Work Function Modification via Combined Charge‐Based Through‐Space Interaction and Surface Interaction

Work function modification of electrodes is an important factor to achieve high performance in organic electronics. However, a clear explanation of the origin of work function modification has remained elusive. Here, it is investigated how the work function of electrodes is affected by the charge‐based through‐space interaction with the well‐known surface interaction. The studies reveal that the formation of a surface dipole leads to a work function shift, even when the work function modifying layer and substrate are separated. A work function shift is also demonstrated by electrophoretic deposition of ionic polyelectrolytes while the same polyelectrolytes do not cause any work function shift when they are spin cast. More noteworthy is that a neutral (nonionic) polymer which has no specific surface‐interacting functional groups can induce work function shift of its substrate by a charge‐based through‐space interaction when deposited by electrospraying. These results provide a more comprehensive understanding of work function modification and motivate the design and selection of a wide range of effective work function modifying layers for organic electronics.Work function modification of indium tin oxide (ITO) by thin‐layer polymer coating is investigated with a set of representative polyelectrolytes. The studies reveal that while direct surface interaction is the major factor affecting work function modification, charge‐based through‐space interaction has also a significant effect on modifying the work function of electrodes by building opposite charges on ITO.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145536/1/admi201800471-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145536/2/admi201800471.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145536/3/admi201800471_am.pd

Design of Organic Materials with Unique Supramolecular Assembly for Optical, Electronic, and Biomedical Applications.

Rational material design is inevitable to fully realize the properties of organic conjugated materials in applications, by regulating their intermolecular packing as well as intramolecular properties. In this dissertation, molecular design strategies to control interactions and assemblies of organic conjugated materials are systematically investigated, which enables unique optoelectronic properties for various optoelectronic applications. In Chapter 2, a molecular design to control intermolecular interactions renders a unique thermally stable supercooled liquid and its shear-triggered lighting-up crystallization with 25-times fluorescence enhancement. The origin of the unique property is systematically scrutinized. Furthermore, possible biosensor application is proposed by demonstrating highly sensitive crystallization of the supercooled liquid by living cell attachment. Insightful design consideration for both intrachain and interchain properties is also critically important for conjugated polymers (CPs). In Chapter 3, molecular design of CPs’ main and side chains is logically investigated to regulate optical properties. Tailored CPs exhibit identical color in solution manifesting the same intramolecular optical properties by conjugated backbone design. Contrastingly, they show distinct color gradation in the solid state due to the coined intermolecular packing propensity difference through side chain design. Latent optical information encoding using CPs as security inks is demonstrated, which reveals and conceals hidden information upon CP aggregation/deaggregation. Furthermore, expansion of the design principles for efficient CP alignment is investigated (Chapter 4). Realization of CP alignment largely affects optoelectronic applications of CPs since it is inevitable to fully utilize CPs’ anisotropic properties in devices. Previously identified molecular design rules to realize directed CP alignment are evaluated, and more detailed design factors are additionally revealed. The properties of organic conjugated materials are also influenced by environmental factors, including characteristics of a substrate and solvent molecules. In Chapter 5, a novel optical sensor is devised based on controlled subtle interaction differences between substrates, fluorescent sensory molecules, and analyte solvents. The highly selective sensor array can clearly distinguish physicochemically similar liquids; ethanol, methanol, ethylene glycol, and water. The thoroughly discussed molecular design principles in this dissertation depict an insightful picture on how unique optoelectronic properties of conjugated organic molecules and polymers can be designed and fully utilized in various applications.PhDMacromolecular Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133216/1/kychung_1.pd

Correlation between solvent composition and materials properties of organohydrogels prepared by solvent displacement

Owing to their tunable functionality, structural flexibility, and biocompatibility, gels are widely utilized as active materials in applications, such as biomedical, sensors, and energy devices. Especially hydrogels, generally comprised of water over 80 vol%, are emphasized as attractive candidates for suggested applications. However, most devices based on hydrogels suffer from drying and freezing under practical operation environments. To overcome these issues, organohydrogels are proposed as good alternatives with improved durability. Although many organohydrogels with various polymer networks are reported, the effects of solvent system design on the properties of organohydrogels are still not conclusive. Here, we investigated the correlation between the solvent system design of organohydrogels and their properties, particularly in terms of rheological characteristics and ion conductivity. With increasing ethylene glycol content, the shear storage modulus and complex viscosity of the organohydrogels decrease sharply, possibly owing to the decrease in metal-carboxylate coordination in the gel network. The ion conductivity of the organohydrogels gradually decreases with increasing ethylene glycol content, owing to the ion conductivity trends of pure water and pure ethylene glycol ionic solution. Insight into the correlation between the solvent system design and properties of organohydrogels will enable the preparation of optimal organohydrogels for various processes and applications

A Novel Mechanism for Chemical Sensing Based on Solvent-Fluorophore-Substrate Interaction: Highly Selective Alcohol and Water Sensor with Large Fluorescence Signal Contrast

Differentiation of solvents having similar physicochemical properties, such as ethanol and methanol, is an important issue of interest. However, without performing chemical analyses, discrimination between methanol and ethanol is highly challenging due to their similarity in chemical structure as well as properties. Here, we present a novel type of alcohol and water sensor based on the subtle differences in interaction among solvent analytes, fluorescent organic molecules, and a mesoporous silica gel substrate. A gradual change in the chemical structure of the fluorescent diketopyrrolopyrrole (DPP) derivatives alters their interaction with the substrate and solvent analyte, which creates a distinct intermolecular aggregation of the DPP derivatives on the silica gel substrate depending on the solvent environment and produces a change in the fluorescence color and intensity as a sensory signal. The devised sensor device, which is fabricated with simple drop-casting of the DPP derivative solutions onto a silica gel substrate, exhibited a completely reversible fluorescence signal change with large fluorescence signal contrast, which allows selective solvent detection by simple optical observation with the naked eye under UV light. Superior selectivity of the alcohol and water sensor system, which can clearly distinguish among ethanol, methanol, ethylene glycol, and water, is demonstrated.clos

Assembly and alignment of conjugated polymers: materials design, processing, and applications

Conjugated polymers (CPs) are widely investigated because of their intriguing optical and semiconducting properties in various optoelectronic device applications. Because of the one-dimensional p-orbital overlap along the main chain, CPs exhibit strong anisotropy in optoelectronic characteristics. Therefore, macroscopic assembly and alignment of CPs are essential to fully utilize their potential properties in real device applications. Here we review various processing strategies and material design principles for efficient CP alignment that result in highly anisotropic optical and electronic characteristics. Furthermore, we thoroughly review the incorporation of aligned CPs layers in organic light-emitting diodes, organic thin film transistors, and organic photovoltaic devices. The achieved macroscopic CP alignment has increased the optoelectronic properties and greatly improved device performanceclos

Temperature-directed fluorescent switchable nanoparticles based on P3OT-PNIPAM nanogel composite

Poly(N-isopropylacrylamide) (PNIPAM) is a unique stimuli-responsive material that exhibits a lower critical solution temperature (LCST). Owing to this characteristic temperature-dependent behavior, PNIPAM has found extensive utilization as an active material in various applications, including sensors, drug delivery, and cellular imaging. Herein, we demonstrate temperature-directed fluorescent switchable nanoparticles based on poly(3octylthiophene-2,5-diyl) (P3OT) nanoaggregate-embedded PNIPAM nanogel composites (POPNs) featuring different crosslinker contents. The amount of P3OT loading in the nanogel composites can be gradually controlled by varying the crosslinking density of the PNIPAM matrix; this may be attributable to the efficient entrapment of P3OT nanoaggregates in case of a dense polymeric network with the increase in crosslinking density. POPNs exhibit dramatic temperature-dependent fluorescence enhancement (by a factor of 2.11). This is based on the environmental changes affecting fluorescent P3OT chains at temperatures below and above the LCST of the PNIPAM matrix. Based on this temperature-directed fluorescent switching capability, POPN could find potential applications in various fields, including biomedical imaging and sensors

Material Design for 3D Multifunctional Hydrogel Structure Preparation

Hydrogels are recognized as one of the most promising materials for e-skin devices because of their unique applicable functionalities such as flexibility, stretchability, biocompatibility, and conductivity. Beyond the excellent sensing functionalities, the e-skin devices further need to secure a target-oriented 3D structure to be applied onto various body parts having complex 3D shapes. However, most e-skin devices are still fabricated in simple 2D film-type devices, and it is an intriguing issue to fabricate complex 3D e-skin devices resembling target body parts via 3D printing. Here, a material design guideline is provided to prepare multifunctional hydrogels and their target-oriented 3D structures based on extrusion-based 3D printing. The material design parameters to realize target-oriented 3D structures via 3D printing are systematically derived from the correlation between material design of hydrogels and their gelation characteristics, rheological properties, and 3D printing processability for extrusion-based 3D printing. Based on the suggested material design window, ion conductive self-healable hydrogels are designed and successfully applied to extrusion-based 3D printing to realize various 3D shapes

3D Antidrying Antifreezing Artificial Skin Device with Self-Healing and Touch Sensing Capability

Hydrogels are attractive, active materials for various e-skin devices based on their unique functionalities such as flexibility and biocompatibility. Still, e-skin devices are generally limited to simple structures, and the realization of optimal-shaped 3D e-skin devices for target applications is an intriguing issue of interest. Furthermore, hydrogels intrinsically suffer from drying and freezing issues in operational capability for practical applications. Herein, 3D artificial skin devices are demonstrated with highly improved device stability. The devices are fabricated in a target-oriented 3D structure by extrusion-based 3D printing, spontaneously heal mechanical damage, and enable stable device operation over time and under freezing conditions. Based on the material design to improve drying and freezing resistance, an organohydrogel, prepared by solvent displacement of hydrogel with ethylene glycol for 3 h, exhibits excellent drying resistance over 1000 h and improved freezing resistance by showing no phase transition down to -60 degrees C while maintaining its self-healing functionality. Based on the improved drying and freezing resistance, artificial skin devices in target-oriented optimal 3D structures are presented, which enable accurate positioning of touchpoints even on a complicated 3D structure stably over time and excellent operation at temperatures below 0 degrees C without losing their flexibility