High-efficiency Large-area Nanocrystalline Silicon Solar Cells Using MVHF Technology
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High-efficiency Large-area Nanocrystalline Silicon Solar Cells Using MVHF Technology Xixiang Xu1, Tining Su1, Scott Ehlert1, David Bobela2, Dave Beglau1, Ginger Pietka1, Yang Li1, Jinyan Zhang1, Guozhen Yue1, Baojie Yan1, Greg DeMaggio1, Chris Worrel1, Ken Lord1, 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 present the progress made in attaining high-efficiency large-area nc-Si:H based multi-junction solar cells using Modified Very High Frequency technology. We focused our effort on improving the spatial uniformity and homogeneity of nc-Si:H film growth and cell performance. We also conducted both indoor and outdoor light soaking studies and achieved 11.2% stabilized efficiency on large-area (≥400 cm2) encapsulated a-Si:H/nc-Si:H/nc-Si:H triplejunction cells. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H), with its superior long-wavelength response and light-soaking stability, has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multi-junction thin-film silicon solar cells [1,2]. However, because of its indirect bandgap, the thickness of nc-Si:H material must be larger than that of a-Si:H material in order to achieve sufficient light absorption at longer wavelengths. Typical thicknesses of the nc-Si:H based multi-junction cells are 2 to 5 µm, while the a-Si:H based cells are ≤0.5 µm thick. Therefore, the main challenges for using nc-Si:H in thin-film PV manufacturing are to (i) attain high deposition rates and (ii) achieve homogenous properties over a large deposition area [3]. In this work, we present the progress made in attaining over 11% stabilized large-area (≥400 cm2) encapsulated nc-Si:H based triple-junction cells at a high deposition rate. EXPERIMENTAL a-Si:H and nc-Si:H multi-junction solar cells were deposited by PECVD in batch deposition systems on Ag/ZnO coated stainless steel substrates. The substrate dimension was 15" x 14". Intrinsic a-Si:H and nc-Si:H layers were deposited using Modified Very High Frequency (MVHF) excited plasma, while n and p doped layers were deposited with a radio frequency (RF) plasma. Typical deposition rates were 5-15 Å/s for the intrinsic a-Si:H and ncSi:H layers. Most studies were conducted on fully encapsulated large-area (≥ 400 cm2) solar cells. However, small-area cells with an active area of 0.25 cm2 were first selected from various locations of the large area to study uniformity across the large area. Current-density versus voltage (J-V) and quantum efficiency (QE) measurements were performed for solar cell characterization.
We studied both a-Si:H/nc-Si:H/nc-Si:H triple-junction and a-Si:H/nc-Si:H doublejunction structures deposited by MVHF technology. Additionally, a-Si:H/a-SiGe:H/a-SiGe:H triple-junction cells deposited by RF were used as reference samples in light soaking tests. RESULTS AND DISCUSSION At the MRS2009 Spr
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