Formation
of Copper Zinc Tin Sulfide Thin Films from Colloidal Nanocrystal Dispersions
via Aerosol-Jet Printing and Compaction
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Abstract
A three-step
method to create dense polycrystalline semiconductor thin films from
nanocrystal liquid dispersions is described. First, suitable substrates
are coated with nanocrystals using aerosol-jet printing. Second, the
porous nanocrystal coatings are compacted using a weighted roller
or a hydraulic press to increase the coating density. Finally, the
resulting coating is annealed for grain growth. The approach is demonstrated
for making polycrystalline films of copper zinc tin sulfide (CZTS),
a new solar absorber composed of earth-abundant elements. The range
of coating morphologies accessible through aerosol-jet printing is
examined and their formation mechanisms are revealed. Crack-free albeit
porous films are obtained if most of the solvent in the aerosolized
dispersion droplets containing the nanocrystals evaporates before
they impinge on the substrate. In this case, nanocrystals agglomerate
in flight and arrive at the substrate as solid spherical agglomerates.
These porous coatings are mechanically compacted, and the density
of the coating increases with compaction pressure. Dense coatings
annealed in sulfur produce large-grain (>1 μm) polycrystalline
CZTS films with microstructure suitable for thin-film solar cells