High Rate Deposition of Amorphous Silicon Based Solar Cells using Modified Very High Frequency Glow Discharge
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0989-A15-04
High Rate Deposition of Amorphous Silicon Based Solar Cells using Modified Very High Frequency Glow Discharge Guozhen Yue, Baojie Yan, Jeffrey Yang, and Subhendu Guha United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084 ABSTRACT We report our recent progress on high rate deposition of hydrogenated amorphous silicon (aSi:H) and silicon germanium (a-SiGe:H) based nip solar cells. The intrinsic a-Si:H and a-SiGe:H layers were deposited using modified very high frequency (MVHF) glow discharge. We found that both the initial cell performance and stability of the MVHF a-Si:H single-junction cells are independent of the deposition rate up to 15 ≈/s. The average initial and stable active-area cell efficiencies of 10.0% and 8.5%, respectively, were obtained for the cells on textured Ag/ZnO coated stainless steel substrates. a-SiGe:H single-junction cells were also optimized at a rate of ~10 ≈/s. The cell performance is similar to those made using conventional radio frequency technique at 3 ≈/s. By combining the optimized component cells made at 10 ≈/s, an a-Si:H/aSiGe:H double-junction solar cell with an initial active-area efficiency of 11.7% was achieved. INTRODUCTION High rate deposition of thin film solar cells is desirable for increasing throughput and reducing cost of manufacturing solar panels. However, hydrogenated amorphous silicon (aSi:H) solar cells made using conventional radio frequency (RF) glow discharge at high rates commonly exhibit poor quality [1]. The material contains high density of defects, microvoids, and di-hydride structures, which lead to low initial solar cell efficiencies and poor stability after prolonged light soaking. Therefore, new deposition techniques are needed for increasing the deposition rate without compromising the material quality. Very high frequency (VHF) glow discharge has been widely used in the deposition of a-Si:H and hydrogenated nanocrystalline silicon (nc-Si:H) materials and devices [2, 3]. Under a similar excitation power density, VHF plasma has a higher ion flux intensity and lower ion energy than the conventional RF plasma, resulting in improved material quality at higher deposition rates [4]. In our laboratory, we used a modified VHF (MVHF) system with an excitation frequency range of 60-75 MHz to make a-Si:H, a-SiGe:H, and nc-Si:H solar cells. An initial efficiency of 11.2% was reported previously in an a-Si:H/a-SiGe:H double-junction solar cell with the a-Si:H intrinsic layer deposited at 8 ≈/s and the a-SiGe:H intrinsic layer at 6 ≈/s [5]. Recently, we have achieved an initial active-area efficiency of 9.0% for a nc-Si:H single-junction solar cell and a stabilized active-area efficiency of 13.3% for an a-Si:H/nc-Si:H/nc-Si:H triple junction solar cell [6, 7]. In this paper, we report our recent results on high rate deposition of a-Si:H and a-SiGe:H based solar cells. The growth parameters are similar to those used for nc-Si:H cells [6, 7] in the high pressure and high power regime. The initial cell performance and stability as a function of the
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