Surface induced orientation and vertically layered morphology in thin films of poly(3-hexylthiophene) crystallized from
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Guiseppe Portale Zernike Institute for Adv. Materials, University of Groningen, NL-9747 AG Groningen, The Netherlands
Ruth H. Lohwasserc) and Mukundan Thelakkat Applied Functional Materials, Macromolecular Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany
Thomas Thurn-Albrechta) Institute of Physics, Martin Luther University, 06120 Halle/Saale, Germany (Received 21 December 2016; accepted 14 March 2017)
The presence of interfaces and geometrical confinement can have a strong influence on the structure and morphology of thin films of semicrystalline polymers. Using surface-sensitive grazing incidence wide angle X-ray scattering and atomic force microscopy to investigate the vertical structure of thin films of poly(3-hexylthiophene) crystallized from the melt, we show that highly oriented crystallites are induced at the air/polymer interface and not as sometimes assumed at the interface to the substrate. These crystallites are oriented with their crystallographic a-axis perpendicular to the plane of the film. While the corresponding orientation dominates in thinner films, for sufficiently thick films (.60 nm) a layer containing unoriented crystals is present below the surface layer. Due to the anisotropic charge transport properties, the observed effects are expected to be of special relevance for potential applications of semiconductor polymers in the field of organic photovoltaics for which vertical transport in thicker films plays an important role.
I. INTRODUCTION
The crystallization of polymers in thin films can be strongly influenced by geometrical confinement and interfacial interactions,1–6 resulting in changes in orientation, crystallinity, and crystallization kinetics. These effects are especially important for functional polymers like semiconductor polymers which in devices are typically used as thin films. Usually, it is assumed that the interface to the substrate plays the dominant role for the abovementioned effects, for example, by acting as a nucleation site7 or by affecting the molecular dynamics in the vicinity of the interface.5,8–11 On the other hand, detailed investigations of materials containing alkyl chains showed surface induced ordered or crystalline structures. Examples are simple n-alkanes,12–15 long-chain alcohols,16 poly(n-alkyl acrylates),17 and ionic liquids.18 In all these examples, the alkyl chains orient perpendicular to the surface with partial or complete in-plane ordering leading to surface freezing. In many cases, though, not much is Contributing Editor: Chris Nicklin a) Address all correspondence to this author. e-mail: [email protected] b) Present Address: Fraunhofer Institute for Applied Polymer Research, 14476 Potsdam, Germany c) Present Address: BASF, Ludwigshafen, Germany DOI: 10.1557/jmr.2017.107
known about the effect of interfacial ordering on the crystal texture of thicker films. Within the class of conjugated polymers regioregular poly(3-hexylthiophene) (P3HT) is among the most extensively studied materials due to its interesting optoelectronic
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