Organic n -channels of substituted phthalocyanine thin films grown on smooth insulator surfaces for organic field effect
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D. Wo¨hrle Institute of Organic and Macromolecular Chemistry, University of Bremen, P.O. Box 330440, D-28334 Bremen, Germany
D. Schlettweina) Physical Chemistry 1, Institute of Pure and Applied Chemistry, University of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany (Received 14 November 2003; accepted 17 February 2004)
Thin films of the perfluorinated phthalocyanines F16PcVO and F16PcCu were grown on insulator substrates by physical vapor deposition under high vacuum conditions to study their growth and electrical properties, analyzing them as possible candidates for n-type channel materials in organic field effect transistors. As insulator substrates, mica, amorphous SiO2, poly(styrene), poly(vinylchloride), poly(vinylcarbazole), poly(methylmetacrylate) and poly(vinylidenefluoride) were chosen, offering chemically different interactions with the molecules, degrees of order, and tribological characteristics. Optical absorption spectroscopy was used to analyze the alignment of the molecules relative to the substrates and the electronic coupling to adjacent molecules in the films. Electrical conduction measurements served to analyze the electronic coupling of the molecules parallel to the insulating substrates and to discuss the growth mode of films. Atomic force microscopy and scanning electron microscopy were chosen to study the morphology of the films. An inclined orientation at an average surface angle between 58° and 86° dependent on the different substrates was found for both F16Pc. In particular, on mica, thin conductive channels of the organic n-semiconductors could be formed at an average film thickness well below 10 nm at conductivities of up to 1.3 × 10−2 S cm−1. Conductive channels could also be formed on the different polymer substrates at, however, at least an order of magnitude smaller conductivity. Before these layers were formed on the polymers, semiconductor material diffused into the polymer substrates, dependent upon the substrate temperature during deposition relative to the glass transition temperature of the polymers. An influence of the viscous state of the polymer substrates was also seen on the intermolecular coupling detected in optical spectra. Based on these results, implications for the applicability of F16Pc as organic n-channels in organic field effect transistors are discussed.
I. INTRODUCTION
Organic n-channels for organic field effect transistors (OFET) are sought to allow the construction of complementary logic circuits with already quite well established a)
Address all correspondence to this author. Present address: Justus-Liebig-University Giessen, Institute of Applied Physics, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany. e-mail: [email protected] DOI: 10.1557/JMR.2004.0268 2040
J. Mater. Res., Vol. 19, No. 7, Jul 2004
p-channel OFET on the basis of, e.g., pentacene or conducting polymers.1–3 Substituted phthalocyanines are promising candidates as n-channels4 since their electronic levels can be shifted in a wide energy range by appropriate substitution
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