Characterization of the Evolution in Metastable Defects Created by Recombination of Carriers Generated by Photo-generation and Injection in p-i-n a-Si:H Solar Cells

Abstract

The creation of metastable defects in the bulk of the intrinsic layers of a-Si:H p-i-n solar cells by recombination of photo-generated carriers as well as those injected under far forward bias has been investigated. The evolutions in the defects, created with volume-absorbed red light at open circuit voltage and constant far forward bias currents, were characterized with the Shockley-Reed-Hall recombination obtained from the dark currents under low forward bias voltages. This allowed the kinetics to be studied at different temperatures in the absence of isothermal annealing. It is found that the form of the kinetics obtained with the two methods are the same and a detailed study was carried out with currents from 1 to 100mA/cm² with carrier injection during which the electron and hole concentrations remain essentially constant. This kinetics is similar to that reported for thin film results exhibiting stretched exponential behavior with a t^1/3 relation over a limited regime before the onset of saturation. By separating the contributions of the metastable defects from the intrinsic defects, their evolution is found to have a t^1/2 dependence over the entire range prior to the onset of saturation. Since these kinetics also exhibit an (intensity)²t dependence they point to a rate equation for the creation of metastable defects, N_ms, dN_ms / dt ~ (intensity)² / N_ms, which is suggestive of new creation mechanisms for SWE defects.