Exciton optical linewidth and ultrafast exciton dynamics via exciton-phonon interactions from first-principles calculations*

Exciton dynamics dictate the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron-phonon and many-electron interactions. We develop and apply here a first-principles approach to exciton dynamics resulting from exciton-phonon coupling in monolayer MoS₂ and WS₂/MoS₂ heterobilayer. We first show that optical absorption processes rigorously require a second-order perturbation theory approach as proposed by Toyozawa and Hopfield. Such a treatment gives rise to off-diagonal exciton-phonon self-energy, which are critical for the description of dephasing mechanisms, and yields exciton linewidths in excellent agreement with experiment. Our large-scale first-principles calculations then reveal the highly selective nature of exciton-phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest energy bright A exciton in MoS₂ and a separation in thermalization time scale for spin-allowed and spin-forbidden excitons and whole exciton populations. The long total thermalization time is dominant by the slow spin-flip scattering process in monolayer MoS₂. In contrast, the intra-channel thermalization can be achieved within few hundred fs. In the WS₂/MoS₂ heterobilayer, we find that the exciton-phonon interaction induced relaxation time of photo-excited excitons at the K valley of MoS₂ and WS₂ is 67 fs and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways which are otherwise inaccessible to non-interacting electrons and holes.