Jun Takeya

Very high-mobility organic single-crystal transistors with in-crystal conduction channels

Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers. The interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, so that the inner channel plays a significant part in the transfer performance. With the in-crystal carriers less affected by scattering mechanisms at the interface, the maximum transistor mobility reaches 18cm2∕Vs and the contact-free intrinsic mobility turned out to be 40cm2∕Vs as the result of four-terminal measurement. These are the highest values ever reported for organic transistors.

Organic field-effect transistors using single crystals

Abstract Organic field-effect transistors using small-molecule organic single crystals are developed to investigate fundamental aspects of organic thin-film transistors that have been widely studied for possible future markets for ‘plastic electronics’. In reviewing the physics and chemistry of single-crystal organic field-effect transistors (SC-OFETs), the nature of intrinsic charge dynamics is elucidated for the carriers induced at the single crystal surfaces of molecular semiconductors. Materials for SC-OFETs are first reviewed with descriptions of the fabrication methods and the field-effect characteristics. In particular, a benchmark carrier mobility of 20–40 cm2 Vs−1, achieved with thin platelets of rubrene single crystals, demonstrates the significance of the SC-OFETs and clarifies material limitations for organic devices. In the latter part of this review, we discuss the physics of microscopic charge transport by using SC-OFETs at metal/semiconductor contacts and along semiconductor/insulator interfaces. Most importantly, Hall effect and electron spin resonance (ESR) measurements reveal that interface charge transport in molecular semiconductors is properly described in terms of band transport and localization by charge traps.

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.

Field-induced charge transport at the surface of pentacene single crystals: A method to study charge dynamics of two-dimensional electron systems in organic crystals

A method has been developed to inject mobile charges at the surface of organic molecular crystals, and the dc transport of field-induced holes has been measured at the surface of pentacene single crystals. To minimize damage to the soft and fragile surface, the crystals are attached to a prefabricated substrate which incorporates a gate dielectric (SiO2) and four probe pads. The surface mobility of the pentacene crystals ranges from 0.1 to 0.5 cm2/V s and is nearly temperature independent above ∼150 K, while it becomes thermally activated at lower temperatures when the induced charges become localized. Ruling out the influence of electric contacts and crystal grain boundaries, the results contribute to the microscopic understanding of trapping and detrapping mechanisms in organic molecular crystals.

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.

High-mobility, low-power, and fast-switching organic field-effect transistors with ionic liquids

We report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid (IL) electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1μF∕cm2 even at 0.1MHz. With high carrier mobility of 1.2cm2∕Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the IL/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.

Direct Observation of Lanthanide(III)-Phthalocyanine Molecules on Au(111) by Using Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy and Thin-Film Field-Effect Transistor Properties of Tb(III)- and Dy(III)-Phthalocyanine Molecules

The crystal structures of double-decker single molecule magnets (SMM) LnPc(2) (Ln = Tb(III) and Dy(III); Pc = phthalocyanine) and non-SMM YPc(2) were determined by using X-ray diffraction analysis. The compounds are isomorphous to each other. The compounds have metal centers (M = Tb(3+), Dy(3+), and Y(3+)) sandwiched by two Pc ligands via eight isoindole-nitrogen atoms in a square-antiprism fashion. The twist angle between the two Pc ligands is 41.4 degrees. Scanning tunneling microscopy was used to investigate the compounds adsorbed on a Au(111) surface, deposited by using the thermal evaporation in ultrahigh vacuum. Both MPc(2) with eight lobes and MPc with four lobes, which has lost one Pc ligand, were observed. In the scanning tunneling spectroscopy images of TbPc molecules at 4.8 K, a Kondo peak with a Kondo temperature (T(K)) of approximately 250 K was observed near the Fermi level (V = 0 V). On the other hand, DyPc, YPc, and MPc(2) exhibited no Kondo peak. To understand the observed Kondo effect, the energy splitting of sublevels in a crystal field should be taken into consideration. As the next step in our studies on the SMM/Kondo effect in Tb-Pc derivatives, we investigated the electronic transport properties of Ln-Pc molecules as the active layer in top- and bottom-contact thin-film organic field effect transistor devices. Tb-Pc molecule devices exhibit p-type semiconducting properties with a hole mobility (mu(H)) of approximately 10(-4) cm(2) V(-1) s(-1). Interestingly, the Dy-Pc based devices exhibited ambipolar semiconducting properties with an electron mobility (mu(e)) of approximately 10(-5) and a mu(H) of approximately 10(-4) cm(2) V(-1) s(-1). This behavior has important implications for the electronic structure of the molecules.

Ambipolar organic field-effect transistors based on rubrene single crystals

We herein report ambipolar organic field-effect transistors based on rubrene single crystals. The transistors operate in both the p- and n-channel regimes depending upon the bias conditions. Hole and electron mobilities of 1.8 and 1.1×10−2cm2∕Vs, respectively, were derived from saturated currents. The appearance of an electron enhancement mode in single crystals of wide-band-gap semiconductors (∼2.6eV) is ascribed to the reduction of electron traps at the semiconductor-dielectric interface using a hydroxyl-free gate dielectric.

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

A comparative study of organic single-crystal transistors gated with various ionic-liquid electrolytes

We report on a comparative study of rubrene single-crystal field-effect transistors with various ionic-liquid electrolytes used for gate insulators. A systematic correlation is found that mobility of the field-effect transistors increases with decreasing electrostatic capacitance of the electric double layers, as the result of highly reproducible comparisons among tens of samples with the variation of anions in the purified ionic liquids. By optimizing the gating ionic liquid, the highest mobility of the electrolyte-gated organic transistors elevated up to 9.5 cm2/V s, which is only a fraction of the value of intrinsic material property, demonstrating an excellent field-effect switching operation.