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(1) Organic Transistors
semiconductors have been used in organic devices such as organic light emitting
diode, organic transistors, and organic photovoltaic cells. We are studying new materials for organic
(TTF) derivatives are strong donors, but show stable transistor operation when
substituted by bulky groups .
Since organic/organic interface gives rise to small injection barriers,
the use of organic charge-transfer salts such as (TTF)(TCNQ) as source and
drain (S/D) electrodes affords high-performance organic transistors
particularly with bottom-contact geometry . This line is further explored to use the
same organic molecule, for instance, HMTTF, both as the active layer and as the
S/D electrode material in the form of (HMTTF)(TCNQ) to make gself-contacth
organic transistors [3,4]. This is
the same concept as the ordinary silicon devices, where the conducting part is
constructed by doping. In this
connection, we have found BTBT makes highly-conducting charge-transfer
complexes such as (BTBT)2PF6, which shows as high
conductivity as 1500 S/cm . This
compound is promising as a thermoelectric material. BTBT makes such mixed-stack
charge-transfer complexes as (BTBT)(TCNQ), which show air-stable n-channel
transistor properties .
We have explored various organic semiconductors. Diphenylindigo is a high-performance ambipolar transistor material . In this paper, operation regions of ambipolar transistors depending on various VG and VD values are discussed, in which the electron and hole threshold voltages play an important role. The region with opposite VG and VD polarity is meaningful. DMDCNQI is an n-channel transistor material , though low-vacuum evaporation is necessary due to the high vapor pressure . Birhodanine is an n-channel materials showing excellent ambient stability .
From the temperature dependent measurements of transistor characteristics, we can estimate the trap density . If we can reduce the trap number less than the applied VG, we observe band transport above the VG. Along this line, we have achieved band transport maintained down to liquid helium temperatures .
 "Stabilization of Organic Field-Effect Transistors in Hexamethylenetetrathiafulvalene Derivatives Substituted by Bulky Alkyl Groups," M. Kanno, Y. Bando, T. Shirahata, J. Inoue, H. Wada, and T. Mori, J. Mater. Chem. 19(26), 6548 (2009).
 "(Tetrathiafulvalene)(Tetracyanoquinodimethane) as a Low-Contact-Resistance Electrode for Organic Transistors," K. Shibata, H. Wada, K. Ishikawa, H. Takezoe, and T. Mori, Appl. Phys. Lett. 90, 193509 (2007).
 "Charge Injection from Organic
Charge-Transfer Salts to Organic Semiconductors," T. Kadoya, D. de Caro, K. Jacob, C.
Faulmann, L. Valade, and T.
Mori, J. Mater.
Chem. 21, 18421 (4
 "All-Organic Self-Contact Transistors," S. Tamura, T. Kadoya, and T. Mori, Appl.
Phys. Lett. 105(2), 023301 (4
 "Benzothienobenzothiophene-Based Molecular Conductors: High
Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability" Y. Kiyota, T. Kadoya, K. Yamamoto, K. Iijima, T. Higashino, T. Kawamoto, K. Takimiya, and T. Mori, J. Am. Chem. Soc. 138(11),
 "Charge-Transfer Complexes of Benzothienobenzothiophene with Tetracyanoquinodimethane and the n-Channel Organic Field-Effect Transistors," R. Sato, M. Dogishi, T. Higashino, T. Kadoya, T. Kawamoto, T. Mori, J. Phys. Chem. C 121(12), 6561-6568 (2017).
 "High Performance Ambipolar
Organic Field-Effect Transistors Based on Indigo Derivatives," O. Pitayatanakul, T. Higashino, M. Tanaka, H. Kojima, M. Ashizawa, T. Kawamoto, H. Matsumoto, K. Ishikawa, and T. Mori,
Mater. Chem. C, 2, 9311-9317 (2014).
 "Contact Resistance and Electrode Material Dependence of Air-Stable n-Channel Organic Field-Effect Transistors Using Dimethyldicyanoquinonediimine (DMDCNQI)," H. Wada, K. Shibata, Y. Bando, T. Mori, J. Mater. Chem. 18, 4165-4171 (2008).
 "Organic Field-Effect Transistors Based
on Small-Molecule Organic Semiconductors Evaporated under Low Vacuum," T.Takahashi, S. Tamura, Y. Akiyama, T.Kadoya, T. Kawamoto, and T. Mori,Appl.
5, 061601 (3 pages) (2012).
 "Birhodanines and their Sulfur Analogues for Air-Stable n-Channel Organic Transistors," K. Iijima, Y. Le Gal, T. Higashino, D. Lorcy, and T. Mori, J. Mater. Chem. C, 5, 9121 - 9127 (2017).
 "Analysing Organic Transistors Based on Interface Approximation," Y. Akiyama and T. Mori, AIP Advances, 4(1), 017126 (2014).
 "Band-like Transport down to 20 K in Organic Single-Crystal Transistors Based on Dioctylbenzothienobenzothiophene," J. Cho, T. Higashino, and T. Mori, Appl. Phys. Lett. 106, 193303 (2015).
(2) Organic Superconductors
Our main interest concerns to syntheses and properties of organic conductors, in particular charge transfer salts composed of small-molecule donors and/or acceptors. Some of these charge-transfer salts exhibit superconductivity so that our interest is particularly focused on superconductors and the related materials. Our research activities extend to systematic works of donor and acceptor molecules, crystal growth of conducting materials, X-ray structure analysis, low-temperature properties such as conductivity and other transport properties, magnetic measurements like ESR, and energy band calculation and other theoretical approaches. It is our policy to proceed from synthesis to physical identification within a unified system, and to feed back the physical results to designing of new materials.
1) Incommensurate Organic Superconductors and Exotic Organic Superconductors
The conventional organic superconductors have definite 2:1 composition like (BEDT-TTF)2Cu(NCS)2, and the control of charge quantity has not been possible. We are exploring organic superconductors with non integer composition like (MDT-TS)(AuI2)0.441 and studying the peculiar physical properties. Organic conductors with non integer composition have been reviewed in  and , and the unusual incommensurate antiferromagnetic insulating state is described in [15,16]. We have calculated superconductivity gap functions based on RPA, and found that d-wave superconductivity is preferable even in one-dimensional conductors .
 "Organic Conductors with Unusual Band Fillings," T. Mori, Chem. Rev. 104(11), 4947-4970 (2004) (Review).
 "Organic Conductors - From Fundamentals to Nonlinear Conductivity," T. Mori and T. Kawamoto, Ann. Rep. Prog. Chem. Sect. C Phys. Chem. 103, 134-172 (2007) (Review).
 "Superconductivity Competing with the Incommensurate Antiferromagnetic Insulating State in the Organic Conductor (MDT-TS)(AuI2)0.441," T. Kawamoto, Y. Bando, T. Mori, K. Takimiya, and T. Otsubo, Phys. Rev. B 71, 052501 (2005).
 "Organic superconductors with an incommensurate anion structure" T. Kawamoto and K. Takimiya, Sci. Tech. Adv. Mater. 10(2) 024303 (2009).
 "Low-Symmetry Gap Functions of Organic Superconductors," T. Mori, J. Phys. Soc. Jpn. 87, 044705 (9 pages) (2018).
2) Nonlinear Conductivity in Organic Conductors
Some organic conductors show remarkable non ohmic conductivity, sometimes due to charge order, and even show spontaneous current oscillation called organic thyristor. A short account is given in , as well as . Among the charge order states, we have emphasized the importance of so-called non stripe state, which has been described in . We have invented a method to calculate voltage-current characteristics in nonlinear conductivity based on the phenomenological energy conservation, the mathematics behind which is based on nonlinear dynamics .
 "New aspects of nonlinear conductivity in organic charge-transfer salts," T. Mori, I. Terasaki and H. Mori, J Mater. Chem. 17, 4343-4347 (2007) (Highlight).
 "Non-Stripe Charge Order in the Ζ-Phase Organic Conductors," T. Mori, J. Phys. Soc. Jpn. 72(6), 1469-1475 (2003).
 "Nonlinear Dynamics of Conduction Electrons in Organic Conductors," T. Mori, T. Ozawa, Y. Bando, T. Kawamoto, S. Niizeki, H. Mori, I. Terasaki, Phys. Rev. B 79, 115108 (2009).
 "Electronic Properties of Organic Conductors" T. Mori, Springer (2016).