Solution processing of chalcogenide functional materials using thiol–amine “alkahest” solvent systems

In the face of anthropogenic climate change we must begin to think and act based on a systems approach rather than addressing problems as discrete events. Regarding the design of products, we should not only design for sustainability in the direct use of the product, but also for low‐impact fabrication as well as possible reuse, upcycling, or recycling. Specifically, for the design of functional inorganic materials, such as catalysts and semiconductors, we must consider the various impacts, beyond functionality, of choosing specific elements and sources to incorporate into materials. A second aspect of a systems approach to functional materials is the design of versatile, low‐impact, high‐throughput fabrication processes. Currently, the industry standards for fabricating functional inorganic materials are energetically and financially burdensome, requiring capital‐intensive hardware and energy‐intensive deposition processes. As such, there is room for improvements toward more cost‐ and energy‐effective manufacturing methods. ❧ The deposition of molecular precursor inks prepared from bulk materials is an ideal approach to material preparation, as it requires only a single dissolution step followed by direct deposition onto a suitable substrate. Additionally, the composition of molecular inks can be easily tuned by simply changing the formulation of the ink, and the very nature of such precursor inks facilitates thorough mixing on the molecular level to yield more compositionally homogeneous films. With these advantages in mind, our group developed a novel binary solvent system comprised of ethylenediamine (en) and a short chain thiol that is capable of dissolving bulk inorganic precursors, including chalcogenides, oxides, and elemental materials to give solution processable molecular inks. Since its initial discovery, this “alkahest” solution processing method has been employed for the deposition of a wide range of Earth‐abundant functional inorganic thin films including semiconductors for photovoltaic (PV) devices, electrocatalysts, thermoelectrics, and photodetectors. This method is an example for which a systems approach has been used to couple Earth‐abundant compositions with sustainable manufacturing and the possibility for reuse or recycling. ❧Since the seminal report on the solvent system, wherein the dissolution of nine bulk V₂VI₃ chalcogenide semiconductors were shown to dissolve at room temperature, under ambient pressure, and over a short time, we and others have worked toward broadening the scope of soluble precursors. We studied the solubility of ten bulk oxides, showing that upon annealing their respective alkahest inks phase pure sulfides could be recovered. In some cases, when elemental selenium or tellurium was added to the ink, selenides or tellurides could be made. Using Sb₂O₃ and selenium as an example system, we showed how varying the nominal content of selenium in the ink formulation could lead to a series of Sb₂Se₃₋ₓSₓ alloys with tunable band gaps from 1.2 – 1.6 eV. Using Sb₂S₃ and Cu₂S as precursors, we prepared high quality thin films of CuSbS₂, an Earth‐abundant alternative to CuInGaS₂ as a PV absorber. We demonstrated that the alkahest‐processed CuSbS₂ films had optoelectronic properties promising for application in PV devices. ❧ In addition to absorber materials for PV, we applied the alkahest method to the preparation of Earth‐abundant alternatives to platinum group metal electrocatalysts for the hydrogen evolution reaction (HER). We first prepared nano‐structured marcasite‐type CoSe₂ by annealing an ink formulated using Co(OH)₂ and elemental selenium as precursors. Thin films of CoSe₂ that were spin coated onto Highly Ordered Pyrolytic Graphite (HOPG) facilitated HER with 100% Faradaic efficiency and key electrocatalysis parameters on par with previous reports for similar materials prepared by other methods. By comparing samples with various CoSe₂ loadings, we were able to demonstrate further value of the alkahest method in the ability to easily vary the film thickness by simply adjusting the number of ink coats, which lead to optimization of catalyst utilization. Next we prepared pyrite‐type NiSe₂ using elemental nickel and selenium as precursors. Comparison of the HER performance for various NiSe₂ loadings showed good catalyst utilization for all loadings, indicating the ability to use thicker films to achieve higher hydrogen production per unit of geometric surface area. ❧ In our most recent work, we reported on the first solution synthesis of Cu₂BaSn(S,Se)₄ (CBTS). This material has gained recent interest as an alternative to Cu₂ZnSn(S,Se)₄ (CZTS) because of its higher theoretical PV performance stemming from the rendering of antisite defects energetically unfavorable upon replacement of zinc with a larger atom, barium. We used Cu₂S, BaS, and SnO as precursors in an ethanedithiol/en‐based formulation, which after several days of mixing gave a thick solution that could be annealed to yield CBTS. Thorough material characterization verified the phase purity, elemental composition, elemental oxidation states, and band gap of the CBTS. Although the as‐made ink was viscous and not able to be spin coated directly, we were able to prepare uniform CBTS thin films by diluting the ink with 2-methoxyethanol. We were also able to show that by incorporating elemental selenium into the ink formulation a set of Cu₂BaSnS₄₋ₓSeₓ alloys with experimentally determined values from x = 0 – 2, could be prepared with tunable band gaps from 1.56-1.86 eV, thus demonstrating a straightforward handle for engineering optimized photovoltaic absorbers.