Insights into the SILAR Processing of Cu_xZn_1-xS Thin Films via a Chemical, Structural, and Optoelectronic Assessment

Careful analysis of the chemical state of Cu_xZn_1-xS thin films remains an underdeveloped topic although it is key to a better understanding of the phase transformations and the linking between structural and optoelectronic properties needed for tuning the performance of Cu_xZn_1-xS-based next-generation energy devices. Here, we propose a chemical formulation and formation mechanism, providing insights into the successive ionic layer adsorption and reaction (SILAR) processing of Cu_xZn_1-xS, in which the copper concentration directly affects the behavior of the optoelectronic properties. Via chemical, optoelectronic, and structural characterization, including quantitative X-ray photoelectron spectroscopy, we determine that the Cu_xZn_1-xS thin films at low copper concentration are composed of ZnS, metastable Cu_xZn_1-xS, and CuS, where the evidence suggests that a depth compositional gradient exists, which contrasts with homogeneous films reported in the literature. The oxidation states for copper and sulfide species indicate that the films grow following a formation mechanism governed by ionic exchange and diffusion processes. At high copper concentrations, the Cu_xZn_1-xS thin films are covellite CuS that grew on a ZnS seed layer. Hence, this work reiterates that future research related to fine-tuning the application of this material requires a careful analysis of the depth-profile compositional and structural characteristics that can enable high conductivity and transparency.