No description available.

<p>As global population continue to rise, the demand for energy is slated to increase substantially. To combat climate change, large amounts of renewable energy will be needed to feed this growing demand. Of renewable energy sources, photovoltaics are well positioned to meet this increasing demand due to the immense abundance of solar energy incident on earth. However, existing energy intensive, low throughput, and costly manufacturing techniques for photovoltaics may pose a barrier to continued large scale implementation.</p> <p>Solution processing has emerged as a promising photovoltaics fabrication technique with high throughput, high materials utilization, and lower cost than existing vacuum-based methods. Thin film photovoltaic materials such as Cu(In,Ga)(S,Se)₂ and CdTe have both been fabricated using various solution processing methods. Of the various solution processing routes, colloidal metal chalcogenide nanoparticles have demonstrated promise as a hydrazine-free route for the solution processing of high efficiency Cu(In,Ga)(S,Se)₂ solar cells. However, conventional solution processing with colloidal nanoparticles has long suffered from anionic and carbonaceous impurities, stemming from legacy synthesis methods. The work in this dissertation aims to solve these issues through the development of novel synthetic methods, ligand engineering, and ultimately improved scalability through slot-die coating.</p> <p> Typical colloidal syntheses rely on the use of metal salts as precursors such as metal halides, nitrates, acetates, and so forth, where the anions may incorporate and alter the electrical properties of the targeted nanomaterials. In this work, the recent advances in amine-thiol chemistry and its unique ability to solubilize many metal containing species are expanded upon. 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 Cu₂S and Ag₂S. These species were then used directly for the synthesis of colloidal nanoparticles without the need for any additional purification. The metal thiolate thermal decomposition pathway 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 including successful syntheses of In₂S₃, (In_xGa_1–x)₂S₃, CuInS₂, CuIn(S_xSe_1–x)₂, Cu(In_xGa_1–x)S₂, Cu₂ZnSnS₄, and AgInS₂. </p> <p>However, further impurities from deleterious carbonaceous residues originating from long chain native ligands were still a persistent problem. 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. An exhaustive hybrid organic/inorganic ligand exchange was developed in this work to remove tightly bound oleyalmine ligands through a combination of microwave-assisted solvothermal pyridine ligand stripping followed by inorganic capping with diammonium sulfide, yielding 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. Scalable blade coating of the ligand exchanged nanoparticle inks from non-toxic dimethyl sulfoxide inks yielded remarkably smooth and crack free films with RMS roughness less than 7 nm. Selenization of ligand exchanged nanoparticle films afforded substantially improved grain growth as compared to conventional non-ligand 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%.</p> <p>While extremely effective in ligand removal, ligand exchange pathways increase process complexity and solvent usage substantially, which may limit the cost advantage solution processing aims to provide. Further synthesis improvement was developed through a ligand exchange free, direct sulfide capped strategy. Using sulfolane as a benign solvent, CuInS₂ nanoparticles with thermally degradable thioacetamide ligands were synthesized using thermal decomposition of isolated metal thiolates from Cu₂S and In precursors. Through gentle thermal treatment, these ligands decomposed into non-contaminating gaseous byproducts leaving carbon free nanoparticle films without the need for ligand exchange.</p> <p>With the development of virtually contamination free colloidal nanoparticle inks, focus was shifted to scalability using slot die coating. Unlike typical lab-scale coating techniques such as spin coating, slot die coating is a widely used industrial coating technique with nearly 100% materials utilization, and high throughput roll-to-roll compatibility. A custom lab-scale slot-die coater was used to rapidly proof coating conditions, which were rapidly analyzed for uniformity using absorbance scanning in conjunction with profilometry. A cosolvent chlorobenzene/dichlorobenzene ink was developed to yield highly uniform, crack free thin films from non-ligand-exchanged Cu(In,Ga)S₂ nanoparticles, which were finished into devices with champion total are efficiencies of 10.7%. To the best of our knowledge, this represents the first report of slot die coated Cu(In,Ga)(S,Se)₂ photovoltaics. The methods presented in this work offer a pathway towards low impurity, high efficiency, scalable solution processed Cu(In,Ga)(S,Se)₂ photovoltaics to enable low cost renewable energy.</p>

<div>Inorganic metal chalcogenide materials are of great importance in the semiconducting field for various electronic applications such as photovoltaics, thermoelectrics, sensors, and many others. Compared to traditional vacuum processing routes, solution processing provides an alternate cost-effective route to synthesize these inorganic materials through its ease of synthesis and device fabrication, higher material utilization, mild processing conditions, and opportunity for roll-to-roll manufacturing. One such versatile solution chemistry involving a mixture of amine and thiol species has evolved in the past few years as a common solvent for various precursor dissolutions including metal salts, metal oxides, elemental metals, and chalcogens.</div><div><br></div><div>The amine-thiol solvent system has been used by various researchers for the fabrication of inorganic materials, but without the complete understanding of the chemistry involved in this system, utilizing its full potential, and overcoming any inherent limitations will be difficult. So, to identify the organometallic complexes and their reaction pathways, the precursor dissolutions in amine-thiol solutions, specifically for elemental metals like Cu, In and chalcogens like Se, Te were studied using X-ray absorption, nuclear magnetic resonance, infrared, and Raman spectroscopy along with electrospray ionization mass spectrometry techniques. These analyses suggested the formation of metal thiolate complexes in the solution with the release of hydrogen gas in the case of metal dissolutions confirming irreversibility of the dissolution. Insights gained for chalcogen dissolutions confirmed the formation of different species like monoatomic or polyatomic clusters when different amine-thiol pair is used for dissolution. Results from these analyses also identified the role of each component in the dissolution which allowed for tuning of the solutions by isolating the complexes to reduce their reactivity and corrosivity for commercial applications.</div><div><br></div><div>After identifying complexes in metal dissolution for Cu and In metals, the decomposition pathway for these complexes was studied using X-ray diffraction and gas chromatography mass spectrometry techniques which confirmed the formation of phase pure metal chalcogenide material with a release of volatile byproducts like hydrogen sulfide and thiirane. This allowed for the fabrication of impurity-free thin-film Cu(In,Ga)S2 material for use in photovoltaic applications. The film fabrication with reduced carbon impurity achieved using this solvent system yielded a preliminary promising efficiency beyond 12% for heavy alkali-free, low bandgap CuInSe2 material. Along with promising devices, by utilizing the understanding of the chalcogen complexation, a new method for CuInSe2 film fabrication was developed with the addition of selenide precursors and elemental selenium which enabled first-ever fabrication of a solution-processed CuInSe2 thin film with thickness above 2 μm and absence of any secondary fine-grain layer.</div><div><br></div><div>Along with thin-film fabrication, a room temperature synthesis route for lead chalcogenide materials (PbS, PbSe, PbTe) with controlled size, shape, crystallinity, and composition of nanoparticle self-assemblies was demonstrated. Micro-assemblies formed via this route, especially the ones with hollow-core morphology were subjected to a solution-based anion and cation exchange to introduced desired foreign elements suitable for improving the thermoelectric properties of the material. Adopting from traditional hot injection and heat up synthesis routes, a versatile synthesis procedure for various binary, ternary, and quaternary metal chalcogenide (sulfide and sulfoselenide) nanoparticles from elemental metals like Cu, Zn, Sn, In, Ga, and Se was developed. This new synthesis avoids the incorporation of impurities like O, Cl, I, Br arising from a traditional metal oxide, halide, acetate, or other similar metal salt precursors giving an opportunity for truly impurity-free colloidal metal chalcogenide nanoparticle synthesis.</div>