Improving the outcoupling efficiency of indium-tin-oxide-free organic light-emitting diodes via rough internal interfaces

Light Technology Institute, Karlsruhe Institute of Technology, Kaiserstrasse 12, D-76131 Karlsruhe, Germany.
Optics Express (Impact Factor: 3.49). 11/2010; 18 Suppl 4(23):A631-9. DOI: 10.1364/OE.18.00A631
Source: PubMed


We present low-cost texturing methods to produce different surface roughnesses on glass substrates. Using sand blasting, abrasion and wet etching we achieve roughnesses of about 50 nm to 250 nm (root mean squared roughness Rq). These textured substrates are used as extraction elements for guided modes and substrate modes in organic light-emitting diodes (OLEDs). We evaporate 50 nm of the high index material Ta₂O₅ on the textured substrate, which acts as waveguide layer, and flatten it with the transparent photoresist SU-8. On top of that, we fabricate indium tin oxide (ITO)-free OLEDs, which are characterized by electroluminescence and photoluminescence measurements. The devices with rough interfaces obtain an up to 37.4% and 15.5% (at 20 mA/cm²) enhanced emission and it is shown that the enhancement is due to an increased outcoupling efficiency.

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Available from: Martina Gerken, Aug 26, 2014
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    • "About 50% of light gets trapped in ITO (n ito = 1.8 + 0.01i)/Glass (n glass = 1.51) interface and 30% is trapped in Glass (n glass = 1.51)/air (n air = 1) interface by TIR [9]. So far lots of attempts were reported to reduce TIR in this interface, such as using rough internal interfaces [10], high index substrate with micro-lenses array [11] [12] or photonic crystals [13–15]. But using quasi-photonic crystals in OLEDs haven't been studied yet, of course in recent years, there are some reports of applying these structures to GaN leds to increase EQE [16] [17] [18]. "
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    ABSTRACT: In this paper, our main purpose is to reduce Total Internal Reflection (TIR) phenomenon that is happening at Indium Tin Oxide (ITO, n(ito) = 1.8 + 0.01i) and Glass (n(glass) = 1.51) interface in WOLED to achieve higher output power. In normal WOLED because of higher refractive index contrast between ITO and Glass, guided waves usually limited into ITO layer and approximately 50% of generated light is trapped into ITO layer. Here, we tried to reduce this portion of trapped light by using 12-fold quasi-photonic crystal in mentioned interface. With some gentle changes in 12-folds structure, we could reduce TIR in this interface to less than 9%. Also, far field results before and after adding the structure were studied, which represents success of this idea and will open a new insight to lightning applications. (C) 2013 Published by Elsevier GmbH.
    Full-text · Article · Nov 2013 · Optik - International Journal for Light and Electron Optics
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    ABSTRACT: Bragg gratings incorporated into organic light-emitting diodes (OLEDs) establish a coupling between waveguide modes and useful light (leaky modes). Here we demonstrate that the net coupling direction depends on the OLED stack design. We fabricated two different device structures with gold Bragg gratings. Angle resolved electroluminescence spectra were recorded. For the first device peaks of enhanced emission due to the Bragg grating are observed corresponding to a net energy transfer in direction of the leaky modes. The second device, on the other hand, exhibits dips in the emission spectrum. This reversed direction of energy transfer from the leaky modes to the waveguide modes is explained considering transfer matrix simulations of modal intensity distributions and device emission simulations. An OLED efficiency enhancement is only achieved, if the waveguide mode extraction is dominant.
    Full-text · Article · Jul 2011 · Optics Express
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    ABSTRACT: We used laser interference lithography to fabricate organic light-emitting diodes (OLEDs) with gold Bragg gratings on top of the indium-tin-oxide layer for efficient light management. We built polymer OLEDs with 15 and 30 nm gold grating thickness as well as reference devices with continuous gold layers and without gold layer. Electrical and optical device characterization was performed and compared to optical simulations. The gratings extract waveguide and substrate modes and change the angular and spectral emission characteristics by cavity effects. The combination of light extraction and cavity effects leads to 25-30% enhancement in power efficiency compared to reference devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3631728]
    Full-text · Article · Sep 2011 · Applied Physics Letters
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