We present novel numerical techniques for the simulation of the light outcoupling from state of the art organic light-emitting diodes (OLED). For the spatial discretization we use the finite element method which we apply in the frequency domain. To account for the large horizontal extension of the OLED we apply a recently proposed approach based on the Floquet transform which allows to restrict the calculations to the unit cell of a (quasi) periodic structure. Optically thick layers are efficiently treated by a plane wave expansion which we combine with the Finite Element Method by the domain decomposition method. We benchmark the new simulation tools for highly efficient state of the art OLED light extraction structures.
Nanostructures, like periodic arrays of scatters or low-index gratings, are used to improve the light outcoupling from organic light-emitting diodes (OLED). In order to optimize geometrical and material properties of such structures, simulations of the outcoupling process are very helpful. The finite element method is best suited for an accurate discretization of the geometry and the singular-like field profile within the structured layer and the emitting layer. However, a finite element simulation of the overall OLED stack is often beyond available computer resources. The main focus of this paper is the simulation of a single dipole source embedded into a twofold infinitely periodic OLED structure. To overcome the numerical burden we apply the Floquet transform, so that the computational domain reduces to the unit cell. The relevant outcoupling data are than gained by inverse Flouqet transforming. This step requires a careful numerical treatment as reported in this paper.
Organic light emitting diodes (OLEDs) provide potential for power-efficient large area light sources that combine
revolutionary properties. They are thin and flat and in addition they can be transparent, colour-tuneable, or flexible. We
review the state of the art in white OLEDs and present performance data for three-colour hybrid white OLEDs on indexmatched
substrates. With improved optical outcoupling 45 lm/W are achieved. Using a half-sphere to collect all the light
that is in the substrate results in 80 lm/W. Optical modelling supports the experimental work. For decorative applications
features like transparency and colour tuning are very appealing. We show results on transparent white OLEDs and two
ways to come to a colour-variable OLED. These are lateral separation of different colours in a striped design and direct
vertical stacking of the different emitting layers. For a striped colour tuneable OLED 36 lm/W are achieved in white with
improved optical outcoupling.
In a conventional bottom emitting organic light emitting diode only about half of the generated photons are emitted into
the glass substrate (out of which 25% are extracted into air), the other half being wave-guided and dissipated in the
OLED stack. This is due to the refractive index mismatch between the organic layers (n=1.7-1.9) and the glass substrate
(n=1.5). By matching the refractive index of the substrate (n=1.8) and organic layers and augmenting the distance of the
emission zone to the cathode to suppress plasmonic losses light extraction into the substrate can be increased to 80-
90%. This is shown by simulation and experiment. Furthermore the effect of pyramidal structures on the light extraction
from the substrate into air is studied by experiment and simulation. Ultimately it is limited by the reflectance of the
OLED stack. The experimental results for monochromatic light are well corroborated by simulations. The main
conclusion is that most photons can be out-coupled from the organic stack into an index matched substrate. The OLED
light extraction problem is thus reduced to an effective extraction from the substrate into air.
An experimental and modelling study has been carried out of the current-voltage-luminance (J-V-L) characteristics of blue
polyfluorene-based organic light emitting devices, with a PEDOT:PSS anode and a Ba/Al cathode. The polymer contains copolymerized
hole transporting units that facilitate hole injection. The luminous efficacy for perpendicular emission as a
function of the voltage shows a pronounced peak; for an 80 nm thick device, it is equal to 3.3 cd/A at 8 V. At the peak
voltage, the external quantum efficiency is 2.2 %. We are working on a comprehensive device model that should provide a
framework within which these results can be understood, and present in this paper our intermediate results. Hole and electron
transport were studied using devices with a Au and Al cathode and anode, respectively. For hole-only devices a fair
description of the temperature and layer thickness dependent J-V curves could be obtained by using a 'conventional' model
for the mobility, involving a Poole-Frenkel factor for the field-dependence. For electron-only devices, the analysis is
complicated by the presence of an approximately 0.5 eV injection barrier. We have found a parametrization scheme that
provides a good description of the experimental J-V curves. A double carrier model that is based on the results of these
studies of single-carrier devices provides a good description of the J-V curves of double carrier devices. We have developed
a numerical model for the light outcoupling from the optical cavity. For the model parameters assumed, the calculated peak
position and shape of the lumunous efficacy as a function of V are in good agreement with the experimental results at room
temperature. An analysis is given of the factors that determine the peak height. We argue that a solid physical basis for the
model used to describe the electron injection and mobility is still lacking, so that continued electron transport studies will be
required.
Regular arrays of scatterers like cylinders or spheres which resemble 2D photonic crystals can increase light extraction from organic leds. In a recent SPIE publication we have shown that the scattering of the light emitted from a dipole source by such structures can be effectively modelled by an integral equation which kernel is essentially given by the Green’s tensor of the layered medium constituting the organic led. Here we extend the scope of this method by making use of the fact that the matrix-vector products arising from the discretization of the integral equation via the coupled dipole approximation can be calculated by the fast 3D Fourier transform. In this way the iterative solution of linear systems with millions of unknowns becomes feasible and large finite arrays comprising about 10 by 10 particles and more can be effectively treated. After giving an outline of the algorithms we present the results of calculations for large arrays of spheres and cylinders of circular and quadratic cross section. The influence of particle shape, type of array (quadratic, hexagonal) and refractive index on the efficiency of light extraction and angular distribution is studied. We also investigate the effect of random departures of the scattering elements from their ideal lattice positions.
The accepted model for light emission and propagation in organic LEDs (OLED) which consists of several optically thin functional layers deposited on a thick substrate is a classical dipole located in the emitting layer. The propagation of the emitted light is commonly described by a Fourier expansion of the dipole field into plane waves which represent the various radiating and bound modes of the layered structure in k-space. To calculate the electric and magnetic fields inside and outside the LED an integration over the individual plane waves has to be performed. This entails numerical difficulties which can be overcome elegantly with the so-called Green’s tensor approach for stratified media recently developed by the second author. In our contribution we demonstrate the applicability of this method to the computation of electromagnetic field distributions in organic LED structures. Visualizations of typical field distributions arising from individual dipoles are presented and discussed thus allowing a more intuitive understanding of effects relating to dipole location and orientation and material absorption. Furthermore it is shown that scattering of bound modes by particle like inhomogeneities of the layer stucture can be effectively modelled with the Green’s tensor approach. Visualizations are presented and discussed with regard to increased light extraction.
Many illumination tasks require large area light sources, e.g. LCD backlighting or general lighting. Depending on the application these sources have to fulfill criteria concerning uniformity, angular distribution and color of light output, brightness, efficiency, flatness and cost. After discussing established solutions with regard to these requirements, light guides which incorporate light sources like light emitting diodes or thin fluorescent lamps in cavity like recesses are introduced. The advantages of this scheme are flatness, scalability in area and reliance on established sources. This will be demonstrated for large area light sources utilizing high power LEDs or thin fluorescent lamps.
The optical performance of interference filters is subject to inevitable variations of the thicknesses and indices of refraction of the layers during or even after deposition. Assuming their statistical distributions and optimizing the expected performance as a function of the nominal layer thicknesses, robust filter designs can be identified which are insensitive to these variations and are therefore easier to manufacture and more robust to use. The underlying stochastic optimization problem and practical methods for its solution using deterministic and random search procedures based on evolutionary strategies are discussed. Finally a number of examples are presented.
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