Light exposure dependence of field-effect mobility of pentacene thin films deposited on very thin polyimide photo-alignment layers

Abstract

We report the anisotropic field-effect hole mobility of 30 nm-thick pentacene films deposited on 1.6 nm-thick polyimide photo-alignment layers with different in-plane anisotropy. To induce different in-plane anisotropy, the light exposure was varied up to 16 J/cm2. In this light exposure range, the long molecular axis of pentacene was standing with respect to the substrate surface (c*-axis oriented “thin film phase”), and the pentacene molecular plane was oriented on average parallel to the average alignment direction of the underlying polyimide backbone structures. For light exposures of 6 and 11 J/cm2, the field-effect mobility for the current flow direction parallel (perpendicular) to the alignment direction of pentacene molecular plane was enhanced (suppressed) compared to that of the pentacene film deposited on non-aligned polyimide film. This field-effect mobility enhancement shows a positive effect of the in-plane alignment control of pentacene molecules. However, the field-effect mobility decreased at a light exposure of 16 J/cm2, even though the in-plane orientation order of the pentacene molecular plane slightly increased. The causes for the mobility enhancement at 6 and 11 J/cm2 and the mobility decrease at 16 J/cm2 were discussed.