Comparison Study of a-SiGe Solar Cells and Materials Deposited Using Different Hydrogen Dilution
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Comparison Study of a-SiGe Solar Cells and Materials Deposited Using Different Hydrogen Dilution H. Povolny, P. Agarwal, S. Han, and X. Deng Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606 ABSTRACT A-SiGe n-i-p solar cells with i-layer deposited via plasma enhanced chemical vapor deposition (PECVD) with a germane to disilane ratio of 0.72 and hydrogen dilution R=(H2 flow)/(GeH4+Si2H6 flow) values of 1.7, 10, 30, 50, 120, 180 and 240 were deposited on stainless steel substrates. This germane to disilane ratio is what we typically use for the i-layer in the bottom cell of our standard triple-junction solar cells. Solar cell current-voltage curves (J-V) and quantum efficiency (QE) were measured for these devices. Light soaking tests were performed for these devices under 1 sun light intensity at 50o C. While device with R=30 showed the highest initial efficiency, the device with R=120 exhibit higher stabilized efficiency after 1000 hours of light soaking. Single-layer a-SiGe films (~500 nm thick) were deposited under the same conditions as the i-layer of these devices on a variety of substrates including 7059 glass, crystalline silicon, and stainless steel for visible-IR transmission spectroscopy, FTIR, and hydrogen effusion studies. It is interesting to note 1) the H content in the film decreased with increasing R based on both the IR and H effusion measurements, and 2) while the H content changes significantly with different R, the change in Eg is relatively small. This is most likely due to a change in Ge content in the film for different R. INTRODUCTION Although the correlation between amorphous silicon based solar cell device performance and a variety of intrinsic layer (i-layer) materials properties have been studied broadly [1-3], a reliable correlation between i-layer properties and device performance is not yet well established. This is partially due to the different ways used for depositing these materials. H dilution has been used extensively to improve a-Si based solar cell materials and device quality [4-6]. However, the exact role of H dilution to the growth of a-Si alloys, in particular narrow bandgap a-SiGe materials, and its impact on the solar cell device performance and single-layer film properties are not yet fully understood and deserve further study. In this paper, we describe our recent studies of a-SiGe n-i-p solar cells and films deposited under a wide range of H dilution. We were particularly interested in the effect of the different hydrogen dilution on long-term stability of these solar cells. EXPERIMENT A-SiGe films, approximately 0.5 µm thick, were deposited on 7059 glass, c-Si substrates and stainless steel (SS) using a hydrogen dilution ratio R of 1.7, 10, 30, 50, 120, 180 and 240 using PECVD. Optical measurements were performed for samples on 7059 glass substrates to obtain sample thickness, refractive index and the bandgap. FTIR absorption measurements were taken for samples on c-Si substrates to obtain information on the H content and bonding. H
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