Nonradiative Recombination via Conical Intersections Arising at Defects on the Oxidized Silicon Surface

Nonradiative recombination of excitations in semiconductors limits the performance of photovoltaics, light-emitting diodes, photocatalysts, and other devices. Herein we investigate the role that two known defects on the oxidized surface of silicon play in nonradiative recombination in silicon nanocrystals. We apply ab initio multiple spawning and multireference electronic structure methods to model the nonradiative processes which follow excitation of two cluster models of silicon epoxide defects that differ in the oxidation state of their respective silicon atoms. We find conical intersections in both clusters, and these intersections are found to be accessible at energies corresponding to visible wavelengths. In both cases, photochemical opening of the epoxide ring precedes nonradiative decay. These results support the hypothesis that conical intersections associated with specific defect structures on the oxidized surface of silicon nanocrystals facilitate nonradiative recombination. Discussion regarding how this hypothesis can be tested experimentally is presented.