High Efficiency, Large Area, Nanocrystalline Silicon Based, Triple-Junction Solar Cells
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High Efficiency, Large Area, Nanocrystalline Silicon Based, Triple-Junction Solar Cells A. Banerjee, T. Su, D. Beglau, G. Pietka, F. Liu, B. Yan, J. Yang, and S. Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084, U.S.A. ABSTRACT We have fabricated large-area, thin-film multijunction solar cells based on hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) made in a large area batch reactor. The device structure consisted of an a-Si:H/nc-Si:H/nc-Si:H stack on Ag/ZnO back reflector coated stainless steel substrate, deposited using our proprietary High Frequency (HF) glow discharge technique. For the nc-Si:H films, we investigated two deposition rate regimes: (i) low rate 1 nm/s. We optimized the deposition parameters, such as pressure, gas flow, dilution, and power. We did SIMS analysis on the optimized films, and found the impurity concentrations were one order of magnitude lower than the films made with the conventional RF process. In particular, the oxygen concentration is reduced to ~10 18 cm-3. This value is among the lowest oxygen concentration reported in literature. The low impurity content is attributed to proprietary cathode hardware and the optimized deposition process. During the initial optimization and investigative phase, we fabricated small-area (0.25 cm2 and 1.1 cm2) cells. The information obtained from the initial phase was used to fabricate large-area (aperture area 400 cm2) cells, and encapsulated the cells using the same flexible encapsulants that are used in our commercial product. We have light soaked the low-rate and high-rate encapsulated modules. The highest initial efficiency of the low-rate modules is 12.0% as confirmed by NREL. The highest corresponding stable efficiency attained for the low-rate samples cells is 11.35%. For the high-rate small-area (1.1 cm2) cells, the highest initial activearea efficiency and corresponding stable efficiency attained are 13.97% and 12.9%, respectively. We present the details of the research conducted to develop the low- and high-rate cells and modules. INTRODUCTION nc-Si:H material is a promising candidate to replace a-SiGe:H in multijunction thin film silicon solar cells [1-3]. In view of its indirect bandgap, the nc-Si:H layer must be much thicker than its amorphous counterparts to effectively absorb the incident radiation. Typical thicknesses for a nc-Si:H based multijunction cell is 2-5 μm, compared with
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