Evolution of Metastable Defects in Intrinsic Layers of A-Si:H Solar Cells and Corresponding Thin Film Materials Characterized by Carrier Recombination Through Midgap States

Direct correlations have been obtained in the evolution of metastable defects in the intrinsic layers of protocrystalline p-i-n a-Si:H solar cells and corresponding thin film material by characterizing their carrier recombination through states located around midgap. For thin film material, this is carried out with photoconductivity measurements with illuminations from 1 to 10^-6 sun; for solar cells, this is done with the Shockley-Reed-Hall recombination in the dark under low forward bias. Because these were carried out in the absence of isothermal annealing so as to characterize the actual creation kinetics, the contributions of different concentration in the "fast/soft" and "slow/hard" defect states could be reliably characterized. The changes in recombination exhibit regions with the extensively reported t^1/3 dependence. However, when the recombination from just the metastable defects is separated out, their evolution exhibits an exponent very close to 0.5. This, however, is not the case for undiluted a-Si:H films, whose exponents are close to 0.7. The significance of these results for identifying possible causes that lead to their difference in the stability as well as on the various models proposed for SWE are discussed