Kazuo Takimiya

Flexible Low‐Voltage Organic Complementary Circuits: Finding the Optimum Combination of Semiconductors and Monolayer Gate Dielectrics

Low-voltage p-channel and n-channel organic transistors with channel lengths down to 0.5 μm using four small-molecule semiconductors and ultra-thin dielectrics based on two different phosphonic acid monolayers are fabricated on plastic substrates and studied in terms of effective mobility, intrinsic mobility and contact resistance. For the optimum materials combination, flexible complementary circuits have signal delays of 3.1 μs at 5 V.

Naphthodithiophenediimide (NDTI)-based triads for high-performance air-stable, solution-processed ambipolar organic field-effect transistors

Two new NDTI-based triad-type ambipolar molecular semiconductors (NDTI-BBT and NDTI-BNT) were designed and synthesized. The triads can afford solution-processed OFETs with well-balanced, high hole and electron mobilities, up to 0.25 and 0.16 cm2 V−1 s−1, respectively, which further leads to the successful demonstration of complementary-like inverters with high voltage gains of 281 and 254 in the first and third quadrants, respectively, under ambient conditions.

Transient nature of graphene quantum dot formation via a hydrothermal reaction

A facile, economic and environmentally friendly one-step approach for the preparation of highly luminescent graphene quantum dots (GQDs) was developed using a hydrothermal reaction between citric acid and urea. Unlike previous reports, we focused on the effect of the transient nature of GQD formation on the photoluminescence (PL) properties and molecular structure changes of the products. We found that the GQDs have an optimum reaction time and require an effective precursor to achieve excellent luminescent properties. The PL, ultraviolet-visible (UV-vis) absorption, zeta potential, and nuclear magnetic resonance (NMR) analyses of the GQDs prepared at various reaction times revealed that the molecular structures responsible for the luminescence of the GQDs are aggregates or condensation products of citric acid amides. We found that urea addition to the precursor drastically enhances the PL intensity of the GQDs, and it is 40 times higher than those prepared using the pure citric acid precursor. Additionally, a GQDs–polyvinyl alcohol composite achieved an excellent quantum yield (QY) of 43.6%.

Reversible Dimerization and Polymerization of a Janus Diradical To Produce Labile C−C Bonds and Large Chromic Effects

Conducting polymers can be synthesized by irreversible diradical monomer polymerization. A reversible version of this reaction consisting of the formation/dissociation of σ-dimers and σ-polymers from a stable quinonoidal diradical precursor is described. The reaction reversibility is made by a quinonoidal molecule which changes its structure to an aromatic species by forming weak and long intermolecular C-C single bonds. The reaction provokes a giant chromic effect of about 2.5 eV. The two opposite but complementary quinonoidal and aromatic tautomers provide the Janus faces of the reactants and products which produces the observed chromic effect. A reaction mechanism is proposed to explain the variety of final products starting with structurally very similar reactants. These reversible reactions, covering an unusual regime of weak covalent supramolecular bonding, yield products which might be envisaged as novel molecular and polymeric soft matter phases.

Naphtho[2,3-b]thiophene diimide (NTI): a mono-functionalisable core-extended naphthalene diimide for electron-deficient architectures

A new π-extended naphthalene diimide with one fused-thiophene ring, naphtho[2,3-b]thiophene diimide (NTI), was synthesized. Taking advantage of its mono-functionalisability, NTI was incorporated into various “NTI-terminated” π-conjugated architectures, which functioned as n-type organic semiconductors for field-effect transistors and solar cells.

A Flexible Cyclophane: Design, Synthesis, and Structure of a Multibridged Tris-tetrathiafulvalene (TTF) Macrocycle

The tris-tetrathiafulvalene (TTF) macrocycles 3 with a large end-cavity were effectively synthesized from the readily available tetrakis(cyanoethylthio)TTF by means of a selective deprotection/realkylation sequence followed by an intramolecular coupling reaction. Crystar structure analyses revealed that the neutral molecules include two (3a) or one chloroform molecule (3b) as solvent of crystallization inside the cavity, whereas the I3- salt of 3b, obtained by electrocrystallization, has a molecular structure which is different from that of the neutral molecule in that the cavity has completely collapsed.

A comprehensive study of charge trapping in organic field-effect devices with promising semiconductors and different contact metals by displacement current measurements

A systematic and comprehensive study on the charge-carrier injection and trapping behavior was performed using displacement current measurements in long-channel capacitors based on four promising small-molecule organic semiconductors (pentacene, DNTT, C10-DNTT and DPh-DNTT). In thin-film transistors, these semiconductors showed charge-carrier mobilities ranging from 1.0 to 7.8 cm2 V−1 s−1. The number of charges injected into and extracted from the semiconductor and the density of charges trapped in the device during each measurement were calculated from the displacement current characteristics and it was found that the density of trapped charges is very similar in all devices and of the order 1012 cm−2, despite the fact that the four semiconductors show significantly different charge-carrier mobilities. The choice of the contact metal (Au, Ag, Cu, Pd) was also found to have no significant effect on the trapping behavior.

The Elusive Ethenediselone, Se=C=C=Se

The neutral ethenediselone, Se=C=C=Se, has been characterised by neutralisation–reionisation mass spectrometry, which implies a minimum lifetime of the order of microseconds. Tetraselenafulvalene 1 and tetramethyltetraselenafulvalene 2 were used as precursor molecules. Flash vacuum thermolysis (FVT) of these compounds with isolation of the products in Ar matrices permitted the identification of ethyne, 2-butyne, CSe2, and selenoketene, H2C=C=Se, but at best traces of Se=C=C=Se survived the FVT/matrix isolation experiment. Multiconfigurational calculations indicate that Se=C=C=Se is a ground state triplet molecule with a very small singlet-triplet gap.

Selective thionation of naphtho[2,3-b]thiophene diimide: tuning of the optoelectronic properties and packing structure

Naphtho[2,3-b]thiophene diimide (NTI) was found to be selectively thionated to afford naphtho[2,3-b]thiophene-4,9-dicarboxy-5,8-dithiocarboxydiimide (NTI-2S). By elucidating its electrochemical, optical, and structural properties, the effects of thionation on the molecular electronic structure and crystal structure were discussed. Although thionation can greatly lower the LUMO energy level and enhance the intermolecular interaction in the solid state, its chemical lability is a drawback as an organic semiconducting material.

Cumulative gain in organic solar cells by using multiple optical nanopatterns

Light wave manipulation by using nanostructures is a promising strategy for enhancing the light absorption of thin photoactive layers in organic photovoltaics (OPVs). Here, we propose a method for nanopatterning the multiple interfaces in bulk heterojunction (BHJ) OPVs by using soft imprint lithography at room temperature. The interfaces in the OPVs were separately modified in the front ZnO layers and the back metal electrodes with a grating pattern. Each nanopattern increased the light absorption and the power conversion efficiency of the OPVs by up to 32.5% depending on the materials. Moreover, the nanopatterning at both the front and the back cumulatively increased the light absorption, resulting in the highest efficiency increase of 38.5%. The increases were observed in various BHJ systems with different properties containing the polymers PTB7, PCE10, P3HT, or PNTz4T. A certified performance of 10.31% was achieved for the PNTz4T:PC71BM system in the presence of the nanopatterns. Detailed analysis by using the absorption spectra and optical simulations indicated that the origins of the optical gains from the nanopatterns on the front and the back are different. The front pattern increases the transmittance and the back pattern increases the scattering and excites the surface plasmon polaritons.