Yuichi Hino

Red Phosphorescent Organic Light-Emitting Diodes Using Mixture System of Small-Molecule and Polymer Host

We study two types of polymer materials, poly(n-vinylcarbazole) (PVK) and starburst small-molecule 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB), as the host for electrophosphorescent organic light-emitting diodes (PHOLEDs) doped with red-emitting phosphor tris(1-phenylisoquinoline) iridium (III) [Ir(piq)3]. PHOLEDs employed a PVK and TDAPB blend as the host exhibited a maximum red light emission at a wavelength of 630 nm. The external quantum efficiency of 6.3% and power efficiency of 3.0 lm/W have been achieved. The electroluminescent (EL) spectra were not changed at high current and the luminance reached 8,800 cd/m2 at the voltage of 13 V. We found that the roughness of the surface as estimated by means of atomic force microscopy (AFM) smoothened with increasing TDAPB doping concentration, and a slight exciplex emission between TDAPB and an electron transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4,-oxadiazole (PBD) was observed at the wavelength of approximately 530 nm.

Organic Light-Emitting Diodes Fabricated by a Solution Process and Their Stress Tolerance

Polymeric organic light-emitting diodes (PLEDs) were fabricated by solution process, and their stress tolerance were studied by continually pressing the PLEDs fabricated on polymeric substrates. Two types of host materials, poly(nvinylcarbazole) (PVCz) and starburst small-molecule 1,3,5tris[4-(diphenylamino)phenyl] benzene (TDAPB) were employed as host for the PLEDs doped with phosphorescent materials. Two iridium complexes are employed as dopants, fac-tris(2-phenyl- pyridine) iridium [Ir(ppy) 3] and tris(1-phenylisoquinoline) iridium (III) [Ir(piq)3] for green-emitting and red-emitting phosphorescent materials, respectively. The external quantum efficiency and power efficiency were 8.2% and 17.3 lm/W, and 6.3% and 3.0 lm/W for a device with Ir(ppy)3 doped TDAPB and Ir(piq)3 doped PVCz-TDAPB mixture host, respectively. Pushing tolerance tests were performed for PLEDs fabricated on polymeric substrates. The device continued to emit light after a pushing test consisting of more than 20 000 steps. In this paper, we discuss the fabrication and characteristics of PLEDs prepared using starburst TDAPB and phosphorescent materials for green and red emissive materials by a solution process. We discuss the pushing tolerance for PLEDs fabricated on a polymeric substrate

Study on Electron Injection of Phosphorescent Polymer Light-Emitting Diodes Utilizing CsF/Metal Cathode

The effect of electron injection in polymer light-emitting diodes based on the phosphorescent material of fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) devices with CsF/MgAg/Ag cathodes were investigated. The device with MgAg (1 nm)/CsF (3 nm)/Ag cathode showed almost similar current density-voltage-luminance characteristics to that with CsF (3 nm)/MgAg (1 nm)/Ag cathode. The existence of interface between the organic layer and Cs or CsF results in low turn on voltage. To achieve the efficient electron injection and the low turn on voltage, it is necessary to exist MgAg and CsF interface at the position of about 1 nm from organic layer. To achieve the efficient electron injection, it is necessary to exist Cs layer just on the organic layer. The electron injection effects between devices with CsF and Cs are different. It is suggested that the dipole of CsF affect the efficient electron injection.

Efficient polymeric light emitting diodes for flexible display

Polymer based efficient red light emission has been demonstrated utilizing red-emitting phosphor Ir(piq)/sub 3/ doped in mixture of PVK and TDAPB by spin coating method. The maximum luminance is obtained 8800 cd/m/sup 2/ and the external quantum efficiency of 6.3%. The impact test of the PLED shows a star like destruction of electrode by impact the EL device.

Fabrication and characteristics of layered polymeric electroluminescent diodes by all wet-process for flexible display

In this paper, we discussed fabrication and characteristics of layered polymeric OLED using water-soluble molecular and polymer with wet process for flexible display. We demonstrated the emission properties of OLEDs with the three-layered structure using water-soluble organic materials. The threshold voltage of an OLED with three-layered structure using water-soluble organic materials is lowered by 30% than that with two-layered structure.

Organic photonic devices utilizing nano-structured materials

The characteristics of organic photonic devices with various kinds of geometrical nano-structures at the interface between organic materials, and also between the organic layer and metal electrode are investigated. A cutoff frequency of more than 20 MHz was observed for single-layer heterostructure organic photodetector (OPD) by applying a reverse bias electric field under the illumination of the red repetition pulse light. The mixed-layer heterostructure OPD with high photocurrent and high speed photoresponse is suitable for the application of optical link devices. An organic light-emitting diode (OLED) with a partial doping layer at the interface of heterostructure device can be expected to improve the modulation characteristics. The existence of interface between the organic layer and Cs or CsF results in low turn-on voltage for OLEDs. To achieve the efficient electron injection, it is necessary to exist Cs layer just on the organic layer. The efficient electron injection and the low turn-on voltage result from the coexistence of MgAg and CsF at the position of approximately 1 nm from organic layer. The poly(3-hexylthiophene) device with a cathode fabricated from Ag nanoparticles shows a photoresponse and a red emission in the reverse and forward bias regions, respectively. We demonstrated the possibility of polymer OLEDs using a cathode fabricated from Ag nanoparticles by wet processing.

Fabrication and characteristics of flexible layered polymeric electroluminescent diodes fabricated by all-wet process

Highly efficiency organic electroluminescent devices employing a phosphorescent dye doped into a wet-processable molecular material have been demonstrated. Methoxy-substituted 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB) was selected as a host polymer for the phosphorescent dopant fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3], and organic films were fabricated by spin-coating method. The peak external quantum efficiency of 8.2 % (29 cd/A), or luminous power efficiency of 17.3 lm/W, and luminance of 33,000 cd/m2 were achieved at a apply voltage of 9.4 V for a 90nm-thick emissive layer. The emission from the host material of TDAPB was not observed in the electroluminescence or photoluminescence spectra. The decrease in efficiencies at a high current is analyzed using the triplet-triplet annihilation model. The high performance is attributed to the simple device structure, excellent film forming properties of the host material, and the efficient energy transfer from the host to dopant.