The Creation and Annealing Kinetics of Fast Light Induced Defect States created by 1 Sun Illumination in a-Si:H
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A13.2.1
The Creation and Annealing Kinetics of Fast Light Induced Defect States created by 1 Sun Illumination in a-Si:H M.L. Albert1, J. Deng1, X. Niu1, J.M. Pearce2, R.W. Collins3, and C.R. Wronski1 1 Center for Thin Film Device, The Pennsylvania State University, University Park, PA 2 Physics Department, Clarion University of Pennsylvania, Clarion, PA 3 Physics Department, University of Toledo, Toledo, OH ABSTRACT Preliminary results are presented on the kinetics of fast states at 25°C created by 1 sun illumination in protocrystalline hydrogen diluted a-Si:H films. The results are for the bulk properties of the a-Si:H films which was confirmed by the similarity of results obtained on corresponding intrinsic layers in p-i-n solar cells. It is found that the kinetics exhibit two regimes. The first regime is in the form of a delay before the onset of an A·log(t) time dependence indicative of a dispersive process. Despite the unexpected effect of a dependence of this first regime on the degradation/annealing history of the samples, it was possible to characterize the highly reproducible logarithmic dependences for different illumination times as well as in the presence of different carrier generation/recombination rates. It is found that for the degradation times studied, the annealing kinetics associated with the second regime are independent of the 1 sun illumination time but are dependent on the recombination introduced by illuminations as low as ~10-5 of 1 sun. These fast states are located close to midgap, similar to the position of dangling bond defects. The results presented raise interesting questions that still need to be answered about the nature and origin of the fast defects in order to assess their contribution to the long term degradation and the overall stability a-Si:H materials. INTRODUCTION Metastable light induced degradation in hydrogenated amorphous silicon (a-Si:H) is still limiting progress in further improving the performance of its solar cells. There is a striking difference in the stability of a-Si:H films and cells between those fabricated with pure and hydrogen diluted silane. This has been attributed to the differences in “fast” and “slow” light induced defects established in studies on the annealing of cells after high intensity illumination[1,2]. It was found that the defects created faster also anneal out more rapidly and that more fast defects are created at a greater rate with higher light intensities. However, no direct information about the kinetics of fast defect creation under 1 sun illumination at room temperature and their annealing kinetics has yet been reported. Recently, J. Deng, et al.[3] provided direct evidence for the annealing out of such states at room temperature after 1 sun illumination of p-i-n cells from the observed relaxation of dark forward bias recombination currents in the i- layers. Subsequently, relaxation at room temperature was also identified in the photoconductivities of corresponding films, consistent with the results on solar cells [4]. We report preli
