CdS Window Layer for Large Open Circuit Voltages of Low Environmental-Load CdS/CdTe Solar Cells
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CdS Window Layer for Large Open Circuit Voltages of Low Environmental-Load CdS/CdTe Solar Cells T. Toyama, H. Oda, K. Nakamura, T. Fujihara, K. Shimizu and H. Okamoto Department of Systems Innovation, Graduate School of Engineering Science, Osaka University Toyonaka, Osaka 560-8531, Japan
ABSTRACT The low “environmental-load” CdS/CdTe solar cells for reducing consumption of Cd compounds have been investigated employing the CdS layers fabricated at various substrate temperatures, TCdS, and a conversion efficiency of 14.1% has been achieved. The nanostructure of CdS crystallites made at different TCdS are compared to the crystallinity of CdS, and CdTe deposited on CdS as well as sulfur fraction in CdTe1-xSx mixed crystal layer unintentionally formed at CdS/CdTe(S) interface. The photovoltaic performances, especially obtained relative high open circuit voltages, are discussed in conjunction with the structural properties as well as electrical properties of the solar cells. The solar cells show a relative high Voc due to the large CdTe grains as well as the narrow depletion layer width. Besides, preventing deterioration of the CdS/CdTe(S) interface is found to be quite effective for achieving high open circuit voltages and fill factors. INTRODUCTION Energy conversion efficiency over 16% has been realized in R&D level with the use of CdS/CdTe thin-film solar cells [1,2]. However, from a viewpoint of “environmental load” related to the Cd compounds, the thickness of the photovoltaic layer, which is usually 5–10 µm, must be reduced. We have proposed low “environmental-load” CdS/CdTe thin-film solar cells with a photovoltaic layer thickness of < 3 µm [3–6], and already achieved a high conversion efficiency of 13.6% [4]. For the further improvement of the photovoltaic performances, control of CdS nanostructures including crystallinity of the CdS layer is one of the key issues because of the following well-known reasons; (1) the CdS layer acts as the window layer which is related to the spectral response at short wavelength region, (2) it also plays crucial roles for growth of the CdTe layer. Furthermore, we have extensively studied CdTe1-xSx mixed-crystal layer unintentionally formed near the CdS/CdTe metallurgical interface, which strongly influences on the photovoltaic performances, and is likely to contribute to the ionized donor distribution. Thus the third reason is that the nature of CdS must strongly affect the formation of the CdTe1-xSx mixed-crystal layer. In this article, we show the results on the structural studies on CdS layers as a function of the deposition temperature in conjunction with the structural properties of CdTe deposited on the CdS layers. Also shown here are the electrical properties and photovoltaic performances of the CdS/CdTe solar cells made with the CdS layers at different deposition temperatures.
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