Electronic Characterization and Light-Induced Degradation in nc-Si:H Solar Cells
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0910-A01-05
Electronic Characterization and Light-Induced Degradation in nc-Si:H Solar Cells P. G. Hugger1, Shouvik Datta1, P. T. Erslev1, Guozhen Yue2, Gautam Ganguly2, Baojie Yan2, Jeffrey Yang2, Subhendu Guha2, and J. D. Cohen1 1 Physics, University of Oregon, 1371 E. 13th Avenue, Eugene, OR, 97403 2 United Solar Ovonics Corporation, 1100 W. Maple Road, Tory, MI, 48084
ABSTRACT The electronic properties of working nanocrystalline silicon (nc-Si:H) solar cell devices with conversion efficiencies up to 8.6% were studied using junction capacitance methods. The set of devices examined were deposited on both specular stainless steel substrates and Ag/ZnO textured back reflectors. These devices included nc-Si:H grown under constant H2 dilution, and also with profiled H2 dilution to control the crystallite sizes and volume fraction. Transient photocapacitance and transient photocurrent spectroscopies were used to obtain sub-band-gap optical spectra. A comparison of these two kinds of spectra also allowed us to deduce the minority carrier collection fractions as a function of temperature and light-induced degradation. Light-soaking was found to cause a distinct decrease in minority carrier collection, as well as a consistent decrease in defects responding to drive-level capacitance profiling. A tentative microscopic model is proposed that accounts for these degradation effects in nc-Si:H. INTRODUCTION Thin film microcrystalline silicon solar cell devices deposited by plasma enhanced CVD processes close to the amorphous phase boundary, usually termed “hydrogenated nanocrystalline silicon” (nc-Si:H), have improved markedly over the past couple years. At United Solar, cell efficiencies of single junction nc-Si:H devices are now approaching 9% [1], while triple junction devices employing a nc-Si:H bottom cell with an amorphous silicon (a-Si:H) top and silicon geranium (a-SiGe:H) middle cells now exceed the performance of the conventional a-Si:H/aSiGe:H/a-SiGe:H triple junction devices [1]. The nc-Si:H films fabricated in this method are recognized to be mixed-phase materials containing up to 40% of Si in the amorphous phase [2], together with crystallites of sizes roughly between 5 to 20 nm. Recently we showed how the electronic properties of these materials could be characterized using junction capacitance methods [3]. For example, using transient photocapacitance (TPC) spectroscopy together with transient photocurrent (TPI) methods, optical transitions within the amorphous component of these mixed phase materials could be identified, and the relative collection efficiencies of the photo-generated majority vs. the minority carriers could be estimated. In this fashion, we also demonstrated that prolonged light-exposure in nc-Si:H caused a substantial decrease in the relative degree of minority carrier collection. Moreover, while it appears that this light-induced degradation arises from optical transitions across the a-Si:H component of this material [4], we could not observe any increase in the density of deep defects wit
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