New Insights into Complex Excited-State Phenomena from Predictive Computational Approaches

New developments in the theory of excited-state phenomena in energy materials can lead to better understanding of nanoscale energy conversion mechanisms and predictions of new materials hosting such phenomena. In this talk, I will discuss recent studies using ab initio many-body perturbation theory to uncover and understand complex excited state mechanisms in energy materials. I will present a new approach to calculate multi-exciton generation processes in solids from first principles, and describe an application of this approach to singlet fission in organic crystals. Focusing on crystalline pentacene, we predict a new exciton—bi-exciton coupling channel and discover a new selection rule for singlet fission associated with symmetry and structure in the solid state. Additionally, I will describe a recent study of excited state properties in monolayer transition metal dichalcogenides with a dilute concentration of chalcogenide vacancies; these vacancies give rise to localized states, introduce strongly-bound excitons below the absorption edge, and reduce the valley-selective circular dichroism. These findings suggest a novel pathway to tune spin-valley polarization and other optical properties through defect engineering. Finally, I will discuss implications for future pathways to explore exciton transport mechanisms in various energy materials from first principles based on this work.