Accurate optical simulation of disordered scattering layers for light extraction from organic light emitting diodes

Diese Arbeit handelt von einer Methode zur umfassenden optischen Simulation von organischen Leuchtdioden (OLEDs) mit internen ungeordneten Streuschichten zur Lichtauskopplung. Die Methode ist exakt in dem Sinne, dass sie die L\"osung der Maxwellgleichungen f\"ur Dipolstrahlung in einem planparallelen Schichtsystem mit mehreren Streupartikeln erm\"oglicht. Dies wird durch die Entwicklung des gestreuten elektromagnetischen Feldes in Kugelvektorwellenfunktionen (zur effizienten Handhabung der Streuung an den einzelnen Partikeln) sowie in ebenen Wellen (zur Handhabung der Propagation des Lichtes durchs D\"unnschichtsystem) erreicht. Nach einer Einf\"uhrung zum Thema OLEDs mit einem besonderen Augenmerk auf Lichtauskopplung werden die relevanten Formeln f\"ur die Mehrfachstreuung in einem Formalismus von Anregung und Systemantwort hergeleitet. Auf diese Weise kann das Streuproblem in ein System von linearen Gleichungen \"uberf\"uhrt werden, das auf der Grundlage des T-Matrix Formalismus\u27 die Berechnung der Streukoeffizienten erlaubt. Es werden auch numerische Aspekte einer effizienten Aufstellung und L\"osung dieses Gleichungssystemes er\"ortert und Gleichungen, welche die Berechnung der interessanten Kenngr\"o\ss{}en erm\"oglichen, werden hergeleitet (elektrische Feldverteilung, dissipierte Leistung, Leistungsfluss durch Grenzfl\"achen, Fernfeld-Intensit\"atsverteilungen). Die hier vorgestellte Simulationsmethode wurde auch in einer frei verf\"ugbaren Software (\emph{Smuthi}) implementiert. Die Programmstruktur von Smuthi wird skizziert, und die Korrektheit der Simulationsergebnisse wird durch einen Vergleich mit Finite-Elemente Rechnungen belegt. Abschlie\ss{}end wird die Simulationsmethode an Hand einer praxisrelevanten Fallstudie illustriert. Hierzu wird eine wei\ss{}e OLED, deren Auskoppeleffizienz bereits durch Anpassen der Schichtdicken und der Lage der Emissionszonen optimiert worden ist, mit einer internen Streuschicht versehen und der erwartbare Zugewinn in der Lichtausbeute durch Simulationen berechnet. Au\ss{}er der Lichtauskopplung aus OLEDs kann der hier pr\"asentierte theoretische Formalismus und die Software f\"ur zahlreiche andere Anwendungen genutzt werden, welche die Streuung von Licht an Partikeln in der N\"ahe von ebenen Grenzfl\"achen beinhalten

Role of packing density and spatial correlations in strongly scattering 3D systems

Discrete random media have been investigated extensively over the past century due to their ability to scatter light. Even so, the link between the three-dimensional (3D) spatial distribution of the scattering elements and the resulting opacity is still lively debated to date due to different experimental conditions, range of parameters explored, or sample formulations. On the other hand, a unified numerical survey with controlled parameters has been impractical up to date due to the sheer computational power required to address samples with representative size. In this work, we exploit a graphics processing unit implementation of the T-matrix method to investigate the complete range of particle volume concentration and packing-induced spatial correlations, allowing us to reveal and elucidate a twofold role played by spatial correlations in either enhancing or suppressing opacity. By applying these findings to the illustrative case of white paint, we determine the optimal combination of density and spatial correlations corresponding to the highest opacity. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Light scattering by oblate particles near planar interfaces: on the validity of the T-matrix approach

We investigate the T-matrix approach for the simulation of light scattering by an oblate particle near a planar interface. Its validity has been in question if the interface intersects the particle’s circumscribing sphere, where the spherical wave expansion of the scattered field can diverge. However, the plane wave expansion of the scattered field converges everywhere below the particle, and in particular at the planar interface. We demonstrate that the particle-interface scattering interaction is correctly accounted for through a plane wave expansion in combination with Fresnel reflection at the planar interface. We present an in-depth analysis of the involved convergence mechanisms, which are governed by the transformation properties between spherical and plane waves. The method is illustrated with the cases of spherical and oblate spheroidal nanoparticles near a perfectly conducting interface, and its accuracy is demonstrated for different scatterer arrangements and materials

CELES: CUDA-accelerated simulation of electromagnetic scattering by large ensembles of spheres

CELES is a freely available MATLAB toolbox to simulate light scattering by many spherical particles. Aiming at high computational performance, CELES leverages block-diagonal preconditioning, a lookup-table approach to evaluate costly functions and massively parallel execution on NVIDIA graphics processing units using the CUDA computing platform. The combination of these techniques allows to efficiently address large electrodynamic problems ($>10^4$ scatterers) on inexpensive consumer hardware. In this paper, we validate near- and far-field distributions against the well-established multi-sphere $T$-matrix (MSTM) code and discuss the convergence behavior for ensembles of different sizes, including an exemplary system comprising $10^5$ particles

SMUTHI: A python package for the simulation of light scattering by multiple particles near or between planar interfaces

SMUTHI is a python package for the efficient and accurate simulation of electromagnetic scattering by one or multiple wavelength-scale objects in a planarly layered medium. The software combines the T-matrix method for individual particle scattering with the scattering matrix formalism for the propagation of the electromagnetic field through the planar interfaces. In this article, we briefly introduce the relevant theoretical concepts and present the main features of SMUTHI. Simulation results obtained for several benchmark configurations are validated against commercial software solutions. Owing to the generality of planarly layered geometries and the availability of different particle shapes and light sources, possible applications of SMUTHI include the study of discrete random media, meta-surfaces, photonic crystals and glasses, perforated membranes and plasmonic systems, to name a few relevant examples at visible and near-visible wavelengths

Illumination angle and layer thickness influence on the photo current generation in organic solar cells: A combined simulative and experimental study

In most future organic photovoltaic applications, such as fixed roof installations, facade or clothing integration, the solar cells will face the sun under varying angles. By a combined simulative and experimental study, we investigate the mutual interdependencies of the angle of light incidence, the absorber layer thickness and the photon harvesting efficiency within a typical organic photovoltaic device. For thin absorber layers, we find a steady decrease of the effective photocurrent towards increasing angles. For 90-140 nm thick absorber layers, however, we observe an effective photocurrent enhancement, exhibiting a maximum yield at angles of incidence of about 50°. Both effects mainly originate from the angle-dependent spatial broadening of the optical interference pattern inside the solar cell and a shift of the absorption maximum away from the metal electrode

Dipole emission in stratified media with multiple spherical scatterers: Enhanced outcoupling from OLEDs

AbstractScattering particles find application in organic light emitting diodes (OLEDs) for an enhanced outcoupling of the generated light. This paper presents a computational scheme to exactly model the electromagnetic fields and the power outcoupling efficiency of a typical OLED geometry, comprising a thin film system with spherical scattering particles inside. The model is based on the expansion of the fields in plane and spherical vector wave functions, as well as the scattering matrix formalism for the layer system reflections. In a numerical application example, the effect of 1000 spherical high index scattering particles on the internal outcoupling from a realistic OLED structure is discussed

On the fabrication of disordered nanostructures for light extraction in corrugated OLEDs

Light scattering OLED substrates relying on disordered self-assemblies are fabricated by microsphere and polymer blend lithography and used for light extraction. We report on a device efficiency enhancement of up to 50 %