Nanostructured, ITO-free electrodes for OLED emission control
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Nanostructured, ITO-free electrodes for OLED emission control Arfat Pradana, Christian Kluge, and Martina Gerken Institute of Electrical and Information Engineering, Christian-Albrechts-Universität zu Kiel, Kaiserstr. 2, D-24143 Kiel, Germany
ABSTRACT We demonstrated nanostructured, ITO-free anodes in flexible OLEDs using a combination of a composite organic-inorganic UV nanoimprint resist and a conductive, transparent polymer layer. Flexible OLEDs with grating anodes were fabricated on polycarbonate substrates. The nanoimprint resist was blended with 30% TiO2 nanoparticles in order to achieve a sufficient refractive index contrast to the polymer anode. It was periodically structured with a 370-nm period linear photonic crystal structure. PEDOT:PSS was spin-coated on as a polymer anode and structured in an oxygen plasma treatment. For OLED demonstration an organic emission layer (PPV-derivative “Super Yellow”) and a metal cathode (LiF/Al) were deposited. We observed successful waveguide mode extraction both in electroluminescence and photoluminescence for flat and bend substrates. The waveguide mode extraction angle varied slightly under bending. The combination of an inorganic-organic composite material with a conductive polymer transparent electrode is promising for improving the performance of ITOfree, flexible OLEDs. INTRODUCTION Nanostructuring the thin-film layer stack of an organic light-emitting diode (OLED) is a promising approach for emission control [1, 2]. Emission efficiency may be improved and light may be guided to specific emission angles using a periodic grating nanostructure [3-6]. The most common approach in fabricating nanostructured OLEDs is nanostructuring the indium tin oxide (ITO) layer [4, 5]. The refractive index contrast between ITO and the commonly employed subsequent PEDOT allows for efficient grating scattering. Due to the mechanical rigidity of ITO, other transparent electrode materials need to be considered for mechanically flexible, nanostructured OLEDs. We demonstrated previously an ITO-free OLED employing a conductive PEDOT:PSS polymer anode fabricated on a Ta2O5 high refractive index layer [6]. Furthermore, we have introduced an organic-inorganic composite nanoimprint resist obtained by blending TiO2 nanoparticles into a polymer nanoimprint resist [7]. This yields an inorganicorganic composite material with an increased optical refractive index suitable for nanoimprint lithography. Here, we present a flexible, nanostructured OLED on a polycarbonate substrate using a combination of the organic-inorganic composite nanoimprint resist and a PEDOT:PSS polymer anode. The combination of the inorganic-organic composite material with a conductive
polymer transparent electrode is a promising approach for improving the performance of ITOfree, flexible OLEDs. The conductive polymer provides the electrical conductivity and the composite material may be nanostructured and provides the high refractive index necessary for emission control. Thus, the combination of the two materials may be used
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