Light Outcoupling Enhancement Research Articles

Microcavity 적용 광자 발광 소자의 광 추출 향상 연구

Recently, microcavity is studied to reduce the optical loss of BLU and OLED. In this paper, we suggest applying microcavity to photo-luminescent lamp with plasma discharge technology to meet the display applications for a BLU for LCD. The structure of photo-luminescent lamp consists of SUS foil and ITO glass with microcavity. The opto-electric characteristics of photo-luminescent lamp with microcavity was evaluated. The brightness of photo-luminescent device was increased over <TEX>$111cd/m^2$</TEX> with the adaptation of patterned microcavity at <TEX>$30{\mu}m$</TEX>. The 3D optical simulation verified the enhanced light outcoupling when microcavity applied to the device.

Device efficiency of organic light‐emitting diodes: Progress by improved light outcoupling

AbstractOrganic light‐emitting diodes (OLEDs) are efficient large‐area light sources facing their market entry. Still, the development of stable and more efficient blue emitters and the enhancement of light outcoupling remain challenges for further device improvements. Here, we review the working principles of OLEDs and highlight ongoing efforts to improve their efficiency, in particular by coupling out more light.

Enhancing Light Outcoupling of Flexible Organic Light Emitting Diodes by Domain Selective-Etching

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Highly efficient bi-directional organic light-emitting diodes by strong micro-cavity effects

We report on highly enhanced light outcoupling of bi-directional red phosphorescent organic light-emitting diodes (BiOLEDs) by strong micro-cavity effects. Inserting additional metal layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) and controlling the thickness of the counter metal electrode result in dramatic changes in electroluminescence spectra and angular distribution. We find that the external quantum efficiency (EQE) for both bottom and top emitting side can be significantly improved: the overall EQE reaches up to 24.24%.

More light from organic light-emitting diodes

20 EPN 42/4 Organic light-emitting diodes (OLEDs) are efficient large-area light sources facing their market entry. Still, the development of stable and more efficient blue emitters and the enhancement of light outcoupling remain challenges for further device improvements. Here, we review the working principles of OLEDs and highlight ongoing efforts to improve their efficiency. Wolfgang Brutting, Jorg Frischeisen, Bert J. Scholz, and Tobias D. Schmidt DOI: 10.1051/epn/2011402 Institute of Physics, University of Augsburg, 86135 Augsburg, Germany Email: wolfgang.bruetting@physik.uni-augsburg.de More light

Light out-coupling enhancement of organic light-emitting devices with microlens array

Light out-coupling efficiency of organic light-emitting devices from high-index glass substrate into air is enhanced by attaching ordered microlens arrays, which are fabricated by a roll-to-roll mold transfer process. The dependence of microlens geometries on light extraction is analyzed experimentally and theoretically. An increase of 60% in the light out-coupling with an optimized elliptical microlens array is achieved over a conventional device without affecting the electroluminescent spectrum.

Highly efficient tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes utilized by balanced energy transfer with cosensitizing fluorescent dyes

We report on the development of highly efficient organic light-emitting diodes (OLEDs) utilized by balancing the energy transfer between multiple dopants, that is, multiple emissions from the multiple dopants were realized by balanced distributed energy transfer. From the cosensitizing fluorescent OLEDs, the peak external quantum efficiency (EQE) of 4.8% at 130 cd/m2 is demonstrated, which realized theoretical limits of ∼5.0% and means that nearly 100% of the singlet excitons are radiative. Also, the optimized device accompanying thickness-modulated electron transport layer for the enhanced light outcoupling demonstrated the highly improved peak EQE and current efficiency of 6.7%, and 23.4 cd/A.

An ambipolar phosphine oxide-based host for high power efficiency blue phosphorescent organic light emitting devices

We report blue electrophosphorescent organic light emitting devices with an ambipolar host material, 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), doped with FIrpic (iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate). The ambipolar nature of the host was verified using single carrier devices. The power efficiency of devices with PO15 (2,8-bis(diphenylphosphoryl)dibenzothiophene) electron transport layer (ETL) showed optimized performance when the ETL thickness was 500 Å, giving a peak power efficiency of 46 lm/W (corresponding external quantum efficiency (EQE) of 17.1%). The EQE and power efficiency at the brightness of 800 cd/m2 were measured with no light outcoupling enhancement and found to be 15.4% and 26 lm/W, respectively.

Full-wave simulation of enhanced outcoupling of organic light-emitting devices with an embedded low-index grid

Enhancement of light outcoupling into substrate modes by a grid of low-refractive-index material embedded into the organic layer of an organic light-emitting device (OLED) is analyzed using full-wave electromagnetic simulations. The low-index grid (LIG) redirects modes normally trapped within the high-index organic and indium tin oxide layers (waveguide modes) into the substrate where they can be further extracted into free space using methods such as microlens arrays or roughened surfaces. This increases the external quantum and power efficiencies without affecting the electroluminescent spectrum. The dependence on grid geometry, dimensions, and refractive index is explored to optimize the structure. Simulations show that up to 50% more light can be extracted from the high-index region using an ultralow-index (n=1.03) grid than a conventional device, and provided efficient substrate-to-air outcoupling, the external quantum efficiencies of LIG OLEDs can reach &amp;gt;50%.

Large-scale patterning of indium tin oxide electrodes for guided mode extraction from organic light-emitting diodes

We describe a cost-efficient and large area scalable production process of organic light-emitting diodes (OLEDs) with photonic crystals (PCs) as extraction elements for guided modes. Using laser interference lithography and physical plasma etching, we texture the indium tin oxide (ITO) electrode layer of an OLED with one- and two-dimensional PC gratings. By optical transmission measurements, the resonant mode of the grating is shown to have a drift of only 0.4% over the 5mm length of the ITO grating. By changing the lattice constant between 300 and 600nm, the OLED emission angle of enhanced light outcoupling is tailored from −24.25° to 37°. At these angles, the TE emission is enhanced up to a factor of 2.14.

Wide-Energy-Gap Electron-Transport Materials Containing 3,5-Dipyridylphenyl Moieties for an Ultra High Efficiency Blue Organic Light-Emitting Device

An ultra high efficiency blue organic light-emitting device using wide-energy-gap electron-transport material containing 3,5-dipyridylphenyl moieties and blue phosphorescent emitter, FIrpic, was developed. The optimized device showed extremely high power efficiencies of 56 lm/W (53 cd/A) at 100 cd/m2 and 42 lm/W (46 cd/A) at 1000 cd/m2 without light outcoupling enhancement.

Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids

The application of organic light-emitting devices to reduce the energy consumption of interior lighting, which now costs more than $230 billion annually1, is attracting interest due to the high efficiency of these devices. Indeed, electrophosphorescent organic light-emitting devices with an internal quantum efficiency ηIQE of 100% (refs 2,3) already approach the efficiency of fluorescent lamps4,5,6. However, because of the high refractive index of organic materials and the optical confinement and internal reflection that results, the light out-coupling efficiency ηout for conventional organic light-emitting devices is limited to ∼20% (refs 2,7). Here we demonstrate that embedding a low-index grid in the organic layers can enhance the out-coupling of the waveguided light without spectral distortion. When combined with microlenses that out-couple the glass modes, the external quantum efficiency ηEQE and power efficiency of a white organic light-emitting device are improved to 34 ± 2% and 68 ± 4 lm W–1. The resulting ηout is 2.3 ± 0.2 times that of a conventional organic light-emitting device used as a comparison, and simulations indicate that the enhancement can be further increased to 3.4 ± 0.2. Extracting light from organic LEDs is difficult owing to the refractive index of the materials used, and the output efficiency is typically limited to around 15–20%. By embedding a grid with a low refractive index into the organic layers and using a microlens array researchers have now managed to increase this figure to 34%, representing an improvement by factor of 2.3 over a conventional device.

Enhanced light outcoupling in a thin film by texturing meshed surfaces

The authors demonstrate a method of texturing a meshed surface on a poly(dimethyl siloxane) (PDMS) film for improving light extraction. This meshed surface is fabricated through a casting process by using a self-organized porous film as a template. Experimental results show that the light outcoupling efficiency increases on the meshed surface of a freestanding PDMS film with large incident angles. The external quantum efficiency of an organic light-emitting diode with the textured PDMS film was also demonstrated to have an enhancement of 46%.

Organic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography

High efficiency white organic light emitting devices (WOLEDs) with optical outcoupling enhanced by hexagonal polymethylmethacrylate microlens arrays fabricated by imprint lithography on a glass substrate are demonstrated. Monte Carlo and finite difference time domain simulations of the emitted light are used to optimize the microlens design. The measured enhancement of light outcoupling and the angular dependence of the extracted light intensity are in agreement with the simulation. Using microlens arrays, we demonstrate a fluorescent/phosphorescent WOLED with a maximum external quantum efficiency of (14.3±0.3)% at 900cd∕m2 and power efficiency of 21.6±0.5lm∕W at 220cd∕m2. The electroluminescent spectra at viewing angles from normal to the substrate plane, to 60° off normal, remain almost unchanged, giving a color rendering index of 87.

General method to evaluate substrate surface modification techniques for light extraction enhancement of organic light emitting diodes

The external light output of organic light emitting diodes (OLEDs) can be increased by modifying the light emitting surface. The apparent light extraction enhancement is given by the ratio between the efficiency of the unmodified device and the efficiency of the modified device. This apparent light extraction enhancement is dependent on the OLED architecture itself and is not the correct value to judge the effectiveness of a technique to enhance light outcoupling due to substrate surface modification. We propose a general method to evaluate substrate surface modification techniques for light extraction enhancement of OLEDs independent from the device architecture. This method is experimentally demonstrated using green electrophosphorescent OLEDs with different device architectures. The substrate surface of these OLEDs was modified by applying a prismatic film to increase light outcoupling from the device stack. It was demonstrated that the conventionally measured apparent light extraction enhancement by means of the prismatic film does not reflect the actual performance of the light outcoupling technique. Rather, by comparing the light extracted out of the prismatic film to that generated in the OLED layers and coupled into the substrate (before the substrate/air interface), a more accurate evaluation of light outcoupling enhancement can be achieved. Furthermore we show that substrate surface modification can change the output spectrum of a broad band emitting OLED.

Role of the dielectric capping layer in enhancement of light outcoupling for semitransparent metal-cathode organic light-emitting devices

We study theoretically the effects of the dielectric capping layer on the light outcoupling efficiency for semitransparent metal-cathode organic light-emitting devices. The outcoupling enhancement is co-determined by the improved transmission, the reduced (or improved) reflection, and the reduced absorption, as well as the suppressed surface plasmon polaritons. For some metals such as aluminium, the outcoupling efficiency can be enhanced largely by capping, but it can hardly be enhanced for some metals such as ytterbium. As a practical cathode, the Ag(15 nm)/TeO2(24 nm) combination achieves an outcoupling efficiency of 21.5%, competitive with bottom-emission devices.

Enhancing light outcoupling of organic light-emitting devices by locating emitters around the second antinode of the reflective metal electrode

Due to generally low conductivity and low carrier mobilities of organic materials, organic light-emitting devices (OLEDs) are typically optimized for light outcoupling by locating emitters around the first antinode of the metal electrode. In this letter, by utilizing device structures containing conductive doping, we investigate theoretically and experimentally the influences of the location of emitters relative to the metal electrode on OLED emission, and show that substantial enhancement in light outcoupling (1.2 times) or forward luminance (1.6 times) could be obtained by placing emitters around the second antinode instead of the first antinode. Depending on the detailed condition, the second-antinode device may also give more directed emission as often observed in strong-microcavity devices yet without suffering a color shift with viewing angles.

35.4: Enhancing Light Outcoupling of Organic Light-Emitting Devices by Locating Emitters around the Second Antinode of the Reflective Metal Electrode

Due to generally low conductivity and low carrier mobilities of organic materials, organic light-emitting devices (OLEDs) are typically optimized for light outcoupling by locating emitters around the first antinode of the metal electrode. In this paper, by utilizing device structures containing conductive doping, we investigate theoretically and experimentally the influences of the location of emitters relative to the metal electrode on OLED emission, and show that substantial enhancement in light outcoupling (1.2 times) or forward luminance (1.6 times) could be obtained by placing emitters around the second antinode instead of the first antinode. Depending on the detailed condition, the second-antinode device may also give more directed emission as often observed in strong-micrcavity devices yet without suffering color shift with viewing angles.

34.4L: Late-News Paper: Light Out-Coupling Enhancement of OLEDs by a Polarized Light Recycling Structure

We have realized a blue light out-coupling enhancement of OLEDs using a polarized light recycling structure. This structure gives increased blue light intensity and color purity, as well as high efficiency and a wide color gamut while suppressing the increase of ambient light reflection.