High Efficiency Large Area a-Si:H and a-SiGe:H Multi-junction Solar Cells Using MVHF at High Deposition Rate
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1153-A07-01
High Efficiency Large Area a-Si:H and a-SiGe:H Multi-junction Solar Cells Using MVHF at High Deposition Rate Xixiang Xu1, Dave Beglau1, Scott Ehlert1, Yang Li1, Tining Su1,Guozhen Yue1, Baojie Yan1, Ken Lord1, Arindam Banerjee1, Jeff Yang1, Subhendu Guha1, Peter G. Hugger2, and J. David Cohen2 1 United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084, U.S.A. 2 Department of Physics, University of Oregon, Eugene, OR, 97403, U.S.A.
ABSTRACT We have developed high efficiency large area a-Si:H and a-SiGe:H multi-junction solar cells using a Modified Very High Frequency (MVHF) glow discharge process. We conducted a comparative study for different cell structures, and compared the initial and stable performance and light-induced degradation of solar cells made using MVHF and RF techniques. Besides high efficiency, the MVHF cells also demonstrate superior light stability, showing 450 cm2) encapsulated cells, grid wires are used to collect current. Current-density versus voltage (J-V) and quantum efficiency (QE) measurements were performed for solar cell characterization. The light-soaking experiments were conducted under an open-circuit condition with 100mW/cm2 white light at 50 oC for 1000 hours.
RESULTS AND DISCUSSION Simulation & cathode design We designed and developed MVHF deposition hardware including cathode and gas distribution system. The design is based on modeling studies to obtain good film thickness uniformity over large areas (15”x14”). We have optimized the deposition parameters and attained high rate deposited a-SiGe:H films exhibiting hydrogen concentration and defect density similar to or slightly lower than their RF counterparts prepared at lower rate (≤3 Å/s). One major challenge for the MVHF plasma process is to obtain spatial uniformity over a large area. We first conducted computer simulation of MVHF electric field distributions over a large-area (15”x14”) cathode, testing different cathode structures and MVHF application techniques. We then designed new MVHF cathodes that resulted in electric field uniformity better than ±5% over the entire area. We also designed a gas distribution assembly to attain a desirable gas flow pattern in the high-flow regime, and a heating assembly to obtain uniform heating over the deposition area. The film thickness uniformity for both a-Si:H and a-SiGe:H deposited using MVHF plasma shows good agreement with that predicted from the simulation. light light light
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Figure 1. Schematic of three multi-junction structures studied in this work.
Multi-junction cell structures and initial performance We use the a-Si:H/a-SiGe:H double-junction structure to develop the MVHF deposition, because the simpler double-junction cell provides a quick turnaround for various experimental conditions yet still contains a-Si:H and a-SiGe:H component cells,
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