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 C-60. ESR studies of PAT-18-C-60 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 g(1) = 2.0022 and Delta H-PP = 3.5 G, corresponding to positive polaron P+ in PAT-18 chains, and the second with g(2) = 1.9993 and Delta H-PP = 1.5 G, corresponding to the C-60(-) anion. The microwave power dependence of the intensity of these two ESR signals differs significantly due to different mechanisms of spin relaxation in C-60 and PAT-18. The ESR study of PAT-18-C-60 composite, additionally doped by I-2, shows at the initial stage the disappearance of the C-60(-) anion signal due to the further step of charge transfer (CT) from C-60(-) to I-2 at a low doping level and then, after heavy doping in I-2 vapor, the increase of the polaronic P+ polymer signal due to strong CT doping of PAT-18 by I-2.