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.

中文翻译：

---

BiFeO3 能带结构的中间带和 p 轨道调谐用于光伏应用


BiFeO₃ 是一种无铅无机铁电材料，有望在光伏应用中应用。然而，其较大的带隙对有效吸收阳光构成了挑战，限制了其在此类应用中的效率。在这项工作中，选择与氧相比具有较低电负性的氮来调节 BiFeO₃ 的带隙。研究发现，BiFeO _3-xN_x（x = 0.125、0.25、0.375 和 0.5）的带隙从母相的原始值 3.1 eV 降低到 1.45-1.77 eV，这源于 Fe 3d 和 N 2p 之间更强的相互作用引起的中间带，以及由于引入 N 2p 引起的价带最大值向更高的能级移动。特别是，BiFeO_2.75N_0.25 和 BiFeO_2.625N_0.375 的带隙分别为 1.45 和 1.49 eV，更接近光伏材料的最佳带隙约 1.4 eV。随着带隙的减小，N-取代的 BiFeO₃ 在可见光范围内的光吸收系数明显增加。值得注意的是，N 取代的 BiFeO₃ 显示出良好的铁电性，带隙减小，光吸收增加。因此，所提出的结果表明，N 取代是改善 BiFeO₃ 铁电光伏性能的一种有前途的方法。