CdTe Thin-Film Solar Cells
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MRS BULLETIN/OCTOBER 1993
to global electricity production, even more than this will be needed in the next 50 years. From the raw materials point of view, CdTe technology can be a significant contributor to PV production for several decades. A commonly heard concern about CdTe technology is the toxicity of cadmium.23 In CdTe PV panels, Cd is in the form of CdTe and CdS compounds, both chemically stable compounds. CdTe thin-film PV cells are incapsulated to protect the device against the environment—and the environment against the device. The environmental risks of the devices are therefore limited to hazards and to the disposal of the devices after their technical lifetime. There are studies on different hazards as well as on the handling of Cd in the cell manufacturing plant. In both cases the
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Figure 1. CdTe thin-film PV cell structure.
situation is managable. The end-of-life recycling of CdTe panels can be foreseen and should therefore be included as a cost factor of the technology. The CdTe Thin-Film Cell A CdTe thin-film PV cell is generally based on a CdS/CdTe heterojunction. A multicrystalline form of undoped CdTe material is typically a high-resistivity p-type semiconductor. Doping CdTe, both p + - and n-type, is difficult. CdS shows ntype conductivity as a polycrystalline thin film. In spite of the 10% difference in the lattice constants of CdS and CdTe, they form an electrically excellent heterojunction, as shown by its high fill factors up to FF = 0.75 in the devices made. CdTe devices are generally made on a glass substrate (Figure 1). The first layer on the glass is a transparent electrode, often referred to as TCO (transparent conductive oxide, such as SnO2). The next thin-film layers are CdS, CdTe, and a back electrode which may be carbon paste or a metallic thin film. In an actual device, the thin-film structure is generally protected with another piece of glass laminated on top of the thin-film structure. Manufacturing a CdTe thin-film PV cell is relatively straightforward. The transparent electrode (TCO) can be made with any known technique. Unlike the surface for amorphous silicon PV cells, a smooth TCO surface is preferable for CdTe cells. A smooth TCO surface minimizes the necessary thickness of the CdS layer and also serves as a good substrate for the crystallization of both CdS and CdTe layers. CdS makes a good heterojunction with CdTe. The bandgap of CdS is 2.42 eV, which results in a loss of the blue part of the solar spectrum (Figure 2). It turns out that the spectral loss due to absorption in the CdS layer depends not only on the physical thickness of the CdS film, but also on the way it has been produced.4 The latter may reflect variations in the density of the CdS film or its interaction with the TCO or the CdTe layer. The methods used in producing the CdS material are chemical bath deposition (CBD), MOCVD, sputtering, and atomic layer epitaxy (ALE). CdTe has been deposited by many different methods such as electrodeposition, close-spaced sublimation (CSS), closespaced
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