Yongwon Kwon

Light outcoupling enhancement from top-emitting organic light-emitting diodes made on a nano-sized stochastic texture surface

An effective method for enhancing the light outcoupling efficiency from top-emitting organic light-emitting diodes (TEOLEDs) with a nano-sized stochastic texture surface (NSTS) is suggested. The broadly distributed pitch and the randomly sized of islands in the NSTS enable the photons that are otherwise trapped to be emitted over the broad emission wavelength range. The NSTS-embedded TEOLEDs have wide angular-dependent emission characteristics and an enhanced external quantum efficiency (EQE). Theoretical and full-wave optical calculations were performed to understand the mechanisms of the efficiency enhancement. Optimized TEOLEDs achieved a 32% EQE enhancement compared with the reference devices without the NSTS.

Non-interlayer hybrid white organic light-emitting diodes via a bipolar mixed host for the blue-fluorescent-emitting layer

ABSTRACT The reduction of the interlayer in hybrid white organic light-emitting diodes (WOLEDs) provides a potential for improving the device efficiency. Reported herein are non-interlayer hybrid WOLEDs prepared using a mixed host for the blue-fluorescent-emitting layer. The proposed device exhibits high efficiency with the maximum external quantum efficiency up to 17.1%, and stable colors. Moreover, the effect of the interlayer on the device lifetime was demonstrated.

Light out-coupling efficiency enhancement in organic light-emitting diodes using a multilayer stacked electrode with sol-gel processed Ta 2 O 5

We demonstrate enhanced light out-coupling efficiency of organic light-emitting diodes by applying a multilayer stacked electrode structure consisting of fast and cost-effective sol-gel processed tantalum pentoxide (Ta2O5), thin layer of Au and molybdenum trioxide (MoO3). The application of the Ta2O5/Au/MoO3 electrode can modulate the optical characteristics of the device due to the optical microcavity effect. The refractive index of the sol-gel processed Ta2O5 thin film varied depending on the annealing temperature and reached a maximum at 400 °C (n = 2.2 at 512 nm). The influence of the refractive index of the Ta2O5 layer and the thickness of the multilayer electrode stack on the optical microcavity effect was systematically investigated. The device with the Ta2O5/Au/MoO3 electrode, fabricated at an optimum condition based on the simulation result by calculating the photon flux, exhibited 52% enhancement in light out-coupling efficiency at 1000 cd/m2 and improved color stability with the viewing angle, having near-Lambertian emission.

Increased light extraction efficiency from top-emitting organic light-emitting diodes employing a mask-free plasma-etched stochastic polymer surface

Top-emitting organic light-emitting diodes(TEOLEDs) are drawing interest as future devices for both high-quality display and lighting. However, the currentTEOLEDs have external quantum efficiencies of approximately 20%, which still need improvement. To attain high device efficiencies in TEOLEDs, waveguide and surface plasmon polariton modes should be minimised. Many efforts have been made using nano- or micrometre-scale periodic gratings to extract the confined photons. However, significant angular distortion or colour shifts occur as the Bragg condition calls for. Here, an effective method is demonstrated for enhancing the light extraction efficiency from TEOLEDs employing a mask-free plasma-etched polymer surface with stochastically distributed nano-hemispheres. TEOLEDs with the stochastic polymer surface allowed us to achieve a device efficiency enhancement up to 1.55-fold relative to conventional devices without introducing spectral changes and angular emission distortion, unlike periodic grating-embedded devices. Because of the independence of the improvement from the particular emission wavelengths, the proposed TEOLEDs are attractive and practical for use in full colour and white lighting as well as display applications.