11.0% Stable Efficiency on Large Area, Encapsulated a-Si:H and a-SiGe:H based Multijunction Solar Cells Using HF Technol
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11.0% Stable Efficiency on Large Area, Encapsulated a-Si:H and a-SiGe:H based Multijunction Solar Cells Using HF Technology A. Banerjee, D. Beglau, T. Su, G. Pietka, G. Yue, B. Yan, J. Yang, and S. Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI 48084, U.S.A. ABSTRACT We report on the investigation of large area a-Si:H/a-SiGe:H double-junction and a-Si:H/aSiGe:H/a-SiGe:H triple-junction solar cells prepared by our proprietary High Frequency (HF) glow discharge technique. For investigative purposes, we initially used the simpler doublejunction structure. We studied the effect of: (1) Ge content, (2) cell thickness, and (3) SiH4 and GeH4 gas flow on the light-induced degradation of the solar cells. Our results show that the double-junction cells with different Ge concentration have open-circuit voltage (Voc) in the range of 1.62-1.75 V. Voc exhibits a flat plateau in the range of 1.65-1.72 V for both initial and stabilized states. The light-induced degradation for cells in this range of Voc is insensitive to the Ge content. In terms of thickness dependence of the intrinsic layers, we found that the initial efficiency increases with cell thickness in the thickness range 2000-4000 Å. However, lightinduced degradation increases with increasing thickness. Consequently, the stabilized efficiency is invariant with cell thickness in the thickness range studied. The results of SiH4 and GeH4 gas flow on cell characteristics demonstrate that the deposition rate decreases by only 20% when the active gas flow is reduced to 0.25 times standard flow. The initial and stabilized efficiencies are similar. The information gleaned from the study was used to fabricate high efficiency, large area (~464 cm2) double- and triple-junction solar cells. The highest stable efficiency, as measured by NREL, was 9.8% and 11.0% for the double- and triple-junction structures, respectively. INTRODUCTION We previously reported [1] a stabilized efficiency of 13.0% on a small-area (0.25 cm2) aSi:H/a-SiGe:H/a-SiGe:H triple-junction solar cell prepared using RF (13.56 MHz) Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. Using low-rate RF process, we also attained [2] stable large-area (aperture area ~900 cm2) efficiency of 10.5% using a similar triplejunction device structure. The RF technique, which has been widely used to deposit a-Si:H and a-SiGe:H films and thin film solar cells, is limited to a deposition rate of 1~3 Å/s in order to make high quality intrinsic layer materials for high efficiency solar cells. It is well known that higher deposition rates, using RF-excitation, usually results in lower material and cell quality [34]. In recent years, Very High Frequency (VHF) excitation has emerged as the preferred technique to deposit a-Si:H and a-SiGe:H based solar cells. Compared to conventional RF glow discharge methodology, VHF excitation has the advantages [5] of: (1) 2-3 times higher deposition rate, (2) provides solar cells exhibiting both superior light stability and high stabilized efficiency. We have developed a propr
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