| Producing efficient photovoltaic and optoelectronic devices is heavily dependent on the ability to understand and control excited state electronic processes. Semiconductors have been shown as ideal materials to harness excited state charge carriers and provide an environment for charge separation and transport. Within semiconductors multiple relaxation processes such as non-radiative relaxation, radiative relaxation, non-radiative recombination, and multiple exciton generation are competing mechanisms following photoexcitation. Dependent on the specific environment and material composition, the resulting electronic structure dictates the time scales of multiple relaxation mechanisms. In searching for material modifications to enhance electronic structures we have added defects, dopants, and specific surface passivation techniques. In addition to chemical structure modifications this work highlights the study of spatial confinement and effects of quantum confinement on the multiple competing relaxation mechanisms. To feasibly study time dependent charge carrier dynamics in a range of semiconducting materials we implement a suite of both commercial and home-grown code to computationally study electronic processes in methylammonium lead halide perovskites, silicon nanocrystals, and polyoxotitanate clusters. Ground state electronic structure, heating, and molecular dynamic calculations are completed within the Vienna ab initio Software Package (VASP) using the Generalized Gradient Approximation Perdew-Burke-Ernzerhof functional in a plane wave basis set with projector augmented wave psuedopotentials. For specific electronic structures the single gamma point, k-point mesh, spin-polarized, and non-collinear spin builds of VASP have been utilized. The calculation of "on-the-fly" non-adiabatic couplings provide a correlation between nuclear and electronic degrees of freedom resulting in the ability to propagate the electron density matrix, as implemented in Redfield "open system" formalism. These dynamics allow the calculation of the rates for non-radiative relaxation. |
