CdS annealing treatments in various atmospheres and effects on performances of CdTe/CdS solar cells

In this work, a systematic research on CdS annealing treatments under various atmospheres had been done to understand their effects on CdS/CdTe solar cells. CdS films were prepared by a standard CBD method and annealed under various atmospheres, including Ar, Ar+H-2, O-2, Ar+S and Ar+CdCl2. Morphological, structural, optical and chemical properties were investigated using Atom force microscope (AFM), X-ray diffraction (XRD), UV-VIS spectroscopy and X-ray photoelectron spectroscopy (XPS). Annealing treatments enhanced modifications of morphology, structure and electrical properties of CdS films. AFM showed different surface morphologies and roughnesses of CdS films annealed under various atmospheres. XRD indicated the transition of CdS films from metastable cubic structure to stable hexagonal structure after annealing treatment, especially annealed in Ar+CdCl2. From XPS analysis, Fermi levels of CdS films shifted closer to conduction band after annealing under O-2 and Ar+CdCl2, while the levels shifted away from conduction band under Ar+H-2 and Ar+S. The relationships between those modifications by annealing treatments and effects on the performance of solar cells were discussed. Solar cell based on CdS annealed with Ar+CdCl2 had the best performance due to the high n-doping of CdS layer introduced by annealing process.