Optimizing the internal electric field distribution of alternating current driven organic light-emitting devices for a reduced operating voltage

This work was funded with financial means of the European Social Fund and the Free State of Saxony through the OrthoPhoto project.The influence of the thickness of the insulating layer and the intrinsic organic layer on the driving voltage of p-i-n based alternating current driven organic light-emitting devices (AC-OLEDs) is investigated. A three-capacitor model is employed to predict the basic behavior of the devices, and good agreement with the experimental values is found. The proposed charge regeneration mechanism based on Zener tunneling is studied in terms of field strength across the intrinsic organic layers. A remarkable consistency between the measured field strength at the onset point of light emission (3-3.1 MV/cm) and the theoretically predicted breakdown field strength of around 3 MV/cm is obtained. The latter value represents the field required for Zener tunneling in wide band gap organic materials according to Fowler-Nordheim theory. AC-OLEDs with optimized thickness of the insulating and intrinsic layers show a reduction in the driving voltage required to reach a luminance of 1000 cd/m2 of up to 23% (8.9 V) and a corresponding 20% increase in luminous efficacy.Publisher PDFPeer reviewe

Exciton Dynamics in White Organic Light-Emitting Diodes comprising Triplet Harvesting

This work comprises different approaches for the efficiency enhancement of white organic light-emitting diodes (OLEDs). In particular, diffusion and transfer processes of excited singlet and triplet states are investigated. Generation of white light is realized by using the so-called triplet harvesting method where the otherwise nonradiatively decaying triplets of a blue fluorescent emitter are transferred to a highly efficient phosphorescent emitter and result in additional emission at lower energies. Triplet harvesting significantly increases the internal quantum efficiency in OLEDs. First, the well-known blue emitter 4P-NPD is investigated as model case. Using time-resolved spectroscopy, triplet harvesting by a yellow and red phosphorescent emitter, respectively is directly proven. However, triplet harvesting by a green emitter is not possible due to the low triplet energy of 4P-NPD. Using quantum chemical calculations, two new emitter molecules, 8M-4P-NPD and 8M-4P-FPD, are synthesized with the aim to rise the triplet energy. Their properties and their ability to facilitate triplet harvesting by a green emitter are studied. For the first time, a white triplet harvesting OLED is demonstrated where triplet harvesting occurs directly from a blue emitter to a green and a red emitter. Furthermore, an additional singlet transfer is observed in the triplet harvesting OLEDs under investigation. Using the phosphorescent emitter as singlet sensor, this effect allows the determination of the singlet diffusion length in 4P-NPD. By varying the distance between singlet generation zone and singlet sensor, a singlet diffusion length of 4.6 nm is found. One further approach to increase the efficiency is the optimization of a tandem OLED which comprises two single OLED units stacked on top of each other. At a luminance of 1,000 cd/m², the white tandem OLED shows an external quantum efficiency of 25%, a luminous efficacy of 33 lm/W, a color rendering index (CRI) of 62, and Commission Internationale de l’Eclairage (CIE) color coordinates of (0.53/0.43). These efficiencies are comparable to state-of-the-art efficiencies of white OLEDs. Finally, the highly efficient white tandem structure is applied on an alternative electrode consisting of flattened silver nanowires. In comparison to the conventional OLED with indium-tin oxide (ITO) electrode, this OLED shows similarly high efficiencies as well as a superior color stability in terms of viewing angles. The color stability can be assigned to the light scattering properties of the nanowires. The OLED with silver nanowire electrode shows efficiencies of 24% and 30 lm/W at 1,000 cd/m² with a CRI of 69 and CIE coordinates of (0.49/0.47).:List of Publications List of Important Abbreviations 1 Introduction 2 White Light and Color 2.1 Radiometry and Photometry 2.2 Color Stimulus Specification 2.3 White Light 2.4 Light Sources 3 Organic Semiconductors 3.1 Molecular Orbitals 3.2 Fluorescence and Phosphorescence 3.3 Singlet-Triplet Splitting 3.4 Energy Transfer Mechanisms 3.5 Exciton Diffusion and Quenching 3.6 Charge Carrier Transport 4 Organic Light-Emitting Diodes 4.1 Electroluminescence 4.2 The pin Concept 4.3 Phosphorescent Emitters 4.4 Triplet Harvesting 4.5 Light Outcoupling 4.6 White OLEDs - State-of-the-Art 5 Experimental and Methods 5.1 Materials 5.2 Device Preparation 5.3 OLED Characterization 5.3.1 IVL and Spectral Emission 5.3.2 Angular Dependence 5.3.3 Efficiencies 5.3.4 Lifetime 5.4 Time-Resolved Spectroscopy 5.5 Photoluminescence Setup 5.6 Theoretical Calculations 5.6.1 Optical Simulation of OLEDs 5.6.2 Calculation of Molecular Orbitals 6 Triplet Harvesting 6.1 The Emitter 4P-NPD 6.1.1 Orientation 6.1.2 Exciton Harvesting 6.1.3 Two-color white TH OLED 6.2 Development of Blue Emitters 6.2.1 8M-4P-NPD 6.2.2 8M-4P-FPD 6.3 Comparison to Quantum Chemical Calculations 6.4 Summary and Outlook 7 Singlet Diffusion Length 7.1 Electroluminescence Quenching 7.1.1 Working principle of the device 7.1.2 Theoretical Considerations 7.1.3 Results 7.2 Photoluminescence Quenching 7.2.1 Preliminary Considerations 7.2.2 Reference Devices 7.2.3 Sample Devices and Discussion 7.3 Summary and Outlook 8 Tandem OLEDs 8.1 Previous Work 8.2 Triplet Harvesting Unit 8.3 Full Phosphorescent Unit 8.4 Charge Generation Layer in Tandem OLEDs 8.5 Tandem OLED with Double Emission Layer 8.6 Conclusions and Outlook 9 Silver Nanowire Electrodes 9.1 Demand for Alternative Electrodes 9.2 Processing and Quality Characteristics 9.3 Influence of Organic Buffer Layers 9.4 Variation of the Electron Transport Layer Thickness 9.5 Highly Efficient OLEDs on Silver Nanowire Electrodes 9.6 Summary and Outlook 10 Concluding Remarks 10.1 Summary of Main Results 10.2 Outlook: White TH OLEDsIn dieser Arbeit werden verschiedene Ansätze zur Effizienzsteigerung in weißen organischen lichtemittierenden Dioden (OLEDs) erforscht. Hierfür werden im Besonderen Diffusions- und Transferprozesse von angeregten Singulett- und Triplettzuständen untersucht. Zur Erzeugung von weißem Licht wird die sogenannte “triplet harvesting” Methode verwendet, bei der die sonst nicht zur Emission beitragenden Triplettzustände eines fluoreszenten blauen Emitters auf einen hocheffizienten phosphoreszenten Emitter übertragen werden. Dieser liefert dann zusätzliche Emission im niederenergetischen Spektralbereich. Durch triplet harvesting kann die interne Quantenausbeute in OLEDs beträchtlich gesteigert werden. Zunächst wird der bekannte blaue Emitter 4P-NPD als Modellbeispiel untersucht. Mittels zeitlich aufgelöster Spektroskopie kann triplet harvesting auf einen gelben bzw. roten Emitter direkt nachgewiesen werden. Allerdings ist auf Grund der niedrigen Triplettenergie triplet harvesting auf einen grünen Emitter nicht möglich. In Anbetracht dieser Tatsache werden unter Zuhilfenahme quantenchemischer Betrachtungen zwei neue Emittermoleküle, 8M-4P-NPD und 8M-4P-FPD, synthetisiert und auf ihre Eigenschaften und ihre Eignung für triplet harvesting untersucht. Dabei wird zum ersten Mal eine weiße OLED realisiert, in der triplet harvesting von einem blauen Emitter direkt auf einen grünen und einen roten Emitter erfolgt. Des Weiteren wird bei den untersuchten triplet harvesting OLEDs ein zusätzlicher Singulettübertrag auf den phosphoreszenten Emitter beobachtet. Dieser Effekt wird zur Bestimmung der Singulettdiffusionslänge in 4P-NPD genutzt. Der phosphoreszente Emitter dient dabei als Singulettsensor. Über eine Variation des Abstands zwischen Singulettgenerationszone und Sensor wird eine Singulettdiffusionslänge von 4,6 nm bestimmt. Ein weiterer Ansatz zur Effizienzsteigerung besteht in der Optimierung einer aus zwei OLEDs zusammengesetzten Tandem OLED. Bei einer Leuchtdichte von 1000 cd/m² erzielt diese weiße Tandem OLED eine externe Quanteneffizienz von 25% und eine Leistungseffizienz von 33 lm/W mit einem Farbwiedergabeindex (CRI) von 62 und Commission Internationale de l’Eclairage (CIE) Farbkoordinaten von (0,53/0,43). Diese Effizienzen sind vergleichbar mit dem aktuellen Forschungsstand weißer OLEDs. Schließlich wird diese hocheffiziente weiße Tandemstruktur auf eine alternative Elektrode bestehend aus flachgedrückten Silbernanodrähten aufgebracht. Im Vergleich zur konventionellen OLED mit Indiumzinnoxid (ITO) Elektrode erreicht diese ähnlich hohe Effizienzen sowie eine verbesserte Farbstabilität bezüglich des Betrachtungswinkels, was auf die Streueigenschaften der Nanodrähte zurückgeführt werden kann. Bei einer Leuchtdichte von 1000 cd/m² zeigt die OLED mit Silbernanodrahtelektrode Effizienzen von 24% und 30 lm/W bei einem CRI von 69 und CIE Koordinaten von (0,49/0,47).:List of Publications List of Important Abbreviations 1 Introduction 2 White Light and Color 2.1 Radiometry and Photometry 2.2 Color Stimulus Specification 2.3 White Light 2.4 Light Sources 3 Organic Semiconductors 3.1 Molecular Orbitals 3.2 Fluorescence and Phosphorescence 3.3 Singlet-Triplet Splitting 3.4 Energy Transfer Mechanisms 3.5 Exciton Diffusion and Quenching 3.6 Charge Carrier Transport 4 Organic Light-Emitting Diodes 4.1 Electroluminescence 4.2 The pin Concept 4.3 Phosphorescent Emitters 4.4 Triplet Harvesting 4.5 Light Outcoupling 4.6 White OLEDs - State-of-the-Art 5 Experimental and Methods 5.1 Materials 5.2 Device Preparation 5.3 OLED Characterization 5.3.1 IVL and Spectral Emission 5.3.2 Angular Dependence 5.3.3 Efficiencies 5.3.4 Lifetime 5.4 Time-Resolved Spectroscopy 5.5 Photoluminescence Setup 5.6 Theoretical Calculations 5.6.1 Optical Simulation of OLEDs 5.6.2 Calculation of Molecular Orbitals 6 Triplet Harvesting 6.1 The Emitter 4P-NPD 6.1.1 Orientation 6.1.2 Exciton Harvesting 6.1.3 Two-color white TH OLED 6.2 Development of Blue Emitters 6.2.1 8M-4P-NPD 6.2.2 8M-4P-FPD 6.3 Comparison to Quantum Chemical Calculations 6.4 Summary and Outlook 7 Singlet Diffusion Length 7.1 Electroluminescence Quenching 7.1.1 Working principle of the device 7.1.2 Theoretical Considerations 7.1.3 Results 7.2 Photoluminescence Quenching 7.2.1 Preliminary Considerations 7.2.2 Reference Devices 7.2.3 Sample Devices and Discussion 7.3 Summary and Outlook 8 Tandem OLEDs 8.1 Previous Work 8.2 Triplet Harvesting Unit 8.3 Full Phosphorescent Unit 8.4 Charge Generation Layer in Tandem OLEDs 8.5 Tandem OLED with Double Emission Layer 8.6 Conclusions and Outlook 9 Silver Nanowire Electrodes 9.1 Demand for Alternative Electrodes 9.2 Processing and Quality Characteristics 9.3 Influence of Organic Buffer Layers 9.4 Variation of the Electron Transport Layer Thickness 9.5 Highly Efficient OLEDs on Silver Nanowire Electrodes 9.6 Summary and Outlook 10 Concluding Remarks 10.1 Summary of Main Results 10.2 Outlook: White TH OLED

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials owning isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.Comment: 11 pages, 6 figures, will be submitted to PR

What are the consequences of the AWG-projections for the adequacy of social security pensions? ENEPRI Research Report No. 65, 16 January 2009

This paper starts by describing the model MIDAS in detail. It next presents and discusses some simulation results for Belgium, Germany and Italy. Finally, the simulation results of two alternative policy scenarios are presented and discussed

Development and evaluation of a titanium-based planar ultrasonic scalpel for precision surgery.

This paper introduces a titanium-based planar ultrasonic microscalpel. The concept of silicon-based planar ultrasonic transducers has already been proven, but they are not yet suitable for clinical use due to material failure. The main objective of this work was to develop a smaller, lighter, and more cost-effective ultrasonic scalpel that could be used as an alternative or supplementary device to current surgical instruments. Various prototypes were fabricated and characterized, differing in bonding by three epoxy adhesives and two solder pastes as well as three variations in tip design. The instruments were designed to operate in the frequency range of commercial instruments and to generate a longitudinal displacement amplitude. The electro-mechanical characterization through impedance analysis and vibration measurements was complemented by an in vitro cutting trial and an acute in vivo animal experiment in comparison to commercial ultrasonic and electrosurgical devices. The operating frequency was around 40 kHz and 48 kHz depending on whether matched or unmatched operation was used. Unmatched operation turned out to be more suitable, achieving displacement amplitudes of 25.3 μm and associated velocity amplitudes of up to 7.9 m/s at an electrical power of 10.2 W. The cutting ability was demonstrated in vivo by successful dissection even under anticoagulation. The geometry of the instrument tip was found to have a major influence on cutting performance by affecting the resonance behaviour and tissue penetration

A small molecule screen identifies novel inhibitors of mechanosensory nematocyst discharge in Hydra

Cnidarians are characterized by the possession of stinging organelles, called nematocysts, which they use for prey capture and defense. Nematocyst discharge is controlled by a mechanosensory apparatus with analogies to vertebrate hair cells. Members of the transient receptor potential (TRPN) ion channel family are supposed to be involved in the transduction of the mechanical stimulus. A small molecule screen was performed to identify compounds that affect nematocyst discharge in Hydra. We identified several [2.2]paracyclophanes that cause inhibition of nematocyst discharge in the low micro-molar range. Further structure–activity analyses within the compound class of [2.2]paracyclophanes showed common features that are required for the inhibitory activity of the [2.2]paracyclophane core motif. This study demonstrates that Hydra can serve as a model for small molecule screens targeting the mechanosensory apparatus in native tissues

N4BP1 functions as a dimerization-dependent linear ubiquitin reader which regulates TNF signalling

Signalling through TNFR1 modulates proinflammatory gene transcription and programmed cell death, and its impairment causes autoimmune diseases and cancer. NEDD4-binding protein 1 (N4BP1) is a critical suppressor of proinflammatory cytokine production that acts as a regulator of innate immune signalling and inflammation. However, our current understanding about the molecular properties that enable N4BP1 to exert its suppressive potential remain limited. Here, we show that N4BP1 is a novel linear ubiquitin reader that negatively regulates NFκB signalling by its unique dimerization-dependent ubiquitin-binding module that we named LUBIN. Dimeric N4BP1 strategically positions two non-selective ubiquitin-binding domains to ensure preferential recognition of linear ubiquitin. Under proinflammatory conditions, N4BP1 is recruited to the nascent TNFR1 signalling complex, where it regulates duration of proinflammatory signalling in LUBIN-dependent manner. N4BP1 deficiency accelerates TNFα-induced cell death by increasing complex II assembly. Under proapoptotic conditions, caspase-8 mediates proteolytic processing of N4BP1, resulting in rapid degradation of N4BP1 by the 26 S proteasome, and acceleration of apoptosis. In summary, our findings demonstrate that N4BP1 dimerization creates a novel type of ubiquitin reader that selectively recognises linear ubiquitin which enables the timely and coordinated regulation of TNFR1-mediated inflammation and cell death

Induction of Histiocytic Sarcoma in Mouse Skeletal Muscle

Myeloid sarcomas are extramedullary accumulations of immature myeloid cells that may present with or without evidence of pathologic involvement of the bone marrow or peripheral blood, and often coincide with or precede a diagnosis of acute myeloid leukemia (AML). A dearth of experimental models has hampered the study of myeloid sarcomas and led us to establish a new system in which tumor induction can be evaluated in an easily accessible non-hematopoietic tissue compartment. Using ex-vivo transduction of oncogenic Kras(G12V) into p16/p19−/− bone marrow cells, we generated transplantable leukemia-initiating cells that rapidly induced tumor formation in the skeletal muscle of immunocompromised NOD.SCID mice. In this model, murine histiocytic sarcomas, equivalent to human myeloid sarcomas, emerged at the injection site 30–50 days after cell implantation and consisted of tightly packed monotypic cells that were CD48+, CD47+ and Mac1+, with low or absent expression of other hematopoietic lineage markers. Tumor cells also infiltrated the bone marrow, spleen and other non-hematopoietic organs of tumor-bearing animals, leading to systemic illness (leukemia) within two weeks of tumor detection. P16/p19−/−; Kras(G12V) myeloid sarcomas were multi-clonal, with dominant clones selected during secondary transplantation. The systemic leukemic phenotypes exhibited by histiocytic sarcoma-bearing mice were nearly identical to those of animals in which leukemia was introduced by intravenous transplantation of the same donor cells. Moreover, murine histiocytic sarcoma could be similarly induced by intramuscular injection of MLL-AF9 leukemia cells. This study establishes a novel, transplantable model of murine histiocytic/myeloid sarcoma that recapitulates the natural progression of these malignancies to systemic disease and indicates a cell autonomous leukemogenic mechanism.Stem Cell and Regenerative Biolog