Kengo Nakayama

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.

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-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 ]

Monolithic complementary inverters based on organic single crystals.

In the history of the silicon semiconductor technology, the invention of integrated circuits (ICs) has yielded tremendous profi t in the practical economy. The integration technology has been based on fabricating numerous device components on a same monolithic single-crystal platform, allocating sets of p and n -channel metal-oxide-semiconductor fi eld-effect transistors (MOSFETs) for complementary NOT-logic elements, for example. Recently, organic semiconductors are argued as an emerging candidate for post-silicon semiconductor materials because of their unique advantages in low-cost fabrication processes near room temperature, light weight and mechanical fl exibility. So far, however, their compatibility to the monolithic circuitry integration is not yet examined on a single-component platform. In this communication, we present a novel monolithic complementary inverter on a single-crystal organic semiconductor in order to present viability in developing organic integrated circuits. This study is strongly motivated by remarkable progress in material development, device fabrication techniques and fundamental understanding of the charge transport mechanism, resulting in much improved performances of recent organic fi eld-effect transistors (OFETs). The intrinsic carrier transport mechanism is proven to be band transport in high-mobility organic single-crystal transistors, [ 1–3 ] as in silicon single crystals used for the ICs. Already several groups reported that mobility of organic single crystals is as high as 20 cm 2 /Vs or even higher, [ 4–6 ] so that 100 MHz clock frequency can be predicted with micrometer-channel devices assuming a standard charging model. Furthermore, techniques of solution-based crystallization is being developed very recently, [ 7–9 ] so that even mobility as high as 5 cm 2 /Vs is achieved from solution, [ 10 ] suggesting that low-cost production routes on fl exible panels can serve for mass-producing the platforms of low-price and mediumperformance integrated circuits. At present, fairly good inverter performances have been reported only for those with two different materials of p and n -channel organic-semiconductor compounds. [ 11–14 ] In this

Reduced contact resistances in organic transistors with secondary gates on source and drain electrodes

Secondary-gate electrodes under source and drain electrodes are introduced in organic thin-film transistors to reduce carrier-injection barriers into air-stable organic semiconductors. The additional gate electrodes buried in the gate insulators form “carrier-rich regions” in the vicinity of the source and drain electrodes with the application of sufficiently high local electric fields. Fabricating the structure with dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, known for its excellent air stability, the contact resistance is drastically reduced especially at low gate voltages in the main channel. The result demonstrates carrier injection from the same material realizing a minimized potential barrier in the absence of interfacial trap levels for metal-to-semiconductor junctions.

Organic single-crystal transistors with secondary gates on source and drain electrodes

Rubrene and tetracyanoquinodimethane single-crystal transistors are fabricated incorporating secondary gates (split gates) on source and drain electrodes to reduce the interfacial barriers at the metal/semiconductor contacts. Separating the effect of the injection barriers, the intrinsic carrier transport in the semiconductor channels is extracted for the p-type rubrene crystal transistors and the n-type tetracyanoquinodimethane crystal transistors. The transconductance of the tetracyanoquinodimethane devices is drastically improved by activating the split-gate electrodes, indicating significant injection barriers in the n-type transistors. The result demonstrates that the technique is useful to improve transistor performance when it is restricted by the injection barriers.

Reduced Contact Resistances in Organic Transistors with Secondary Gates on Source and Drain Electrodes

Secondary-gate electrodes are introduced in organic thin-film transistors to reduce carrier-injection barriers into air-stable organic semiconductors. The additional gate electrodes buried in the gate insulators under the source and drain electrodes form “carrier-rich regions” in the vicinity of source and drain electrodes with the application of sufficiently high local electric fields. Fabricating the structure with dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, known for its excellent air-stability, it turned out that the contact resistance is drastically reduced especially when operated at low gate voltage in the main channel. The result demonstrates carrier injection with a minimized potential barrier realizing that from the same semiconductor material in the absence of peculiar interfacial trap levels at metal-to-semiconductor junctions.

Three-dimensional Organic Field-effect Transistors on Plastic Substrates: Flexible Transistors with Very High Output Current

Attractiveness of organic field-effect transistors are in their low-cost and easy fabrication processes as well as their mechanical flexibility, while a significant drawback has been their poorer transistor performances than those of silicon and oxide semiconductors because of lower carrier mobility in organic semiconductors. We have developed an easy MEMS-based process to fabricate three-dimensional organic transistors with metal-insulator-semiconductor structures of multiple vertical channels on plastic platforms. The design maximizes the space availability and the output current per area. The flexible three-dimensional organic transistors indeed present outstanding current of ∼ 0.5 A/cm 2 , which is more than sufficient for driving pixels of typical organic light-emitting diodes. High on-off ratio up to 10 7 is also demonstrated.