A process study of high-quality Zn(O,S) thin-film fabrication for thin-film solar cells

The growth mechanism and process optimization of Zn(O,S) films fabricated using the chemical bath deposition method are systematically investigated. A device with a Zn(O,S)-CIGS solar cell after post-treatment showed near conversion efficiency comparable to that of a device with the CdS-CIGS cell. The Zn(O,S) thin film is considered a most promising candidate for a cadmium-free buffer layer of the Cu(In,Ga)Se-2 (CIGS) thin-film solar cell due to its advantages of optical responses in the short-wavelength region and adjustable bandgap. In this paper, the thin-film growth mechanism and process optimization of Zn(O,S) films fabricated using the chemical bath deposition method are systematically investigated. The thickness and quality of Zn(O,S) films were found to be strongly affected by the concentration variation of the precursor chemicals. It was also revealed that different surface morphologies of Zn(O,S) films would appear if the reaction time were changed and, subsequently, the optimum reaction time was defined. The film-growth curve suggested that the growth rate varied linearly with the deposition temperature and some defects appeared when the temperature was too high. In addition, to further improve the film quality, an effective post-treatment approach was proposed and the experimental results showed that the microstructure of the Zn(O,S) thin film was improved by an ammonia etching process followed by an annealing process. For comparison purposes, both Zn(O,S)-based and CdS-based devices were fabricated and characterized. The device with a Zn(O,S)-CIGS solar cell after post-treatment showed near conversion efficiency comparable to that of the device with the CdS-CIGS cell.