Shogo Saito

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.

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.

Organic Electroluminescent Device with a Three-Layer Structure

An electroluminescent device with a three-layer structure was constructed using an intensely fluorescent material, 12-phthaloperinone derivative, for the emitter material. The cell structure of Mg-electrode/amorphous electron-transport layer/polycrystalline emitting layer/amorphous hole-transport layer/Au-electrode was constructed on a glass substrate. Yellow EL emission was clearly observed in normal room lighting at the dc bias voltage of 60 V. The emission intensity was proportional to the injection current over a wide current range of 10-7 to 10-3 A/cm2. The EL intensity exceeded 1 µW/cm2 at the injection current of 2 mA/cm2.

Polarized electroluminescence from oriented p‐sexiphenyl vacuum‐deposited film

Oriented sexiphenyl (6P) film was prepared through epitaxial growth on rubbed substrates by vacuum deposition. Polarized absorption spectra demonstrated that the long axis of 6P molecule was highly oriented along the rubbing direction. Using the epitaxial growth film of 6P, we fabricated electroluminescent (EL) devices consisting of indium‐tin‐oxide anode, emissive layer of 6P, electron‐transporting layer of an oxadiazole derivative, and MgAg alloy cathode. Owing to the orientation of 6P molecules, EL polarized in the rubbing direction was observed in the devices.

Sharply directed emission in organic electroluminescent diodes with an optical‐microcavity structure

An optical microcavity structure was introduced into organic three‐layer electroluminescent (EL) diodes with a europium complex as an emission layer. The device structure consisted of a dielectric reflector composed of SiO2/TiO2 bilayers, an indium‐tin‐oxide electrode, a hole transport layer, a europium complex as an emission layer, an electron transport layer, and a MgAg electrode. The dielectric reflector and the MgAg metal electrode constituted a planer microcavity. Sharply directed emission from the europium complex was observed when operated under dc drive voltage. Both angular dependences of intensity of emitted light and emission spectra of the EL diodes with the microcavity were compared with those of conventional EL diodes without microcavity.

ELECTROLUMINESCENCE IN ORGANIC THIN FILMS

Mechanism of electroluminescence in organic dye films, mechanisms of carrier injection, carrier transport, carrier recombination, creation of molecular excitons, movement of molecular excitons, and emission from molecular excitons, is described. Using high performance electroluminescent devices, three attempts towards novel fields of photophysics and photochemistry of organic solids were performed. First, confinement of molecular excitons within a molecular-size area was reported. Second, emission from triplet excitons produced by electric excitation was observed. Third, the presence of quantum optical size effect of radiation field in organic thin films was demonstrated.

Electron drift mobility of oxadiazole derivatives doped in polycarbonate

Charge drift mobilities of five oxadiazole derivatives doped in polycarbonate (PC) were evaluated with the time‐of‐flight technique. It is demonstrated that oxadiazoles incline to having electron‐transport characteristics. In particular, an oxadiazole with naphthyl substituent (BND) was found to possess high potential of electron transport; the electron drift mobility of 50 wt % BND doped PC was 2.2×10−5 cm2 V−1 s−1 at an electric field of 7.5×105 V cm−1 at room temperature. In addition, incorporating strong electron‐releasing substituents into oxadiazoles was demonstrated to add hole transport characteristics to oxadiazoles.