Conjugated polymers came to an unprecedented epoch that the charge transport is limited only by small disorder within aggregated domains. Accurate evaluation of transport performance is thus vital to exploring the charge transport for further optimization of molecular design. Yet the routine method by means of the conventional field-effect transistors (FETs) may not satisfy such a requirement. Here, we show that the commonly used top-gate, bottom-contact polymer transistors suffer from the extrinsic effects in particular the ambipolar charge transport, which significantly distorts device characteristics and makes trouble to parameter extractions. By elevating contact electrodes and meanwhile incorporating a thin p-type contact doping layer, the charge injection and the channel transport can all take place at the top surface of the polymer semiconductor film, exactly alike the planar-processed Si metal-oxide-semiconductor FETs (MOSFETs). The devices are therefore termed as planar polymer transistors. Compared to the conventional counterparts, the planar polymer transistors exhibit ideal p-FET characteristics with a subthreshold slope as low as 85 mV/dec, because the planar ohmic contacts eliminate the injection barrier, the access transport, and the concurrent ambipolar conduction, i.e., better capability of exploring the charge transport in the donor-acceptor (D-A) copolymers studied.
We then, for the first time, examine quantitatively the Schottky barrier and find out that the charge injection is indeed seriously affected by the high energetic barrier. In the conventional polymer transistors, the Schottky barrier is quite high and strongly depends on the gate and drain voltages, which is the origin of many misconceptions remaining in this filed, such as mobility overestimation. For the planar polymer transistors, on the other hand, their injection barrier is much lower and is nearly independent of the gate and drain voltages, thanks to the ohmic contacts formed that enable high-efficiency field emission and/or thermionic field emission without and/or with a negligibly small barrier—typical ohmic-contact characteristics. In the subsequent transport analyses, we find that the planar polymer transistors deliver much superior reliability in fitting into the transport models as compared with the conventional rivals. However, only the planar transistors operating in the low-field regime are robust to explore the inherent transport properties due to the energetic disorder lowering by the lateral field induced by high drain voltage. Hence, this work opens up a reliable and robust approach to comprehend the delicate charge transport in conjugated polymers so as to develop high-performance semiconducting polymers for the promising plastic electronics.