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

摘要

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 DELTAH_pp=3.5 G, corresponding to positive polaron P~+ in PAT-18 chains, and the second with g_2=1.9993 and DELTAH_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.