Evaluating the role of precursor concentration in facile conformal coating of sub-micrometer thick Cu2ZnSnS4 films using non-toxic ethanol based solutions

Abstract

Although non-toxic solution processing offers a scalable approach for growth of Cu2ZnSnS4 (CZTS) thin films, it must suppress the formation of secondary phases and yield facile conformal large grained films with minimum process steps. While growing CZTS films from environment-friendly ethanol based solutions, here we demonstrate how precursor concentration and post sulfurization critically affect the reaction pathway and morphology of the films. Deposition from lower ionic concentration precursor solution and sulfurization with nominal sulfur amount yielded phase-pure films. However, the thickness and grain size of these films were far lower than that desired for solar cell application, thus necessitating overlay depositions to achieve desired thickness and grain size. This issue was alleviated by increasing the ionic concentration in the precursor solution and the sulfur amount during sulfurization. It was found that coating from 1 M Cu-solution in just two dipping cycles followed by sulfurization at 500 °C yielded phase pure kesterite CZTS films of ~850 nm thickness. In addition, differently with the reported bilayered structure, the films exhibit a uniform microstructure with reasonably large grains. The resulting films showed the bandgap of 1.42 eV and very high photosensitivity upon illumination of white light.