Isao Takasu

A host material consisting of a phosphinic amide directly linked donor–acceptor structure for efficient blue phosphorescent organic light-emitting diodes

A host material with a high triplet excited-state energy level and a deep HOMO energy level has been developed by directly linking the donor and acceptor units through a phosphinic amide. Phosphorescent organic light-emitting diodes containing this material and a deep-blue dopant achieved an external quantum efficiency of 19.7%.

LED packaging by ink-jet microdeposition of high-viscosity resin and phosphor dispersion

— An ink-jet-printing method applied to the microdeposition of high-viscosity resin, including optimization of phosphor dispersion for light-emitting-diode (LED) packaging was examined for the first time. An ultrasonic ink-jet-printing method was used, in which ink droplets are ejected by a focused ultrasonic beam from a nozzle-less printhead. To fabricate white LEDs, high-viscosity phosphor-dispersed resin was deposited to form an encapsulant dome. Two types of methods to control phosphor sedimentation for color uniformity were examined; one is heating the lead frame during the resin deposition, and the other is hydrophobic surface treatment of the lead frame base enabling the fabrication of a small encapsulant dome. For light direction control, a silicone micro lens was deposited on an encapsulant dome using the ink-jet method. The results show that ultrasonic ink-jet printing is an applicable technique to optimize and modify on-demand optical characteristics of LED devices.

Comparison of transient state and steady state exciton–exciton annihilation rates based on Förster-type energy transfer

We investigated differences between the transient state and steady state exciton–exciton annihilation rates based on Forster-type energy transfer. The exciton–exciton annihilation rate of an organic semiconductor is usually determined by transient state photoluminescence measurements using a pulsed laser or steady state photoluminescence measurements using a continuous wave laser. However, it is unclear that the respective annihilation rates determined by their rate equations are the same. In calculations with platinum-octaethylporphyrin (PtOEP) parameters, Monte Carlo simulations gave two different annihilation rates for the transient state and the steady state. The analytical models based on Forster-type energy transfer also showed the same result. These results indicate that the exciton–exciton annihilation rates in transient state and steady state are distinguished.

25.4: High-Brightness Large-Area White OLED Fabricated by Meniscus Printing Process

We demonstrated high-brightness large-area, white organic light-emitting diode (OLED) consisting of printing-processed organic semiconductor layers. Meniscus printing process was applied to the substrate with 2 μm-high stripe-shape auxiliary electrodes. The OLED panel showed white emission all over the whole emitting area of 58 mm × 52 mm, high average luminance of 10,000 cd/m2, luminance uniformity of 40 %, and high luminous flux of 95 lm.

54.1: LED Packaging by Ink‐jet Microdeposition of High‐Viscosity Resin and Phosphor Dispersion

The applicability of an ink-jet printing method to microdeposition of high-viscosity resin and phosphor dispersion for light-emitting-diode (LED) packaging was examined for the first time. An ultrasonic ink-jet printing method was used, in which ink droplets are ejected by focused ultrasonic beam from nozzle-less printhead. for fabricating white LEDs, high-viscosity phosphor-dispersed resin was deposited to form an encapsulant dome, and hydrophobic surface treatment of the lead frame substrate enabled fabrication of a small encapsulant dome to improve color uniformity. A silicone microlens was deposited on an encapsulant dome using the ink-jet method for light directivity control. The results show the ultrasonic ink-jet printing is an applicable technique to optimize and modify on demand optical characteristics of the LED devices.

Effect of horizontal molecular orientation on triplet-exciton diffusion in amorphous organic films

Triplet harvesting is a candidate technology for highly efficient and long-life white OLEDs, where green or red phosphorescent emitters are activated by the triplet-excitons diffused from blue fluorescent emitters. We examined two oxadiazole-based electron transport materials with different horizontal molecular orientation as a triplet-exciton diffusion layer (TDL) in triplet-harvesting OLEDs. The device characteristics and the transient electroluminescent analyses of the red phosphorescent emitter showed that the triplet-exciton diffusion was more effective in the highly oriented TDL. The results are ascribed to the strong orbital overlap between the oriented molecules, which provides rapid electron exchange (Dexter energy transfer) in the TDL.

Fluorinated Poly( N -vinylcarbazole) Host for Triplet Energy Confinement on Phosphorescent Emitter in Organic Light-emitting Diodes

Fluorinated carbazoles as host materials have been investigated for highly efficient organic light emitting diodes (OLEDs). By molecular orbital calculations, we found that fluorinations at position 2, 4, 5 and 7 of carbazole ring were effective for widening HOMO-LUMO energy gap. The energy gaps of our synthesized 2,7-difluorocarbazole (F2-Cz) and 2,4,5,7-tetrafluorocarbazole (F4-Cz), were estimated to be 3.71 eV and 3.87 eV by the absorption spectra, respectively. These energy gaps were higher than that of the non-substituted carbazole (Cz, 3.59 eV). We synthesized poly( N -vinyl-2,7-difluorocarbazole) (F2-PVK) and poly( N -vinyl-2,4,5,7-tetrafluorocarbazole) (F4-PVK) as solution processable polymer host materials. However, the F4-PVK was found to be an unsolved polymer. The F2-PVK could be compared with non substituted poly( N -vinylcarbazole) (PVK) in OLEDs. The emission layer (EML) contained iridium(III) bis [(4,6-di-fluorophenyl)-pyridinato- N,C2 ′] picolinate (FIrpic) as a blue phosphorescent dopant, and iridium(III) bis [2-(9,9-dihexylfluorenyl)-1-pyridine] acetylacetonate as a yellow dopant. The white OLED with the F2-PVK showed 1.4 times higher luminous current efficiency (24 cd/A) than the PVK (17 cd/A). These data show that the excitation energy is confined on dopants by using fluorinated polymer host material with higher T1 corresponding to wider HOMO-LUMO energy gap.