Jun’ya Tsutsumi

Inkjet printing of single-crystal films

The use of single crystals has been fundamental to the development of semiconductor microelectronics and solid-state science. Whether based on inorganic or organic materials, the devices that show the highest performance rely on single-crystal interfaces, with their nearly perfect translational symmetry and exceptionally high chemical purity. Attention has recently been focused on developing simple ways of producing electronic devices by means of printing technologies. ‘Printed electronics’ is being explored for the manufacture of large-area and flexible electronic devices by the patterned application of functional inks containing soluble or dispersed semiconducting materials. However, because of the strong self-organizing tendency of the deposited materials, the production of semiconducting thin films of high crystallinity (indispensable for realizing high carrier mobility) may be incompatible with conventional printing processes. Here we develop a method that combines the technique of antisolvent crystallization with inkjet printing to produce organic semiconducting thin films of high crystallinity. Specifically, we show that mixing fine droplets of an antisolvent and a solution of an active semiconducting component within a confined area on an amorphous substrate can trigger the controlled formation of exceptionally uniform single-crystal or polycrystalline thin films that grow at the liquid–air interfaces. Using this approach, we have printed single crystals of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) (ref. 15), yielding thin-film transistors with average carrier mobilities as high as 16.4 cm2 V−1 s−1. This printing technique constitutes a major step towards the use of high-performance single-crystal semiconductor devices for large-area and flexible electronics applications.

Ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy of [6,6]-phenyl-C61-butyric acid methyl ester in gas and solid phases

The electronic structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was studied using ultraviolet photoelectron spectroscopy of vapor and thin film and inverse photoemission spectroscopy of thin film. The threshold ionization energy of PCBM was found to be 7.17±0.04 eV in gas phase and 5.96±0.02 eV in solid phase. The threshold electron affinity was 3.9±0.1 eV in solid phase. These values are 0.4−0.6 eV smaller than C60. The density functional theory calculations gave consistent results with these trends and suggested that the electron donation from the side chain to C60 backbone raised the C60-backbone-derived π orbitals of PCBM. The polarization energy of PCBM is 1.21 eV, which is almost the same as C60 but is about 0.5 eV smaller than the value of typical aromatic hydrocarbons.

Simple push coating of polymer thin-film transistors

Organic semiconductors may be processed in solution under ambient conditions; however, liquid manipulation on hydrophobic surfaces is difficult, which may hinder development of devices. Here, a push-coating technique is used to produce large-area semiconducting polymer films over hydrophobic surfaces.

N-type field-effect transistors based on layered crystalline donor–acceptor semiconductors with dialkylated benzothienobenzothiophenes as electron donors

We report the structural, electronic, and field-effect transistor characteristics of a series of molecular donor–acceptor charge-transfer (CT) compounds composed of 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophenes (diCn-BTBT; n = 4, 8, and 12) as donors and optionally fluorinated derivatives of tetracyanoquinodimethane (FmTCNQ; m = 0, 2, and 4) as acceptors. (diCn-BTBT)(FmTCNQ) form isomorphous layered crystalline structures consisting of alternating π-conjugated layers and alkyl chain layers, irrespective of the length of the alkyl chains. The compounds exhibit unique double-peaked CT absorption-band features that are polarized along the donor–acceptor interaction. Field-effect transistors based on these single crystals show a relatively high field-effect mobility (0.4 cm2 V−1 s−1), the p- and n-type operation of which can be tuned by altering the fluorine substitution in TCNQ.

Nanoparticle chemisorption printing technique for conductive silver patterning with submicron resolution

Silver nanocolloid, a dense suspension of ligand-encapsulated silver nanoparticles, is an important material for printing-based device production technologies. However, printed conductive patterns of sufficiently high quality and resolution for industrial products have not yet been achieved, as the use of conventional printing techniques is severely limiting. Here we report a printing technique to manufacture ultrafine conductive patterns utilizing the exclusive chemisorption phenomenon of weakly encapsulated silver nanoparticles on a photoactivated surface. The process includes masked irradiation of vacuum ultraviolet light on an amorphous perfluorinated polymer layer to photoactivate the surface with pendant carboxylate groups, and subsequent coating of alkylamine-encapsulated silver nanocolloids, which causes amine–carboxylate conversion to trigger the spontaneous formation of a self-fused solid silver layer. The technique can produce silver patterns of submicron fineness adhered strongly to substrates, thus enabling manufacture of flexible transparent conductive sheets. This printing technique could replace conventional vacuum- and photolithography-based device processing.

Fabrication and Raman scattering study of epitaxial VO2 films on MgF2 (001) substrates

Effects of epitaxial strain on metal–insulator transitions (MITs) of epitaxial VO2 films grown on MgF2 (001) substrates were examined. A partially tensile-strained film deposited at 420 °C showed an MIT temperature (TMI) of 318 K whereas that of a relaxed film deposited at 520 °C was about 331 K. Raman scattering measurements showed that the epitaxial strain affects the V–V vibration modes in the insulating phase. The TMI in the strained film is lower as a result of the shorter V–V distance and the reduced twisting angle of the V–V dimer caused by in-plane tensile strain.

Polymorphism in the 1:1 Charge-Transfer Complex DBTTF–TCNQ and Its Effects on Optical and Electronic Properties

The organic charge-transfer (CT) complex dibenzotetrathiafulvalene - 7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ) is found to crystallize in two polymorphs when grown by physical vapor transport: the known α-polymorph and a new structure, the β-polymorph. Structural and elemental analysis via selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and polarized IR spectroscopy reveal that the complexes have the same stoichiometry with a 1:1 donor:acceptor ratio, but exhibit unique unit cells. The structural variations result in significant differences in the optoelectronic properties of the crystals, as observed in our experiments and electronic-structure calculations. Raman spectroscopy shows that the α-polymorph has a degree of charge transfer of about 0.5e, while the β-polymorph is nearly neutral. Organic field-effect transistors fabricated on these crystals reveal that in the same device structure both polymorphs show ambipolar charge transport, but the α-polymorph exhibits electron-dominant transport while the β-polymorph is hole-dominant. Together, these measurements imply that the transport features result from differing donor-acceptor overlap and consequential varying in frontier molecular orbital mixing, as suggested theoretically for charge-transfer complexes.

Uniaxially oriented polycrystalline thin films and air-stable n-type transistors based on donor-acceptor semiconductor (diC8BTBT)(FnTCNQ) [n = 0, 2, 4]

We report the fabrication of high quality thin films for semiconducting organic donor-acceptor charge-transfer (CT) compounds, (diC8BTBT)(FnTCNQ) (diC8BTBT = 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene and FnTCNQ [n = 0,2,4] = fluorinated derivatives of 7,7,8,8,-tetracyanoquinodimethane), which have a high degree of layered crystallinity. Single-phase and uniaxially oriented polycrystalline thin films of the compounds were obtained by co-evaporation of the component donor and acceptor molecules. Organic thin-film transistors (OTFTs) fabricated with the compound films exhibited n-type field-effect characteristics, showing a mobility of 6.9 × 10−2 cm2/V s, an on/off ratio of 106, a sub-threshold swing of 0.8 V/dec, and an excellent stability in air. We discuss the suitability of strong intermolecular donor–acceptor interaction and the narrow CT gap nature in compounds for stable n-type OTFT operation.

Crystal structure of asymmetric organic semiconductor 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene

A full X-ray crystal structure analysis of 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), reported to be a promising semiconducting component for high-performance organic thin-film transistors, has been carried out. The analysis revealed the formation of a bilayer-type crystal structure composed of alternating antiparallel polar monomolecular layers that are formed by the herringbone packing of asymmetric molecules. The large and highly isotropic calculated intermolecular interactions within the two-dimensional semiconducting layers indicate the potentially excellent semiconducting performance of Ph-BTBT-C10.

An Accurate Calculation of Electronic Contribution to Static Permittivity Tensor for Organic Molecular Crystals on the Basis of the Charge Response Kernel Theory

We have developed a new method to calculate the static permittivity tensors of organic molecular crystals by applying the charge response kernel theory (Morita, A.; Kato, S. J. Am. Chem. Soc. 1997, 119, 4021) in which all the parameters were obtained with the density functional theory. The accuracy together with the requirements of the computation was discussed in terms of positions of the charge response sites and choice of a basis set. The calculated permittivities of typical organic compounds turned out to agree with the experimentally obtained values in the deviation of about 7% when a reasonable computational cost was maintained.