Light-Induced Defects in Medium and Low Gap a-Si,Ge:H,F Alloys
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LIGHT-INDUCED DEFECTS IN MEDIUM AND LOW GAP a-Si,Ge:H,F ALLOYS*)
V. CHU, J.P. CONDE, D.S. SHEN, S. ALJISHP) AND S. WAGNER Department of Electrical Engineering Princeton University, Princeton, New Jersey 08544
ABSTRACT A systematic study of the electronic and optical properties of a-Si,Ge:H,F alloys in the initial and light-soaked states is presented. The three alloys have optical gaps of 1.20, 1.34 and 1.40eV. Constant photocurrent method (CPM) spectra show that the higher gap samples begin to exhibit increased subgap absorption due to light soaking much sooner than the low gap sample. Dark arid photoconductivity remain essentially constant as functions of light soaking time up to approximately 100 hours of illumination, when the dark conductivity begins to increase and the photoconductivity begins to decrease simultaneously. The electron mobility-lifetime product measured by the time-of-flight technique shows much scatter but no significant change with light-soaking. The hole mobility-lifetime product on the other hand decreases immediately upon light soaking and continues to decrease gradually with increasing illumination time. A discussion of these effects and a possible explanation in terms of the density of states is presented.
INTRODUCTION Reversible, light-induced changes in the properties of a-Si:H, called the Staebler-Wronski effect [1], have been studied extensively since they were observed and reported in 1977. However, the process of light-induced degradation in a-Si,Ge:H alloys is still a mystery. Early work in the alloys found little light-induced degradation, due to high initial defect densities [2-5]. With the improved quality of recent alloys, the realistic prospect of making an efficient low gap solar cell has made the Staebler-Wronski effect in these materials an important issue. Aljishi ot al. found that the high and medium gap alloys (E.,, ý1.4eV) seem to degrade by a mechanism different from that which controls degradation in the low gap alloys (E,, 0 00
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decrease continues for subsequent illumination. This implies that the light-induced defects act as efficient hole recombination centers. However, in the case of RC295, there is no corresponding increase in sub-gap absorption until 200 hours of illumination. According to the Shockley-Read-Hall theory of indirect recombination, r = 1/N, t r where N, is the number of available recombination centers, vCAis the carrier thermal velocity, and a is the capture cross-
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