Organische, lichtemittierende Dioden (OLEDs) bezeichnen neuartige Lichtquellen, welche zur Beleuchtung oder für Displayanwendungen nutzbar sind. Im Allgemeinen ist die Lichtausbeute durch den hohen Brechungsindex und die Dünnschichtgeometrie der OLED begrenzt. Der hohe Brechungsindex sorgt dafür, dass ein signifikanter Anteil des emittierten Lichts in der OLED durch Totalreflexion (TIR) gefangen ist. Durch den Dünnschichtaufbau der OLED wird außerdem die Lichterzeugung für resonante Moden der kohärenten optischen Mikrokavität erhöht. Dies gilt im Besonderen für die nichtstrahlenden Moden. In dieser Arbeit wurden zwei Methoden untersucht, um die Lichtausbeute aus OLEDs zu erhöhen.
Zuerst wurde die Implementierung von Materialien mit niedrigem Brechungsindex angrenzend zum undurchsichtigen metallischen Rückkontakt untersucht. Die Modifizierung des Brechungindexes verändert die Dispersionsrelation der an der Grenzfläche zwischen Metall und Dielektrikum angeregten nicht-strahlenden Oberflächenplasmonpolariton-Resonanz (SPP). Dadurch wird der Phasenraum verkleinert, in welchen effizient Strahlung abgegeben werden kann. Da die SPP-Resonanz eine nichtstrahlende Verlustquelle der Mikrokavität darstellt, wird so die Auskopplungseffizienz der OLED erhöht. In experimentellen Umsetzungen konnte die externe Quanteneffizienz (EQE) sowohl für einen Emitter gesteigert werden, welcher eine isotrope Verteilung der Strahlungsquellen besitzt (Ir(ppy)3 , +19 %), als auch für eine vorzugsweise horizontale Ausrichtung (Ir(ppy)2 (acac), +18 %). Die Steigerung der EQE korrespondiert sehr gut mit der berechneten Steigerung der Auskopplungseffizienz für die jeweiligen Mikrokavitäten (+23 %, bzw. +19 %). Weitere optische Simulationen legen den Schluss nahe, dass dieser Ansatz ebenso für perfekt horizontale Ausrichtung der Quellen sowie für weiße OLEDs anwendbar ist.
Als zweiter Ansatz wurde die erhöhte Lichtausbeute durch Bragg-Streuung an periodische Linienstrukturen untersucht. In dieser Arbeit wurden Methoden untersucht, bei denen die Oberflächen strukturiert wurde, auf welche die organischen Halbleiterschichten der OLEDs aufgebracht wurden. Für bottom-OLEDs (durch ein Substrat emittierende OLEDs), wurde direkt die transparente Elektrode durch ein Laserinterferenzablationsverfahren (DLIP) modifiziert. Zusätzlich wurden top-OLEDs untersucht (direkt aus der Mikrokavität Licht emittierende OLEDs), für welche alle Schichten auf eine periodisch strukturierte Photolackschicht aufgedampft wurden.
Für die bottom-OLEDs konnte für eine Gitterkonstante von 0.71 μm eine Steigerung der EQE um 27 %, verglichen zu einer optimierten unstrukturierten Referenz, ermittelt werden. Eine Vergrößerung der Gitterkonstante führt zu einer Abnahme der EQE. Die erhöhte EQE wird auf die Überlagerung des planaren Emissionsspektrums mit Beiträgen von Bragg-gestreuten, ursprünglich nicht-strahlenden Moden zurückgeführt, wobei die Intensitäten der Anteile von der Gitterkonstante und der Strukturhöhe abhängen. Für die top-OLEDs konnte eine Steigerung der EQE um 13 % für eine Gitterkonstante von 1.0 μm festgestellt werden.
Im Gegensatz zu den bottom-OLEDs wird für kleinere Gitterkonstanten (0.6 μm) hier die EQE nicht erhöht. Vielmehr kommt es durch die starke Veränderung des Emissionsspektrums zu einer Erhöhung der photometrischen Lichtausbeute um 13.5 %. Die starke Veränderung des Emissionspektrums wird auf eine kohärente Kopplung zwischen den Bragg-gestreuten Moden zurückgeführt, bedingt durch die starke optische Mikrokavität dieses OLED-Typs.
Um diese Effekte quantitativ zu beschreiben, wurde ein entsprechendes Modell entwickelt und implementiert. Die Qualität der Simulationsergebnisse wird anhand von Literaturreferenzen überprüft, wobei eine gute Übereinstimmung zu experimentell gemessenen Spektren erzeugt wird. Mit dem Simulationsmodell werden Vorhersagen über das Emissionspektrum und die resultierenden Effizienzen möglich. Für bottom-OLEDs wurde festgestellt, dass eine starke Veränderung des Emissionspektrums für Gitterkonstanten unterhalb von 0.5 μm erzeugt werden kann.
Hingegen sind für top-OLEDs sehr schwache Strukturen oder große Gitterkonstanten notwendig, um eine nur schwache Veränderung des Emissionsspektrums und damit einen allgemeinen Effizienzgewinn zu erzeugen. Bezüglich der Gitterkonstante, ist diese Erkenntnis ist im Gegensatz zur üblichen Herangehensweise zur Implementierung periodischer Streuschichten in OLEDs. Mit der implementierten Simulationsmethode werden jedoch Aussagen bzgl. Emissionspektrum und Effizienz für eine breite Spanne an OLED-Strukturen vor der experimentellen Umsetzung möglich.:1. Introduction
1.1. Motivation
1.2. Scope and outline of this work
2. Organic light emitting diodes - basic concepts
2.1. Amorphous organic semiconductors – electronic properties and transport of charge carriers
2.2. Charge injection into organic semiconductors
2.3. Doping of organic semiconductors and the p-i-n concept for OLEDs
2.4. Charge carrier recombination mechanisms
2.4.1. Displaced harmonic oscillator model for the photoluminescence
spectrum of organic emitters
3. OLEDs from thin homogeneous films - theoretical introduction into the optics
3.1. Maxwell’s equations
3.1.1. Boundary conditions
3.1.2. Poynting’s theorem, energy density, and energy flux density
3.2. Optics of thin planar films
3.2.1. Plane waves solution for the homogeneous Maxwell’s equations
3.2.2. Transfer-matrix formalism
3.3. Radiation from electric dipoles embedded into stratified media
3.4. Remarks on the normalized power dissipation
3.5. Description of outcoupled light as resonances
3.6. Basics of optimizing efficiency of OLEDs
3.6.1. Efficiencies for OLED characterization
3.6.2. Optimization of light outcoupling of OLEDs
3.6.2.1. Optimization of the basic cavity layout
3.6.2.2. Optimization of the emitter distribution
3.6.3. Enhancing OLED efficiency beyond the planar limit
4. Experimental fabrication and characterisation methods
4.1. Fabrication methods for organic semiconductors
4.2. Electrical and optical characterisation
4.3. Experimental realization of periodic corrugated surfaces
5. Enhancing the outcoupling efficiency by introducing low-refractive index layers
5.1. Dispersion relations of surface plasmon polaritons in thin film optical
microcavities
5.1.1. Bulk surface plasmon polaritons
5.1.2. Coupled surface plasmon polariton states for thin film geometries
5.2. Theoretical potential for outcoupling enhancement due to low refractive
index interlayers
5.3. Experimental validation for top-emitting OLEDs with isotropic or anisotropic green phosphorescent emitter
6. Bragg scattering for improved light outcoupling from OLEDs
6.1. Field expansion for periodic photonic crystals
6.2. Bragg scattering in weakly periodically perturbed bottom-emitting monochrome OLEDs
6.2.1. Device details and experimental characterization
6.2.2. Quantitative assignment of Bragg scattering effects within electroluminescence spectrum
6.3. Bragg scattering in top-emitting monochrome OLEDs
6.3.1. Device details and experimental characterization
6.3.2. Analysis of electroluminescence spectrum and description of scattered light from resonance model
6.4. Simulation of the spectral radiant intensity for periodically corrugated
OLEDs
6.4.1. Theoretical flowchart
6.4.2. Preliminaries and source representation
6.4.2.1. Plane wave expansion and z-depended field coefficient
representation
6.4.2.2. Pseudo-periodic polarization source
6.4.2.3. Solution to the inhomogeneous Maxwell’s equations for a pseudo-periodic source
6.4.3. Field propagation and scattering matrix
6.4.3.1. Reducing the Maxwell’s equation
6.4.3.2. Representation-transformation matrices
6.4.3.3. Formulation of transfer-matrix formalism for periodically perturbed media
6.4.3.4. Iterative calculation of the scattering matrix in plane
wave basis
6.4.4. From electromagnetic fields to measurement
6.4.4.1. Far-field solutions in superstrate/substrate media
6.4.4.2. System matrix for pseudo-periodic sources within periodically corrugated cavity
6.4.4.3. Radiant intensity from far-field solutions
6.4.4.4. Treatment of incoherent thick superstrate and substrate
6.4.4.5. Including electroluminescence spectra of organic emitter materials
6.5. Simulation of light emission from emitters embedded into periodically perturbed microcavities
6.5.1. Comparison to experimental data and existing simulation approaches
6.5.2. Simulated light emission for periodically perturbed microcavities
6.5.2.1. Simulation of light emission from corrugated bottom-emitting OLEDs
6.5.2.2. Comparing simulation to experiment for top-emitting OLEDs on corrugated photoresist
6.5.3. A-priori simulation of optical microcavities
6.5.3.1. Variation of lattice constant and aspect ratio to maximize
total radiant intensity
7. Conclusions
8. Outlook
Appendices
A. Materials – Abbreviations and optoelectronic modeling parameters
A.1. Organic semiconductors and ZnO:Al
A.1.1. Active emitter materials
A.1.2. Dielectric functions
A.2. Metals
B. Power dissipation spectra and oSPP shifts for top- and bottom-emitting
OLEDs incorporating silver and aluminum anode layers
C. Further comments on theoretical derivations leading to the simulation of
emission from photonic crystal optical microcavities
C.1. Numerical approximation of the integration of Maxwell’s equation
C.2. Details on the far-field approximation of the periodic plane wave expansion
C.3. Derivation of the efficient iterative calculation scheme for the scattering-matrix
C.4. On the equality of the two system matrices
C.5. Calculation of the complete scattering-matrix for the passive periodically perturbed microcavity
Bibliography / Organic light-emitting diodes (OLEDs) are an attractive new light source for display and lighting applications. In general, the light extraction from OLEDs is limited due to the high refractive index of the active emitter material and the thin film geometry. The high refractive index causes the trapping of a significant portion of the emitted light due to total internal reflection (TIR). Due to the thin film layout, the light emission is enhanced for resonant modes of the coherent optical microcavity, in particular for light affected by TIR. In this work two approaches are investigated in detail in order to increase the light extraction efficiency of OLEDs.
In a first approach, the implementation of a low refractive index material next to the opaque metallic back-reflector is discussed. This modifies the dispersion relation of the non-radiative surface plasmon polariton (SPP) mode at the metal / dielectric interface, causing a shift of the SPPs dispersion relation. Thereby, the phase space into which power can be efficiently dissipated by the emitter is reduced. For the SPP this power would have been lost to the cavity, such that in total the outcoupling efficiency is increased.
In experiment, an increased external quantum efficiency (EQE) is observed for an emitter exhibiting isotropic orientation of the sources (Ir(ppy)3 ,+19 %), as well as for an emitter which shows preferential horizontal orientation (Ir(ppy)2 (acac), +18 %), compared to an optimized device which uses standard material. This corresponds very well to the enhancement of the outcoupling efficiencies of the corresponding microcavities (+23 %, resp. +19 %) reducing the refractive index of the hole transport layer by 15 %. Optical simulations indicate that the approach is generally applicable to a wide range of device architectures. These in particular include OLEDs with emitters showing a perfectly horizontal alignment of their transition dipole moments. Furthermore, the approach is suitable for white OLEDs.
Bragg scattering was investigated as second option to increase the light extraction from OLEDs. The method requires a periodically structured surface. For the bottom-emitting OLEDs, this is achieved by a direct laser interference patterning (DLIP) of the transparent electrode. Additionally, top-emitting devices were fabricated onto periodically corrugated photoresist layers. Using a periodic line pattern with a lattice constant of 0.71 μm, the EQE of the bottom-emitting devices was enhanced by 27 % compared to an optimized planar reference. For the bottom-emitting layout, increasing the lattice constant leads to lower EQEs. The increased EQE is attributed to the superposition of the radiative cavity resonances by Bragg scattered intensities of trapped modes. The intensities depend on the lattice constants as well as the height of the periodic surface perturbation.
For top-emitting OLEDs comprising a lattice constant of 1.0 μm the EQE was increased by 13 %. Reducing the lattice constant (0.6 μm) decreases the EQE, albeit the luminous efficacy is increased by 13.5 % due to a heavily perturbed emission spectrum. The perturbation is attributed to a coherent interaction of the Bragg scattered modes due to the strong optical microcavity for the top-emitting OLEDs. Thus, for strong perturbation specific emission patterns can be achieved, but an overall enhanced efficiency is difficult to obtain.
To investigate the observed results theoretically, a detailed simulation approach is outlined. The simulation method is carefully evaluated using reference data from literature. Using the simulation approach, the emission patterns as well as the efficiencies of the devices can be estimated. The emission spectra reproduced from simulation are in good agreement with the experiment. Furthermore, for the bottom-emitting layout, a strong interaction can be found from simulations for lattice constants below 0.5 μm. For top-emitting OLEDs, the weak interaction regime seems to be more likely to result in an overall enhanced emission. This requires, in contrast to conventional opinion, very shallow perturbations or lattice constants which exceed the peak wavelength of the emission spectrum. However, with the established simulation approach a-priori propositions on the emission spectrum or particular beneficial device layouts become feasible.:1. Introduction
1.1. Motivation
1.2. Scope and outline of this work
2. Organic light emitting diodes - basic concepts
2.1. Amorphous organic semiconductors – electronic properties and transport of charge carriers
2.2. Charge injection into organic semiconductors
2.3. Doping of organic semiconductors and the p-i-n concept for OLEDs
2.4. Charge carrier recombination mechanisms
2.4.1. Displaced harmonic oscillator model for the photoluminescence
spectrum of organic emitters
3. OLEDs from thin homogeneous films - theoretical introduction into the optics
3.1. Maxwell’s equations
3.1.1. Boundary conditions
3.1.2. Poynting’s theorem, energy density, and energy flux density
3.2. Optics of thin planar films
3.2.1. Plane waves solution for the homogeneous Maxwell’s equations
3.2.2. Transfer-matrix formalism
3.3. Radiation from electric dipoles embedded into stratified media
3.4. Remarks on the normalized power dissipation
3.5. Description of outcoupled light as resonances
3.6. Basics of optimizing efficiency of OLEDs
3.6.1. Efficiencies for OLED characterization
3.6.2. Optimization of light outcoupling of OLEDs
3.6.2.1. Optimization of the basic cavity layout
3.6.2.2. Optimization of the emitter distribution
3.6.3. Enhancing OLED efficiency beyond the planar limit
4. Experimental fabrication and characterisation methods
4.1. Fabrication methods for organic semiconductors
4.2. Electrical and optical characterisation
4.3. Experimental realization of periodic corrugated surfaces
5. Enhancing the outcoupling efficiency by introducing low-refractive index layers
5.1. Dispersion relations of surface plasmon polaritons in thin film optical
microcavities
5.1.1. Bulk surface plasmon polaritons
5.1.2. Coupled surface plasmon polariton states for thin film geometries
5.2. Theoretical potential for outcoupling enhancement due to low refractive
index interlayers
5.3. Experimental validation for top-emitting OLEDs with isotropic or anisotropic green phosphorescent emitter
6. Bragg scattering for improved light outcoupling from OLEDs
6.1. Field expansion for periodic photonic crystals
6.2. Bragg scattering in weakly periodically perturbed bottom-emitting monochrome OLEDs
6.2.1. Device details and experimental characterization
6.2.2. Quantitative assignment of Bragg scattering effects within electroluminescence spectrum
6.3. Bragg scattering in top-emitting monochrome OLEDs
6.3.1. Device details and experimental characterization
6.3.2. Analysis of electroluminescence spectrum and description of scattered light from resonance model
6.4. Simulation of the spectral radiant intensity for periodically corrugated
OLEDs
6.4.1. Theoretical flowchart
6.4.2. Preliminaries and source representation
6.4.2.1. Plane wave expansion and z-depended field coefficient
representation
6.4.2.2. Pseudo-periodic polarization source
6.4.2.3. Solution to the inhomogeneous Maxwell’s equations for a pseudo-periodic source
6.4.3. Field propagation and scattering matrix
6.4.3.1. Reducing the Maxwell’s equation
6.4.3.2. Representation-transformation matrices
6.4.3.3. Formulation of transfer-matrix formalism for periodically perturbed media
6.4.3.4. Iterative calculation of the scattering matrix in plane
wave basis
6.4.4. From electromagnetic fields to measurement
6.4.4.1. Far-field solutions in superstrate/substrate media
6.4.4.2. System matrix for pseudo-periodic sources within periodically corrugated cavity
6.4.4.3. Radiant intensity from far-field solutions
6.4.4.4. Treatment of incoherent thick superstrate and substrate
6.4.4.5. Including electroluminescence spectra of organic emitter materials
6.5. Simulation of light emission from emitters embedded into periodically perturbed microcavities
6.5.1. Comparison to experimental data and existing simulation approaches
6.5.2. Simulated light emission for periodically perturbed microcavities
6.5.2.1. Simulation of light emission from corrugated bottom-emitting OLEDs
6.5.2.2. Comparing simulation to experiment for top-emitting OLEDs on corrugated photoresist
6.5.3. A-priori simulation of optical microcavities
6.5.3.1. Variation of lattice constant and aspect ratio to maximize
total radiant intensity
7. Conclusions
8. Outlook
Appendices
A. Materials – Abbreviations and optoelectronic modeling parameters
A.1. Organic semiconductors and ZnO:Al
A.1.1. Active emitter materials
A.1.2. Dielectric functions
A.2. Metals
B. Power dissipation spectra and oSPP shifts for top- and bottom-emitting
OLEDs incorporating silver and aluminum anode layers
C. Further comments on theoretical derivations leading to the simulation of
emission from photonic crystal optical microcavities
C.1. Numerical approximation of the integration of Maxwell’s equation
C.2. Details on the far-field approximation of the periodic plane wave expansion
C.3. Derivation of the efficient iterative calculation scheme for the scattering-matrix
C.4. On the equality of the two system matrices
C.5. Calculation of the complete scattering-matrix for the passive periodically perturbed microcavity
Bibliography
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:29030 |
| Date | 14 July 2015 |
| Creators | Fuchs, Cornelius |
| Contributors | Leo, Karl, Hoffmann, Karl Heinz, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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