Meso-Epitaxial Solution-Growth of Self- Organizing Discotic Liquid-Crystalline Semiconductors** By Anick M. van de Craats , Natalie Stutzmann, Oliver Bunk, Martin M. Nielsen, Mark Watson, Klaus Müllen, Henri D. Chanzy , Henning Sirringhaus , and Richard H. Friend* Direct processing from solution of organic semiconductors presents a new paradigm for low-cost device manufacture, and considerable progress has been made using conjugated polymers for light-emitting diodes and displays, [1±3] and, by ink-jet printing, for transistor circuits. [4] However, ordered thin films are required for efficient charge transport, and it has been a challenge to establish optimal structural organiza- tion using small molecules. We show that substituted hexaben- zocoronenes (HBCs) can be processed from solution, to form films with supramolecularly ordered columnar stacks which lie parallel to the substrate and can be oriented uniaxially onto poly(tetrafluoroethylene) (PTFE) alignment layers. Re- markably, we find epitaxial growth of a crystalline HBC model compound on the oriented PTFE over micrometer dimensions. This high interfacial order achieved by solution processing under ambient conditions is as required for field- effect transistors (FETs). We find FET mobilities for p-type accumulation mode devices up to 10 ±3 cm 2 V ±1 s ±1 on imple- menting these aligned semiconducting films. The combination of solution processing and high interfacial order together with useful semiconductor properties shows considerable scope for practical applications of discotic liquid-crystalline materials. The intermolecular coupling of p-electrons on adjacent mol- ecules along the columnar stack in discotic liquid crystals has been recognized as an efficient structure to allow facile charge movement along the stack, as indicated in Figure 1b. Adam and co-workers [5] have shown time-of-flight drift mobilities of up to 0.1 cm 2 V ±1 s ±1 in a triphenylene derivative, and van de Craats and co-workers [6] have found intrinsic mobilities of up to 1 cm 2 V ±1 s ±1 using microwave conductivity measurements in bulk samples. However, little is known about the arrangement of columnar stacks in thin films and at substrate interfaces. We show here that discotic-phase-forming materials based on hex- abenzocoronene can be oriented parallel to a substrate to form well-ordered, supramolecular architectures. We processed oriented thin hexa-peri-hexabenzocoronene films of both a crystalline derivative with branched alkyl chains, HBC-C8,2, and a material that is in its discotic liquid- crystalline phase at room temperature, HBC-PhC12 (see Fig. 1a), by casting from solution onto oriented PTFE layers, [7] and slow evaporation of the solvent. We find that this alignment procedure works equally well for both materials. However, the film formation appears to rely on somewhat dif- ferent growth and orientation mechanisms, as optical micros- copy indicates (Fig. 1c,d). Using for instance, the liquid-crys- talline material, well-aligned films have been grown on PTFE-coated surfaces, while randomly oriented morphologies are observed on the substrate where the alignment layer is ab- sent (top-left corner of Fig. 1c, left panel). For the crystalline HBC derivative, the growth mechanism appears to be regu- lated by preferential, oriented crystal nucleation, [8±10] as the HBC-C8,2 crystallized almost exclusively on the PTFE. Re- markably, in this case, no adsorption on the uncoated glass was found (top-left corner of image Fig. 1d, left panel). This crystallization behavior obviously is highly desirable and con- trasts, for instance, the observation reported by Amundson and co-workers [11] regarding the film formation on PTFE for the semiconducting polymer, poly(3-hexylthiophene) (P3HT). For the latter case, the material purportedly deposited prefer- entially in regions where the PTFE coverage was low , which was attributed to the presumed poor wetting behavior of the P3HT/chloroform solution on PTFE. [11] Finally, we like to em- phasize that for both HBC compounds the friction-transferred alignment layer promoted the direct formation of a highly ori- ented supramolecular structure, i.e., without the need of an additional annealing step in the mesophase, as apparently re- quired for example for a discotic triphenylene derivative. [12] Electron diffraction results are shown in Figure 2. These reveal that the columnar stacks in both HBC films are orient- ed parallel to the underlying PTFE chains (Fig. 2c). For the HBC-PhC12 film (Fig. 2a) a relatively high degree of order, typical for oriented liquid-crystalline phases has been ob- Adv. Mater. 2003, 15, No. 6, March 17 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0935-9648/03/0603-0495 $ 17.50+.50/0 495 COMMUNICATIONS ± [*] Prof. R. H. Friend, Dr. A. M. van de Craats, [+] Dr. N. Stutzmann, [++] Dr. H. Sirringhaus Cavendish Laboratory, University of Cambridge Madingley Road, Cambridge CB3 0HE (UK) E-mail: rhf10@cam.ac.uk Dr. O. Bunk Materials Research Department Risù National Laboratory PO Box 49, DK-4000 Roskilde (Denmark) Dr. M. M. Nielsen The Danish Polymer Centre, Risù National Laboratory PO Box 49, DK-4000 Roskilde (Denmark) Dr. M. Watson, Prof. K. Müllen Max-Planck-Institut für Polymerforschung Ackermannweg 10, D-55128 Mainz (Germany) Dr. H. D. Chanzy Centre de Recherches sur les MacromolØcules VØgØtales, CNRS, BP 53, F-38041 Grenoble, Cedex 9 (France). [+] Present address: Nederlands Forensisch Instituut (NFI), Volmerlaan 17, NL-2288 GD Rijswijk, The Netherlands. [++] Present address: Philips Research, Prof. Holstlaan 4, NL-5656 AA Eindhoven, The Netherlands. [**] We are deeply indebted to Paul Smith (ETH Zurich) for many helpful dis- cussions, and use of the infrastructure of the Polymer Technology group. The HASYLAB staff is thanked for experimental assistance with the X-ray diffraction experiments. A.M.C. would like to thank the European Commission for financial support by means of a Marie Curie Fellowship (HPMF-CT-2000-00737). N.S. was funded by the Swiss National Science Foundation through a young researcher's grant, for which she is grateful. M.M.N. and O.B. would like to thank DanSync for financial support. M.W. and K.M. gratefully acknowledge financial support from the ªZen- trum für Multifunktionelle Werkstoffe und Miniaturisierte Funktionsein- heitenº (BMBF03N 6500), EU-TMR project SISITOMAS, and the ªDeutsche Forschungsgemeinshaftº (Schwerpunkt Feldeffekt-Transisto- ren). Research reported in this article is performed within the EU pro- gramme ªDISCELº( Nr. G5RD-CT-2000-00321). Finally, A. L. Forno, Zürich, is acknowledged for providing the facilities permitting efficient conduction of the project.