A combined Na and Cl treatment to promote grain growth in MOCVD grown CdTe thin films

The role of sodium (Na) in cadmium telluride (CdTe) thin film photovoltaic absorbers deposited on a metal-coated substrate is investigated. Introducing Na during the growth of the structure influences the morphological and crystallographic properties of the CdTe layer grown by metal-organic chemical vapour deposition (MOCVD). It is observed that the introduction of Na between the metal and CdTe layers induces a slight randomisation via promotion of (400) and (220) orientations. It is shown that the inclusion of Na between the substrate and the Mo back metal contact enlarges the CdTe grains following a CdCl2 treatment by 50% but weakens the adhesion to the substrate. The introduction of Na between the Mo back contact and CdTe layer promotes the formation of large faceted grains for the as-grown films with an average grain size ten times larger than in the case of Na free deposition while maintaining good adhesion to the substrate. There is no further grain growth following CdCl2 treatment however the CdTe grain size is still double that of the Na-free samples.