Use of inorganic fluxes to control morphology and purity of crystalline késterite and related quaternary chalcogenides

An inherently safe method of preparing crystalline quaternary chalcogenides, which are of interest to photovoltaic applications, has been developed. Mixed sulfide/selenide derivatives with a general formula of Cu2ZnSnSxSe4−x, where x ranges from x=4 to x=0, were prepared in good chemical and phase purity from binary sulfides or elements in molten cesium-containing salts, and the products were isolated and characterized by X-ray analysis. The unit cells for the sulfide/selenide series show a linear expansion with increasing levels of selenium. Morphology of Cu2ZnSnS4 crystals can be controlled by the nature of the flux, e.g., from trilled needles in CsCl flux to isotropic polyhedrons in CsCl/BaCl2. Purity estimates by Rietveld refinements of X-ray data were found to be 99% for Cu2ZnSnS4, 97% for Cu2ZnGeS4, and 98% for Cu2CdSnS4.

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► Morphology of késterite crystals can be controlled by the nature of the flux.
► Purity estimates by Rietveld refinements were found to be 99 % for Cu2ZnSnS4, 97 % for Cu2ZnGeS4.
► Fluxes of heavier elements, e.g., Cs, Ba, La, and Sn, gave the most crystalline késterite samples.