Our group has previously shown that small changes to molecular structure result in large changes to device properties and stability in organic electronic applications. By functionalizing aromatic heteroacenes with group 14 and group 16 elements, it is possible to control morphology and improve stability for a variety of applications such as thin film transistors and solar cells. Functionalization within the heteroacene core led to changes in electronic structure as observed by electrochemistry and light absorption. By substituting down the periodic table, the carbon heteroatom bond length increased, leading to subtle changes in crystal packing. Absorption maxima were red-­‐shifted and stability to light decreased. Substitution of group 14 elements to the solubilizing ethynyl groups attached to the heteroacene also had an effect on crystallization and stability. Substitution of silicon with carbon decreased solubility as well as stability to light. Substitution with germanium also decreased stability to light, but close contacts within the crystal structure and solubility in nonpolar organic solvents increased.

Small molecule organic semiconductors such as phthalocyanines and their derivatives represent a very interesting alternative to inorganic semiconductor materials for the development of flexible electronic devices such as organic thin field effect transistors, organic Light Emitting Diodes and photo-voltaic cells. Phthalocyanine molecules can easily accommodate a variety of metal atoms as well in the central core of the molecule, resulting in wide range of magnetic properties. Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to sub-micron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. However, this can be overcome by fabricating macroscopically ordered semiconductor films by solution processing. Presence of fewer grain boundaries and defects while modifying the pi orbital overlap by controlling the molecular stacking by forming macroscopic long range ordered grains, was essential in understanding the delocalization and diffusion length and its role in magnetic exchange mechanisms in this system. The origins of magnetic exchange mechanism between delocalized ligand electrons and spins in organic semiconductors has been of key interest as it underlies many complex optical and transport properties and is investigated in this thesis. The interaction between magnetic ions in organic magnetic semiconductors is quite challenging to interpret due to competing exchange mechanisms present in crystalline thin films. Better understanding of these exchange mechanisms is essential for tuning of magnetic exchange interaction to suit the need of practical magnetic storage and spintronic devices. Optical techniques such as linear dichroism, magnetic circular dichroism and magneto-photoluminescence are used and provided key understanding about the relation between excitons, spin exchange mechanisms and collective magnetic behavior of delocalized electrons in organic semiconductors. An enhancement in the collective magnetization of the crystalline thin films with strong exchange coupling between the delocalized ligand electrons and d-electrons is observed. The electronic states responsible for magnetic exchange are identified utilizing magnetic field and temperature dependent studies. Furthermore, soluble organics allowed engineering of organic analogues to diluted magnetic semiconductors (DMS) by creation of metal/metal-free Pc alloys. Optical studies provided crucial information about delocalization and diffusion lengths in these systems allowing fine tuning of this delocalization length scale and metal-metal distance. The exploration of magnetic behavior in metal/metal-free Pc alloys opens an avenue for tuning magnetic properties through an exchange similar to Ruderman-Kittel-Kasuya Yosida (RKKY) type interaction in organic magnetic semiconductors.

L’industrialisation des cellules photovoltaïques organiques implique le développement de plusieurs aspects. Une augmentation des rendements de conversion, une amélioration de la stabilité et la mise au point de procédés de dépôt en ligne. Ce dernier point va passer par le développement de dépôt par voie liquide des différentes couches composant les dispositifs. Dans ma thèse je vais m’intéresser à un type de couche, les couches de transport de charges. Ces couches sont disposées entre la couche photo-active et les électrodes afin d’améliorer l’extraction des charges générées au sein de la première vers ces dernières. Je vais focaliser mon étude sur les couches de transport de trous. Afin de remplacer le matériau couramment utilisé (PEDOT:PSS), on utilise souvent les oxydes de métaux de transition.Ces matériaux habituellement évaporés, sont déposables en voie liquide à partir de suspensions de nanoparticules, ou de précurseurs (ex: sol-gel). J’ai développé 3 approches au cours de ma thèse. Dans la première, un dépôt par voie sol-gel d’oxyde de tungstène ou de vanadium a permis d’obtenir des rendements similaires à ce qui est obtenu avec les mêmes matériaux évaporés. Dans la deuxième approche un dépôt d’oxyde de cobalt (II, III),m’a permis d’améliorer l’extraction des charges. Néanmoins le matériau présente des difficultés de mise en forme ne permettant pas d’atteindre des rendements à l’état de l’art.Finalement une approche plus originale a été développée, une diffusion induite thermiquement d’un dopant, déposé par voie liquide à l’interface organique/métal m’a permis d’obtenir des rendements similaires à ce qui est obtenu avec des structures classiques. / In order to allow the industrialisation of organic photovoltaic cells, power conversion efficiency must be increased, stability must be improved, and in-line deposition processing (solution processing of each layer) must be developed. This work presents the development of solution-processed interlayers, layers inserted between the photoactive organic layer and electrodes in order to enhance charge extraction. This study is focused on the hole transport layer and, in particular, the replacement of the commonly used material PEDOT:PSS. A frequent approach to achieve this is the use of transition metal oxide layers such as MoO3 orV2O5. These oxides are usually deposited by evaporation but can be solution-processed from precursor solutions (e.g. sol-gel) or nanoparticle suspensions. This work considers three approaches. In the first, the use of sol-gel deposited tungsten or vanadium oxide led to an enhancement of hole extraction, allowing efficiencies in the range of what is expected for state of the art materials to be reached. The second approach involved the use of solution processed cobalt oxide. Although the use of this material enhanced charge extraction, due to a deposition issue, efficiency did not reach expected value. Finally, thermally induced diffusion of a solution-processed dopant was utilised, which is a novel approach. The dopant deposited at the organic/metal interface enhances hole extraction and leads to power conversion efficiencies similar to reference cells incorporating an evaporated metal oxide interlayer.

Cu(In,Ga)(Se,S)2 (CIGS) solar cells have attracted a lot of attention due to their high performance and the prospect for lower manufacturing costs over conventional crystalline silicon solar cells. All recent record efficiency CIGS absorbers have been deposited using vacuum processing which introduces high manufacturing costs. CIGS can also be compatible with low cost, atmospheric processing which can significantly reduce manufacturing costs. Recently, there has been some progress in developing atmospheric solution-based processes for CIGS. Among different solution approaches, deposition of molecular precursors can be advantageous in terms of simplicity and straightforward compositional control. Nonetheless, the developed methodologies involve highly toxic reagents or large impurity content in the device, limiting the potential for commercialisation. This thesis describes the development of a novel solution-based approach for the deposition of CIGS absorber layers. Metal chalcogenides are used as the starting precursors, which are free from detrimental impurities. These compounds contain strong covalent bonds and, consequently, they are insoluble in common solvents. Until recently, hydrazine, which is highly toxic and explosive, was the only solvent to effectively dissolve these types of precursors, limiting the feasibility of this approach for industrial applications. In this work, metal chalcogenides are dissolved in a safer solvent combination of 1,2-ethanedithiol and 1,2-ethylenediamine, completely eliminating hydrazine from the process. By using this solvent system, optically transparent solutions are formed which exhibit long-term stability. The precursor solutions are decomposed cleanly and they are converted to single phase CIGS upon selenisation. CuIn(S,Se)2 solar cells with power conversion efficiencies up to 8.0% were successfully fabricated by spray depositing the precursor solution, followed by a selenisation step. This progress has been made by continuously optimising the deposition, drying, and especially the selenisation configuration. Among other parameters, the working pressure during selenisation was found to have a dramatic effect on the material crystalline quality. Rapid thermal processing was also explored as an alternative selenisation configuration to tube furnace annealing and it was shown to improve the back contact/absorber interface. It has been demonstrated that Ga can easily be incorporated in the absorber for band-gap tuning and, consequently, for VOC enhancement of the solar cells. The structural properties of the films were investigated with Ga content, as well as the opto-electronic characteristics of the corresponding solar cells. The band-gap of the material was conveniently varied by simply adjusting the precursor ratio, allowing for fine compositional control. By using this technique, Cu(In,Ga)(Se,S)2 solar cells with conversion efficiencies of up to 9.8% were obtained. The solar cell performance in this work is limited by the porosity of the absorber and the back contact quality. Despite a significant improvement during the course of this work, the remaining porosity of the absorber causes selenium to diffuse towards the back forming a thick MoSe2 layer and causing a high series resistance in the device. A low cost, solution-based technique was also developed for the deposition of aluminium-doped zinc oxide films that can be used as the transparent conductive oxide layer in thin film solar cells. This methodology involves the use of an ultrasonic spray pyrolysis system, which is a very versatile and easily controlled deposition technique. Although the presence of oxygen makes the film closer to stoichiometric (fewer oxygen vacancies) good electronic and optical properties have been obtained by process optimisation. Films deposited with optimum conditions exhibited a sheet resistance of 23 Ω/sq, which can be further reduced by increasing the thickness with minimal transmittance losses. The simplicity, low toxicity and straightforward control make the proposed methodologies extremely potential for low cost and scalable deposition of thin film solar cells.

Inorganic nanoscale clusters have garnered significant interest for many practical applications within the fields of materials chemistry, inorganic chemistry, geochemistry, and environmental chemistry. However, the fundamental inner workings of how these materials interact in the solid state and solution continues to be a very elusive problem for scientists. My dissertation focuses on taking non-traditional approaches and characterization techniques to further understand the dynamic interactions of some of the aforementioned clusters. Chapter I is a comprehensive survey and perspective on selected characterization techniques used to study Group 13 aqueous nanoscale clusters and other polyoxometalates in solution. Chapter II focuses on utilizing Raman spectroscopy, infrared spectroscopy, and quantum mechanical computations to unambiguously identify Group 13 tridecameric species in the solid state and aqueous solution. Chapter III discusses the first instance of transmetalation of aqueous aluminum clusters via salt addition of In(NO3)3 in aqueous or methanol. Chapters IV and V explore the effects that aprotic and protic solvents can have on the solution speciation of the flat aluminum tridecamer. Chapter VI discusses the utility of using electrochemically synthesized gallium tridecamer and its functional use as a thin film semiconductor. Chapter VII describes a unique graduate level chemistry course designed to allow students to conduct and generate publication-worthy research within the timeframe of the course. Chapter VIII ventures out beyond the group 13 cluster and introduces techniques used to study the formation and stability of aqueous hafnium clusters. Chapter IX details the synthesis and characterization of rhombic structured copper clusters in the solid state. Finally, chapter X highlights my unfinished projects that can propel future research within the lab. This dissertation includes previously published and unpublished co-authored material.

abstract: Satisfying the ever-increasing demand for electricity while maintaining sustainability and eco-friendliness has become a key challenge for humanity. Around 70% of energy is rejected as heat from different sectors. Thermoelectric energy harvesting has immense potential to convert this heat into electricity in an environmentally friendly manner. However, low efficiency and high manufacturing costs inhibit the widespread application of thermoelectric devices. In this work, an inexpensive solution processing technique and a nanostructuring approach are utilized to create thermoelectric materials. Specifically, the solution-state and solid-state structure of a lead selenide (PbSe) precursor is characterized by different spectroscopic techniques. This precursor has shown promise for preparing thermoelectric lead selenide telluride (PbSexTe1-x) thin films. The precursor was prepared by reacting lead and diphenyl diselenide in different solvents. The characterization reveals the formation of a solvated lead(II) phenylselenolate complex which deepens the understanding of the formation of these precursors. Further, using slightly different chemistry, a low-temperature tin(II) selenide (SnSe) precursor was synthesized and identified as tin(IV) methylselenolate. The low transformation temperature makes it compatible with colloidal PbSe nanocrystals. The colloidal PbSe nanocrystals were chemically treated with a SnSe precursor and subjected to mild annealing to form conductive nanocomposites. Finally, the room temperature thermoelectric characterization of solution-processed PbSexTe1-x thin films is presented. This is followed by a setup development for temperature-dependent measurements and preliminary temperature-dependent measurements on PbSexTe1-x thin films. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Photovoltaics provide a feasible route to fulfilling the substantial increase in demand for energy worldwide. Solution processable organic photovoltaics (OPVs) have attracted attention in the last decade because of the promise of low-cost manufacturing of sufficiently efficient devices at high throughput on large-area rigid or flexible substrates with potentially low energy and carbon footprints. In OPVs, the photoactive layer is made of a bulk heterojunction (BHJ) layer and is typically composed of a blend of an electron-donating (D) and an electron-accepting (A) materials which phase separate at the nanoscale and form a heterojunction at the D-A interface that plays a crucial role in the generation of charges. Despite the tremendous progress that has been made in increasing the efficiency of organic photovoltaics over the last few years, with power conversion efficiency increasing from 8% to 13% over the duration of this PhD dissertation, there have been numerous debates on the mechanisms of formation of the crucial BHJ layer and few clues about how to successfully transfer these lessons to scalable processes. This stems in large part from a lack of understanding of how BHJ layers form from solution. This lack of understanding makes it challenging to design BHJs and to control their formation in laboratory-based processes, such as spin-coating, let alone their successful transfer to scalable processes required for the manufacturing of organic solar cells. Consequently, the OPV community has in recent years sought out to better understand the key characteristics of state of the art lab-based organic solar cells and made efforts to shed light on how the BHJ forms in laboratory-based processes as well as in scalable processes. We take the view that understanding the formation of the solution-processed bulk heterojunction (BHJ) photoactive layer, where crucial photovoltaic processes take place, is the one of the most crucial steps to developing strategies towards the implementation of organic solar cells with high efficiency and manufacturability. In this dissertation, we investigate the mechanism of the BHJ layer formation during solution processing from common lab-based processes, such as spin-coating, with the aim of understanding the roles of materials, formulations and processing conditions and subsequently using this insight to enable the scalable manufacturing of high efficiency organic solar cells by such methods as wire-bar coating and blade-coating. To do so, we have developed state-of-the-art in situ diagnostics techniques to provide us with insight into the thin film formation process. As a first step, we have developed a modified spin-coater which allows us to perform in situ UV-visible absorption measurements during spin coating and provides key insight into the formation and evolution of polymer aggregates in solution and during the transformation to the solid state. Using this method, we have investigated the formation of organic BHJs made of a blend of poly (3-hexylthiophene) (P3HT) and fullerene, reference materials in the organic solar cell field. We show that process kinetics directly influence the microstructure and morphology of the bulk heterojunction, highlighting the value of in situ measurements. We have investigated the influence of crystallization dynamics of a wide-range of small-molecule donors and their solidification pathways on the processing routes needed for attaining high-performance solar cells. The study revealed the reason behind the need of empirically-adopted processing strategies such as solvent additives or alternatively thermal or solvent vapor annealing for achieving optimal performance. The study has provided a new perspective to materials design linking the need for solvent additives or annealing to the ease of crystallization of small-molecule donors and the presence or absence of transient phases before crystallization. From there, we have extended our investigation to small-molecule (p-DTS (FBTTh2)2) fullerene blend solar cells, where we have revealed new insight into the crucial role of solvent additives. Our work has also touched upon modern polymers, such as PBDTTPD, where we have found the choice of additives impacts the formation mechanism of the BHJ. Finally, we have performed a comparative study of the BHJ film formation dynamics during spin coating versus wire-bar coating of p-DTS(FBTTh2)2: fullerene blends that has helped in curbing the performance gap between lab-based and scalable techniques. This was done by implementing a new apparatus that combines the benefits of rapid thin film drying common to spin coating with scalability of wire-bar coating. Using the new apparatus, we successfully attain similar performance of solar cell devices to the ones fabricated by spin coating with dramatically reduced material waste.

<p>Kyle Weideman PhD Thesis</p>

Polyacrylonitrile (PAN) and its copolymers are used in a wide variety of applications ranging from textiles to purification membranes, packaging material and carbon fiber precursors. High performance polyacrylonitrile copolymer fiber is the most dominant precursor for carbon fibers. Synthesis of very high molecular weight poly(acrylonitrile-co-methyl acrylate) copolymers with weight average molecular weights of at least 1.7 million g/mole were synthesized on a laboratory scale using low temperature, emulsion copolymerization in a closed pressure reactor. Single filaments were spun via hybrid dry-jet gel solution spinning. These very high molecular weight copolymers produced precursor fibers with tensile strengths averaging 954 MPa with an elastic modulus of 15.9 GPa (N = 296). The small filament diameters were approximately 5 'm. Results indicated that the low filament diameter that was achieved with a high draw ratio, combined with the hybrid dry-jet gel spinning process lead to an exponential enhancement of the tensile properties of these fibers. Carbon fibers for polymer matrix composites are currently derived from polyacrylonitrile copolymer fiber precursors where solution spinning accounts for ~40 % of the total fiber production cost. To expand carbon fiber applications into the automotive industry, the cost of the carbon fiber needs to be reduced from $8 to ~$3-5. In order to develop an alternative melt processing route several benign plasticizers have been investigated. A low temperature, persulfate-metabisulfite initiated emulsion copolymerization was developed to synthesize poly(acrylonitrile-co-methyl acrylate) copolymers with acrylonitrile contents between 91-96 wt% with a molecular weight range of 100-200 kg/mol. This method was designed for a potential industrial scale up. Furthermore, water was investigated as a potential melting point depressant for these copolymers. Twenty-five wt% water lead to a decrease in the Tm of a 93/7 wt/wt % poly(acrylonitrile-co-methyl acrylate) of Mw = 200 kg/mol to 160 0C as measured via DSC. Glycerin, ethylene glycol and glycerin/water combinations were investigated as potential plasticizers for high molecular weight (~200,000 g/mol), high acrylonitrile (93-96 mole:mole %) content poly(acrylonitrile–co-methyl acrylate) statistical copolymers. Pure glycerin (25 wt %) induced crystallization followed by a reduced "Tm" of about 213 °C via DSC. However this composition did not melt process well. A lower MW (~35 kg/mol) copolymer did extrude with no apparent degradation. Our hypothesis is that the hydroxyl groups in glycerin (or water) disrupt the strong dipole-dipole interactions between the chains enabling the copolymer endothermic transition (Tm) to be reduced and enable melting before the onset of degradation. Additionally high molecular weight (Mw = 200-230 kg/mol) poly(acrylonitrile–co-methyl acrylate) copolymers with lower acrylonitrile content (82-85 wt %) were synthesized via emulsion copolymerization and successfully melt pressed. These materials will be further investigated for their utility in packaging applications. / Ph. D.

Solution chemistry is a versatile and powerful tool in the synthesis of designed, complex nano-level high-tech materials. Normally, the technique is considered too expensive for large-scale production of complex multi-component ceramic materials. This thesis describes the expansion of the useful area of solution processing to multi-component bulk materials such as ZnO-based high-field varistors and WC–Co cutting tools, by developing novel techniques for solution-based coating of conventionally prepared metal and ceramic powders. The chemistry and microstructure development in the preparation of coatings, and the sintering of the coated powders to compacts, were studied in detail by SEM-EDS, TEM-EDS, XRD, IR-spectroscopy, dilatometry, TGA and DSC chemical analysis. ZnO powder with a ca 20 nm thick, homogeneous oxide coat of Bi–Sb–Ni–Co–Mn–Cr–Al oxide was prepared. After sintering to dense varistor bodies, much improved microstructures with much reduced ZnO-grain sizes were obtained. This shows that the oxides added as liquid sintering aid and grain-growth inhibitor become much more active when added homogeneously as a skin on the ZnO powder. After sintering of cobalt-coated WC, much improved micro-structures were obtained with a much more narrow WC grain-size distribution than that obtained from starting powders mixed by a conventional milling route. Coated powders also obviate the need for the extensive milling of WC and Co powders used in conventional mixing. The novel solution route was also applied to preparation of porous sponges and thin films on metal, glass and Al2O3 of sub 20 nm sized Co- or Ni-particles.