Light-Induced Electron Paramagnetic Resonance Study of Poly(3-alkylthiophene)/Fullerene Composites

Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with an energy of 1.98−2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) and poly(3-dodecylthiophene) (P3DDT) with [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) and 2-(azahomo[60]fullereno)-5-nitropyrimidine (AFNP) fullerene derivatives were studied by a direct light-induced electron paramagnetic resonance (LEPR) method in a wide temperature range. LEPR spectra of the polymer/fullerene composites consist of contributions of mobile and trapped charge carriers. Concentration and magnetic resonance parameters of these charge carriers were found to depend on the energy of initiated photons. Spin−lattice and spin−spin relaxation times of polarons and fullerene anion radicals were determined by the steady-state saturation method. The interaction of most charge carriers with the lattice is characterized by monotonic temperature dependence, whereas the spin−lattice relaxation time of fullerene anion radicals trapped in the P3DDT matrix demonstrates sharper temperature dependence. Spin−spin interaction is shown to be nearly temperature independent and to be governed by structural properties of polymer/fullerene composites. Longitudinal diffusion of polarons and pseudorotation of fullerene derivatives was shown to follow the activation Elliot hopping model. The replacement of the P3HT matrix by P3DDT accelerates polaron dynamics and increases its anisotropy. The energetic barrier required for polaron interchain hopping mainly prevails upon that of its intrachain diffusion in all composites except P3DDT/AFNP one. Spin dynamics becomes easier when the PCBM fullerene derivative is replaced by the AFNP one.