Organic n-channels of substituted phthalocyanine thin films grown on smooth insulator surfaces for organic field effect transistors applications

Abstract

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.