published_or_final_version / Electrical Engineering / Master / Master of Philosophy

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Boronsubphthalocyanines (BsubPcs) are a class of organic semiconducting materials which are relatively underdeveloped in their synthetic methods and organic semiconducting applications. A comprehensive investigation of these materials is explored in a rigorous and strategic manner progressing through each stage of the materials development cycle: materials selection from computational screening, organic/organometallic synthesis of target materials using known methods or by the development of new synthetic methods, physical and chemical analysis of new materials, and device implementation in organic light emitting diodes and organic photovoltaic cells. The result is the formation of new compositions of BsubPc specifically engineered for application as organic semiconductors in devices. Specifically, phenoxy-boronsubphthalocyanine derivatives are investigated starting with a computational study of their molecular orbitals – a property that dictates their function (donor or acceptor behaviour) in organic electronic devices. The nature of the axial phenoxylate is found to vary the energy level of the frontier molecular orbitals minimally, by up to ~0.4 eV while the nature of the BsubPc periphery can shift the energy levels of the frontier molecular orbitals by >1 eV. The differential sensitivity of the axial phenoxylate and the BsubPc periphery becomes a key design element allowing controlled adjustments of the frontier molecular orbitals by peripheral modification and isolating the design physical chemical properties essential to device fabrication to the axial phenoxylate. Subsequently, an investigation into the solubility and sublimability of these materials is performed, which leads to their investigation in OLED and OPV devices. The success from the phenoxy-BsubPcs study has led to the exploration of new chemistry to expand the available axial nucleophiles beyond phenoxylates. Previously unattainable sulphur and nitrogen nucleophiles are synthesised using two methods (1) the condensation of Cl-BsubPc with phthalimides and (2) the activation of Cl-BsubPc using aluminum chloride to access thiols and anilines. The phthalimido-BsubPcs synthesized from this method are incorporated in OLEDs.

Because of its potential application for flat panel displays, solid-state lighting and 1.5 µm emitter for fiber optical communications, organic light-emitting diodes (OLEDs) have been intensively researched. One of the major problems with current OLED technology relates to inefficient electron injection at the cathode interface, which causes high driving voltage and poor device stability. Making a low resistance cathode contact for electron injection is critical to device performance. This work mainly focuses on cathode interface design and engineering. The Ohmic contact using a structure of C60/LiF/Al has been developed in electron only devices. It is found that application of the C60/LiF/Al contact to Alq based OLEDs leads to a dramatic reduction in driving voltages, a significant improvement in power efficiency, and a much slower aging process. A new cathode structure based on metal-organic-metal (MOM) tri-layer films has been developed. It is found that MOM cathodes reduce reflection by deconstructive optical interference from two metal films. The absolute reflectance from the MOM tri-layer films can be reduced to as low as 7% in the visible light spectrum. In actual working devices, the reflectance can be reduced from ~80% to ~ 20%. MOM cathodes provide a potential low-cost solution for high contrast full-color OLED displays. Low voltage Erq based OLEDs at 1.5 µm emission have been developed. The Erq/Ag cathode interface has been found to be efficient for electron injection. Dramatic improvement in driving voltage and power efficiency has been realized by implementing Bphen and C60 into Erq devices as an electron transport layer. Integration of Erq devices on Si wafers has also been demonstrated.

Boronsubphthalocyanines (BsubPcs) are a class of organic semiconducting materials which are relatively underdeveloped in their synthetic methods and organic semiconducting applications. A comprehensive investigation of these materials is explored in a rigorous and strategic manner progressing through each stage of the materials development cycle: materials selection from computational screening, organic/organometallic synthesis of target materials using known methods or by the development of new synthetic methods, physical and chemical analysis of new materials, and device implementation in organic light emitting diodes and organic photovoltaic cells. The result is the formation of new compositions of BsubPc specifically engineered for application as organic semiconductors in devices. Specifically, phenoxy-boronsubphthalocyanine derivatives are investigated starting with a computational study of their molecular orbitals – a property that dictates their function (donor or acceptor behaviour) in organic electronic devices. The nature of the axial phenoxylate is found to vary the energy level of the frontier molecular orbitals minimally, by up to ~0.4 eV while the nature of the BsubPc periphery can shift the energy levels of the frontier molecular orbitals by >1 eV. The differential sensitivity of the axial phenoxylate and the BsubPc periphery becomes a key design element allowing controlled adjustments of the frontier molecular orbitals by peripheral modification and isolating the design physical chemical properties essential to device fabrication to the axial phenoxylate. Subsequently, an investigation into the solubility and sublimability of these materials is performed, which leads to their investigation in OLED and OPV devices. The success from the phenoxy-BsubPcs study has led to the exploration of new chemistry to expand the available axial nucleophiles beyond phenoxylates. Previously unattainable sulphur and nitrogen nucleophiles are synthesised using two methods (1) the condensation of Cl-BsubPc with phthalimides and (2) the activation of Cl-BsubPc using aluminum chloride to access thiols and anilines. The phthalimido-BsubPcs synthesized from this method are incorporated in OLEDs.

Organic semiconductors have the advantage over traditional inorganic semiconductors, such as Si or GaAs, in that they do not require perfect single crystal films to operate in real devices. Complicated multi-layer structures with nanometer scale thicknesses can thus be easily fabricated from organic materials using low-cost roll-to-roll manufacturing techniques. However, the discrete nature of organic semiconductors also implies that they typically contain almost no intrinsic charge carriers (i.e., electrons or holes), and thus act as insulators until electrical charges are injected into them. In electrical device applications this means that all of the holes and electrons within a device must be injected from the anode and cathode respectively. As a result, device stability, performance, and lifetime are greatly influenced by the interface between the organic materials and the electrode contacts. Despite the fundamental importance of the electrode/organic contacts, much of the basic physical understanding of these interfaces remains unclear. As a result, the current design of state-of-the-art organic optoelectronic devices tends to be based on trial and error experimentation, resulting in overly complicated structures that are less than optimal. In the present thesis, various electrode/organic interfaces relevant to device applications are studied using a variety of different techniques, including photoelectron spectroscopy and the iii temperature dependent current-voltage characteristics of single carrier devices. The fundamental understanding gleaned from these studies has been used to develop new strategies for controlling the energy-level alignment at electrode/organic interfaces. A universal method for tuning the work function of electrode materials using a halogenated organic solvent and UV light has been developed. Application of this technique in organic light emitting diodes enabled the first highly simplified two-layer device with a state-of-the-art record breaking efficiency.

We have synthesized norbornene-functionalized side-chain monomers for potential use as hole transporting and electron transporting/hole blocking materials in organic light-emitting diodes. TPD-norbornenes were prepared. The monomers demonstrated similar electrochemical and absorbance spectra to the parent TPD small molecule. The similarity is promising for using the monomers in OLEDs because TPD is a known blue-emitter with relatively high hole mobility in amorphous thin films. 1,10-Phenanthroline small molecules and monomers were synthesized to explore their potential as hole blocking materials in multilayer devices. We had difficulty purifying the monomers; however, the small molecules were slightly easier to reduce than commonly used hole blocking materials 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline.

This thesis aims to provide a detailed understanding of side-chain functionalized polymers as emissive materials for OLEDs. The syntheses and photophysical properties of these solution-processable materials as well as the effects of metal types, polymer backbones, chain lengths, spacer types and lengths, host types, and concentrations of the metal complexes on the emission properties and device performance will be dicussed. The polymers were functionalized with host materials along with the metal complexes to enhance the charge transport and to obtain energy transfer from the host to the complex. The physical and photophysical properties of the polymers were tuned by changing the backbone and the metal complex. Poly(norbornene)s, poly(cyclooctene)s, and poly(styrene)s were studied. The differences in the glass transition temperatures and PDIs of the polymers indicated that device performances might be affected by the polymer type due to the differences in the processability of the polymers. In addition to the backbone, it was found that device performance is dependent on various parameters such as molecular weight, metal loading, spacer type, and spacer length. In each case, it was found that the polymer backbone does not interfere with the basic photopysical properties of the metal complexes. The two main classes of metal complexes studied in this thesis are metalloquinolates and iridium complexes. It was shown that the emission properties of poly(cyclooctene)s containing 8-hydroxyquinolines in their side-chains could be altered by simply changing the metal. Green- and near IR-emitting polymers were synthesized by employing aluminum and ytterbium, respectively. On the other hand, for the iridium complexes, changes in color were achieved by varying the ligands. Iridium containing polymers with emission spectra that span the entire visible spectrum were synthesized by employing the appropriate ligands. It was demonstrated that OLEDs with high efficiencies can be fabricated by using these polymers as the emissive layer.

Thesis (Ph. D.)--University of Hong Kong, 2008. / Also available in print.