325 research outputs found
The role of quantum coherence and dissipation in cosmology
This thesis looks at different manifestations that the non-unitary dynamics proper of dissipation can have during the inflationary era and the late-time universe. For starters, we formalise the calculation of the primordial spectrum in warm inflation, which significantly differs in terms of phenomenology from the standard inflationary picture because of the extra degrees of freedom that originate due to the dissipation of energy from the inflaton field into a thermal radiation bath.
Then, turning our attention to inflation in the context of string theory, we point out how the fluctuation-dissipation dynamic in warm inflation makes it robust against most of the so-called swampland conjectures. Nevertheless, that is not the case for the trans-Planckian censorship conjecture (TCC), which severely limits the duration of inflation to avoid trans-Planckian (TP) modes becoming observable, threatening the EFT description of inflation. In general, only models of inflation with a small energy scale can satisfy the TCC, effectively destroying any hope of experimental confirmation of inflation. To deal with this, we proposed a multi-stage warm inflation scenario with radiation-dominated eras in between. Such a model proved successful in opening a wider range of available energies that could make a model satisfying the TCC produce sizeable tensor perturbations. However, we also argue in favour of refinements of TCC that could make most high-energy models consistent with the conjecture. To do this, we looked at several mechanisms of subhorizon decoherence, like preheating and warm inflation itself, ultimately proving that keeping TP modes hidden inside the horizon is not enough to prevent their classicalisation, negating in this way the original premise of TCC.
Next, in a different direction, we study inflationary perturbations as an open quantum system, with an environment composed of subhorizon fluctuations and a system of superhorizon modes. We argue that this is the most appropriate way to study the physics of inflationary perturbations, as opposed to standard Wilsonian EFTs. We use the technology of open quantum systems in two big setups: scalar and tensor perturbations. In both cases, we compute the corrections to the two-point correlation function due to gravitational nonlinearities present in the Einstein-Hilbert action. This allowed us to explore topics such as long-time IR behaviour, (non-)Markovian behaviour, and the relation between this method and a standard loop expansion.
Finally, we changed our gears and looked at the viability of establishing quantum communication channels mediated by photons across astronomical and cosmological distances. For this, we survey multiple factors that could potentially disrupt the quantum state of the photons, like charged particles in space or the gravitational field of astrophysical bodies. We concluded that the x-ray portion of the electromagnetic spectrum would be ideal for establishing a quantum communication channel
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 â 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 fĂŒr 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 nĂ€chsten Jahren wohl den gröĂten Marktanteil ausmachen. Da sie nicht auf der starren Kristallstruktur klassischer Halbleiter beruhen und als planare DĂŒnnschichtmodule produziert werden können, ermöglichen sie auĂerdem die Fertigung groĂer FlĂ€chenstrahler auf flexiblen Substraten â ein sehr attraktives Szenario fĂŒr zukĂŒnftige Beleuchtungssysteme. Trotz dieser vielversprechenden Eigenschaften steht der Durchbruch der OLED-Technologie als Leuchtmittel noch aus und erfordert weitere Forschung. Die Dynamik der LadungstrĂ€ger (Elektronen und Löcher) in einer OLED charakterisiert wichtige Teile der Bauteilfunktion und ermöglicht daher das VerstĂ€ndnis grundlegender physikalischer Konzepte und PhĂ€nomene. Diese Arbeit leitet anhand dieser Beschreibung experimentelle Methoden und Bauteilkonzepte ab, um die Effizienz und StabilitĂ€t von OLEDs zu optimieren.
OLEDs zeichnen sich dadurch aus, dass ein elektrischer Strom aus LadungstrĂ€gern (Elektronen und Löchern) möglichst effizient unter Aussendung von Licht rekombiniert. Diesem Prozess geht eine LadungstrĂ€gerinjektion und deren Transport zur Emissionsschicht voraus. Diese drei Aspekte werden in dieser Arbeit zusammenhĂ€ngend diskutiert. Als erstes wird eine Methode vorgestellt, die InjektionswiderstĂ€nde anhand eines einfachen Experimentes quantifiziert. Sie bildet eine wertvolle Möglichkeit, Injektionsschichten besser zu verstehen und zu optimieren. AnschlieĂend wird der LadungstrĂ€gertransport bei hohen elektrischen Strömen untersucht, wie sie fĂŒr OLEDs als helle Beleuchtungselemente nötig sind. Hier werden elektro-thermische Effekte vorgestellt, die physikalische Grenzen fĂŒr das Design und die Funktion von OLED Modulen bilden und deren plötzliches Versagen erklĂ€ren. AbschlieĂend wird die Dynamik der stark elektrostatisch gebundenen LadungstrĂ€gerpaare, 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 lĂ€sst.: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
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