263958 Synthesis of CZTS Nanomaterials Via a Continuous Flow Supercritical Carbon Dioxide Process
Synthesis of CZTS Nanomaterials Via a Continuous Flow Supercritical Carbon Dioxide Process
Michael J. Casciato*, Galit Levition, Dennis W. Hess, Martha A. Grover
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA
Direct band gap thin film solar cells have attracted attention over the last 20 years as a potential alternative to indirect band gap silicon-based solar cells. In particular, copper indium gallium selenide (CuInxGa1-xSe2, CIGS) thin film solar cells have achieved solar conversion efficiency nearing 20%. However, CIGS thin films require the use of expensive, scarce, and toxic compounds which may inhibit wide scale implementation of CIGS devices. A related chalcogenide material, copper zinc tin sulfide (Cu2ZnSnS4, CZTS), has been explored recently because it is composed of relatively less expensive, more abundant, and less toxic materials compared to CIGS. The highest photoconversion efficiency reported using the CZTS material is 10.1%, although this device was based on CZT(S,Se), which possesses selenium in addition to CZTS and therefore is less green than CZTS alone.
Sputtering, electrodeposition, and coevaporation of metals followed by sulfurization and annealing are three widely used techniques to fabricate CZTS. Metal dithiocarbamate precursors have been used in liquid and chemical vapor deposition (CVD) processes to form CZTS nanocrystals and thin films directly, without sulfurization. However, none of these techniques is well-suited for depositing CZTS particles or thin films onto structures that possess high aspect ratios or high tortuosity due to solubility and/or transport limitations. Nonetheless, it may still be desirable to decorate high aspect ratio/tortuosity nanostructures with CZTS particles or thin films. For example, CZTS particles could be anchored on Si nanowires or carbon nanotubes to improve photoelectrochemical cell performance. Thus, a technique that can efficiently cover high aspect ratio/tortuosity nanostructures with CZTS particles or films would be advantageous.
Much work has been performed detailing the deposition of metal thin films and nanoparticles into high aspect ratio/tortuosity structures using supercritical carbon dioxide (sc-CO2) because sc-CO2 possesses liquid-like solubility and gas-like diffusivity. Furthermore, sc-CO2 is a green, sustainable processing technique, employing recycled CO2 as a solvent for precursors.
In this work, a sc-CO2 continuous flow reactor (CFR) is employed to deposit CZTS micro- and nanoparticles on a silicon wafer from metal dithiocarbamate precursors. This work demonstrates that the sustainable sc-CO2 CFR process is a viable technique for fabricating CZTS particles, establishing the potential for future studies to deposit CZTS in high aspect ratio/tortuosity nanostructures. Moreover, no postprocessing was required to form the kesterite CZTS phase, although annealing at high temperature and selenization may still be necessary to yield an efficient solar device, as mentioned above.
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