Evolution of D 0 and non-D 0 Light Induced Defect States in a-Si:H Materials and Their Respective Contribution to Carrie
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Evolution of D0 and non-D0 Light Induced Defect States in a-Si:H Materials and Their Respective Contribution to Carrier Recombination J. M. Pearce, J. Deng, V. Vlahos, R. W. Collins, and C. R. Wronski Center for Thin Film Devices The Pennsylvania State University J. Whitaker and P. C. Taylor University of Utah A study has been carried out on the evolution of light induced defects in protocrystalline (diluted) a-Si:H films under 1 sun illumination. A room temperature reversal is observed in the photocurrents at 25˚C, which is consistent with the relaxation in the recombination currents on corresponding p-i-n solar cells. It is also consistent with the pressure of “fast” states such as have been observed after high intensity illumination. Even with the limitations imposed by the relaxation in the light induced changes on the subgap absorption measurements, the evolution of distinctly different gap states centered around 0.9 and 1.15eV from the conduction band was identified. The kinetics of the electron occupied states, kN(E), at these two energies is compared with that of the neutral dangling bond (D0) densities as measured with electron spin resonance. Because of the similarity between the preliminary results of these kinetics it has not been possible to identify which states correspond to the D0 nor to draw any reliable conclusions about the nature of the different states. INTRODUCTION Despite over twenty-five years of scientific studies on light induced defects in hydrogenated amorphous silicon (a-Si:H) materials and solar cells there are still unanswered questions regarding their nature. It was found early on that light induced degradation created dangling bonds. These defect states, and in particular that of the neutral dangling bond (D0) which can be identified and its densities measured with electron spin resonance (ESR), has received the most attention. There are, however, copious results reported on a-Si:H thin films and solar cells which clearly point to the presence of multiple light induced defects in a-Si:H. These include: A disproportion between light-induced changes in the defect densities as determined by optical absorption and those by ESR [1]; absence of consistent correlations between the changes in electron mobility-lifetime (µτ) products and those in subgap absorption [α(E)] [2,3,4]; and isochronal annealing between the recoveries of µτ and α(1.25eV) [5,6]. The presence of “fast” and “slow” defect states has been well established by the studies on the recoveries of the fill factors after degradation with high intensity illumination [7,8]. These studies also showed that those states created faster also anneal out faster. The presence of such defect states is also reflected in the degradation kinetics of cells and films under 1 sun illumination where the initial and subsequent regimes exhibit distinctly different dependence on temperature [9]. There is also evidence in the results on subgap absorption for the presence of multiple light induced defects indicated by their changes not only in
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