An in situ ESR study on poly(3-methylthiophene): charge transport due to polarons and bipolarons before the evolution of metallic conduction

In situ ESR measurements with ClO4−-doped poly(3-methylthiophene) (PMT) are carefully performed over a wide range of doping level from 0.02% to 23% in order to correlate a drastic increase of carrier mobilities by doping with the nature of charged species in the π-conjugated polymer. Either or both of a Gaussian and Lorentzian components with different g-factors are observed, depending on the doping level. The broad Gaussian signal ascribable to structural defects in the polymer remains almost unchanged at various doping stages while the Lorentzian signal varies greatly with the doping level. From spin concentrations for the Lorentzian signal, molar fractions of polarons and diamagnetic species (bipolarons) are evaluated as a function of doping level. Combination of the molar fractions and the mobility data obtained earlier demonstrates that the conduction process in the lightly doped PMT film (0.02%–1%) can be expressed by independent driftings of polarons and bipolarons under electric field, whose mobilities are 2×10−5 and 10−3 cm2 V−1 s−1, respectively. At higher doping levels of 1%–23%, conductivity is much more enhanced than expected from this model and the line width of the Lorentzian signal increases from 1.2 to 7.5 G, suggesting the evolution of a metallic conduction.