Evidencing Prominent Involvement of Bi Orbitals of BiFeO3 in Eliciting Charge-Transfer Excitation

BiFeO3 (BFO) finds applications in spintronics, multiferroics, sensors, piezoelectricity, emerging ferroelectric solar cells, and photocatalysis. However, the performance of BFO depends on its electronic properties and charge-transfer (CT) excitation, which are governed by the extent/choice of orbitals involved in hybridization. The CT excitation is often assumed to occur due to electron transitions from the O 2p to Fe 3d orbitals of the conduction band (CB), while completely disregarding the role of Bi 6p orbitals. For the first time, using near-ambient x-ray photoelectron spectroscopy under light illumination, valence states of BFO constituents are deconvoluted. By selectively probing different orbitals of the CB under illumination, the often-ignored role of Bi orbitals in CT excitation is rationalized. In addition, ultrafast transient absorption spectroscopy unravels the distinctive decay behavior of charge carriers in CB states compared with that in in-gap defects. Such comprehensive correlation evidences on the significant involvement of Bi orbitals in eliciting CT excitation from a chemical bonding perspective. Results aid in establishing missing links among band structure, optoelectronic properties, and function/performance of BFO. Such insights are key to improving the performance of next-generation multifunctional applications of BFO, particularly in solar-energy conversion.