Byoung-Ki Choi

High-efficiency stacked white organic light-emitting diodes

We report efficient tandem white organic light-emitting diodes (WOLEDs) by using bathocuproine:Cs2CO3∕MoO3 as an effective interconnecting layer. We utilized two primary colors of sky blue and orange fluorescent emitters to obtain efficient white electroluminescence. Although single WOLEDs using two adjacent emitting layers showed a maximum current efficiency of 7.96cd∕A with Commission Internationale d’Eclairage (CIE) coordinates of (0.28, 0.34), the tandem WOLED device made by stacking two single color OLEDs in series demonstrated doubled maximum current efficiency of 17.14cd∕A with CIE coordinates of (0.28, 0.41). The stacking of different single color OLEDs in series instead of double stacking of WOLEDs can be useful to achieve highly efficient WOLEDs because it can reduce the number of layers of the devices.

Small molecule based and solution processed highly efficient red electrophosphorescent organic light emitting devices

The authors report the very high luminous efficiency in solution processed red electrophosphorescent organic light emitting devices using small molecular host and guest materials. The maximum luminous efficiency reached 12.7cd∕A, corresponding to external quantum efficiency of 15.1%, with its emission peak wavelength of 620nm and the Commission Internationale de l’Eclairage coordinates of (0.65, 0.33). Along with these excellent performances of the solution processed device, which were comparable to those of the vacuum deposited counterpart device with similar structure and materials, the comparative study on both devices suggests the merits of the solution process adopting robust small molecular materials only.

A stable blue host material for organic light-emitting diodes

We have developed a high performance fluorescent host material, 6-anthracene-9-yl-2,3-di-p-tolyl benzo[b]thiophene (ATB), for blue organic light-emitting diodes. ATB formed a stable amorphous solid state film with a high glass transition temperature (Tg=116°C). The multilayer devices fabricated using ATB as a blue host material showed higher power efficiency (6.4lm∕W at 1000cd∕m2) than the conventional device using 2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN) (4.3lm∕W at 1000cd∕m2). The half lifetime of the ATB device (6480h at 1000cd∕m2) was also enhanced compared to the TBADN device (5341h at 1000cd∕m2).

Enhancing the Kinetic Stability and Lifetime of Organic Light‐Emitting Diodes based on Bipolar Hosts by using Spiroconjugation

We examined how to enhance the lifetime of organic light-emitting diodes (OLEDs) based on bipolar host molecules ET-HT, where ET and HT refer to electron- and hole-transporting units, respectively, by analyzing their thermodynamic and kinetic stabilities. Our DFT calculations reveal that the thermodynamic stability of ET-HT is determined by that of its anion, which is difficult to improve by chemical modifications of ET and HT. The kinetic stability of ET-HT can be enhanced by the spiroconjugation between ET and HT, which occurs when their π-frameworks are extended and have an orthogonal arrangement. Green OLED devices were fabricated by using ET-HTs with and without spiroconjugation, to find that the device with spiroconjugation has a lifetime that is approximately 6 times longer than the one without spiroconjugation.