Fabrication of Cu2ZnSnS4 thin films using a Cu-Zn-Sn-O amorphous precursor and supercritical fluid sulfurization

Publication date: 30 September 2017
Source:Thin Solid Films, Volume 638
Author(s): Yuta Nakayasu, Takaaki Tomai, Nobuto Oka, Kanako Shojiki, Shigeyuki Kuboya, Ryuji Katayama, Liwen Sang, Masatomo Sumiya, Itaru Honma
A supercritical ethanol (scEtOH) environment, in which sulfur is highly soluble, can lead to safety improvements and cost reductions in the sulfurization process. In this study, the feasibility of the sulfurization process in scEtOH for the preparation of Cu2ZnSnS4 (CZTS) thin films at low temperature was verified with the purpose of creating a sustainable and cost-effective process for fabricating metal-sulfide solar cells. We found that to promote atomic diffusion of sulfur and achieve sulfurization at a low temperature, the presence of defects in the amorphous oxide thin film was preferable. Moreover, in thin film fabricated by sulfurization under ethanol, the crystal size was strongly affected by ethanol density. The grain size increased up to about 1μm as the ethanol density increased, and grain growth was remarkable, particularly in the high-density conditions of over 3.0mol/L. Finally, we fabricated a crystalline CZTS thin film, which exhibited structural and optical properties comparable to those of a film fabricated using conventional vapor-phase sulfurization. For the prepared disordered-kesterite CZTS thin film with a Cu-poor and Zn-rich composition, photoluminescence measurements confirmed that the donor or acceptor defects engage in the emission of about 1.24eV at 5K, and UV–Vis measurement revealed a bandgap of 1.38eV at room temperature.