Received: May 25, 2012 Revised: August 8, 2012 Published online: 12 Sep 2012 Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS) State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University Beijing 100871, PR China E-mail: guoxf@pku.edu.cn DOI: 10.1002/adma.201202099 Molecular engineering and chemical self-assembly are combined with materials fabrication to achieve air-stable solution-processable oligothiophene-based field-effect transistors with mobilities up to 6.2 cm2 V−1 s−1, which ranks as the highest among oliogthiophene-based semiconducting materials.

High field effect mobilities of OFETs that are beyond 1 cm2 V−1 s−1 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) 3.0 cm2 V–1 s–1 J. Am. Chem. Soc., 2011, 133 (22), pp 8732–8739 N ature V O L 4 8 0, 2 2 / 2 9 D E C E M B E R 2 0 1 1 Upto 3.7 cm2 V–1 s–1 4.6 cm2 V−1 s−1 Zhenan Bao et al Adv. Mater.2011, 23, 1222–1225 Kazuo Takimiya et al Adv. Mater. 2007, 19, 678–682 pentacene 1.8 cm2 V–1 s–1 Kilwon Cho et al. 6.30 cm2 V−1 s−1 Appl. Phys. Lett. 94, 263303 (2009); S. G. Mhaisalkar et al

organic semiconductors (OSCs) for Field-Effect transistors Generally required complex physical vapor or vacuum deposition techniques To enable low-cost manufacturing, materials fabrication through simple solution processing under ambient conditions is highly desirable OFETs fabricated with solution techniques; electrical properties are inferior to those of the vacuum-deposited devices More defects and carrier traps in thin films and/or the instability in air. oligothiophenes and polythiophenes Promising class of organic semiconductors for solution-processed OFETs Relatively straightforward synthesis and flexible structural modification Defects in their solution-processed thin films Sensitivity to photo-oxidative doping/bleaching hamper the intrinsic charge carrier transport and yield poor transistor performance - Only very few liquid crystalline semiconductors have been recently demonstrated with improved properties (mobility 10 − 2 ~ 10 − 1 cm2 V−1 s−1).

Structure design strategy lamella-like layered π -stacking structural orders long-range intermolecular side-chain interdigitation long alkyl side-chains - integrate self-healing capabilities of LC into oligothiophene molecules self-organize into highly ordered molecular stacks during thermal annealing Unsubstitution in quaterthiophene or pentathiophene backbones Some rotational freedom Oxidative doping stability

Synthetic scheme

Characterizations λ max = 413 nm (1) and 437 nm (2) formation of H-aggregates with close π -stacking in a face-to-face alignment

The DSC thermogram of 2 at a scan rate of 10 oC min − 1 A POM image of 2 ∼ 135 oC; crystalline-to-liquid crystalline ∼ 160 oC; liquid crystalline-to-isotropic phase transitions Fanlike texture in the temperature ranges of 140-158 and 155–120 oC for the heating and cooling procedures, respectively. Smectic liquid crystalline phase

microribbons from a CHCl 3 solvent via a slow solvent evaporation process. XRD STM images of 1 and 2 interlayer distance ( d -spacing) of 40.2 Å for 1 and 47.2 Å for 2 -> slightly larger than the molecular lengths (34.8 Å for 1 and 38.7 Å for 2 ) with the fully-extended alkyl groups

The channel length (L) and width (W) were 100 μm and 20 μm, respectively. Where W/L is the channel width/length, Ci is the insulator capacitance per unit area, and VG and VT are the gate voltage and threshold voltage, respectively. Thin film based device; spin-coating Single crystal based device; polydimethoxysilane (PDMS) stamp transfer method

enhanced molecular ordering π – π stacking induced by thermally-assisted self-assembly of liquid crystals

Photoresponsive behavior on/off ratio on the order of ∼ 10 3 mobility distribution average mobility of 4.0 cm2 V–1 s–1 With the highest up to 6.2 cm2 V–1 s–1

Conclusion - Oligothiophene based OTFT exhibits air-stable thin-film mobilities as high as 0.2 cm2 V−1 s−1 and an average single-crystal mobility of 4.0 cm2 V−1 s−1 with the highest up to 6.2 cm2 V−1 s−1. - These results provide new insights into how to combine molecular engineering and chemical self-assembly with materials fabrication to achieve properties that are desirable for device applications. - The long alkyl side chains not only increase the molecular solubility allowing for low-cost device fabrication, but also improve the self-assembly ability to form high-quality single crystals.