Structural homogenization and cation ordering in CZTS films during sulfurization as probed via in-situ Raman

Abstract

In this work, we investigate the production of copper-zinc-tin-sulfide (CZTS) thin films via the sintering of nanoparticle coatings. CZTS nanoparticles are produced via spray pyrolysis starting from a solution of metal salts and thiourea. This approach yields small particles that are easy to disperse and apply onto the desired substrate. Sintering these coatings into large-grained films, however, is not trivial. We have used a sodium disulfide-dipping technique that facilitates large-scale grain growth and high-phase purity after annealing in a sulfur atmosphere. We have also utilized a custom-built apparatus to perform in situ Raman spectroscopy on closed systems in volatile and corrosive environments at high temperatures. With this apparatus, we have observed the growth of long-range-ordered crystal domains at high temperature. We have found that precise control of the cooling rate is highly beneficial for cation ordering in large grain CZTS, and that cation re-ordering and phase re-homogenization in large grain CZTS are possible via low-temperature post-processing. This study highlights the fact that much is still not understood about the kinetics of phase evolution for this complex quaternary system during thermal processing, and that well-designed in-situ characterization techniques are necessary to overcome this fundamental lack of knowledge.