Equilibrium Between Charge Carriers, Charged Centers, and Dangling Bonds in Amorphous Silicon
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EQUILIBRIUM BETWEEN CHARGE CARRIERS, CHARGED CENTERS, AND DANGLING BONDS IN AMORPHOUS SILICON C. FORTMANN Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown, PA 18940 ABSTRACT The recombination rate dependence of the high temperature saturated dangling bond density is explored. Solar cell RBA (reverse bias anneal) and FBA (forward bias anneal) are studied as a function of temperature and bias voltage. The defects that are responsible for the SWE (Staebler-Wronski effect)[1] are shown also to be related to the RBA and FBA effects. At annealing temperatures greater than 150 0 C, forward bias increases the density of dangling bonds in the i-layer, while reverse bias decreases the density. The temperature dependence of FBA and RBA are investigated. The density of dangling bonds was found not to be a function of cooling rate when bias is maintained during cooling. The dangling bond density is proportional to the square root of the recombination rate, rather than the linear relationship predicted by simple kinetic models. The results are more consistent with a equilibrium description of the SWE. INTRODUCTION The effect of RBA has been used to enhance the performance of amorphous silicon based solar ce~ls [2]. Field effects including RBA in a-Si:H have been described by Smith et al. [3]. The RBA effect has been attributed to effects in doped layers [4]. In this work the possibility that the RBA effect is related to changes in the i-layer is investigated. The degradation of a-Si:H is also considered. The rate of degradation due to the SWE in a-Si:H has been related to the recombination rate of carriers L5], and a balance of recombination driven degradation, thermal annealing, recombination driven annealinq [b], and with thermal degradation also taken into account [7]. In general these recombination rate degradation models lead to a high temperature saturated concentration of dangling bonds that is proportional to the recombination rate. For simplicity, the dangling bond is assumed to be the SWE related defect, although there is not complete agreement on this identification. The relationship between recombination and degradation was explored at temperatures great enough kT>15nOC) to insure saturation in reasonable period of time (
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