Highly Efficient Color Stable Inverted White Top‐Emitting OLEDs with Ultra‐Thin Wetting Layer Top Electrodes

Schwab, Tobias; Schubert, Sylvio; Hofmann, Simone; Fröbel, Markus; Fuchs, Cornelius; Thomschke, Michael; Müller‐Meskamp, Lars; Leo, Karl; Gather, Malte C.

doi:10.1002/adom.201300241

Cited by 88 publications

(77 citation statements)

References 33 publications

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“…Within the experiment, such ultra-thin metal layers are realized by using a recently adapted wetting layer electrode approach 48,49 , where a seed metal is used to form a closed conductive and highly transparent metal electrode. With this approach, the combined thickness of the smooth metal electrode does not exceed 10 nm, enabling very high outcoupling efficiencies.…”

Section: Optimizing Thin Film Spps and Power Dissipation Of Implemmentioning

confidence: 99%

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

et al. 2015

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We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (∼ 1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials owning isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.

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Section: Optimizing Thin Film Spps and Power Dissipation Of Implemmentioning

confidence: 99%

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

et al. 2015

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“…In contrast to the majority of publications on this topic, our devices are driven by an alternating current (AC) signal, which allows us to reduce the required number of independently addressable electrodes from three to only two. 8,[16][17][18] By using an ultrathin and highly transparent layer of highly conductive metal as the intermediate electrode, [19][20][21] our devices achieve efficiencies comparable to those of state-of-the-art non-color-tunable white OLEDs with the additional advantage of providing continuous color tuning at constant brightness from deep blue through cold-white and warm-white to saturated yellow. Our device efficiencies of up to 36.8 lm W 21 at warm-white color coordinates are-to the best of our knowledge-the highest values achieved thus far for any freely color-tunable white OLED.…”

Section: Introductionmentioning

confidence: 91%

Get it white: color-tunable AC/DC OLEDs

et al. 2015

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Organic light-emitting diodes (OLEDs) have gained considerable attention because of their use of inherently flexible materials and their compatibility with facile roll-to-roll and printing processes. In addition to high efficiency, flexibility and transparency, reliable color tunability of solid state light sources is a desirable feature in the lighting and display industry. Here, we demonstrate a device concept for highly efficient organic light-emitting devices whose emission color can be easily adjusted from deep-blue through cold-white and warm-white to saturated yellow. Our approach exploits the different polarities of the positive and negative half-cycles of an alternating current (AC) driving signal to independently address a fluorescent blue emission unit and a phosphorescent yellow emission unit which are vertically stacked on top of each other. The electrode design is optimized for simple fabrication and driving and allows for two-terminal operation by a single source. The presented concept for color-tunable OLEDs is compatible with application requirements and versatile in terms of emitter combinations. INTRODUCTIONIn recent years, organic light-emitting diodes (OLEDs) have evolved into a mature technology and OLEDs are now used in various display applications. OLEDs provide an internal charge-to-photon conversion efficiency of nearly 100% and deliver homogeneous emission over large areas, making them promising candidates for new and innovative lighting applications. 1 White OLEDs, in particular, offer great potential for energy-efficient general illumination: luminous efficacies of more than 90 lm W 21 , comparable to the best fluorescent tubes, have already been reported. 2,3 Furthermore, OLED-based light sources can be made mechanically flexible and transparent, offering new opportunities for architecture, visual art and decoration. 4 The reliable realtime tunability of the OLED emission color would impart further momentum to OLED technology on its way to becoming a widespread source of general illumination. Thus far, two different color-tuning concepts have prevailed in the literature. One exploits voltagedependent changes in emission color and was demonstrated as early as 1994 for OLEDs fabricated from polymer blends. 5 Voltage-dependent color shifts are the result of a variety of mechanisms, e.g., voltagedependent charge trapping, a spatial shift of the recombination zone, a modified exciton distribution, or exciton quenching at high current densities. [5][6][7] However, this approach has several drawbacks: not only are the mechanisms that lead to voltage-dependent color-shifts difficult to control, but adjusting the driving voltage also unavoidably results in a dramatic and undesired change in device brightness. The second concept overcomes the disadvantages of the voltage-controlled approach by using a stacked tandem OLED structure with two (or more) independently addressable units emitting light of different colors. 8,9 In comparison with the first method, this approach provides

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“…To enhance this effect even more we will use a recently developed wetting layer electrode 22,23 as transparent top contact. These electrodes consist of a very thin layer of metal deposited onto a seed metal thus forming a closed planar film.…”

Section: Theoretical Conceptmentioning

confidence: 99%

Surface plasmon polariton modification in top-emitting organic light-emitting diodes for enhanced light outcoupling

et al. 2014

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We report on the enhanced light outcoupling efficiency of monochrome top-emitting organic light-emitting diodes (OLEDs). These OLEDs incorporate a hole transport layer (HTL) material with a substantially lower refractive index (∼ 1.5) than the emitter material or the standard HTL material (∼ 1.8) of a reference device. This low-index HTL is situated between the opaque bottom metal contact (anode) and the active emission layer. Compared to an HTL with common refractive index, the dispersion relation of the surface plasmon polariton (SPP) mode from the opaque metal contact is shifted to smaller in-plane wavenumbers. This shift enhances the outcoupling efficiency as it reduces the total dissipated power of the emitter. Furthermore, the excitation of the coupled SPPs at the thin transparent metal top contact (cathode) is avoided by using an ultrathin top electrode. Hence, the coupling of the electroluminescence from the emitter molecules to all non-radiative evanescent modes, with respect to the emitter material, is reduced by at least a factor of two, additionally increasing the outcoupling efficiency.Furthermore, for sufficiently high refractive index contrast the shift of the SPP at the anode/organic interface can lead to in-plane wavenumbers smaller than the wavenumber within the organic emitter layer and outcoupling of all excited modes by high index light extraction structures, e.g. microlens, seems feasible. In accordance to optical simulations, the external quantum efficiency is enhanced by about 20 % for monochrome green emitting OLEDs with low refractive index HTL compared to a reference sample.

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Highly Efficient Color Stable Inverted White Top‐Emitting OLEDs with Ultra‐Thin Wetting Layer Top Electrodes

Cited by 88 publications

References 33 publications

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

Get it white: color-tunable AC/DC OLEDs

Surface plasmon polariton modification in top-emitting organic light-emitting diodes for enhanced light outcoupling

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