First-principles modeling of excited states at finite temperatures: phonon-induced localization, dissociation and screening*

The excited state properties of semiconductors and insulators are critical to optoelectronic devices such as photovoltaics. These devices typically operate at room temperature, however, most first-principles excited state calculations ignore finite temperature effects, due to the challenges that their inclusion into current theoretical methods poses, leading to a disconnect between theory and practical applications. 

In this talk I will present extensions of the ab initio GW-Bethe Salpeter Equation (BSE) approach within many-body perturbation theory to account for finite temperature effects on excited states. In the first part of the talk I will demonstrate how finite displacement methods can be used alongside GW-BSE, to understand the temperature-dependent exciton bandwidth of molecular crystals due to phonon-induced exciton localization [1], including the important role of anharmonic phonons [2]. High-frequency phonons are found to couple weakly to delocalized excitations [3], allowing us to demonstrate design rules for minimizing non-radiative recombination losses in organic systems [4]. In the second part of the talk, building on a framework introduced in prior work [5], I will present a first-principles extension of GW-BSE to include phonon screening of excited states at finite temperatures. Using an efficient computational implementation of this scheme [6], we demonstrate strong temperature dependence of exciton binding energies in several systems, as well as accurate predictions of exciton dissociation rates [7]. 

[1] Alvertis, Haber, Engel, Sharifzadeh, Neaton, Phys. Rev. Lett. 130, 086401, (2023)

[2] Alvertis, Engel, Phys. Rev. B 105, L180301, (2022)

[3] Alvertis et al. Phys. Rev. B 102, 081122(R), (2020)

[4] Ghosh, Alvertis et al. submitted (2023)

[5] Filip, Haber, Neaton, Phys. Rev. Lett. 127, 067401 (2021)

[6] Li, Alvertis, Gant, Neaton, Louie, in preparation

[7] Alvertis, Haber, Li, Coveney, Louie, Filip, Neaton, submitted (2023)