Investigation of collection efficiencies much larger than unity in a-Si:H p-i-n structures

Abstract

An apparent quantum efficiency much larger than unity is observed under reverse bias voltage conditions, in hydrogenated amorphous silicon p-i-n structures. High collection efficiencies are measured for low-level probe illumination of the device n side with red light, with simultaneous bias illumination from the device p side with strongly absorbed blue light. The photogating effect responsible varies experimentally with reverse bias voltage, and collection efficiencies for the probe excitation of up to 50 are obtained. Detailed computer simulations corroborate such high values of quantum efficiency and the underlying mechanisms for the effect are revealed. We present the influence on quantum efficiency of bias light wavelength and photon flux, probe light photon flux, applied voltage, and defect density.

Intensity dependence of quantum efficiency and photo‐gating effects in thin film silicon solar cells

“…Computer simulations used in support of this work have been described in previous publications [6,10]. Here we have used the 'standard' defect model, with a spatiallyindependent Gaussian distribution of deep states.…”

“…We hope to discover more about the changes in electronic properties which accompany changes in composition, particularly for lowcrystallinity materials which hitherto have not been studied widely, and under conditions more relevant to solar cell operation. One technique we employ is 'photogating' [6,7], in which a photodiode is subject to simultaneous illumination by a strongly-absorbed 'bias' beam, and a more weakly absorbed 'probe' beam, such that the increase in terminal photocurrent due to the probe beam in the pres- ence of the bias beam, ΔI P+B , exceeds that due to the probe beam alone, ΔI P . This may be quantified in terms of a collection efficiency:…”

Recent progress in the preparation and application of quantum dots/graphene composite materials

“…They also demonstrated that the photogating effect was responsible for quantum efficiencies greater than unity. The photogating effect was further analyzed in forward biased a-Si:H Schottky barrier structures, 37 reverse biased a-Si:H based p-i-n junctions 38 and microcrystalline silicon (mc-Si:H) based n-i-p devices. 39 It was found that the photogating effect was derived from band tail states in the conduction and valence bands of semiconductors, which were caused by structural defects and disordered interfaces.…”

Photogating effect as a defect probe in hydrogenated nanocrystalline silicon solar cells

“…5 Most of the reports about the PG effect published in the literature refer to amorphous silicon ͑a-Si:H͒ based thin film structures. [3][4][5][6][7] In single p-i-n solar cells the effect can be observed in samples degraded by light soaking, 3,5 or in an a-Si:H p-i-n structure with a very thick ͑up to a few micron͒ i-layer. 7 In Schottky barriers it was observed also in the annealed state at moderate forward voltages.…”

“…[3][4][5][6][7] In single p-i-n solar cells the effect can be observed in samples degraded by light soaking, 3,5 or in an a-Si:H p-i-n structure with a very thick ͑up to a few micron͒ i-layer. 7 In Schottky barriers it was observed also in the annealed state at moderate forward voltages. 6 Enhanced QE were not only measured in thin film silicon devices but also on other structures like CdS/CdTe solar cells.…”