Comparative Study of MVHF and RF Deposited Large Area Multi-junction Solar Cells Incorporating Hydrogenated Nano-Crystal
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1153-A22-02
Comparative Study of MVHF and RF Deposited Large Area Multi-junction Solar Cells Incorporating Hydrogenated Nano-Crystalline Silicon Xixiang Xu1, Yang Li1, Scott Ehlert1, Tining Su1, Dave Beglau1, David Bobela2, Guozhen Yue1, Baojie Yan1, Jinyan Zhang1, Arindam Banerjee1, Jeff Yang1, and Subhendu Guha1 1 United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084, U.S.A. 2 National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401, U.S.A.
ABSTRACT We report our investigations of large area multi-junction solar cells based on hydrogenated nano-crystalline silicon (nc-Si:H). We compared results from cells deposited by RF (13.56 MHz) at lower deposition rate (~3 Å/s) and by Modified Very High Frequency (MVHF) at higher rate (≥ 10 Å/s). With optimized process conditions and cell structures, we have obtained ~12% initial small active-area (~0.25 cm2) efficiency for both RF and MVHF cells and 10~11% large aperture-area (~400 cm2) encapsulated MVHF cell efficiency for both aSi:H/nc-Si:H double-junction and a-Si:H/nc-Si:H/nc-Si:H triple-junction structures on Ag/ZnO coated stainless steel substrate.
INTRODUCTION Hydrogenated nano-crystalline silicon (nc-Si:H) thin films have been widely used in multi-junction silicon based thin film solar cells. Compared with hydrogenated amorphous silicon (a-Si:H) thin films, nc-Si:H thin films have superior long wavelength response and low light-induced degradation [1-3]. These films are materials of choice for middle and bottom cells in multi-junction solar cells. In order to achieve higher light absorption at longer wavelengths, nc-Si:H cells must be much thicker than the a-Si:H based cells. Typical thickness of the nc-Si:H based multi-junction cell structure ranges from 2 to 4 µm, while the a-Si:H based cells are typically ~0.5 µm thick. Such a thick cell requires higher deposition rate for nc-Si:H to be practical for manufacturing. Currently, the most commonly used method to increase deposition rate for plasma enhanced chemical vapor deposition (PECVD) is to use MVHF (30 to 150 MHz) excited plasma instead of RF (13.56 MHz) plasma. Key issues of using MVHF are: 1) whether one can achieve good spatial uniformity over a large area at higher rate; and 2) whether the performance of MVHF deposited high rate solar cells is comparable to RF deposited lower rate solar cells. Previously, we have reported achieving good spatial uniformity over large areas using MVHF PECVD process [2]. In this work, we present results of achieving high deposition rate (≥ 10Å/s) and high quality nc-Si:H over a large area using the MVHF technique. We also present results from a comparative study on multi-junction cells incorporating nc-Si:H component cells deposited by RF and MVHF techniques.
EXPERIMENTAL Large area multi-junction solar cells were deposited in batch deposition systems with the substrate dimension of 15” x 14”. Both RF (13.56 MHz) and MVHF (40-100 MHz) techniques were used for the comparative study. Details of the deposition system can be found elsewhere [2]. Typical
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