Highly efficient organic light-emitting diodes with enhanced outcoupling efficiency

Abstract

This dissertation discusses on the efforts to increase the optical outcoupling efficiencies of organic light-emitting diodes (OLEDs), one of the most significant performance bottlenecks constraining the ultimate poten-tials of OLEDs as future displays and lightings. Many researches have been focusing on improving device effi-ciency - and there have been significant advances especially in developing intrinsically efficient light-emitting and carrier transporting organic materials, establishing efficient charge injection contacts and interfaces, and optical optimization of the device structure. Even though those efforts partially enabled OLEDs with higher opti-cal efficiency, there is still a significant amount of light trapped within OLEDs, which is even several times larger than the light being actually outcoupled. Hence, finding ways to enhance optical outcoupling efficiency of OLEDs has recently been widely studied - to realize OLEDs with efficiencies close to the theoretical limits. In the first part of the dissertation, various OLED device structures for higher optical outcoupling effi-ciency are proposed and verified. At the beginning, a novel anode structure consisting of a conductive low-index layer and a micropatterned indium tin oxide (ITO) layer is introduced. Refractive index contrast between the conductive low-index layer and the surrounding ITO/organic layers allows a certain portion of light trapped in ITO/organic layers to be outcoupled into air by disturbing the waveguided modes. Bottom-emitting OLEDs with this novel anode structure show ~25% enhanced outcoupling efficiency, compared to the conventional planar OLEDs. Further optimization of a fabrication process leading to a favorable taper angle of an ITO micro-structure enables even larger enhancement up to 50%. Another concept, micro-mesa structure is also proposed for enhancement of outcoupling efficiency in top-emitting OLEDs, and it is theoretically and experimentally verified. Moving on ...