The impact of hole polaron formation on the excitonic properties of MgO from first principles.

Excitons—neutral electron-hole pairs bound by Coulomb attraction—are crucial to understanding the optoelectronic behavior of materials. In systems with strong electron-phonon coupling, excitons can interact with lattice vibrations (phonons), leading to spatial localization and modified transition energies. These exciton-phonon interactions impact material properties, leading to newly allowed optical transitions, enhanced photoluminescence, and Stokes shift. In this study, we investigate the excitonic properties of MgO, an ionic material characterized by strong electron-phonon interactions and formation of a hole polaron. We utilize density functional theory (DFT) and many-body perturbation theory within the GW/BSE approximation to understand the excitonic properties in the presence of the hole polaron. By comparison of the equilibrium structure without the presence of phonons and a polaron-distorted structure computed within a Delta-SCF type approach, we show that the hole polaron forms upon electron removal from an oxygen atom, primarily involving oxygen p orbitals, and results in an in-gap occupied state 0.762 eV above the valence band maximum. While the lowest energy transition within undistorted MgO is predicted to be 7.6 eV, consistent with optical measurements at 7.7 eV, the presence of the polaron leads to a low-energy transition at 6.9 eV, consistent with measured photoluminescence at 6.76 eV, validating our approach to studying the polaronic exciton. This study highlights the importance of polaronic effects in the excitonic properties of MgO and provides a pathway for calculating them from first-principles.