Space-charge-limited current diode model for amorphous silicon solar cells and their degradation

Abstract

A quantitative space-charge limited-current (SCLI) diode model for the rectifying current-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 (a-Si:H) solar cells indicates that their light-exposed degradation causes two major changes in their high resistance, intrinsic regions. These are a sharp increase in the equilibrium and steady-state carrier concentrations and a change in the distribution of traps in the energy band gap. The measured trap concentration densities span the 1014–1018 cm−3eV−1 magnitude range at 0.8–0.2 eV below the conduction-band edge. The measured carrier concentrations cover the 105–1012 cm−3 range. The model indicates that higher device efficiencies are obtained when the quasi-Fermi level in the high resistance region lies just above a peak in the trap concentration density in the band gap. This peak steady-state density in a-Si:H is above 1017 cm−3 eV−1 and is located approximately 0.5–0.6 eV below the conduction-band edge. Fermi–Dirac statistics are employed in the model and quantitative explanations are given for the dark and light current-voltage curves of a-Si:H under forward and reverse bias. Conditions are defined that predict maximum a-Si:H fill factors in the 0.74–0.76 range and device efficiencies at the 11%–13% 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 light-exposed a-Si:H by Staebler and Wronski would tend to increase the fill factor and efficiency of a-Si:H solar cells.