Ground state charge transfer in fullerene-polyalkylthiophene composites: ESR and iodine doping effect

Charge transfer in the ground state has been observed in poly(3-octadecylthiophene) (PAT-18), doped by C60. ESR studies of PAT-18-C60 composite reveal for the first time two ESR signals without photoexcitation. For the detection of two ESR signals an increase of microwave power is important. Only then can the two ESR signals be resolved: one with g1 = 2.0022 and ΔHPP = 3.5 G, corresponding to positive polaron P+ in PAT-18 chains, and the second with g2 = 1.9993 and ΔHPP = 1.5 G, corresponding to the C60− anion. The microwave power dependence of the intensity of these two ESR signals differs significantly due to different mechanisms of spin relaxation in C60 and PAT-18. The ESR study of PAT-18-C60 composite, additionally doped by I2, shows at the initial stage the disappearance of the C60− anion signal due to the further step of charge transfer (CT) from C60− to I2 at a low doping level and then, after heavy doping in I2 vapor, the increase of the polaronic P+ polymer signal due to strong CT doping of PAT-18 by I2.