Excited-state dynamics in crystals

Understanding photogenerated excited-state phenomena in extended materials is of emerging interest, with applications ranging from materials characterization and energy conversion to the design of photon emitters and identification of coherent states to store quantum information. Excited-state dynamics and lifetimes are strongly coupled to the material's structural properties, such as dimensionality, homogeneity, and symmetry. The establishment of structural design principles demands a comprehensive understanding of the quantum interactions and selection rules involved, and their relation to the underlying mechanisms dominating the processes of interest.

Our group develops new ab initio computational frameworks to account for exciton dynamics in semiconductors. Our main framework is many-body perturbation theory within the GW and Bethe-Salpeter equation (GW-BSE) approach.  We evaluate the excited-state and excitonic properties of structurally complex systems and incorporate the calculated properties to study exciton relaxation and diffusion processes in functional materials. 

We have recently developed a GW-BSE-based approach to study multiexciton generation processes in molecular crystals. We found coherent exciton-exciton coupling mechanisms, dominating the observed singlet-fission decay. In our ongoing work, we extend our studies to include other excitonic processes. For example, we have recently studied the relationship between exciton crystal dispersion and its time-resolved propagation properties in several systems of reduced dimensionality.  We are deriving a general scheme to explore exciton scattering in these systems.

 

GW-BSE calculated quasi-1D exciton wavefunctions of spin-triplet states in the pentacene molecular crystal. Triplet states can be formed in this crystal through a multiexciton generation process called singlet fission, and are of great interest for photogeneration due to their relatively long lifetimes, at the order of nanoseconds.