Toshiki Koyama

High mobility n-type thin-film transistors based on N,N′-ditridecyl perylene diimide with thermal treatments

The authors demonstrated that N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) thin-film transistors (TFTs) exhibited high field-effect electron mobility of 2.1cm2∕Vs by just annealing at an adequate temperature (140°C) after the TFT fabrications. While PTCDI-C13 formed c-axis oriented thin films, the thermal treatments improved crystallinity of the thin films as revealed by x-ray diffraction. The thermal treatment also affected thin-film morphologies; the morphologies changed from oval ball-like grains to flat and large tilelike grains, which had molecular height steps and whose size reached several micrometers.

Charge carrier trapping effect by luminescent dopant molecules in single-layer organic light emitting diodes

We investigated electroluminescent (EL) characteristics of single-layer organic light emitting diodes (SOLEDs). Our SOLED devices are composed of an inert polymer as a binder, in which hole transport molecules, emissive electron transport molecules (ETMs), and highly fluorescent dopants as luminescent centers are dispersed. We examined two typical dopants: rubrene and coumarin 6. These exhibited different charge carrier recombination and emission mechanisms. The dopant concentration dependence of the current density–voltage–luminance relationships clearly showed the importance of carrier trapping by dopant molecules for obtaining high luminance. When the dopant was rubrene, we observed that charge carriers were well trapped by the dopant molecule. This means that direct recombination of holes and electrons occurred on the dopant molecules and trapping significantly enhanced the external EL quantum efficiency ΦEL. For coumarin 6, on the other hand, we observed that charge carriers primarily recombined at t...

Doped organic light emitting diodes having a 650-nm-thick hole transport layer

We have succeeded in fabricating a thick-film organic light emitting diode having a doped hole transport layer (DHTL). The basic cell structure is anode DHTL/emitter layer/cathode. The DHTL is composed of a hole transporting polycarbonate polymer (PC-TPB-DEG) and tris(4-bromophenyl)aminium hexachloroantimonate (TBAHA) as a dopant. As an emitter, we used tris(8-hydroxyquinoline) aluminum (Alq). With a 650-nm-thick DHTL, the device showed considerable reduction in cell resistance compared with an anode/nondoped HTL/Alq/cathode device with the same HTL thickness. Although the electroluminescent quantum efficiency ΦL was rather low in the doped device, we should be able to increase it by interposing a thin tetraphenylbendidine (TPB) layer between the DHTL and the emitter layer while keeping the driving voltage low. The anode/DHTL (650 nm)/TPB(50 nm)/Alq(50 nm)/cathode showed luminance of more than 4004 cd/m2 at 10.0 V and 220 mA/cm2.

Emission gain narrowing from single crystals of a thiophene/phenylene co-oligomer

Emission gain narrowing has been observed for single crystals of a thiophene/phenylene co-oligomer. The hexagon flake crystals were placed on a quartz substrate with the crystals’ face in close contact with the substrate plane. These crystals were irradiated with a N2 laser with a 337.1 nm wavelength at a repetition rate 10 Hz that tuned its intensity to 100–1150 μJ/cm2. The emission gain narrowing takes place at 21490 (465.4 nm) and 20220 cm−1 (494.5 nm) with increased intensities, with their half width at half maxima reaching ∼50 cm−1. On the basis of the nonlinear relationship between the emission peak intensities and the laser light intensity, the gain narrowing has been attributed to the amplified spontaneous emission.

Photopumped laser oscillation and charge-injected luminescence from organic semiconductor single crystals of a thiophene/phenylene co-oligomer

We have demonstrated that single crystals of a thiophene/phenylene co-oligomer [α,ω-bis-biphenyl-4-yl-terthiophene (BP3T)] show interesting photonic aspects: (1) the self-waveguided amplified spontaneous light emissions with a comparable low threshold of 8μJ∕cm2 to other optimized organic solid-state laser systems, and (2) the laser oscillation based on the optical self-confinement effect in the crystals. We have also presented electroluminescence from the crystals based on bipolar injection and the crystals’ tolerance for intense current driving. These achievements strongly imply that BP3T crystals are a promising candidate for organic laser diodes.

Significant improvement of device durability in organic light-emitting diodes by doping both hole transport and emitter layers with rubrene molecules

We have developed highly durable organic light-emitting diodes. The basic structure of the diodes is anode/hole injection layer/hole transport layer+dopant/emitter layer+dopant/cathode. Both the hole transport and the emitter layers were doped with the highly fluorescent rubrene molecules. With the doping of both layers, 85% of the initial luminance was successively maintained even after 1000 h of continuous operation under constant current driving. Doping of only one of these layers, either the hole transport layer or emitter layer, on the other hand, resulted in shorter lifetime. We mention the possible mechanisms of the doping that enhance the device duration.

Bipyridyl oxadiazoles as efficient and durable electron-transporting and hole-blocking molecular materials

We have demonstrated new and efficient electron transporting (ET) and hole blocking (HB) materials for organic light-emitting devices (OLEDs). The bipyridyl-substituted oxadiazole derivatives can form a stable glassy state with glass transition temperatures greater than 100 °C. The bipyridyl-substituted oxadiazole derivatives have excellent ET, HB and electron accepting ability and also exhibit electrical operation durability. Bipyridyl-substituted oxadiazoles are a promising candidate for ET and HB materials for OLEDs.

Transparent organic light-emitting diodes using metal acethylacetonate complexes as an electron injective buffer layer

We studied transparent organic light-emitting diodes, which had a transparent top electrode deposited by sputtering, for possible application to a transparent light-emitting display. In the fabrication of a transparent electrode on an organic layer, steps must be taken to reduce the damage incurred by the organic layer during the sputter deposition process. We report the results of our study where we found that we could reduce the damage to the organic layer by suppressing the temperature rise of substrate resulting from the intermittent plasma irradiation. We also found that a thin film of metal acethylacetonate complexes [Mt(acac)2] is useful as a buffer layer to prevent an underlying emission layer from incurring damage in the sputter process. In previous reports, a thin film of copper phthalocyanine (CuPc) was used as an electron injective buffer layer. However, the absorption of the CuPc Q bands at λ=620 and 665 nm decreased the total optical transmission of the device. Transmittance in the visible r...

Electroluminescence of organic light emitting diodes with a thick hole transport layer composed of a triphenylamine based polymer doped with an antimonium compound

We investigated the electroluminescence (EL) performance of organic light emitting diodes having a thick doped hole transport layer [(DHTL):650 nm–1.5 μm]. The basic cell structure is an anode/DHTL/hole transport layer [(HTL):50–60 nm]/emitter layer [(EML):50–60 nm]/cathode. We examined various combinations of host polymers and guest molecules as a component of DHTL in this device structure. During the course of the materials’ search, we found that the best combination of a hole transport polycarbonate polymer (PC–TPD–DEG) and a tris (4-bromophenyl) aminium hexachroloantimonate (TBAHA) as a dopant enabled us to form a uniform thick DHTL (typically 650 nm–1.5 μm thick), which resulted in excellent EL performance. The thick DHTL not only showed considerable reduction in cell resistance compared with a conventional anode/DHTL (without doping)/HTL/EML/cathode device with the same thicknesses of the organic layers, but also greatly contributed to the enhancement of the device stability, particularly to pinhole...

Bright red organic light-emitting diodes doped with a fluorescent dye

We have evaluated a synthetic red fluorescent dye, 6-methyl-3-[3-(1,1,6,6-tetramethyl-10-oxo2,3,5,6-tetrahydro-1H,4H,10H-11 -oxa-3a-aza-benzo[de]anthracen-9-yl)-acryloyl]-pyran-2,4-dione (AAAP), as a dopant for an organic light-emitting diode (LED). Bright emission of a good red (maximum luminance: 5600 cd/m2, chromaticity coordinates: x=0.63, y=0.36) was obtained. The device consisted of ITO/TPD(50 nm)/Alq3 doped with AAAP(1.5 mol %,15 nm)/bOXDF(20 nm)/Alq3(25 nm)/Mg:Ag (ITO: indium tin oxide, TPD: N, N′-diphenyl- N,N′-di(3-methylphenyl)-1, 1′biphenyl-4,4′-diamine, Alq3: tris (8-hydroxyquinolinato)-aluminum (III), bOXDF:2,2-bis[5-(4-biphenyl)-1,3,4-oxadiazole-2-yl-4,1 -phenylene]-hexafluoropropane). The bands of fluorescence of the Alq3 host and of absorption of the AAAP dopant overlap in an advantageous way, and insertion of the bOXDF layer between the emission layer, doped with AAAP, and Alq3 layer, made for a device with good properties.