Young Pyo Jeon

Blue phosphorescent organic light-emitting devices based on carbazole/thioxanthene-S,S-dioxide with a high glass transition temperature

Carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) bipolar molecules were synthesized by incorporating electron-donating carbazole and electron-accepting thioxanthene-S,S-dioxide into one molecule at 85 °C using a solventless green reaction method. The EBCz-ThX bipolar molecule exhibited a high glass transition temperature of 227 °C and a high triplet energy of 2.94 eV. The highest occupied molecular orbital (HOMO, −5.95 eV) and lowest unoccupied molecular orbital (LUMO, −2.63 eV) energy levels of the molecule matched the HOMO energy level of the hole transporting layer and the LUMO energy level of the electron transporting layer, respectively, which facilitated the injection of holes and electrons. Phosphorescent organic light-emitting devices (PHOLEDs) were fabricated with EBCz-ThX as a blue host material and with bis((4,6-difluorophenyl)-pyridinate-N,C2′)picolinate as a blue dopant, and the doping ratio was controlled to optimize the performance of the blue PHOLEDs. Blue PHOLEDs with a 15% doping ratio at 100 cd m−2 showed excellent performances and low operation voltages with quantum and current efficiencies of about 12% and 24 cd A−1, indicating that the EBCz-ThX acted as an excellent host for the blue PHOLEDs. The external quantum efficiency at 1000 cd m−2 was improved up to 30% in comparison with that of a conventional PHOLED with an mCP of 8% doping ratio.

Electrical and optical properties of blue organic light-emitting devices fabricated utilizing color conversion CdSe and CdSe/ZnS quantum dots embedded in a poly(N-vibyl carbazole) hole transport layer

Blue organic light-emitting devices (OLEDs) with color conversion quantum dots (QDs) embedded in a poly(N-vinyl carbazole) (PVK) hole transport layer (HTL) were fabricated. The absorbance and the photoluminescence spectra for the CdSe and the CdSe/ZnS QDs showed dominant exciton peaks. Current densities as functions of the voltage showed enhanced hole trapping and a decreased hole current in the OLEDs containing CdSe and CdSe/ZnS QDs embedded in a HTL. The phenomena were intensified due to the existence of the ZnS shell. The luminance-voltage curve and the electroluminescence spectra showed that the brightness of the blue OLEDs fabricated utilizing the HTL based on CdSe and CdSe/ZnS QDs embedded in a PVK layer reached over 3,000 cd/m2 and that the dominant exciton peak was shifted to longer wavelength.

Enhancement of the Luminance Efficiency due to an Embedded Self-Assembled Monolayer in Organic Light-Emitting Devices

The electrical and optical properties of organic light-emitting devices (OLEDs) with and without a self-assembled monolayer (SAM) were investigated, and the effect of an embedded SAM was clarified. The contact angles of COOH-terminated alkanethiol SAMs were smaller than those of CH3-terminated alkanethiol SAMs. The operating voltage of OLEDs with COOH-terminated alkanethiol SAMs was 4.4 V lower than that with CH3-terminated alkanethiol SAMs. The luminance of OLEDs with COOH-terminated alkanethiol SAMs was higher than that with CH3-terminated alkanethiol SAMs. The enhancement of the luminance efficiency of OLEDs with COOH-terminated alkanethiol SAMs was attributed to the increase in hole injection due to a decrease in the chain length of the COOH-terminated alkanethiols, resulting in a better balance between the numbers of holes and electrons at an emitting layer in the OLEDs.