Numerical investigation of a 2D-grating for light extraction of a bottom emitting OLED

An important limiting factor for efficient white light emitting organic LEDs is the total internal reflection occurring at each interface. In a bottom emitting OLED light is trapped by reflection at the interface between the organic layers and glass substrate and at the interface between the glass substrate and air. We investigate the use of a grating at the glass substrate-air interface. In this paper we will discuss the developed 3D-simulation method and several important simulation results. Our simulation method shows that the grating extracts approximately 50 % more power in comparison with a planar device. These results are comparable with the us

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

Blue organic LEDs with improved power efficiency

High-power-efficiency blue fluorescent organic lightemitting devices have been demonstrated by simultaneously doping two hole-conduction layers of 4, 4′, 4″-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA), and N, N′-diphenyl-N, N′-bis(1-naphthyl)-(1, 1′-biphenyl)-4, 4′-diamine (NPB) with 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) as well as doping two ambipolar emission layers with p-bis(p-N, N-diphenyl-aminostyryl) benzene (DSA-Ph). By doping the two host layers with DSA-Ph to form the double emission layer, the current efficiency is enhanced due to the extended emission zone. We further increase the performance by introducing the doping F4-TCNQ into the hole-injection and transporting layers to reduce the transport barrier at the m-MTDATA:F4TCNQ/NPB and to enhance the hole injection and conduction. The luminance and power efficiencies reach 8.9 cd/A and 4.5 lm/W, respectively. © 2009 IEEE.published_or_final_versio

Ketohydrazone complexes as potential emitting material in OLED

Abstract Ketohydrazone is a molecule that is able to act as a bidentate ligand through the O of C=O and N of N=C in the molecule. Three ketohydrazone ligands had been fully synthesized through the condensation reaction between 2- hydroxynaphthaldehydes with various hydrazides: salisylic hydrazide, benzyhydrazide and 2-furoic acid hydrazide in a 1:1 stoichiometry. The ligands had been characterized using infrared, 1HNMR and ultraviolet-visible spectrometer. Complexation reaction between all ligands and metals, with a stoichiometry of 1:2 for Zn (II) : ligand and 1:3 for Al(III) : ligand were carried out. All six complexes obtained were characterized using FTIR and UV-Vis spectrometer. The fluorescence properties of each ligands and complexes were investigated using luminescence spectrofluorometer excited at 406 nm. It was found that the compounds emitted blue light at ? max = 470 nm. Results showed that all the ligands and molecules synthesized have the fluorescence properties and complexation with metal enhanced the intensity of the fluorescence. It was observed that complex of Al(NDB)3 showed the best potential as an emitting material for OLED as it has the highest fluorescence intensity compared to others. Abstrak Ketohidrazon merupakan sebatian molekul yang berupaya berfungsi sebagai ligan bidentat melalui O daripada C=O dan N daripada N=C di dalam molekulnya. Tiga ligan ketohidrazon telah berjaya disintesis melalui proses kondensasi antara 2- hidroksinaftaldehida dan beberapa kumpulan hidrazida, salisilik hidrazida,, 2-furoik hidrazida, dan benzihidrazida.mengikut nisbah stoikiometri 1 : 1. Semua ligan yang telah disintesis dicirikan melalui spektroskopi IR, UV-Vis dan 1H-RMN. Tindak balas pengkompleksan antara ligan yang telah disintesis dengan dua logam yang berasingan, iaitu aluminium dan zink telah dijalankan dengan nisbah ion ligam : ligan 1 : 3 bagi pengkompleksan dengan aluminium dan 1 : 2 bagi pengkompleksan dengan zink. Enam kompleks yang terhasil dicirikan melalui spektroskopi IR dan UV-Vis. Ciri-ciri pendarfluor bagi tindak balas pengkompleksan dan ligan yang terhasil telah dikaji menggunakan spektrometer pendarfluor pada panjang gelombang pemancaran 405nm. Didapati sebatian-sebatian tersebut memancarkan cahaya biru pada ? max = 470 nm. Hasil yang diperolehi menunjukkan bahawa semua ligan dan kompleks yang disintesis menunjukkan sifat pendarfluor dan pengkompleksan dengan logam dapat meningkatkan keamatan pendarfluornya.. Kompleks Al(NDB)3 menunjukkan potensi yang terbaik sebagai bahan pemancar dalam diod pemancar cahaya organik (OLED) memandangkan kompleks ini menunjukkan keamatan pendarfluor yang tertinggi

Cathodic and Anodic Material Diffusion in Polymer/Semiconductor-Nanocrystal Composite Devices

In the present day, the information technologies and telecommunications sector continually increase their demand for low cost, low power consumption, high performance electroluminescent devices for display applications. Furthermore, general lighting applications, such as white light and large array colour displays, would also benefit from an increase in the overall efficiency. Several technologies are being investigated to fulfill these needs, such as organic light emitting diodes (OLED), polymeric light emitting diodes (PLED) and field effect emission devices. A new and promising technology is light emitting devices (LEDs) based on nanostructured materials. With organic LEDs (OLEDs) already making an impact on the market in an increasingly large number of applications, hybrid technologies based on organic/inorganic nano-composites are a potential the next step. The incorporation of highefficiency fluorescent semiconductor nanoparticles has been shown to have a beneficial effect on device performance, [1] modify the colour output from the device 2 and provide a simplified route to generation of LED type devices. [3

Visible Light Communications towards 5G

5G networks have to offer extremely high capacity for novel streaming applications. One of the most promising approaches is to embed large numbers of co-operating small cells into the macro-cell coverage area. Alternatively, optical wireless based technologies can be adopted as an alternative physical layer offering higher data rates. Visible light communications (VLC) is an emerging technology for future high capacity communication links (it has been accepted to 5GPP) in the visible range of the electromagnetic spectrum (~370–780 nm) utilizing light-emitting diodes (LEDs) simultaneously provide data transmission and room illumination. A major challenge in VLC is the LED modulation bandwidths, which are limited to a few MHz. However, myriad gigabit speed transmission links have already been demonstrated. Non line-of-sight (NLOS) optical wireless is resistant to blocking by people and obstacles and is capable of adapting its’ throughput according to the current channel state information. Concurrently, organic polymer LEDs (PLEDs) have become the focus of enormous attention for solid-state lighting applications due to their advantages over conventional white LEDs such as ultra-low costs, low heating temperature, mechanical flexibility and large photoactive areas when produced with wet processing methods. This paper discusses development of such VLC links with a view to implementing ubiquitous broadcasting networks featuring advanced modulation formats such as orthogonal frequency division multiplexing (OFDM) or carrier-less amplitude and phase modulation (CAP) in conjunction with equalization techniques. Finally, this paper will also summarize the results of the European project ICT COST IC1101 OPTICWISE (Optical Wireless Communications - An Emerging Technology) dealing VLC and OLEDs towards 5G networks

Optical design of organic/polymer solar cells and light emitting devices

InvitedIn this talk, we will study the effects of the microcavity structures and plasmonic resonances on the quantum efficiencies and photon lifetime. Theoretical and experimental results will be discussed on organic LEDs and solar cells (SCs). © 2011 IEEE.published_or_final_versionThe 11th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD 2011), Rome, Italy, 5-8 September 2011. In Proceedings of the 11th NUSOD, 2011, p. 1-

Feel the heat: Nonlinear electrothermal feedback in organic LEDs

For lighting applications, Organic light-emitting diodes (OLED) need much higher brightness than for displays, leading to self-heating. Due to the temperature-activated transport in organic semiconductors, this can result in brightness inhomogeneities and catastrophic failure. Here, we show that due to the strong electrothermal feedback of OLEDs, the common spatial current and voltage distribution is completely changed, requiring advanced device modeling and operation concepts. Our study clearly demonstrates the effect of negative differential resistance (NDR) in OLEDs induced by self-heating. As a consequence, for increasing voltage, regions with declining voltages are propagating through the device, and even more interestingly, a part of these regions show even decreasing currents, leading to strong local variation in luminance. The expected breakthrough of OLED lighting technology will require an improved price performance ratio, and the realization of modules with very high brightness but untainted appearance is considered to be an essential step into this direction. Thus, a deeper understanding of the control of electrothermal feedback will help to make OLEDs in lighting more competitive