Perfectness of the main-chain alignment in the conjugated polymer films prepared by the floating film transfer method

Polymer semiconductor/gate dielectric interface is well known to influence the charge transport behaviors in organic field-effect transistors (OFETs). In the unidirectional floating-film transfer method (UFTM), thin floating films of uniaxially aligned π-conjugated polymers are solidified at the air/liquid interface before transferring them onto device substrates. Therefore, packing structures in the films prepared by UFTM are not affected by the strong interactions with the device substrate as usually seen in other film growth methods. Here, one question arises: will the difference in interactions between the liquid substrate and air impact the orientational structure of the polymer chain near the two surfaces or not? To answer this question, we have studied the charge-transport anisotropy in top- and bottom-gate OFETs by changing the film thickness. As a result, the field-effect carrier mobility and its anisotropy did not significantly differ for both surfaces and were constant against the film thickness. These results indicate that the UFTM can provide unidirectional alignment of polymer chains throughout the film thickness and gives a similar superior performance either for a bottom- or a top-gate OFET.