Hiroaki Tokuhisa

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.

Revisit on the role of oxadiazole hole blocking layer in organic multilayer electroluminescent devices

Abstract Hole-blocking function of an oxadiazole derivative layer in a triple-layer organic electroluminescent device was revisited. Triple-layer devices with the structure of ITO/4,4′-bis[(3-methylphenyl)phenylamino]biphenyl/9,10-bis(4-cyanostyryl)anthracene (BSA-l)/1,3-bis[(4- tert -butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7)/MgAg were fabricated The OXD-7 layers were partially doped with a fluorescent squarilium dye, 2,4-bis[4-(diethylamino)-2-hydroxyphenyl]cycrobutenediylium-1,3-dioxide (Sq) or wholly doped with a dye which quenches fluorescence from OXD-7, 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (TIN). The emission from Sq was observed only in the device doped with Sq at the BSA-1/OXD-7 boundary region. The emission intensity from the emitter BSA-1 was not reduced by doping with the quencher TIN within the OXD-7 layer. These results gave direct evidence for the absence of injection of holes into the OXD-7 layer, in other words, the effective hole-blocking capability of the OXD-7 layer.

Electric Conduction of Electroluminescent Metal Chelate Thin Films

Abstract Electric conduction in vacuum-deposited films of electroluminescent metal-chelates, tris-(8-hydroxyquinoline) aluminum (Alq3) and zinc bis-(2-(o-hydroxypheyl)-benzoxazolate)) (ZnPBO), were studied by current (I)-voltage (V) measurement. The I-V characteristics demonstrated that the films exhibited trap-controlled conduction. For the Alq3 film, conductivity σ was estimated to be 3.0 × 10−15 Scm−1 from the characteristic in ohmic current region. From the I-V characteristic in space charge limited current region, further, carrier mobility μ of the Alq3 film was estimated to be 5.0 × 10−7 cm2V−1s−1; this value is in good agreement with electron mobility evaluated by the time-of-fight technique. For ZnPBO, the values of σ and μ were estimated to be 2.0 × 10−14 Scm−1 and 7.0 × 10−6 cm2V−1s−1, respectively.

Molecular organic light-emitting diodes based on a guest-host active layer: Approaches for enhancing device performance

Abstract We present a comprehensive study of enhancement mechanisms in electroluminescence devices where the active layer is doped with fluorescent dye. We demonstrate two typical systems where the emission mechanism is based on either energy transfer or direct carrier recombination. For the energy transfer mechanism, we find that the maximum quantum efficiency of multilayered devices depends on the nature of the host in addition to the luminescent properties of the guest, by comparing electroluminescence and absolute photoluminescence quantum yields in various guest-host systems. For the direct carrier recombination mechanism using rubrene as the dopant and TPD as the host, we find that the dual electronic nature of rubrene acting as a hole trap and an electron hopping site is responsible for maximizing the carrier recombination on the dopant molecule.

Characterization of electron transport in naphthyl-oxadiazole doped polycarbonate film

Electron-transport behavior in 2.5-bis(4-naphthyl)-],3,4-oxadiazole (BND) doped polycarbonate (PC) film was characterized through the analysis of the dependences of electron mobility on electric field, temperature, and dopant concentration according to Scheins procedure. In BND doped PC film, the zero-field activation energy (E () ) was found to depend on the distance between hopping site (p). Further, the zero-field electron-mobility μ(F=0) was represented by the equation μ(F=0) = a () p 2 exp(-E 1 (p)/kT), where a()=3.6 x 10 14 V -1 s -1 . These results demonstrate that electron transport in BND doped PC is described as adiabatic hopping on the basis of the small polaron theory.