Mayumi Uno

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.

Very High Mobility in Solution-Processed Organic Thin-Film Transistors of Highly Ordered [1]Benzothieno[3,2-b]benzothiophene Derivatives

Field-effect mobility as high as 5 cm2/(V s) is achieved in solution-processed organic thin-film transistors with the development of a method for growing highly-oriented crystalline films of [1]benzothieno[3,2-b]benzothiophene derivatives. A droplet of the solution is sustained at an edge of a structure on an inclined substrate, so that the crystalline domain grows in the direction of inclination. The oriented growth realizes excellent molecular ordering that manifests itself in micrometer-scale molecular terraces on the surface as a result of the self-organizing function of the material. The unprecedented performance achieved using an easy fabrication process has increased attractiveness of organic thin-film transistors for industrial applications.

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

Rewritable Dual-Layer Phase-Change Optical Disk Utilizing a Blue-Violet Laser

We have demonstrated for the first time the feasibility of using a rewritable dual-layer phase-change optical disk utilizing a blue-violet laser. For the first medium, we adopted a very thin recording layer with a new phase-change material Ge–Sn–Sb–Te, and a 10-nm-thick silver-alloy reflective layer to obtain a large transmittance and high-quality signals. For the second medium, we optimized the thickness of each layer to obtain both a large optical absorption of the recording layer and a small heat capacity. Carrier-to-noise ratios of more than 50 dB, erasability of more than 30 dB and recording powers of 8 mW for the first medium and 11 mW for the second medium were obtained under typical recording conditions corresponding to a capacity of 27 GB per one side of a 120 mm disk and a user data transfer rate of 33 Mbps.

High-mobility double-gate organic single-crystal transistors with organic crystal gate insulators

High-mobility organic transistors are fabricated on both surfaces of approximately 1-μm-thick rubrene crystals, molecularly flat over an area of 10×10μm2. A thin platelet of 9,10-diphenylanthracene single crystal and surface-passivated SiO2 are used for the gate insulators. Because of the minimized densities of hole-trapping levels at the interfaces and in the rubrene crystal, the field-induced carriers do not necessarily reside near the interface but are distributed in the bulk of the semiconductor by adjusting the two gate voltages. Making use of the highly mobile carriers in the inner crystal, the mobility is maximized to ∼43cm2∕Vs.

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 ]

Flexible Three‐Dimensional Organic Field‐Effect Transistors Fabricated by an Imprinting Technique

IO N Implementing semiconductor devices of soft organic materials on substrates of plastic materials gives rise to semiconductor technology in fl exible electronics, [ 1–6 ] which can induce such innovative products as bendable displays, wearable computers, and low-cost soft radio-frequency identifi cation tags for future smart society. Performances of the fl exible switching devices are, however, not necessarily satisfactory yet to broadly attract industrial interest, requiring further advance in material research both on the organic semiconductor devices and on the plastic substrates. Here, we disclose newly developed three-dimensional organic fi eld-effect transistors (3D-OFETs), where multiple semiconductor channels of dinaphtho[2,3b :2 ′ ,3 ′ f ]thieno[3,2-b]thiophenes (DNTT) [ 7 ] are vertically implemented on mass-producible plastic 3D structures fabricated by using an ultraviolet (UV) imprinting technique. The 3D-OFETs are characterized by densely integrated vertical channels with much shorter length than conventional planar devices, so that they exhibit extremely high current density and high-speed switching on hard substrates, [ 8 ] both of which are essential in high-performance fl exible electronics devices. Developing a method to combine the high performance with the simple and low-cost molding technique using a plastic material, the present report launches the devices towards practical applications. 3D plastic molding is now widely used for manufacturing various industrial products of plastics such as optical disks, opticalswitching components, and medical sensors, all of which are indeed mass-produced at low cost. Imprint lithography is also a well-known simple and high-resolution process, which enables fabrication of nano/micro-meter-scale patterns on plastic substrates. In the imprinting process, 3D structures on the surface of hard materials such as silicon or fused glass are translated to photo-cured plastics at room temperature, so that fi ne patterns are properly preserved without thermal deformation. The method is highly effi cient in production because translation can be repeated using the same mother patterns of hard materials. Since S. Chou’s group reported the nano-imprint technology

Three-dimensional organic field-effect transistors: Charge accumulation in the vertical semiconductor channels

A three-dimensional organic field-effect transistor is developed to accumulate charge in its vertical semiconductor channel so that space availability for the field-induced carriers is essentially multiplied. A multicolumnar structure is built incorporating a vertical layer of soluble benzothieno-benzothiophene derivative. Pronounced field-effect performance is realized with the well-defined saturation in the output, where extremely large value of channel width divided by length enables current amplification up to ∼2 μA in a square pixel even with relatively low carrier mobility of 10−4 cm2/V s. The result demonstrates usefulness of the three-dimensional structure in achieving sufficient current for matrix-controlling devices in flat-panel displays.

Moderately anisotropic field-effect mobility in dinaphtho[2,3-b:2′,3′-f]thiopheno[3,2-b]thiophenes single-crystal transistors

Anisotropy of carrier mobility is measured for dinaphtho[2,3-b:2′,3′-f]thiopheno[3,2-b]thiophenes single-crystal transistors. We have developed a method of “local gating” to restrict carrier-accumulated channels elongated radially within the herringbone planes of submillimeter crystals so that mixture of conductivity off the intended directions is minimized in the measurement. The highest mobility 4 cm2/V s is achieved for the a-axis direction due to the highest orbital overlaps, while the lowest mobility measured in the perpendicular direction is still as high as 2.5 cm2/V s. The moderate anisotropy favors high performance in polycrystalline thin-film transistors of the compound, where charge transport is inevitably mixed for all directions.

Phase-change material for use in rewritable dual-layer optical disk

A thin film of Sn-doped and GeTe-rich GeTe-Sb2Te3 shows characteristics that make it suitable for use in rewritable dual-layer optical disks employing a violet laser. By increasing the GeTe component form Ge2Sb2Te5 to Ge4Sb2Te7, and Ge8Sb2Te11, optical changes were increased. By substituting Sn for a proposition of Ge in these compositions, crystallization rates are greatly increased and even a 5 nm-thick film showed a very short laser-crystallization time of less than 50 ns. The material film was successfully applied to Layer 0 of rewritable dual-layer disk: capacity of 27 GB and a 33 Mbps data transfer rate were confirmed for a disk using a conventional 0.6 mm substrate, and 45 GB capacity and the same data transfer rate were obtained for another disk using thin cover layer 0.1 mm thick.