Paper EN+PS+TF-MoM9
Microstructure Evolution During Annealing of Copper Zinc Tin Sulfide Colloidal Nanocrystal Coatings to form Large-Grain Polycrystalline Thin Films

Monday, October 28, 2013, 11:00 am, Room 101 A

Copper zinc tin sulfide (Cu₂ZnSnS₄, or CZTS), copper zinc tin selenide (Cu₂ZnSnSe₄, or CZTSe), and their alloys are candidates for environmentally sustainable light absorbing materials for thin film solar cells because they are composed of abundant elements. In one potentially low-cost approach to making solar cells, coatings drop-cast from colloidal dispersions of CZTS nanocrystals (NCs) (inks) are annealed to form 1-3 micron thick polycrystalline films with 1-3 micron grains. We synthesize CZTS NC inks using rapid thermal decomposition of copper, zinc, and tin diethyldithiocarbamate precursors in presence of hot (150-300 ˚C) oleylamine. The synthesis temperature determines the NC size, which can be tuned from 2 to 25 nm. The formation of CZTS is confirmed using X-ray diffraction (XRD), Raman spectroscopy, and energy dispersive X-ray spectroscopy (EDS). Following synthesis, the NCs are dispersed in toluene and NC films are cast on various substrates including quartz and soda lime glass. One micron or thicker films cast from <10 nm NCs crack due to capillary forces during drying. Cracks are reduced significantly when films are cast from 25 nm nanoparticles. The NC films are then placed in quartz tubes with pure S, Se, or a mixture of the two, evacuated to 10^-6 Torr, sealed, and then heated for the desired times (1-8 hours) at the desired temperatures (500-800 ^oC). This annealing approach provides excellent and very reproducible control of the annealing temperature and S and/or Se pressure over the film. The resulting films are characterized using a suite of techniques, including XRD, electron microscopy, EDS, and Raman scattering. We have explored the extent to which vapor species, vapor pressure, substrate choice, carbon content in the NC film, annealing time, and annealing temperature affect the mechanisms by which the polycrystalline films form and how their microstructure evolves. Depending on the annealing conditions, the CZTS NCs sinter and grow to sizes ranging from a hundred nanometers to a few microns. In addition to sintering, we observe abnormal grain growth, which, if encouraged, can lead to formation of single-crystal CZTS grains up to 10 microns in size. Raman scattering spectra collected from these large CZTS crystals show that the crystals have texture, preferring two orientations. The surface energy difference between the NCs and the large grains is the driving force for abnormal grain growth, which appears to be enhanced at high temperatures but reduced significantly on SLG and at high S pressures. The abnormal grain growth can be turned on or off via control of these variables.