Masashi Mamada

Fabrication of Ultra-Thin printed organic TFT CMOS logic circuits optimized for low-voltage wearable sensor applications

Ultrathin electronic circuits that can be manufactured by using conventional printing technologies are key elements necessary to realize wearable health sensors and next-generation flexible electronic devices. Due to their low level of power consumption, complementary (CMOS) circuits using both types of semiconductors can be easily employed in wireless devices. Here, we describe ultrathin CMOS logic circuits, for which not only the source/drain electrodes but also the semiconductor layers were printed. Both p-type and n-type organic thin film transistor devices were employed in a D-flip flop circuit in the newly developed stacked structure and exhibited excellent electrical characteristics, including good carrier mobilities of 0.34 and 0.21 cm2 V−1 sec−1, and threshold voltages of nearly 0 V with low operating voltages. These printed organic CMOS D-flip flop circuits exhibit operating frequencies of 75 Hz and demonstrate great potential for flexible and printed electronics technology, particularly for wearable sensor applications with wireless connectivity.

Synthesis, Physical Properties, and Field-Effect Mobility of Isomerically Pure syn-/anti-Anthradithiophene Derivatives

Isomerically pure syn-/anti-isomers of 2,8-dimethylanthradithiophene (DMADT) were synthesized in five steps and characterized using thermogravimetry, X-ray single crystal analysis, UV-vis absorption, and electrochemical measurements. The physical properties in solution were slightly different for each isomer, whereby the more obvious differences were observed in the solid state. A field-effect transistor using the anti-isomer showed a much higher performance than that using the syn-isomer.

High performance organic field-effect transistors based on [2,2′]bi[naphtho[2,3-b]thiophenyl] with a simple structure

A novel semiconductor with naphtho[2,3-b]thiophene rings was synthesized and characterized by using single crystal X-ray structure analysis, absorption and emission spectra, electrochemical measurements, quantum chemical calculations, thin-film X-ray diffraction and AFM studies. FET devices using the molecule as the active layer showed high mobilities and high air stability. The hole mobility was enhanced to 0.67 cm2V−1 s−1 in air.

syn-/anti-Anthradithiophene Derivative Isomer Effects on Semiconducting Properties

Isomerically pure syn-/anti-anthradithiophene derivatives have been developed in the past few years. Although anti-isomers showed higher field-effect mobilities than mixture of isomers have been reported, a detailed comparison of syn-isomer and anti-isomer molecules has not been carried out. In this study, we took newly synthesized pure unsubstituted syn-/anti-anthradithiophenes (ADTs) and compared their single crystal structures, physical properties and semiconducting behavior with a previously studied syn-/anti-dimethylanthradithiophenes (DMADTs). Although the both isomers were typical herringbone packing structures with similar parameters, anti-isomers involved less disordered atoms in the crystal packing. The results from thermal analysis, UV-vis spectra, photo luminescence spectra and cyclic voltammograms of syn-/anti-anthradithiophenes were nearly the in the solid state as well as in solution. However, field-effect transistors showed obvious differences with mobilities of 0.12 cm(2) V(-1) s(-1) for anti-anthradithiophene and 0.02 cm(2) V(-1) s(-1) for syn-anthradithiophene. Because the crystallinity of thin-films measured by X-ray diffraction (XRD) and atomic force microscopy (AFM) seems to be better in syn-isomers, the differences in transistor performance are likely attributed to local defects affecting intermolecular interactions, such as disorder in the crystal packing and charge-dipole interactions.

A Solution-Processed Organic Thin-Film Transistor Backplane for Flexible Multiphoton Emission Organic Light-Emitting Diode Displays

An active matrix backplane based on solution-processed organic thin-film transistors (OTFTs) has been developed for flexible displays having multiphoton organic light-emitting diodes (OLEDs). The OTFT device has a bottom-gate/bottom-contact type structure consisting of a polyethylene naphthalate-based flexible substrate covered with patterned gate electrodes and a gate dielectric of UV-cured cardo-polymer. Teflon AF was used for a hydrophobic bank structure that defines active regions of the OTFTs, and the organic semiconductor was coated by solution shearing. The OTFT backplane fabricated here had 384 × 128 pixels at a resolution of 300 dpi; images and video were successfully displayed on the resulting flexible multiphoton-emission OLED display.

Benzimidazole Derivatives: Synthesis, Physical Properties, and n‐Type Semiconducting Properties

A series of new benzimidazole derivatives were synthesized by the solid-state condensation and direct sublimation (SSC-DS) method and their physical properties were investigated. The reaction yields and product stability were significantly affected by the identity of the diamine and anhydride substituents. On the other hand, the substituents of the benzimidazole ring allowed fine tuning of the emission maxima, fluorescence quantum yields, and redox potentials. The HOMO-LUMO levels were estimated by cyclic voltammetry in film on indium tin oxide (ITO) and compared with values obtained by other methods. The described benzimidazoles showed high crystallinity, which is attributed to a high planarity and interactions between carbon and heteroatoms. These compounds showed n-type semiconducting behavior in organic field-effect transistors (OFETs). Optimized devices for fluorinated NTCBI (naphthalene tetracarboxylic bisbenzimidazole) showed respectable electron mobilities of ∼10(-2)  cm(2)  V(-1)  s(-1) .

Low-Voltage and Hysteresis-Free N-Type Organic Thin Film Transistor and Complementary Inverter with Bilayer Gate Insulator

In this paper, we report on a low-voltage-operation n-type organic thin film transistor (OTFT) and a complementary inverter circuit using a thiazolothiazole derivative as an organic semiconductor (OSC). To achieve the low voltage and stable operation of the n-type OTFT, a bilayer structure consisting of a thin low-k polymer layer and a high-k Ta2O5 layer was used and investigated as the gate insulator. Atomic force microscopy images of the OSC film on the hydrophobic polymer showed densely distributed small grains, and the corresponding OTFT exhibited hysteresis-free and excellent n-type performance, for example, an electron mobility of 0.23–0.35 cm2 V-1 s-1. In addition, the n-type OTFT with a thinner polymer layer exhibited a low operating voltage of less than 10 V while maintaining excellent characteristics. A complementary inverter based on the thiazolothiazole derivative and pentacene was also fabricated using the bilayer gate insulator. The switching operation of the inverter was achieved at a low voltage of 5–10 V.

Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System

Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor–acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs.

Triclinic polymorph of dibenzotetra-thia-fulvalene.

Crystals of the title compound (DBTTF), C14H8S4, feature a triclinic polymorph different from two known monoclinic polymorphs. In this form, there are two independent centrosymmetric half-molecules in the asymmetric unit. Although the molecular orientations are relatively similar to one of monoclinic polymorphs, the packing motif is different.

Charge transport, carrier balance, and blue electrophosphorescence in diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide devices

Diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (EMPA1) displays a wide highest occupied molecular orbital–lowest unoccupied molecular orbital gap (4.1 eV), singlet (4.3 eV) and triplet (3.4 eV), and an electron-dominated charge transport that follows a trap-free space charge limited model with an average electron mobility of 5.7×10−6 cm2 V−1 s−1 and a hole mobility of 1.1×10−6 cm2 V−1 s−1. At high driving voltages (>6 V), ambipolar charge transport is observed, resulting in a balanced charge density in the active layer. Highly efficient blue phosphorescent organic light-emitting diodes were realized, showing a high external quantum efficiency (21%) and a luminance efficiency of 45 cd/A using a bis[2-(4′,6′-difluorophenyl)-pyridinato-N,C2′]iridium(III) picolinate dopant.