Masanao Era

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.

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.

Highly efficient electroluminescence from a heterostructure device combined with emissive layered-perovskite and an electron-transporting organic compound

Abstract Two Pbl-based layer perovskite compounds, which possess cyclohexenylethylamine or phenylbutylamine as an organic ammonium layer, were newly found to exhibit efficient exciton emission due to their self-organized quantum well structure where a lead halide semiconducting layer and an organic ammonium dielectric layer are alternately piled up. We prepared heterostructure electroluminescent devices using the combination of the emissive layered perovskite and an electron-transporting oxadiazole. When the heterostructure devices were driven at 110 K, greenish emission, which corresponded well to the exciton emission, was observed. In the device using the perovskite with an organic layer of cyclohexenythylamine, a high luminance exceeding 4000 cd m −2 and high external EL quantum efficiency of 2.8% were attained at a current density of 50 mA cm −2 at an applied voltage of 24 V.

Polarized electroluminescence from smectic mesophase

Electrically pumped light emission was observed from a smectic mesophase of liquid-crystalline oxadiazole; 2,5-hexyloxybiphenyl–hexyloxyphenyl–oxadiazole (HOBP-OXD), which exhibits high electron transport capability. The light-emitting diode consisted of the HOBP-OXD film, a copper phthalocyanine vacuum deposited film as a hole injection layer, and a tris-(8-hydroxyquinoline) aluminum vacuum deposited film as an electron injection layer. The emission was linearly polarized originating from highly oriented molecular textures in the smectic mesophase with a degree of orientational order of 0.32.

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.

Preparation of highly oriented polyarylenevinylene thin films by the Langmuir-Blodgett technique

We obtained three kinds of polyarylenevinylene(poly (p-phenylenevinylene), poly(2,5-dimethoxy-p-phenylenevinylene) and poly(2,5-thienylenevinylene)) thin films with the Langmuir-Blodgett (LB) technique. Stable polyarylenevinylene precursor-anionic amphiphile complex monolayers were formed at the air-precursor aqueous solution interface through adsorption of the precursors onto an amphiphilic monolayer. The polyion complex monolayers could be transferred on substrates by the LB technique. The polyarylenevinylene precursor LB films were converted to polyarylenevinylene films with a heat treatment under nitrogen flow. Extension of the mean n conjugation length and planar orientation of the conjugation sequences were realized in the poly (p-phenylenevinylene) and poly(2,5dimethoxy-p-phenylenevinylene) LB films.

Double-heterostructure electroluminescent device with cyanine-dye bimolecular layer as an emitter

Abstract Using the Langmuir-Blodgett technique, we fabricated a double-heterostructure electroluminescent (EL) device with a J -aggregated cyanine-dye bimolecular layer as an emitting layer. The EL cells were composed of an indium tin oxide electrode, a diamine hole-transport layer, a J -aggregated cyanine-dye emitting layer, an oxadiazole electron-transport layer and an MgAg electrode. Efficient EL was observed in the EL device owing to the confinement of charge carriers and excitons within the cyanine-dye bimolecular layer. The EL spectrum corresponded well to resonance fluorescence of the J -aggregate.

Progress in organic multilayer electroluminescent devices

Survey of dye materials for the emission layer in the multilayer organic electroluminescent (EL) device is discussed in terms of emission color and fluorescent efficiency. Organic semiconductors for the electron or the hole transport layer in the EL device are proposed for preparing stable homogeneous thin layer. Requirement of accomplishing the confinement of the singlet excitons generated by the recombinations of injected electrons and holes is discussed by using three layer EL devices with extremely thin bimolecular emission layer. Then the emphasis is laid on the size effects in three layer EL device with double heterojunctions on the spontaneous emission. Variations of the intensity and pattern of outer emission through semitransparent ITO glass substrate with the spacing between the emission layer and the metallic electrode are discussed theoretically and experimentally. And variation of the fluorescent lifetime or the radiative decay rate with the spacing is also discussed theoretically and experimentally.

Enhanced phosphorescence from naphthalene-chromophore incorporated into lead bromide-based layered perovskite having organic–inorganic superlattice structure

Abstract Strongly enhanced phosphorescence from the naphthalene-chromophore was observed in lead bromide-based layered perovskites with a superlattice structure where a lead bromide semiconductor layer and a naphthalene-linked ammonium layer are alternately piled up. From the photoluminescence excitation spectrum of the layered perovskites, it is suggested that the enhanced phosphorescence is caused by efficient energy transfer from an exciton state in the lead bromide layer to the excited triplet state of the naphthalene-linked ammonium molecules. This result is experimental evidence that new functionality due to strong interaction between organic and inorganic layers arises in the perovskite with a functional organic layer.

Electroluminescent device using two dimensional semiconductor (C6H5C2H4NH3)2PbI4 as an emitter

Abstract Layered Perovskite compounds expressed by the formula (C6H5C2H4NH3)2PbX4 (X: halogen) naturally formed a dielectric quantum well structure in their spin-coated films. Strong exciton absorption and sharp photoluminescence from the exciton were observed in the spin-coated films even at room temperature; exciton absorption and emission bands of the layered Perovskite compounds were located at 3.65 eV and 3.57 eV for X=Cl, 3.08 eV and 3.05 eV for X = Br, and 2.41 eV and 2.38 eV for X = I, respectively, Using the combination of (C6H5C2H4NH3)2PbI4 (PhEPbI4), one of the luminescent layered Perovskite compounds, and an electron-transporting oxadiazole derivative (OXD7), a multilayered electroluminescent (EL) device consisting of indium-tin-oxide (ITO)/PhEPbI4/OXD7/MgAg was prepared. Intense EL corresponding well to the exciton emission of PhEPbI4 was observed in the device. The EL intensity was reached to luminance of about 12,000 cd m−2 at current density of 2 A cm−2 at liquid nitrogen temperature.