Thermally enhanced quantum efficiency in hydrogenated amorphous silicon p-i-n photodiodes studied by intensity-modulated photocurrent spectroscopy

It is shown that at zero bias or slightly reverse bias the photocurrent qua ntum yield of hydrogenated amorphous Si (a-Si:H) p(+)-i-n(+) photodiodes in creases with decreasing absorbed photon flux. The quantum yield can exceed unity. The dynamics of photogenerated charge carriers in the intrinsic laye r of a-Si:H were studied by intensity-modulated photocurrent spectroscopy ( IMPS). A model is proposed, which accounts for the enhanced quantum yield a t low light intensities. Basically, the increase in the quantum yield is ex plained by thermal generation of charge carriers mediated by a one-electron defect state formed under illumination. The model is supported by the resu lts of IMPS measurements.