Structural and Electronic Properties of Hydrogenated Nanocrystalline Silicon Films Made with Hydrogen Dilution Profiling
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Structural and Electronic Properties of Hydrogenated Nanocrystalline Silicon Films Made with Hydrogen Dilution Profiling Technique Keda Wang1, Daxing Han1, D. L. Williamson2, Brittany Huie3, J. R. Weinberg-Wolf3 Baojie Yan4, Jeffrey Yang4, and Subhendu Guha4 1 Physics Department, Boston College, Chestnut Hill, MA 02467 2 Department of Physics, Colorado School of Mines, Golden, CO 80401 3 Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 4 United Solar Ovonic Corporation, 1100 West Maple Road, Troy, MI 48084 ABSTRACT We used X-ray diffraction (XRD), Raman scattering and photoluminescence (PL) spectroscopy to characterize structural and electronic properties of nc-Si:H films made with different hydrogen dilution ratios and hydrogen dilution profiling with continuously reduced hydrogen dilution during the deposition. The XRD results show that the crystalline volume fraction (fc) is in the range of 60-70% with grain size of 22-26 nm for the nc-Si:H films studied. Comparing the sample made using hydrogen dilution profiling to that with constant hydrogen dilution, the hydrogen dilution profiling promotes the (220) preferential orientation due to a very high hydrogen dilution in the initial growth. The Raman results show that the fc is in the range of 60-90%, depending on the sample and excitation wavelength. For the samples with constant hydrogen dilution, the fc measured by Raman increases along the growth direction. The hydrogen dilution profiling reverses this trend, which affirms that the hydrogen profiling controls the nanocrystalline structure evolution along the growth direction. The PL results show only one peak around 0.8-0.9 eV for the samples made with constant hydrogen dilution, but an additional peak at 1.4 eV appears in the sample made with the hydrogen dilution profiling. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H) solar cells have attracted remarkable attention due to the higher long wavelength response and improved stability over hydrogenated amorphous silicon cells [1]. However, the poor grain boundary passivation and post-growth impurity diffusion for cells with high crystalline volume fraction (fc) and large grain size degrade the cell performance [2]. The best nc-Si:H solar cells are deposited under conditions close to the nanocrystalline/amorphous transition [2] where the materials are compact. However, it is still difficult to grow thick nc-Si:H with good cell performance because of the increase of fc and grain size along the growth direction. Such crystalline evolution may cause high microvoid/microcrack density in the top surface layer [3]. To solve this problem, a hydrogen dilution profiling technique has been developed and proven to be an effective method for improving the cell performance [4]. For example, an initial cell efficiency of 8.4% was achieved with a profiled hydrogen dilution compared to 6.7% with a constant hydrogen dilution [4]. In order to elucidate the microscopic mechanism of improving cell performance
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