Electrical Conductivity and ESR Studies in Iodine-Doped Polythiophene from Semiconductor to Metallic Regime

Dependences of electrical conductivity and ESR of iodine-doped polythiophene films, (C 4 H 2 SI y ) x , on iodine concentrations which are precisely determined by a neutron activation technique (from a dilute level of y ∼4×10 -4 to a deep level of y ∼4×10 -1 ) are reported. A drastic increase of conductivity from 10 -8 to 10 -2 S/cm is observed at the fairly narrow dopant concentration region of 2×10 3 < y <5×10 -3 , accompanied by a remarkable decrease in the activation energy of conductivity. At this transitional region, the ESR signal also changes significantly, showing a maximum spin density and a minimum linewidth. These results are discussed to understand the semiconductor-to-metal transition in iodine-doped polythiophene, taking polaron and bipolaron models into account.