Effect of CdCl2 annealing treatment on thin CdS films prepared by chemical bath deposition

In order to study the CdS recrystallization mechanism, a comparative study was carried out on thin films prepared by chemical bath deposition. The CdS films were annealed in air with or without a CdCl2 coating layer. In-situ Raman spectra obtained during the annealing showed that both the air- and the CdCl2-annealing did not cause rearrangement of the neighboring atoms in the CdS clusters below ~ 300 °C. CdS thin film was partially oxidated to CdO and CdSO4 on the cluster surface when annealed in air. The oxides and the sulfur stoichiometric deficiency prevented the clusters to coalesce at higher temperatures. Coating thin CdS film with a thin CdCl2 layer protected it from oxidation during annealing in air and promoted formation of ClS and VCd point defects in CdS. The anti-oxidation was attributed to the incorporation of a significant amount of Cl into CdS to form the ClS, which prevented the oxygen in-diffusion and chemical bonding during the annealing. The anti-oxidation at the CdS nano-crystalline surface and the point defects formed in the CdS promoted coalescence of the neighboring clusters without the need of long-range redistribution of the atoms. Large CdS grains with good crystalline quality formed through recrystallization during the CdCl2 heat treatment, which provided the solid basis for the subsequent CdTe growth and high efficient CdS/CdTe solar cell fabrication.