Research Seminar - Antonios Alvertis

The optoelectronic response of organic semiconductors is dominated by bound electron-hole pairs called excitons. The molecular character of these materials often leads to very strong exciton-phonon interactions, which can make perturbation theory insufficient for their description, making their first-principles description challenging. In this talk, I will present a non-perturbative methodology for accurately capturing exciton-phonon interactions to all orders, based on a combination of finite differences methods for phonons with Green’s function-based methods (GW-BSE) or time-dependent density functional theory, for crystalline or isolated systems respectively. I will demonstrate that the spatialextent of an exciton state is the main parameter controlling the magnitude of its coupling to phonons, as well as the nature of the phonons (low- vs high-frequency) that dominate this interaction. This allows for a unified picture of exciton-phonon interactions in organic semiconductors, reconciling the complementary views of finite molecular clustersand periodic molecular solids, which are commonly considered in the quantum chemistry and theoretical condensed matter physics communities respectively.

In the second part of my talk, I will discuss some applications of the presented methodology. Firstly, I will show that accounting for the coupling of excitons to nuclear quantum motion leads to unprecedented quantitative accuracy in the prediction of exciton energies, both for crystalline molecular systems1, and for single organic molecules in vacuum/solution2. Moreover, I will apply these methods to explaining the microscopic mechanism of the temperature dependence of exciton energies1. Finally, I will discuss recent results obtained in collaboration with the Neaton group at Berkeley, which reveal the localising effect that phonons have on exciton wavefunctions, and provide a roadmap towards a deeper understanding of dynamic processes such as singlet exciton fission in organic semiconductors.

[1] Antonios M. Alvertis, Raj Pandya, Loreta A. Muscarella, Nipun Sawhney, Malgorzata Nguyen, Bruno Ehrler, Akshay Rao, Richard H. Friend, Alex W. Chin. Bartomeu Monserrat “Impact of exciton delocalization on exciton-vibration interactions in organic semiconductors” Physical Review B, 102, 081122(R) (2020)

[2] Timothy J. H. Hele, Bartomeu Monserrat, Antonios M. Alvertis. “Systematic improvement of molecular excited state calculations by inclusion of nuclear quantum motion: a mode-resolved picture and the effect of molecular size.” The Journal of Chemical Physics, 154, 244109 (2021)

Antonios received his Ph.D. in Physics from the Cavendish Laboratory of the University of Cambridge, where he focused on the theoretical modelling of organic semiconducting materials, and specifically on understanding the interactions of fundamental energy carriers called excitons, with the molecular and crystal vibrations of these systems. To achieve this, he employed a variety of theoretical methods and worked very closely with experimental groups. For his Ph.D. thesis, he was awarded the Cavendish Ph.D. prize in computational physics, and a Springer Thesis prize. He recently joined the Neaton group on a Winton-Kavli postdoctoral exchange fellowship, where he tries to understand exciton physics in a variety of different systems beyond organic materials., and to learn a lot about many-body perturbation theory.