Akiko Nakao

Patternable Solution‐Crystallized Organic Transistors with High Charge Carrier Mobility

Development of high-performance printed semiconductor devices is highly desired with the expectation for the nextgeneration technologies of “printable electronics” providing simply fabricated, fl exible, large-area, low-cost, and environmentally friendly electronic products such as paper-like fl exible displays. Patterned arrays of printed organic fi eld-effect transistors (OFETs) based on chemically stable solutionprocessed organic semiconductors are regarded as key devices that operate as fundamental switching components in, for example, pixel-controlling active-matrix elements. However, performance of conventional solution-coated noncrystal organic thin-fi lm transistors has yet to be improved for practical use in general electronic circuitry. Here, newly developed arrays of patterned crystalline OFETs of air-stable compound 2,9-didecyl-dinaphtho[2,3-b:2’,3’-f ]thieno[3,2-b]thiophene (C 10 -DNTT) formed from hot solution are presented. A method of oriented growth is introduced to provide the singlecrystalline fi lms of C 10 -DNTT that regulates the crystallizing direction and positions in a single process. The benchmark value, 10 cm 2 V − 1 s − 1 , of the charge mobility is achieved for the present OFETs, far exceeding the performance of former devices and opening a practical way to realize printed and fl exible electronics with suffi cient switching speed. The result is attributed to almost perfect molecular periodicity in the crystal fi lms, which allows effective intermolecular charge transport of the electrons.

Three-way switching in a cyanide-bridged [CoFe] chain

Bistable compounds that exist in two interchangeable phases under identical conditions can act as switches under external stimuli. Among such switchable materials, coordination complexes have energy levels (or phases) that are determined by the electronic states of their constituent metal ions and ligands. They can exhibit multiple bistabilities and hold promise in the search for multifaceted materials that display different properties in different phases, accessible through the application of contrasting external stimuli. Molecular systems that exhibit both thermo- and photoinduced magnetic bistabilities are excellent candidates for such systems. Here we describe a cyanide-bridged [CoFe] one-dimensional chiral coordination polymer that displays both magnetic and electric bistabilities in the same temperature range. Both the electric and magnetic switching probably arise from the same electron-transfer coupled spin-transition phenomenon, which enables the reversible conversion between an insulating diamagnetic phase and either a semiconducting paramagnetic (thermoinduced) or a type of ferromagnetic single-chain magnet (photoinduced) state.

Solution-crystallized organic field-effect transistors with charge-acceptor layers: high-mobility and low-threshold-voltage operation in air.

For the development of low-cost fl exible electronic devices organic fi eld-effect transistors (OFETs) are highly anticipated for use in fundamental switching components because OFETs allow easy production routes from solution at low temperatures, which do not damage the plastic substrates. Processes such as the spin-coating of polymers or polycrystalline thin fi lms are indeed very advantageous because they allow mass production on large-area plastic backplanes. However, the typical performance of solution-coated organic thin-fi lm transistors is not yet satisfactory for their expected use in common applications such as active matrices in large-area fl exible displays. Though mobility of more than 10 cm 2 V − 1 s − 1 is achieved for devices based on vapor-grown organic single crystals, [ 1–3 ] these “hand-made” devices are not suitable for industrial production. In addition, an equally important requirement for their practical usage is stable operation in ambient atmosphere. Here, we report high-mobility organic single-crystal transistors of air-stable compound 2,7-dioctyl[1]benzothieno[3,2b ][1]benzothiophene (C 8 -BTBT) treated with a 2,3,5,6-tetrafl uoro-7,7,8,8tetracyanoquinodimethane (F 4 -TCNQ) solution. A method of oriented growth is employed to provide fully single-crystal domains of the C 8 -BTBT main channels, regulating crystallographic direction during the fi lm growth. Charge mobility as high as 3.5–6 cm 2 V − 1 s − 1 is achieved in the saturation regime, owing to the almost perfectly periodic crystal packing that allows effective intermolecular exchange of π electrons. Excellent air stability due to the high ionization potential is reported for C 8 -BTBT, [ 4 ] though it had the drawback of a relatively high

High Electron Mobility in Air for N,N′‐1H,1H‐Perfluorobutyldicyanoperylene Carboxydi‐imide Solution‐Crystallized Thin‐Film Transistors on Hydrophobic Surfaces

After remarkable progress in developing organic semiconductors during the last decade, it remains challenging to achieve highmobility, air-stable and solution-processed organic fi eld-effect transistors (OFETs) based on electron-transporting (n-type) materials with a performance comparable to that of hole-transporting (p-type) organic semiconductors. [ 1 ] Both types of semiconductor are needed for industrial applications based on high-speed complementary logic devices. As seen for complementary metaloxide-semiconductor (CMOS) devices in current silicon technology, high-performance, organic complementary circuits will enable the development of low-cost and large-area electronic devices, even on plastic substrates via simple printing processes. For this purpose, it is necessary to develop high-performance devices processed at near room temperature from solution. Since the performance of solution-processed p-type organic semiconductors has advanced recently, it is now needed to improve the performance of the corresponding n-type OFETs. Although the highest mobility for molecular semiconductors that has been reported for solution-crystallized p-type OFETs is > 10 cm 2 V − 1 s − 1 , [ 2a ] the maximum value reported for n-type devices is only 0.16 cm 2 V − 1 s − 1 . [ 2b ] We have developed a technique to form highly crystalline, n-type, organic semiconductor fi lms on low-surface-energy gate dielectrics so that a carrier mobility > 1 cm 2 V − 1 s − 1 is achieved for an important material, N , N′ -1 H ,1 H -perfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN 2 ), when processed in ambient atmosphere.

Hydrogen bond-promoted metallic state in a purely organic single-component conductor under pressure

Purely organic materials are generally insulating. Some charge-carrier generation, however, can provide them with electrical conductivity. In multi-component organic systems, carrier generation by intermolecular charge transfer has given many molecular metals. By contrast, in purely organic single-component systems, metallic states have rarely been realized although some neutral-radical semiconductors have been reported. Here we uncover a new type of purely organic single-component molecular conductor by utilizing strong hydrogen-bonding interactions between tetrathiafulvalene-based electron-donor molecules. These conductors are composed of highly symmetric molecular units constructed by the strong intra-unit hydrogen bond. Moreover, we demonstrate that, in this system, charge carriers are produced by the partial oxidation of the donor molecules and delocalized through the formation of the symmetric intra-unit hydrogen bonds. As a result, our conductors show the highest room-temperature electrical conductivity and the metallic state under the lowest physical pressure among the purely organic single-component systems, to our knowledge.

Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism: A Phase Transition Caused by Deuterium and Electron Transfer in a Hydrogen-Bonded Purely Organic Conductor Crystal

A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

High-Speed Flexible Organic Field-Effect Transistors with a 3D Structure

Organic semiconductor materials offer fl exible platforms for charge current due to their weak van der Waals interaction between π -conjugated organic molecules such that the transport of electrons or holes is activated with modest mobility. Making use of such material properties, technologies of fl exible organic fi eld-effect transistors (OFETs) are in the process of developing attractive devices with fl exible, stretchable, light-weight, low-cost, and low-power-consumption switching components, such as active-matrix elements for plastic displays, [ 1–4 ] sensor arrays, [ 5 , 6 ]

Frustration-Induced Valence-Bond Ordering in a New Quantum Triangular Antiferromagnet Based on (Pd(dmit)2)

The temperature dependence of magnetic susceptibility, χ( T ), of the P 2 1 / m crystal of a novel [Pd(dmit) 2 ] salt, (C 2 H 5 )(CH 3 ) 3 P[Pd(dmit) 2 ] 2 , has been measured. The observed χ( T ) above 30 K is reproduced by the model of a spin-1/2 Heisenberg antiferromagnet on a triangular lattice with the exchange coupling, J / k B = 250 K. A phase transition at 25 K to a spin-gapped state has been found. As well as a spin-Peierls transition, this transition breaks the translational symmetry, leading to the formation of a columnar arrangement of the spin-singlet pairs. A significant role of the lattice structure in stabilizing the spin-singlet ordering is pointed out. To our knowledge, this is the first report of a two-dimensional (2D) spin system exhibiting a spin-Peierls-like transition.

Structural Study of Low Temperature Charge-Separated Phases of Pd(dmit)2-Based Molecular Conductors

We have studied crystal structures of two Pd(dmit) 2 -based molecular conductors, Et 2 Me 2 Sb[Pd(dmit) 2 ] 2 and Cs[Pd(dmit) 2 ] 2 , above and below their phase transition temperatures. The Et 2 Me 2 Sb salt undergoes a first-order transition from a Mott insulator to a non-magnetic state at ca. 70 K, while the Cs salt exhibits a metal-to-insulator (M–I) transition at ca. 56 K with decreasing temperature. These salts are composed of crystallographically equivalent dimeric units [Pd(dmit) 2 ] 2 - above the transition temperatures. The low temperature structures have been revealed to have two crystallographically independent dimers arranged alternately along the b axis. The structural changes of the dimers have been analyzed by calculations of overlap integrals, to show that the degree of the dimerization changes through the phase transitions. This indicates that both of the phase transitions are characterized by the charge separation, 2dimer - →dimer 0 + dimer 2- . Crystal structure of a related compound, ...

Electron doping in the cubic perovskiteSrMnO3: Isotropic metal versus chainlike ordering of Jahn-Teller polarons

Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distortion, accompanied by a quasi-one-dimensional 3z^2-r^2 orbital ordering with the C-type antiferromagnetism. The self-organization of such dilute polarons may reflect the critical role of the cooperative Jahn-Teller effect that is most effective in the originally cubic system.