Manipulating the Atomic Structure of Bismuth Ferrite for Improved Photovoltaic Output

Ferroelectric materials, with their intrinsic and switchable electrical polarization, provide a natural asymmetry to separate photo-excited electrons and holes without resorting to engineering interfacial built-in electric fields. This effect, called the Bulk Photovoltaic Effect (BPVE), is considered capable of surpassing the Shockley-Queisser limit that has hampered the conversion efficiency of traditional p-n junction solar cells. Yet, much remains unknown about the relationship between the atomic structure and the BPV response. Bismuth ferrite (BiFeO3, or BFO for short) is a ferroelectric material that has garnered considerable attention due to (i) being one of the only ferroelectrics with optical absorption in the visible range and (ii) its multifunctional properties that can be tuned by epitaxial strain. More specifically, under compressive bi-axial strain, BFO is known to adopt a strongly anisotropic T-phase over the bulk-like R-phase [1]. It is our hypothesis that, near the phase transition, the shift current part of the BPVE is significantly enhanced. Our hypothesis rests on recent theoretical works which predict (1) that strongly anisotropic systems favor large shift vectors [2] and (2) large photostrictive responses at the R-phase/T-phase boundary [3]. We employ computational density functional theory (DFT) and Wannier-based interpolation of the band structure [4] to investigate how epitaxial biaxial strain affects the electronic structure and shift photovoltaic response of BFO thin films near the phase transitions of the R-phase and T-phase. Our preliminary results provide theoretical insight into the photovoltaic performance of BFO near such phase transitions and direct the competitiveness of BFO for energy harvesting applications.

[1] Sando, D., Xu, B., Bellaiche, L. & Nagarajan, V. A multiferroic on the brink: Uncovering the nuances of strain-induced transitions in BiFeO3. Applied Physics Reviews 3, 011106 (2016).

[2] Tan and Rappe, Phys Rev B 100, 085102 (2019)

[3] Yang, Y. et al. Large photostriction near the phase boundary in BiFeO3 under varying epitaxial strain. Physical Review B 109, 184111 (2024).

[4] Ibañez-Azpiroz, J., Tsirkin, S. S. & Souza, I. Ab initio calculation of the shift photocurrent by Wannier interpolation. Physical Review B 97, 245143 (2018).