Polymer "bulk heterojunctions" based on poly(3-alkylthiophenes) (P3AT) and 6,6-phenyl-C₆₁-butanoic acid methyl ester (PCBM) attract great research interest due to their perspective applications in e.g. plastic solar cells. The illumination of the P3AT/PCBM composit by a laser leads to the simultaneous formation of additional polarons P⁺ on the polymer chains and fullerene ion-radicals due to charge transfer from polymer chain to fullerene molecule. Besides, charge transfer by polarons along (Q1D) and between (Q3D) polymer chains and also rotational motion of fullerene molecules are realized in such system. These paramagnetic centers formed posses unpaired electron spins, therefore, the light-induced electron paramagnetic resonance (LEPR) method seems to be a promising method to investigate charge transport mechanisms in conducting polymers and polymer/acceptor composites. Molecular and electronic processes are correlated in P3AT/PCBM systems, however, they has not been studied so far.

We used the LEPR method for the study of magnetic, relaxation and dynamics parameters of polarons and fullerene ion-radicals photoinduced by laser beam with wave-length of 660, 530, and 450 nm in different P3AT/PCBM (A = hexyl, octyl, dodecyl) systems at 77 - 300 K temperature region.

Radical pairs photoinduced in the systems under study at low temperature region demonstrate two Lorentzian lines of diffusing polarons P⁺ and rotating ion-radicals. These paramagnetic centers start to recombine faster at temperatures higher than a critical temperature T_c due probably to the acceleration of spin dynamics and/or phase transition in the systems resulting in the decrease of their total paramagnetic susceptibility. Since the signals were found to be independent of one another with different electron relaxation, both the spin-lattice and spin-spin relaxation times of these paramagnetic centers were determined separately by the steady-state saturation method. These parameters as well the quantum effectiveness of the P3AT/PCBM system were shown to depend mainly on the length of alkyl group A and the energy of the light quantum. It was shown that the polarons and fullerene ion-radicals interact differently with own microenvironment. The activation barrier for reorientational hopping or rotation of fullerene ion-radicals depends on the structure of P3AT and energy of light quantum and lies near 20 - 40 meV that is lower considerably as compared with that obtained for fullerene rotating in more crystalline systems. The mechanism of charge transfer in different P3AT/PCBM systems is discussed as well.