A possible phase transition in metallic polythiophene, and poly(3-methylthiophene) at low temperature

Summary form only given. Polythiophene (PT) and poly(3-methylthiophene) (PMeT) films prepared by electrochemical synthesis are flexible and highly conducting (/spl sigma/>100S/cm). We have studied the transport properties in as-grown PT and PMeT doped with PF/sub 6//sup -/. Precise measurements were made of the Voltage-Shorted-Compaction (VSC) resistivity (P/sub vsc/) and the absolute thermoelectric power S(T) between 1.6-300K, both of which are unique in abstracting the transport properties inherent to the crystallized regions of polymeric materials. In PT, (P/sub vsc/) exhibited a metallic behaviour at high temperatures between 30-100K with the temperature dependence approximately expressed by T/sup 2.5/, characteristic of the low dimensional electronic structure. Unlike the behaviour above 30K, (P/sub vsc/) increased steeply with decreasing temperature below 30K. Corresponding to (P/sub vsc/), S(T) also showed metallic behaviour at high temperatures, while at low temperatures an anomalous behaviour was observed as in TTF-TCNQ. The results were strongly indicative that a metal-insulator transition would occur in the crystallized regions at 30K. In PMeT, (P/sub vsc/) was metallic between 5-300K, with a T/sup 2/-dependence especially between 5-40K. Below 5K it became almost independent of temperature. The difference of the transport property of the bulk material of PT and PMeT is to be understood by their different transition temperatures.