New Insights into Single- and Multi-Exciton Phenomena in Complex Materials from Ab Initio Many-Body Perturbation Theory

Theoretical predictions of excited-state phenomena in complex materials can lead to better understanding of nanoscale energy conversion mechanisms, for instance in emerging photovoltaic and photocatalytic systems. In this talk, I will discuss recent studies using new ab initio many-body perturbation theory methods within the GW approximation and the Bethe Salpeter equation approach (GW-BSE) to understand and uncover such mechanisms. In one example, I will present a new approach to calculate multi-exciton generation processes in solids from first principles, without empirical input, used to study singlet fission in organic crystals. Applying this approach to crystalline pentacene, we discovered a new exciton—bi-exciton coupling channel, one that is purely Coulombic, with a predicted decay rate comparable to experiments; our results led to new understanding of the role of symmetry and structure in the singlet fission mechanism in the solid state. Additionally, I will discuss our recent progress in calculating excited state properties in complex systems of reduced dimensionality. Selected results will be presented for two materials: monolayer transition metal dichalcogenides with point defects, where the calculated GW-BSE absorbance is strongly affected by the presence of localized defect states; and a new class of multilayered hybrid chalcogenides with 2D-like excitonic behavior that is strongly coupled to their unique structure and chemistry.