Transistor Performance and Film Structure of Hexabenzocoronene Derivatives
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1091-AA11-41
Transistor Performance and Film Structure of Hexabenzocoronene Derivatives Tomohiko Mori, Yoshihiro Kikuzawa, Kazuhiko Umemoto, Hiroshi Nozaki, Yoshiki Seno, and Hisato Takeuchi Information & Electronics Dept., TOYOTA Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi, 480-1192, Japan ABSTRACT Hexabenzocoronene (HBC) derivatives that are designed to self-assemble into lamellar aggregates were synthesized. The derivatives were deposited as an active layer in an organic field-effect transistor (OFET) using vacuum sublimation. The dihexyl and tetrahexyl derivatives (2H-HBC, 4H-HBC) increased the field-effect mobilities and on/off ratios by a factor of 10 or more compared to unsubstituted HBC and hexahexyl-hexabenzocoronene (6H-HBC). The crystal and thin film structures were determined by powder x-ray diffraction and grazing incidence X-ray diffraction (GIXD). The data indicate that 2H-HBC and 4H-HBC self-assemble into lamellar aggregates. 2H-HBC forms layers of aromatic cores that are sandwiched by the layers of hexyl groups, which is a preferable crystal structure for carrier transport. The good OFET performance could be explained by the self-assembly in lamellar aggregates of 2H-HBC and 4H-HBC, in contrast to self-assembly in the columnar aggregate of 6H-HBC and the low self-assembling properties of unsubstituted HBC. INTRODUCTION Organic field-effect transistors (OFETs) have recently attracted considerable attention, because of their use in lightweight, low-cost, large-area and flexible electronics products such as flat-panel displays, smart cards and radio-frequency identification (RFID) tags [1]. The requirement for high charge carrier mobility in organic thin films is satisfied by the overlap of π electrons of neighboring molecules. Graphene are rich in π electrons, and one would expect a high degree of overlap between adjacent π orbitals. Hexa-peri-hexabenzocoronenes (HBCs) are graphenes of well-defined size and shape. HBCs actually exhibit some of the highest values for intrinsic charge carrier mobility, which makes them promising candidates for organic electronic devices [2]. HBCs substituted peripherally with alkyl chains have been recognized as a molecule that self-assembles into columnar aggregates [3]. The aromatic cores are stacked in the direction of the columnar axis, which allows facile carrier transport along the stack [2]. These alkylsubstituted HBCs display intrinsic carrier mobilities of up to 1 cm2/Vs, measured using the pulse-radiolysis time-resolved microwave conductivity technique for bulk samples [2], while they show low FET mobilities up to 10-3 cm2/Vs in a thin film sample [4]. The remaining problem is that application of these HBCs will only be realized when the pathways for carrier transport can be uniaxially aligned parallel to the surfaces in thin-film geometries, because the columnar aggregates form one-dimensional axially insulated nanowires. On the other hand, lamellar aggregates form two-dimensional conducting pathways that do not require uniaxial alignment for ca
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