Polymer and Fullerene Zwitterions: From Synthesis to Solar Cells

This thesis describes the synthesis and applications of hydrophilic conjugated polymers and fullerenes containing dipole-inducing pendent groups. The pendent groups include tertiary amines, sulfobetaine (SB) zwitterions, quaternary ammoniums, and sulfonates, providing solubility in polar solvents. Particular emphasis is placed on zwitterions functalized structures. Suzuki-Miyaura (SM) and Horner-Wadsworth-Emmons (HWE) coupling reactions proved valuable for the preparation of the hydrophilic conjugated polymers, while the Prato reaction afforded the functional fulleropyrrolidines. Ultraviolet photoelectron spectroscopy (UPS) probed the interactions between the hydrophilic conjugated polymers and conductive metal substrates. In particular, UPS revealed that conjugated polymer zwitterions (CPZs) substantially reduce work function (Φ) of metals, represented by a negative interfacial dipole (Δ). Their solubility properties and interactions with metals make CPZs attractive for integration into solar cells, specifically at the interface between a photoactive layer and high Φ metal cathode. This thesis thus provides routes to improve polymer-based solar cell (PSC) technology through the implementation of novel hydrophilic semiconductors. Initial syntheses focused on the preparation of polythiophene with pendent SB groups, producing CPZs that were incorporated into PSCs as cathode modification layers. Tuning the electronic properties of CPZs with different polymer backbones further enhanced their effectiveness as interlayers in PSCs. Diketopyrrolopyrrole (DPP), iso-indigo (iIn) and naphthalene diimide (NDI) were functionalized with SB, followed by SM polymerization to provide the corresponding CPZs. Unprecedented power conversion efficiency (PCE) values (\u3e 10%) were achieved for devices containing the NDI CPZs, and improved electron transport of the interlayers was found central to this efficiency enhancement. Fulleropyrrolidines functionalized with tertiary amines and SB groups represent an alternative, non-polymeric, class of materials studied as interfacial modifiers in PSCs. The intrinsic n-type properties of fullerene provide an ideal platform for such interlayers, and led to state-of-the-art devices with record PCE values, irrespective of the selection of conductive cathode (Al, Ag, Cu and Au), while eliminating the need for precise control over interlayer thickness. Finally, HWE coupling was investigated as a new approach to hydrophilic CPZs. The methodology presented afforded room temperature production of a variety of hydrophilic poly(arylene vinylene)s (PAVs) from water, including zwitterionic, cationic and anionic derivatives. The scope and limitations of the HWE reaction in water is discussed, along with the utility of the resulting PAVs in sensing and PSCs

Chemical Stabilization of Perovskite Solar Cells with Functional Fulleropyrrolidines.

While perovskite solar cells have invigorated the photovoltaic research community due to their excellent power conversion efficiencies (PCEs), these devices notably suffer from poor stability. To address this crucial issue, a solution-processable organic chemical inhibition layer (OCIL) was integrated into perovskite solar cells, resulting in improved device stability and a maximum PCE of 16.3%. Photoenhanced self-doping of the fulleropyrrolidine mixture in the interlayers afforded devices that were advantageously insensitive to OCIL thickness, ranging from 4 to 190 nm. X-ray photoelectron spectroscopy (XPS) indicated that the fulleropyrrolidine mixture improved device stability by stabilizing the metal electrode and trapping ionic defects (i.e., I-) that originate from the perovskite active layer. Moreover, degraded devices were rejuvenated by repeatedly peeling away and replacing the OCIL/Ag electrode, and this repeel and replace process resulted in further improvement to device stability with minimal variation of device efficiency

Additives for Ambient 3D Printing with Visible Light

With 3D printing we desire to be “limited only by our imagination”, and although remarkable advancements have been made in recent years the scope of printable materials remains narrow compared to other forms of manufacturing. Light-driven polymerization methods for 3D printing are particularly attractive due to unparalleled speed and resolution, yet the reliance on high energy UV/violet light in contemporary processes limits the number of compatible materials due to pervasive absorption, scattering, and degradation at these short wavelengths. Such issues can be addressed with visible light photopolymerizations. However, these lower-energy methods often suffer from slow reaction times and sensitivity to oxygen, precluding their utility in 3D printing processes that require rapid hardening (curing) to maximize build speed and resolution. Herein, multifunctional thiols are identified as simple additives to enable rapid high resolution visible light 3D printing under ambient (atmospheric O2) conditions that rival modern UV/violet-based technology. The present process is universal, providing access to commercially relevant acrylic resins with a range of disparate mechanical responses from strong and stiff to soft and extensible. Pushing forward, the insight presented within this study will inform the development of next generation 3D printing materials, such as multicomponent hydrogels and composites.We thank the Robert A. Welch Foundation (F-2007) and the Center for Dynamics and Control of Materials: an NSF MRSEC (DMR-1720595) for financial support. The authors acknowledge the use of shared research facilities supported in part by the Texas Materials Institute, the Center for Dynamics and Control of Materials: an NSF MRSEC (DMR-1720595), and the NSF National Nanotechnology Coordinated Infrastructure (ECCS-1542159).Center for Dynamics and Control of Material

Visible light-responsive DASA-polymer conjugates

A modular synthesis of Donor-Acceptor Stenhouse Adduct (DASA) polymer conjugates is described. Pentafluorophenyl-ester chemistry is employed to incorporate aromatic amines into acrylate and methacrylate copolymers, which are subsequently coupled with activated furans to generate polymers bearing a range of DASA units in a modular manner. The effect of polymer glass transition temperature on switching kinetics is studied, showing dramatic rate enhancements in going from a glassy to a rubbery matrix. Moreover, tuning the DASA absorption profile allows for selective switching, as demonstrated by ternary photopatterning, with potential applications in rewriteable data storage

Multimorphic Materials: Spatially Tailoring Mechanical Properties via Selective Initiation of Interpenetrating Polymer Networks

Access to multimaterial polymers with spatially localized properties and robust interfaces is anticipated to enable new capabilities in soft robotics, such as smooth actuation for advanced medical and manufacturing technologies. Here, orthogonal initiation is used to create interpenetrating polymer networks (IPNs) with spatial control over morphology and mechanical properties. Base catalyzes the formation of a stiff and strong polyurethane, while blue LEDs initiate the formation of a soft and elastic polyacrylate. IPN morphology is controlled by when the LED is turned „on‟, with large phase separation occurring for short time delays (~1-2 minutes) and a mixed morphology for longer time delays (>5 minutes), which was supported by dynamic mechanical analysis, small angle X-ray scattering, and atomic force microscopy. Through tailoring morphology, tensile moduli and fracture toughness can be tuned across ~1-2 orders of magnitude. Moreover, a simple spring model is used to explain the observed mechanical behavior. Photopatterning produces “multimorphic” materials, where morphology is spatially localized with fine precision (<100 µm), while maintaining a uniform chemical composition throughout to mitigate interfacial failure. The fabrication of hinges represents a possible use-case for multimorphic materials in soft robotics.This work was primarily supported by the National Science Foundation under Grant No. DMR- 2045336 (M.J.A., C. B., and Z.A.P., synthesis and mechanical characterization). Partial support was provided from the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0022050 (N.P. and X. G., morphology characterization related to scattering and AFM-IR) and through the Center for Materials for Water and Energy Systems (M-WET), an Energy Frontier Research Center under Award #DE-SC0019272 (M.J.A. and B.D.F., nanoindentation characterization), the National Science Foundation under Grant No. CMMI-2038512 (L.M.C., AFM fast force distance mapping characterization), NSF Graduate Research Fellowship under Grant No. DGE-1610403 (M.J.A.), and the Robert A. Welch Foundation under Grant No. F-2007 (Z.A.P., partial materials and supplies support). The authors acknowledge the use of shared research facilities supported in part by the Texas Materials Institute and the Center for Dynamics and Control of Materials (NSF MRSEC) under Grant No. DMR-1720595.Center for Dynamics and Control of Material

Rapid High-Resolution Visible Light 3D Printing

Light-driven 3D printing to convert liquid resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest resolution of any additive manufacturing process. However, the reliance on high-energy UV/violet light limits the materials scope due to degradation and attenuation (e.g., absorption and/or scattering). Chemical innovation to shift the spectrum into more mild and tunable visible wavelengths promises to improve compatibility and expand the repertoire of accessible objects, including those containing biological compounds, nanocomposites, and multimaterial structures. Photo- chemistry at these longer wavelengths currently suffers from slow reaction times precluding its utility. Herein, novel panchromatic photopolymer resins were developed and applied for the first time to realize rapid high-resolution visible light 3D printing. The combination of electron-deficient and electron-rich coinitiators was critical to overcoming the speed-limited photocuring with visible light. Furthermore, azo-dyes were identified as vital resin components to confine curing to irradiation zones, improving spatial resolution. A unique screening method was used to streamline optimization (e.g., exposure time and azo-dye loading) and correlate resin composition to resolution, cure rate, and mechanical performance. Ultimately, a versatile and general visible-light-based printing method was shown to afford (1) stiff and soft objects with feature sizes <100 μm, (2) build speeds up to 45 mm/h, and (3) mechanical isotropy, rivaling modern UV-based 3D printing technology and providing a foundation from which bio- and composite-printing can emerge.We thank the ARO STIR program of the Department of Defense (W911NF1910310) and Robert A. Welch Foundation (F-2007) for financial support. The authors acknowledge the use of shared research facilities supported in part by the Texas Materials Institute, the Center for Dynamics and Control of Materials, an NSF MRSEC (DMR-1720595), and the NSF National Nanotechnology Coordinated Infrastructure (ECCS- 1542159).Center for Dynamics and Control of Material

Organic electronics by design: the power of minor atomic and structural changes

Fundamental to the field of organic electronics is the understanding that structure begets function. For conjugated polymers, monomer structure determines overall energy levels while also influencing interchain interactions. These interchain interactions induce aggregation and creates higher order morphology, greatly influencing the ultimate performance of electronic devices. Understanding the interplay of morphological changes with device efficiency is imperative to improving the performance of organic electronics with minor differences in molecular structure, linear versus branched side-chains, carbon versus silicon bridgehead atoms or hydrogen versus fluorine substitution, having dramatic effects on the energetics, aggregation, morphology, and, ultimately, performance of these materials. This report highlights the power of minor structural changes in conjugated polymers and the associated design rules for the preparation of next generation electronic materials

Conjugated Thiophene-Containing Polymer Zwitterions: Direct Synthesis and Thin Film Electronic Properties

We report a direct and facile synthesis of novel conjugated polymeric zwitterions (CPZs) as a simple route to electronically active homopolymers and copolymers containing dipole-inducing pendent zwitterions. Sulfobetaine-containing polythiophenes (<b>PTSB-1</b> and <b>PTSB-2</b>) and alternating thiophene–benzothiadiazoles (<b>PTBTSB-1</b> and <b>PTBTSB-2</b>) were prepared and characterized relative to alkylated polymer analogues (<b>POT-</b><i><b>a</b></i><b>-T</b> and <b>POT-</b><i><b>a</b></i><b>-BT</b>). The polar zwitterionic side chains make these polymers hydrophilic and salt-responsive, with interesting electronic properties that depend on zwitterion distance from the conjugated polymer backbone (tether length), as characterized by UV–vis absorption and ultraviolet photoelectron spectroscopy (UPS). Close proximity (CH₂ spacer) of the sulfobetaine groups to the polymer backbone results in increased ionization potential and enlarged band gaps of 2.19 and 2.04 eV for <b>PTSB-1</b> and <b>PTBTSB-1</b>, respectively. On Au and Ag surfaces, the zwitterionic pendent groups significantly alter the work function due to the presence of an interfacial dipole, with the largest interfacial dipoles measuring −1.29 eV (<b>PTBTSB-1</b>/Au) and −0.69 eV (<b>PTBTSB-1</b>/Ag)

Modular synthesis of asymmetric rylene derivatives

The modular synthesis of asymmetric rylenes from naphthalic anhydride derivatives is presented. Imidization, Suzuki-Miyaura coupling and cyclodehydrogenation reactions are utilized for the generation of novel functional rylenes with these three core transformations providing significant flexibility over the final structure. The combination of simple purification and high yields enables access to asymmetric rylenes with functional handles at the imide-position and site-specific incorporation of bay position substituents. The resulting library of perylenes and bisnapthalimide-anthracene derivatives showcase the presented methodology and the ability to tune optoelectronic and electrochemical properties

Chemical Stabilization of Perovskite Solar Cells with Functional Fulleropyrrolidines.

While perovskite solar cells have invigorated the photovoltaic research community due to their excellent power conversion efficiencies (PCEs), these devices notably suffer from poor stability. To address this crucial issue, a solution-processable organic chemical inhibition layer (OCIL) was integrated into perovskite solar cells, resulting in improved device stability and a maximum PCE of 16.3%. Photoenhanced self-doping of the fulleropyrrolidine mixture in the interlayers afforded devices that were advantageously insensitive to OCIL thickness, ranging from 4 to 190 nm. X-ray photoelectron spectroscopy (XPS) indicated that the fulleropyrrolidine mixture improved device stability by stabilizing the metal electrode and trapping ionic defects (i.e., I-) that originate from the perovskite active layer. Moreover, degraded devices were rejuvenated by repeatedly peeling away and replacing the OCIL/Ag electrode, and this repeel and replace process resulted in further improvement to device stability with minimal variation of device efficiency