Temperature-Dependent Excitons and Phonon-Driven Dissociation in BiVO₄

The role of phonon screening in the formation and dissociation of excitons in materials has been a topic of growing interest, with prior first-principles calculations showing phonon exchange between electrons and holes can both renormalize exciton binding energies and result in the decay of bound electron-hole pairs [1-2]. Here, we perform ab initio Bethe-Salpeter equation calculations that include the effects of phonon screening on the monoclinic phase of BiVO4, an indirect gap energy material known for its photocatalytic properties. At T=0 K, we find that the lowest-energy exciton binding energy is reduced by nearly a factor of two when including phonon screening, and we compute this state to have a finite lifetime, of a few ps, due to exciton dissociation accompanied by spontaneous phonon emission. At room temperature, the renormalization of the exciton energy is even more dramatic, and phonon-driven exciton dissociation proceeds on much faster timescales, on the order of a few femtoseconds. Additionally, we discuss a scheme for the incorporation of phonon screening into the calculation of optical absorption spectra, and show that the use of our approach leads to greatly improved agreement relative to the state of the art with available room temperature experiments. We conclude by discussing general criteria for when phonon screening will play an important role in the calculation of optical properties.

[1] A. M. Alvertis, et al., PNAS 121, e2403434121 (2024).

[2] C. J. N. Coveney, et al., Phys. Rev. B 110, 054307 (2024).