pubs.acs.org/Macromolecules Published on Web 06/11/2009 r 2009 American Chemical Society Macromolecules 2009, 42, 4651–4660 4651 DOI: 10.1021/ma900021w Bimodal Temperature Behavior of Structure and Mobility in High Molecular Weight P3HT Thin Films Siddharth Joshi, † Patrick Pingel, ‡ Souren Grigorian,* †, Tobias Panzner, † Ullrich Pietsch, † Dieter Neher, ‡ Michael Forster, § and Ullrich Scherf § † Solid State Physics, University of Siegen, Walter Flex Strasse 3, D-57068, Siegen, Germany, ‡ Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Potsdam, Germany, and § Macromolecular Chemistry, University of Wuppertal, Gauss-Strasse 20, D-42097 Wuppertal, Germany Received January 6, 2009; Revised Manuscript Received May 15, 2009 ABSTRACT: We report a temperature dependent crystalline structure of spin-coated thin films of high molecular weight regioregular poly(3-hexylthiophene) (P3HT) (M n ∼ 30000 g/mol) and its correlation with charge carrier mobility. These investigations show a reversible change of the crystalline structure, where the interlayer lattice spacing (100) along the alkyl side chains continuously increases up to a temperature of about 220 °C; in contrast, the in-plane π-π distance reduces with increasing temperature. These changes in structure are reversible and can be repeated several times. The temperature-induced structural properties differ for thick and thin films, pointing to a surface/interface role in stabilization of the layer morphology. In contrast to the structural changes, the carrier mobility is rather constant in the temperature range from room temperature up to 100-120 °C, followed by a continuous decrease. For thick layers this drop is significant and the transistor performance almost vanishes at high temperature, however, it completely recovers upon cooling back to room temperature. The drop of the charge carrier mobility at higher temperatures is in contrast with expectations from the structural studies, considering the increase of crystalline fraction of the polycrystalline layer. Our electrical measurements underscore that the reduction of the macroscopic mobility is mostly caused by a pronounced decrease of the intergrain transport. The thermally induced crystallization along (100) direction and the creation of numerous small crystallites at the film-substrate interface reduce the number of long polymer chains bridging crystalline domains, which ultimately limits the macroscopic charge transport. Introduction Among many conjugated polymers, poly(3-alkylthiophenes) (P3ATs), a class of organic semiconductors with good solubility and processability and environmental stability accompanied by a high charge-carrier mobility, have been found. This makes the material potentially useful for fabricating inexpensive, flexible, and large-area electronic devices. Potential applications include organic light emitting diodes, nonlinear optical devices, recharge- able batteries, and organic field-effect transistors (OFETs). 1,2 In particular, thiophene-based polymers have shown field-effect mobilities between 0.1 and 0.5 cm 2 V -1 s -1 , 3-5 which approaches a-Si mobility of 1.0 cm 2 V -1 s -1 . Typically, solution-processed polymers form complex micro- structures that are deposited to form a thin film on top of a solid substrate. In the case of poly(3-hexylthiophene), P3HT, it was found that the polymer chains self-organize into a well-ordered structure for a broad range of molecular weights (M w =2500- 30000 g/mol). 6-11 Depending on the preparation conditions and molecular parameters, these nanofibrils can spatially extend up to several micrometers in lengths and few hundred nanometers in width. Under different conditions, P3HT molecules can arrange into crystalline domains with an isotropic size of 10- 100 nm. 6,12-15 However, in both cases the fibrillar structures or the crystalline nanodomains are separated by a significant and often underestimated amount of amorphous material which is presumably formed by expelled segments during chain folding and chain ends. 7,15-18 Note that thin film crystallization depends subtly on processing conditions and molecular parameters such as molecular weight, regioregularity, and polydispersity. For a P3HT fraction with very low molecular weight and moderate polydispersity we previously found the formation of mostly amorphous films, in which sharply distinguished crystallites are embedded. 7,8,16 On the other hand, it has been found that low molecular weight films of a polythiophene derivative (PQT-12) with a very low polydispersity are highly crystalline with a lateral coherence in the range of a micrometer and a high field-effect mobility. 19 It is generally accepted, that charge carrier mobility in polymer thin films is a function of molecular weight. 8,12 At the same time, Meijer et al., 13 have shown that there can be a molecular weight optimum for the order parameters in fluorene-type copolymers and the picture of a simple increase in order with increasing molecular weight may be not true for all cases. For P3HT, the charge carrier mobility was found to increase by 4 orders of magnitude changing molecular weight between 3000 and 30000 g/mol. This was particularly inter- preted in terms of better connectivity of high-mobility crystal- line domains by long polymer chains in high molecular weight samples. Extending the model to the previously mentioned low molecular weight polythiophenes, we found that the degree of crystallinity;defined as the volume portion of polymer chains and segments which are arranged in an ordered fashion - is a major parameter for charge carrier mobility and that the amorphous phase ultimately limits the charge transport in thin polymer films. *To whom correspondence should be addressed. E-mail: grigorian@ physik.uni-siegen.de. Downloaded by UNIV POTSDAM on September 15, 2009 | http://pubs.acs.org Publication Date (Web): June 11, 2009 | doi: 10.1021/ma900021w