Energy-resolved defect kinetics in a-Si:H investigated by modulated photocurrents

The gap state distribution in a-Si:H for solar cells was investigated by phase shift analysis of modulated photocurrents. Two peaks in the density of states at 0.6 and 0.38 eV below E_c were detected. Upon light soaking the peaks of shallow states were quenched and the peaks of deep states were enhanced; the original distribution was restored by annealing above 120 degrees C. The formation and annealing kinetics of the deep states were studied. A power law was obtained for the formation. Activation energies for annealing were centered around 1.1 eV, and a correlation of the activation energies with the energetic position of defects in the gap was obtained. To explain the results, a model of the defect structure involving charged and neutral Si defects in different hybridization states is discussed.