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Quantum efficiencies exceeding unity in amorphous silicon solar cells

The experimental observation of internal quantum efficiencies above unity in crystalline silicon solar cells has brought up the question whether the generation of multiple electron/hole pairs also has to be taken into consideration in solar cells based on direct gap amorphous semiconductors. To study photogenerated carrier dynamics, we have applied intensity modulated photocurrent spectroscopy (IMPS) to hydrogenated amorphous silicon p-i-n solar cells. In the reverse voltage bias region at low illumination intensities it has been observed that the low frequency limit of the AC quantum yield Y increases significantly above unity with decreasing light intensity, indicating that more than one electron per photon is detected in the external circuit. This phenomenon can be explained by considering trapping and thermal emission of photogenerated carriers at intragap amphoteric dangling bond defect centers.