Microcrystalline Silicon Solar Cell Deposited Using Modified Very-High-Frequency Glow Discharge and Its Application in M
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Microcrystalline Silicon Solar Cell Deposited Using Modified Very-High-Frequency Glow Discharge and Its Application in Multi-junction Structures Guozhen Yue, Baojie Yan, Jessica M. Owens, Jeffrey Yang, and Subhendu Guha United Solar Ovonic Corporation, 1100 West Maple Rd., Troy, MI 48084, U.S.A. ABSTRACT We have used the modified very-high-frequency glow discharge technique to deposit hydrogenated microcrystalline silicon (µc-Si:H) solar cells at high rates for use as the bottom cell in a multi-junction structure. We have investigated µc-Si:H single-junction, a-Si:H/µc-Si:H double-junction, and a-Si:H/a-SiGe:H/µc-Si:H triple-junction solar cells and achieved initial active area efficiencies of 7.7%, 12.5%, and 12.4%, respectively. Issues related to improving material properties and device structures are addressed. By taking advantage of a lower degradation in µc-Si:H than a-Si:H and a-SiGe:H alloys, we have minimized the light induced effect in multi-junction structures by designing a bottom-cell-limited current mismatching. As a result, we have obtained a stable active-area cell efficiency of 11.2% with an a-Si:H/aSiGe:H/µc-Si:H triple-junction structure. INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) has received great interest as a low cost replacement for a hydrogenated amorphous silicon germanium (a-SiGe:H) bottom cell in a multijunction structure. The main reason for this is its better stability against light soaking and higher current capability resulting from an excellent long wavelength response compared with aSiGe:H. The elimination of germane from the fabrication process also reduces manufacturing cost. However, due to the nature of the indirect bandgap in the crystalline phase, the absorption coefficient of µc-Si:H in the visible part of the solar spectrum is low. A relatively thick intrinsic layer (> 1 µm) is usually needed for obtaining a high current. Therefore, from a manufacturing point of view, a high rate deposition technique for µc-Si:H solar cells is essential for increasing the throughput and reducing the production cost. Although many high rate methods have been investigated, only two methods appear to be successful - very high frequency (VHF) glow discharge [1-2] and RF glow discharge in a high pressure depleting regime [3-4]. In our previous study, we have shown that, by using a modified very high frequency (MVHF) glow discharge, the deposition rate can be increased by a factor of ten in making a-Si:H solar cells without compromising the cell performance [5]. Using the same technique, we previously have obtained an initial active-area cell efficiency of 12.3% and a stable efficiency of 10.4% using an aSi:H/µc-Si:H double-junction structure [6]. In this paper, we present our recent improvements for the µc-Si:H single-junction solar cell and its application in multi-junction structures. In addition, the stability against light soaking for a-Si:H/a-SiGe:H/µc-Si:H triple-junction solar cells is also presented. EXPERIMENTAL A multi-chamber system with three RF chambers and
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