Abstract

New developments in the theory of excited-state phenomena can lead to better understanding of the involved nanoscale mechanisms and to predictions of new materials hosting such phenomena. In this talk, I will present recent studies using first-principles computational methods to model and understand such mechanisms in extended materials. Specifically, I will discuss the effect of point defects on excited-state properties and selection rules in monolayer transition metal dichalcogenides. These impurities can give rise to localized states, introduce strongly-bound excitons below the absorption edge, and reduce the valley-selective circular dichroism, suggesting a novel pathway to tune spin-valley polarization and other optical properties through defect engineering. Additionally, I will present a new approach to explore multi-exciton generation in solids from first principles and describe an application of this approach to singlet fission, a multi-exciton generation process in organic crystals. Focusing on crystalline acenes, I will discuss a newly discovered exciton—bi-exciton coupling channel, revealing selection rules for singlet fission that are associated with the crystal symmetry and structure.