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

Abstract

CdS/CdTe solar cells have long been recognized for their terrestrial applications. These cells are usually fabricated on soda-lime glass substrates, but the poor mechanical and thermal properties of glass make it difficult to incorporate it in continuous fabrication processes involving high temperatures, typically above 500°C. In addition, prospective space power applications 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 been deposited on light-weight molybdenum foils by the close spaced sublimation (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 temperatures were varied in steps and the film thickness and grain sizes were studied. The films were characterized by SEM, EDS, XRD and AFM as well as contact resistance measurements between the CdTe and molybdenum substrate. Films deposited with source and substrate temperatures of 650°C and 575°C respectively, spaced 0.5 mm apart in helium ambient at 70 Torr yielded CdTe films of 4–5 μm thickness. SEM studies showed CdTe films to be continuous and of uniform grain size. The grains are well faceted and the grain size is of the order of the film thickness. EDS studies indicated stoichiometric CdTe. XRD analysis showed a cubic phase with a preferred orientation in the (1 1 1) direction. A cadmium chloride dip followed by annealing treatment did not alter the grain size or the orientation of the film.