Mi Jin Park

Small single–triplet energy gap bipolar host materials for phosphorescent blue and white organic light emitting diodes

We report extremely high efficiency tandem white organic light-emitting diodes (OLEDs) with newly synthesized host materials. Our new host materials have well balanced bipolar characteristics and very small singlet to triplet splitting energies (∼0.4 eV) due to almost no orbital overlapping between ground and excited states. The fabricated blue phosphorescent device with one of these host materials shows very high external quantum efficiency of 25.7%, low onset and driving voltages of 2.47 V and 3.49 V at 1 and 1000 cd m−2, respectively. High performance asymmetric tandem OLEDs could be fabricated by adopting these efficient and low driving voltage blue unit and very efficient yellow unit devices. Power efficiency of 63.1 lm W−1 and current efficiency of 128.8 cd A−1 at 1000 cd m−2 were achieved without any light out-coupling technology. We expect that blue phosphorescent OLEDs with our new narrow band-gap bipolar host materials could be a promising solution to make high efficiency white OLEDs for display and lighting applications.

High efficiency red top-emitting micro-cavity organic light emitting diodes

In this study, we present optical simulation versus real fabricated device results in the micro-cavity red top-emitting organic light emitting diodes (TEOLEDs). The optical simulation results indicate that the two kinds of possible emissive layer (EML) positions exist in the second order micro-cavity effect and each EMLs could emit the similar radiance with near National Television System Committee (NTSC) color coordinate. Expected current efficiency and external quantum efficiency by the optical simulation toward the surface normal in the red tandem TEOLED are 98.8 cd/A and 22.6% for two EMLs, while fabricated device shows 95.8 cd/A and 26.5%, respectively.

Efficient micro-cavity top emission OLED with optimized Mg:Ag ratio cathode

Micro-cavity top-emitting organic light emitting diodes (TEOLEDs) are now receiving prominence as a technology for the active matrix display applications. The semi-transparent metal cathode plays the crucial role in realizing TEOLEDs structure. Here, we report the optimization results on Mg:Ag ratio as the semitransparent cathode deposited by vacuum thermal evaporation. The optimized Mg:Ag cathode with 1:10 ratio (wt %) shows a sheet resistance value as low as 5.2 Ω/□, an average transmittance of 49.7%, reflectance of 41.4%, and absorbance of 8.9% over the visible spectral region (400~700 nm). The fabricated red TEOLEDs device implemented using LiF (1nm)/Mg:Ag (1:10) cathode shows the voltage value of 4.17 V at a current density of 10.00 mA/cm2, and current efficiencies variation from 55.3 to 50.1 cd/A over the brightness range 2,000 - 12,000 cd/m2. The electroluminescence (EL) spectrum displays the light emission at 608 nm wavelength with a half width of 29.5 nm. The narrow half-width of red light emission is attributed to the micro-cavity effects due to the semitransparent cathode.

Low Driving Voltage and High Efficiency Blue Phosphorescent OLEDs with Mixed Host System

We report both low driving voltage and high efficiency characteristics in blue phosphorescent organic light-emitting diodes with a simple device structure. A mixed host system consisting of narrow band-gap host material of 4,4’,4’’-tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting material of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) shows low driving voltage performances with high luminance stability. A very low onset voltage of 3.0 V and a driving voltage of 3.8 V to obtain a brightness of 1000 cd/m2are achieved with a current efficiency of 44.2 cd/A.