Time Dependence of Spatial Defect Profiles in a-Si:H Solar Cells with Light-Soaking
- PDF / 357,271 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 98 Downloads / 172 Views
ABSTRACT We report on modeling the evolution of the spatial defect profile in the i-layer of a-Si:H solar cells combined with tests of the model against the efficiency decay of amorphous silicon (aSi:H) p-i-n solar cells under various light-soaking conditions. We use a finite-element device simulator. The defect density during light-soaking is calculated as a function of time and of position. The defect density evolves due to the combined effects of light-induced generation, assumed proportional to the product of free carrier densities, and of light-induced annealing, assumed proportional to the sum of carrier densities. We find that in thick cells defects close to the p-i interface affect cell efficiency less than bulk defects, and that in thin cells the defect density increases significantly in the central portion of the i-layer. However, in thin cells the high electric field masks the effect of the defect density increase. INTRODUCTION p-i-n a-Si:H solar cells lose efficiency under illumination [1, 2]. This degradation is ascribed either to an increase of the density of metastable defects in the intrinsic zone or to enhanced degradation of the p-i interface [3]. Recent work [4, 5] demonstrates that, using appropriate material parameters, the current-voltage (I-V) characteristics and quantum efficiency (QE) spectra before and after light-soaking can be explained well by an increase of the defect density in the active layer, without invoking additional interface losses. On the other hand, the defect evolution in i-layer films has been carefully investigated [6, 7, 8, 9], leading to a phenomenological description of both the creation and annealing of dangling bonds, which is able to explain many experimental results. In this paper, we apply these defect kinetics for undoped a-Si:H films under illumination to the defect evolution JLnthe i-layer of solar cells, including both light-induced generation and light-induced annealing processes. Since the defect kinetics depend on the carrier concentrations, defects will evolve as a function of position in the intrinsic zone. Various experimental light-soaking conditions are interpreted by using this model, and are discussed in detail. EXPERIMENT We carried out experiments on cells of the structure glass/textured SnO2/p+a-Si:H/i aSi:H/n+a-Si:H/A1. The i-layers were 2000 ,• and llam thick. The cells were light-soaked with blue (486 nm, 1.3 Wcm-2) or red (647 nm, 1.0 Wcm-2) light from a Kr+ ion laser in open circuit. The temperature of the cells, measured with a thermocouple, was around 50 °C under both light-soaking conditions. We characterized the degradation process by I-V at AM I.5 and QE without bias light, at each light-soaking step. The initial efficiencies of all cells lay around 8.5%.
669
Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
Different light-soaking conditions and i-layer thicknesses will create different defect profiles. In fact, the absorption of blue light is so high that the light is absorbed in the first -1000 A of 1 the intrin
Data Loading...
