Low temperature transport in semiconducting polymer thin films

Abstract

Since Heeger, MacDiarmid, and Shirakawa reported that plastic can be electrically conductive in 1977, the era of conducting polymer has started. Because conducting polymers have both the electronic advantages and the material advantages, the applicability becomes enormously increasing. The future devices made of conducting polymers will be properly working while folded, rolled, or even crumpled. This specialty will result in a new generation of various portable devices, such as smart phones, tablet devices, or electronic books. Moreover, due to the printability on the large-area surface, we can readily realize large-screen displays, low-cost solar cells or high-capacity batteries soon. Furthermore, once a conducting polymer printer gets into general use, everyone can print the displays, logic circuits, or sensors on a A4 copy paper at home. However, despite its industrial development potential, how the charge carrier transport occurs inside of conducting polymers is still veiled. In this thesis, we investigated the transport mechanism in semiconducting polymer thin films from a physicist point of view through the low temperature experiments.In the first experiment, the high-mobility amorphous copolymer, DTCPD-alt-CB, was studied. Generally, amorphous polymers are known to have higher disorder than crystalline polymers, resulting in lower charge carrier mobility. However, The transistors made of DTCPD-alt-CB revealed pretty high mobility in spite of its amorphous nature. Through the low temperature experiments, we could determine the charge transport mechanism and compare the resulting physical parameters with those obtained from other polymers. As a result, this polymer has low disorder and low barrier height for transport, reflecting the weak charge localization in the films. This breaks the consensus that amorphous nature always indicates high disorder in polymer systems.In the second experiment, we could actively control the microstructure in polymer thin f...