Hisayoshi Fujikawa

Highly efficient pure blue electroluminescence from polyfluorene: Influence of the molecular weight distribution on the aggregation tendency

A dependency between the molecular weight distribution of polyfluorene (PFO) and its aggregation tendency on operation in organic light-emitting devices (OLED) is described. As a result of these findings, low molecular parts of the polymer could be made responsible for the aggregation in liquid crystalline PFO. Consequently, a procedure was developed which leads to PFO-based OLEDs which do not show aggregation on operation anymore but exhibit highly efficient and stable blue electroluminescence.

Electronic structure of 8-hydroxyquinoline aluminum/LiF/Al interface for organic electroluminescent device studied by ultraviolet photoelectron spectroscopy

Electronic structures of the 8-hydroxyquinoline aluminum (Alq3)/LiF/Al and Alq3/Al interfaces were measured by ultraviolet photoelectron spectroscopy. Shifts of the highest occupied molecular orbital level and the vacuum level of the Alq3 layer due to insertion of a thin LiF layer were observed. This result indicates that the thin LiF layer at the Alq3/Al interface reduces barrier height for electron injection from the Al to Alq3. We, therefore, conclude that lowering of the driving voltage in an organic electroluminescent device with a thin LiF layer is attributable to the reduction of the barrier height.

Polarization independent visible color filter comprising an aluminum film with surface-plasmon enhanced transmission through a subwavelength array of holes

The enhanced transmission through periodic arrays of sub-wavelength holes in optically-thick metallic films has many potential applications, such as in wavelength filters, light extraction from light emission diodes, and subwavelength photolithography. A color filter comprising arrays of subwavelength holes in an aluminum film has been fabricated. In addition to the simplicity of the process, the aluminum film enables the excitation of visible-range surface plasmons due to its high plasma frequency. Periodic nanostructures in the aluminum film open the way for new visible color filters.

Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules

Adding low-molecular-weight hole-transporting molecules (HTM) with different oxidation potentials to the polyfluorene emission layer of single-layer light-emitting diodes causes significant changes in the device properties. The pronounced increase in luminance efficiency combined with a decrease in current is attributed to significant hole trapping, as further suggested by thermoluminescence experiments. Using a oligo-triphenylamine HTM with an ionization potential of ∼4.9 eV, light-emitting diodes with stable blue emission, a brightness of 800 cd/m2 and an efficiency of 0.87 cd/A were realized. Further, the red-emitting contribution to the spectra as observed in the pure polymer devices could be fully suppressed.

Design of multiwavelength resonant cavities for white organic light-emitting diodes

We have developed a multiwavelength resonant cavity (MWRC) optical device designed for organic light-emitting diodes (OLEDs). This device consists of two or more wavelength-selective mirrors, where each resonant peak can be adjusted independently, resulting in enhancement of the luminance and modification of the color of emission. We have designed a white OLED with a MWRC by using optical simulation and have confirmed that the white MWRC device is 1.3 times as bright as that of the conventional noncavity one.

Effects of additive elements on electrical properties of tantalum oxide films

Ta2O5‐based composite films prepared by magnetron sputtering have been investigated with respect to their dielectric properties. As additive third oxides, Y2O3 and WO3 were found to be effective materials for improving insulating properties. In these composite films, the dielectric constant remained unchanged. The maximum storage charge of the composite films was twice that of Ta2O5 films. The main reason for improving the insulating properties could be explained by the charge compensation of excess oxygen by these additive oxides.

New 9-fluorene-type trispirocyclic compounds for thermally stable hole transport materials in OLEDs

A novel trispirocyclic hydrocarbon having three 9-fluorene moieties around the core of truxene (5) was prepared readily via coupling of truxenone with 2-bromobiphenyl; 5 was a high melting (>500 °C) solid. For the application of 5 to an effective hole transport material (HTM) in the OLED, a triphenylamine derivative carrying six diphenylamino groups at the 2- and 7-positions of each 9-fluorene moiety (6) was designed in order to get high thermal stability as an improved material of the TPD type HTM. The synthesis of 6 was easily achieved using 4,4′-bis(diphenylamino)-2-bromobiphenyl (9). The trispirocyclic 6 was found to show a glass transition temperature as high as 170 °C. It effects the formation of its stable cation radical upon electrooxidation in solution, and amorphous thin films in solid. A multi-layered EL device for 6 as an HTM using Alq3 as an electron transporting emitter showed good EL characteristics such as the maximum luminance of 37 000 cd m−2 at 14 V. Thus, the hexakis(diphenylamino)substituted trispirocycle 6 (TX-F6S) can be used as an efficient and thermally stable HTM in OLEDs.

Improving the thermal stability of organic light-emitting diodes by using a modified phthalocyanine layer

Remarkable improvement in thermal stability has been demonstrated in an organic light-emitting diode (OLED) using a metal-free phthalocyanine (H2Pc)-doped copper phthalocyanine (CuPc) layer as a hole injection layer. Compared to an OLED using a CuPc layer, approximately twice the lifetime has been achieved in the OLED using the H2Pc-doped CuPc layer at a high temperature of 85 °C, operating under a constant current and starting at a luminance of 400 cd/m2. Atomic force microscopy measurements show that the dopant of H2Pc depresses the crystallization of a CuPc layer. It is suggested that the improved thermal stability of the OLED is attributable to that of the phthalocyanine layer in morphology.

Organic electroluminescent devices using alkaline-earth fluorides as an electron injection layer

Abstract We investigated the properties of organic electroluminescent (EL) devices using alkaline-earth fluorides as an electron injection layer, and found that the alkaline-earth fluorides are desirable materials to reduce the turn-on voltage of the EL devices with Al cathode. In the devices using the fluorides as the electron injection layer, we found that the turn-on voltage can be associated with electronegativity of metal ion of the fluoride; the fluoride with a low electronegativity metal ion results in lowering the barrier height at the organic/Al interface and enhanced the electron injection.

Highly efficient blue-green emission from organic light-emitting diodes using dibenzochrysene derivatives

We have fabricated highly efficient organic light-emitting diodes (OLEDs) using new hole-transporting emissive materials based on dibenzochrysene. Hole drift mobilities of the dibenzochrysene derivatives were measured in the vacuum-deposited films and found to be 5×10−4–2×10−3 cm2/V s (at 5×105 V/cm). The OLEDs consist of an emitting layer of the dibenzochrysene derivative and an electron-transport layer of tris(8-quinolinolato) aluminum. Emission colors of the OLEDs were blue-green and their spectra were consistent with the photoluminescence spectra with a peak wavelength of 490 nm. High external quantum efficiency of 2% was obtained at a luminance of 300 cd/m2, and good durability in a continuous operation at room temperature and high temperatures was achieved.