Microstructure of local defects in amorphous Si:H: A quantum chemical study

Abstract

The mechanism of the Staebler-Wronski effect, which is responsible for the light-induced degradation of photovoltaic devices based on amorphous, hydrogenated silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}),$ is still in question. It is, e.g., assumed that a defect precursor A is transformed into the actual, metastable defect B by excitation and subsequent electron trapping. We present calculations on large, distorted silicon clusters containing vacancies/voids of up to three missing atoms. A sophisticated embedding procedure is employed where the atoms surrounding the vacancy/void are treated by means of a density functional (BP86) and the outer atoms are taken into account semiempirically (AM1). Full geometry optimization results in novel and unprecedented structure information. Excitation energies and electron attachment energies are presented. These findings substantiate ideas regarding the nature of A and B.