Controlled alignment of polymer chains near the semiconductor-dielectric interface

Alignment of semiconducting polymer chains is crucial for achieving desired charge transport in organic field effect transistors (OFETs). Although the transistor performance is known to be critically affected by the charge accumulation layer of the semiconducting polymer film next to the semiconductor-gate dielectric interface, it is challenging to investigate the effect of polymer chain alignment specifically at the interface due to experimental limitations. In this work, the unique capability of multilayer formation and polymer chain alignment of the floating film transfer method (FTM) is utilized to study the alignment effect on the charge transport properties of OFETs. Discrete modulation of polymer chain alignment for the top half and bottom half layers of the films was achieved in a multilayered structure. When the bottom layer, which is close to the interface with the gate dielectric, has a parallel orientation of the polymer chains and the chains in the top layer are oriented perpendicular to the source-drain direction in the OFET devices, the average hole mobility is increased by a factor of 3.3 compared to that in the opposite case where the bottom layer has a perpendicular orientation and a parallel orientation in the top layer, even though the two cases have very similar bulk polymer chain alignments. In addition, no matter which polymer chain orientation is chosen for the top layer, the polymer chain orientation direction of the bottom layer governs the overall device performance. These experimental findings evidently support that the polymer chain orientation adjacent to the semiconductor/dielectric interface is a decisive factor in charge transport of OFETs.