Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS), copper zinc tin selenide (Cu2ZnSnSe4 or CZTSe) are being considered as sustainable alternatives to cadmium telluride and copper indium gallium selenide, the dominant light absorbing materials in commercial thin film solar cells. Thin CZTS films can be synthesized in many different ways including coevaporation from elemental sources in vacuum or sulfidation of copper-zinc-tin films deposited by sputtering. Recent advances in synthetic routes to CZTS nanocrystals and CZTS nanocrystal dispersions are beginning to enable alternative low-cost routes to making thin CZTS films for solar cells. In one strategy, CZTS layers are formed by coating appropriate substrates with colloidal nanocrystals from their dispersions in various liquids, evaporating the liquid and heating for sintering and grain growth. We have been exploring routes to making 1-3 micrometer thick polycrystalline thin films with 1-3 micrometer grains (e.g., a monolayer of large grains). First, we synthesize 2-25 nm diameter CZTS nanocrystals from copper, zinc and tin dithiocarbamate complexes in presence of oleic acid and oleylamine. Dispersions of these CZTS nanocrystals are stabilized either in organic nonpolar liquids such as toluene or in polar liquids such as water by steric and electrostatic stabilization, respectively. Following, we cast thin films from these dispersions onto various substrates including quartz and soda lime glass. After deposition, these colloidal nanocrystal films are placed in quartz tubes with pure S or pure Se, evacuated to 10-6 Torr, sealed, and heated for a variable time period at the desired annealing temperature. We find that the CZTS nanocrystals sinter and grow to sizes ranging from a few hundred nanometers to a few microns. In addition to sintering we observe both normal and abnormal grain growth. The extent to which each mechanism determines the final film microstructure depends on the temperature, the annealing atmosphere and the substrate.

Thin films from colloidal dispersions of copper zinc tin sulfide nanocrystals

246th National Meeting (2013)

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