The Status of and Challenges in CdTe Thin-Film Solar-Cell Technology
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The Status of and Challenges in CdTe Thin-Film Solar-Cell Technology Alvin D. Compaan Department of Physics and Astronomy, The University of Toledo, Toledo, OH, 43606, USA ABSTRACT Polycrystalline CdTe thin-film solar cells have shown high potential for low cost, large-area module fabrication. But successful large-scale commercial production has been elusive. Fabrication of the basic n-CdS / p-CdTe heterojunction is possible by a wide variety of methods, including close-spaced sublimation, vapor-transport deposition, electrodeposition, chemical bath deposition, and magnetron sputtering. An overview of these methods is presented as well as the role of the postdeposition "activation" treatment using CdCl2 and issues related to the difficulty of obtaining low resistance back contacts to CdTe. We present some of our recent fabrication results using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses the challenges of fabricating tandem cells with efficiencies over 25%.
INTRODUCTION In the context of this symposium focused on amorphous and nanocrystalline silicon it is useful to set the environment for polycrystalline thin-film solar cells, particularly CdTe, in the broader field of photovoltaics, both traditional crystalline as well as amorphous thin film. The efficiencies of the laboratory best photovoltaic (PV) cells have shown steady advances over the past two decades so that triple-junction, epitaxially grown III-V semiconductor-based cells have reached efficiencies (at air mass 1.5 global) of about 34%,[1] single-crystal Si cells with precise texturing of the surface and buried grid lines have reached ~24% efficiency,[2] and multicrystal Si grown by ingot casting has reached ~18%.[2,3] In addition, there are three competitive inorganic thin-film technologies which have achieved efficiencies well above 10%. These are: CuIn(Ga)Se2 with an efficiency of 19.8%, CdTe (16.5%), and amorphous silicon (12% stabilized).[4] However, in most applications the cost per peak watt is more important than utilizing cells with the highest efficiency and thus the thin-film technologies appear to be quite viable commercially, although thin-film PV manufacturing has not generally achieved profitable high-volume production. As the production of PV has increased, the cost has fallen. Analyses by Margolis[5] and by Surek[6] have shown that the retail cost of solar panels has closely followed an 80% learning curve since the beginning of cost tracking in 1976. Thus for every doubling in cumulative production the cost has dropped to 80% of the previous cost. (For example, the cost of modules has dropped from ~$70/Wp in 1976 to ~$7/Wp in 1992 to under $3/Wp in 2002.) Today the traditional crystalline (and multicrystalline silicon) accounts for about 80% of PV production and sales. However, Surek has predicted that for a variety of reasons,
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