Light-induced ESR studies in conjugated polymer-fullerene composites

In this contribution we discuss photo-induced electron transfer in conjugated polymer/fullerene composites and compare this with light induced effects in pure components. Two overlapping LESR lines are observed, from positive polarons on the polymer chains and negative charges on the fullerene moieties. Microwave power saturation studies show different relaxation times for these two spins, giving clear evidence for independent spins. The unusually high relaxation rate of the fullerene monoanionic spins is of intrinsic origin and discussed in terms of a splitting of the T-1u level by a Jahn-Teller type distortion as proposed in the literature. We observed two distinct contributions to LESR signals, a prompt and a persistent one. Excitation light intensity dependence of the prompt contributions into P+ and C-60(-) ESR signals is of bimolecular type (I-0.5) and implies the mutual annihilation within (P+...C-60(-)) pair. The persistent contribution is found to be excitation intensity independent and proposed to originate from deep traps due to disorder.