A computer simulation model of amorphous silicon solar cells using a Scharfetter and Gummel solution of Poisson’s equation and Taylor and Simmons occupancy statistics for the dangling bond gap states (D states), has been developed. With a suitable choice of parameters, the numerical results for solar cell collection efficiency and dark and illuminated I-V characteristics agree well with experimental values. The model has been used specifically to study the influence of interface states at the TCO-p (transparent conductive oxide), p-i, i-n, and n-metal interfaces and to explain the beneficial role of a graded-band-gap layer at the p-i interface.