ESR and ESE studies on polymer semiconductors of weakly doped poly(tetrathiafulvalene)

In the studied poly(tetrathiafulvalenes), PTTF, the TTF units are linked via phenyl groups or via tetrahydroanthracene bridges. The PTTF are doped with oxygen and with iodine respectively. The electrical conductivity strongly depends on temperature as it is typical of semiconducting materials. The existence of various types of paramagnetic centres is confirmed by ESR saturation experiments, by spectra simulation and by electron spin echo (ESE) measurements in the temperature region of 2 K to 400 K. At room temperature the doped PTTF have a dc conductivity between 10⁻⁹ Scm⁻¹ and 10⁻⁴ Scm⁻¹. Relations between the concentration of various radicals and the electrical conductivity of the polymers are discussed by means of a model based on mobile and immobile radicals, polarons and bipolarons. There are experimental indications for temperature-dependent formation and break-up of bipolarons which are discussed in the paper.