Improving the efficiency of organic light-emitting diodes using textured polymer foils fabricated by R2R hot-embossing method

LASAGNI, ANDRÉS F.; LIU, YUAN; SOLDERA, MARCOS; LEO, KARL; RANK, ANDREAS; LANG, VALENTIN

Darmstadt

Conferencia; Materials Science and Engineering; 2018

DGM

Recently, a lot of effort has been put in the research and development of organic light-emitting diodes (OLEDs), which can be used e.g. for the realization of high efficiency color displays. However, the overall efficiency is strongly limited by the low light outcoupling efficiency because only a small fraction of light can leave the multilayer structure due to the large difference in refractive index between air, glass and organic layers, which produce guided modes and plasmon coupling loss at the metal cathode. In a typical optimized OLED only about 20% of the light is emitted directly into air and roughly the same amount is coupled to the glass substrate.One path to couple the guided modes to useful far field radiation is using periodic gratings in the OLED front surface, which also reduces the power loss associated to surface plasmon modes. A process capable of producing suitable micro and nanostructures in a fast and cost efficient way is Roll-to-Roll Nanoimprint Lithography (R2R-NIL). Nevertheless, fabrication of large area seamless structured R2R tools, such as metallic sleeves, with traditional techniques like electron beam lithography (EBL) is still a challenge due to the 3D shape of the tools as well as the long processing time. A method capable of producing surface structures with high resolution and quality in a cost efficient way is Direct Laser Interference Patterning (DLIP). For the improved OLED hole like interference patterns with spatial periods smaller than 3.0 µm and structure depths of up to 400 nm are fabricated on nickel sleeves employing the DLIP method in combination with a picosecond (ps) laser source. The sleeves have a width and a diameter of 300 mm. The DLIP processed sleeves are later used in a R2R-Thermal-NIL unit to transfer the produced micro pillars on polyethylene terephthalate (PET) polymer foils. Pillar like structures were produced with web speeds up to 30 m/min.The produced OLED consists of a stack of AlL, OLEDorganic transport and emitting layers, ITO, glass and the structured foil on top to improve the outcoupling efficiency. In this way the outcoupling efficiency for the OLED can be improved by at least 6% compared to the one without the structured foil.We also studied numerically flat and structured OLEDs to identify and quantify the loss mechanisms that limit the device outcoupling efficiency. The simulations are in good agreement with the experimental results and allow us to further optimize the pattern geometry.Acknowledgement: This Work was supported by the German Research Foundation (DFG), Excellence Initiative by the German federal and state governments to promote top-level research at German universities (Grant No.: F-003661-553-71A-1132104).