Guiding Principle to Develop Intrinsic Microcrystalline Silicon Absorber Layer For Solar Cell By Hot-Wire Cvd
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GUIDING PRINCIPLE TO DEVELOP INTRINSIC MICROCRYSTALLINE SILICON ABSORBER LAYER FOR SOLAR CELL BY HOT-WIRE CVD A. R. Middya1, U. Weber, C. Mukherjee and B. Schroeder Department of Physics and Center of Materials Research, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautren, Germany ABSTRACT We report on ways to develop device quality microcrystalline silicon (µc-Si:H) intrinsic layer with high growth rate by hot-wire chemical vapor deposition (HWCVD). With combine approach of controlling impurities and moderate H-dilution [H2/SiH4 ≈ 2.5], we developed, for the first time, highly photosensitive (103) µc-Si:H films with high growth rate (>1 nm/s); the microstructure of the film is found to be close to amorphous phase (fc ≈ 46 ± 5%). The photosensitivity systematically decreases with fc and saturates to 10 for fc > 70%. On application of these materials in non-optimized pin µc-Si:H solar cell structure yields 700 mV open-circuit voltage however, surprisingly low fill factor and short circuit current. The importance of reduction of oxygen impurities [O], adequate passivation of grain boundary (GB) as well as presence of inactive GB of (220) orientation to achieve efficient µc-Si:H solar cells are discussed.
INTRODUCTION Since the report on 7.7% single junction microcrystalline silicon (µc-Si:H) solar cells and 13.1% a-Si:H/µc-Si:H tandem cells by University of Neuchatel in 1996, the low-temperature microcrystalline solar cells technology has taken significant stride over last five years [1]. Recently Kaneka Corporation, Japan has demonstrated a-Si:H/poly-Si tandem solar cells at 550°C substrate temperature with 11.6% initial and 9.2% stable module efficiency (910 x 455 mm2) using unknown modified PECVD [2]. However both Neuchatel group and Kaneka Corporation have been investigating without much success to increase the growth rate of microcrystalline and polycrystalline films more than 0.1 nm/s, so that the entire device can be fabricated within reasonable time. On the other hand, in last five years hot-wire CVD appeared as a low-cost technique for one-step deposition of microcrystalline and polycrystalline thin films with high growth rate (> 1.5 nm/s) on low cost substrate like glass [3]. In addition scale-up potential of HWCVD has also been demonstrated [4]. Thus with proper control of materials engineering and device technology, HWCVD should be a promising technique for the fabrication of low cost thin silicon film solar cells both from throughput and capital cost point of view. Here we report our effort to develop high growth rate device quality µc-Si:H absorber layer and solar cells so that it can be integrated into our a-Si:H/a-Si:H tandem-junction solar cells technology [5] as a bottom cells to increase short circuit current and hence overall efficiency of the tandem devices. EXPERIMENTAL DETAILS 1
Present address: Department of Physics, Syracuse University, Syracuse, NY 13244-1130 A25.6.1
Deposition rate (A/s)
In this work an ultra-high vacuum (10-7 mbar) multi-chamber hot-wire chemical vapor
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