Resolving the effects of compositional change on structures in Cu2ZnSnS4 nanocrystals by X-ray absorption fine structure

Abstract

Renewable energy sources, and solar energy in particular, are a high impact research topic in the push for sustainable, long-term energy alternatives to fossil fuels. Cu2ZnSnS4 (CZTS) is one of the attractive, cost-effective materials that meets these needs. The quaternary nature makes the structure prone to defects and crystal alignment disorder. Some of these defects create advantageous electronic effects through antisite substitutions of Zn for Cu, [Formula: see text]. Others such as Sn for Zn replacements are detrimental. Synchrotron-based X-ray absorbance fine structure (XAFS) analysis was used to identify specific patterns in the antisite contributions to the structure of low-cost CZTS films that produced the highest photoresponse in each of our samples. Correlations were found between the Cu/(Zn + Sn) ratio and advantageous antisite formations, though at the cost of increased alignment disorder. Similarly, the Zn/Sn ratio showed relationships between both advantageous and disadvantageous antisite and vacancy pairs. Variations in the local surroundings for each metal center were confirmed through X-ray absorption near-edge structures (XANES). Extended X-ray absorption fine structures (EXAFS), verified through FEFF fitting of the EXAFS, confirmed the patterns in crystal alignment disorder, and the effects each antisite had on the overall crystal structure. The precision and unique nature of such synchrotron techniques offers opportunities to identify these trends at each metal center, providing guidance to balance negative and positive structural components during fabrication. Each minor change in stoichiometry has been shown to affect several interactions within the structure.