Study of Grain Growth of CZTS Nanoparticles Annealed in Sulfur Atmosphere

The kesterite material, Cu₂ZnSn(S_xSe_1-x)_{4 (}CZTS), is very promising as absorber material in future thin filmsolar cells. The elements are abundant, the material has a high absorption coefficient, and the pure sulfideCZTS is non-toxic. These properties make CZTS a potential candidate also for large-scale applications. Here,solution processing allows for comparatively fast and inexpensive fabrication and solution processing alsoholds the record efficiency in the kesterite family, however for the selenized compound. The current challenges are, (1) that the high carbon content in nanoparticle thin films is one of the main limitations for this approach, and (2) that grain boundaries and defects are believed to be a site for recombination that limits the efficiency. Annealing in vacuum and/or a nitrogen atmosphere facilitates grain growth and improves the electronic properties. Conventionally selenization (annealing in selenium) shows the best results, however sulfurization (annealing in sulfur) has the advantage of leading to a non-toxic material. In this work, nanocrystals of CZTS with a targeted Cu-poor/Zn-rich composition are synthesized through a hot-injection method with oleylamine as the solvent. The nanocrystal inks are deposited through doctor blading in octanethiol, and annealed in a vacuum furnace using a graphite box with sulfur. The surface morphology and thus grain growth are studied for various annealing conditions in a 10-mbar nitrogen atmosphere with a varying amount of sulfur.The films are characterized with scanning electron microscopy (SEM), and an example before and after annealing is displayed in Fig. 1 (a) and (b), respectively. Compositional changes are monitored by energy dispersive X-ray spectroscopy (EDX) and the crystallinity by X-ray diffraction (XRD).A photovoltaic device of the structure soda lime glass (SLG)/Mo/CZTSSe/CdS/ZnO:Al/Ag has been built, and our preliminary results show a power conversion efficiency of 1.41% for the nanoparticles annealed inselenium.