Tetsuo Tsutsui

Blue light-emitting organic electroluminescent devices

Organic electroluminescent (EL) devices with multilayered thin‐film structures which emitted bright blue light were constructed. Two empirical guides for the selection of blue‐emitting materials were established. The keys to obtain the EL cells with high EL efficiency were excellent film‐forming capability of an emitter layer and the appropriate combinations of emitter and carrier transport materials for avoiding the formation of exciplexes. In one of our organic electroluminescent devices, blue emission with a luminance of 700 cd/m2 was achieved at a current density of 100 mA/cm2 and a dc drive voltage of 10 V.

Organic electroluminescent device having a hole conductor as an emitting layer

We have succeeded in fabricating a novel thin‐film electroluminescent device with a luminescent hole transport layer as an emitter. The cell structure is composed of an indium‐tin‐oxide substrate, a luminescent hole transport layer (emitter), an electron transport layer, and a MgAg electrode. The most essential feature of our device owes for adoption of an oxadiazole derivative as an electron transport layer. The emission intensity of 1000 cd/m2 was achieved at a current of 100 mA/cm2.

Electroluminescence in Organic Films with Three-Layer Structure

We have succeeded in the fabrication of a stable organic electroluminescent (EL) device with a three-layer structure; hole transport layer/emitting layer/electron transport layer. The EL device was prepared by vacuum evaporation. Efficient carrier double injection into the emitting layer was realized by the use of separate hole and electron transport layers. Bright EL emission was observed in a darkened room at the dc bias voltage of 50 V. Stable emission lasted for more than five hours at this condition. The emission spectrum could be changed with variation of the organic material for the emitting layer.

Confinement of charge carriers and molecular excitons within 5‐nm‐thick emitter layer in organic electroluminescent devices with a double heterostructure

Organic electroluminescent devices with a double‐heterostructure indium‐tin‐oxide substrate/hole transport layer/emitter layer/electron transport layer/MgAg have been fabricated by vacuum vapor deposition. The organic carrier transport and emitter layers were composed of amorphous films. In the double‐heterostructure devices, the luminance continued to lie in high level, even when the emitter thickness was 50 A. The confinement of charge carriers and molecular excitons within a narrow emitter layer was achieved.

Organic‐inorganic heterostructure electroluminescent device using a layered perovskite semiconductor (C6H5C2H4NH3)2PbI4

Using the combination of a layered perovskite compound (C6H5C2H4NH3)2PbI4 (PAPI), which forms a stable exciton with a large binding energy owing to its low‐dimensional semiconductor nature and exhibits sharp and strong photoluminescence from the exciton band, and an electron‐transporting oxadiazole derivative, we fabricated an organic–inorganic heterostructure electroluminescent (EL) device. The EL spectrum of the device corresponded well to the photoluminescence spectrum of the PAPI film; the emission was peaking at 520 nm and half‐width of the emission was about 10 nm at liquid‐nitrogen temperature. Further, highly intense EL of more than 10 000 cd m−2 was performed at 2 A cm−2 at liquid‐nitrogen temperature in the device.

Organic light-emitting device with an ordered monolayer of silica microspheres as a scattering medium

Periodic dielectric structures, consisting of hexagonally closed-packed arrays of silica microspheres with the diameter of 550 nm, were incorporated into organic light-emitting devices with a conventional two-layer structure made with vacuum-sublimation. The arrays acted as a two-dimensional diffraction lattice which behaved as a light scattering medium for the light propagated in waveguiding modes within the device. Strongly scattered light emission through the front surface of the devices was observed. An increase in the device coupling-out factor for electroluminescent efficiency by using the scattering structure is demonstrated.

Blue-Light-Emitting Organic Electroluminescent Devices with Oxadiazole Dimer Dyes as an Emitter

Multilayer thin-film electroluminescent devices with blue emission color have been fabricated using new oxadiazole dimer dyes for both emission and electron transport layers. Emission characteristics of three types of cells, SH-A: ITO/HTL/EML/MgAg, SH-B: ITO/EML/ETL/MgAg and DH: ITO/HTL/EML/ETL/MgAg, in which ITO, HTL, EML, ETL and MgAg represent indium-tin-oxide cathode, hole transport layer, emission layer, electron transport layer and magnesium-silver alloy anode, respectively, are compared. Luminances exceeding 1000 cd/m2 are observed and emission peaks are located at 470-480 nm. Among the three cells, the DH cell shows the best performance. The DH cell is stable for more than 40-day storage and can also be continuously driven for more than one hour at room temperature and 7 h at liquid nitrogen temperature.

High electron mobility in bathophenanthroline

We have measured electron mobility in vacuum-deposited films of 4,7-diphenyl-1,10phenanthroline (bathophenanthroline, or BPhen) using a time-of-flight technique. Electron transport was highly dispersive for BPhen with a dispersion parameter of a value 0.30. The electron mobility in excess of 10−4 cm2/V s has been observed at electric fields of the order of 105 V/cm with weakly dependent on the electric field. The characteristic energy of the distribution is obtained a value 0.09 eV. It is directly confirmed that the BPhen has superior electron-transport capability.

Carrier transport properties of organic materials for EL device operation

We have measured drift mobilities in vacuum deposited films of tris(8-quinolinolato)aluminum (Alq3), a triphenylamine derivative (TPD) and a naphtyl-substituted benzidine derivative (α-NPD) using a time-of-flight (TOF) technique. The hole mobilities of TPD and α-NPD are about two orders of magnitude higher than the electron mobility of Alq3. This implies that the current density versus applied voltage characteristics is dominated by the electron mobility of Alq3 rather than the hole mobility of TPD or α-NPD in double-layered devices ITO/TPD or α-NPD (50 nm)/Alq3 (50 nm)/MgAg.

Control of emission characteristics in organic thin‐film electroluminescent diodes using an optical‐microcavity structure

An electroluminescent diode with a microcavity structure which comprised a reflective Ag anode (36 nm), a hole transport dye layer (250 nm), an emission dye laser (15 nm), an electron transport dye layer (240 nm), and a reflective MgAg cathode was fabricated. A diode without the microcavity structure with a transparent ITO anode was also prepared for reference. The diode with microcavity was driven in the electric excitation mode and emission spectra at fixed detection angles were measured together with the angular dependence of emission intensity at fixed wavelengths. A sharpening of emission spectra and a marked alteration of emission patterns in the diode with microcavity were observed.