Cu ₂ZnSnS ₄ thin films prepared with a Joule-heated graphite closed-space sulfurization system

Non-stoichiometric Cu ₂ZnSnS ₄ (CZTS) thin films were prepared using a closed-space sulfurization process of vacuum-evaporated stacks of ZnS, Cu and Sn in a custom-designed Joule-heated graphite reactor. In the first sulfurization step, at 250 °C, Cu and Sn transform into CuS and SnS. In a second sulfurization step, at 510 °C, the stack finally transforms into CZTS. Sample compositions were determined by energy-dispersive X-ray spectroscopy. Three different Cu-poor/Zn-rich samples and one slightly Cu-rich/Zn-poor sample were obtained, with Cu/(Zn+Sn) ratios of 0.64, 0.85, 0.91 and 1.02. The effect of the closed-space sulfurization process in the morphology was analyzed by scanning electron microscopy, allowing to obtain estimates of grain sizes in the range from 0.50 to 0.75 μ m. X-ray diffraction confirmed the polycrystalline structure of the kesterite CZTS thin films with crystallite sizes that vary from 48 to 64 nm as the Cu content increases in the samples. An estimate of lattice microstrain indicates larger values for samples with larger grains and higher Cu/(Zn+Sn) ratios. Raman spectroscopy reveals the characteristic kesterite structure with the main peak shifted to a lower wave number, a signature associated with partially disordered kesterite. Optical characterization shows a decreasing bandgap from 1.47 to 1.39 eV and an increasing Urbach energy from 77 to 200 meV as the relation Cu/(Zn+Sn) increases, indicating a decrease in the localized states in the bandgap for Cu-poor samples. Results indicate that CZTS thin films obtained from the custom-made Joule-heated graphite reactor have similar characteristics to the films obtained from more conventional methods like sulfurization in a tubular furnace with the advantage of a precise and fast-response temperature control that could allow novel sulfurization strategies.