High Efficiency Hydrogenated Nanocrystalline Silicon Solar Cells Deposited at High Rates
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High Efficiency Hydrogenated Nanocrystalline Silicon Solar Cells Deposited at High Rates Guozhen Yue, Laura Sivec, Baojie Yan, Jeffrey Yang, and Subhendu Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI 48084, U.S.A. ABSTRACT We report recent progress on hydrogenated nanocrystalline silicon (nc-Si:H) solar cells prepared at different deposition rates. The nc-Si:H intrinsic layer was deposited, using a modified very high frequency (MVHF) glow discharge technique, on Ag/ZnO back reflectors (BRs). The nc-Si:H material quality, especially the evolution of the nanocrystallites, was optimized using hydrogen dilution profiling. First, an initial active-area efficiency of 10.2% was achieved in a nc-Si:H single-junction cell deposited at ~5 Å/s. Using the improved nc-Si:H cell, we obtained 14.5% initial and 13.5% stable active-area efficiencies in an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. Second, we achieved a stabilized total-area efficiency of 12.5% using the same triple-junction structure but with nc-Si:H deposited at ~10 Å/s; the efficiency was measured at the National Renewable Energy Laboratory (NREL). Third, we developed a recipe using a shorter deposition time and obtained initial 13.0% and stable 12.7% active-area efficiencies for the same triple-junction design. INTRODUCTION nc-Si:H is a good candidate for high efficiency multi-junction solar cells because of its superior long wavelength response and improved stability as compared to hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) alloys. However, its indirect bandgap inherently results in lower absorption coefficients; thus a thicker intrinsic layer is required to achieve high photocurrents. It is known that thick intrinsic layers will weaken the built-in electrical field, reduce carrier collection, and consequently decrease the fill factor (FF) of solar cells. A good light trapping technique using a textured BR can enhance the short-circuit current density (Jsc) without the need of increasing the intrinsic layer thickness; thus playing a critical role in improving the efficiency of nc-Si:H cells. However, nc-Si:H materials deposited on textured surfaces usually have a high defect density due to crystallite collisions [1,2] and result in a poor cell performance. An optimized BR surface morphology is needed to minimize this effect. In previous work [3,4], we systematically studied the effect of Ag and ZnO texture and thicknesses on cell performance and found that textured Ag with thin ZnO was the most desirable BR structure for nc-Si:H. Using such an optimized BR, we have further optimized the deposition parameters for nc-Si:H solar cells and significantly improved efficiencies for single-junction and a-Si:H/nc-Si:H/nc-Si:H triple-junction. In this paper, we report our recent progress on nc-Si:H based cell efficiencies made at different rates. In addition, a comparison study of the stability against prolonged light-soaking for nc-Si:H single- and triplejunction solar cells made at d
