A systematic approach has been used to optimise the fabrication process of pentacene-based nonvolatile organic thin film memory transistors (OTFMTs) operating at low programming voltages. In the first part of this work, reliable, reproducible and hysteresis free organic metal-insulator-semiconductor (OMIS) devices and organic thin film transistors (OTFTs) were fabricated and characterised. All devices were based on poly(methyl methacrylate) (PMMA) and poly(vinyl phenol) (PVP) as the organic insulators. The second part of this work focused on optimising the evaporation parameters to fabricate high-performance pentacene-based devices. About 50 nm thickness of pentacene film with a deposition rate of 0.03 nm s-1 on ~ 300 nm of PMMA was found to produce large, uniform and condense grains leading to high quality devices. OTFTs with high mobility of 1.32 cm2 V−1 s−1, on/off current ratio of 106, and negligible hysteresis and leakage current were demonstrated. The effect of the environment on the OTFTs obehaviour was also investigated. The bias stress effect was also investigated in terms of threshold voltage shift ΔVT at various conditions and times. The results show ΔVT increases with the increase of stress voltage. A negligible hysteresis is evident between the forward and reverse direction of the transfer characteristics and the shape of the transfer characteristics does not change with the bias stress. Floating gate memory structures with thin layer of gold, gold nanoparticles (AuNPs) and single walled carbon nanotubes (SWCNTs) were fabricated and characterised during this investigation. Hysteresis in memory structures was a clear indication of the memory effect and charge storage in these devices. Also, the hysteresis was centred close to 0 V for SWCNTs-based structures, which indicate that a low operation voltage is needed to charge the devices. A memory window of about 40 V was observed for AuNPs-based memory devices based on PVP; while the memory windows for devices based on PMMA with thin layer of Au and AuNPs floating gates were 22 V and 32 V, respectively. The electrical properties of the OTFMTs were improved by the use of the Au nanoparticles as the floating gate compared with that of an Au thin film. Using appropriate negative or positive voltages, the floating gate was charged and discharged, resulting in a clear shift in the threshold voltage of the memory transistors. Negative and positive pulses of 1 V resulted in clear write and erase states, respectively. Additionally, these organic memory transistors exhibited rather high carrier mobility of about μ = 0.319 cm2 V-1 s-1. Furthermore the data retention and endurance measurements confirmed the non-volatile memory properties of the memory devices fabricated in this study.

Since the discovery of organic semiconductors (OSCs) over four decades ago, the field of organic electronics has broken our misconceptions regarding the possibilities of modern electronics. The synthetic toolkit of organic chemistry enables the creation of a limitless number of unique OSCs that can be specifically tailored and engineered with the specific and desired properties for unique applications. The rapid adoption of modern information systems, “Internet of Things,” in which smart devices and sensors ubiquitously collect and exchange data has resulted in a need for low-cost sensors to be deployed everywhere from the monitoring of food supply chains, environmental conditions, to human health. Organic thin-film transistors (OTFTs) are a necessary component to support these technologies. However, their mass adoption will require reduction in cost and improved compatibility with low voltage generating printed batteries or flexible and ultrathin photovoltaics. This thesis is focused on the development of high performing solid state polymer electrolytes to be employed as the gating medium in OTFTs. The choice of conventional gating materials often leads to a tradeoff between high capacitance, operating speed and material softness. For example liquid electrolyte gating materials have high capacitance but low operating speed and are liquid at room temperature which is unacceptable for many electronics application. Polymer gating materials often have lower capacitance but fast operating conditions and solid at room temperature. In this thesis we establish structure property relationships which aid in the design of novel block copolymer-based gating materials which simultaneously enable the increase in capacitance and switching speed while remaining solid at room temperature. In the first study I established a styrene-based ionic liquid monomer can be using as a controlling monomer in the nitroxide mediated copolymerization of methacrylates. The second study then focuses on the integration of these materials into OTFT devices; the morphology (block vs random copolymers) effect on device performance is assessed. The last study builds on the findings of the previous study and further explores the structural elements of block copolymers on device performance. The work presented here outlines the development of advanced poly(ionic liquid) based solid electrolyte materials that enables both reduced operating voltages and fast switching. Finally, we establish structure-property relationships that relate the molecular architecture to OTFT device performance paving the way for the adoption of a new generation of high performing, printable and flexible electronics.

Research on pi-conjugated organic materials in the recent past has produced enormous developments in the field of organic electronics and it is mainly due to their applications in electronic devices such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). The primary goal of this research work is to design and synthesize high performing charge transport organic semiconductors. One of the criteria for better performance of the organic thin film transistor (OTFT) is to have high uniform thin film morphology of the organic semiconductor layer on the substrate. The first project in this dissertation has been directed towards improving the thin film morphology of the functionalized pentacenes through liquid crystalline behaviour. The results have suggested the possibility of thermotropic liquid crystalline phases in 6,13-bis(diisopropylhexylsilylethynyl) pentacene which has no pi-stacking in its solid state and the presence of silyl group at the peri-position is crucial for the stability of the functionalized pentacenes. In the second project, i have investigated the effect of alkyl groups with varying chain length on the anthradithiophene chromophore on the performance of the charge transporting devices. Organic blend cell based on solution processable 2,8-diethyl-5,12-bis(triethylsilylethynyl) anthradithiophene has showed 1% power conversion efficiency and the performance is mainly attributed to the large crystalline phase segregation of the functionalized anthradithiophene from the amorphous soluble fullerene derivative matrix. OTFT study on alkyl substituted functionalized anthradithiophenes suggested the need of delegate balance between thin film morphology and the crystal packing. Third project has been directed towards synthesizing halogen substituted functionalized anthradithiophenes and their influence in the performance of OFETs. OTFT made of 2,8-difluoro-5,12-bis(triethylsilylethynyl) anthradithiophene produced devices with thin film hole mobilities greater than 1 cm2/Vs. The result suggested that the device is not contact limited rather this high performance OTFTs are due to the contact induced crystallinity of the organic semiconductor.

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.

Soluble organic semiconductors have attracted interest due to their potential in making flexible and cheap electronics. Though their use is being implemented in electronics today, the conduction mechanism is still under investigation. In order to study the charge transport, this study examines the position, voltage, and frequency dependence of charge induced changes in far infrared absorption in soluble organic semiconductors in thin-film transistor structures. Measurements are compared to a simple model of a one-dimensional conductor which gives insight into the charge distribution and timing in devices. Main results of the study are dynamic measurements of charge taken by varying the frequency of the applied gate voltage while observing signal at one position within the transistor; mobility values obtained from a comparison to the one-dimensional model compare well with standard current-voltage measurements. Two small molecule soluble organic semiconductors were studied: 6,13 bis(triisopropylsilylethynyl)-pentacene and fluorinated 5,11 bis(triethylsilylethynyl) anthradithiophene.

This thesis presents the development of all-inkjet-printed low-voltage organic thin-film transistors. Organic thin-film transistors (OTFTs), taking advantage of low-temperature printability, mechanical flexibility, and multi-functionality, are promising for a wide range of emerging applications such as wearable electronics. Printed OTFTs provide great benefits in fabrication cost reduction, but they need a very high operating voltage and exhibit severe instability during storage and operation in ambient environment. In this study, all-inkjet-printed OTFTs with a low operating voltage of less than 3 V are demonstrated through reducing trap density in the fabricated devices. The transistors use 6,13-bis(triisopropylsilylethynyl)pentacene as the semiconductor, poly(4-vinylphenol) as the dielectric, silver as the electrodes, and CYTOP as the encapsulation. Several aspects of physical and chemical properties of polymer dielectrics are studied to achieve this goal, including cross-linking, wetting, and moisture affinity. Through the careful selection of device architecture and control of the inkjet-printing processes, the semiconductor-dielectric interface trap density of the fabricated OTFTs is significantly reduced. The applicability of this approach to different materials is also investigated and confirmed, including polyvinyl cinnamate as the dielectric, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene as the semiconductor, and anisole as the solvent for semiconductor inks. Based on the investigation of different materials, the characteristics and parameters of all-inkjet-printed OTFTs are optimised, demonstrating an ultra-steep subthreshold of 60.2 mV/decade approaching the theoretical limit and a low operating voltage of 1 V. In order to explore their feasibility in real-world applications, the stability of all-inkjet-printed OTFTs is investigated and the factors of instability are analysed. Based on these findings, the stability of the fabricated device is improved, such that the threshold voltage shift is less than 0.1 V in ambient environment storage for 3 months and operation for 1 hour. The electrical characteristics of OTFTs in the subthreshold regime are studied for analogue circuit design. Based on the developed low-voltage stable transistors, an ultra-low-power (< 1 nW) high-gain (> 200 V/V) amplifier is presented and utilised to detect electrophysiological signals from the human body.

Silicon phthalocyanines (R₂-SiPcs) are an emerging class of high-performance n-type or ambipolar organic semiconductors which have found application in organic electronic devices, including organic thin-film transistors (OTFTs), organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). Owing to their tetravalent silicon metal centre, R₂-SiPcs can be substituted with a range of axial ligands including phenols, carboxylic acids, and silanes to tune their intermolecular interactions, optical properties, electronic properties and solubility. While early reports of R₂-SiPcs have demonstrated promising results, the relationship between their structure and performance in OTFTs is poorly understood. Additionally, many OTFTs with R₂-SiPcs as semiconductor only demonstrate n-type behaviour under inert atmospheres due to their shallow lowest unoccupied orbital level below -4.1 eV making them susceptible to electron trapping by moisture and oxygen. This thesis presents developments in both the understanding of how R₂-SiPc structure influences performance, device engineering and exploration of these materials in ammonia sensors. First, I develop of structure-property relationships for a catalogue of fifteen R₂-SiPcs integrated into OTFTs including eleven materials used in OTFTs for the first time. I then explore the influence of dielectric surface chemistry on the texture of R₂-SiPc films and their resulting performance in OTFTs using silane self-assembled monolayers and para-sexiphenyl to understand the weak epitaxial growth behaviour of this class of materials. Next, I report eight novel peripherally fluorinated and axially substituted silicon phthalocyanines (R₂-FₓSiPcs) to investigate the influence of peripheral and axial fluorination on air-stable electron transport and determine the threshold for achieving air-stable n-type OTFTs. Finally, I integrate R₂-FₓSiPcs into organic heterojunction ammonia gas sensors to understand the influence of peripheral fluorination on the majority charge carrier in this device architecture.

<p>Organic thin film transistors (OTFTs) have attracted great attention in the last couple of decades due to their potential of cost reductions in manufacturing low-end electronic devices through solution processes. Currently, one of the major challenges facing the field of OTFTs is lack of high performance functional organic materials including both organic semiconductors and gate dielectrics for effective device integrations by solution deposition technologies. This thesis focuses on material designs, interfacial compatibilities, and device integrations for high performance OTFTs.</p> <p>Research progresses in the following areas are presented in this thesis. First, novel liquid-crystalline organic semiconductors, 2,5‟-bis-[2-(4-pentylphenyl)vinyl]-thieno(3,2-</p> <p><em>b</em>) thiophene and 2,5‟-bis-[2-(4-pentylphenyl)vinyl]-(2,2‟)bithiophene for OTFT applications were developed. Mobilities of the OTFTs fabricated from these semiconductors reached 0.15 cm2/V.s with high environmental stability. Such high performance is attributed to their ability to form highly ordered molecular structures. Second, a simple effective approach was developed for tuning solubility of a high mobility polythiophene system through engineering its molecular structure. OTFTs fabricated with the newly developed copolythiophenes from an environmentally benign non-chlorinated solvent showed excellent performance with mobility up to 0.18 cm2/V.s. Third, an effective approach to a solution processed gate dielectric Ph.D. Thesis – P. Liu, McMaster University, Chemical Engineering iv</p> <p>design was developed for all solution-processed flexible OTFTs. This was achieved through a dual-layer dielectric structure design comprised of a bottom layer with a UV-crosslinked poly(4-vinyl phenol-co-methyl methacrylate), (PVP-PMMA), and a top layer with a thermally crosslinked polysiloxane. This solution-processed dual-layer dielectric structure enabled all solution-processed high performance flexible OTFTs. Finally, flexible OTFTs were successfully integrated on plastic substrates (PET) from non-chlorinated solvents by using the copolythiophenes and the dual-layer dielectric. The integrated flexible devices showed good OTFT characteristics with mobility up to about 0.1 cm²/V.s.,</p> <p>well defined linear and saturated regions, and a close to zero turn-on voltage.</p> / Doctor of Philosophy (PhD)

Organic semiconductors are widely studied as potential active components for consumer electronics due largely to their easily tuned properties and the promise of lower-cost solution-based processing technology. Imide-functionalized organic small molecule compounds have been one of the more important and studied organic semiconductors. However, very few imide-functionalized conjugated polymers have been reported in the literature. The body of this dissertation focuses on the synthesis, structure-property and device studies of imide-functionalized conjugated polymers. Reasons for choosing arylene imides as polymer building blocks include: a) they impart low-lying LUMOs to polymers, allowing band-gap engineering through choice of comonomers with variable electron-donating ability; b) imide-nitrogens provide points to attach side chains to manipulate solubility and solid-state packing; c) they are easily prepared. Structure-property studies include electrochemical measurements, UV-Vis absorption spectroscopy, differential scanning calorimetry (DSC), x-ray diffraction, and in some cases evaluation as active components in field-effect transistors (OFETs) and photovoltaic devices (PVDs). The published method to synthesize 3,6-dibromo-pyromellitic bisimides (PMBI) was streamlined and poly(phenylene ethynylene)s (PPEs) with variable band gaps were prepared from them (Chapter 2). As noted in all the chapters, electrochemical and optical measurements reveal that the LUMO of the polymers is indeed dictated by the arylene imide, while the HOMO, and therefore the optical energy gap is controlled through varying the electron donor monomer. Intramolecular hydrogen bonding was employed for increasing backbone coplanarity and therefore the polymer could have higher conjugation. One of these polymers demonstrated the narrowest band gap (1.50 eV) for any published PPE. Chapter 3 describes the first published conjugated copolymers from naphthalene bisimides (NBI), here using thiophene-based comonomers as donor units. Polymers with high molecular weight and decent solubility were obtained by choosing appropriate side chains. The optical energy gaps could be tuned across the visible and into the near IR. Preliminary OFET studies revealed electron mobility as high as ~0.01 cm2/Vs. One low band gap polymer provided OFETs with electron mobility of ~0.04 cm2/Vs and hole mobility of ~0.003 cm2/Vs, which is also among the highest mobilities of ambipolar polymeric semiconductors. Using the same approach as in Chapter 3, phthalimide-based monomers were incorporated into polymer backbones for developing new high performance p-type polymer semiconductors for OFETs and PVDs (Chapter 4). Some analogues based on benzothiadiazole, PMBI, and thiophene imides as acceptors were prepared for comparison. Again, high molecular weight, soluble polymers with band gaps spanning the visible and into the near IR were obtained. OFETs from one of the polymers yielded hole mobility ~0.3 cm2/Vs under ambient atmosphere without post-processing thermal annealing, which places it squarely within the state-of-the-art for conjugated polymers. Due to the high mobility and low band gap, this polymer also leads to PVDs with moderately good power conversion efficiency (PCE: ~2%).

Donor-acceptor copolymers have received a great deal of attention for application as organic semiconductors, in particular as the active layers in low-cost consumer electronics. The functional groups grafted to the polymer backbones generally dictate the molecular orbital energies of the final materials as well as aid in self-assembly. Additionally, the side chains attached to these functional groups not only dictate the solubility of the final materials, but also their morphological characteristics. The bulk of the research presented in this dissertation focuses on the synthesis and structure-property relationships of polymers containing novel acceptor motifs. Chapter 2 focuses on the synthesis of 1,2-disubstituted cyanoarene monomers as the acceptor motif for copolymerization with known donors. It was found that cyanation of both benzene and thiophene aromatic cores resulted in a decrease of the molecular orbital energy levels. Additionally, the small size of this functional group allowed favorable self-assembly and close π-stacking to occur relative to related acceptor cores carrying alkyl side chains as evidenced by UV-Vis and WAXD data. Chapter 3 describes the systematic variation of side chain branching length and position within a series of phthalimide-based polymers. Branching of the side chains on bithiophene donor units resulted in the expected increase in solubility for these materials. Furthermore, a correlation was found between the branching position, size, and the HOMO energy levels for the polymers. Additionally, it was demonstrated that branching the alkyl side chains in close proximity to polymer backbones does not disrupt conjugation in these systems. A novel acceptor motif based on the 1,3-indanedione unit is presented in Chapter 4. Despite the stronger electron withdrawing capability of this functional group relativeto phthalimide, it was found that polymers based on this unit have the same HOMO molecular orbital energy levels as those presented in Chapter 3. It was found, however, the presence of orthogonal side chains greatly enhanced the solubility of the final polymers. Additionally, UV-Vis and WAXD measurements revealed that thermal annealing had a profound effect on the ordering of these polymers. Despite the presence of orthogonal side chains, long range order and close π-stacking distances were still achieved with these materials. Finally, alkynyl “spacers” were used in Chapter 5 to separate the solubilizing alkyl side chains from the polymer backbones on bithiophene donor monomers. The alkynyl groups allowed for conjugated polymer backbones to be achieved as well as low HOMO energy levels. A correlation between the side chain size, π-stacking distances and HOMO-LUMO energy levels was measured in this polymer series.