OBSERVATION AND EXPLANATION OF QUANTUM EFFICIENCIES EXCEEDING UNITY IN AMORPHOUS-SILICON SOLAR-CELLS

The experimental observation of internal quantum efficiencies above un ity in crystalline silicon solar cells has brought up the question whe ther the generation of multiple electron/hole pairs has to be taken in to consideration also in solar cells based on direct gap amorphous sem iconductors. 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 regio n at low illumination intensities it has been observed that the DC qua ntum yield and the low frequency limit of the AC quantum yield Y incre ases significantly above unity with decreasing light intensity, indica ting that more than one electron per photon is detected in the externa l circuit. This phenomenon can occur due to the presence of dangling b ond defect centers in the band gap, which have an amphoteric character and can enhance the photocurrent due to trapping and thermal emission processes of photogenerated carriers.