CdTe Thin Films from Nanoparticle Precursors by Spray Deposition

Abstract

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 (T dep ) 280-440°C) with no further thermal treatment. Two-step films were sprayed at lower temperatures (T dep ) 25-125°C) and were subjected to subsequent thermal treatments (T anneal ) 250-500°C) in argon or forming gas (10% H 2 in N 2 ) ambients. The effects of a CdCl 2 treatment were also investigated for CdTe films on both 7059 glass and CdS on SnO 2 -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 CdCl 2 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