Die vorliegende Arbeit beschäftigt sich mit drei Ansätzen der hocheffizienten Erzeugung von weißem Licht mit organischen Leuchtdioden (OLEDs) auf der Basis kleiner Moleküle. Ein Ansatz kombiniert die Emission eines fluoreszenten und zweier phosphoreszenter Emitter in einer einzelnen Emissionsschicht. Da das Triplettniveau des verwendeten Blauemitters niedriger ist als die Triplettniveaus der phosphoreszenten Emitter, werden die Konzentrationen der Emitter so gewählt, dass ein Exzitonenübertrag zwischen ihnen unterbunden wird. Die strahlungslose Rekombination von Tripletts auf dem fluoreszenten Blauemitter begrenzt die Effizienz dieses Ansatzes, jedoch besticht die resultierende weiße OLED durch eine bemerkenswerte Farbstabilität. Der zweite Ansatz basiert auf dem “Triplet Harvesting” Konzept. Ansonsten ungenutzte Triplett Exzitonen werden von einem fluoreszenten Blauemitter auf phosphoreszente Emitter übertragen, wodurch interne Quanteneffizienzen bis zu 100 % möglich sind. Der zur Verfügung stehende Blauemitter 4P-NPD erlaubt aufgrund seines niedrigen Triplettniveaus nicht den Triplett übertrag auf einen grünen Emitter. Daher wird das “Triplet Harvesting” auf zwei unterschiedliche phosphoreszente Emitter, anhand des gelben Emitters Ir(dhfpy)2acac und des roten Emitters Ir(MDQ)2acac untersucht. Es wird gezeigt, dass beide phosphoreszente Emitter indirekt durch Exzitonendiffusion angeregt werden und nicht durch direkte Rekombination von Ladungsträgern auf den Emittermolekülen. Eine genaue Justage der Anregungsverteilung zwischen den phosphoreszenten Emittern ist durch Schichtdickenvariation in der Größenordnung üblicher Schichtdicken möglich. Spätere Produktionsanlagen brauchen daher keinen speziellen Genauigkeitsanforderungen gerecht zu werden. Der dritte und zugleich erfolgreichste Ansatz beruht auf einer Weiterentwicklung des zweiten Ansatzes. Er besteht zunächst darin den Tripletttransfer auf den Übertrag von einem fluoreszenten blauen auf einen phosphoreszenten roten Emitter zu beschränken. Die sich ergebende spektrale Lücke wird durch direktes Prozessieren einer unabhängigen voll phosphoreszenten OLED auf diese erste OLED gefüllt. Verbunden sind beide OLEDs durch eine ladungsträgererzeugende Schicht, in welcher durch das angelegte Feld Elektron/Loch-Paare getrennt werden. Dieser Aufbau entspricht elektrisch der Reihenschaltung zweier OLEDs, welche im Rahmen dieser Arbeit individuell untersucht und optimiert werden. Dabei ergibt sich, dass die Kombination von zwei verschiedenen phosphoreszenten Emittern in einer gemeinsamen Matrix die Ladungsträgerbalance in der Emissionszone sowie die Quanteneffizienz der vollphosphoreszenten OLED stark verbessert. Als Ergebnis steht eine hocheffiziente weiße OLED, welche durch die ausgewogene Emission von vier verschiedenen Emittern farbstabiles Licht mit warm weißen Farbkoordinaten (x, y) = (0.462, 0.429) und ausgezeichneten Farbwiedergabeeigenschaften (CRI = 80.1) erzeugt. Dabei sind die mit diesem Ansatz erreichten Lichtausbeuten (hv = 90.5 lm/W) mit denen von voll phosphoreszenten OLEDs vergleichbar.

The use of Raman spectroscopy in ultra high vacuum to assess structure and reactivity at the interface of tris-(8-hydroxyquinoline) aluminum (Alq3) with vapordeposited metals is presented. Understanding the structure of the interface between electron transport layer materials such as Alq3 and low work function metals such as Al, Mg and Ca is vital for engineering organic light emitting diodes with high efficiency and low driving voltage. Reactivity at the interface of Al, Mg and Ca with Alq₃ thin films is examined with Raman spectroscopy along with the non-reactive Ag/Alq₃ interface for comparison. Additionally, the effect of a thin LiF barrier layer on reactivity at the Al/Alq₃ and Mg/Alq₃ interfaces is also examined. Raman spectroscopy of post-deposited Ag on Alq3 films confirms preservation of the Alq₃ structure along with evolution of simple surface enhancement of Alq₃ spectral intensities. Changes in key vibrational modes of Alq₃ upon Ag deposition are consistent with weak interaction of Ag with the conjugated ring of the ligand. In contrast, vapor deposition of Al onto Alq₃ films results in the appearance of new Raman modes linked to the formation of an Al-Alq₃ adduct. Additionally, Raman modes associated with graphitic carbon are also noted for the Al/Alq₃ interface and are attributed to partial degradation of the organic film. The Raman spectral results for deposition of Mg onto Alq3 films also indicate formation of a complex interfacial region composed primarily of Mg-Alq₃ adducts and small-grained amorphous or nanocrystalline graphite. Raman spectroscopy of the Ca/Alq₃ interface is also indicative of formation of a Ca-Alq₃ complex; however, the graphitic carbon in this system is noted to be more disordered, sp³-type carbon compared to that observed for Al/Alq₃ and Mg/Alq₃. Examination of the Al/LiF/Alq₃ and Mg/LiF/Alq₃ interfaces illustrates that 5 Å-thick LiF layers partially block reaction chemistry between the metal and organic, while 10 Å thick LiF films completely eliminates reactivity at these interfaces. Implications of the presence of chemical species observed at these metal/organic interfaces on charge transport in devices are also discussed.

In dieser Arbeit wird die Photophysik von Einzelphotonenemittern unterschiedlicher Materialklassen, wie Fehlstellen in Diamant und Siliziumcarbid sowie organischer Moleküle bei Raumtemperatur untersucht. Zu diesem Zweck wurde ein hochauflösendes konfokales Mikroskop konzipiert und konstruiert, welches die optische Detektion einzelner Quantensysteme ermöglicht. Zusätzlich werden verschiedene Methoden wie die Rotationsbeschichtung, das Inkjet-Printing und das Inkjet-Etching in Bezug auf die Reproduzierbarkeit und Strukturierbarkeit von organischen Leuchtdioden (OLEDs) verglichen. Im weiteren Verlauf werden die optoelektronischen Prozesse in dotierten OLEDs untersucht, ausgehend von hohen Dotierkonzentrationen bis hin zur Dotierung mit einzelnen Molekülen. Dadurch kann die Exzitonen-Ladungsträger Wechselwirkung auf und in der Umgebung von räumlich isolierten Molekülen analysiert werden. / In this work the room-temperature photophysics of single-photon sources of different material systems such as NV-centers, vacancies in silicon carbide and organic molecules are investigated. A high resolution home-built confocal microscope is used to detect and analyse the isolated single quantum emitters. Additionally, different methods and techniques for production of organic light emitting diodes (OLEDs) such as spin-coating, inkjet-printing and inkjet-etching are compared concerning their reproducibility and feasibility for structured OLED preparation. Subsequently, the opto-electronic processes in dye-doped polymeric OLEDs are examined for various doping concentrations ranging from high concentrations down to the doping by single molecules. This provides access to the investigation of the exciton-charge carrier interaction of single organic molecules in organic matrices.

Zugl.: Erlangen, Nürnberg, Univ., diss, 2007

Organic Light Emitting Diodes (OLEDs) are widely viewed as next generation platform for flat panel displays and solid state lighting. Currently, OLED efficiency is not high due to high driving voltage. Molybdenum trioxide (MoO3) is ideal for p-type doping of the wide bandgap organic semiconductor 4,4’-bis-9-carbozyl biphenyl (CBP). With p-type doped CBP layer as Hole Transport Layer (HTL), driving voltage can be significantly reduced. Effective design for doped OLED structure consists of a HTL with doped layer from 20nm to 40nm and MoO3 concentration above 5%, the optimized OLED with doped CBP HTL present an 18% improvement over a standard device with CBP HTL at 100mA/cm2. Injection is found to be the principle cause of the reduction of driving voltage and shows close relations to doped layer thickness. Also charge balance is an important factor for high current efficiency, doped layer can be used as tools to promote charge balance.

Organic Light Emitting Diodes (OLEDs) are widely viewed as next generation platform for flat panel displays and solid state lighting. Currently, OLED efficiency is not high due to high driving voltage. Molybdenum trioxide (MoO3) is ideal for p-type doping of the wide bandgap organic semiconductor 4,4’-bis-9-carbozyl biphenyl (CBP). With p-type doped CBP layer as Hole Transport Layer (HTL), driving voltage can be significantly reduced. Effective design for doped OLED structure consists of a HTL with doped layer from 20nm to 40nm and MoO3 concentration above 5%, the optimized OLED with doped CBP HTL present an 18% improvement over a standard device with CBP HTL at 100mA/cm2. Injection is found to be the principle cause of the reduction of driving voltage and shows close relations to doped layer thickness. Also charge balance is an important factor for high current efficiency, doped layer can be used as tools to promote charge balance.

Energy accounting determines how much a software process contributes to the total system energy consumption. It is the foundation for evaluating software and has been widely used by operating system based energy management. While various energy accounting policies have been tried, there is no known way to evaluate them directly simply because it is hard to track every hardware use by software in a heterogeneous multicore system like modern smartphones and tablets. This work provides the ground truth for energy accounting based on multi-player game theory and offers the first evaluation of existing energy accounting policies, revealing their important flaws. The proposed ground truth is based on Shapley value, a single value solution to multi-player games of which four axiomatic properties are natural and self-evident to energy accounting. This work further provides a utility optimization formulation of energy management and shows, surprisingly, that energy accounting does not matter for existing energy management solutions that control the energy use of a process by giving it an energy budget, or budget based energy management (BEM). This work shows an optimal energy management (OEM) framework can always outperform BEM. While OEM does not require any form of energy accounting, it is related to Shapley value in that both require the system energy consumption for all possible combination of processes under question. This work reports a prototype implementation of both Shapley value-based energy accounting and OEM based scheduling. Using this prototype and smartphone workload, this work experimentally demonstrates how erroneous existing energy accounting policies can be, show that existing BEM solutions are unnecessarily complicated yet underperforming by 20% compared to OEM.

Recently, ¡§Pyrene¡¨ is a well-known substitutive group because the rigid structure and hole-injection ability of pyrene units can improve the thermal and electronic properties of blue OLED materials. The aromatic ring of pyrene not only improves the thermal and hole-injection ability of derivatives, but also provides high photoluminescence (PL) efficiency and high carrier mobility. Through these characteristics we can understand that the electron-rich pyrene derivatives can enhance the properties of OLED device. In this study, we developed low-molecular-weight structures with spiro-type molecules based on 9,9-spirobifluorene and fabricated the OLED device with the structure of ITO(170 nm)/PEDOT:PSS(50 nm)/PVK:TPSBF (40%)(90 mm)/BPhen (30 nm)/ LiF(1 nm)/Al(200 nm). It exhibited a maximum luminance at 500 mA/cm2 of 4130 cd/m2 with the Commission Internationale de l'Eclairage (CIE) of (0.16 , 0.16), and the maximum current and power efficiency were 1.9 cd/A and 0.9 lm/W, respectively. We concluded that TPSBF has good emission efficiency according to device performances. The PL maximum of the thick film of TPSBF, prepared by vacuum vapor deposition appears broadband spectrum, that resulted from the aggregation of the pyrene moieties. From Atomatic Force Microscopy (AFM) results, we propose three emission mechanisms. 1. Intramolecular spectrum ¡÷ 450 nm 2. Molecular aggregation spectrum ¡÷ 500 nm 3. Longer conjugation spectrum of intermolecular ¡÷ 550 nm We could fabricate the organic white light emitting device with a single and thicker emitting layer TPSBF and the more broadband emitting spectrum can be obtained. Finally, we fabricated the organic white light emitting device with a structure of ITO(170 nm)/2T-NATA(15 nm)/NPB(65 nm)/TPSBF(50 nm)/Alq3(30 nm)/LiF(0.8 nm)/Al(200 nm). It exhibited a maximum luminance at 1300 mA/cm2 of 57680 cd/m2, the maximum current and power efficiency were 6.51 cd/A and 4.07 lm/W, respectively, and with good CIE coordinate of (0.29 , 0.36).

Tandem structure for OLED application with HAT-CN:Alq3 interlayer is studied. It has been found that tandem cell with such interlayer structure has a low turn-on voltage as comparing to other types of interlayer structure. Three research topics are included in this research: (1) Study of carrier generation , carrier transport , and other optoelectronic properties for tandem cells with HAT-CN:Alq3 interlayer. (2) Study of white OLED device with single emitting layer and HAT-CN:Alq3 interlayer. (3) Study of electroluminescence property of top and bottom device of a tandem cell with HAT-CN:Alq3 interlayer. Experimental results show that electron transport is slowing down with increasing Alq3 concentration in the HAT-CN:Alq3 interlayer. However concentration of Alq3 in the interlayer does not affect transport property of hole. A white OLED with HAT-CN:Alq3 interlayer can reach 40,500 cd/m2 at 2180 mA/cm2 with a power efficiency of 1.96 lm/W and CIE coordicate of (0.34,0.32).

In this study, the dependence of metal/Alq3 Schottky contact barrier on the current¡Vvoltage characteristics of organic light emitting diodes was investigated to know the charge injection mechanism of OLED with the single-layer metal/Alq3/ metal structures, and the current density increases obviously with the reduction of contact potential barrier. As the thin LiF layer is inserted between the Al electrode and the Alq3 layer, it shows that the electron injection was promoted, and higher electroluminescence efficiency was also obtained. Both the energetic barrier and the tunneling integral parameter are reduced when the LiF layer thickness increases. For very thin films of LiF, the beneficial effect of the barrier reduction is dominant. When the film grows thicker, the negative insulating effect becomes dominant. Besides of simulating the current¡Vvoltage characteristics of organic light emitting diodes (OLED) based on Alq3 in combination with different cathodes, it was simulated that the current¡Vvoltage characteristics of OLED with an inserted LiF layer between metal and organic material, and then the OLED with various thicknesses of LiF films were also simulated. Finally, the result of simulations was compared to achieve a better description for the characteristics of current¡Vvoltage for the single carrier and layer based OLED.