Electronic processes in bulk heterojunctions between poly(3-dodecylthiophene) (P3DDT) and fullerene derivative, [1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁] (PCBM) attracts growing interest during last years due to their perspective applications in molecular electronic and photonic. The irradiation of the P3DDT/PCBM system photoinduces electron transfer from polymer chain to the fulleren molecule accompained by the formation of the paramagnetic polaron P^+. and fullerene ion-radical C₆₁^-.. The power conversion efficiency of such solar cells should obviously depend on dynamics properties of these paramagnetic centers. In the present report we discuss first results of the light-induced EPR (LEPR) study of the P3DDT/PCBM system irradiated by laser with photon energy hν_ph of 1.88, 2.22, and 2.75 eV and 200K > T > 77K.

LEPR spectra of radical pairs photoinduced in the P3DDT/PCBM system consists of two lines at g₁ =2.00230 and g₂ =2.00011 attributed to P^+. and fullerene ion-radicals C₆₁^-., respectively. These radicals recombine at 180 - 200 K and the recombination rate is governed by the laser photon energy. At the increase of magnetic term B₁ these spectra are broadened, so then both the spin-lattice T₁ and spin-spin T₂ relaxation times were determined by the steady-state saturation method.

The polaron diffusion along polymer chains with diffusion coefficient D_1D and the rotational diffusion of fullerene ion-radical with coefficient D_r induce an additional magnetic field in both spin ensembles. This allows us to determine the above dynamics parameters. The data obtained can be interpreted in terms of a carrier hopping over barriers E_b.

The analysis showed that at the increase of hν_ph from 1.88 eV up to 2.22 eV and then up to 2.75 eV the value of E^diff_a first decreases from 65 meV down to 44 meV and then increases up to 95 meV. In the same time the E^rot_a decreases monotonically from 24 meV down to 9 meV and then down to 6 meV. Therefore, a charge photoinduced in the P3DDT/PCBM is transferred by polaron and fullerene anion. The energies for activation of these processes differ one from other that allows to conclude an independence of charge carriers in the system studied.