Building blocks for n-type molecular and polymeric electronics. Perfluoroalkyl- versus alkyl-functionalized oligothiophenes (nT; n = 2-6). Systematics of thin film microstructure, semiconductor performance, and modeling of majority charge injection in field-effect transistors

Abstract

The solid-state properties and FET electrical behavior of several series of alpha,omega- and beta,beta'-fluorocarbon- and alkyl-substituted and unsubstituted oligothiophenes nTs (n = 2-6) are compared and contrasted. The thin films were grown by slow vacuum deposition over a range of substrate temperatures and/or by casting from solution and were investigated by X-ray diffraction and scanning electron microscopy. Our results indicate that vacuum deposition at 60-80 degrees C affords films with remarkably similar microstructures despite the extensive H --> F substitution. Trends in observed d spacing versus molecular core extension provide quantitative information on molecular orientation. Field-effect transistor measurements performed for all systems and having the same device structure, components, and fabrication conditions demonstrate that all nTs functionalized with fluorocarbon chains at the thiophene termini are n-type semiconductors, in contrast to the p-type activity of the remaining systems. One of these systems, alpha,omega-diperfluorohexyl-4T, exhibits a mobility of 0.22 cm2/(V s) and an Ion:Ioff ratio of 10(6), one of the highest so far reported for an n-type organic semiconductor. The effect of substitution regiochemistry on FET majority charge carrier was additionally studied, in the case of a 6T core, by shifting the fluorocarbon substituents from the terminal to the central thiophene units. Finally, we propose a simple theoretical model for electrode/organic interfacial carrier injection. The results suggest why modest substituent-induced changes in the injection barrier can produce working n-type materials.