Elementary Degradation Aspects of a-Si Solar Cells
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ELEMENTARY DEGRADATION ASPECTS OF a-Si SOLAR CELLS W. KUSIAN•, H. PFLEIDERER' AND B. BULLEMER• *Siemens Research Laboratories, Otto-Hahn-Ring 6, D-8000 Mfnchen 83 **Fed. Armed Forces University Munich, D-8014 Neubiberg ABSTRACT The degradation of amorphous silicon (a-Si) pin solar cells under prolonged illumination (Stabler-Wronski-Effect SWE) is investigated. We first measured the spectral response due to low-intensity illumination. The external collection efficiency Q is a function of the light wavelength A and the applied voltage U. The functions Q(U) saturate for increasing reverse bias. Normalization to the saturation values defines the internal collection efficiency q. The functions q(U) for different A cross through a nodal point with coordinates Uf, qf. The voltage Uf represents the flat-band voltage of the uniform-field model. Our first SWE aspect consists of a substantial qf reduction caused by enhanced photocarrier recombination near the pi- and in-junctions. We additionally measured the photocharacteristics due to white illumination at a variable intensity f. The photocurrents vanish at the transition voltage Ut which is independent of f for an undegraded cell. With advancing degradation, Ut decreases but Ut(,) becomes an increasing function. The latter effect represents our second degradation aspect, supplementing the first one. INTRODUCTION The efficiency of an a-Si (amorphous silicon) pin solar cell degrades under prolonged AMI illumination. The original efficiency is restorable by annealing. The annealed state "A" of an a-Si solar cell has to be distinguished from a more or less illumination-stressed state "B". We highlight the degradation process, the A-B transition, by observing the variation of two complementary aspects of the photocurrent. The photocurrent I p represents the difference between the light and dark currents at any applied voltage U /l/. An I < 0 is called primary and an Ip > 0 secondary. The condition I" = 0 defines the transition voltage Ut /2/. We measure the I -U characteristics for two particular cases: Firstly, under-weak monochromatic illumination by light of different wavelengths A and secondly, under white light of different intensities f. In the first case, all the characteristics, when plotted in a suitable way, cross through a nodal point that shifts with the transition from A to B. In the second case, state A is distinguished by Ut(f) = const. With the transition to state B, Ut generally decreases, but Ut(.) becomes an increasing function. These degradation aspects are interpreted by means of the uniform-field model /3/ for charge carrier collection. We investigated a standard pin cell /4/. The B state was commonly reached after 64h at AMI illumination. The Ip(A) and I (f) measurements were performed with the lock-in technique (chopper frequency 182 Hz) in order to suppress the dark current. The highest f level was AM1/100, but Ut(f)=const in state A holds up to the AMI intensity /1/. EXPERIMENT Normalization of Ip (,U) to the incoming photon flux yields the external co
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