Tuning Electrical, Optical, and Thermal Properties through Cation Disorder in Cu2ZnSnS4

Chemical disorder in semiconductors is important to characterize reliably because it affects materials performance, for instance by introducing potential fluctuations and recombination sites. It also represents a means to control material properties, to far exceed the limits of equilibrium thermodynamics. We present a study of highly disordered Cu-Zn-Sn-S (d-CZTS) films along the Cu₂SnS₃-Cu₂ZnSnS₄-ZnS binary line, deposited by physical vapor deposition. Deposition at low temperature kinetically stabilizes compositions that are well outside of the narrow, equilibrium solid solution of kesterite (Cu₂ZnSnS₄). Here we study d-CZTS and its thermal treatment using complementary characterization techniques: X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). We find that cations in d-CZTS are highly disordered while the sulfur anions remain in a well-defined, cubic close-packed lattice. On the atomic scale, composition fluctuations are accommodated preferentially by stacking faults. Kinetically-stabilized cation disorder can produce nonequilibrium semiconductor alloys with a wide range of band gap, electronic conductivity, and thermal conductivity. d-CZTS therefore represents a processing route to optimizing materials for optoelectronic device elements such as light absorbers, window layers, and thermal barriers.