This thesis reports on the design, fabrication, and testing of state-of-the-art, high-performance inverted top-emitting organic light-emitting diodes (OLEDs). The vast majority of research reports focuses on a device architecture referred to as a conventional OLED which has its anode on the bottom of the device and its cathode on the top. Moreover, most conventional OLEDs are bottom-emitting such that light exits the structure through both a semitransparent bottom electrode of indium-tin oxide and a glass substrate. The particular device architecture developed in this thesis is one in which the devices are inverted (i.e. their cathode is on the bottom as opposed to on top) and top-emitting. Despite the advantages that inverted top-emitting OLEDs possess over conventional bottom-emitting OLEDs, their development has been relatively slow. This is because inverted OLEDs have traditionally been hampered by the difficulty of injecting electrons effectively into the device. In this work, a novel method of injecting electrons from bottom cathodes into inverted OLEDs is discovered. In several previous reports, bottom Al/LiF cathodes had been used with the electron-transport material Alq3 to produce inverted OLEDs, but the resulting inverted OLEDs exhibited inferior performance to conventional OLEDs with top cathodes of Al/LiF. A new route for the development of highly efficient inverted OLEDs is shown through the use of electron-transport materials with high electron mobility values and large electron affinities. After systematic device optimization, inverted top-emitting OLEDs are demonstrated that currently define the state-of-the-art in terms of device efficiency. Optimized green and blue inverted top-emitting OLEDs are demonstrated that have a current efficacies of 92.5 cd/A and 32.0 cd/A, respectively, at luminance values exceeding 1,000 cd/m2. Finally, this discovery has enabled the development of the first stacked inverted top-emitting OLEDs ever made, combining all of the advantages offered by an inverted architecture, a top-emissive design, and a stacked structure. These OLEDs have a current efficacy of 200 cd/A at a luminance of 1011 cd/m2, attaining a maximum current efficacy of 205 cd/A at luminance of 103 cd/m2.

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The piezo-phototronics effect is the three way coupling of semiconductor properties, photonics and piezoelectricity in the same material. Research on piezo-phototronics effect has illustrated its application on various Zinc Oxide (ZnO) nanowire based devices, yet a systematical study with comprehensive theoretical model is still missing. Here we have designed experiments on wider variety of materials to investigate the mechanism of the piezo-phototronics effect, and then built up a theoretical model for more thorough understanding. Experimental results are shown for Cadmium Sulfide (CdS) photodetectors for visible light detection, inorganic/organic hybrid Light Emitting Diodes (LEDs) and LED arrays, and it is demonstrated that strain can significantly tune the performance of these optoelectronic devices. Theoretical methodologies are proposed for Metal-Semiconductor-Metal (MSM) structure and p-n junctions, including analytical solutions and Finite Element Method (FEM) simulations. For Schottky contacts in photodetectors, barrier height change is determined as the main reason for the effect, and an exponential relationship between applied external strain and the device current is discovered, and is qualitatively confirmed from experimental results. For p-n junctions in LEDs, change in size of depletion region under strain is credited for the current change, and a charge channel is predicted for large strain, which gives explanation for the observed gigantic enhancement of light emission efficiency in experiments.

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

Metal/organic/indium tin oxide (ITO) structures, including OLEDs, are demonstrated to contain multiple nonvolatile conductance states that can be programmed by the application of an external bias above a certain threshold voltage (Vth). These conductance states are stable and in turn can be probed by the use of a bias lower in value than Vth. The unbiased retention time of states is greater than several weeks, and more than 48,000 write-read-rewrite-read cycles have been performed with minimal degradation. It is found that the programming of a continuum of conductance states is possible, and techniques to do so are outlined. The electrical conductivity of the highest and lowest states can differ by six orders of magnitude. Switching speeds below 50 ns are shown, resulting in an energy requirement of about 100 pJ to switch from one conductance state to another. The memory phenomenon is shown to be influenced by the active layer thickness and anode/surface roughness while temperature dependence is limited. The electrical characteristics of these devices are consistent with metal diffusion or filament phenomena found in metal-insulator-metal structures, suggesting a possible mechanism by which the states are stored.Electroluminescent devices employing several new organic-inorganic lumophore-functionalized macromolecules are presented. In this study, macromolecules incorporating several lumophores covalently bonded to the vertices of a cubical core structure based on Polyhedral Oligomeric Silsesquioxane (POSS) in multiple configurations are implemented as light-emitting centers. The hole-transporting polymer poly(N-vinylcarbazole) (PVK) and electron-transporting additive 2-(4-biphenylyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (PBD) are used as a two-part host to enhance the carrier transport in these simple solution-processed single-layer devices. A study of energy transfer in several systems is carried out to understand the requirements needed to create white-light emission from a single macromolecule. A single macromolecule incorporating twenty-one blue and one yellow lumophore is shown to exhibit field-independent stable white-light electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31, 0.37). An external quantum efficiency of 0.55 percent and a maximum brightness of 1600 cd/m2 are attained with simple solution-processed single-layer devices. High solubility and ease of purification give these macromolecule white-light emitters advantages over their small molecule and polymeric type counterparts.

White LEDs (Light Emitting Diodes) in Visible Light Communication (VLC) is an emerging technology that is being researched so it can eventually be used for common communications systems. LEDs have a number of advantages, one of which is long life expectancy. However, like many emerging technologies, VLC has many technical issues that need to be addressed. We proposed an optical indoor wireless communication system that used white LEDs like plug-in devices. We developed a practical implementation of VLC and demonstrated it experimentally. In particular we focused on designing a prototype of VLC that can be used without having to make major changes to the present infrastructure with two types of protocol — namely RS-232 and USB — for data transmission.

Surface plasmon is a collective oscillation behavior of electrons in metal nanoparticle induced by the excitation of incident light, which can create an enhanced localized electric field near the surface of metal nanoparticle. To date, metal nanoparticle surface plasmon resonances have been extensively studied in the photoluminescence domain; little work however was devoted to electroluminescent and photovoltaic research. In this thesis, as a fundamental study we firstly investigated surface plasmon enhanced europium complex luminescence and obtained an improved understanding of the importance of optical spacer in metal enhanced fluorescence phenomenon. Under this guideline, we incorporated metal NPs into organic light emitting diodes (OLED) and organic solar cells, by means of thermal evaporation and wet chemistry. Metal nanoparticles are demonstrated to enhance the efficiency of both OLEDs and solar cells only under tailored device architecture. The surface plasmon enhanced local electric field plays an important and comprehensive role in enhancing device performance. In Alq3 based OLED we observed increased charge carrier injection by depositing Ag nanoparticles underneath the Al cathode; in Ir(ppy)3 based OLED we gained enhanced luminous efficiency via doping silica functionalized Ag nanoparticles into emitting layer; in P3HT based organic polymer solar cell we noticed an increased polymer absorption by incorporating Ag nanoparticles over the active layer. On the other hand, adverse effects such as metal nanoparticle induced charge carrier recombination and light extinction are also observed. The study of surface plasmon effects in organic optoelectronic devices reveals interesting surface plasmon features and permits to optimize optoelectronic devices from a novel point of view. / Thesis (Ph.D, Chemistry) -- Queen's University, 2012-01-05 17:22:40.074

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Over the past decade, intensive academic and commercial interests have been paid on compounds possessing photochemical properties, namely for their preparation, chemical properties, high efficiency and potential low-cost. Compounds having intense photochemical properties gained great interest due to wide range of potential applications. The sensitizers are one of the key components for high power-conversion efficiency in the dye sensitized solar cells (DSSCs). They are the core components in the organic light-emitting devices (OLEDs) due to their ability to emit light with the wavelengths largely red- shifted from their absorption wavelength. Ruthenium based sensitizers have been tagged “molecular light switches” because, although the fluorescence of these complexes in aqueous solutions is negligible, it increases of greater than 10000 fold in the presence of DNA. Many polypyridyl and dipyrido phenazine ruthenium complexes have achieved high power conversion efficiencies and therefore are of practical interest. Several research groups stated that the dipyrido phenazine ligand may be thought of as comprising two components: a bipyridyl unit and a phenazine unit. These two subunits behave essentially separately, with many molecular orbitals being localised over only one subunit and a redox properties of central phenazine moiety in the dipyrido phenazine ligand are important for the photochemical applications. Therefore a phenazine ligand was selected as a model for the present investigation. The chemistry of phenazine ligand is mostly limited to the late transition metal and f - element complexes. Our laboratory has a rich backgroung in the aluminum and early transition metal chemistry. The aluminum chemistry and early transition metal chemistry are of great interest since aluminum and early transition metal complexes are environmentally friendlier and cheaper than the late transition metal compounds. Another drawback of the ruthenium-based sensitizers is the lack of absorption in the red region of the visible spectrum, and also low molar extinction coefficients. An essential requirement for efficient conversion of solar energy is the good spectral match of the sensitizer absorption to the emission spectrum of solar radiation. In this regard, the ruthenium sensitizers’ spectral response in the lower energy regions is not sufficient. The current project has three parts. In the first part we collected and reviewed known literature regarding the certain classes of non-innocent ligands containing the six-membered carbon- nitrogen heterocycles and regarding the ligands potentially important for the photochemical applications. We also reviewed all available to the data information about the complexes supported by the phenazine ligand. In the second part we have investigated interaction of alkylaluminum compounds and phenazine and observed reduction of phenazine accompanied by formation of dialuminum cage type compounds containing two formally mononegative phenazine ligand. The derivatization of phenazine has been also observed. It resulted in formation of compounds having a stable organic radical. In a third part of our project we have explored interaction of phenazine or thiophenazine with the alkylaluminum compounds and chromium dichloride. The reaction in the three component system resulted in reduction of phenazine ligand and lead to the heterometallic Cr(II) - aluminum complexes containing a formally dinegative phenazine or thiophenazine ligands. When a large excess of triethylaluminum was taken, reduction of phenazine and chromium has been observed leading to the heterometallic multinuclear Cr(I) - aluminum complex containing a formally dinegative phenazine ligands and two chromium atoms in one complex in the rare oxidation state one.