Characterization of the Evolution in Metastable Defects Created by Recombination of Carriers Generated by Photo-generati
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0910-A02-02
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 Jingdong Deng1, Benjamin Rross1, Mathew Albert1, Robert Collins2, and Christopher Wronski1 1 Center for Thin Film Materials and Devices, Pensylvania State University, University Park, 16802 2 University of Toledo, Toledo, 43606 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 t1/3 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 t1/2 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, Nms, dNms / dt ~ (intensity)2 / Nms, which is suggestive of new creation mechanisms for SWE defects. INTRODUCTION Despite extensive studies in the past, SWE remains a big challenge in the further development of high performance hydrogenated amorphous silicon (a-Si:H) solar cells. The extensive studies on SWE carried out on thin films have established that recombination of photogenerated carriers is responsible for the creation of light induced metastable defects [1]. Insights have also been obtained from studies on solar cells such as the presence of “fast” and “slow” states, with the effect of their relative densities on the rates of degradation as well as the degraded steady state [2, 3, 4]. However since these studies were carried out with measurements of cell efficiencies or fill factors, such results could not be directly related to the densities of defect states in the intrinsic layers so it was not possible to characterize in detail the kinetics of their changes during the degradation. Recently it has been shown that in solar cells, in which the recombination at the p/i interface is sufficiently reduced, the dark current forward bias (JD-V) characteristics can be directly related to the gap states in the intrinsic layer [5] and this has been utilized in characterizing the gap states
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