Design and structure–property relationship of benzothienoisoindigo in organic field effect transistors

• Published in: RSC advances 2015-01, Vol.5 (75), p.61035-61043
• Main Authors: Hasegawa, Tsukasa, Ashizawa, Minoru, Matsumoto, Hidetoshi
• Format: Article
• Language: English
• Subjects: Atomic force microscopy; Backbone; Carrier transport; Columnar structure; Field effect transistors; Hole mobility; Nanostructure; Semiconductor devices
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C5RA07660A

A novel planar π-conjugated small molecule, benzothienoisoindigo (BTII), in which additional benzene rings are fused with the thieoisoindigo (TII) unit, has been designed and synthesized. We report the impact of the planar π-framework and π-conjugation length on the carrier transport properties using three sets of molecules, BTII, isoindigo (II) and TII, bearing the same hexyl-side chain. The absorption spectra are remarkably red-shifted in the order of II < TII < BTII along with the enhanced molar extinction coefficient in the low-energy region, leading to the reduced bandgap. The single-crystal structure analyses revealed that all molecules have a planar backbone, and II and BTII are packed into a slipped columnar structure showing highly one-dimensional π–π interactions, while TII did not form, any noticeable intermolecular overlaps. The carrier transport properties were investigated in field-effect transistors (FETs). All molecules exhibited typical ambipolar properties. Among them, BTII showed the highest FET p-dominant ambipolar performance with the hole mobility of 0.095 cm 2 V −1 s −1 and electron mobility of 5.8 × 10 −3 cm 2 V −1 s −1 on the tetratetracontane (TTC)-modified substrate and p-type performance with the hole mobility of 0.18 cm 2 V −1 s −1 on the octadecyltrimethoxysilane (OTMS)-modified substrate. The microstructure of thin films was characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. These results indicated that smooth and densely packed nanorod-like crystalline grains are formed by extension of the π-conjugation in BTII. Due to the π-extension of planar organic semiconductors, the novel BTII unit can be extended for the rational design of high performance FET materials.