1]Benzothieno[3,2-b][1]benzothiophene-based liquid crystalline organic semiconductor for solution-processed organic thin film transistors

Abstract

Liquid crystalline organic semiconductors have drawn tremendous interest due to potentials of applications in organic thin-film transistors (OTFTs) with their high electrical performance and solution-processability. However, alkylated liquid crystals often encounter poor thermal durability and mobility drop at smectic E (SmE) phase. Here, we introduce new solution-processable [1]benzothieno[3,2-b][1]benzothiophene-based small molecules with different types of alkyl side chains for OTFTs. All BTBT derivatives showed smectic phase; dialkylated 2-octyl-7-(5′-octyl-[2,2′-bithiophen]-5-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (O-2Th-BTBT) and 2-(5′-(2-ethylhexyl)-[2,2′-bithiophen]-5-yl)-7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (E-2Th-BTBT) suffered mobility degradation at SmE phase. However, monoalkylated 2-([2,2′-bithiophen]-5-yl)-7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (2Th-BTBT) did not exhibit SmE phase over temperature range, resulting in high carrier mobility up to 0.31 cm2/Vs through molecular bilayer transition by thermal annealing. Physicochemical and liquid crystal properties were analyzed, and thin film characterization by atomic force microscopy and X-ray diffraction was discussed.