High-Mobility Aligned Pentacene Films Grown by Zone-Casting

We investigate the growth and field-effect transistor performance of aligned pentacene thin films deposited by zone-casting from a solution of unsubstituted pentacene molecules in a chlorinated solvent. Polarized optical microscopy shows that solution processed pentacene films grow as large crystalline domains with pronounced anisotropy in the substrate plane, in contrast to vacuum sublimed pentacene films, which consist of small crystalline grains with random in-plane orientation. The high structural alignment is confirmed by in-plane and out-of-plane X-ray diffraction analysis, with out-of-plane 00n reflections up to at least the seventh order, and a pronounced in-plane anisotropy with the a-axis of the triclinic unit cell predominantly aligned parallel to the zone-casting direction and the ab-plane parallel to the substrate. The average charge carrier mobility of the zone-cast pentacene devices depends strongly on the underlying dielectric. Divinylsiloxane-bis-benzocyclobutene (BCB) resin is found to be a suitable gate dielectric allowing reproducible film deposition and high field-effect mobilities up to 0.4−0.7 cm2/(V s) and on/off ratios of 106−107. A small mobility anisotropy is observed for devices with channels aligned along and perpendicular to the zone-casting direction.