A Comparison of the Degradation and Annealing Kinetics in Amorphous Silicon and Amorphous Silicon-Germaniumsolar Cells
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degradation and annealing kinetics, we also performed some experiments to examine the diffusion of hydrogen in these materials. EXPERIMENT The a-Si:H and a-SiGe:H cells in this study were all p-i-n diodes (0.27 cm2 in area) deposited on Asahi (type U) tin oxide coated glass using DC PECVD. The p-layers consisted of about 10 nm of boron-doped a-SiC:H, and the n-layers were phosphorus-doped a-Si:H (- 200 nm thick). A thin hydrogen-rich buffer layer (- 10 nm thick) was deposited between the p- and i-layers. The i-layers were deposited using 10:1 hydrogen dilution, and the germane to silane ratio was - 0.28 during the deposition of the a-SiGe i-layers. The back contacts were formed by sputter depositing - 100 nm of zinc oxide and then - 300 nm of aluminum. Accelerated light soaking was performed using a xenon arc lamp to illuminate the cells with high intensity white light (up to 60 suns), and the temperature was controlled by adjusting the flow of nitrogen gas that was directed onto the cell. The temperature of cells exposed to intense illumination was measured using a thin foil (0.0005") thermocouple that was cemented to the back contact of the cell with a high thermal conductivity, electrically insulating paste.
395 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
RESULTS We first looked at the relative rate of light-induced degradation in both a-Si:H and aSiGe:H single-junction cells when exposed to intense illumination (60 suns at 60'C) under open-circuit conditions. As shown in Fig. 1, the normalized conversion efficiency fell more slowly over the first 20 minutes for an a-SiGe:H cell as compared to an a-Si:H cell of comparable thickness (- 150 - 170 nm). While the a-Si:H cell tended to reach a steady state after about an hour of illumination, the a-SiGe:H cell did not exhibit saturation. When the thickness of the i-layer in the a-Si:H cell was increased from 150 to 300 nm, the cell degraded even more rapidly and took longer to saturate. We exposed some of the cells to 60 suns illumination at 60'C for times up to 73 hours. An a-Si:H cell with a 300 nm thick i-layer saturated at a normalized efficiency of- 0.6 after about 10 hours of illumination. However, the a-SiGe:H with a 170 nm thick i-layer exhibited a normalized efficiency of 0.48 after 73 hours and still showed no evidence of saturation. 1.0 Z
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TIME (min.) Fig. 1. The normalized conversion efficiency of a-Si:H and a-SiGe:H cells as a function of time exposed to 60 suns illumination at 60 °C under open circuit. Several experiments were performed to investigate the effects of strong reverse biases on the recovery of light-soaked a-SiGe:H single-junction cells [2]. Inthe first set of reverse bias on the annealing of degraded aexperiments, we looked at the effects of a strong Si:H and a-SiGe:H cells inth
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