Blade Coating Aligned, High-Performance, Semiconducting-Polymer Transistors

Recent demonstration of mobilities in excess of 10 cm² V^-1 s^-1 have energized research in solution deposition of polymers for thin film transistor applications. Due to the lamella motif of most soluble, semiconducting polymers, the local mobility is intrinsically anisotropic. Therefore, fabrication of aligned films is of interest for optimization of device performance. Many techniques have been developed to control film alignment, including solution deposition via directed flows and deposition on topologically structured substrates. We report device and detailed structural analysis (ultraviolet-visible absorption, IR absorption, near-edge X-ray absorption (NEXAFS), grazing incidence X-ray diffraction, and atomic force microscopy) results from blade coating two high performing semiconducting polymers on unpatterned and nanostructured substrates. Blade coating exhibits two distinct operational regimes: the Landau-Levich or horizontal dip coating regime and the evaporative regime. We find that in the evaporative deposition regime, aligned films are produced on unpatterned substrates with the polymer chain director perpendicular to the coating direction. Both NEXAFS and device measurements indicate the coating induced orientation is nucleated at the air interface. Nanostructured substrates produce anisotropic bottom contact devices with the polymer chain at the buried interface oriented along the direction of the substrate grooves, independent of coating regime and coating direction. Real time studies of film drying establish that alignment occurs at extremely high polymer volume-fraction conditions, suggesting mediation via a lyotropic phase. In all cases the final films appear to exhibit high degrees of crystalline order. The independent control of alignment at the air and substrate interfaces via coating conditions and substrate treatment, respectively, enable detailed assessment of structure-function relationships that suggest the improved performance of the nanostructure aligned films arise from alignment of the less ordered material in the crystallite interphase regions.