Intermediate bands and p-orbital tuning of the band structure of BiFeO3 for photovoltaic application

Abstract

BiFeO₃ is a lead-free inorganic ferroelectric material that holds promise for photovoltaic applications. However, its larger band gap poses a challenge for efficient sunlight absorption, limiting its efficiency in such applications. In this work, nitrogen, which has lower electronegativity compared to oxygen, was selected to tune the band gap of BiFeO₃. It was discovered that the band gap of BiFeO_3−xN_x (x = 0.125, 0.25, 0.375, and 0.5) was reduced from the original value of 3.1 eV for the parent phase to 1.45–1.77 eV, which originated from the intermediate bands caused by the stronger interaction between Fe 3d and N 2p and the valence band maximum shifted towards higher energy levels caused by the introduction of N 2p. In particular, the band gaps of 1.45 and 1.49 eV for BiFeO_2.75N_0.25 and BiFeO_2.625N_0.375, respectively, were closer to the optimal band gap of approximately 1.4 eV for photovoltaic materials. Accompanied with the reduction in the band gap, the optical absorption coefficient of N-substituted BiFeO₃ in the visible range was evidently increased. Noticeably, N-substituted BiFeO₃ showed good ferroelectricity with a decreased band gap and increased optical absorption. The presented results thus demonstrate that N substitution is a promising way to improve the ferroelectric photovoltaic properties of BiFeO₃.