13 research outputs found

    Optimizing Performance and Operational Stability of CsPbI3 Quantum-Dot-Based Light-Emitting Diodes by Interface Engineering

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    Perovskite light-emitting diodes (PeLEDs) have emerged as a promising candidate for next-generation display technology and lighting applications owing to their high current efficiency, low operating voltage, narrow spectral emission, and tunable emission color. Keys to achieving efficient PeLEDs are, besides an emitter layer with high optical quality, a negligible charge injection barrier between charge injecting layers (CILs) and an optimized thickness of these CILs for a controlled flow of charge carriers through the device. In this study, we systematically optimized hole transport layers and electron transport layers (ETLs) in PeLEDs employing CsPbI3 quantum dots as an emitter layer. We also investigated two bilayer cathodes (Liq/Ag and LiF/Al) with the various ETLs employed in our study and observed that 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) as an ETL improves the band alignment, leading to better electron injection. The improved electron/hole current balance results in ∼63% higher external quantum efficiency (EQE) in PO-T2T-based devices compared to PeLEDs employing other ETLs. In addition, we tracked the operational stability of the different devices observing a correlation with the EQE, where samples with higher EQE (PO-T2T-based devices) also present the highest stable operation at elevated current densities

    Efficient electron injection in organic light-emitting diodes using lithium quinolate/Ca/Al cathodes

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    Device performances of green devices with cathode structure of lithium quinolate Liq /Ca/Al were investigated and electron injection mechanism was studied using ultraviolet photoelectron spectroscopy. Power efficiency could be improved by 70% by using Liq/Ca/Al cathode structure due to efficient electron injection, and interfacial energy barrier lowering by Liq/Ca/Al metal cathode was observed

    Optimization of Organic Light Emitting Diode Structures

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    In this work we present detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq3) based OLEDs as a function of: the choice of cathode, the thickness of organic layers, and the position of the hole transport layer/Alq3 interface. The calculations fully take into account dispersion in glass substrate, indium tin oxide anode, and in the organic layers, as well as the dispersion in the metal cathode. Influence of the incoherent transparent substrate (1 mm glass substrate) is also fully accounted for. Four cathode structures have been considered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer, N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) was considered. As expected, emitted radiation is strongly dependent on the position of the emissive layer inside the cavity and its distance from the metal cathode. Although our optical model for an OLED does not explicitly include exciton quenching in vicinity of the metal cathode, designs placing emissive layer near the cathode are excluded to avoid unrealistic results. Guidelines for designing devices with optimum emission efficiency are presented. Finally, the optimized devices were fabricated and characterized and experimental and calculated emission spectra were compared

    Comparative study of zinc bis-quinolates and lithium mono-quinolates: Investigation of the effect of coordination geometry on electroluminescence performance

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    Metal (8-quinolinolato) (Mqn) chelates have proven to be viable for use in organic light-emitting devices (OLEDs), both as the active emitter layer and as the electron-transporting host in dye-doped OLEDs. Whether serving as the emitter layer or host material the energy level-alignment affecting charge injection efficiency and charge mobility properties are of equal importance. Substitution of the 8-quinolinolato ligand or substitution of different metal ions has been shown to shift absorption and emission energies, which can modify the relative energy level alignments of the HOMO and LUMO levels at charge injection interfaces in an OLED. Furthermore, substitution of metal ions of different oxidation states will result in differing coordination geometries of the resulting metal chelates. For Alq3, the electron transport properties and the good thermal stability of vapor-deposited films have been attributed to its octahedral geometry. In this work, we present a comparative study of the photophysical (absorption and emission) properties and thermal stability properties of methylated zinc bis-quinolates and lithium mono-quinolates. Results are related to aluminum tris-quinolates and implications on electroluminescence performance will be discussed

    Organic Light Emitting Devices

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    This book describes the state-of-the-art advancement in the field of organic electroluminescence contributed by many researchers with internationally established expertise in the field. It includes original contributions on the synthesis of suitable organic materials, fabrication of organic light emitting devices (OLEDs) and organic white light emitting devices (WOLEDs), characterization of these devices and some designs for optimal performance. All chapters are self-sufficient in presenting their contents. The cost effective chemical technology offers many exciting possibilities for OLEDs and organic solar cells (OSCs) to be futuristic solutions for lighting and power generation. A common flexible substrate can be used to fabricate OLEDs on one side facing a room and OSCs on the other side facing the sun. The device thus fabricated can generate power in the day time and light a room/house at night. The book covers developments on OLEDs, WOLEDs and briefly on OSCs as well

    Organic light-emitting diodes with doped charge transport layers

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    Organische Farbstoffe mit einem konjugierten pi-Elektronen System zeigen überwiegend ein halbleitendes Verhalten. Daher sind sie potentielle Materialien für elektronische und optoelektronische Anwendungen. Erste Anwendungen in Flachbildschirmen sind bereits in (noch) geringen Mengen auf dem Markt. Die kontrollierte Dotierung anorganischer Halbleiter bereitete die Basis für den Durchbruch der bekannten Halbleitertechnologie. Die Kontrolle des Leitungstypes und der Lage des Fermi-Niveaus erlaubte es, stabile pn-Übergänge herzustellen. LEDs können daher mit Betriebsspannungen nahe dem thermodynamischen Limit betrieben werden (ca. 2.5V für eine Emission im grünen Spektralbereich). Im Gegensatz dazu bestehen organische Leuchtdioden (OLEDs) typischerweise aus einer Folge intrinsischer Schichten. Diese weisen eine ineffiziente Injektion aus Kontakten und eine relative geringe Leitfähigkeit auf, welche mit hohen ohmschen Verlusten verbunden ist. Andererseits besitzen organische Materialien einige technologische Vorteile, wie geringe Herstellungskosten, große Vielfalt der chemischen Verbindungen und die Möglichkeit sie auf flexible große Substrate aufzubringen. Sie unterscheiden sich ebenso in einigen fundamentalen physikalischen Parametern wie Brechungsindex, Dielektrizitätskonstante, Absorptionskoeffizient und Stokes-Verschiebung der Emissionswellenlänge gegenüber der Absorption. Das Konzept der Dotierung wurde für organische Halbleiter bisher kaum untersucht und angewandt. Unser Ziel ist die Erniedrigung der Betriebsspannung herkömmlicher OLEDs durch den Einsatz der gezielten Dotierung der Transportschichten mit organischen Molekülen. Um die verbesserte Injektion aus der Anode in die dotierte Löchertransportschicht zu verstehen, wurden UPS/XPS Messungen durchgeführt (ultraviolette und Röntgen-Photoelektronenspektroskopie). Messungen wurden an mit F4-TCNQ dotiertem Zink-Phthalocyanin auf ITO und Gold-Kontakten durchgeführt. Die Schlussfolgerungen aus den Experimenten ist, das (i) die Fermi-Energie sich durch Dotierung dem Transportniveau (also dem HOMO im Falle der vorliegenden p-Dotierung) annähert, (ii) die Diffusionspannung an der Grenzfläche durch Dotierung entsprechend verändert wird, und (iii) die Verarmungszone am Kontakt zum ITO sehr dünn wird. Der Kontakt aus organischem Material und leitfähigem Substrat verhält sich also ganz analog zum Fall der Dotierung anorganischer Halbleiter. Es entsteht ein stark dotierter Schottky-Kontakt dessen schmale Verarmungszone leicht durchtunnelt werden kann (quasi-ohmscher Kontakt). Die Leistungseffizienz von OLEDs mit dotierten Transportschichten konnte sukzessive erhöht werden, vom einfachen 2-Schicht Design mit dotiertem Phthalocyanine als Löchertransportschicht, über einen 3-Schicht-Aufbau mit einer Elektronen-Blockschicht bis zu OLEDs mit dotierten 'wide-gap' Löchertransport-Materialien, mit und ohne zusätzlicher Schicht zur Verbesserung der Elektroneninjektion. Sehr effiziente OLEDs mit immer noch niedriger Betriebsspannung wurden durch die Dotierung der Emissionsschicht mit Molekülen erhöhter Photolumineszenzquantenausbeute (Laser-Farbstoffe) erreicht. Eine optimierte LED-Struktur weist eine Betriebsspannung von 3.2-3.2V für eine Lichtemission von 100cd/m2 auf. Diese Resultate entsprechen den zur Zeit niedrigsten Betriebsspannungen für OLEDs mit ausschließlich im Vakuum aufgedampften Schichten. Die Stromeffizienz liegt bei ca. 10cd/A, was einer Leistungseffizienz bei 100cd/m2 von 10lm/W entspricht. Diese hohe Leistungseffizienz war nur möglich durch die Verwendung einer Blockschicht zwischen der dotierten Transportschicht und der Lichtemissions-Schicht. Im Rahmen der Arbeit konnte gezeigt werden, dass die Dotierung die Betriebsspannungen von OLEDs senken kann und damit die Leistungseffizienz erhöht wird. Zusammen mit einer sehr dünnen Blockschicht konnte einen niedrige Betriebsspannung bei gleichzeitig hoher Effizienz erreicht werden (Blockschicht-Konzept).Organic dyes with a conjugated pi-electron system usually exhibit semiconducting behavior. Hence, they are potential materials for electronic and optoelectronic devices. Nowadays, some applications are already commercial on small scales. Controlled doping of inorganic semiconductors was the key step for today's inorganic semiconductor technology. The control of the conduction type and Fermi-level is crucial for the realization of stable pn-junctions. This allows for optimized light emitting diode (LED) structures with operating voltages close to the optical limit (around 2.5V for a green emitting LED). Despite that, organic light emitting diodes (OLEDs) generally consist of a series of intrinsic layers based on organic molecules. These intrinsic organic charge transport layers suffer from non-ideal injection and noticeable ohmic losses. However, organic materials feature some technological advantages for device applications like low cost, an almost unlimited variety of materials, and possible preparation on large and flexible substrates. They also differ in some basic physical parameters, like the index of refraction in the visible wavelength region, the absorption coefficient and the Stokes-shift of the emission wavelength. Doping of organic semiconductors has only been scarcely addressed. Our aim is the lowering of the operating voltages of OLEDs by the use of doped organic charge transport layers. The present work is focused mainly on the p-type doping of weakly donor-type molecules with strong acceptor molecules by co-evaporation of the two types of molecules in a vacuum system. In order to understand the improved hole injection from a contact material into a p-type doped organic layer, ultraviolet photoelectron spectroscopy combined with X-ray photoelectron spectroscopy (UPS/XPS) was carried out. The experimental results of the UPS/XPS measurements on F4-TCNQ doped zinc-phthalocyanine (ZnPc) and their interpretation is given. Measurements were done on the typical transparent anode material for OLEDs, indium-tin-oxide (ITO) and on gold. The conclusion from these experiments is that (i) the Fermi-energy comes closer to the transport energy (the HOMO for p-type doping), (ii) the built-in potential is changed accordingly, and (iii) the depletion layer becomes very thin because of the high space charge density in the doped layer. The junction between a doped organic layer and the conductive substrate behaves rather similar to a heavily doped Schottky junction, known from inorganic semicondcutor physics. This behavior favors charge injection from the contact into the organic semiconductor due to tunneling through a very small Schottky barrier (quasi-ohmic contact). The performance of OLEDs with doped charge transport layers improves successively from a simple two-layer design with doped phthalocyanine as hole transport layer over a three-layer design with an electron blocking layer until OLEDs with doped amorphous wide gap materials, with and without additional electron injection enhancement and electron blocking layers. Based on the experience with the first OLEDs featuring doped hole transport layers, an ideal device concept which is based on realistic material parameters is proposed (blocking layer concept). Very high efficient OLEDs with still low operating voltage have been prepared by using an additional emitter dopant molecule with very high photoluminescence quantum yield in the recombination zone of a conventional OLED. An OLED with an operating voltage of 3.2-3.2V for a brightness of 100cd/m2 could be demonstrated. These results represent the lowest ever reported operating voltage for LEDs consisting of exclusively vacuum sublimed molecular layers. The current efficiency for this device is above 10cd/A, hence, the power efficiency at 100cd/m2 is about 10lm/W. This high power efficiency could be achieved by the use of a blocking layer between the transport and the emission layer

    Role of polythiophene- based interlayers from electrochemical processes on organic light-emitting diodes

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    In this work, well-defined and stable thin films based on polythiophene and its derivative, are employed as the hole-injection contact of organic light-emitting diodes (OLED). The polymer films are obtained by the electropolymerization or the electrochemical doping/dedoping of a spin-coated layer. Their electrical properties and energetics are tailored by electrochemical adjustment of their doping levels in order to improve the hole-injection from the anode as well as the performance of small molecular OLEDs. By using dimeric thiophene and optimizing the electrodeposition parameters, a thin polybithiophene (PbT) layer is fabricated with well-defined morphology and a high degree of smoothness by electro-polymerization. The introduction of the semiconducting PbT contact layer improves remarkably the hole injection between ITO anode and the hole- transport layer (NPB) due to its favourable energetic feature (HOMO level of 5.1 eV). The vapor-deposited NPB/Alq3 bilayer OLEDs with a thin PbT interlayer, show a remarkable reduction of the operating voltage as well as enhanced luminous efficiency compared to the devices without PbT. Investigations have also been made on the influence of PbT thickness on the efficiency and I-V feature as well as device stability of the OLED. It is demonstrated that the use of an electropolymerization step into the production of vapor deposited molecular OLED is a viable approach to obtain high performance OLEDs. The study on the PbT has been extended to poly(3,4-ethylenedioxythiophene) (PEDT) and the highly homogenous poly(styrenesulfonate) (PSS) doped PEDT layer from a spin-coating process has been applied. The doping level of PEDT:PSS was adjusted quantitatively by an electrochemical doping/dedoping process using a p-tuoluenesulfonic acid containing solution, and the redox mechanism was elucidated. The higher oxidation state can remain stable in the dry state. The work function of PEDT:PSS increases with the doping level after adjusting at an electrode potential higher than the value of the electrochemical equilibrium potential (Eeq) of an untreated film. This leads to a further reduction of the hole-injection barrier at the contact of the polymeric anode/hole transport layer and an ideal ohmic behavoir is almost achieved at the anode/NPB interface for a PEDT:PSS anode with very high doping level. Molecular Alq3-based OLEDs were fabricated using the electrochemically treated PEDT:PSS/ITO anode, and the device performance is shown to depend on the doping level of polymeric anode. The devices on the polymer anode with a higher Eeq than that for the unmodified anode, show a reduction of operating voltage as well as a remarkable enhancement of the luminance. Furthermore, it is found that the operating stability of such devices is also improved remarkably. This originates from the removal of mobile ions such as sodium ions inside the PEDT:PSS by electrochemical treatment as well as the planarization of the ITO surface by the polymer film. By utilizing an Al/LiF cathode with an enhanced electron injection and together with a high Eeq- anode, a balanced injection and recombination of hole and electron is achieved. It leads to a further reduction of the operating voltage and to a drastic improvement of EL efficiency of the device as high as 5.0 cd/A. The results demonstrate unambiguously that the electrochemical treatment of a cast polymer anode is an effective method to improve and optimize the performance of OLEDs. The method can be extended to other polythiophene systems and other conjugated polymers in the fabrication of the OLEDs as well as organic transistors and solar cells

    Enhanced performance of organic light-emitting diodes (OLEDs) and OLED-based photoluminescent sensing platforms by novel microstructures and device architectures

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    Organic light emitting diodes (OLEDs) have advanced dramatically since they exhibit great promise in various applications such as displays, solid-state lighting, and (bio)chemical sensing. In this dissertation, multiple approaches were employed to enhance the performance of OLEDs and OLED-based sensing platforms. Comprehensive investigations were conducted on electroluminescence (EL) spikes and tails in charge trapping guest-host OLEDs and their influence on OLED-based sensor performance. Novel microstructures and device architectures were developed to construct OLED sources with spectrally selective and enhanced emission. The peak emission wavelength of the multicolor microcavity devices with MoO3 as the HIL/spacer was tunable from 493 to 639 nm. The controlled microporous structures formed by polystyrene (PS):polyethylene glycol (PEG) was able to enhance the forward light extraction of the OLED by up to ~60%. The combination of the PtOEP:PS:PEG sensing film coupled with the multicolor microcavity OLEDs and the appropriate OPD, and the possibility to combine time- and intensity-domain analyses have shed light on the opportunities to realize simple, compact, potentially disposable sensors for the detection of O2, pH and other (bio)chemical analytes and parameters

    Understanding and Controlling the Morphology of Organic Thin Films

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    Display panels today open the door to the repository of knowledge, and their use is expanding from conventional home appliances to transparent, wearable and mobile devices. Organic semiconductors are a perfect candidate for this ubiquitous use of display panels since organics are flexible, transparent with high brightness and capable of good color rendering. The expanding outdoor use of organic devices poses questions whether: i) the devices are bright enough to be visible under daylight, and ii) the devices can withstand extreme conditions such as the interior of an automobile in the summer. Organic light emitting devices (OLEDs) achieved ~100% internal quantum efficiency with phosphorescence in 1998 [1], and since then the bottle neck for OLED brightness has been the outcoupling efficiency. The capability of organics to withstand severe conditions is closely related to their morphological stability. Thus, improving the outcoupling efficiency and controlling the morphology of OLEDs are the two crucial aspects in the future display technology. In this sense, this thesis mainly deals with the methods to improve the light outcoupling of OLEDs by morphological control. Also, methods for understanding organic film morphology are discussed. In this thesis, we demonstrate a measurement technology to obtain precise nanoscale information about the morphologies of several organic thin film structures using Fourier plane imaging microscopy (FIM). We use FIM to detect the orientation of molecular transition dipole moments from an extremely low density (i.e. small fractions of a monolayer) of luminescent dye molecules, which we call “morphology sensors.” The orientation of the sensor molecules is driven by the local film structure, and thus can be used to determine details of the host morphology without influencing it. We use symmetric planar phosphorescent dye molecules as the sensors that are deposited into the bulk of organic film hosts during the growth. Furthermore, we monitor morphological changes arising from thermal annealing of metastable organic films that are commonly employed in photonic devices. Methods to control the organic film morphology to improve the light outcoupling are also demonstrated. Here we control the orientation of Pt complex molecules during the growth of emissive layers by two different methods: modifying the molecular structure, and using structural templating. Pt complex dopant molecules whose structures are modified by adjusting the ligands show an approximately 20% increased fraction of horizontally aligned transition dipole moments compared to the original molecule while being diluted in the host matrix. Alternatively, we pre-deposit a highly ordered structural template layer, which results in a 60% increase in horizontally aligned transition dipole moments compared to the film deposited in the absence of the template. Finally, we employ a 2-dimensional transition metal dichalcogenides as the active luminescent layer due to its optimum emission profile for efficient outcoupling. Therefore, a hybrid light emitting device (LED) is fabricated employing a chemical-vapor-deposition grown, centimeter-scale monolayer of WS2 (mWS2), embedded within conductive organic layers. As a result, LEDs with an average external quantum efficiency of 0.3 ± 0.3% and with the highest efficiency of 1% were achieved. Also, we show that negatively charged excitons, also known as trions, are generated in the mWS2 with the injected current, causing an efficiency roll-off at high current densities.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169696/1/jongckim_1.pd
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