Light-Soaking Effects on the Open-Circuit Voltage of a-Si:H Solar Cells
- PDF / 139,480 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 59 Downloads / 209 Views
A13.6.1
Light-Soaking Effects on the Open-Circuit Voltage of a-Si:H Solar Cells Jianjun Liang,1 E. A. Schiff,1 S. Guha,2 B. Yan,2 and J. Yang2 1 Department of Physics, Syracuse University, Syracuse, NY 13244-1130 USA 2 United Solar Ovonic Corp., Troy, MI 48084 USA ABSTRACT We present measurements on the decline of the open-circuit voltage VOC in a-Si:H solar cells during extended illumination (light-soaking) at 295 K. We used a near-infrared laser that was nearly uniformly absorbed in the intrinsic layer of the cell. At the highest photogeneration rate (about 2x1021 cm-3), a noticeable decline (0.01 V) occurred within about 10 minutes; VOC stabilized at 0.04 V below its initial value after about 200 hours. We found that both the kinetics and the magnitudes of VOC are reasonably consistent with the predictions of a calculation combining a bandtail+defect picture for recombination and a hydrogen-collision model for defect generation. The version of the hydrogen-collision model that we used assumes that only bandtail recombination drives the hydrogen collision processes. Within this picture, the crossover between bandtail and defect recombination occurs on the same timescale as the “light-induced annealing” process that accounts for stabilization of the optoelectronic properties for long lightsoaking times. INTRODUCTION The metastable degradation of the optoelectronic properties of a-Si:H has been actively studied for more than 25 years. Important progress has been made by many researchers, but a fully satisfactory picture for the effect has never emerged. The difficulty of the metastability problem may, at least in part, be ascribed to the fact that there are probably two difficult, linked problems that must be understood simultaneously. First, it is likely that photocarrier recombination drives metastability, but - even for a single light-soaking state of an a-Si:H sample - no consensus viewpoint on these recombination processes has emerged. Second, even if one accepts the role of recombination in mediating metastability, there is no consensus viewpoint about how recombination leads to degradation by defect generation or other metastabilities. The present paper exploits the close connection between the open-circuit voltage VOC measured in an nip solar cell and photocarrier recombination in the bulk, intrinsic-layer material. In particular we report a series of measurements of the time-dependent degradation of VOC in United Solar cells under near-solar illumination. In recent work [1] we reported the temperaturedependence of VOC at solar illumination intensities. We concluded from these studies that, for their as-deposited state, the recombination traffic in our samples flowed predominantly through the valence bandtail states. For the stabilized, light-soaked state recombination traffic appears to be split fairly evenly between the valence bandtail and defects. We emphasize that these conclusions apply to our materials, under near-solar illumination, and near room-temperature; these are, of course, the conditions of
Data Loading...
