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This dissertation is concerned with the fabrication and characterization of organic electrochemical transistors (OECTs) made from polyelectrolytes based on the material PCPDTBT-SO3-Na (CPE-Na). OECTs are electrolyte-gated transistors that allow the electrolyte to permeate into the bulk of the film and directly changes the doping state of the bulk of the semiconducting layer. The relationship between the molecular structure of the polyelectrolytes and the material properties are explored in the lens of optimizing device characteristics in OECTs. The first chapter of this thesis focuses on the development of OECTs made using PCPDTBT-SO3-K (CPE-K) as the active layer. While the large majority of OECT research still utilizes poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the active layer, a few new OECT materials have been recently reported. CPE-K has been used in the past for applications such as DNA-detection, thermoelectric devices and as interlayers in organic photovoltaic devices. Conjugated polyelectrolytes (CPEs) represent a promising and unique class of materials which are characterized by their conjugated carbon backbone and pendant ionic chains. Their water solubility in addition to their electronic and ionic conductivity makes them an ideal candidate for OECT applications. Using various spectroscopic and electric characterization techniques, CPE-K was fully characterized so that it could be accurately compared to other OECT materials reported in the literature. In fact, CPE-K is among the short list of high-performance OECT materials. The techniques used in this work was used to shed light on the operational mechanism of CPE-based OECTs. In addition, the effective use of interdigitated contacts was used to increase the transconductance of OECT devices.The second portion of this thesis expands on the work of CPE OECTs, by exploring their structure-property-relationship. A series of CPE-Na polymers with varying sidechain lengths were synthesized by collaborator Luana Llanes from the lab of Professor Guillermo Bazan. A series of CPE-Na with increasing alkyl chain distances (2-5 methylene units) were synthesized. The purpose of synthesizing these materials was to explore the effect of varying the distance of the pendant charged sulfonate on the side-chain. CPE-Na is a self-doped polymer, due to the negative charge on the sulfonate stabilizing the positively charged polaron. It was hypothesized that reducing the side-chain length would result in easier doping of the CPE due to the proximity of the sulfonate. To our surprise, the opposite trend was observed. By using a wide range of characterization techniques, the structure-property-relationship between the sidechain length and ease of doping of the CPE backbone was explored. The next chapter of the thesis switches topics to organic field-effect transistors (OFETs). Due to convenience and low leakage current, many OFETs use silicon dioxide as the gate dielectric material. As demonstrated by Dr. Hung Phan, trap sites at the dielectric-semiconductor interface result charge trapping. This charge trapping results in unwanted artifacts such as hysteresis, double-slope nonideality characteristics, and reduction in current when the device is continuously biased. The fluoropolymer P(VDF-HFP) was used as a dielectric material. We found that the device transfer characteristics are heavily dependent on the processing conditions and choosing the proper grade of P(VDF-HFP). We presented the correct choice of processing conditions and material grade results in ideal transfer characteristics such as high on-off ratio, low threshold voltage, low hysteresis and high current stability. The final section of the thesis discusses a project to explore the structure-property relationship of a group of 12 different n-type non-fullerene acceptor small molecules. Non-fullerene acceptors (NFAs) have recently become an area of interest to researchers aiming to develop high performance organic solar cells due to their promising electronic properties and greater tunability relative to fullerene-based acceptors. Despite the success of NFAs in organic solar cells, there have only been a few studies looking at the electron mobility of these materials. A collection of 12 NFAs with a variety of core groups and sidechain configurations were tested as the active layer in an organic field-effect transistor (OFET) to measure the mobility of horizontal charge transport.

The ability to readily synthesize derivatives of semi-conducting organic molecules through iterative synthesis allows for the facial tuning of both electron and physical properties. This has led to their use in devices such as field-effect transistors, light emitting diodes, and photovoltaics, bringing organic devices from the bench top to everyday use. Acenes, molecules consisting of annulated benzenes, have garnered interest for use in both single-crystals and thin films devices. However, if organic semiconductors (OSCs) are ever to compete with inorganic semiconductors, a complete structure function relationship (SFR) is required. This will require the development of efficient synthetic strategies, evaluation of solid-state packing, and device performance. Charge transport in OSCs is greatly affected by HOMO/LUMO energies of the molecule, solid state molecular packing, and thin film morphology. Through functionalization of the acene core, HOMO/LUMO energies can be modified yielding n-¬type (electron-acceptor) or p-type (electron-donating) materials. However, the addition of functional groups to acenes can also drastically alter solid-state packing resulting in new solid-state morphologies which can profoundly affect device efficiency. To vertically advance the field of organic electronics a complete understanding of SFR of new OSC must be determined in order to allow for the rational design of the next generation of OSC molecules. Over the course of my research I have synthesized two modified acene system to study SFRs of organic semiconductors: asymmetrically substituted diarylindenotetracenes (ASIs) and functionalized rubrenes. Derivatives of ASIs containing either electron donating or electron withdrawing groups were synthesized and their electronic properties were studied using UV/Visible light spectroscopy, fluorescence spectroscopy, and cyclic voltammetry (CV). Stability of these new OSC was determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). To examine the solid-state packing, single crystals of ASIs were grown and crystal morphologies were determined via single-crystal X-¬ray diffraction experiments. Organic photovoltaic devices were prepared using four ASI derivatives as the active layer n-type material and the electronic properties were determined via UV/Visible light spectroscopy, fluorescence spectroscopy, and OPV performance was measured via I-V curves. To correlate thin film morphology and OPV performance solid-state characterization was performed utilizing atomic force microscopy (AFM), grazing incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). With both the electronic and physical properties of ASIs determined correlations between ASI molecular structure, thin film morphologies and OPV device performance could be made. Through our study of ASIs we have advanced the field of OSC research and increased our understanding of SFRs of n-type semiconductors. Two different studies were done to examine the effect of functionalization on the p-type semiconductor rubrene. First, the effect of fluorination on the solid-state packing of rubrene derivatives was studied. Several fluorinated rubrene derivatives were synthesized and single crystals were grown for single-crystal X-ray experiments to examine changes in molecular solid-state packing. By closely studying both inter and intramolecular interactions in each crystal we found that intermolecular interaction, and specifically C–FX intermolecular interactions, are a major contributor to crystal packing and molecular orientation in the solid state. Second, to study the effect of nitrogen substitution on both electronic and physical properties of rubrene, a synthetic strategy was developed to prepare 5,6,11,12-¬tetraphenylnaphtho [2,3-g] quinoline, azarubrene. The total synthesis of this rubrene derivative is still in progress.

The first part of this dissertation focuses on interface and morphology engineering in polymer- and small molecule-based organic solar cells. High-performance devices were fabricated, and the device performance was correlated with nanoscale structures using various electrical, spectroscopic and microscopic characterization techniques, providing guidelines for high-efficiency cell design. The second part focuses on perovskite solar cells (PSCs), an emerging photovoltaic technology with skyrocketing rise in power conversion efficiency (PCE) and currently showing comparable PCEs with those of existing thin film photovoltaic technologies such as CIGS and CdTe. Fabrication of large-area PSCs without compromising reproducibility and device PCE requires formation of dense, pinhole-free and highly uniform perovskite thin films over large area, which remains a big challenge as of today. In this work, a scalable process, called ultrasonic spray-coating (USC), was thoroughly optimized to deposit dense and uniform perovskite thin films for high-efficiency PSCs. In order to realize high-performance flexible PSCs, a unique photonic curing technique was demonstrated to achieve highly conductive TiO2 as electron transport layer on flexible substrates. Moreover, the effect of processing conditions on perovskite film growth was evaluated and taken into account to increase PCE to more than 15%. In addition, a series of high-performance organic field-effect transistors (OFETs) were fabricated en route to demonstrate the versatility of the USC process. Several different polymer binders were used to modulate the lateral and vertical phase morphologies in OFETs, significantly improving the device performance. In summary, this research provides guidelines for the design and fabrication of high-performance solution-processed solar cells and field-effect transistors based on organic materials and hybrid perovskites, while presenting a viable route for large-scale fabrication.

Organic field-effect transistors (OFETs) are one of the key components of the ubiquitous flexible electronics in the near future. Since the first report in the mid-1980s, OFETs have been intensively studied for more than 20 years. However, most of these studies are based on steady state or quasi steady state DC measurements, which are not sensitive to transient processes including the formation and depletion of the conducting channel. These transient processes are essential parts of the device operation, and determine the response frequency of OFETs. For these reasons, the transient carrier behaviors during these processes warrant examination. This thesis develops displacement current measurement (DCM) as a technique to probe transient carrier behaviors. Instead of using OFETs for measuring displacement current, long-channel capacitors (LCCs) are used. A LCC can be viewed as a simplified OFET with only one channel contact to limit the carrier injection/extraction to one direction only. The channel of a LCC is elongated to millimeter range to increase the transient time and the displacement current associated with charging/discharging the channel. Displacement current has been measured from LCCs under cyclic gate voltage sweeps. The number of the injected, extracted, and trapped carriers can be calculated by integrating the displacement current with respect to time. A current peak has always been observed in the charging sweep, and it is attributed to the transient process of conducting channel formation. Analytical and numerical device models have been developed to understand the transient carrier behaviors. It is found that carrier mobility can be calculated from the slope of the displacement current peak. In addition, the evolution of the carrier distribution in the long channel during the conducting channel formation and depletion are presented in detail. Further more, the effects of carrier traps on the transient carrier behaviors are discussed. DCM has also been used to study the contact effects at metal-pentacene interfaces. It is found that the carrier trapping in the long channel of the LCCs with Au contacts is indirectly caused by the deep trap states at the pentacene-dielectric interface in the contact region generated by Au penetration. Low trapped carrier density is found in the LCCs with Cu contacts due to the shallow penetration of Cu atoms. In addition, ambipolar injection and transport are observed in a LCC with Al contact and a PMMA buffer layer between pentacene and SiO2. Thus DCM can be used to characterize the quality of metal-organic contacts. The conducting channel depletion dynamics under constant gate voltages has also been examined. It is found that the discharging displacement current can either follow an exponential decay or a power law decay, depending on the discharging gate voltage. A simple RC model has been given to explain these different decay behaviors. For the power law decay, the decay exponent measured from the experimental data is around 1.2 to 1.3, while the exponent predicted by the RC model is 2. The smaller exponent observed in experiment might be attributed to the effect of carrier traps.

The practical uses of pentacene, the most promising polyacene in the area of organic field-effect transistors, are limited by its sensitivity to oxygen and poor solubility in organic solvents. To overcome these disadvantages, new organic semiconductors, 2,3,6,9,10,13-hexa(trimethylsilylethynyl)pentacene, 2,3,9,10-tetrakis(triisopropylsilylethynyl)-6,13-bis(trimethylsilylethynyl)pentacene, and 2,3,9,10-tetrakis(3,5-di-t-butylphenylethynyl)-6,13-bis(trimethylsilylethynyl)pentacene were designed, synthesized, and fully characterized.In comparison with pentacene, they are stable and soluble and have extended pi-conjugation. Each demonstrates reduction potentials that are essentially unchanged, but different oxidation potentials. The HOMO-LUMO gaps estimated from the electrochemical data are ca. 1.7 eV, which is among the lowest HOMO-LUMO gaps reported in the literature for pentacene derivatives. The compounds fluoresce in the deep red region, with the width at half height being less than 40 nm. In thin films the compounds show strong aggregation and adopt a favorable stacking geometry with the molecular long axis upright to the substrate surface.Modifying pentacene at the central and terminal rings with ethynyl moieties proves viable in synthesizing new pentacene derivatives and tuning their electronic properties. The three new pentacene derivatives may be useful for electronic devices such as organic field-effect transistors or organic light-emitting diodes.Six novel D-π-D fluorophores were also synthesized. They are 1,2,3-trimethoxy-5-(2-{4-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)benzene, 5-(2-{3-fluoro-4-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)-1,2,3-trimethoxybenzene, 1,2,3-trimethoxy-5-[2-(4-{4-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}phenyl)-1-ethynyl]benzene, 1,2,3-trimethoxy-5-(2-{3-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)benzene, 1,2,3-trimethoxy-5-(2-{2-[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]phenyl}-1-ethynyl)benzene, and 9,10-di[2-(3,4,5-trimethoxyphenyl)-1-ethynyl]anthracene.These compounds fluoresce in the blue-violet region in both solution and the solid state. In solution, they exhibit significant positive solvatochromism in fluorescence, which is ascribed to the intramolecular charge transfer upon excitation. The last D-π-D fluorophore is an exception: it exhibits insignificant solvatochromism in solution and in the solid state, only weak fluorescence in the green-yellow region was observed, which is probably due to aggregation. The last D-π-D fluorophore also shows strong two-photon excited fluorescence in the blue region with a two-photon absorption cross-section of 124 GM. The others do not.These six D-π-D fluorophores may be useful in photonic applications where blue light is desired.

The electrical performance and device stability of a patterned-gate, gate-planarized, inverted, coplanar thin-film transistor with an organic polymer F8T2 active layer semiconductor was studied. The validity of electrical performance parameter extraction methods was studied and the inclusion of a field-dependent mobility provided the most reliable results. A gate-bias dependent activation energy near 0.2eV for the field-effect mobility of holes in F8T2 was found. The source and drain contact and channel resistances were characterized and are similar with values near 109O. Additionally, these devices showed promise for use in high-voltage applications. The effects of broadband and monochromatic illumination were characterized and strongly absorbed illumination reduced the threshold voltage. Application of a photo-field effect theory provided an estimation of the density and slope of the gap states. This method also provided an estimate of the flat-band voltage of -10V. The performance of the device as a photodetector showed a responsivity of 1A/W, a photosensitivity greater than 100, an external quantum efficiency greater than 100%, a noise equivalent power of 10-14 WHz-0.5 and a specific detectivity of approximately 2x1011 cmHz0.5W-1. Hysteresis in the transfer characteristics was characterized by the dependence on the applied gate-bias, temperature and illumination. It was observed that the hysteresis is a charge trapping effect that is dependent on the charge density within the channel. The hysteresis was eliminated by incorporating an organic insulator layer between the inorganic insulator and the organic semiconductor. Bias temperature stress effects were characterized and the major effect was an increase in threshold voltage. Analysis using a stretched exponential behavior for DC bias stress effects showed that there exists a distribution of trap states centered at 0.25eV above the valence band. Negative AC BTS resulted in a dependence on the total stress time and demonstrated the combination of several physical phenomena, including slow carrier transport and the existence of few reversible and many irreversible trap states. A relatively low (65°C) optimal operating temperature of organic-based devices was observed. The trap states were further characterized using the photodischarge method to investigate the kinetics and distribution of trap states. A narrow distribution of trap states at 0.3eV above the valence band was found, which is consistent with field-effect mobility and bias temperature stress results.

The performance of organic field-effect transistors (OFETs) is determined by both carrier injection and transport in organic semiconductors. To improve injection in bottom contact OFET devices, self-assembled monolayers (SAMs) of octadecanethiol (ODT), 2-mercapto-benzothiazole (BZT), or 5-mercapto-2,2 '-bithienyl (BTT) were made on Au electrodes, followed by evaporation of pentacene. The devices with the SAMs showed better transistor performance, and apparent carrier mobility in the devices with electron-rich BTT SAMs was 10 times higher than in the devices without SAMs. BTT SAM devices were also better than ODT and BZT SAM devices. This increased performance is attributed to a combination of better adhesion between pentacene and the electrodes and to matching the energy levels of the BTT SAMs to the semiconducting pentacene. Novel organic semiconductors with a pi-stacked structure were synthesized to understand structural and transport properties of pi-stacked materials. Variable-temperature X-ray diffraction patterns showed that a cooligomer of 4,4'-dinonyl-2,2'-bithiazole and 3,4-ethylenedioxythiophene (NTZN) had a crystal-crystal transition at 110°C. Variable-temperature UV-Vis spectra showed that NTZN had less exciton coupling in the high temperature phase as shown by loss of fine structure. A mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V·s in single-crystal transistors. The pi-stacked structure, which provides more efficient orbital overlap and thereby facilitates carrier transport, is responsible for this high mobility. On the other hand, the mobility in thin-film devices is affected by film morphology. Oligofluorene-thiophene derivatives with dodecyl side chains were prepared to study the effect of the side chains on the film morphology. The interactions between the side chains promote two-dimensional crystal growth, leading to large and well-interconnected grains. Poly(phenyleneethynylene) with end-capping thiol groups (PPE-SH) was synthesized to explore 'intra'molecular transport in one molecular wire, which is connected between two electrodes. PPE-SH was self-assembled onto a device with nano-gap Au structures. SEM images of the device before and after the self assembly showed that polymer chains lay flat on the Au electrodes. This may be due to highly polarizable pi electron clouds of conjugated cores, which substantially induce dispersion force attraction towards polarizable Au surfaces.