In this contribution, we demonstrate a new strategy to build electron deficient centers and electron accepting compounds through fusion of cyclopentene units. Two series of cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) were designed and synthesized as possible n-type semiconductors. One series of CP-PAHs were prepared by a palladium catalyzed cyclopentannulation reaction from precursors derived from the low-cost dye Vat Orange 3 (4,10-dibromoanthanthrone). The new cyclopenta-anthanthrene compounds possessed reduced optical and electrochemical gaps compared to anthanthrene derivatives and were electron acceptors with lowest unoccupied molecular orbital energies of ~ -3.4 eV to -3.9 eV. In a second research direction, CP-PAH chromophores were linked through two five-membered rings to create a new semiconductor linkage strategy. We show this new linker design can access planarized structures with reduced optical gaps and redox potentials. Two aceanthrylene chromophores were connected into dimer model systems with the chromophores either projected outward (2,2’-biaceanthrylene) or inward (1,1’-biaceanthrylene) and the optical and electronic properties were compared. Only the planar 2,2’-biaceanthrylene system showed significant reductions of the optical gaps (1 eV) and redox potentials in relation to the aceanthrylene monomer.

Hybrid organic/inorganic solar cells such as ZnO/P3HT offer promise in increasing efficiency of organic-based devices. However there are many unresolved issues such as poor short-circuit current and open-circuit voltage that are hampering their widespread, commercial use. It is thought that surface trap states on ZnO are providing an open avenue for carrier recombination thus creating devices with poor current transport characteristics. Using self assembled monolayers (SAMs) may provide some key answers and solutions to this problem by passivating trap states. In the course of this work, benzoic acid, 4-aminobenzoic acid, 4-methoxybenzoic acid, phenylphosphonic acid, and 4-methoxyphenylphosphonic acid SAMs were studied in large part due to their commercial availability. It was found that the phenylphosphonic acids had a clear impact on decreasing dark current; therefore strongly suggesting that exciton recombination has been inhibited to some degree. These molecules also caused a decrease in efficiency by an order of magnitude as compared to a plain ZnO/P3HT bilayer cell (standard). There were pronounced negative effects on the other device parameters such as open circuit voltage and short circuit current. In the case of 4-methoxybenzoic acid and benzoic acid the effects are not so clear in that parts of the dark J-V curve indicate a decrease in dark current while other regions show an increase. Interestingly for the negative effect on efficiency and other device parameters was not as pronounced as the phenylphosphonic acids. In both cases it is hypothesized that because of their wide band gaps and poor energy level matching, they ultimately impact device performance negatively. In the future, use of simulations to determine optimal SAM molecular structures that can be synthesized in the lab or purchased commercially is suggested.

Organic semiconductors are of interest because low processing temperatures and cost which make such materials candidates for flexible electronics. The charge transport properties of the material are largely dependent on solid-state arrangement of the molecules. This thesis focuses on co-crystallization as a means to achieve [2+2] photodimerization with organic semiconductors, the impact the co-crystal former has on mobility, the use of a co-crystal former to obtain different conformations of a flexible system, and the ability to detect the change in conformation by infrared spectroscopy. [2+2] photodimerization is studied as a way to alter the orientation of the π-systems in the solid state. To align a semiconductor building block into an orientation suitable for [2+2] photodimerization a co-crystallization method was used. The result of the photoreaction is the formation of a dimer in which the π-systems of the semiconductor building block are in a different orientation then before. Changes in the physical properties of the material through photodimerization are explored as a method for patterning thin films. The impact the second component has on the overall mobility in our system is examined. The second component is not expected to act as a semiconductor and the impact on mobility the by its inclusion in the solid is unknown. The impact of a second component on mobility is studied by observing the mobility of multiple co-crystals along with the mobility of the single component. It was found that the mobility could be increased by a factor of approximately 200 with addition of a second component. The mobility change seen in the two-component crystals is equated to the changes observed in the crystal packing. The conformation a molecule adopts in a solid can vary. It was discovered that the addition of a second component can be used to select the major conformation a bithiophene adopts in a solid. The change in conformation changes the orientation of the π-systems between molecules within the solid. The ability for a second component to alter the conformation of a bithiophene is explored. Infrared spectroscopy is used as a facial method to detect the change in the bithiophene conformation.

This PhD thesis deals with research activity in organic electronics, an extremely exciting field offundamental science, which is complemented by relevant applications in every-day life: optoelectronics,solar energy conversion (photovoltaics), sensing and information technology, just to mentiona few. In this area, charge transfer/energy transfer processes, as well as light matter interaction,play a major role. Within this topic, this work focus on the theoretical study of the electronic structureand related charge transfer/spectroscopic properties of various molecular systems: Coronenecrystals, F4TCNQ/perylene co-crystals, polythiophenes, PCPDT/PCPDT-BT homo- and copolymerbased on thiophene, chemisorbed ferrocene on Si(111) substrate. Particular attention wasdevoted to the calculation of infrared (IR) and electronic spectra associated with the so-called “polaronsignature” in doped organic semiconductors. To assess the reliability and physical meaning of electronic quantities involved in the calculations(like the HOMO/LUMO gap, theoretical spectroscopic intensity and vibrational modes, pursuingthe comparison between neutral and charged systems) a number of different levels of the theoryhave been systematically varied from HF (Hartree-Fock) to “pure” DFT (Density Functional Theory),as well as hybrid B3LYP (Becke 3-Parameter (Exchange), Lee, Yang and Parr 1,2) and longrange corrected Coulomb-Attenuating Method (CAM-B3LYP) functionals. In the attempt to untanglethe physics underlying the polaron formation in charged molecular systems (as evidenced by theoccurrence of giant intensity bands found in IR spectra), the mapping of the vibrational mode betweenneutral and charged systems has been calculated. A major achievement in this work is thatthe vibrational (giant) mode associated with the “polaron” emerges as a new mode (peculiar of thecharged state) not related to any mode existing in the neutral system.

Organic semiconductors have recently been found to have a comparably large spin diffusion time and length. This makes them ideal candidates for spintronic devices. However, spin injection, transport and detection properties in organic materials have yet to be fully understood. This work studies spin injection from ferromagnets into organic semiconductors via spin pumping. Furthermore, work towards thermal spin injection, and detection is presented and discussed. The first part of this thesis comprises the spin pumping experiments. Measuring linewidth broadening of the microwave absorption at ferromagnetic resonance due to increase in effective Gilbert damping by spin pumping from a ferromagnetic substrate into an adjacent non-magnetic semiconductor allows us to quantify the spin-mixing conductance. This technique is employed to demonstrate spin injection from a ferromagnetic metal, permalloy (Ni81Fe19), into organic small molecules and conjugated polymers as well as to quantify the spin injection efficiency. The results highlight the importance of structural properties of organic semiconductors at the interface to permalloy. Significant suppression of spin injection due to alkyl side-chains separating the core of the small molecules from the interface is exemplary for this finding. Furthermore, the spin-mixing conductance depends very sensitively on the charge carrier density within a certain range of doping level. This suggests a strong link between spin injection efficiency and spin concentration in the organic semiconductor at the interface to permalloy. The second part of the thesis aims to explore spin caloritronic effects. We study spin injection into organic semiconductors by probing the spin Seebeck effect by making use of the inverse spin Hall effect for spin-to-charge conversion. Moreover, we present experimental work towards observation of a novel effect, the inverse spin Nernst effect, for thermal spin detection.

This research project consists of a critical analysis of the role of intellectual property amongst other factors in the successful commercial development at the Cavendish Laboratory of optoelectronic light emitting diode display devices based on novel organic semiconductor materials. It begins by giving the background to the quantum mechanical properties upon which the technology is based, followed by a discussion of the path of innovation, describing the interaction between the different socioeconomic factors that influence this path. It then draws an analogy with the development of an analogous technology - inorganic semiconductors - to signpost the factors that may affect the developmental history of the technology. This is followed by an analysis of a chronology derived initially from patents downloaded from the World Patents Database of the European Patent Office to showcase the technology's development steps, and to study the patenting strategy of Cambridge Display Technology (CDT) - the company that was set up to commercialise the novel technology - through a patent trends analysis. From that, the major socioeconomic factors critical to the technology's development are analysed, followed by a test and extension of an existing Black Box mathematical model for studying the dynamics of innovation that is based on the interaction of those factors. Finally, through a patent citation analysis, CDT's commercial strategy for the technology is shown as being based on its patents portfolio to build an extensive licensing programme that pooled major academic, industry and commercial partners for the furtherance of the technology. This later evolved into a new ecosystem for the innovation, of which CDT occupied a central and indispensable position.

It has been found that a small magnetic field (<300 mT) can substantial change the electroluminescence, photoluminescence, photocurrent, electrical injection current in nonmagnetic organic semiconductors. It is generally believed that these magnetic field effects (MFE) are related to the spin dependent processes in organic semiconductor. However, the origin of MFE is still not well understood. In this dissertation, we investigate the underlying mechanism for magnetic field effects on electroluminescence (MFEEL) and magnetoresistance (MR) and demonstrate the complete tuning of MFEEL and MR based on our theoretical understanding. We consider MFE arising from magnetic field sensitive intersystem crossing (ISC) and triplet charge reaction. Magnetic field can increase the singlet ratios through ISC, accounting for positive MFEEL. Magnetic field modulated ISC strongly depends on the electron-hole pair separation distance. MFE can be enhanced by increasing the electron hole pair distance through material mixing and interplaying the electric dipole-dipole interaction. Meanwhile, two possible mechanisms corresponding for negative MFEEL: triplet-triplet annihilation and triplet charge reaction are also discussed. The negative MFEEL is achieved through adjusting triplet density charge confinement and exciton/charge ratio, which indicates that triplet charge reaction is a dominate process accountable for negative MFEEL. Significant MR and MFEEL are observed in strong spin orbital coupling iridium complex based OLED device after introducing the non-magnetic insulating blocking PVA layer. A possible mechanism for this new interface induced MR and MFEEL is proposed based on magnetic field perturbed spin-spin interaction at short capture distance of inter-charge carriers. The comparative study of two strong spin orbital coupling materials Ir(ppy)3 and Ir(ppy)2(acac) with different electrical dipole moments indicate the electric dipole-dipole interaction can change MR and MFEEL from short distance capture based regime to long distance intersystem-crossing regime. At last, we demonstrate the fully tuning sign of magnetic field effect on the fluorescence (MFEFEL) and phosphorescence (MFEPEL) by using the ISC, energy transfer and spin-spin interaction. In addition, we demonstrate a giant MFEEL (400%) in electrochemical cells and attribute this giant MFEEL to Lorentz force driven ion transport and Lorentz force dependent diffusion layer thickness through convection.

Abstract Organic semiconductors (OSCs) and graphene are two classes of conjugated materials that hold promise to create flexible electronic displays, high speed transistors, and low-cost solar cells. Crucial to understanding the behavior of these materials is understanding the effects metallic contacts have on the local charge environment. Additionally, characterizing the charge carrier transport behavior within these materials sheds light on the physical mechanisms behind transport. The first part of this thesis examines the origin of the low-temperature, high electric field transport behavior of OSCs. Two chemically distinct OSCs are used, poly-3(hexylthiophene) (P3HT) and 6,13- bis(triisopropyl-silylethynyl) (TIPS) pentacene. Several models explaining the low-temperature behavior are presented, with one using the Tomonaga-Luttinger liquid (TLL) insulator-to-metal transition model and one using a field-emission hopping model. While the TLL model is only valid for 1-dimensional systems, it is shown to work for both P3HT (1D) and TIPS-pentacene (2D), suggesting the TLL model is not an appropriate description of these systems. Instead, a cross-over from thermally-activated hopping to field-emission hopping is shown to explain the data well. The second part of this thesis focuses on the interaction between gold and platinum contacts and graphene using suspended graphene over sub-100 nanometer channels. Contacts to graphene can strongly dominate charge transport and mobility as well as significantly modify the charge environment local to the contacts. Platinum electrodes are discovered to be strong dopants to graphene at short length scales while gold electrodes do not have the same effect. By increasing the separation distance between the electrodes, this discrepancy is shown to disappear, suggesting an upper limit on charge diffusion from the contacts. Finally, this thesis will discuss a novel technique to observe the high-frequency behavior in OSCs using two microwave sources and an organic transistor as a mixer. A theoretical model motivating this technique is presented which suggests the possibility of retrieving gigahertz charge transport phenomena at kilohertz detection frequencies. The current state of the project is presented and discrepancies between devices made with gold and platinum electrodes measured in the GHz regime are discussed.

Using X-ray photoelectron (XPS) and X-ray absorbtion spectroscopy (XAS) we demonstrate charge transfer at an interface between two transition metal phthalocyanines, MnPc and F16CoPc, resulting in charged MnPc + and F16CoPc -, species. Density functional theory calculations reveal that a hybrid state is formed between the two types of phthalocyanines, which causes this charge transfer. For the hybrid state the Mn3dxz interacts with the Co 3dz2 orbital leading to a two-level system. Moreover, we have prepared mixed films out of this pair, which were characterized also by means of electron energy-loss spectroscopy. Our data reveal the formation of MnPc/F16CoPc charge transfer dimers in analogy to the heterojunction. The electronic excitation spectrum of these blends is characterized by a new low energy excitation at 0.6 eV.

The formulation induced photooxidative stability of substituted 6, 13 trialkylsilylethynylpentacene derivatives is report for the first time. It is known that 6,13 trialkylsilylethynylpentacene derivatives undergo rapid oxidation through singlet oxidation and is believed to be primarily to the 6,13 endoperoxide. The effect of solvent, organic semiconductor concentration and the presence of polymeric binder on the rate of photooxidative degradation in oxygen saturated solutions on degradation kinetics have been investigated. The photochemical stability of 6,13 trialkysilylethynylpentacene has been studied in toluene, THF, dichloromethane, chloroform, decane and dodecane at concentrations ranging from 2.5×10-6 M to 5×10-2 M by measurement of the half-life by UV-visible spectroscopy. The predominant degradation products of TIPS-Pentacene are the same regardless of solvent and concentration, being predominantly an endo-peroxide resulting from O2 addition across the 6, 13 position of the pentacene. At low concentration (< 1×10 -3 M), the half- life of TIPS-Pentacene is observed to vary with solvent and the amount of dissolved oxygen, with half-life showing a strong, positive correlation between the product of the Hansen Solubility Parameter (HSP) and the mole-fraction oxygen solubility. In this range, the half-life is observed to increase markedly with increasing solution concentration, showing a linear correlation with mean intermolecular distance within the solution. From intermediate to high concentration, the half-life increase more rapidly with increasing concentration, which is attributed to aggregation. In most solvents, this behaviour shows a specific onset point, suggesting a cooperative rather than isodesmic aggregation mechanism. Photooxidative half-lives of TMTES-pentacene and fluorinated soluble pentacene derivatives were determined from 2.5 × 10-6 M to 5.0 × 10-3 M solutions in THF, toluene, chloroform and CH2Cl2. Independent of solvent and at constant concentration the relative ordering of photo-oxidative resistance was: α-di-FTIPS-pentacene > β-di-FTIPS-pentacene, α-mono -FTIPS-pentacene, β-mono-FTIPS-pentacene > TIPS-pentacene ≥ TMTES-pentacene. The same rank order for a particular derivative was generally maintained over the concentration range 2.5 x 10-6 M to 5 x 10-3 M the stability to photooxidation for all derivative increased with increasing concentration. All derivatives showed a mixture of endooxidation products as shown by APCI mass spectrometry, and 1H NMR.Added polystyrene and isotactic polystyrene were found to accelerate photooxidative degradation whist isotactic polymethylmethacrylate (i-PMMA) and poly (triaylamine) (PTAA) had no effect adding weight to the argument that anything which can act as an acceptor to the solution slows down the degradation and anything which impairs pair-wise energy transfer speeds up the degradation. Understanding of these phenomena is therefore essential for the further application of solution processing of organic materials.