi(Fe,Co)O₃. Moreover, it is suggested that the changes in the optoelectronic properties are mainly dictated by the hybridisation between unoccupied Co 3d and O 2p states along with the presence of Co³⁺/Co²⁺ species. This thorough study on (Bi,La)(Fe,Co)O₃ thin films coupled with the use of a cost-effective and facile solution deposition synthesis increases the motivation to continue exploiting the potential of these perovskite materials.