Modeling of OLED degradation for prediction and compensation of AMOLED aging artifacts

Degradation is still the most challenging issue for OLED, which causes the image-sticking artifact on AMOLED displays and limits their lifetime. To overcome the demerit, OLED degradation is modeled in this thesis, and compensation based on the models is applied for AMOLEDs. A data-counting model is firstly developed to quantitatively evaluate the degradation on OLEDs, with consideration of the accumulation stress during operation. An electro-optical model is further built, based on an equivalent circuit. It can simulate the electro-optical characteristic (I-V, Eff-V) and the degradation behaviors in aging process. Besides, the correlation model is aimed to derive the current efficiency decay with measurable electrical values, delivering more dependable results at strongly aged state. The prediction and compensation are implemented based on developed models. The results show that the models exactly predict the efficiency decay during operation. The image-sticking aging artifact on AMOLED can be suppressed by applying compensation, so that the display lifetime is extended.Durch das Einbrennen von Bildern in AMOLED Displays wird deren Lebensdauer verringert; dieser Qualitätsverlust stellt nach wie vor die größte Herausforderung für die OLED Technologie dar. In dieser Thesis wird die Degradation der OLEDs modelliert und eine Kompensierung anhand der Modelle erreicht. Zunächst wurde ein Data-counting Modell entwickelt, um die Degradation von OLEDs unter Berücksichtigung der akkumulierten Belastung während des Betriebs quantitativ zu bewerten. Des Weiteren wurde ein elektro-optisches Modell entwickelt, das auf einem äquivalenten Schaltungsmodell basiert. Es kann die elektro-optischen Eigenschaft (I-V, Eff-V) und das Degradationsverhalten im Alterungsprozess simulieren. Außer den beiden Modellen wird noch ein Korrelationsmodell entwickelt, das darauf abzielt, die Abnahme der Stromeffizienz aus den messbaren elektrischen Werten abzuleiten. Dieses Modell liefert im stark gealterten Zustand zuverlässigere Ergebnisse. Aufbauend auf die entwickelten Modelle wurden die Vorhersage und die Kompensierung implementiert. Die Ergebnisse zeigen, dass die Modelle den Effizienzverlust während des Betriebes genau vorhersagen. Das Einbrennen des Bildes in das AMOLED-Display kann durch das Anwenden der Kompensierung unterdrückt werden, so dass die Lebensdauer des Displays verlängert wird

Advanced Technologies for Large-Sized OLED Display

Five years have passed, since the first 55″ full high-definition (FHD) OLED TV fabricated on Gen 8.5 glass was successfully launched into the TV market. For the time being, the size of OLED TV became diverse from 55″ to 77″, and the resolution was doubled into ultrahigh definition (UHD). The brightness and color gamut were enhanced, while the lower power consumption was realized. Utmost picture quality and slim form factor of OLED TV as well as the improved performance have made OLED TV recognized as the best premium TV. In this chapter, we describe the recent progress in three key technologies, which enable such an enhancement of performance in OLED TV, i.e., oxide thin-film transistor (TFT) and white organic light-emitting diode (WOLED), compensation circuit, and method to compensate the nonuniformity of oxide TFTs, OLED devices, and luminance

Fabrication of Highly Reflective Bottom Electrodes for Inverted Top-emission OLED Applications

The demand for more air-stable and high resolution devices are opening the space for inverted top-emission OLEDs in the displays market. To realize these high-performance devices, a combination of a highly reflective bottom electrode with an efficient electron injection layer is essential to achieve high luminance efficiency and low driving voltage, respectively. The aim of this thesis was to develop a new highly reflective stack and a corresponding method for high resolution patterning to enable future top-emission iOLED applications. First, the deposition methods for a multilayer reflective stack were developed. Initially, a structure using silver as reflective thin film and ITZO as electron injection layer was used. Reflective and transmission measurements together with optical simulation were executed to optimize the thickness of each sputtered thin film in the final stack. During the development, the oxidation of Ag prevented the high reflectivity of the layer. To mitigate this issue, a new stack with an ITO interlayer between Ag and ITZO layers was created to prevent silver oxidation from the ITZO deposition process. Finally, optimized samples with a configuration of Ag(50 nm)/ITO(5 nm)/ITZO(80 nm) were demonstrated with a reflectivity > 78% in the visible range. To obtain high resolution feature sizes on the developed stack, the photolithography recipe to pattern the multilayer stacks with resolution down to 5um was also developed. During this project, different liquid etchants were tested to pattern the structure in two wet-etching steps. The quality of the etch were analysed by atomic force microscope and optical microscope images. Images of individual ITZO layers demonstrated the importance of using Ti Prime before photoresist spin-coating to improve the quality of the etch. At the end, cross-sections of the full stack showed that well-defined hills and valleys were achieved when samples were dipped in 10% mixed acid etchant for 30sec at RT followed by a dip in 75% PWES solution for 5 sec at 45C

Charge-carrier dynamics in organic LEDs

Anyone who decides to buy a new mobile phone today is likely to buy a screen made from organic light-emitting diodes (OLEDs). OLEDs are a relatively new display technology and will probably account for the largest market share in the upcoming years. This is due to their brilliant colors, high achievable display resolution, and comparably simple processing. Since they are not based on the rigid crystal structure of classical semiconductors and can be produced as planar thin-film modules, they also enable the fabrication of large-area lamps on flexible substrates – an attractive scenario for future lighting systems. Despite these promising properties, the breakthrough of OLED lighting technology is still pending and requires further research. The charge-carrier dynamics in an OLED determine its device functionality and, therefore, enable the understanding of fundamental physical concepts and phenomena. From the description of charge-carrier dynamics, this work derives experimental methods and device concepts to optimize the efficiency and stability of OLEDs. OLEDs feature an electric current of charge carriers (electrons and holes) that are intended to recombine under the emission of light. This process is preceded by charge-carrier injection and their transport to the emission layer. These three aspects are discussed together in this work. First, a method is presented that quantifies injection resistances using a simple experiment. It provides a valuable opportunity to better understand and optimize injection layers. Subsequently, the charge carrier transport at high electrical currents, as required for OLEDs as bright lighting elements, will be investigated. Here, electro-thermal effects are presented that form physical limits for the design and function of OLED modules and explain their sudden failure. Finally, the dynamics and recombination of electro-statically bound charge carrier pairs, so-called excitons, are examined. Various options are presented for manipulating exciton dynamics in such a way that the emission behavior of the OLED can be adjusted according to specific requirements.:List of publications . . . . . . . . . . . . . . . . . v List of abbreviations . . . . . . . . . . . . . . . . . ix 1 Introduction . . . . . . . . . . . . . . . . . 1 2 Fundamentals . . . . . . . . . . . . . . . . . 5 2.1 Light sources and the human society . . . . . . . . . . . . . . . . . 5 2.1.1 Human light perception . . . . . . . . . . . . . . . . . . . . 8 2.1.2 Physical light quantification . . . . . . . . . . . . . . . . . . 10 2.1.3 Non-visual light impact . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Implications for modern light sources . . . . . . . . . . . . . 15 2.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.1 Molecular energy states . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Intramolecular state transitions . . . . . . . . . . . . . . . . 24 2.2.3 Molecular films . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.4 Electrical doping . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2.5 Charge-carrier transport . . . . . . . . . . . . . . . . . . . . 36 2.2.6 Exciton formation and recombination . . . . . . . . . . . . . 38 2.2.7 Exciton transfer . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.3 Organic light-emitting diodes . . . . . . . . . . . . . . . . . . . . . 44 2.3.1 Structure and operation principle . . . . . . . . . . . . . . . 44 2.3.2 Metal-semiconductor interfaces . . . . . . . . . . . . . . . . 47 2.3.3 Typical operation characteristics . . . . . . . . . . . . . . . . 49 2.4 Colloidal nanocrystal emitters . . . . . . . . . . . . . . . . . . . . . 52 2.4.1 Terminology: Nanocrystals and quantum dots . . . . . . . . 52 2.4.2 The particle-in-a-box model . . . . . . . . . . . . . . . . . . 54 2.4.3 Surface passivation . . . . . . . . . . . . . . . . . . . . . . . 55 3 Materials and methods . . . . . . . . . . . . . . . . . 57 3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.1 OLED materials . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.2 Materials for photoluminescence . . . . . . . . . . . . . . . . 60 3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.2.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . 62 3.2.2 Solution processing . . . . . . . . . . . . . . . . . . . . . . . 64 3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.3.1 Absorbance spectroscopy . . . . . . . . . . . . . . . . . . . . 66 3.3.2 Photoluminescence quantum yield . . . . . . . . . . . . . . . 66 3.3.3 Excitation sources . . . . . . . . . . . . . . . . . . . . . . . 67 3.3.4 Sensitive EQE for absorber materials . . . . . . . . . . . . . 68 3.4 Exciton-lifetime analysis . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.1 Triplet lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.2 Singlet-state lifetime . . . . . . . . . . . . . . . . . . . . . . 70 3.4.3 Lifetime extraction . . . . . . . . . . . . . . . . . . . . . . . 70 3.5 OLED characterization . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.5.1 Current-voltage-luminance and quantum efficiency . . . . . . 73 3.5.2 Temperature-controlled evaluation . . . . . . . . . . . . . . . 74 4 Charge-carrier injection into doped organic films . . . . . . . . . . . . . . . . . 77 4.1 Ohmic injection contacts . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2 Device symmetry . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.3 Device homogeneity . . . . . . . . . . . . . . . . . . . . . . . 83 4.3 Resistance characteristics . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Equivalent-circuit development . . . . . . . . . . . . . . . . 85 4.4 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Measurement fundamentals . . . . . . . . . . . . . . . . . . 92 4.4.2 Thickness dependence . . . . . . . . . . . . . . . . . . . . . 93 4.4.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . 95 4.5 Depletion zone variation . . . . . . . . . . . . . . . . . . . . . . . . 97 4.6 Molybdenum oxide as a case study . . . . . . . . . . . . . . . . . . 99 5 Charge-carrier transport and self-heating in OLED lighting . . . . . . . . . . . . . . . . .101 5.1 Joule self-heating in OLEDs . . . . . . . . . . . . . . . . . . . . . . 104 5.1.1 Electrothermal feedback . . . . . . . . . . . . . . . . . . . . 104 5.1.2 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1.3 Cooling strategies . . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 Self-heating causes lateral luminance inhomogeneities in OLEDs . . 108 5.2.1 The influence of transparent electrodes . . . . . . . . . . . . 108 5.2.2 Luminance inhomogeneities in large OLED panels . . . . . . 110 5.3 Electrothermal OLED models . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 Perceiving an OLED as thermistor array . . . . . . . . . . . 112 5.3.2 The OLED as a single three-layer thermistor . . . . . . . . . 114 5.3.3 A numerical 3D model of heat and current flow . . . . . . . 116 5.4 OLED stack and experimental conception . . . . . . . . . . . . . . 118 5.5 The Switch-back effect in planar light sources . . . . . . . . . . . . 120 5.5.1 Predictions from numerical 3D modeling . . . . . . . . . . . 121 5.5.2 Experimental proof . . . . . . . . . . . . . . . . . . . . . . . 124 5.5.3 Variation of vertical heat flux . . . . . . . . . . . . . . . . . 127 5.5.4 Variation of the OLED area . . . . . . . . . . . . . . . . . . 131 5.6 Electrothermal tristabilities in OLEDs . . . . . . . . . . . . . . . . 133 5.6.1 Observing different burn-in schematics . . . . . . . . . . . . 133 5.6.2 Bistability and tristability in organic semiconductors . . . . 134 5.6.3 Experimental indications for attempted branch hopping . . . 138 5.6.4 Saving bright OLEDs from burning in . . . . . . . . . . . . 144 5.6.5 Taking another view onto the camera pictures . . . . . . . . 145 6 Charge-carrier recombination and exciton management . . . . . . . . . . . . . . . . .147 6.1 Optical down conversion . . . . . . . . . . . . . . . . . . . . . . . . 149 6.1.1 Spectral reshaping of visible OLEDs . . . . . . . . . . . . . 149 6.1.2 Infrared-emitting OLEDs . . . . . . . . . . . . . . . . . . . . 155 6.2 Dual-state Förster transfer . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.3 Singlet fission and triplet fusion in rubrene . . . . . . . . . . . . . . 161 6.3.1 Photoluminescence of pure and doped rubrene films . . . . . 163 6.3.2 Electroluminescence transients of rubrene OLEDs . . . . . . 172 6.4 Charge transfer-state tuning by electric fields . . . . . . . . . . . . . 177 6.4.1 CT-state tuning via doping variation . . . . . . . . . . . . . 177 6.4.2 CT-state tuning via voltage . . . . . . . . . . . . . . . . . . 180 6.5 Excursus: Exciton-spin mixing for wavelength identification . . . . 183 6.5.1 Characteristics of the active film . . . . . . . . . . . . . . . . 184 6.5.2 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.5.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.5.5 Application demonstrations . . . . . . . . . . . . . . . . . . 192 6.5.6 All-organic device . . . . . . . . . . . . . . . . . . . . . . . . 195 6.5.7 Device limitations and prospects . . . . . . . . . . . . . . . . 198 7 Conclusion and outlook . . . . . . . . . . . . . . . . . 207 7.1 Charge-carrier injection into doped films . . . . . . . . . . . . . . . 207 7.2 Charge-carrier transport in hot OLEDs . . . . . . . . . . . . . . . . 208 7.2.1 Prospects for OLED lighting facing tristable behavior . . . . 209 7.2.2 Outlook: Accessing the hidden PDR 2 region . . . . . . . . . 210 7.3 Charge-carrier recombination and spin mixing . . . . . . . . . . . . 211 7.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.3.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bibliography. . . . . . . . . . . . . . . . . 215 Acknowledgements . . . . . . . . . . . . . . . . . 249Wer sich heute für ein neues Mobiltelefon entscheidet, kauft damit wahrscheinlich einen Bildschirm aus organischen Leuchtdioden (OLEDs). Durch ihre brillanten Farben, die hohe erreichbare Auflösung und eine vergleichsweise einfache Prozessierung werden OLEDs als relativ neue Bildschirmtechnologie in den nächsten Jahren wohl den größten Marktanteil ausmachen. Da sie nicht auf der starren Kristallstruktur klassischer Halbleiter beruhen und als planare Dünnschichtmodule produziert werden können, ermöglichen sie außerdem die Fertigung großer Flächenstrahler auf flexiblen Substraten – ein sehr attraktives Szenario für zukünftige Beleuchtungssysteme. Trotz dieser vielversprechenden Eigenschaften steht der Durchbruch der OLED-Technologie als Leuchtmittel noch aus und erfordert weitere Forschung. Die Dynamik der Ladungsträger (Elektronen und Löcher) in einer OLED charakterisiert wichtige Teile der Bauteilfunktion und ermöglicht daher das Verständnis grundlegender physikalischer Konzepte und Phänomene. Diese Arbeit leitet anhand dieser Beschreibung experimentelle Methoden und Bauteilkonzepte ab, um die Effizienz und Stabilität von OLEDs zu optimieren. OLEDs zeichnen sich dadurch aus, dass ein elektrischer Strom aus Ladungsträgern (Elektronen und Löchern) möglichst effizient unter Aussendung von Licht rekombiniert. Diesem Prozess geht eine Ladungsträgerinjektion und deren Transport zur Emissionsschicht voraus. Diese drei Aspekte werden in dieser Arbeit zusammenhängend diskutiert. Als erstes wird eine Methode vorgestellt, die Injektionswiderstände anhand eines einfachen Experimentes quantifiziert. Sie bildet eine wertvolle Möglichkeit, Injektionsschichten besser zu verstehen und zu optimieren. Anschließend wird der Ladungsträgertransport bei hohen elektrischen Strömen untersucht, wie sie für OLEDs als helle Beleuchtungselemente nötig sind. Hier werden elektro-thermische Effekte vorgestellt, die physikalische Grenzen für das Design und die Funktion von OLED Modulen bilden und deren plötzliches Versagen erklären. Abschließend wird die Dynamik der stark elektrostatisch gebundenen Ladungsträgerpaare, sogenannter Exzitonen, kurz vor deren Rekombination untersucht. Es werden verschiedene Möglichkeiten vorgestellt sie so zu manipulieren, dass sich das Abstrahlverhalten der OLED anhand bestimmter Anforderungen einstellen lässt.:List of publications . . . . . . . . . . . . . . . . . v List of abbreviations . . . . . . . . . . . . . . . . . ix 1 Introduction . . . . . . . . . . . . . . . . . 1 2 Fundamentals . . . . . . . . . . . . . . . . . 5 2.1 Light sources and the human society . . . . . . . . . . . . . . . . . 5 2.1.1 Human light perception . . . . . . . . . . . . . . . . . . . . 8 2.1.2 Physical light quantification . . . . . . . . . . . . . . . . . . 10 2.1.3 Non-visual light impact . . . . . . . . . . . . . . . . . . . . . 13 2.1.4 Implications for modern light sources . . . . . . . . . . . . . 15 2.2 Organic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . 17 2.2.1 Molecular energy states . . . . . . . . . . . . . . . . . . . . . 18 2.2.2 Intramolecular state transitions . . . . . . . . . . . . . . . . 24 2.2.3 Molecular films . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.2.4 Electrical doping . . . . . . . . . . . . . . . . . . . . . . . . 34 2.2.5 Charge-carrier transport . . . . . . . . . . . . . . . . . . . . 36 2.2.6 Exciton formation and recombination . . . . . . . . . . . . . 38 2.2.7 Exciton transfer . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.3 Organic light-emitting diodes . . . . . . . . . . . . . . . . . . . . . 44 2.3.1 Structure and operation principle . . . . . . . . . . . . . . . 44 2.3.2 Metal-semiconductor interfaces . . . . . . . . . . . . . . . . 47 2.3.3 Typical operation characteristics . . . . . . . . . . . . . . . . 49 2.4 Colloidal nanocrystal emitters . . . . . . . . . . . . . . . . . . . . . 52 2.4.1 Terminology: Nanocrystals and quantum dots . . . . . . . . 52 2.4.2 The particle-in-a-box model . . . . . . . . . . . . . . . . . . 54 2.4.3 Surface passivation . . . . . . . . . . . . . . . . . . . . . . . 55 3 Materials and methods . . . . . . . . . . . . . . . . . 57 3.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.1 OLED materials . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.1.2 Materials for photoluminescence . . . . . . . . . . . . . . . . 60 3.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.2.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . 62 3.2.2 Solution processing . . . . . . . . . . . . . . . . . . . . . . . 64 3.3 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.3.1 Absorbance spectroscopy . . . . . . . . . . . . . . . . . . . . 66 3.3.2 Photoluminescence quantum yield . . . . . . . . . . . . . . . 66 3.3.3 Excitation sources . . . . . . . . . . . . . . . . . . . . . . . 67 3.3.4 Sensitive EQE for absorber materials . . . . . . . . . . . . . 68 3.4 Exciton-lifetime analysis . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.1 Triplet lifetime . . . . . . . . . . . . . . . . . . . . . . . . . 69 3.4.2 Singlet-state lifetime . . . . . . . . . . . . . . . . . . . . . . 70 3.4.3 Lifetime extraction . . . . . . . . . . . . . . . . . . . . . . . 70 3.5 OLED characterization . . . . . . . . . . . . . . . . . . . . . . . . . 73 3.5.1 Current-voltage-luminance and quantum efficiency . . . . . . 73 3.5.2 Temperature-controlled evaluation . . . . . . . . . . . . . . . 74 4 Charge-carrier injection into doped organic films . . . . . . . . . . . . . . . . . 77 4.1 Ohmic injection contacts . . . . . . . . . . . . . . . . . . . . . . . . 79 4.2 Device architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.1 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.2 Device symmetry . . . . . . . . . . . . . . . . . . . . . . . . 80 4.2.3 Device homogeneity . . . . . . . . . . . . . . . . . . . . . . . 83 4.3 Resistance characteristics . . . . . . . . . . . . . . . . . . . . . . . . 84 4.3.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . 84 4.3.2 Equivalent-circuit development . . . . . . . . . . . . . . . . 85 4.4 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4.1 Measurement fundamentals . . . . . . . . . . . . . . . . . . 92 4.4.2 Thickness dependence . . . . . . . . . . . . . . . . . . . . . 93 4.4.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . 95 4.5 Depletion zone variation . . . . . . . . . . . . . . . . . . . . . . . . 97 4.6 Molybdenum oxide as a case study . . . . . . . . . . . . . . . . . . 99 5 Charge-carrier transport and self-heating in OLED lighting . . . . . . . . . . . . . . . . .101 5.1 Joule self-heating in OLEDs . . . . . . . . . . . . . . . . . . . . . . 104 5.1.1 Electrothermal feedback . . . . . . . . . . . . . . . . . . . . 104 5.1.2 Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 5.1.3 Cooling strategies . . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 Self-heating causes lateral luminance inhomogeneities in OLEDs . . 108 5.2.1 The influence of transparent electrodes . . . . . . . . . . . . 108 5.2.2 Luminance inhomogeneities in large OLED panels . . . . . . 110 5.3 Electrothermal OLED models . . . . . . . . . . . . . . . . . . . . . 112 5.3.1 Perceiving an OLED as thermistor array . . . . . . . . . . . 112 5.3.2 The OLED as a single three-layer thermistor . . . . . . . . . 114 5.3.3 A numerical 3D model of heat and current flow . . . . . . . 116 5.4 OLED stack and experimental conception . . . . . . . . . . . . . . 118 5.5 The Switch-back effect in planar light sources . . . . . . . . . . . . 120 5.5.1 Predictions from numerical 3D modeling . . . . . . . . . . . 121 5.5.2 Experimental proof . . . . . . . . . . . . . . . . . . . . . . . 124 5.5.3 Variation of vertical heat flux . . . . . . . . . . . . . . . . . 127 5.5.4 Variation of the OLED area . . . . . . . . . . . . . . . . . . 131 5.6 Electrothermal tristabilities in OLEDs . . . . . . . . . . . . . . . . 133 5.6.1 Observing different burn-in schematics . . . . . . . . . . . . 133 5.6.2 Bistability and tristability in organic semiconductors . . . . 134 5.6.3 Experimental indications for attempted branch hopping . . . 138 5.6.4 Saving bright OLEDs from burning in . . . . . . . . . . . . 144 5.6.5 Taking another view onto the camera pictures . . . . . . . . 145 6 Charge-carrier recombination and exciton management . . . . . . . . . . . . . . . . .147 6.1 Optical down conversion . . . . . . . . . . . . . . . . . . . . . . . . 149 6.1.1 Spectral reshaping of visible OLEDs . . . . . . . . . . . . . 149 6.1.2 Infrared-emitting OLEDs . . . . . . . . . . . . . . . . . . . . 155 6.2 Dual-state Förster transfer . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 6.2.2 Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 6.3 Singlet fission and triplet fusion in rubrene . . . . . . . . . . . . . . 161 6.3.1 Photoluminescence of pure and doped rubrene films . . . . . 163 6.3.2 Electroluminescence transients of rubrene OLEDs . . . . . . 172 6.4 Charge transfer-state tuning by electric fields . . . . . . . . . . . . . 177 6.4.1 CT-state tuning via doping variation . . . . . . . . . . . . . 177 6.4.2 CT-state tuning via voltage . . . . . . . . . . . . . . . . . . 180 6.5 Excursus: Exciton-spin mixing for wavelength identification . . . . 183 6.5.1 Characteristics of the active film . . . . . . . . . . . . . . . . 184 6.5.2 Conception . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.5.3 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.5.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.5.5 Application demonstrations . . . . . . . . . . . . . . . . . . 192 6.5.6 All-organic device . . . . . . . . . . . . . . . . . . . . . . . . 195 6.5.7 Device limitations and prospects . . . . . . . . . . . . . . . . 198 7 Conclusion and outlook . . . . . . . . . . . . . . . . . 207 7.1 Charge-carrier injection into doped films . . . . . . . . . . . . . . . 207 7.2 Charge-carrier transport in hot OLEDs . . . . . . . . . . . . . . . . 208 7.2.1 Prospects for OLED lighting facing tristable behavior . . . . 209 7.2.2 Outlook: Accessing the hidden PDR 2 region . . . . . . . . . 210 7.3 Charge-carrier recombination and spin mixing . . . . . . . . . . . . 211 7.3.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 7.3.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bibliography. . . . . . . . . . . . . . . . . 215 Acknowledgements . . . . . . . . . . . . . . . . . 24

유기발광 디스플레이 수명 모델 제안 및 모델 검증 체계 연구

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 기계항공공학부, 2018. 2. 윤병동.Despite the advantages of organic light-emitting diode (OLED) displays over liquid crystal displays, OLED displays suffer from reliability concerns related to luminance degradation and color shift. In particular, existing testing schemes are unable to reliably estimate the lifetime of large OLED displays (i.e., displays of 55 inches or larger). The limited number of test samples and the immature technology result in great hurdles for timely product development. This study proposes a statistical approach to develop a lifetime model for OLED panels. The proposed approach incorporates manufacturing and operational uncertainties, and accurately estimates the lifetime of the OLED panels under normal usage conditions. The proposed statistical analysis approach consists of: (1) design of accelerated degradation tests (ADTs) for OLED panels, (2) establishment of a systematic scheme to build bivariate lifetime models for OLED panels, (3) development of two bivariate lifetime models for OLED panels, and (4) statistical model validation for the heat dissipation analysis model for OLED TV design. This four-step statistical approach will help enable accurate lifetime prediction for large OLED panels subjected to various uncertainties. Thereby, this approach will foster efficient and effective OLED TV design to meet desired lifespan requirements. Furthermore, two bivariate acceleration models are proposed in this research to estimate the lifetime of OLED panels under real-world usage conditions, subject to manufacturing and operational uncertainties. These bivariate acceleration models take into account two main factors—temperature and initial luminance intensity. The first bivariate acceleration model estimates the luminance degradation of the OLED panelthe second estimates the panels color shift. The lifespan predicted by the proposed lifetime model shows a good agreement with experimental results. Ensuring the color shift lifetime is a great hurdle for OLED product development. However, at present, there is no effective way to estimate the color shift lifetime at the early stages of product development while the product design is still changing. The research described here proposes a novel scheme for color shift lifetime analysis. The proposed method consists of: (1) a finite element model for OLED thermal analysis that incorporates the uncertainty of the measured surface temperature, (2) statistical model validation, including model calibration, to verify agreement between the predicted results and a set of experimental data (achieved through adjustment of a set of physical input variables and hypothesis tests for validity checking to measure the degree of mismatch between the predicted and observed results), and (3) a regression model that can predict the color shift lifetime using the surface temperature at the early stages of product development. It is expected that the regression model can substantially shorten the product development time by predicting the color shift lifetime through OLED thermal analysis.Chapter 1. Introduction 1 1.1 Background and Motivation 1 1.2 Overview and Significance 2 1.3 Thesis Layout 6 Chapter 2. Literature Review 8 2.1 Accelerated Testing 8 2.2 Luminance Degradation Model for OLEDs 12 2.3 Color Shift of OLEDs 14 2.4 Verification and Validation Methodology 16 Chapter 3. OLED Degradation 28 3.1 Chromaticity and the Definition of Color Shift Lifetime 30 3.2 Degradation Mechanism 31 3.2.1 Luminance Degradation Mechanism 33 3.2.2 Color Shift Mechanism 34 3.3 Performance Degradation Models 36 3.3.1 Performance Degradation Model 36 3.3.2 Performance Color Shift Model 38 3.4 Acceleration Model 38 Chapter 4. Acceleration Degradation Testing (ADT) for OLEDs 42 4.1 Experimental Setup 42 4.2 Definition of the Time to Failure 46 4.2.1 The Time to Failure of Luminance 46 4.2.2 The Time to Failure of Color Shift 47 4.3 Lifespan Test Results 50 Chapter 5. Bivariate Lifetime Model for OLEDs 53 5.1 Fitting TTF Data to the Statistical Distribution 53 5.1.1 Estimation of Lifetime Distribution Parameters 53 5.1.2 Estimation of the Common Shape Parameter 58 5.1.3 Likelihood-Ratio Analysis 62 5.2 Bivariate Lifetime Model 64 5.2.1 Luminance Lifetime Model 64 5.2.2 Color Shift Lifetime Model 66 5.3 Validation of the Lifetime Model 67 Chapter 6. Statistical Model Validation of Heat Dissipation Analysis Model 77 6.1 Estimation Method for TTF using Surface Temperature 79 6.2 Thermal Analysis Model for OLED Displays 81 6.3 Statistical Calibration using the EDR Method 82 6.4 Validity Check 87 6.5 Results and Discussion 90 Chapter 7. Case Study 93 7.1 Computational Modeling 93 7.2 Estimation of Color Shift 95 7.3 Estimation of Luminance Degradation 96 Chapter 8. Contributions and Future Work 98 8.1 Contributions and Impacts 98 8.2 Suggestions for Future Research 103 References 104Docto

Perception-Aware Optimisation Methodologies for Quantum Dot Based Displays and Lighting

Human colour vision acuity is limited. This presents opportunities to leverage these perceptual limits to achieve engineering optimisations for devices and systems that interact with the human vision system. This dissertation presents the results of few investigations we carried out into quantifying these limits and several optimisation methodologies that we devised. The first step in this process is to quantify the acuity of human colour vision. We obtained a large corpus of colour matching data from a mobile video game called Specimen. We examine what questions about human vision this dataset allows us to answer and explore global statistics about colour vision based on this data on 41,000 players from 175 countries. We show that we can use the information in this dataset to infer potential candidate functions for the spectral sensitivities of each person in the dataset. The human eye acts like a many to one function; quantifiably different spectra can look like the same colour. This is referred to as metamerism. From a device perspective, different spectra consume different amounts of energy to generate. We show that we can use these two properties to elicit the same colour sensation using less energy. In the colour samples we evaluated, we show that we can achieve up to 10 times less power consumption while achieving a colour match. Given that one cannot change the emission spectrum of a display after fabrication, we propose the use of a multi-primary colour display to achieve this. We present two indices for quantifying the metameric capacity of such a display and its ability to save energy. The emission spectrum of a quantum dot (QD) based device is very narrow. Previous work in the literature suggested that narrow bandwidth spectra can lead to observer metameric breakdown; different observers disagreeing on the perceived ‘colour’ of a spectrum. We show that this might not be the case, using modern colour science tools, and show how metameric breakdown in a display could be minimised by carefully choosing the primary emission wavelengths. The limited colour acuity of human vision implies that people cannot notice small differences in colour. This fact has been used to create approximate colour transformation algorithms that subtly change colours in images such that they consume less energy when displayed on an emissive pixel display without causing unacceptable visual artefacts. We conducted a user study to gather information about the effect of one such colour transform called Crayon. We present a method for effectively picking the optimal transform parameters for Crayon, based on the user study results. The method presented calculates these parameters based on the properties of the image being transformed such that the power saving can be maximised while minimising the loss of image quality. The user study results show that we can achieve up to 50% power saving with a majority of the study participants reporting a negligible degradation in image quality in the transformed images. We additionally investigate a hypothesis that was presented stating that images with large amounts of highly luminous pixels cause increased power consumption in OLED displays due to localised display heating. We show that this hypothesis is wrong. We also investigate if sub-pixel rendering in Pentile displays can be used to reduce display power consumption by intentionally turning off random sub-pixels. However, we present a negative result showing that even single-pixel artefacts are observable on the test platform and thus, this cannot be used to improve display power efficiency. The narrow-band optical emissions of QD based devices mixed with their ability to be fabricated through solution processing can be used to mix multiple QDs together to build devices that generate arbitrary spectral shapes. We show how to use this property in an numerical optimisation based design framework to create lighting devices with a high colour rendering index (CRI). We evaluate the effects of different cost functions and initialisation strategies, and show that, we are able to design devices with a CRI > 96 using only four different QD primaries. We use a charge-transport based simulator to asses the electric properties of the designed devices. We also showcase initial work done on a modular software interface and a material library we developed for this simulator.EPSRC DTP studentship award RG84040:EP/N509620/

Thin-Film Transistor Integration for Biomedical Imaging and AMOLED Displays

Thin film transistor (TFT) backplanes are being continuously researched for new applications such as active-matrix organic light emitting diode (AMOLED) displays, sensors, and x-ray imagers. However, the circuits implemented in presently available fabrication technologies including poly silicon (poly-Si), hydrogenated amorphous silicon (a-Si:H), and organic semiconductor, are prone to spatial and/or temporal non-uniformities. While current-programmed active matrix (AM) can tolerate mismatches and non-uniformity caused by aging, the long settling time is a significant limitation. Consequently, acceleration schemes are needed and are proposed to reduce the settling time to 20 µs. This technique is used in the development of a pixel circuit and system for biomedical imager and sensor. Here, a metal-insulator-semiconductor (MIS) capacitor is adopted for adjustment and boost of the circuit gain. Thus, the new pixel architecture supports multi-modality imaging for a wide range of applications with various input signal intensities. Also, for applications with lower current levels, a fast current-mode line driver is developed based on positive feedback which controls the effect of the parasitic capacitance. The measured settling time of a conventional current source is around 2 ms for a 100-nA input current and 200-pF parasitic capacitance whereas it is less than 4 μs for the driver presented here. For displays needed in mobile devices such as cell phones and DVD players, another new driving scheme is devised that provides for a high temporal stability, low-power consumption, high tolerance of temperature variations, and high resolution. The performance of the new driving scheme is demonstrated in a 9-inch fabricated display intended for DVD players. Also, a multi-modal imager pixel circuit is developed using this technique to provide for gain-adjustment capability. Here, the readout operation is not destructive, enabling the use of low-cost readout circuitry and noise reduction techniques. In addition, a highly stable and reliable driving scheme, based on step calibration is introduced for high precision displays and imagers. This scheme takes advantage of the slow aging of the electronics in the backplane to simplify the drive electronics. The other attractive features of this newly developed driving scheme are its simplicity, low-power consumption, and fast programming critical for implementation of large-area and high-resolution active matrix arrays for high precision

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

AMOLED Displays with In-Pixel Photodetector

The focus of this chapter is to consider additional functionalities beyond the regular display function of an active matrix organic light-emitting diode (AMOLED) display. We will discuss how to improve the resolution of the array with OLED lithography pushing to AR/VR standards. Also, the chapter will give an insight into pixel design and layout with a strong focus on high resolution, enabling open areas in pixels for additional functionalities. An example of such additional functionalities would be to include a photodetector in pixel, requiring the need to include in-panel TFT readout at the peripherals of the full-display sensor array for applications such as finger and palmprint sensing