Impurity Study of Optical Properties in Fluorine-Doped Tin Oxide for Thin-Film Solar Cells
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1165-M06-04
Impurity Study of Optical Properties in Fluorine-Doped Tin Oxide for Thin-Film Solar Cells Xiaonan Li, Joel Pankow, Yanfa Yan National Renewable Energy Laboratory, Golden, CO 80401, USA ABSTRACT Tin oxide (SnO2) is a durable, inexpensive transparent conducting oxide (TCO) material used for thin-film photovoltaic devices. However, the optical properties of conducting SnO2:F are generally not as good as in other conducting TCO materials such as ITO and ZnO:Al. Our previous analyses indicate that for thin-film solar cells, improving the optical properties of SnO2coated glass could enhance photon collection and gain up to 10% additional photocurrent. Previously, we showed that some commercial SnO2 samples could have much higher optical absorption than others [2]. In this work, we continue our study on causes that could contribute to the high optical absorption of SnO2 films. The SnO2:F samples are fabricated by low-pressure metal-organic chemical vapor deposition or atmospheric-pressure chemical vapor deposition with tin precursors that includes different amounts of chlorine. Optical, electrical, and compositional analyses were performed. In addition to the free-carrier-introduced optical absorption, the non-active dopant also impacts the optical absorption. Among the SnO2 films fabricated with different precursors, the optical properties show a relationship based on the level of chlorine in the precursors and films. With a low-optical-absorption SnO2 layer, the solar cell could have better photon collection and a higher short-circuit current density. INTRODUCTION In recent years, solar energy, as an alternative energy source that provides clean power, has experienced significant success. The photovoltaic (PV) industry has grown 30%~50% annually in the past decade. This has been especially dramatic for thin-film PV: 10 years ago, the market share for thin-film PV modules was negligible, but by the close of 2006, it had increased to 44% of U.S. PV market share [1]. Thus, improving the quality of the front-window material for thinfilm PV cells has become an important issue for researchers. In this paper, we examine the possible impact of impurities on the optical properties of SnO2. SnO2 film is used extensively as a transparent conducting oxide (TCO) material based on its excellent electrical and optical properties. In particular, because of its stability under extreme conditions (high temperature and acidic environment), it is commonly used as construction glass and is thus well suited for use with outdoor PV panels. In the PV industry, SnO2 is mainly used as a transparent electrode for a-Si and CdTe thin-film solar panels. As a front-window layer in aSi and CdTe thin-film solar cells, its optical properties play an important role in energy conversion efficiency. Therefore, obtaining high-quality SnO2 is critical for high-efficiency thinfilm PV. Our previous study indicated that current loss due to the SnO2 optical absorption could be 5%~10% of the photocurrent of the solar device [2]. Commercial SnO2 typic
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