Article pubs.acs.org/journal/apchd5
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*,†,⊥,# †
Light Technology Institute, Karlsruhe Institute of Technology, Engesserstraße 13, 76131 Karlsruhe, Germany Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Department of Microsystems Engineering, Albert-Ludwigs-University, Georges-Köhler-Allee 102, 79110 Freiburg, Germany ∥ OSRAM OLED GmbH, Wernerwerkstraße 2, 93049 Regensburg, Germany ⊥ Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany # InnovationLab, Speyererstraße 4, 69115 Heidelberg, Germany
ACS Photonics 2017.4:928-933. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/24/19. For personal use only.
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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 × 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. KEYWORDS: organic light-emitting diode, internal extraction layer, screen printing, light outcoupling, nanoparticle, scattering layer, OLED
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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 multiperiodic 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
rganic 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 materials5−7 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 © 2017 American Chemical Society
Received: December 22, 2016 Published: February 24, 2017 928
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screen with a fineness of 510 lines/inch (200 lines/cm) and a line diameter of 0.025 mm was used. The maximum theoretical fluid volume is calculated to be 14 cm3/m2, which corresponds to a theoretical wet layer thickness of 14 μm. The printing layout has 5 × 5 squares each 15.8 × 15.8 mm2 (2.5 cm2 each) in size, at a distance of 9.2 mm. A working blade with a rectangular shape and a hardness of 85 shore was used, which was tilted by 15° to the surface normal. The printing speed was 100 mm/s for best homogeneity. A total volume of 5 mL was distributed at the beginning and at the end of the blading area, which covers 15 × 15 cm2. After the material was printed on the glass substrates, the plates were heated to 40 °C for 60 min in a vacuum oven to improve the layer homogeneity. Afterward, they were transferred into a nitrogen atmosphere (glovebox system) and illuminated by a mercury UV lamp at an intensity of 8 mW/cm2 to start the photoinitiation and cross-link the monomer. For a more thorough polymerization, the samples were further heated to 150 °C for 20 min to achieve solvent stability. A 130 nm thick transparent electrode made of indium tin oxide (ITO) was deposited by sputtering at room temperature using face-to-face sputtering targets to avoid high annealing temperatures, which could damage the composites. The organic layers of the white-emitting OLED stack were thermally evaporated at deposition rates between 0.01 and 0.1 nm/s at a pressure of 10−7 mbar, followed by a silver cathode with a thickness of 200 nm, and last an encapsulation glass. For white emission, a tandem architecture was used, consisting of a phosphorescent yellow emission unit (Y unit) and a fluorescent blue one (B unit) separated by a charge generation layer (CGL),33 as well as electron and hole injection and transport layers. The stack layout is shown in the inset of Figure 5c. The final OLEDs have a total area of 13.4 × 13.4 mm2 (1.8 cm2 each).
and/or UV radiation. Nanoparticles with a high refractive index contrast, such as high-index TiO2, Al2O3, or ZrO223−25 or lowindex nanoscaled 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 ≪ λ have a very limited scattering effect, but can be used to increase the effective refractive index of the matrix material.19 In combination, the embedded nanoparticles can be employed to simultaneously achieve a good spatial overlap of the (internal) modes with the scattering layer, as well as a high scattering coefficient (referred to as “haze” in the following). In this article, we report on the development of such a scattering composite layer that is printed over a 15 × 15 cm2 substrate and integrated within a WOLED stack for light extraction purposes. We first introduce a method for fabricating a high refractive index scattering material that complies with large-area screen-printing requirements. This technique was selected since it enables to cover large areas and has the ability for lateral structuring of our layers.26−30 We further discuss key parameters to achieve 10 μm thick homogeneous scattering layers and analyze their resulting optical properties. Lastly, we demonstrate that the luminous efficacy of WOLEDs can be increased by 56% following this route while preserving the desired spectral and angular invariance of a Lambertian emitter.
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EXPERIMENTAL METHODS The mixture used to print the scattering layers contained a 52.9 wt % methacrylate-based monomer system, which in turn consisted of 80 wt % ethoxylated (2) bisphenol A dimethacrylate (Sartomer SR348), 10 wt % benzyl methacrylate (BMA, Sigma-Aldrich), and 10 wt % cross-linking agent 1,3butanediol dimethacrylate (BDDMA, Sigma-Aldrich). The BMA was added in order to lower the viscosity for better nanoparticle dispersion, whereas the BDDMA was used to improve the hardness of the final layers. To increase the refractive index of the matrix material, 35.27 wt % of the total mixture contained 9-vinylcarbazole (Sigma-Aldrich).31,32 Furthermore, 8.82 wt % titanium oxide (TiO2) nanoparticles (P25, Evonik, mean primary particle diameter 21 nm) was added to further increase the mean refractive index of the extraction layer, as well as to introduce volume scattering by agglomerated nanoparticles. Further additives were 0.88 wt % 2-(2methoxyethoxy)ethoxy]acetic acid (Clariant) for particle stabilization, 1.59 wt % benzophenone (Sigma-Aldrich) for UV polymerization, and 0.54 wt % lauroyl peroxide (SigmaAldrich) for further thermal hardening. All components were mixed and dispersed with a highperformance stirrer (IKA T10 basic Ultra Turrax) for 5 min at 30 000 rpm, followed by a 1 min treatment with a sonifier (Branson Digital W450) at 40% power, 50% duty cycle, 1 s repetition time. The rheological characterization was performed with a Bohlin CVO50 rheometer (Malvern Instruments), equipped with a cone-type measuring system (diameter of 40 mm, inclination angle 4°, gap size of 150 μm) for shear rates between 1 and 200 s−1 at varying temperatures. For the preparation of the substrates, glass plates with a size of 15 × 15 cm2 and a thickness of 0.7 mm were cleaned for 10 min in acetone and 2-propanol and dried with nitrogen. Afterward, they were treated with oxygen plasma for 5 min for further cleaning and for adapting their surface energy. Screen printing was performed by using a Kammann K15 Q-SL machine. The screen was adjusted such that a film thickness of around 10 μm could be achieved. Therefore, a stainless steel
Figure 1. Viscosity measurements of the scattering layer mixture at varying temperatures.
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CHARACTERIZATION METHODS OLED Electro-optical Measurements. The luminous flux was measured using an integrating sphere coupled to a spectrometer (Instrument Systems CAS140), which was calibrated using a halogen lamp. The electrical characteristics were measured by a source-measurement unit (Keithley 2400). The current efficiencies were calculated by assuming Lambertian emission. The edges of the devices were blackened to avoid substrate mode emission. The angle-dependent 929
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Figure 2. Influence of the O2 plasma and thermal treatments on the screen-printed layer homogeneity. Photographs of the printed layer surface (a) without any treatment, (b) with 1 min O2 plasma pretreatment, (c) with 5 min O2 plasma pretreatment, and (d) after a thermal post-treatment at 40 °C.
scattering layer thickness of 10 μm was employed, because it results in a trade-off between the aforementioned requirements. Experimentally, this thickness can be varied by choosing the proper screen layout and also by adapting the viscosity of the liquid material. In Figure 1, the measured viscosity of the material is given for several shear rates and varying temperatures. As can be seen, the viscosity follows Newtonian behavior at room temperature and is constant at shear rates between 1 and 200 s−1, whereas the viscosity exhibits a strong temperature dependency. Besides, it is lowered from 20 Pa·s at room temperature to less than 1 Pa·s at 60 °C while following a more non-Newtonian behavior, giving the possibility to easily adapt the material’s rheological properties for the desired printing requirements without changing its composition. The as-deposited layer shows a very low homogeneity of the material distribution, which is notably attributed to the unfavorable surface energy of the glass substrate (Figure 2a). To ameliorate the mixture’s wetting, the glass substrates were treated with oxygen plasma for several minutes. As indicated in Figure 2b and c, the printed layer homogeneity was maximized using a pretreatment of 5 min. Longer exposure times of the substrates to the oxygen plasma did not further improve the resulting homogeneity. Since the layers still included macroscopic air voids and visible inhomogeneities directly after printing, the temperature dependency of the viscosity was additionally exploited to reflow the material and correct those defects. The improved homogeneity of the layer’s optical properties can be qualitatively appreciated in Figure 2d after a final thermal treatment in a vacuum oven at 40 °C. Following this process, a square array of identical IELs was screen-printed over a 15 × 15 cm2 substrate (see Figure 3). After cross-linking and annealing as described in the Experimental Methods section, the optical properties of the scattering films were then quantitatively assessed and revealed an overall (diffuse and specular) transmission ranging between 50% and 75% over the visible range (see Figure 4). The reduced transmission values below 400 nm originate from the enhanced parasitic absorption, as expected in TiO2-loaded acrylate polymers and the presence of the bisphenol A dimethacrylate and BMA, which contains UV-absorbing aromatic moieties. Moreover, we calculated the haze, corresponding to the ratio of the diffused transmitted light over the overall transmitted light. Values around 30% ramping up to 90% were achieved at 700 and 400 nm, respectively. Together with the mean refractive index approaching that of the ITO anode, and calculated to be 1.655, those haze values enable an efficient extraction of the WOLED guided modes, as will be demonstrated in the next section. Those values are mainly attributed to volume scattering as a consequence of the TiO2 nanoparticles introduced. Indeed, atomic force microscopy measurements (not reported here) performed over a
Figure 3. Photographic image of the complete screen-printed IEL layout.
Figure 4. Optical properties of the bare IEL layers printed on a glass substrate. The values reported were obtained by illuminating the sample from the substrate side.
emission spectra were measured using a goniometric setup (Instrument Systems Gon360). The layer morphology was characterized by an atomic force microscope (Bruker Dimension Icon). Film thicknesses were measured by a profilometer (Bruker DektakXT). The optical characterization of the scattering layers was performed using a PerkinElmer Lambda 1050 UV/vis/NIR spectrometer with an attached integrating sphere for total transmission, as well as a PerkinElmer 3D module for specular transmission. Measurements were conducted using unpolarized light.
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RESULTS AND DISCUSSION We first aimed at fabricating homogeneous volumetric scattering layers that feature a low surface roughness, high optical transmission, and appropriate haze. For that purpose, a 930
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Figure 5. Improved optoelectronic properties of the WOLEDs following the introduction of the IEL. (a) Current efficiency, (b) luminous efficacy, (c) luminance, and (d) current density of the devices with (red dots) and without (white squares) the IEL. The error bars indicate the statistics obtained from 5 OLEDs per configuration.
Figure 6. Angularly robust illumination characteristics of the WOLEDs integrating the IEL. Angular-dependent EL spectra (a) without IEL and (b) with IEL. All spectra are normalized to their respective maximum. CIE coordinates of the devices (c) without IEL and (d) CIE coordinates with IEL. The arrows indicate an increasing viewing angle from 0° to 75°. The black line indicates the Planckian locus.
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30 × 30 μm2 area highlighted a root-mean-square value of only 23 nm, leading to a limited light scattering by the surface roughness. The good planarity of the IELs allowed for the deposition of the complete WOLED stacks. The OLED characteristics were measured according to the methods presented in the Characterization Methods section Figure 5a and b show the current and luminous efficiencies of the devices with and without the IELs. The reference devices without IELs show efficiencies of 35 lm/W (68 cd/A). Their performance could be significantly improved by the scattering layer, whereby the mean efficiency was increased to 57 lm/W (102 cd/A) at a luminance of 2000 cd/m2. Hence, a relative enhancement of 56% was demonstrated for both cd/A and lm/W efficiencies, indicating an efficient waveguide and substrate mode outcoupling. Furthermore, as displayed in Figure 5c, the onset voltage was kept constant at 4.75 V. Lastly, Figure 5d shows the current−voltage characteristics, where only a slight increase of current density is observed for the IEL-based WOLEDs. This can be explained by the surface roughness of the scattering layers that leads to a slightly increased surface area of the anode and therefore to a larger amount of injected charge carriers. However, this effect plays a minor role only, since the current and luminous efficiencies do not show a stronger roll-off and the enhancement factors for luminous and current efficiencies are at the same values. By taking into account the current densities below the onset voltage (inset in Figure 5d), we furthermore conclude that no increased leakage currents occur in the devices. To gain further insights into the impact of the IEL on the WOLED illumination characteristics, electroluminescence (EL) emission spectra were acquired over a broad range of viewing angles (Figure 6a and b). All spectra are normalized to their respective maximum. As clearly seen in Figure 6a, the reference devices exhibit strong angular spectral variations, which are attributed to thin film interferences. This becomes even more obvious by analyzing the CIE1931 color space plots in Figure 6c. For the devices without IELs a strong color shift from warm-white/red to a more blue-shifted white is observed with increasing viewing angle, as expected from white emitters in a weak microcavity. In contrast, this angular dependency is almost suppressed by introducing the IELs. As evidenced in both Figure 6b and d, a more stable spectrum is achieved, and the emission fits to the blackbody spectrum at a color temperature of 3065 K and a color rendering index of 70.
It should be noted that this process can easily be integrated in large-scale roll-to-roll processes for flexible WOLEDs. Further improvements could be achieved by using higher nanoparticle contents for better refractive index adjustments, and therefore an even more improved efficiency, or by further improving the scattering properties by using dual-size nanoparticle composites.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Uli Lemmer: 0000-0001-9892-329X Author Contributions ¶
J. B. Preinfalk and T. Eiselt contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial supported by the German Federal Ministry of Education and Research (BMBF) under the contract FKZ 13N12240 (project “OLYMP”). J.B.P. acknowledges the financial support of the Karlsruhe School of Optics & Photonics (KSOP). G.G. gratefully acknowledges funding from the Helmholtz Postdoc Programme.
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CONCLUSION In this study, we presented a universal and up-scalable method to fabricate internal extraction layers for improved light outcoupling in OLEDs. A material consisting of high refractive index TiO2 nanoparticles embedded into a multicomponent polymer host was presented. The composition matched the requirements necessary for a large-area layer deposition via screen printing and resulted in a high degree of volumetric light scattering. Additional substrate treatment measures were implemented to improve the layer wetting and reflow for a maximum spatial homogeneity. An internal extraction layer was printed over an area of 15 × 15 cm2 using single pixel sizes of 2.5 cm2 each. With a white OLED stack deposited on top, a relative efficiency enhancement of 56% was achieved without introducing electrical defects. Lastly, it was shown that the introduction of scattering layers leads to a better angular stability of the emission spectra. 932
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