title: Thermal Evaporation versus Spin-Coating: Electrical Performance in Columnar Liquid Crystal OLEDs.
abstract: The electrical responses of a columnar liquid crystal (a diimidodiester derivative of benzo[ghi]perylene) deposited either by spin-coating or by thermal evaporation into a typical OLED device are compared. For the spin-coated film, homeotropic alignment was induced by thermal annealing, which enhanced the charge carrier mobility significantly. For the evaporated films, homeotropic alignment could not be obtained by annealing. However, a degree of rectification higher than 3 orders of magnitude was achieved, even without annealing, with an electrical response similar to the response of the aligned spin-coated film. A trap-limited space-charge-limited current model was used to extract the charge carrier mobility directly from the current-voltage curves. Grazing incidence wide-angle X-ray scattering confirmed the homeotropic alignment of the annealed spin-coated film, whereas the columns are mostly oriented parallel to the surface in the evaporated case. In a field-effect transistor with bottom-gate bottom-contact geometry, the evaporated film exhibited a typical behavior of an n-type transistor. The degree of intermolecular order is thereby strongly dependent on the deposition method where vacuum deposition leads to a higher order. This higher order, however, impedes reorientation by annealing of the evaporated film but leads to improved charge transport between the electrodes even without homeotropic alignment of columnar liquid crystal.
authors.affiliation: †Departamento de Física, Universidade Federal de Santa Catarina-UFSC, 88040-900 Florianópolis, Santa Catarina, Brazil.|‡Department of Electronic Materials, Institute of Materials Science, Technical University of Darmstadt, Petersenstrasse 23, D-64287 Darmstadt, Germany.
authors.forename: Juliana
authors.initials: J
authors.lastname: Eccher
authors.affiliation: ∥Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Zajaczkowski
authors.affiliation: §Instituto de Física de São Carlos, Universidade de São Paulo, PoB.:369, 13560-970 São Carlos/SP, Brazil.
authors.forename: Gregório C
authors.initials: GC
authors.lastname: Faria
authors.forename: Harald
authors.initials: H
authors.lastname: Bock
authors.affiliation: ‡Department of Electronic Materials, Institute of Materials Science, Technical University of Darmstadt, Petersenstrasse 23, D-64287 Darmstadt, Germany.
authors.forename: Heinz
authors.initials: H
authors.lastname: von Seggern
authors.affiliation: ∥Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.|#Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Pisula
authors.affiliation: †Departamento de Física, Universidade Federal de Santa Catarina-UFSC, 88040-900 Florianópolis, Santa Catarina, Brazil.
authors.forename: Ivan H
authors.initials: IH
authors.lastname: Bechtold
keywords: columnar liquid crystals; current rectification; molecular packing; optoelectronic devices; processing