CDTE THIN-FILMS FROM NANOPARTICLE PRECURSORS BY SPRAY DEPOSITION

The formation of CdTe thin films by spray deposition using nanoparticl e colloids has been investigated. Employing a metathesis approach, cad mium iodide is reacted with sodium telluride in methanol solvent, resu lting in the formation of soluble NaI and insoluble CdTe nanoparticles . After appropriate chemical workup, methanol-capped CdTe colloids wer e isolated. CdTe colloids prepared by this method exhibit a dependence of the nanoparticle diameter upon reaction temperature as determined by UV-visible spectroscopy (UV-vis), X-ray diffraction (XRD), and tran smission electron microscopy (TEM). CdTe thin-film formation was perfo rmed by spray depositing the 25-75 Angstrom diameter nanoparticle coll oids according to a one- or two-step method. Films derived from a one- step approach were sprayed onto substrates at elevated temperatures (T -dep = 280-440 degrees C) with no further thermal treatment. Two-step films were sprayed at lower temperatures (T-dep = 25-125 degrees C) an d were subjected to subsequent thermal treatments (T-anneal = 250-500 degrees C) in argon or forming gas (10% H-2 in N-2) ambients. The effe cts of a CdCl2 treatment were also investigated for CdTe films on both 7059 glass and CdS on SnO2-coated 7059 glass. The CdTe films were cha racterized by XRD, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Phase-pure cubic CdTe formation was observed b y XRD for two-step derived films (400 degrees C in forming gas) while one-step films were composed of the cubic CdTe and an oxide phase. XPS analysis of five films processed at 400 degrees C and a variety of co nditions showed that while CdTe films produced by the one-step method contained no Na or C and substantial O, two-step films subjected to a CdCl2 treatment showed reduced O but increased C content. AFM gave CdT e grain sizes of similar to 0.1-0.3 and similar to 0.3-0.7 mu m for a one-step film sprayed at 400 degrees C and a two-step film annealed at 400 degrees C, respectively. The potential of employing CdTe nanopart icles toward photovoltaic technologies is discussed.