CdTe Thin Films from Nanoparticle Precursors by Spray Deposition

Authors: Douglas L. Schulz, Martin Pehnt, Doug H. Rose, Ed Urgiles, Andrew F. Cahill, David W. Niles, Kim M. Jones, Randy J. Ellingson, Calvin J. Curtis, David S. Ginley

In: Chemistry of Materials 9 (1997), 889-900. The formation of CdTe thin films by spray deposition using nanoparticle colloids has been investigated. Employing a metathesis approach, cadmium iodide is reacted with sodium telluride in methanol solvent, resulting in the formation of soluble NaI and insoluble CdTe nanoparticles. After appropriate chemical workup, methanol-capped CdTe colloids were 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 transmission electron microscopy (TEM). CdTe thin-film formation was performed by spray depositing the 25−75 Å diameter nanoparticle colloids according to a one- or two-step method. Films derived from a one-step approach were sprayed onto substrates at elevated temperatures (Tdep = 280−440 °C) with no further thermal treatment. Two-step films were sprayed at lower temperatures (Tdep = 25−125 °C) and were subjected to subsequent thermal treatments (Tanneal = 250−500 °C) in argon or forming gas (10% H2 in N2) ambients. The effects 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 characterized by XRD, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Phase-pure cubic CdTe formation was observed by XRD for two-step derived films (400 °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 °C and a variety of conditions 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 CdTe grain sizes of ∼0.1−0.3 and ∼0.3−0.7 μm for a one-step film sprayed at 400 °C and a two-step film annealed at 400 °C, respectively. The potential of employing CdTe nanoparticles toward photovoltaic technologies is discussed.