Flux-Boosted Sulfide Crystal Growth: Growth of CuInS₂ Crystals by NaCl-InCl₃ Evaporation

Copper-indium-gallium-sulfide-selenide (CIGSSe) is used in photovoltaic cells and photocathodes, because of its tunable optoelectronic properties, but the fabrication of CIGSSe samples usually requires a multistage process under vacuum. Herein we used a flux growth technique for the sulfide system and achieved efficient flux growth of idiomorphic copper-indium-sulfide CuInS₂ crystals of size ∼5 μm from a NaCl-InCl₃ flux under mild conditions at ambient pressures. We first examined the flux growth conditions such as holding temperature, solute concentration, and holding time for growing highly crystalline CuInS₂ crystals. A moderate holding temperature (∼550 °C) and high solute concentration (∼70 mol %) yielded idiomorphic pure CuInS₂ crystals. High-resolution transmission electron microscopy showed clear electron diffraction spots, indicating that the resultant CuInS₂ crystals had a highly crystalline, intrinsic tetragonal crystal structure. Thermogravimetry-differential thermal analysis showed that the CuInS₂ crystals grew efficiently during flux evaporation at 550 °C, at which the flux evaporation degree reached ∼81%. The CuInS₂ crystal growth mode is discussed based on the characterization results.