High Voltage Amorphous Silicon Solar Cells by Hot-Wire Chemical Vapor Deposition
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High Voltage Amorphous Silicon Solar Cells by Hot-Wire Chemical Vapor Deposition Qi Wang and Eugene Iwaniczko Electronic Materials and Device Division National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401, U.S.A. ABSTRACT We have achieved the best open-circuit voltage (Voc = 0.94 V) to-date in hydrogenated amorphous silicon (a-Si:H) photovoltaic cells deposited entirely by hot-wire chemical vapor deposition. The fill factor (FF = 0.74) remained high and a current density of 8-9 mA/cm2 with about 1800 Å i-layer was obtained in our n-i-p cells on untextured stainless-steel substrates. The Voc improvement of about 60 mV in compared to our previous best Voc was obtained by incorporating materials grown with H-dilution close to the phase transition from amorphous to microcrystalline silicon in the i-layer and at the i-p interface. A low substrate temperature of 150 °C for the i-layer was also essential, most likely to widen the bandgap of the i-layer. A brief atomic H-treatment after grown the i-layer increases the Voc further by improving the p-i interface. The last 60 Å of the i-layer before p-layer is extremely close to the transition to microcrystallinity, though it remains mainly amorphous. Our p-layers are also close to the phase transition. INTRODUCTION The device physics of open circuit voltage (Voc) of solar cells continues to be an interesting topic. From the textbook [1], Voc depends on the built-in potential—the difference between Fermi-energy of n-layer and p-layer, the photo and dark current, and operating temperature. However, the fundamental understanding of Voc in a-Si:H solar cell still remains unclear. The limitations of Voc can be attributed to the bulk i-layers, dopant layers, and interfaces [2-4]. The understanding of Voc will lead to the improvement of solar cells performance not just for the top cell in multi-junction configuration but also for µc-Si solar cells in which low Voc is currently a predominant problem. The highest initial Voc for a-Si:H based solar cell is just over 1.00 V using plasma enhanced CVD technique [5]. Successful approaches utilize a wide-gap thin buffer layer between p- and ilayer, a wide-gap i-layer of a-SiC:H, a-SiO:H and hydrogen diluted a-Si:H that near the edge of the phase transition from amorphous to microcrystalline silicon, and a wide-gap conductive Hdiluted p-layer [6-8]. For the hot-wire (HW) CVD solar cells, the present best initial Voc are 0.92 V in the hybrid devices [9] that the i-layer was deposited using HWCVD, and 0.88 V for all-HW solar cells [10]. Many groups have achieved the Voc just below 0.90 V for their best HW solar cells [11-13]. In this paper, we present our experimental research for high voltage a-Si:H solar cells where all the layers were deposited using the HWCVD technique. We focus on optimizing the i-layer using H-dilution so that the materials are near or on the edge of phase transition from amorphous to microcrystalline silicon at a high deposition rate of 10 Å/s. A13.1.1 Downloaded from https://www.cambridge.org/core. HKUS
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