Charge transport anisotropy in a pentacene transistor with an underlying photo-alignment layer

Improving molecular packing of semiconductor materials in the vicinity of an insulating layer is of primary interest for efficient charge transport in an organic thin-film transistor. Self-assembled monolayers (SAMs) are often employed for this purpose. In other cases, a thin alignment layer is placed on a gate insulator in an attempt to order organic molecules along a specific direction. We fabricated bottom-gate, top-contact pentacene transistors with an underlying photo-alignment layer as follows. Azobenzene-polyamic acid was spin-coated on SiO2 and was irradiating with linearly polarized ultraviolet light at various energy densities. The material was converted to polyimide by heating and a thin pentacene layer was evaporated on it. Finally, source and drain electrodes were formed by sputtering gold through a shadow mask. The transistors fabricated with the polarization perpendicular to the current direction in the channel showed higher field-effect mobility and the maximum value was 1.0cm2/Vs. This is close to the value reported for the conventional pentacene transistors having SAMs. The mobility decreased as the irradiation energy density increased. Hence, we attribute the anisotropy introduced by the photo-alignment layer to degradation in charge transport in the specific direction.