Paramagnetic defects in hydrogenated amorphous silicon

Abstract

This thesis presents a detailed study of paramagnetic defects in hydrogenated amorphous silicon (a-Si:H). The defect states act as recombination centers for excess charge carriers and limit the efficiency of thin-film solar cells based on a-Si:H materials. The defect density drastically increases by light irradiation of the solar cell, which is a well-known degradation effect (Staebler Wronski effect - SWE). This effect is the major limitation of solar cells based on a-Si:H. In order to understand the process which leads to the generation of the defects states, detailed knowledge of the microscopic origin of the defect states is mandatory. In this work, the paramagnetic defects present after deposition (native defects) and after degradation with light (light-induced defects) are investigated with electron paramagnetic resonance (EPR). This technique measures the g-value of the defects and hyperfine interaction with magnetic nuclei in their vicinity and therefore allows mapping out the wave function of the defects. Native defects in a-Si:H are localized coordination defects, which were attributed to threefold-coordinated silicon atoms (dangling bonds). A measurement of the EPR spectrum of native defects at various microwave frequencies (multifrequency EPR) is presented in this work and allows a precise determination of the g-values and the hyperfine interactions. The values obtained by the experiment are compared to theoretical density-functional theory calculations of dangling bonds to test if the microscopic origin of native coordination defects can be ascribed to dangling bonds. Defect states generated by light in the SWE are investigated by EPR measurements at high-field (Q-Band). Due to the increased resolution of these experiments, a new type of light-induced defect state could be observed, which exhibits different g-values and a faster primary-echo decay as compared to native coordination defects. In order to perform these advanced EPR experiments on fully-processed thin- film solar cells, electrically detected magnetic resonance (EDMR) is used with its superior sensitivity. A novel EDMR microwave pulse sequence is presented and tested on µc-Si:H solar cells to detect weak hyperfine interactions.