Critical Influence of Annealing Configuration in Kesterite Phase Evolution During Growth of Cu2ZnSnS4 Thin Films from Non-Toxic Environment-Friendly Solutions

While preparing Cu2ZnSnS4 (CZTS) thin films via non-toxic solution based approaches, it is critical to suppress the formation of secondary phases as they adversely affect solar cell performance. During post-deposition annealing of the precursor film—typical of solution processing—the annealing parameters must be carefully controlled first to weaken the bonds in the organic–inorganic complexes, and then to allow reactions to yield the eventual films. While growing CZTS thin films from ethanol based solutions containing common metal salts, we have studied the influencing mechanisms of phase evolution by employing ten possible annealing settings with sulfur flakes that include systematic variations in ramp rate, sulfur source temperature and reaction temperature. Phase pure films were obtained in the only instance when the precursor film and the sulfur flakes were at the same temperature zone in the furnace and the annealing geometry allowed the confinement of the sulfur vapor flux nearer to the reacting surface of the precursor film that promoted growth of kesterite CZTS and prevented its decomposition reaction at elevated temperatures. The results highlight the promise of the approach including the proposed annealing strategy for reproducible growth of phase pure kesterite CZTS films.