Large-Area Hydrogenated Amorphous and Microcrystalline Silicon Double-Junction Solar Cells
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Large-Area Hydrogenated Amorphous and Microcrystalline Silicon Double-Junction Solar Cells Baojie Yan, Guozhen Yue, Arindam Banerjee, Jeffrey Yang, and Subhendu Guha United Solar Ovonic Corporation, 1100 West Maple Road, Troy, Michigan 48084 ABSTRACT Hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µcSi:H) double-junction solar cells were deposited on a large-area substrate using a RF glow discharge technique at various rates. The thickness uniformity for both a-Si:H and µc-Si:H is well within ± 10% and the reproducibility is very good. Preliminary results from the large-area a-Si:H/µc-Si:H double-junction structures show an initial aperture-area efficiency of 11.8% and 11.3%, respectively, for 45 cm2 and 461 cm2 size un-encapsulated solar cells. The 11.3% cell became 10.6% after encapsulation and stabilized at 9.5% after prolonged light soaking under 100 mW/cm2 of white light at 50 °C. High rate deposition of the µc-Si:H layer in the bottom cell was made using the high-pressure approach. An initial active-area (0.25 cm2) efficiency of 11.3% was achieved using an a-Si:H/µc-Si:H double-junction structure with 50 minutes of µc-Si:H deposition time. INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) as a long wavelength absorber layer in the bottom cell of hydrogenated amorphous silicon (a-Si:H) based multi-junction solar cells has attracted significant attention because of its higher current capability and better stability against light soaking [1]. In recent years, significant progress has been made in many groups [2-8]. At United Solar, we have explored various techniques for µc-Si:H deposition at different rates [6-8]. We have shown an initial efficiency of 13.4% with an a-Si:H/µc-Si:H double-junction cell and 13.6% with an a-Si:H/a-SiGe:H/µc-Si:H triple-junction cell with an active-area of 0.25 cm2 [8]. In order to use the multi-junction structure with µc-Si:H in the bottom cell, an increase in the deposition rate and improvement of the uniform deposition on large area substrates are two major issues that need to be investigated. In this paper, we will present our recent progress on the development of large-area a-Si:H/µc-Si:H double-junction solar cells and modules. EXPERIMENTAL µc-Si:H films and solar cells were deposited using a multi-chamber RF glow discharge system with gas mixtures of H2 and SiH4 on substrates with an area of 35 × 33 cm2. a-Si:H/µcSi:H double-junction solar cells with an n-i-p structure were deposited on Ag/ZnO back reflector coated stainless steel substrates. The thicknesses of the intrinsic a-Si:H and µc-Si:H layers are about 2000-3000 Å and 1-2 µm, respectively. Indium-Tin-Oxide was deposited on top of the p layer as a transparent contact. Metal grids or wires were used to reduce the series resistance and improve the collection of current. The thickness uniformity was measured on different locations using an optical method, and the uniformity of cell performance was characterized by evaluating small-area cells defined by ITO dots with a
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