A quantitative spacehyphen;charge limitedhyphen;current (SCLI) diode model for the rectifying currenthyphen;voltage properties of diodes with a discrete trap distribution is extended to the case of diode solar cells with a continuous distribution of traps in the energy band gap. Analyzing hydrogenated amorphous silicon (ahyphen;Si:H) solar cells indicates that their lighthyphen;exposed degradation causes two major changes in their high resistance, intrinsic regions. These are a sharp increase in the equilibrium and steadyhyphen;state carrier concentrations and a change in the distribution of traps in the energy band gap. The measured trap concentration densities span the 1014ndash;1018cmminus;3eVminus;1magnitude range at 0.8ndash;0.2 eV below the conductionhyphen;band edge. The measured carrier concentrations cover the 105ndash;1012cmminus;3range. The model indicates that higher device efficiencies are obtained when the quasihyphen;Fermi level in the high resistance region lies just above a peak in the trap concentration density in the band gap. This peak steadyhyphen;state density inahyphen;Si:H is above 1017cmminus;3eVminus;1and is located approximately 0.5ndash;0.6 eV below the conductionhyphen;band edge. Fermindash;Dirac statistics are employed in the model and quantitative explanations are given for the dark and light currenthyphen;voltage curves ofahyphen;Si:H under forward and reverse bias. Conditions are defined that predict maximumahyphen;Si:H fill factors in the 0.74ndash;0.76 range and device efficiencies at the 11percnt;ndash;13percnt; levels. The trap distributions that give higher device performance are defined, and a method for measuring these distributions is described and illustrated. The model indicates that the specific conductivity changes measured on lighthyphen;exposedahyphen;Si:H by Staebler and Wronski would tend to increase the fill factor and efficiency ofahyphen;Si:H solar cells.
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