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Large-area screen-printed internal extraction layers for organic light emitting diodes Jan B. Preinfalk, Thomas Eiselt, Thomas Wehlus, Valentina Rohnacher, Thomas Hanemann, Guillaume Gomard, and Uli Lemmer ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b01027 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017
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ACS Photonics
Large-area screen-printed internal extraction layers for organic light emitting diodes Jan B. Preinfalk1, Thomas Eiselt2,3, Thomas Wehlus4, Valentina Rohnacher1, Thomas Hanemann2,3, Guillaume Gomard1,5,* and Uli Lemmer1,5,6,* 1
Light Technology Institute, Karlsruhe Institute of Technology (KIT), Engesserstr. 13, 76131
Karlsruhe, Germany, 2
Institute for Applied Materials, KIT, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-
Leopoldshafen, Germany, 3
Dept. of Microsystems Engineering, Albert-Ludwigs-University, Georges-Köhler-Allee 102,
79110 Freiburg, Germany, 4
OSRAM OLED GmbH, Wernerwerkstrasse. 2, 93049 Regensburg, Germany,
5
Institute of Microstructure Technology, KIT, Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany, 6
InnovationLab, Speyererstr. 4, 69115 Heidelberg, Germany
KEYWORDS: Organic light-emitting diode, internal extraction layer, screen printing, light outcoupling, nanoparticle, scattering layer, OLED.
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ABSTRACT
To unleash the full potential of white organic light emitting diodes (OLEDs) as large-area light sources, guided optical modes have to be efficiently outcoupled which calls for internal extraction layers (IELs) that can be easily integrated into a scalable manufacturing process. To realize such IELs, we developed a high refractive index scattering polymer:TiO2-nanoparticle mixture that can be deposited onto a large area by using the cost-effective screen-printing method. We exploited this approach to produce a 10 µm thick IEL covering the exact area of active pixels distributed over a 15 x 15 cm² glass substrate. By optimizing the initial mixture composition, we achieved screen printing-compatible rheological properties as well as tailored light scattering and transmission over the visible spectrum. The spatial homogeneity of those optical properties was obtained by additional substrate treatments to improve the wetting behavior and to allow reflow after printing. The devices were finalized by depositing a high efficiency white OLED stack atop the IEL. We demonstrated a luminous efficacy increase up to 56% due to the scattering layer. The IEL also ensured a Lambertian emission profile without any angular color shift.
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ACS Photonics
Organic light emitting diodes (OLEDs) have been the subject of intense research efforts in industry and academia over the last years.1–4 Indeed, their unique thin film architecture allows new design principles such as large-area luminaires in flexible and transparent devices. Moreover, the device efficiency has been drastically increased over the years and a wide range of materials
5–7
and colors can be realized which enables display applications with a wide color
gamut and contrast, as well as lighting applications with excellent color quality and rendering. Even though the internal quantum efficiency of the emitting materials could be increased up to 100% in the last years,8 the external quantum efficiency of OLEDs as such is limited to around 20%. The weak light extraction stems from the coupling of the generated photons with substrate, waveguide and surface plasmon-polariton modes and subsequent parasitic absorption. 9
Photons propagating in those internal modes confined within the thin film stack can be outcoupled by introducing periodic Bragg gratings, yet this approach results in spectral and angular dependent properties and as such, is not suited for white OLEDs (WOLEDs) aiming at a homogeneous illumination.10,11 These shortcomings are addressed by using multi-periodic gratings12–14 or randomly disordered structures.15–18 Nevertheless, the obtained optical benefits come at the expense of the complexity of the necessary implementation in a high-throughput manufacturing process. An alternative up-scalable, cost-effective and industrially relevant technique consists in using polymer-nanoparticle composite layers19,20 that can be processed using screen printing, inkjet printing or slot-die coating.21 Herein, a high refractive index contrast between the matrix polymer and scattering particles is necessary to achieve a strong volumetric scattering effect.17,22 Typical materials for the polymeric matrix are cross-linkable acrylates, which, initially found as a monomer in solution phase, can be polymerized and cross-linked by heat and/or UV radiation. Nanoparticles with a high refractive index contrast, such as high-index
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TiO2, Al2O3 or ZrO223–25 or low-index nano-scaled air voids17 can be embedded to introduce scattering, if their diameter d is comparable to the OLED emission wavelength λ. On the other hand, small nanoparticles with d
