Junichi Nishide

Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence

Organic light-emitting diodes (OLEDs) are attractive for next-generation displays and lighting applications because of their potential for high electroluminescence (EL) efficiency, flexibility and low-cost manufacture. Although phosphorescent emitters containing rare metals such as iridium or platinum produce devices with high EL efficiency, these metals are expensive and their blue emission remains unreliable for practical applications. Recently, a new route to high EL efficiency using materials that emit through thermally activated delayed fluorescence (TADF) was demonstrated. However, it is unclear whether devices that emit through TADF, which originates from the contributions of triplet excitons, are reliable. Here we demonstrate highly efficient, stable OLEDs that emit via TADF by controlling the position of the carrier recombination zone, resulting in projected lifetimes comparable to those of tris(2-phenylpyridinato)iridium(III)-based reference OLEDs. Our results indicate that TADF is intrinsically stable under electrical excitation and optimization of the surrounding materials will enhance device reliability.

Real-time three-dimensional holographic display using a monolithic organic compound dispersed film

Organic holographic materials such as photorefractive polymers are one of the promising candidates for the next generation three dimensional (3D) real-time display. Recently, we found that polymer composite of monolithic organic compound of 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) (30 wt%) doped transparent polymethylmethacrylate (PMMA) had capability of recording and displaying new images within a few seconds and fixed at ten seconds and viewing for a longer time without applying electric field. Here, we present 3D holographic display using monolithic organic compound NACzE dispersed transparent PMMA film sandwiched between two glass plates with size of 7.5 × 5 cm2. The thickness of film is ca. 50 μm. Images are easily and completely erased by over recording and it is accelerated by slight heating.

Asymmetric Energy Transfer in Photorefractive Polymer Composites Under Non-Electric Field

We prepared photorefractive composites using poly(methyl methacrylate) (PMMA) as a host material doped with 3-[(4-Nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) as an NLO chromophore. Photorefractive properties were characterized by means of a two beam coupling (2BC) and a degenerated four wave mixing (DFWM) methods performed at 632.8 nm and room temperature. We successfully observed asymmetric 2BC under non-electric field: photorefractive characteristics obtained are a net 2BC gain of 426 cm−1 and a diffraction efficiency of 35%. We could also demonstrate its high performance by DFWM image processing experiment. In order to reduce the glass transition temperature Tg of photorefractive composites we selected poly(ethyl methacrylate) (PEMA) as a host material. Photorefractive properties obtained are a net 2BC gain of 398 cm−1 and a diffraction efficiency of 40%.

Non-Electric Field Photorefractive Effect Using Polymer Composites

We prepared photorefractive composites using poly(3,4-ethylenedioxythiophene)-block-polyethylene glycol (PEDOT-block-PEG) as a key material doped with nonlinear optical chromophore ((S)-(-)-l-(4-Nitrophenyl)-2-pyrrolidinemethanol (NPP). To adjust the optical absorbancc PMMA was used as a matrix. Photorefractive properties were characterized by means of a two beam coupling (2BC) method and a degenerated four wave mixing (DFWM) method. We successfully observed asymmetric 2BC under non-electric field: Photorefractive characteristics obtained are a net 2BC gain of 175 cm−1, a diffraction efficiency of 49%, and a phase shift of 90°. Furthermore, we could demonstrate its high performance by DFWM image processing experiment.

Photoinduced grating formation in a polymer containing azo-carbazole dyes.

Although some azo-carbazole derivatives attached on or doped into inert polymers are known to show photorefractive effect without external electric field, the origin of their asymmetric energy transfer in two-beam coupling experiments were unknown. We made the two-beam coupling experiment followed by sample translation and one-beam diffraction at 633 nm for thick films composed of 3-[(4-nitrophenyl)]azo-9H-carbazole-9-ethanol (NACzEtOH) and poly(methylmethacrylate), finding that photoinduced gratings grew in several minutes accompanied with phase displacement of the gratings, but the phase shift was not always synchronized with the refractive index modulation. We reformulated the Kogelniks coupled-wave theory with strict energy conservation law for analysis. Comparison of the grating growth and erasure at 532 nm to Disperse Red 1 (DR1), the most well-known azo dye showed that the photoisomerization was dominant at this wavelength and that the azo-carbazole dyes were faster in response time and more resistive to erasure than DR1.

Control of Crystal Morphology of Aromatic Electron Donors and Acceptors for Organic Electronics

Low-molar-mass organic compounds can form oriented crystal fibers on various substrates by a simple casting technique. A dilute solution of an aromatic organic compound, such as a tetrathiofulvalene derivative, an annulated aromatic such as tetracene, or a perylene diimid, is cast on a substrate, and the solvent is allowed to evaporate while rolling a glass bar over the substrate. Because the crystal formation takes place at the solution edge, crystals grow in the roller direction. Optical microscopy, fluorescence microspectrometry, and scanning nearfield optical microscopy were used to characterize the resulting fibers and crystals. Casting on interdigitated electrodes enabled us to measure conductivity and field effect transistors in these oriented crystals.

Organic Field-Effect Transistors Using Polydiacetylene Derivatives as an Active Layer

To enhance the field-effect mobility, we tried to control the interchain distance (or packing density of polymer chains) in polydiacetylene thin films by changing the side chain length of diacetylene monomers. We selected five different alkyl chain lengths of diacetylene monocarboxylic acid. An OFET device was prepared by a vacuum deposition of diacetylene monomer, followed by a photopolymerization under Xe-lamp excitation (bottom-contact type device). In this series of samples, we obtained a maximum field-effect mobility of 4.4 × 10−2 cm2/Vs in the OFET using 10, 12-heptacosadiynoic acid (HDA). Controlling the substrate temperature from 28 to 70°C, we found the optimum substrate temperature of 40°C for HDA monomer deposition, and obtained a high field-effect hole mobility of 0.1 cm2/Vs in the bottom source-drain electrode configuration.

Formation of photo-induced index grating in azo-carbazole dye-doped polymer

An azo-carbazole dye, 3-[(4-Nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzEtOH), and its relatives doped in polyacrylate films have been known to show ‘photorefractive’ effect without external electric field even in symmetric optical alignment. We have already observed strong energy transfer due to phase-shifted grating by two-beam coupling experiments made for NACzEtOH doped PMMA films. Although the operation mechanism is still unknown, the high efficiency of diffraction is very promising for the application to real-time holography. For the convenience of analysis, we modified Kogelnik’s theory for thick grating by correcting the constraint condition and applied it to evaluate refractive index modulation and other parameters quantitatively. In order to clarify the operation mechanism, we conducted the writing and erasing of gratings by using red and green laser beams and analyzed the diffraction magnitude and response dynamics, showing that the response time strongly depended on the writing wavelength, and that the grating formation and its phase shift were not always synchronized.

Roll-Off Characteristics of Electroluminescence Efficiency of Organic Blue Electrophosphorescence Diodes

We studied the relaxation of electroluminescence roll-off characteristics by changing the dopant concentration and thickness of the emitting layer in organic blue electrophosphorescence diodes composed of a 2,6-dicarbazolo-1,5-pyridine host doped with iridium(III)-bis[(4,6-di-fluorophenyl)-pyridinato-N,C2]picolinate. Analyzing the roll-off characteristics using the theoretical models of triplet–triplet annihilation and triplet–polaron annihilation (TPA), we found that the roll-off characteristics of these devices were determined primarily by TPA.

PHOTOREFRACTIVE POLYMER WITH HIGH OPTICAL GAIN UNDER NON-ELECTRIC FIELD

We developed new composite polymer system with monolithic azo dye substances (NACzE and NACzH) and high optical quality polymer hosts (PMMA and PEMA), and obtained quite high photorefractive performances of two beam coupling (2BC) gain 451 cm-1 (PMMA/NACzE) and 340 cm-1 (PEMA/NACzH), respectively, without applying external electric field.