(656a) 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

*michael.casciato@chbe.gatech.edu

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^[1]. In
particular, copper indium gallium selenide (CuIn_xGa_1-xSe₂,
CIGS) thin film solar cells have achieved solar conversion efficiency nearing
20%^[2].
However, CIGS thin films require the use of expensive, scarce, and toxic
compounds which may inhibit wide scale implementation of CIGS devices^[3]. 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%^[4],
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^[5],
electrodeposition^[6], and
coevaporation^[7] of
metals followed by sulfurization and annealing are three widely used techniques
to fabricate CZTS. Metal dithiocarbamate precursors have been used in liquid^[8] and
chemical vapor deposition^[9] (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^[10]. 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^[11].
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^[4].

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