(574g) Understanding the Formation and Pyrolysis of Metal Thiolate Complexes for Solution-Processed Thin Film Photovoltaics
Recent work conducted on the fabrication of CIGSe photovoltaic devices using amine-thiol mixtures yielded a champion efficiency of 13.12%, achieved through the implementation of a gallium gradient within the device. Despite these successes, there has been no work that the authors are aware of that has investigated the underlying chemistry of amine-thiol reactions with pure metals and the way in which they form the desired molecular precursors. This represents a significant gap in the understanding of these promising systems.
We will present our recent work determining the structure of the compounds formed when metallic copper, indium, and gallium are dissolved in a mixture consisting of a primary alkylamine (1-hexylamine or 1-propylamine) and 1,2-ethanedithiol. Data collected from XANES, EXAFS, Mass Spectrometry and Nuclear Magnetic Resonance spectroscopy shows that indium and gallium each form a complex in which they are chelated by two 1,2-ethanedithiolate ligands and suggests that copper may be forming a polymer consisting of Cu(I) cations and 1,2-ethylenedithiolate anions. Based on the identification of these reaction products, we have proposed a potential reaction pathway governing the dissolution of these metals. The resulting insights in to the underlying mechanisms of these reactions has enabled greater control over the use of the amine-thiol system, allowing us to reduce amine-thiol usage down to the stoichiometric quantities necessary for the reaction, minimizing residual carbon contamination in the final films and preventing the formation of undesirable by-products.
Following dissolution, these compounds are cast on molybdenum-coated glass using a blade-coating approach and annealed at 300ºC to drive off the organic ligands and form a copper indium gallium disulfide film, which can then be further processed into a complete device. We have done preliminary work to investigate the products released in the pyrolysis of these metal thiolate complexes, and initial results from pyrolysis-mass spectrometry suggests that ethylene sulfide is released upon heating. Further work is underway to confirm the identity of species evolved during decomposition, allowing for greater control over the processing of films formed with these complexes. Such an understanding will lead to greater control over the final device absorber layer, resulting in high-efficiency photovoltaic devices. Thus far, in our lab, champion device efficiencies of 11.8% for the CIGSe material system and 11.5% for the copper indium diselenide system (an amine-thiol record) have been achieved with this approach and both are expected to rise as further insights in to this system are uncovered and processing conditions are tailored accordingly.
Our work can be easily extended to other semiconductor materials. Other photovoltaic materials such as copper zinc tin sulfoselenide (CZTSSe) and CdTe have been processed with amine-thiol chemistry in our group, and lead chalcogenide thermoelectric materials are also under development. Future work extending this newly understood chemistry to these systems will help clear the way for high quality semiconductor films and particles. The chemistry described during this presentation has the potential to significantly improve the understanding of the amine-thiol system and represents a substantial contribution to the field.
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