Path integral approach for lattice effect on excitonic properties in semiconductors
The application and design of photovoltaic devices rely on understanding the photophysical properties of semiconducting materials. While it has been suggested that the interplay between charge carriers and a fluctuating lattice plays an important role in determining optoelectronic properties, away from weak coupling limit with harmonic lattices, little is known about the effects of electron-phonon coupling on excitonic properties. In our work, we employ quasiparticle path integral molecular dynamics to study how the excitonic properties of model semiconductors are altered by electron-phonon coupling. We describe ways within a path integral representation to evaluate numerically exact role of phonons through an imaginary time influence functional to incorporate phonon-induced effects on effective mass, binding energy, and radiative recombination rate of excitons nonperturbatively.
We apply path-integral approach to MAPbI3 perovskite to study their exceptionally low recombination rates and small exciton binding energies. In this material, since both electrons and holes are diffusive and strongly couple to an anharmonic lattice, elucidating the nature of how photogenerated electrons and holes bind, dissociate and recombine is theoretically difficult. Combined with an explicit atomistic representation of MAPbI3 perovskite lattice, allowing us to capture all orders and anharmonicity, we extract effective electron-hole interaction. With these simulations and theory, we find that the inclusion of a spatially dependent (nonlocal) screening produces a repulsive exciton interaction at intermediate distances which can be responsible for the decrease in exciton binding energy and the increase in free carrier lifetimes.
Yoon Park is a 5th year graduate student in the department of Chemistry at UC Berkeley, working with Professor David Limmer, where her research has focused on studying the interplay between structural dynamics and optoelectronic properties of semiconducting materials. She completed her bachelor’s in Chemistry and Mathematics from Sogang University in South Korea and obtained her MS in theoretical chemistry from the same university in 2018 where she worked on the structural motif of glass-forming liquids.