Swapnil D. Deshmukh

Solution-processed CuInSe₂ films have generally relied on sulfide or sulfoselenide precursor films that, during the grain growth process, hamper the growth of thicker films and lead to the formation of a fine-grain layer. However, recent research has indicated that sulfur reduction in the precursor film modifies the grain growth mechanism and may enable the fabrication of thicker absorbers that are free of any fine-grain layer. In this work, we pursue direct solution deposition of sulfur-free CuInSe₂ films from the molecular precursor approach. To this end, we tune the amine-thiol reactive solvent system and study the changes to the resulting soluble complexes through a combination of analytical techniques. We show that by reactively dissolving indium(III) selenide and selenium in solutions of n-butylamine and 1,2-ethanedithiol, a metal thiolate species is formed, and that this metal thiolate can be modified by isolation from the thiol-containing solvent via precipitation. As the quantity of selenium in the ink increases, the thiol content in the complex decreases, eventually producing soluble [InSe_x]^- species. Extending this method to be used with copper selenide as a copper source, molecular precursor inks can be made for solution-processed, sulfur-free CuInSe₂ films. We then show that these CuInSe₂ precursor films can be fully coarsened without a fine-grain layer formation, even at the desired thicknesses of 2 μm and greater.

Solutions containing n-alkylammonium polyselenides constitute a new reactive solvent system. This reactive dissolution chemistry is broadly generalizable and can enable solution processed synthesis of a wide range of metal chalcogenide semiconductors, both as thin films and nanoparticles.

Fine grain layer formation in solution processed CuInSe ₂ /CuInGaSe ₂ (CISe/CIGSe) photovoltaic devices is controlled through surface modifications and ink manipulations to achieve improved performance and fine grain free morphologies.

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 high-resolution 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_nTe_1–n material and also provided a general knob to alter various metal chalcogenide material syntheses.

Large-scale deployment of photovoltaic modules is required to power our renewable energy future. Kesterite, Cu₂ZnSn(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)Se₂ 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 solution-processed. 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 state-of-the art of each and provide perspective on the different strategies.