Growth and characterization of kesterite Cu₂ZnSn(S_xSe_1−x)₄ crystals for photovoltaic applications

Highlights: • Cu₂ZnSn(S_xSe_1−x)₄ crystals were grown using vertical gradient freeze technique. • Grown CZTSSe crystals exhibited highly crystalline tetragonal kesterite structure. • Grown ingot showed a homogeneous, stoichiometric composition along the growth axis. • Cu-rich particulates were observed along the polycrystalline grain boundaries. • Large volume of high quality CZTSSe can be grown using this novel VGF method. - Abstract: Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe) bulk crystals have been grown for the first time, in multi-gram quantities, using a novel vertical gradient freeze (VGF) technique. Large grain polycrystalline CZTSSe ingot with grain sizes ∼0.2–0.5 mm was grown in a vacuum sealed quartz ampoule. Structural, compositional, and electronic properties of the ingot were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), and van der Pauw measurements. XRD analysis showed highly crystalline tetragonal structure corresponding to kesterite CZTSSe with lattice constants of a = 5.563 Å, and c = 11.09 Å (c/2a = 0.997) and Raman spectra confirmed the formation of single phase CZTSSe. High-resolution XPS spectra confirmed the formation of metal-sulfoselenides. The ingot along the growth axis showed a homogeneous near stoichiometric elemental distribution corresponding to Cu/(Zn + Sn) = 0.94 and Zn/Sn = 0.98. EDS mapping on the ingot's outer surface revealed formation of islanded Cu-rich droplets at the grain boundaries. Grown crystals exhibited p-type conductivity with a measured bulk resistivity of ∼51 Ω cm