Katsuhiko Fujita

Ambipolar pentacene field-effect transistors with calcium source-drain electrodes

Field-effect transistors consisted of vacuum-sublimed polycrystalline pentacene films and calcium source-drain electrodes were prepared and device characteristics were evaluated in an oxygen-free condition. The field-effect transistor showed typical ambipolar characteristics and field-effect hole mobility of 4.5×10−4cm2/Vs and field-effect electron mobility of 2.7×10−5cm2∕Vs were estimated from saturation currents. Appearance of an electron enhancement mode in pentacene field-effect transistors was ascribed to the lowering of barrier for electron injection at source-drain electrodes. Effective elimination of electron traps using an oxygen-free condition was found to be another requirement for the observation of ambipolar behavior in pentacene.

Carbazole Dendrimers as Solution‐Processable Thermally Activated Delayed‐Fluorescence Materials

Recently, thermally activated delayed fluorescence (TADF) materials have received increasing attention as effective emitters for organic light-emitting diodes (OLEDs). However, most of them are usually employed as dopants in a host material. In this report, carbazole dendrimers with a triphenyl-s-triazine core are reported, which are the first solution-processable, non-doped, high-molecular-weight TADF materials. The dendrimers were obtained by a new and facile synthetic route using the tert-butyldimethylsilyl moiety as a protecting group. All dendrimers showed TADF in toluene. Measurements of the temperature-dependent luminescence lifetime revealed that spin-coated neat films also showed TADF with moderate quantum yields. OLED devices incorporating these dendrimers as spin-coated emitting layers gave external quantum efficiencies of up to a 3.4 %, which suggests that this device is harvesting triplet excitons. This result indicates that carbazole dendrimers with attached acceptors are potential TADF materials owing to their polarized electronic structure (with HOMO-LUMO separation).

Organic Field-Effect Transistors with Gate Dielectric Films of Poly-p-Xylylene Derivatives Prepared by Chemical Vapor Deposition

We have fabricated organic field-effect transistors (OFETs) using various dielectric films of poly-p-xylylene derivatives to investigate the correlation of field-effect mobility and the surface properties of the dielectric films under constant conditions of fabrication process and molecular backbone. The OFET using pentacene as the semiconductor and poly-chloro-p-xylylene as the dielectric film showed good performance for an OFET using a polymer dielectric film; the field-effect mobility was 0.81 cm2/Vs and the on/off current ratio was 1.4×106. We observed an obvious tendency for the hydrophobic dielectric layers to give a higher field-effect mobility for both crystalline organic semiconductors and amorphous polymer semiconductor, though a significant correlation of field-effect mobility with the dielectric constant and surface roughness of the dielectric films was not observed.

Preparation of organic bulk heterojunction photovoltaic cells by evaporative spray deposition from ultradilute solution

An organic bulk heterojunction photovoltaic cell (OPC) having a thin layer consisting of a functionalized fullerene and a poly-p-phenylenevinylene (PPV) derivative was prepared from a highly diluted tetrahydrofuran solution at the concentration below 1 ppm by the evaporative spray deposition from ultradilute solution (ESDUS) technique. The power conversion efficiency of the OPC prepared from 1:1 mixture solution of fullerene/PPV derivative at the concentration of 1 ppm was 0.69%, which was almost comparable to that prepared by the conventional spin coating technique from 0.5 wt % chlorobenzene solution of the mixture. Although the bulk heterojunction OPCs are known to show relatively high power conversion efficiency, they have two major problems for development of higher device performance. One is a solubility problem of used materials and the other is a strong dependence of the device performance on the preparation solvent. The former has been a stumbling block to improve the performance by examining var...

The Shift from “Hard” to “Soft” Electronics

Organic electronics have emerged as a promising new technology, with likely applications in areas such as flexible displays and electronic labels. This is reflected in a shift in research and development efforts from traditional hard materials such as silicon to soft, organic materials. In this article, the significant breakthroughs achieved in this field over the last 50 years are summarized and the major differences between hard and soft electronics are highlighted. It is envisaged that organic electronics will play an important role in the 21st century.

Synthesis of Copper–Antimony-Sulfide Nanocrystals for Solution-Processed Solar Cells

The p-type nanocrystals (NCs) of copper-based chalcogenides, such as CuInSe2 and Cu2ZnSnS4, have attracted increasing attention in photovoltaic applications due to their potential to produce cheap solution-processed solar cells. Herein, we report the synthesis of copper-antimony-sulfide (CAS) NCs with different crystal phases including CuSbS2, Cu3SbS4, and Cu12Sb4S13. In addition, their morphology, crystal phase, and optical properties were characterized using transmission electron microscopy, X-ray diffractometry, UV-vis-near-IR spectroscopy, and photoemission yield spectroscopy. The morphology, crystal phase, and electronic structure were significantly dependent on the chemical composition in the CAS system. Devices were fabricated using particulate films consisting of CAS NCs prepared by spin coating without a high-temperature treatment. The CAS NC-based devices exhibited a diode-like current-voltage characteristic when coupled with an n-type CdS layer. In particular, the CuSbS2 NC devices exhibited photovoltaic responses under simulated sunlight, demonstrating its applicability for use in solution-processed solar cells.

Charge recombination electroluminescence in organic thin-film devices without charge injection from external electrodes

Organic thin-film electroluminescent (EL) devices with a double-insulated structure, Al electrode/polymer insulator layer/ambipolar EL layer/ITO nanoparticles layer/ambipolar EL layer/polymer insulator layer/ITO electrode, were fabricated. The ambipolar EL layer was the composition of poly (N-vinylcarbazole) as a hole transport material and 2,5-bis(4-naphthyl)-1,3,4-oxadiazole as an electron transport material in 65∕35 molar ratio with added 0.6mole% coumarin-6 for emissive centers. When the devices were driven with ac voltage, bright surface emission (81cd∕m2 at 250V and 300kHz) was observed. Emission is due to the recombination of holes and electrons that are generated from ITO nanoparticles embedded in organic layers. Charge recombination EL is observed without charge injection from external electrodes in double-insulated organic EL devices.

Novel Method for Polymer Thin Film Preparation: Spray Deposition of Highly Diluted Polymer Solutions

A spray deposition method is proposed as a novel way to prepare organic thin films. In this method, a highly diluted solution of a functional organic material, such as a π-conjugated polymer, is nebulized into air and concentrated under a controlled evaporation condition. The resulting aerosol is transported by a carrier gas and deposited onto a solid substrate. We successfully prepared a thin film of a poly-p-phenylenevinylene derivative from a highly diluted solution at the concentration of 10-4 wt% and fabricated an electroluminescent device exhibiting surface emission.

Flexible organic field-effect transistors fabricated by the electrode-peeling transfer with an assist of self-assembled monolayer

We propose a way to fabricate a flexible organic field-effect transistor: an electrode-peeling transfer method. In this method, source–drain metal electrodes were formed finely on rigid temporary substrate, where a micropatterning process such as photolithography is applicable. The electrodes were treated with an alkane thiol to form a self-assembled monolayer followed by complete covering of the temporary substrate via chemical vapor deposition of an organic dielectric layer. After the gate electrode was deposited on the top, the multilayer of the source–drain electrodes/dielectric layer/gate electrode was peeled off from the temporary substrate by an adhesive Scotch tape substrate without deletion. The peeling-transfer was completed with an assist of a self-assembled monolayer as a connecting buffer layer between the electrodes and the dielectric layer. Any organic semiconductor materials can be deposited on freshly peeled-off surface of the flexible substrate. In the present case, pentacene was used as...

Mulifunctional Dendritic Emitter: Aggregation-Induced Emission Enhanced, Thermally Activated Delayed Fluorescent Material for Solution-Processed Multilayered Organic Light-Emitting Diodes

Thermally activated delayed fluorescence (TADF) materials emerged as promising light sources in third generation organic light-emitting diodes (OLED). Much effort has been invested for the development of small molecular TADF materials and vacuum process-based efficient TADF-OLEDs. In contrast, a limited number of solution processable high-molecular weight TADF materials toward low cost, large area, and scalable manufacturing of solution processed TADF-OLEDs have been reported so far. In this context, we report benzophenone-core carbazole dendrimers (GnB, n = generation) showing TADF and aggregation-induced emission enhancement (AIEE) properties along with alcohol resistance enabling further solution-based lamination of organic materials. The dendritic structure was found to play an important role for both TADF and AIEE activities in the neat films. By using these multifunctional dendritic emitters as non-doped emissive layers, OLED devices with fully solution processed organic multilayers were successfully fabricated and achieved maximum external quantum efficiency of 5.7%.