Effect of supporting electrolyte concentration on one-step electrodeposited CuInS2 films for ZnS/CuInS2 solar cell applications

Abstract

A one-step electrodeposition process is used to obtain CuInS2 (CIS) films on a molybdenum substrate by varying the supporting electrolyte (lithium chloride, LiCl) concentration. The as-deposited samples are characterized by scanning electron microscopy, energy dispersive spectroscopy, profilometry and diffuse reflectance spectroscopy. It is found that different concentrations of LiCl mainly lead to a morphological change and that the chemical composition of the obtained CIS films shifts to the stoichiometric composition for high concentrations of supporting electrolyte. After annealing, the structural analysis from X-ray diffraction reveals that all samples crystallized in the tetragonal phase of CIS. In addition, it is found that the crystallite size increased for samples grown at higher concentrations of LiCl. Optical studies carried out by diffuse reflectance spectroscopy reveal that the band gap values increase from ~1.40 to ~1.45 eV (average) after the annealing process. Finally, zinc sulfide (ZnS) thin films are chemically deposited onto electrodeposited CIS films in order to evaluate the photovoltaic response of ZnS/CIS bilayer systems. We discover that ZnS thin films cover the surface of CIS more effectively for the highest concentration of LiCl and that only the ZnS/CIS bilayer with the CIS film obtained at the highest concentration of LiCl shows a photovoltaic response.