Thermal and Mechanical Cracking in Bis(triisopropylsilylethnyl) Pentacene Thin Films JIHUA CHEN, 1 CHEE KEONG TEE, 2 JUNYAN YANG, 3,* CHARLES SHAW, 1 MAX SHTEIN, 3 JOHN ANTHONY, 5 DAVID C. MARTIN 1,3,4 1 Macromolecular Science and Engineering Center, University of Michigan, Ann Arbor, Michigan 48109 2 Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109 3 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109 4 Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109 5 Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506 Received 6 June 2006; revised 30 August 2006; accepted 4 September 2006 DOI: 10.1002/polb.21007 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Bis(triisopropylsilylethnyl) pentacene (TIPS pentacene) was synthesized to increase its solubility in common liquid solvents and, at the same time, enhance the p–p stacking between neighboring acenes in the crystallized state in comparison with unmodified pentacene. Hot-stage microscopy experiments revealed that during heating voids develop along the long axis of the TIPS pentacene films {along the [210] direction/parallel to the ( 120) planes} and crystals overlap along the short axis {along the [ 120] direction/parallel to the (210) planes}. From molecular mechanics simulations, the predominant twin boundaries of ( 120) and commonly observed crack- ing planes of (120), ( 120), and (210) had relatively low surface energies in comparison with planes with similar Miller indices. Organic thin-film transistors with TIPS pen- tacene as the active layer were fabricated, and the mobility values decreased from 0.4–1.0 cm 2 /V s before cracking to 0.2 cm 2 /V s after cracking. To maintain the high charge carrier mobility of TIPS pentacene devices, these cracks should be avoided. V V C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3631–3641, 2006 Keywords: annealing; charge transport; functionalized pentacene; organic semicon- ductors; organic thin-film transistors; structure-property relations; surface energy INTRODUCTION Pentacene is an organic semiconductor with one of the highest measured hole mobilities in com- parison with other similar materials, such as polythiophene, oligothiophenes, and phthalocya- nines. However, the experimentally observed mobilities in thin films have not increased sig- nificantly since devices with 1.5 cm 2 /V s were reported. 1,2 To improve its solid-state stacking and solubility in organic solvents, bis(triisopro- pylsilylethnyl) pentacene (TIPS pentacene) has been designed and synthesized with the addition of two side groups. 3 The herringbone crystal structure of pentacene polymorphs is disrupted by the bulky side groups, and the TIPS penta- cene molecules assume two-dimensional p–p This article includes Supplementary Material available from the authors upon request or via the Internet at www. interscience.wiley.com/jpages/0887-6266/suppmat *Present address: Dow Chemical Company, Freeport, Texas. Correspondence to: D. C. Martin (E-mail: milty@umich.edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 3631–3641 (2006) V V C 2006 Wiley Periodicals, Inc. 3631