Amos Egel

Photon management in solution-processed organic light-emitting diodes: a review of light outcoupling micro- and nanostructures

Abstract. To allow a greater acceptance in the display and lighting markets, organic light-emitting diode (OLED) technology is currently the subject of intensive research efforts aimed at manufacturing cost-effective devices with higher efficiencies. In this regard, strategies matured in the field of photonics and nanophotonics can be applied for photon management purposes to improve the outcoupling of the generated light and to control the emission pattern. In this review, we report on the recent experimental and numerical advances to pursue those goals by highlighting the example of bottom-emitting devices. The cases of periodical micro- and nanostructures, as well as of stochastic ensembles that can be easily implemented using printing techniques, are covered herein. It is shown that beyond the sole optical properties, such additional elements can simultaneously improve the electrical characteristics of solution-processed OLEDs, and thus enable an optimization of the devices at different levels.

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.

RC-Constant in Organic Photodiodes Comprising Electrodes With a Significant Sheet Resistance

Organic photodiodes provide prospects for the fabrication of arbitrarily shaped photodetectors. However, the enlarged detection area in conjunction with their minuscule absorber layer thickness increases the capacitance of these devices when compared with convential silicon photodiodes. The mandatory transparency of at least one electrode can, so far, only be provided by materials with significant sheet resistances. These factors lead to nonnegligible RC-constants where high frequency signal detection is ultimately RC-limited. In this letter, we devise a method to determine the effective RC-constant for an extended rectangular device comprising electrodes with a significant sheet resistance and show that it is up to 59% smaller than estimated from the geometric device dimensions.

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 particles 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.

Efficient evaluation of Sommerfeld integrals for the optical simulation of many scattering particles in planarly layered media.

A strategy for the efficient numerical evaluation of Sommerfeld integrals in the context of electromagnetic scattering at particles embedded in a plane parallel layer system is presented. The scheme relies on a lookup-table approach in combination with an asymptotic approximation of the Bessel function in order to enable the use of fast Fourier transformation. Accuracy of the algorithm is enhanced by means of singularity extraction and a novel technique to treat the integrand at small arguments. For short particle distances, this method is accomplished by a slower but more robust direct integration along a deflected contour. As an example, we investigate enhanced light extraction from an organic light-emitting diode by optical scattering particles. The calculations are discussed with respect to accuracy and computing time. By means of the present strategy, an accurate evaluation of the scattered field for several thousand wavelength scale particles can be achieved within a few hours on a conventional workstation computer.

Tuning the Microcavity of Organic Light Emitting Diodes by Solution Processable Polymer–Nanoparticle Composite Layers

In this study, we present a simple method to tune and take advantage of microcavity effects for an increased fraction of outcoupled light in solution-processed organic light emitting diodes. This is achieved by incorporating nonscattering polymer-nanoparticle composite layers. These tunable layers allow the optimization of the device architecture even for high film thicknesses on a single substrate by gradually altering the film thickness using a horizontal dipping technique. Moreover, it is shown that the optoelectronic device parameters are in good agreement with transfer matrix simulations of the corresponding layer stack, which offers the possibility to numerically design devices based on such composite layers. Lastly, it could be shown that the introduction of nanoparticles leads to an improved charge injection, which combined with an optimized microcavity resulted in a maximum luminous efficacy increase of 85% compared to a nanoparticle-free reference device.

Plane-wave coupling formalism for T-matrix simulations of light scattering by nonspherical particles

The computation of light scattering by the superposition T-matrix scheme has been restricted thus far to systems made of particles that are either sparsely distributed or of near-spherical shape. In this work, we extend the range of applicability of the T-matrix method by accounting for the coupling of scattered fields between highly nonspherical particles in close vicinity. This is achieved using an alternative formulation of the translation operator for spherical vector wave functions, based on a plane-wave expansion of the particles scattered electromagnetic field. The accuracy and versatility of the present approach is demonstrated by simulating arbitrarily oriented and densely packed spheroids, for both dielectric and metallic particles.

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

Abstract 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.

Extracting Substrate Modes from Flexible OLEDs

We study microlens arrays for light extraction from flexible OLED substrates by ray-tracing simulations and experiment. Results show that index matching is not necessarily optimal for substrates coated with barrier layers of lower refractive index.

Simulation of light scattering in clusters of nonspherical nanoparticles: an adapted T-matrix approach

Being restricted to the domain of validity of spherical wave expansions, the T-matrix method has been in question to model light scattering by nonspherical particle systems when inter-particle distances are low. In this work, we discuss a formalism to account for multiple scattering between nonspherical particles in close vicinity. Accurate coupling between adjacent particles’ scattered fields is achieved by an alternative plane-wave formulation of the translation operator for spherical vector wave functions. The accuracy of the presented approach is demonstrated by a far field simulation of a large particle cluster. The near field of nonspherical particles has not been accessible in T-matrix simulations thus far. Utilizing the benefits of plane-wave expansions, the near field of nonspherical particles can be constructed in T-matrix simulations. We hereby show that the T-matrix method is also applicable for the analysis of localized resonances, making it suitable for the description of plasmonic systems.