Yutaka Ohmori

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.

Frequency response properties of organic photo-detectors as opto-electrical conversion devices

Frequency performances and sensitivities for three types of organic photo-detector (OPD) were studied for an opto-electrical conversion device. A high efficiency of 19.4 % of the external conversion efficiency and a high cutoff frequency response of 16 MHz were achieved using a mixed-layer between copper phthalocyanine (CuPc) and N, N-bis (2,5-di-tert-butylphenyl) 3,4,9,10-perylene dicarboximide (BPPC) at a reverse bias voltage of 8 V under red incident light. These results were achieved by increasing exciton dissociation and charge carrier generation and to reduce the number of trapped carriers at the interfaces between CuPc and BPPC. The transmission of a moving picture was successfully demonstrated using mixed-layer OPD as an opto-electrical conversion device. These results indicate that it is possible for an OPD to be used as an opto-electrical conversion device in high-speed optical transmission systems

Semitransparent Organic Photodetectors Utilizing Sputter-Deposited Indium Tin Oxide for Top Contact Electrode

Semitransparent organic photodetectors with the heterostructure of copper phthalocyanine (CuPc) and N,N-bis(2,5-di-tert-butylphenyl) 3,4,9,10-perylenedicarboximide (BPPC) were fabricated using sputter-deposited indium tin oxide (ITO) for a top contact electrode, and the I–V and frequency characteristics were investigated. To prevent ITO penetration into an organic layer, an amorphous carbon nitride (a-C:N) buffer layer was placed between the organic layer and an ITO electrode. A cutoff frequency of more than 2 MHz was obtained for a semitransparent photodetector by applying a reverse bias voltage of -2 V under visible light illumination. We demonstrated that the fabricated semitransparent organic photodetectors can be applicable to a photoreflector with light-emitting diodes for integrated devices with organic light-emitting devices.

Effects of CsF/Metal Interface on Electron Injection in Polymer Light-Emitting Diodes

The effect of electron injection in polymer light-emitting diodes has been investigated at the interface between the CsF layer and metals of the cathode. Efficient electron injection was achieved when the thickness of the CsF layer deposited on the organic layer was 1.5 nm. When the thickness of the MgAg layer deposited on the CsF layer was more than 1 nm, efficient electron injection was achieved. For the device with a 1.5-nm-thick CsF and a 1-nm-thick MgAg cathode, the deposition orders of CsF and MgAg were independent of efficient electron injection. That is, the coexistence of CsF and MgAg near the organic layer resulted in efficient electron injection. To achieve efficient electron injection, we found that was sufficient device performance was obtained when the surface of the organic layer with CsF was covered with approximately 25% MgAg.

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.