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.

The solution processing of Cu­(In,Ga)­(S,Se)2 photovoltaics from colloidal nanoparticles has long suffered from deleterious carbonaceous residues originating from long chain native ligands. This impurity carbon has been observed to hinder grain formation during selenization and leave a discrete residue layer between the absorber layer and the back contact. In this work, organic and inorganic ligand exchanges were investigated to remove tightly bound native oleylamine ligands from Cu­(In,Ga)­S2 nanoparticles, thereby removing the source of carbon contamination. However, incomplete ligand removal, poor colloidal stability, and/or selective metal etching were observed for these methods. As such, an exhaustive hybrid organic/inorganic ligand exchange was developed to bypass the limitations of individual methods. A combination of microwave-assisted solvothermal pyridine ligand stripping followed by inorganic capping with diammonium sulfide was developed and yielded greater than 98% removal of native ligands via a rapid process. Despite the aggressive ligand removal, the nanoparticle stoichiometry remained largely unaffected when making use of the hybrid ligand exchange. Furthermore, highly stable colloidal ink formulations using nontoxic dimethyl sulfoxide were developed, supporting stable nanoparticle mass concentrations exceeding 200 mg/mL. Scalable blade coating of the ligand-exchanged nanoparticle inks yielded remarkably smooth and microcrack free films with an RMS roughness less than 7 nm. Selenization of ligand-exchanged nanoparticle films afforded substantially improved grain growth as compared to conventional nonligand-exchanged methods, yielding an absolute improvement in device efficiency of 2.8%. Hybrid ligand exchange nanoparticle-based devices reached total area power conversion efficiencies of 12.0%, demonstrating the feasibility and promise of ligand-exchanged colloidal nanoparticles for the solution processing of Cu­(In,Ga)­(S,Se)2 photovoltaics.

Large-scale deployment of photovoltaic modules is required to power our renewable energy future. Kesterite, Cu 2 ZnSn(S, Se) 4 , is a p-type semiconductor absorber layer with a tunable bandgap consisting of earth abundant elements, and is seen as a potential 'drop-in' replacement to Cu(In,Ga)Se 2 in thin film solar cells. Currently, the record light-to-electrical power conversion efficiency (PCE) of kesterite-based devices is 12.6%, for which the absorber layer has been solutionprocessed. This efficiency must be increased if kesterite technology is to help power the future. Therefore two questions arise: what is the best way to synthesize the film? And how to improve the device efficiency? Here, we focus on the first question from a solution-based synthesis perspective. The main strategy is to mix all the elements together initially and coat them on a surface, followed by annealing in a reactive chalcogen atmosphere to react, grow grains and sinter the film. The main difference between the methods presented here is how easily the solvent, ligands, and anions are removed. Impurities impair the ability to achieve high performance (>∼10% PCE) in kesterite devices. Hydrazine routes offer the least impurities, but have environmental and safety concerns associated with hydrazine. Aprotic and protic based molecular inks are environmentally friendlier and less toxic, but they require the removal of organic and halogen species associated with the solvent and precursors, which is challenging but possible. Nanoparticle routes consisting of kesterite (or binary chalcogenides) particles require the removal of stabilizing ligands from their surfaces. Electrodeposited layers contain few impurities but are sometimes difficult to make compositionally uniform over large areas, and for metal deposited layers, they have to go through several solid-state reaction steps to form kesterite. Hence, each method has distinct advantages and disadvantages. We review the stateof-the art of each and provide perspective on the different strategies.

The amine−thiol solvent system has been used extensively to synthesize metal chalcogenide thin films and nanoparticles because of its ability to dissolve various metal and chalcogen precursors. While previous studies of this solvent system have focused on understanding the dissolution of metal precursors, here we provide an in-depth investigation of the dissolution of chalcogens, specifically Se and Te. Analytical techniques, including Raman, X-ray absorption, and NMR spectroscopy and highresolution tandem mass spectrometry, were used to identify pathways for Se and Te dissolution in butylamine−ethanethiol and ethylenediamine−ethanethiol solutions. Se in monoamine−monothiol solutions was found to form ionic polyselenides free of thiol ligands, while in diamine−monothiol solutions, thiol coordination with polyselenides was predominately observed. When the relative concentration of thiol is increased to that of Se, the chain length of polyselenide species was observed to shorten. Analysis of Te dissolution in diamine−thiol solutions also suggested the formation of relatively unstable thiol-coordinated Te ions. This instability of Te ions was found to be reduced by codissolving Te with Se in diamine−thiol solutions. Analysis of the codissolved solutions revealed the presence of atomic interaction between Se and Te through the identification of Se−Te bonds. This new understanding then provided a new route to dissolve otherwise insoluble Te in butylamine−ethanethiol solutions by taking advantage of the Se 2− nucleophile. Finally, the knowledge gained for chalcogen dissolutions in this solvent system allowed for controlled alloying of Se and Te in PbSe n Te 1−n material and also provided a general knob to alter various metal chalcogenide material syntheses.

Colloidal metal chalcogenide nanoparticles have emerged as a promising hydrazine-free route for the fabrication of solution processed electronic devices. While a wide variety of synthetic pathways have been developed for these nanomaterials, typical colloidal syntheses rely on the use of metal salts as precursors, which contain anionic impurities such as halides, nitrates, acetates, and so forth, that may incorporate and alter the electrical properties of the targeted nanoparticles. In this report, the recent advances in aminethiol chemistry and its unique ability to dissolve pure metals, chalcogens, and metal chalcogenides is expanded upon for the fabrication of metal chalcogenide nanoparticles. Alkylammonium metal thiolate species are easily formed upon addition of monoamine and dithiol to elemental Cu, In, Ga, Sn, Zn, Se, or metal chalcogenides such as Cu 2 S and Ag 2 S. These species were then used directly for the synthesis of colloidal nanoparticles without the need for any additional purification. The thermal decomposition pathway of one such representative alkylammonium metal thiolate species was studied, verifying that only metal chalcogenides and volatile byproducts are formed, providing a flexible route to compositionally uniform, phase pure, and anionic impurity-free colloidal nanoparticles. Synthetic methods were developed from these precursors to yield pure phase colloidal nanoparticles of binary, ternary, and quaternary materials and their alloys including In 2 S 3 , (In x Ga 1−x ) 2 S 3 , CuInS 2 , CuIn(S x Se 1−x ) 2 , Cu(In x Ga 1−x )S 2 , Cu 2 ZnSnS 4 , and AgInS 2 . Successful synthesis with various experimental methods such as heat up, hot injection, and microwave assisted solvothermal reactions were also demonstrated, showing the flexibility and greater scope for this new synthesis route.