Photoinduced charge carriers in conjugated polymer–fullerene composites studied with light-induced electron-spin resonance

Abstract

Detailed studies on photoinduced spins in conjugated polymer/fullerene composites using (cw) light-induced electron-spin-resonance (LESR) technique are reported. 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, ruling out spin-exchange correlations, giving clear evidence of 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. Further, we observed two distinct contributions to LESR signals: a prompt one and a persistent one. The excitation light intensity dependence of the prompt contributions into the $P^{+}$ and ${\mathrm{C}}_{60}^{\mathrm{-}}$ ESR signals is of bimolecular type ${(I}^{0.5}),$ and implies mutual annihilation within the created ${(P}^{+}\dots {\mathrm{C}}_{60}^{\mathrm{-}})$ pair. The persistent contribution is found to be excitation intensity independent, and is proposed to originate from deep traps due to disorder.