title: Interplay between Long-Range Crystal Order and Short-Range Molecular Interactions Tunes Carrier Mobility in Liquid Crystal Dyes.
abstract: We investigated the influence of molecular packing on the optical and electrical properties of the liquid crystalline dye 4,7-bis[5-(2-fluoro-4-pentyl-phenyl)-2-thienyl]-2,1,3-benzothiadiazole (FPPTB). FPPTB is crystalline at room temperature, exhibits a nematic phase at temperatures above 149 °C and is in an isotropic melt at temperatures above 230 °C. Solution processed FPPTB films were subject to thermal annealing through these phase transition temperatures and characterized with X-ray diffraction and polarized optical microscopy. Cooling FPPTB films from the nematic and isotropic phases increased crystal domain size, but also induced local structural variations in the molecular packing of crystalline FPPTB. The decrease in long-range order was correlated with an increase in short-range π-π interactions, leading to changes in molecular aggregation which persisted even when the FPPTB films were cooled to room temperature. Annealing-induced changes in molecular aggregation were confirmed with optical spectroscopy. The carrier mobility in FPPTB films increased over 2 orders of magnitude from (2.2 ± 0.4) × 10-5cm2V-1s-1in as-spun films to μ = (5.0 ± 0.8) × 10-3cm2V-1s-1in films cooled from the isotropic melt. We discuss the relationship between thermal stability and high carrier mobility values in terms of the interplay between long-range molecular order and increased π-π interactions between molecular pairs in the FPPTB film.
authors.affiliation: Physics of Energy, Department of Physics and Astronomy, Vrije Universiteit Amsterdam , De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.|Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky University of Oldenburg , Carl-von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany.
authors.forename: Nadine
authors.initials: N
authors.lastname: Tchamba Yimga
authors.affiliation: Physics of Energy, Department of Physics and Astronomy, Vrije Universiteit Amsterdam , De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
authors.forename: Charusheela
authors.identifier: http://orcid.org/0000-0001-8603-6853
authors.initials: C
authors.lastname: Ramanan
authors.affiliation: Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky University of Oldenburg , Carl-von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Borchert
authors.affiliation: Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky University of Oldenburg , Carl-von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Parisi
authors.affiliation: Liquid Crystals R&D Chemistry, Merck KGaA , Frankfurter Strasse 250, 64293 Darmstadt, Germany.
authors.forename: Harald
authors.initials: H
authors.lastname: Untenecker
authors.affiliation: Liquid Crystals R&D Chemistry, Merck KGaA , Frankfurter Strasse 250, 64293 Darmstadt, Germany.
authors.forename: Peer
authors.initials: P
authors.lastname: Kirsch
authors.affiliation: Physics of Energy, Department of Physics and Astronomy, Vrije Universiteit Amsterdam , De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
authors.forename: Elizabeth
authors.identifier: http://orcid.org/0000-0002-6269-0540
authors.initials: E
authors.lastname: von Hauff
keywords: dye; liquid crystal; mobility; molecular packing; organic semiconductor; thermal stability