A simplified treatment of recombination, quasi-Fermi levels and photoconductivity in pure (undoped) amorphous semiconductors is presented assuming a symmetrical Cohen-Fritzsche-Ovshinsky distribution of localized bandgap states, with donorlike states of density decreasing exponentially with increases in energy from the valence band (mobility) edge: and acceptorlike states of exponentially decreasing density with decreases in energy from the conduction band edge. Recombination proceeds by Shockley-Read-Hall capture processes, but due account is taken of the dependence of cross sections on whether Coulombic or neutral capture is present in each case The ratio of photoconductivity to dark conductivity (σph/σdark) is calculated for parameters expected to correspond to (hdrogenated) intrinsic amorphous silicon. The dependence of this ratio upon the photogeneration rate (Gph) involves a single adjustable parameter, which represents the rate of decay of the density of gap states with energy away from the band edges. Comparison of experimental data for σph/σdark vs. Gph with this model provides for an estimate of the gap-state energy distribution.
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