Kavli Affiliate: Jeffrey B. Neaton
| First 5 Authors: Stephen E. Gant, Antonios M. Alvertis, Christopher J. N. Coveney, Jonah B. Haber, Marina R. Filip
| Summary:
Monoclinic bismuth vanadate (m-BiVO$_4$) is a promising indirect band gap
semiconductor for photoelectrochemical water splitting, yet the characteristics
of its low-lying photoexcitations, or excitons, remain poorly understood. Here,
we use an ab initio Bethe-Salpeter equation approach that incorporates phonon
screening to compute the nature and lifetimes of the low-lying excitons of
m-BiVO$_4$. Our calculations indicate that at 0 K, the lowest-lying exciton
energy exceeds the indirect band gap, enabling spontaneous dissociation into
free carriers via phonon emission within picoseconds. At 300 K, both phonon
emission and absorption effects reduce this timescale to only a few
femtoseconds. Phonon screening also greatly reduces the binding energy of the
lowest-lying exciton, leading to an optical absorption spectrum that better
reproduces experimental measurements. Overall, our findings establish the
general conditions under which phonon emission-driven exciton dissociation can
occur in indirect gap semiconductors, and they emphasize the critical role
phonon screening can play in predictive calculations of photophysical
properties of complex materials.
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