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Utilizing Amine-Thiol Molecular Precursors for Ag2ZnSnSe4 Thin Films
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Thin film photovoltaic materials have garnered much interest recently due to their processability in addition to good properties for conversion of solar photons to usable energy. Amine-thiol chemistry has shown the ability to produce solution processed materials such as Cu2ZnSn(S,Se)4 (CZTSSe), a thin film absorber composed of earth abundant metals. Using similar solution processing methods as those used to produce CZTS, we wish to synthesize a phase pure solution processed material from molecular precursors of metals and metal chalcogenides into an Ag2ZnSnSe4 absorber which lacks the electronic defects that plague CZTSSe. Additionally, we will utilize the reactive dissolution of metal in amine-thiol solution chemistry for a more detailed understanding of how metal-sulfur complexes form and then decompose into films, to gain insight about the conditions that produce stable solutions and high quality films for a better ability to optimize processing conditions.
We find we are able to individually
dissolve zinc metal, tin metal, and silver sulfide precursors to produce
solutions of metal thiolate complexes. Based on results from electrospray
ionization mass spectrometry (ESI-MS), proton nuclear magnetic resonance (1H-NMR),
and extended X-ray absorption fine structure (EXAFS)/ X-ray absorption near
edge spectra (XANES) we propose that these structures contain thiolate
molecules coordinated with Ag, Zn, and Sn in the +1, +2, and +2 oxidation
states respectively. However, mixing these produces an AZTS solution which is
only stable for 3 hours, due to a redox reaction between Ag+ and Sn2+
which forms Sn4+ and insoluble Ag metal. To solve this, we
synthesize SnS2 and show this produces a different Sn-thiolate
complex with fully oxidized Sn4+. This is then used to produce the
first stable AZTS solution, an essential step to fabricating reproducible films.
We use this AZTS solution to fabricate films containing AZTS, and selenize
these films in a tube furnace to produce films which contain AZTSe as well as
secondary phases. We then use rapid thermal processing furnace to remove some
of these secondary phases, and discuss ways to further improve our material
Collaborative Research: NRT-INFEWS: Sustainable Food, Energy, and Water Systems (SFEWS)
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