Takashi Minakata

A scanning Kelvin probe study of charge trapping in zone-cast pentacene thin film transistors

We have used scanning Kelvin probe microscopy (SKPM) as a local probe to study charge trapping in zone-cast pentacene field effect transistors on both SiO(2) and benzocyclobutene (BCB) substrates. Annealing at 130 degrees C was found to reduce the threshold voltage, susceptibility to negative gate bias stress and trapping of positive charges within single pentacene grains. We conclude that oxygen is able to penetrate and disassociatively incorporate into crystalline pentacene, chemically creating electrically active defect states. Screening of a positive gate bias caused by electron injection from Au into pentacene was directly observed with SKPM. The rate of screening was found to change significantly after annealing of the film and depended on the choice of gate dielectric.

High-mobility solution-processed organic thin-film transistor array for active-matrix color liquid-crystal displays

Abstract— A solution-processed organic thin-film-transistor array to drive a 5-in.-diagonal liquid-crystal display has been fabricated, where semiconductor films, a gate dielectric film, and passivation films have all been formed using solution processes. A field-effect mobility of 1.6 cm2/V-sec, which is among the highest for solution-processed organic thin-film transistors ever reported, was obtained. This result is due to semiconductor material with large-grain-sized pentacene crystals formed from a solution and adoption of three-layered passivation films that minimize the performance degradation of organic thin-film transistors.

Evidence for Charge‐Trapping Inducing Polymorphic Structural‐Phase Transition in Pentacene

Trapped-charge-induced transformation of pentacene polymorphs is observed by using in situ Raman spectroscopy and molecular dynamics simulations reveal that the charge should be localized in pentacene molecules at the interface with static intermolecular disorder along the long axis. Quantum chemical calculations of the intermolecular transfer integrals suggest the disorder to be large enough to induce Anderson-type localization.

P‐3: Organic Thin‐Film Transistors with Solution‐Processed Pentacene and Polysilazane Using Self‐Aligned Device Structure

We have fabricated organic thin-film transistors that had a high field-effect mobility of 0.39 cm2/Vs, in which both the semiconductor and the gate-dielectric were solution-processed using pentacene and polysilazane. These materials were also applied to a self-aligned structure for the first time and exhibited good device performance with solution-processed electrodes.

Transistors formed with solution-processed thin films of pentacene

Solution-processed pentacene thin films could be prepared directly from pentacene solution without using precursor molecules. The solution-processed pentacene films showed highly oriented and highly crystalline structure. It appeared that large crystal-like platelets grown in the films, and it was assumed that crystalline size of solution-processed films was larger than that of sublimed films determined by in grazing incidence X-ray diffraction method and powder X-ray diffraction method. FETs formed with solution-processed films exhibited good switching properties with the mobility of 0.8cm2/Vs and on/off ratio above 105. Maximum carrier mobility above 1cm2/Vs was confirmed. The solution-processed film transistors showed lower threshold voltage due to lower carrier density in the films compared with sublimed films. It was confirmed that the solution-processed transistors showed more stable properties for the storage for months than sublimed film transistors. These aspects of transistor performance could be explained by structural difference between solution-processed and sublimed films. Transistors of solution-processed films of pentacene were fabricated by printing technique using a temperature controllable dispenser and the switching operations of isolated transistors were confirmed.

Chemical-State Analysis of Organic Semiconductors Using Soft X-ray Absorption Spectroscopy Combined with First-Principles Calculation

The chemical states of organic semiconductors were investigated by total-electron-yield soft X-ray absorption spectroscopy (TEY-XAS) and first-principles calculations. The organic semiconductors, pentacene (C(22)H(14)) and pentacenequinone (C(22)H(12)O(2)), were subjected to TEY-XAS and the experimental spectra obtained were compared with the 1s core-level excited spectra of C and O atoms, calculated by a first-principles planewave pseudopotential method. Excellent agreement between the measured and the calculated spectra were obtained for both materials. Using this methodology, we examined the chemical states of the aged pentacene, and confirmed that both C-OH and C═O chemical bonds are generated by exposure to air. This result implies that not only oxygen but also humidity causes pentacene oxidation.

63.4L: Late News Paper: High Mobility Solution-Processed Organic Thin-Film Transistor Array for Active-Matrix Color Liquid Crystal Displays

We have fabricated a solution-processed 240 × 960 organic thin-film transistor array to drive a liquid crystal display, where the semiconductor, gate dielectric and passivation films have been all formed using solution processes. A field effect mobility of 1.6 cm2/Vs obtained is the highest value for solution-processed TFTs ever reported.

High-performance organic field-effect transistors, textured by self-assembly

FA4-MS04-P11 High Performance Organic Field Effect Transistors, Textured by Self-assembly. Jens W. Andreasena, Claudia M. Duffyb, Hoi N. Tsaoc, Dag W. Breibyd, Wojciech Pisulac, Masahiko Andoe, Takashi Minakataf, Don Choc, Ali Rouhanipourc, Martin M. Nielseng, Henning Sirringhausb, Klaus Müllenc. aSolar Energy Programme, Risø National Laboratory for Sustainable Energy, Technical University of Denmark. bOptoelectronics Group, Cavendish Laboratory, University of Cambridge. cMax Planck Institute for Polymer Research. dDepartment of Physics, Norwegian University of Science and Technology. eHitachi Cambridge Laboratory. fAsahiKASEI Corporation. gCentre for Molecular Movies, Niels Bohr Institute, University of Copenhagen. E-mail: jewa@risoe.dtu.dk

Thin films of pentacene formed by transferring crystals dispersed in liquid media

We fabricate thin films of pentacene by coating flake crystals dispersed in a liquid media. The films are structured with oriented crystals and have a well-defined grain structure obtained from the source crystals. The films assembled with large or small crystals show the carrier mobility of 1 cm2/V s and above 0.01 cm2/V s, respectively. Small electrode contact resistance of the films and transport barrier heights below 56 meV were confirmed from the temperature dependence of the mobility.

In-plane Molecular Alignment in Thin Films of Pentacene Grown by Solution Casting and Performance of Thin Film Transistors

We have fabricated solution-processed thin films of pentacene by casting solution on a substrate and vaporizing solvent. The films with large oriented platelet domains were obtained by directionally grown condition. Molecular alignment in the directionally grown grains has been studied by several kinds of structural analysis. Oriented domains with the width of several hundreds microns and the length in an order of cm of the films were confirmed by polarized microscopy. In-plane crystalline structure of the domain has been studied by grazing incidence X-ray diffraction (GIXD) and strong anisotropy of in-plane crystalline structure was confirmed. Crystalline growth direction of the film was determined to be b-axis from both transmission electron diffraction and GIXD. Thin films transistors (TFTs) with directionally oriented domains of the films were fabricated on electrode patterned substrate. The observed maximum carrier mobility of 2.7 cm 2 /Vs was comparable to that of single crystal, which indicated that the quality of the film was almost identical with the single crystal. Correlation between FET performance and growth direction was studied and preferred performance of TFTs with the film grown perpendicularly to the channel was observed.