Toshimitsu Tsuzuki

Highly efficient, deep-blue phosphorescent organic light emitting diodes with a double-emitting layer structure

We have demonstrated a highly efficient, deep-blue organic light-emitting diode (OLED) using a host material with a high triplet energy. The OLED device that we have prepared utilizes a phosphorescent guest material, iridium(III)bis(4′,6′,-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate, exhibits a peak quantum efficiency of about 15.7%. We employed a double-emitting layer (DEL) structure that distributes the carrier recombination region within the device. In this DEL structure, the emission mechanism is such that the energy transfers from the host material in one emitting layer, and the other emitting layer provides for direct charge trapping in the guest material. This DEL structure proved to be quite useful in achieving the reported device characteristics.

Organic Light-Emitting Diodes Using Multifunctional Phosphorescent Dendrimers with Iridium-Complex Core and Charge-Transporting Dendrons

We report a novel class of light-emitting materials for use in organic light-emitting diodes (OLEDs): multifunctional phosphorescent dendrimers that have a phosphorescent core and dendrons based on charge-transporting building blocks. We synthesized first-generation and second-generation dendrimers consisting of a fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] core and hole-transporting phenylcarbazole-based dendrons. Smooth amorphous films of these dendrimers were formed by spin-coating them from solutions. The OLEDs using the dendrimer exhibited bright green or yellowish-green emission from the Ir(ppy)3 core. The OLEDs using the film containing a mixture of the dendrimer and an electron-transporting material exhibited higher efficiency than those using the neat dendrimer film. The external quantum efficiency of OLEDs using the film containing a mixture of the first-generation dendrimer and an electron-transporting material was as high as 7.6%.

Photoelectrical conversion of p-n heterojunction devices using thin films of titanyl phthalocyanine and a perylene pigment

Two types of p-n heterojunction devices consisting of thin films of titanyl phthalocyanine (TiOPc) and N,N′-dimethyl-3,4:9,10-peryle-nebis(dicarboximide) (MPCI), sandwiched between indium tin oxide (ITO) and gold (Au), have been fabricated, and their performance characteristics investigated. The devices showed a response to light over the whole visible wavelength region from 400 to 900 nm. The cell with a structure of ITOMPCITiOPcAu (Type A) was superior to the cell with a structure of ITOTiOPcMPCIAu (Type B) in the conversion efficiency. The cell of type A exhibited conversion efficiencies of ca. 0.7% for the monochromatic light of 470 or 720 nm reaching the organic layer through the electrode, and ca. 0.35% for incident white light (8–130 mW cm−2).

Fabrication of 5.8-in. OTFT-driven flexible color AMOLED display using dual protection scheme for organic semiconductor patterning

— A 5.8-in. wide-QQVGA flexible color active-matrix organic light-emitting-diode (AMOLED) display consisting of organic thin-film transistors (OTFTs) and phosphorescent OLEDs was fabricated on a plastic film. To reduce the operating voltage of the OTFTs, Ta2O5 with a high dielectric constant was employed as a gate insulator. Pentacene was used for the semiconductor layer of the OTFTs. This layer was patterned by photolithography and dry-etched using a dual protection layer of poly p-xylylene and SiO2 film. Uniform transistor performance was achieved in the OTFT backplane with QQVGA pixels. The RGB emission layers of the pixels were formed by vacuum deposition of phosphorescent small molecules. The resulting display could clearly show color moving images even when it was bent and operated at a low driving voltage (below 15 V).

Efficient organic light-emitting devices using an iridium complex as a phosphorescent host and a platinum complex as a red phosphorescent guest

We demonstrated efficient organic light-emitting devices (OLEDs) using a phosphorescent host/guest system consisting of bis(2-phenylpyridinato-N,C2′)iridium(acetylacetonate) [(ppy)2Ir(acac)] as a host and a platinum complex (Pt-SA-1) as a guest. The OLED using (ppy)2Ir(acac) film doped with Pt-SA-1 (1wt%) showed an ideal red emission via efficient energy transfer from the host to the guest. The external quantum efficiency of the device was as high as 8.3%. The driving voltage was significantly reduced compared with a device using a conventional host of 4,4′-di(carbazole-9-yl)biphenyl, which resulted from the enhancement of the hole injection from the hole-transport layer to the host.

Highly efficient and stable organic light-emitting diode using 4,4′-bis(N-carbazolyl)-9,9′-spirobifluorene as a thermally stable host material

We demonstrated efficient and stable organic light-emitting diodes (OLEDs) using 4,4′-bis(N-carbazolyl)-9,9′-spirobifluorene (CFL), which has spirobifluorene and two carbazole moieties, as a thermally stable host and tris(1-phenylisoquinolinolato-C2,N) iridium(III) [Ir(piq)3] or tris(2-phenylpyridinne)iridium(III) [Ir(ppy)3] as a guest. Glass transition temperature of CFL was 151 °C. The efficiency of OLEDs that use CFL as a host and Ir(piq)3 as a guest were higher than that of an OLED, which uses conventional 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) as a host. The external quantum efficiency of the OLED using CFL and Ir(piq)3 was 13%. The lifetime of the OLED using CFL as the host was longer than that of the OLED using CBP.

Fabrication and performances of a double-layer organic electroluminescent device using a novel starburst molecule, 1,3,5-tris[N-(4-diphenylaminophenyl)phenylamino]benzene, as a hole-transport material and tris(8-quinolinolato)aluminum as an emitting material

A double-layer organic electroluminescent (EL) device was fabricated using a novel starburst molecule, 1,3,5-tris[N-(4-diphenylaminophenyl)phenylamino]benzene (p-DPA-TDAB), as a hole transport material and tris(8-quinolinolato) aluminum (Alq/sub 3/) as an emitting material, and its performance characteristics were investigated. It was found that p-DPA-TDAB, which forms a stable amorphous glass with a glass-transition temperature of 108/spl deg/C, functions as a good hole-transport material and that the EL device is thermally stable, operating at a temperature of 120/spl deg/C.

Characterization of ultrathin films of titanyl phthalocyanine on graphite: PIES and UPS study

Abstract Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS) were used to characterize ultrathin films (0.2–3 MLE) of titanyl phthalocyanine (OTiPc) deposited on a graphite substrate. The change in the molecular orientation at the outermost surface layer was selectively probed by PIES during the variations of the thickness and temperature of the films.

A 5.8‐in. phosphorescent color AMOLED display fabricated by ink‐jet printing on plastic substrate

— A flexible phosphorescent color active-matrix organic light-emitting-diode (AMOLED) display on a plastic substrate has been fabricated. Phosphorescent polymer materials are used for the emitting layer, which is patterned using ink-jet printing. A mixed solvent system with a high-viscosity solvent is used for ink formulation to obtain jetting reliability. The effects of evaporation and the baking condition on the film profile and OLED performances were investigated. An organic thin-film-transistor (OTFT) backplane, fabricated using pentacene, is used to drive the OLEDs. The OTFT exhibited a current on/off ratio of 106 and a mobility of 0.1 cm2/V-sec. Color moving images were successfully shown on the fabricated display.

Penning ionization electron spectroscopy of titanyl phthalocyanine ultrathin films: electronic state and molecular orientation

Abstract Penning ionization electron spectroscopy (PIES) was used to investigate the molecular orientation, aggregation of molecules, and electronic structure of ultrathin films of titanyl phthalocyanine (OTiPc) vapor deposited on graphite and MoS 2 substrates. For the monolayer film, the molecular orientation was remarkably different between the two substrates. At room temperature, molecules spread over and almost cover the substrate on graphite, whereas they aggregate and form islands on MoS 2 . The PIES band due to the nonbonding orbital of the oxygen atom extending perpendicular to the molecular plane is extraordinary strong for the He * (2 3 S) metastable atom but not for the He * (2 1 S), indicating that the potential for the entrance channel is quite different between the two species.