Investigating the Role of Electron-Phonon Interactions and Reduced Dimensionality on Optical Excitations in Monolayer GeSe

Two-dimensional (2D) van der Waals-bonded layered materials are promising as inexpensive, light-weight solar energy conversion materials. Better understanding the role of electron-phonon interactions in 2D will be necessary for understanding the optical properties of this class of materials. Here, we use first-principles theory to investigate, the role of electron-phonon interactions on the optical absorption spectrum of monolayer germanium selenide GeSe, a direct gap 2D semiconductor with promising optoelectronic properties. We utilize density functional theory, density functional perturbation theory, and many-body perturbation theory to study both bulk and monolayer GeSe. We determine that the optical gap at room temperature is lower than that of the zero temperature for both systems. For the bulk, an indirect gap semiconductor, this reduction is mainly due to phonon-assisted transitions. For the monolayer, we attribute the reduced gap to the localization of the excited-state in the presence of phonons. The significant influence of electron-phonon interactions in the monolayer suggests that this phenomenon should be better understood for 2D material technology.