Photoresponse in amorphous semiconductors as dark conductivity varies

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Abstract
The variations in conductivity, under illumination of amorphous semiconductors, is examined as a function of the dark Fermi level (EF) using a simple, generally accurate approach which eliminates the need for a complete numerical solution of equations covering generation, recombination, and trapping. The enhanced photoconduction on doping is readily quantified along with the reduced photosensitivity (total to dark conductivity ratio). Various models can be more readily assessed. Predictions are examined in detail for the often used exponential gap state model. The associated Rose model result for the intensity dependence is seen to be an asymptotic limit which becomes less applicable as dark conductivity increases. Temperature dependence is also examined. Emphasis is on doping, but contrasts are also drawn between the effects of a homogeneous and inhomogeneous shift in dark activation energy. The reduction in the exponent γ for the intensity dependence is far more pronounced when external fields modify the dark conductivity.