(135h) Solution Phase Fabrication of Cu(In,Ga)(S,Se)2 Thin-Films Using Amine-Thiol Solvent System

Among various PV materials, Cu(In,Ga)(S,Se)₂ is a promising thin-film material not just because it has achieved cell efficiencies above 23% but also because it allows for bandgap tuning to as low as 1 eV (CuInSe₂) bandgap which makes it an ideal candidate for tandem solar cell applications. Currently, solution-based Cu(In,Ga)(S,Se)₂ has a record cell efficiency of ~18% which was realized using highly toxic and explosive hydrazine solvent.¹ While the various hydrazine-free molecular precursor deposition approaches are being researched, the amine-thiol solvent system appears to be an attractive route due to its ability to dissolve pure metals and metals chalcogenides² which avoid incorporation of any impurity arising from metal precursors like halides, oxides, nitrates, acetates, and other similar precursors. In this work, detailed speciation of the amine-thiol solvent system, its chemical transformation to phase pure material, and optimization of thin-film deposition is performed in order to develop a scalable high-efficiency PV fabrication process.

Precursor dissolutions in amine-thiol solutions, specifically elemental metals like Cu, In and chalcogen like Se, were studied using X-ray absorption, Mass spectrometry, NMR, Raman, and XRD analysis to identify the organometallic complexes and their reaction pathways.³ Based on the insights gained from these analyses, we eliminated the necessity of using amine-thiol mixture as a fabrication solvent and rather utilized it as a reactant species to generate metal thiolate compounds. These compounds were then diluted with benign solvents like DMSO, DMF, acetonitrile etc. in order to reduce the overall reactivity and corrosivity associated with the system, making it a feasible route for scalable film deposition. The thiolates formed in these dissolutions were then studied for the decomposition mechanism using a heat treatment that realized its clean conversion into metal sulfide along with the formation of volatile by-products.⁴ Sulfide films fabricated from the solution of Cu/Cu₂S and In/Ga precursors were then annealed in selenium atmosphere to obtain desired selenide material (CuIn_xGa_1-xSe₂) and analyzed under STEM-EDS which confirmed the removal of carbonaceous fine grain layer from the film architecture. Devices fabricated with this impurity-free fabrication process produced total area efficiencies >12% for Ga free CuInSe₂ material without any heavy alkali doping or bandgap grading.

While promising performance was achieved with this approach, the vapor phase selenization of sulfide material always poses a challenge for uniform conversion throughout the film as well as for uniform film morphology, especially when thicker and smoother films are needed for better PV performance. Unlike other hydrazine-free routes, the amine-thiol solvent system allows for dissolution of elemental selenium which provides an opportunity to incorporate selenium in the film prior to selenization. Using techniques like SEM, Raman, XRD, TRPL, PL, CV, etc., we studied this selenium incorporation approach in precursor ink and compared it with traditional Se-free precursor films for morphological as well as electrical properties improvement. Based on our understanding of Se incorporation, we have developed a process which eliminates the need for vapor phase selenization and can grow micron-size grains by just high-temperature annealing, making it an attractive advancement for scaling up of the film fabrication process.

The impurity-free benign ink formulation strategies, thin-film deposition techniques, and device fabrication methods developed in this work can be further applied to a variety of semiconducting chalcogenide material for scalable fabrication of electronic devices.

(1) Zhang, T.; Yang, Y.; Liu, D.; Tse, S. C.; Cao, W.; Feng, Z.; Chen, S.; Qian, L. Energy Environ. Sci. 2016, 9 (12), 3674–3681.

(2) Zhang, R.; Cho, S.; Lim, D. G.; Hu, X.; Stach, E. A.; Handwerker, C. A.; Agrawal, R. Chem. Commun. 2016, 846 (1–4), 31–39.

(3) Zhao, X.; Deshmukh, S. D.; Rokke, D. J.; Zhang, G.; Wu, Z.; Miller, J. T.; Agrawal, R. Chem. Mater. 2019, 31 (15), 5674–5682.

(4) Deshmukh, S. D.; Ellis, R. G.; Sutandar, D. S.; Rokke, D. J.; Agrawal, R. Chem. Mater. 2019, 31 (21), 9087–9097.