Organic thin-film transistors (OTFTs) have been widely studied because of their promising potential for application in low-cost, large-area and flexible electronics. However, several challenges remain on the way towards practical OTFT devices, such as a high operating voltage (> 20 V) induced by the low charge carrier mobility of organic semiconductors and low capacitance of organic gate dielectrics. A low operating voltage is essential for various OTFTs applications, such as portable displays, radio frequency identification tags (RFIDs), smart textiles and sensors. The key to low voltage operation of OTFTs is reduction of the threshold voltage, inverse subthreshold slope which can be fulfilled by using a high-capacitance gate dielectric with superior interface properties. Since field-effect current is proportional to field-induced charge density, using a gate dielectric layer with high dielectric constant (high-k) enhances output current densities at much lower applied voltages. Very thin dielectric layers have reportedly suffered from poor dielectric properties, while very high-k gate dielectrics have led to inferior dielectric-semiconductor interface. As a result, unsatisfactory device performance, such as low charge carrier mobility and high gate leakage current, has been obtained. In addition, solution-processability on a variety of substrates and compatibility with most common semiconducting materials make high-k dielectric materials an unrivalled candidate for low-voltage, low-cost applications. Consequently, the aim of this project was to produce a high-quality, high-capacitance gate dielectric with excellent properties which is consistent with cheap, basic solution-processing manufacturing techniques. With great promise in hybrid materials, a novel, high-k dielectric material based on alternative organic-inorganic nanocomposites that combine very high dielectric constant values intrinsic to ferroelectric ceramic materials (nanoparticles) with mechanical flexibility, low-cost and easy processing of polymers was developed. Both low- and high-k polymer matrices have been used in formulating high-k nanocomposite dielectric suspensions. The uniformity of suspensions has been improved by surface modification of nanoparticles in the case of low-k polymers, while a combination of polymer choice, solvents and nanoparticle-to-polymer ratio led to homogenous suspensions based on high-k polymers. The nanocomposite preparation technique was also unique to this work and gave reproducibly stable nanocomposite suspensions. Finally, ultralow-voltage (~ 1) OTFTs have been successfully demonstrated by integrating nanocomposite bilayer dielectrics using a high-k fluorinated polymer. Bilayer dielectrics were formed by (partially) capping the surface of the nanocomposite films with an ultrathin capping layer. The capping layer was the key to the operation of low-voltage OTFTs as it allowed remarkable and advantageous use of the nanocomposite surface roughness while improving the dielectric-semiconductor surface roughness. Ultimately, such nanocomposite bilayers have a potential to pave the way towards low-cost fabrication and integration of low-voltage components and circuits on flexible substrates.

Organic electronics are devices based on semiconductors derived from carbon based molecules and polymers. These devices can be made flexible, lightweight and potentially inexpensive with the development of economies of scale. Specific examples of organic electronics include organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs) and organic photovoltaic (OPVs). Anthracene-based semiconductors are materials that have generated great interest primarily because of their structural planarity, potential for strong intermolecular interactions, air stability and ideal frontier molecular orbital energy levels. In this thesis, we detail two publications that examined functionalized anthracene molecules integrated into OTFTs, along with their thermal, electrochemical and optical properties. We started by examining seven novel 9,10-anthracene-based molecules. It was found that functionalization of the 9,10-positions with different phenyl derivatives resulted in negligible variation in the optical properties with minor (±0.10 eV) changes in electrochemical behaviour, while the choice of phenyl derivative greatly affected the thermal stability whereby the decomposition temperatures (Td) varied by as much as 128 °C between certain functionalized derivatives. The findings suggested that functionalization of the 9,10-position of anthracene leads to an effective handle for tuning of the thermal stability while having little to no effect on the optical properties and the solid-state arrangement. We continued with the synthesis of several novel anthracene derivatives which were di-substituted at the 2,6-positions. It was found that 2,6-functionalization with various fluorinated phenyl derivatives led to negligible changes in the optical behaviour while influencing the electrochemical properties (±0.10 eV). Furthermore, the choice of fluorinated phenyl moiety had noticeable effects on melting point and thermal stability (ΔTm < 55 °C and ΔTd < 65 °C). OTFTs were fabricated and characterized using the 2,6-anthracene derivatives as the semiconducting layer. The addition of fluorine groups on the phenyl groups led to a transition from p-type behaviour to n-type behaviour in BGBC OTFTs. The results indicated that the choice of functional group as well as its functionalization location, at the 9,10- and 2,6-positions, can act as powerful handles to engineer high performance OTFTs.

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.

The high-frequency and low-voltage operation of organic thin-film transistors (OTFTs) is a key requirement for the commercial success of flexible electronics. Significant progress has been achieved in this regard by several research groups highlighting the potential of OTFTs to operate at several tens or even above 100 MHz. However, technology maturity, including scalability, integrability, and device reliability, is another crucial point for the semiconductor industry to bring OTFT-based flexible electronics into mass production. These requirements are often not met by high-frequency OTFTs reported in the literature as unconventional processes, such as shadow-mask patterning or alignment with unrealistic tolerances for production, are used. Here, ultra-short channel vertical organic field-effect transistors (VOFETs) with a unity current gain cut-off frequency (fT) up to 43.2 MHz (or 4.4 MHz V−1) operating below 10 V are shown. Using state-of-the-art manufacturing techniques such as photolithography with reliable fabrication procedures, the integration of such devices down to the size of only 12 × 6 μm2 is shown, which is important for the adaption of this technology in high-density circuits (e.g., display driving). The intrinsic channel transconductance is analyzed and demonstrates that the frequencies up to 430 MHz can be reached if the parasitic electrode overlap is minimized.