Model for amorphous-silicon solar cells

Owing to the short lifetimes and low mobilities of carriers in amorphous silicon, diffusion is not the dominant mechanism responsible for determining the collection of photogenerated electrons and holes. It has been established that the photocurrent is determined by the photogeneration of carriers in the depletion region and their subsequent removal with the aid of the built-in field. This is confirmed here through an analysis of the p-i-n cell spectral response curve published by Carlson and Wronski. From the analysis, a value of 10-8 cm2/Vis estimated for the product of lifetime and mobility for photogenerated holes, which is consistent with a short diffusion length. A calculation is presented of the current/voltage characteristic for a model Schottky-barrier solar cell under illumination. This gives a short-circuit current in agreement with the measured values, and demonstrates the reduced fill factor which can be expected from the voltage dependence of the photocurrent. This model predicts an a.m.l efficiency of about 8%.