Carrier recombination and differential diode quality factors in the dark forward bias current-voltage characteristics of a-Si:H solar cells

Abstract

A careful study has been carried out on dark forward bias current-voltage characteristics in high-quality well-controlled a-Si:H solar cell structures. Contributions of potential barriers in the intrinsic layers adjacent to the p and n contacts on carrier injection have been clearly identified and carrier recombination in the p∕i regions systematically controlled and clearly separated from that in the bulk of the intrinsic layers. It is found that the recombination in the p∕i regions results in voltage-independent diode quality factor, n, with values very close to 1 whereas recombination in the bulk results in bias-dependent differential diode quality factors, n(V). These n(V) characteristics are consistent with Shockley-Read-Hall recombination through a continuous distribution of gap states in the intrinsic layers which have spatially uniform distributions of gap states and electric field. Based on an analytical model the n(V) characteristics are interpreted in terms of Gaussian-like energy distributions of gap states in both undiluted and diluted protocrystalline a-Si:H intrinsic layers. Gaussian-like distributions are identified centered around as well as ∼0.3eV away from midgap with differences in their distributions for the two materials in the annealed states and their evolution upon introducing light-induced defects. These results demonstrate that forward bias dark currents and, in particular, n(V) characteristics offer a reliable probe for characterizing the gap states of the native- and light-induced defect states in a-Si:H solar cells as well as mechanisms limiting their performance.