Utilizing Amine-Thiol Molecular Precursors for Ag2ZnSnSe4 Thin Films

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 quality.