Controlling Thin-film Morphology and Incorporating Novel Semiconducting Molecules toward High Performance Organic Optoelectronic Devices

Organic optoelectronic devices have been widely used in display, energy-storage, and consumer electronics. Insightful understanding on material properties, device architecture, and fabrication processes is inevitable to improve the performance of organic optoelectronic devices. My PhD research focuses on improving the performance of organic photovoltaics (OPV) and organic light-emitting diode (OLED) through the systematic processing and material design. The first part of the dissertation describes how to construct a highly conductive morphology of mixed donor:acceptor heterojunction. Organic vapor phase deposition (OVPD) was utilized to enhance crystallinity of C70 acceptor in the mixed tetraphenyldibenzoperiflanthen (DBP):C70 thin-film. Forming the face-center-cubic (fcc) structure of C70 facilitated charge extraction, thereby improving fill factor (FF) of the corresponding OPVs. The second part presents the study on the morphological stability and reliability of OPVs. The cathode buffer, bathophenanthroline (BCP), undergoes significant morphological degradation. This morphological degradation was successfully suppressed by making the underlying DBP:C70 layer rougher via the moving N2 carrier gas in OVPD. The open-circuit voltage (Voc) of the obtained heterojunction OPVs of DBP:C70 grown by OVPD experienced a negligible drop (< 3 % change) while the equivalent OPVs grown by VTE showed a significant decrease in Voc from 0.91±0.01 V to 0.74±0.01 after 1 Sun illumination for 250 h. The third part explains a more precise way to control the morphology of organic mixed layer. It was found that increase in the growth pressure of OVPD induced reorganization of molecules to form the equilibrium morphology. The morphology of the electron-filtering buffer layer of 3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (BP4mPy):C60 was optimized to achieve the highest electron mobility by means of the control of the growth pressure. Consequently, the resulting OPVs with optimized BP4mPy:C60 buffer showed FF = 0.65±0.01 and a much higher PCE = 8.0±0.2 % compared to PCE = 6.6±0.2 % of the equivalent OPVs with the same composition buffer layer grown by VTE. The fourth part summarizes the effects of the inclusion of novel block-copolymers on the performance of the polymer bulk-heterojunction photovoltaic cells. The block-copolymers were composed of thiophene units with and without a dangling phenyl-C61-butyric acid methyl ester (PCBM) side chain. The added copolymer into the poly(3-hexylthiophene) (P3HT): PCBM active layer resulted in greatly improved thermal stability of P3HT:PCBM. Furthermore, electron conductivity also increased since the fullerene units of the copolymers contribute to the formation of a percolation pathway for electron transport. While PCE of conventional P3HT:PCBM bulk-heterojunction solar cells decreases significantly from 2.6±0.2 to 1.2±0.2% after 90-min of thermal annealing, the equivalent OPVs with the copolymer shows a much smaller decrease in PCE from 3.1±0.2% to 2.7±0.2%. The last section of this dissertation covers the design of phosphorescent OLED employing a metal-free purely organic phosphor. Owing to their much longer triplet lifetime in the millisecond regime compared to microseconds of organometallics, a more careful consideration should be given in the device design. The requirements for the host materials in metal-free purely organic phosphor OLEDs are identified to be a high triplet energy, suitable HOMO and LUMO energy levels, and large spectral overlap with the absorption of the phosphors. Systematic investigation on various host molecules, electron transporting molecules, and the layer thickness of each layer allows us to demonstrate an optimized phosphorescent OLED having an external quantum efficiency (EQE) of 2.5 % at 1 mA/cm2.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144195/1/bssong_1.pd

Factors determining thermally activated delayed fluorescence performance beyond the singlet-triplet gap

Thermally activated delayed fluorescence (TADF) has been proposed as a pathway to achieve high efficiency organic light-emitting diodes (OLEDs) without the use of heavy metal atoms in molecular structures. Many different factors can be decisive for efficient light emission from TADF emitters. However, a complete picture of the working mechanisms behind TADF is still missing and further research exploring novel material and device ideas is required. This thesis aims to extend the understanding of TADF emitter and OLED design considerations by investigating photophysical properties of novel materials as well as fabricating, optimizing and characterizing devices. TADF emitters have great potential of being used in OLEDs because they allow for high quantum efficiencies by utilizing triplet states via reverse intersystem crossing (RISC). In small molecules this is done by spatially separating the frontier orbitals, forming an intramolecular charge-transfer state (iCT) and leading to a small energy difference between lowest excited singlet and triplet states (ΔST). In polymer emitters, sufficient frontier orbital separation is harder to achieve, and typical strategies usually include adding known TADF units as sidechains onto a polymer backbone. In this thesis, a novel pathway of TADF polymer design is explored. A shift from a non-TADF monomer to TADF oligomers is explored. The monomer shows non-TADF emission and the delayed emission is shown to be of triplet-triplet annihilation (TTA) origin. An iCT state is formed already in the dimer, leading to a much more efficient TADF emission. This is confirmed by an almost two-fold increase of photoluminescence quantum yield (PLQY), a decrease in the delayed luminescence lifetime and the respective spectral line shapes of the molecules. Recently, intermolecular effects between small-molecule TADF emitters have been given more attention, revealing strong solid-state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. A thorough investigation of a novel TADF emitter 5CzCO2Me is conducted. Steady-state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same, thus the redshift is attributed to the solid-state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66 %. The best OLED in terms of current-voltage-luminance and external quantum efficiency parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5 %. It is shown that the emitter transports holes and that charge carrier recombination does not take place on the bandgap of the host, but rather, a mixed host-guest concentration dependent recombination is seen. The hole transporting properties of 5CzCO2Me allows for a new dimension in tuning the device performance by controlling the emitter concentration

Self-assembled nanostructures for photon management in optoelectronic devices

Für den Erfolg der Photovoltaik (PV) und der organischen Leuchtdioden (OLED) sind erhebliche Fortschritte bei der Kostensenkung und Effizienzsteigerung erforderlich. Beide Ziele können durch den Einsatz von Nanostrukturen für das Photonenmanagement gleichzeitig erreicht werden. Die grundlegenden Ziele des Photonenmanagements sind die Verringerung der Reflexion des einfallenden Lichts, die Verbesserung der Absorption oder die Verstärkung der Auskopplung sowie die Anpassung der optischen Eigenschaften eines Bauelements für den Einsatz in verschiedenen Arten von Energieumwandlungssystemen. Für eine optimale Effizienz von Solarzellen und OLEDs sollten die Nanostrukturen einen erweiterten Spektral- und Winkelbereich aufweisen. In dieser Hinsicht hat das Photonenmanagement auf der Grundlage ungeordneter Nanostrukturen in vor kurzem große Aufmerksamkeit erregt. Solche photonischen Schichten arbeiten in einem breiteren Spektralbereich als vergleichbare periodische Strukturen und besitzen optische Eigenschaften, die im Gegensatz zu rein zufälligen Strukturen leicht vorhergesag- und eingestellbar sind. In dieser Arbeit wird das Photonenmanagement durch planare, ungeordnete 2D-Nanostrukturen für optoelektronische Dünnschichtbauelemente vorgestellt. Die Nanostrukturen sind so konzipiert, dass sie als antireflektierende oder effizient streuende Strukturen fungieren, deren primäres Ziel es ist, die optische Absorption von Dünnschicht-Solarzellen zu verbessern und ihre Energieumwandlungseffizienz zu erhöhen. Die entwickelte Methodik und die Strukturen haben direkte Auswirkungen auf den Bereich der OLED-Bauelemente. Die entwickelten Strukturen eingesetzt, um die Auskopplungseigenschaften von OLEDs zu verbessern, die einen vergleichbaren spektralen Wirkungsbereich wie Solarzellen besitzen. Voraussetzung für alle experimentellen Untersuchungen ist eine ausgereifte Herstellungsmethode zur Erzeugung von Nanostrukturen mit kontrollierbaren Störungseigenschaften, bei der eine vielseitige, großflächige nasschemische Methode eingesetzt wird, die auf der lateralen Phasentrennung einer Polymermischung beruht. Diese nasschemische Methode wird häufig durch Schleuderbeschichtung durchgeführt, die es nicht erlaubt, phasengetrennte Nanostrukturen (PSN) in beliebige 2D-Designs einzubauen, wie es für ihren Einsatz in kommerziellen Produkten erforderlich ist. Andererseits können additive Fertigungsverfahren wie Tintenstrahldrucker nahezu jede geometrisch komplexe Form im Mikrometer- bis Makromaßstab herstellen. Da die meisten herkömmlichen Tintenstrahldrucker jedoch nur eine Auflösung im Mikromaßstab aufweisen, sind sie für die Entwicklung von nanostrukturierten Materialien und Bauelementen noch nicht geeignet. In dieser Studie werden erstmals beide Mängel behoben und gleichzeitig die kostengünstige Attraktivität und Vielseitigkeit der Phasentrennung durch Homopolymermischungen bewahrt, indem die einzigartigen Vorteile des Tintenstrahldrucks genutzt werden. Unter optimierten Bedingungen werden digital druckbare PSN vom Mikrometer- bis in den Sub-100 nm-Bereich nach einem vorgegebenen 2D-Layout realisiert. Diese PSN können auf verschiedenen starren und flexiblen Substraten mit einer Geschwindigkeit von 45 cm/s hergestellt werden. Der vorgeschlagene Ansatz eröffnet außerdem zahlreiche neue Möglichkeiten für die Nanofabrikation, einschließlich der dynamischen Variation von PSN während des Tintenstrahldrucks, entweder durch Anpassung der Druckauflösung von Pixel zu Pixel für eine bestimmte Tintenformulierung oder durch die Verwendung mehrerer Polymer-Tinten. Darüber hinaus werden PSN in der Regel aus Polymeren mit niedrigen Glasübergangstemperaturen hergestellt, was ihre praktische Bedeutung für die Nanoimprint-Lithografie (NIL) einschränkt, da solche PSN bei hohem Druck und hoher Temperatur zu Verformungen in der Prägeebene neigen. Um dieses Manko zu überwinden, werden in dieser Arbeit die einzigartigen Vorteile eines anorganisch-organischen Hybridpolymers (OrmoStamp) genutzt, welches in der Industrie bereits als Material für Prägestempel in der UV- und thermisch basierten NIL bekannt geworden ist. In dieser Arbeit wird zum ersten Mal gezeigt, dass Nanostempel auf der Basis von PSN (aus OrmoStamp) direkt auf verschiedenen starren und flexiblen Substraten mit Hilfe eines Phasentrennungsprozesses hergestellt werden können. Dies ermöglicht den direkten Einsatz von PSN in der NIL ohne zusätzliche lithographische oder replikative Zwischenschritte. So können schleuderbeschichtete und gedruckte sowie geprägte PSN das Photonenmanagement in vielfältigen nanophotonischen Anwendungen verbessern, wie hier durch ihren Einsatz in Solarzellen und OLEDs zur Steigerung der Leistungseffizienz demonstriert wird. Für Solarzellen werden zwei verschiedene optische Managementtechniken erforscht. Die erste Methode konzentriert sich auf die Entwicklung von lichtstreuenden Schichten für Solarzellen, entweder durch eine Bottom-up- oder eine Top-down-Strategie. Bei der Bottom-up-Strategie werden PSN in die Rückseite von Solarzellen aus hydrogeniertem amorphem Silizium (a-Si:H) eingebracht, bevor ein Reflektor abgeschieden wird, um lichtstreuende Reflektoren zu realisieren. Diese lichtstreuenden Reflektoren erzielen einen besseren Wirkungsgrad als ein Bauelement, das auf einem kommerziellen lichtstreuenden Substrat basiert. Darüber hinaus werden ergänzende optische Simulationen an einem akkuraten 3D-Modell durchgeführt, um die überlegenen Lichtsammeleigenschaften der entwickelten Streureflektoren zu analysieren und allgemeine Designregeln abzuleiten. In der Top-Down-Strategie werden PSN verwendet, um eine Resist-Ätzmaske zu strukturieren, die für die Übertragung ungeordneter Nanolöcher in eine dünne a-Si:H-Schicht durch Trockenätzung verwendet wird. Die Studie begann mit der Durchführung dreidimensionaler optischer Simulationen, um die Auswirkungen der Unordnung auf die ursprünglich periodischen Anordnungen von Nanostrukturen systematisch zu untersuchen. Die Ergebnisse dieser Simulationen zeigen, dass quasi-ungeordnete Strukturen zu breiteren Spektral- und Winkelantworten führen, was für PV-Anwendungen eindeutig von Vorteil ist. Nach dem Top-Down-Ansatz wird eine Verbesserung der integralen Absorption um bis zu 93% bei normalem Einfall und um bis zu 200% bei großen Einfallswinkeln im Vergleich zu einem ungemusterten Absorber gezeigt.Darüber hinaus kann eine ähnliche Struktur als Nanostempel in einer Top-Down-Strategie dienen, wobei die Perowskit-Schichten durch die Nanostempel unter Verwendung eines thermischen NIL-Systems nanogeprägt werden. Für den nanostrukturierten Perowskitfilm wird eine erhöhte integrierte Absorption und eine gesteigerte Photolumineszenz von 7%$_{rel}$ bzw. 121%$_{rel}$ erzielt. Dieser Weg ebnet den Weg für Rolle-zu-Rolle verarbeitbare "photonisierte" Absorber. Die zweite Methode konzentriert sich auf die Entwicklung von Antireflexionsschichten durch zusätzliche Anpassung des PSN an die Abmessungen unterhalb der Wellenlänge. Das hier betrachtete Design besteht aus einer Frontelektrode, Indium-Zinn-Oxid (ITO), die formschlüssig auf die PSN aufgebracht wird. Im optimalen Fall führen die nanostrukturierten ITO-Elektroden zu einer Erhöhung des Transmissionsgrads um 7%$_{rel}$ im Vergleich zu planaren Referenzstrukturen. Die Antireflexionseigenschaften werden genutzt, um die Photostromdichte von 4-poligen Perowskit/Kristallsilizium (Perowskit/c-Si)-Tandemsolarzellen zu erhöhen. Perowskit/c-Si-Tandem-Zellen mit nanostrukturiertem ITO weisen eine höhere Kurzschlussstromdichte (2,9 mA/cm$^{2}$ absolute Verstärkung) und PCE (1,7% absolute Verstärkung) in der unteren c-Si-Solarzelle im Vergleich zur Referenz auf. Schließlich wird in dieser Arbeit die Bedeutung der genannten Erkenntnisse für das umgekehrte Problem - die Lichtextraktion in OLEDs - aufgezeigt. In der ersten untersuchten Konfiguration nutzt diese Arbeit die leicht abstimmbaren Lichtstreueigenschaften von ungeordneten Titandioxid-Nanosäulen, die aus einer selbstorganisierenden Struktur und einem lösungsmittelbasierten Lift-off-Prozess resultieren. Die anschließende Planarisierung dieser Nanosäulen durch eine dünne Epoxidschicht gewährleistet eine hervorragende Reproduzierbarkeit der Bauelemente - ein Aspekt, der bei nanowelligen Substraten oft kritisch ist - und bewahrt eine starke räumliche Überlappung der eingefangenen optischen Moden mit den lichtstreuenden Strukturen. Zur Veranschaulichung wird gezeigt, dass das vorgeschlagene Design die Effizienz einer von unten emittierenden OLED ($\mathbf{\lambda_{peak}}$=520 nm) um +22%$_{rel}$ und die Winkelemissionscharakteristik im Vergleich zu planaren Bauelementen verbessert. In der zweiten untersuchten Konfiguration werden im Tintenstrahldruckverfahren hergestellte lichtauskoppelnde PSN mit verschiedenen 2D-Designs getestet, wie sie für den Einsatz in gedruckten OLED-Bauelementen vorgesehen sind. Dabei wird ein transparentes Anodenmaterial direkt auf das PSN aufgebracht, was zu einer Strukturierung der Grenzfläche zwischen Anode und organischen Schichten und einer resultierenden Streuung der Wellenleitermoden führt. Eine OLED ($\mathbf{\lambda_{peak}}$=520 nm), die ein gedrucktes PSN enthält, weist bei einer Leuchtdichte von 1000 cd/m$^{2}$ im Vergleich zu einem planaren Referenzelement eine um 57% höhere Effizienz auf. Dieser Ansatz lässt sich in eine Hochdurchsatz-Fertigungsroutine integrieren und kann leicht auf andere OLED-Layouts erweitert werden

Fabrication and Characterization of Hybrid Metal-Oxide/Polymer Light-Emitting Diodes

Hybrid metal-oxide/polymer light-emitting diodes (HyLEDs) are a novel class of electronic devices based on a combination of electroluminescent organic and charge-injecting metal-oxide components. These devices employ air-stable electrodes, such as ITO and Au, and are therefore well suited for fabrication of encapsulation-free light-emitting devices. The current work is intended to provide an insight into operating mechanisms and limitations of the HyLEDs, and, on the basis of this knowledge, aims at modifying the device architecture in order to improve the performance. The choice of optically transparent metal-oxide charge-injection layers appears to be critical in this respect in order to optimize the electron-hole balance within the polymer layer. Starting from the original device architecture, ITO/TiO2/F8BT/MoO3/Au, which uses ITO as a cathode and Au as an anode, we follow different approaches, such as the use of dipolar self-assembled monolayers and nanoscale structuring of the electron-injecting interface, pursuing the goal of enhancing electron injection into the emissive layer. However, substitution of the electron-injecting layer of TiO2 with ZrO2 is demonstrated to be the most efficient of the approaches employed herein. Further, optimization of the device utilizing the latter metal oxide is demonstrated in terms of deposition and post-deposition treatment of the electron-injecting and electroluminescent layers. Substrate temperature during spray pyrolysis deposition of the electron-injecting layer is found to have a strong influence on the HyLED performance, as well as the precursor solution spraying rate and the layer thickness. On the other hand, post-deposition annealing of the polymer layer is shown to improve the device efficiency and brightness significantly, possible explanations lying in enhancement in polymer luminescence efficiency and formation of a more intimate contact between the electron-injecting and the active polymer layers. Combining electron-transporting (TiO2 and ZnO) and hole-blocking (Al2O3 and ZrO2) materials into a single electron-injecting layer is demonstrated to be an effective strategy of enhancing efficiency in the HyLEDs. The search for a hole-injecting electrode alternative to the conventionally used MoO3/Au leads to the device employing the PEDOT:PSS/VPP-PEDOT system, which though resulting in a poorer device efficiency, provides route for fabrication of vacuum deposition-free organic light-emitting devices. Finally, the HyLED architecture is demonstrated to offer better stability than the conventional architecture using LiF/Al as a cathode. It is hoped that the current work provides a better understanding of the requirements for fabrication of encapsulation-free organic light-emitting devices

Modelling and characterization of Quantum Dots as QLED devices for automotive lighting systems

This work reports the design, manufacturing and numerical simulation approach of an electroluminescent quantum dot light emitting device (QLED) based on quantum dots as an active layer. In addition, the electrical I-V curve was measured, observing how the fabrication process and layer thickness have an influence in the shape of the plot. This experimental device enabled us to create a computational model for the QLED based on the Transfer Hamiltonian approach to calculate the current density J(mA/cm2), the band diagram of the system and the accumulated charge distribution. Thanks to the QLED simulator developed, it would be possible to model the device and anticipate the electrical performance in a theoretical design step before going to QLED manufacturing at the laboratory. Eventually, particular automotive lighting system demonstrators were designed to integrate the theoretical and experimental research carried out in an industrial automotive product.Tesis Univ. Granada

Novel Dicyano-Phenylenevinylene Fluorophores for Low-Doped Layers: A Highly Emissive Material for Red OLEDs

Two efficient deep red (DR)-emitting organic dicyano-phenylenevinylene derivatives with terminal withdrawing or donor groups were synthesized. The spectroscopic properties of the neat solids and the low-doped layers in polystyrene or polyvinylcarbazole host matrixes were analyzed, and the luminescence performance was explained using density functional theory (DFT) analysis. A noteworthy 89% fluorescence quantum yield was observed for the brightest red-emissive polyvinylcarbazole (PVK) blend. This result pushed us to successfully produce an emissive red organic light-emitting device (OLED) as a preliminary feasibility test

The Impacts of Solvents, Heat Treatments and Hole Injection Layers on the Electroluminescent Lifetime of Organic Light-Emitting Devices

Since their invention over three decades ago, organic light-emitting devices (OLEDs) have attracted tremendous interest for display and solid-state lighting applications and have already been commercialized in smartphones, tablets and television screens. However, the most coveted potential of OLED technology is to enable ultra-low cost, roll-to-roll manufacturing of large-area panels on flexible substrates. To date, commercial OLED products rely on high-cost vacuum deposition techniques and thus fail to realize this potential. In particular, the lifetime, of solution-based (and thus printable) devices remains well below commercially acceptable standards. The significant lifetime limitations of solution-based devices demand a more thorough understanding of the impact of the unique factors involved in the fabrication of these devices. Solution-processable hole injection layers (HILs), solvents, and heat or drying treatments are three such factors that play a crucial role in solution-processed devices. The principle aim of this work is to understand the influence of these factors on OLED lifetime in vacuum-deposited devices, independent of the multitude and variability of other parameters (e.g.: drying conditions, solubility, solution concertation) involved in most solution-processing methods; and to demonstrate proof-of-concept strategies to mitigate potentially adverse effects for application in solution-processed OLEDs. Results show that solution-processed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) HILs are susceptible to electron-induced degradation, a mechanism that can lead to relatively short OLED electroluminescent (EL) lifetimes. This degradation can be minimized by selecting hole transporting materials and device structures that minimize electron leakage to the HIL, resulting in a lifetime improvement of up to 20x. The effects of solvent and heat treatments on device efficiency and EL lifetime across a variety of hole injection and hole transport materials were found to vary considerably depending on the specific material combination. The extent of the morphological changes induced by the two treatments is highly material-dependent and does not necessarily correlate with device efficiency and EL lifetime. This suggests that additional, material-specific factors should be likely be considered in future correlations of device characteristics to the morphology of corresponding organic films for solution-processed devices. Finally, solvent treatment of carbazole hole transport layers was found to induce substantial aggregation and lead to shorter EL lifetimes and lower device efficiency. The origin of this effect was found to be a decrease in photoluminescence quantum yield resulting from this aggregation. Material intermixing was shown to suppress this aggregation and resulted in improved device efficiency and a 2.5x increase in EL lifetime

TADF OLEDs: characterization, modelling and detailed degradation analysis

The need of high quality and efficient displays is continuously increasing. The organic light-emitting diode (OLED) technology is certainly one of the most important in this sense, thanks to their high contrast, excellent color purity and wide viewing angle. Despite being already widely used in commercial products, scientific research on OLED materials is still ongoing to improve their efficiency and durability. A new technology which might replace currently used emissive materials in OLED is the so called thermally-activated delayed fluorescence (TADF). With these emitters, the display efficiency can be improved without the need of expensive and pollutant heavy metal atoms. This PhD project is focused on these materials and their use in OLEDs. The study of TADF OLEDs presented in this thesis has been structured in three main parts. The first study allows to clarify a frequent misconception about these emitters: the portion of excitons leading to an emissive event, usually approximated to 100%, can actually be much lower when electrically excited. A method to estimate this value is provided from the analysis of transient and steady state optical measurements. In a state-of-the-art OLED, the emission layer (EML) consists of two or more components. The adjustment of each material component and the optimization of the concentration largely impact the OLED performance. In the second study, OLEDs containing different concentrations of the TADF molecule in the emissive layer are investigated. Several experimental techniques are used and, with the use of software simulations, the effect of emission layer composition on the charge and excitonic processes is analysed. The key aspect which must be improved in order to make TADF a suitable technology in commercial products, is the lifetime. Two studies about this topic have been included in this thesis. To effectively measure the lifetime of emissive devices, one would need to operate them for several thousands of hours. This approach is definitely not applicable on a large scale, when a multitude of different devices need to be tested, since it requires a lot of time and resources. Such characterization is therefore typically done under accelerated stressing conditions, with high currents and/or temperatures. The use of appropriate scaling laws allows to estimate the durability of the device in standard operating conditions from the accelerated ones. In the first study described in this work, several identical TADF OLEDs have been stressed with different current at different temperature, and the complete set of luminance decay is fed into a global fitting algorithm. With this approach the expected lifetime can be estimated in a shorter amount of time, yet with a high accuracy. To improve the device lifetime, a detailed understanding of the processes causing it is necessary. The second study on device lifetime goes more into detail of the degradation processes occurring in a specific TADF OLED stack. The devices are stressed with constant current, and during stressing interruption a series of experimental techniques are used. Electrical device simulations are used to model these OLEDs and qualitatively identify the degradation causes. Specifically, it is found that the generation of trap states causes a variation of the charge injection and accumulation inside the device

Chapter Advanced Numerical Simulation of Organic Light-emitting Devices

Materials scienc