Theoretical analysis of trapping and recombination of photogenerated carriers in amorphous silicon solar cells

Abstract

We have made theoretical studies on the trapping and recombination of photogenerated carriers in hydrogenated amorphous silicon (a-Si∶H) p-i-n solar cells. We discuss in detail the following points: 1) The limitations of the assumptions in the previous analysis. It has been clarified that the single-level Shockley-Read-Hall model for carrier recombination and the treatment of trap occupation in terms of quasi-Fermi levels are inadequate for exact analysis. 2) The superlinear dependence of carrier recombination rate on the free-carrier density which can explain the enhancement of photo-induced changes ina-Si∶H under high intensity light. 3) The estimation of capture cross section of the tail states ina-Si∶H. We show that the charged and neutral tail states have rather small capture cross sections of less than 10⁻¹⁶ cm² and of less than 10⁻¹⁹ cm², respectively. 4) The effect of the recombination of photogenerated (PG) carriers at the p/i and the n/i interfaces. We estimate the recombination velocityS of PG carriers at these interfaces to be about 10³ cm/s. It has been also clarified that the decrease inS is effective to improve the cell performance, especially the open circuit voltage.