Investigating Chemistry of Metal Dissolution in Amine–Thiol Mixtures and Exploiting It toward Benign Ink Formulation for Metal Chalcogenide Thin Films

Abstract

Solution processing of metal chalcogenides using elemental metals dissolved in an amine–thiol solvent mixture has recently received a great deal of attention for the fabrication of thin-film optoelectronic devices. However, little is known about the dissolution pathway for metallic precursors in such mixtures. To exploit the full potential of this method, it is essential that a detailed understanding of the dissolution chemistry be developed. In this study, we use several characterization techniques to examine these solutions and then propose reaction mechanisms for In and Cu dissolutions in a hexylamine/1,2-ethanedithiol mixture. These dissolutions are rather found to be reactions resulting in metal oxidation with coevolution of H2 forming bis­(1,2-ethanedithiolate)­indium­(III) in the case of indium dissolution and high nuclearity Cu­(I) thiolate compounds in case of copper dissolution. This understanding allowed us to address the issue of toxicity and corrosivity associated with amine–thiol solvent by utilizing it as a reactant rather than a solvent for ink formulation. Here, we demonstrate a new approach whereby metal complexes formed by dissolving a range of metals including Cu, In, Zn, Sn, Se, and Ga with Se in amine–thiol solution can first be isolated by evaporation of the precursor solution and then dissolved in a variety of weakly coordinating organic solvents to provide a benign and stable solution free of unreacted amine and thiol for thin-film fabrication of various chalcogenide semiconductors. We utilize this new approach to demonstrate the fabrication of CuIn­(S,Se)2 solar cells using dimethyl sulfoxide as a fabrication solvent.

Present Status of Solution‐Processing Routes for Cu(In,Ga)(S,Se)2 Solar Cell Absorbers

“…Molecular inks (MIs) supply most or all the constituent elements for the final semiconductor, i.e., Cu, In, Ga, and in some cases Se and/or S. They are commonly prepared from metal salts and volatile solvents but can include additional agents such as cosolvents and binders. [ 24,54–56 ] This is in contrast to colloidal nanoparticle (NP) inks, which require carefully prepared solid nanoparticles that undergo several washing steps prior to ink formulation and are stabilized in the ink by ways of dispersants, charge, and/or Brownian motion. [ 57–59 ] Unlike NP dispersions, slurries, or suspensions, molecular inks can be considered a single‐phase homogenous liquid, i.e., a true solution.…”

“…[ 50,62,88 ] Similarly, even anions from metal salt precursors (e.g., NO 3 − ) can oxidize volatile alcohol solvents to create residual carbonaceous impurities in the absorber. [ 89 ] In fact, a major challenge in MI routes entails the adequate removal of residual and nonvolatile impurities such as oxygen (2 wt%, [ 90 ] 5–7 at%, [ 91 ] 15 wt% [ 92 ] ), chlorine (3 wt% [ 90 ] ), carbon (4–7 wt%, [ 55,93 ] 2–3 at% [ 91 ] ), and organic moieties, [ 94 ] from the deposited film.…”

“…[ 24,76 ] More commonly, however, protic organic solvents thiol–amine “alkahest” and alcohols (R‐OH) and aprotic organic solvents such as dimethylformamide and dimethyl sulfoxide have been employed for MI routes. [ 24,45,55,89 ] In the case of protic alcohols, oxygen and carbon impurities were often prevalent in final absorber films, limiting device performance. [ 62,71,88,104 ] The protic amine–thiol (R‐S‐H) solvent system, for example, exhibits a reactive H termination which can make it difficult to remove metal‐sulfur‐rest (M‐S‐R) groups created from addition reactions with the R‐S‐H group.…”

“…To develop a universal solvent system having the solvation capability and volatility of hydrazine, while addressing concerns about the latter's toxicity, Brutchey's group at the University of Southern California (USC) reported en/EDT (amine–thiol) mixtures for the dissolution of bulk chalcogenide compounds at room temperature, ambient pressure, and in air. [ 146 ] For PV applications, the amine–thiol processing route offers several advantages including i) permission of a wide range of solvent volatilities (monoamine–monothiol, monoamine–dithiol, diamine–dithiol, and diamine–monothiol), which can be tailored to design specific routes for deposition of different chalcogenide materials, [ 147 ] ii) control of chalcogenides solubility by adjusting the amine–thiol ratio in the solvent mixture, [ 55 ] iii) dissolution of a variety of metal reactants such as pure metals, metal salts, metal oxides, and elemental chalcogenides to prepare stable molecular solutions, [ 32 ] and iv) flexibility of choice of four binary solvent pairs with different solvent properties, which can be used for washing and separation processes. [ 147 ] Notably, record PCEs of 16.4% (with ARC) have been reported from amine–thiol molecular inks in 2020 (see Table 5 ).…”

“…For example, metal thiolate ions can be separated from the amine–thiol solvent system and resolvated in benign solutions (e.g., DMSO, DMF, and 2‐methoxyethanol) to fabricate chalcogenide semiconductors. [ 55 ] This may provide a less toxic alternative route to use this solvent system.…”