Organic Light-emitting Diodes Stack Research Articles

Multiscale Fabrication Process Optimization of DFB Cavities for Organic Laser Diodes.

In the context of the quest for the Organic Laser Diode, we present the multiscale fabrication process optimization of mixed-order distributed-feedback micro-cavities integrated in nanosecond-short electrical pulse-ready organic light-emitting diodes (OLEDs). We combine ultra-short pulsed electrical excitation and laser micro-cavities. This requires the integration of a highly resolved DFB micro-cavity with an OLED stack and with microwave electrodes. In a second challenge, we tune the cavity resonance precisely to the electroluminescence peak of the organic laser gain medium. This requires precise micro-cavity fabrication performed using e-beam lithography to pattern gratings with a precision in the nanometer scale. Optimal DFB micro-cavities are obtained with 300 nm thick hydrogen silsesquioxane negative-tone e-beam resist on 50 nm thin indium tin oxide anode exposed with a charge quantity per area (i.e., dose) of 620 µC/cm2, developed over 40 min in tetramethylammonium hydroxide diluted in water. We show that the integration of the DFB micro-cavity does not hinder the pulsed electrical operability of the device, which exhibits a peak current density as high as 14 kA/cm2.

Planarized and Compact Light Scattering Layers Based on Disordered Titania Nanopillars for Light Extraction in Organic Light Emitting Diodes

AbstractIn this work, the extraction of waveguided and substrate modes in organic light emitting diodes (OLEDs) is improved by using compact light scattering layers composed of a disordered 2D array of TiO2 nanopillars. The TiO2 nanopillars are fabricated by combining a self‐assembly and a solvent‐assisted lift‐off process, and are further planarized by a 250 nm thin epoxy‐based photoresist layer to facilitate their anode deposition and integration within the OLED stack. This fabrication route allows engineering internal light outcoupling elements with a limited amount of parasitic absorption and with easily tunable light scattering properties that are effective over a broad spectral and angular range. Taking the example of a monochromatic bottom emitting OLED ( = 520 nm), the authors demonstrate an efficiency enhancement of +22%rel upon the incorporation of the planarized light extraction layer as well as ameliorated angular emission characteristics. This approach can be integrated in a high‐throughput fabrication routine and straightforwardly extended to other OLED layouts.

Optimization of Transparent Organic Light-Emitting Diodes by Simulation-Based Design of Organic Capping Layers.

In this work, we use the transfer matrix method to optimize TPBi capping layers deposited on organic light emitting diodes with respect to light extraction and transmittance. The green transparent organic light emitting diodes comprise three organic semiconductors (CBP, Ir(ppy)₃ and TPBi) forming an efficient simplified phosphorescent organic light emitting diode stack. A transparent cathode of 2 nm Cs₂CO₃, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The diode stack as well as the capping layer are deposited by organic vapor phase deposition. The refractive indices and extinction coefficients of all materials in the transparent organic light emitting diodes (glass, indium tin oxide, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance and reflectance measurements. With these spectrally resolved data, we calculate the transmittance of transparent organic light emitting diodes with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm-1000 nm) by a series of experiments. By choosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated transparent organic light emitting diodes with an optical transmittance which was strongly enhanced from 47% (reference without capping layer) to 65%, measured at 555 nm.

Molecular Orientation Effects in Organic Light‐Emitting Diodes

AbstractIt is well known that by horizontally aligning the transition dipole moments of exciton dipoles in the emitter films of organic light‐emitting diodes (OLEDs), a larger fraction of the radiative power can escape from the OLED stack, increasing the light outcoupling efficiency by up to 50 % compared to the isotropic counterparts. In this account, we review recent advances in understanding this phenomenon, with a special focus on the practical strategies to control the molecular orientation in vacuum‐deposited films of thermally activated delayed fluorescent (TADF) dyes. The role of molecular orientation in efficient OLED design is discussed, which has been experimentally proven to increase the external quantum efficiency exceeding 30 %. We outline the future challenges and perspectives in this field, including the potential to extend the concept to the solution‐processed films. Finally, the development of multiscale computer simulations is reviewed to assess their potential as a complementary approach to systematically screening OLED molecules in silico.

84‐3: Invited Paper: Organic Light‐Emitting Diode Beam Shaping: Pixel Design for Variable Angular Emission Profile Control

Organic light‐emitting diodes (OLEDs) are the leading self‐emitting pixel technology in current and future small and large area displays. Once integrated with a certain layer architecture into the backplane layout, their emission colour and angular distribution is set by the optical properties of the layered system. In this paper, we demonstrate a pixel design that allows for actively controlled variation of the angular emission profile of the individual vertical pixel. For this, a tandem device is developed that comprises two units optimized for different angular emission pattern. We constrained the system to operate in a narrow emission band to maintain monochromaticity of the individual pixel. We discuss this concept for a red phosphorescence‐based OLED stack and give an outlook based on simulations for the other primary display colours green and blue. The tandem unit can be operated with only two electrodes making use of the AC/DC driving concept, where the outer electrodes are in direct connection. In this paper, we will discuss the potential, status, and technology challenges for this concept.

Inkjet-printed internal light extraction layers for organic light emitting diodes

We have developed inkjet-printed scattering layers for enhanced light extraction in organic light emitting diodes (OLEDs). These layers are based on scattering polymer/nanoparticle composites, which are prepared from a solvent-free process and used for the outcoupling of waveguide modes to free propagating modes, thus improving the device efficiencies. Two different monomers are examined and an inkjet-printing process is developed for each of them. We first analyze the inks’ rheological properties, followed by the optical and morphological properties of the resulting layers. Upon integration of these printed layers into an OLED stack, the device efficiencies are increased by up to 40% with respect to an unpatterned reference device. Furthermore, we show that these internal light extraction layers improve the angular and spectral stability of the devices.

Adjusting the emission color of organic light-emitting diodes through aluminum nanodisc arrays

With the introduction of phosphorescent and thermally activated delayed fluorescence emitter materials, organic light-emitting diodes (OLEDs) with internal quantum yields of up to 100% can be realized. Still, light extraction from the OLED stack is a bottleneck, which hampers the availability of OLEDs with large external quantum efficiencies. Many different strategies to enhance the outcoupling of the light have been suggested, for instance, the use of collective lattice resonances induced by arrays of plasmonic nanodiscs. Here, we investigate the usability of these nanodisc arrays to tune the emission color of an organic blue-emitting material. By means of extinction and photoluminescence spectroscopy, we show a correlation of the sharp features observed in extinction with a selective fluorescence enhancement. At the same time, the nanodisc array also modifies the microcavity of an OLED stack. For one exemplarily lattice constant of an aluminum nanodisc array directly integrated into an OLED stack, we show that a combination of these effects allows the modification of the emission color from CIE1931 (x,y) chromaticity coordinates of (0.149, 0.225) to (0.152, 0.352). Importantly, the OLED exhibited a similar emission color modification under optical as well as electrical excitation.

Coherent mode coupling in highly efficient top-emitting OLEDs on periodically corrugated substrates

Bragg scattering at one-dimensional corrugated substrates allows to improve the light outcoupling from top-emitting organic light-emitting diodes (OLEDs). The OLEDs rely on a highly efficient phosphorescent pin stack and contain metal electrodes that introduce pronounced microcavity effects. A corrugated photoresist layer underneath the bottom electrode introduces light scattering. Compared to optically optimized reference OLEDs without the corrugated substrate, the corrugation increases light outcoupling efficiency but does not adversely affect the electrical properties of the devices. The external quantum efficiency (EQE) is increased from 15 % for an optimized planar layer structure to 17.5 % for a corrugated OLED with a grating period of 1.0 μm and a modulation depth of about 70 nm. Detailed analysis and optical modeling of the angular resolved emission spectra of the OLEDs provide evidence for Bragg scattering of waveguided and surface plasmon modes that are normally confined within the OLED stack into the air-cone. We observe constructive and destructive interference between these scattered modes and the radiative cavity mode. This interference is quantitatively described by a complex summation of Lorentz-like resonances.

Plasmonically Enhanced Optical Characteristics From Europium Organometallic Complex

We demonstrated that the plasmonic effect can enhance the photoluminescence of the europium organometallic complex in conventional organic light emitting diodes stack from an anode to emissive layer with solution processing. The aggregated gold nanoparticles (A-Au NPs) were incorporated in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) layer to increase the luminescent quantum efficiency of the emissive layer. An enhancement of 31% was achieved in the emission intensity at 614 nm for samples with A-Au NPs. The reduced exciton lifetime measured by time-resolved photoluminescence comply with the Purcell effect. These improvements are attributed to the localized surface plasmon of A-Au NPs increasing the electric dipole transition rate from Eu3+ ions.

Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll‐Off of Phosphorescent Organic Light‐Emitting Diodes

This article describes the first systematic investigation of how the efficiency roll‐off in organic light‐emitting diodes (OLEDs) is influenced by the position and orientation of the emitter molecules within the OLED cavity. The efficiency roll‐off is investigated for two OLED stacks containing either the phosphorescent emitter Ir(MDQ)2(acac) or Ir(ppy)3 by varying the distance between emitter and metal cathode; a strong influence of emitter position and orientation on roll‐off is observed. The measurements are modeled by triplet‐triplet‐annihilation (TTA) theory yielding the critical current density and the TTA rate constant. It is found that Ir(MDQ)2(acac) shows the lowest roll‐off when the emitter is located in the first optical maximum of the electromagnetic field, whereas the roll‐off of the Ir(ppy)3 stack is lowest when the emitter is positioned closer to the metal cathode. Measurement and modeling of time‐resolved electroluminescence show that the different roll‐off behavior is due to the different orientation and the corresponding change of the decay rate of the emissive dipoles of Ir(MDQ)2(acac) and Ir(ppy)3. Finally, design principles are developed for optimal high‐brightness performance by modeling the roll‐off as a function of emitter‐cathode distance, emissive dipole orientation, and radiative efficiency.

Efficiency Analysis of Organic Light-Emitting Diodes Based on Optical Simulations

Organic light-emitting diodes (OLEDs) are promising new large-area light sources on their way to commercialization. However, there is still much room for improvement in terms of device efficiency and long-term stability under electrical operation. In this paper, we review the current issue of efficiency analysis based on optical simulations of state-of-the-art OLED stacks. In detail, we present a method to determine the radiative quantum efficiency of the emitter, figure out the crucial points for nonisotropic emitter orientation, and discuss the application of the developed method to analyze degradation effects during electrical operation.

P‐177: Characterization of Orange OLED Stack for Bidirectional Active‐Matrix OLED Microdisplays

AbstractHighly efficient, low‐voltage organic light emitting diodes (OLEDs) are well suitable for the integration into a CMOS‐process.An orange emitting phosphorescent OLED‐stack with doped charge transport layers was prepared on different types of CMOS substrates. First OLED deposition was done on a single diode 6″‐Wafer substrate and afterwards on a micro structured two metal layer CMOS test substrate without active transistor area.The different test substrates were measured and compared with respect to their performance (current, luminance, voltage, luminance dependence on viewing angle, optical outcoupling etc.)For bidirectional microdisplay applications the dynamic characteristics of the OLED stack are also of interest. Therefore the charge and discharge behaviour and the switch on and switch off delay time of voltage apply to luminance output are measured and summarised in a simple model.