Growth and characterization of CdTe by close spaced sublimation on metal substrates

CdS/CdTe solar cells have long been recognized for their terrestrial applic ations. These cells are usually fabricated on soda-lime glass substrates, b ut the poor mechanical and thermal properties of glass make it difficult to incorporate it in continuous fabrication processes involving high temperat ures, typically above 500 degrees C. In addition, prospective space power a pplications would benefit if a lighter flexible substrate can be used. With a view to making light-weight modules suitable for large-scale production, a new device structure on metal foils has been proposed. CdTe films have b een deposited on light-weight molybdenum foils by the close spaced sublimat ion (CSS) technique. Firstly, the salient features of the CSS technique as well as the design and construction of a CSS system will be described. CdTe films were deposited using this CSS system. The source and substrate tempe ratures were varied in steps and the film thickness and grain sizes were st udied. The films were characterized by SEM, EDS, XRD and AFM as well as con tact resistance measurements between the CdTe and molybdenum substrate. Fil ms deposited with source and substrate temperatures of 650 degrees C and 57 5 degrees C respectively, spaced 0.5 mm apart in helium ambient at 70 Torr yielded CdTe films of 4-5 mu m thickness. SEM studies showed CdTe films to be continuous and of uniform grain size. The grains are well faceted and th e grain size is of the order of the film thickness. EDS studies indicated s toichiometric CdTe. XRD analysis showed a cubic phase with a preferred orie ntation in the (1 1 1) direction. A cadmium chloride dip followed by anneal ing treatment did not alter the grain size or the orientation of the film. (C) 1999 Elsevier Science B.V. All rights reserved.