Electronic processes carrying out in bulk heterojunctions formed by poly(3-alkylthiophene) (P3AT) and fullerene derivative, [1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁] (PCBM), attracts growing interest during last years due to perspective applications of the P3AT/PCBM composite in molecular electronics and photonic. The irradiation of such a system by visible light excites electron transfer from polymer chain to the fullerene molecule accompanied by the formation of the paramagnetic polaron P^+. and fullerene anion-radical C₆₁^-.. The power conversion efficiency of such solar cells should obviously depend on dynamics properties of these paramagnetic centers. In the present report are discussed the first results of the light-induced EPR (LEPR) study of P3AT/PCBM composites with hexyl (P3HT) and dodecyl (P3DDT) alkyl substitutes irradiated by laser beam with photon energy of 1.8 - 3.4 eV at 77 - 200 K.

LEPR spectra of radical pairs photoinduced in the P3AT/PCBM system directly in the spectrometer cavity was shown to consist of two lines at g₁ = 2.0023 and g₂ = 2.0001 attributed to the polaron P^+o and fullerene anion-radical C₆₁^-., respectively. These radicals recombine at T > 180 K and the rate of such a process is governed by the laser photon energy. A decay of long-living radical pairs photoinduced in P3AT/PCBM composites was interpreted to depend strongly on a spatial distance between photoinduced charge carriers. The long lifetimes are solely ascribed to the long spatial distances that build up among the remaining photoinduced charge carriers, which did not recombine at a shorter time.

At the increase of microwave field in a spectrometer cavity these spectra are broadened, so then both the spin-lattice and spin-spin relaxation times were determined by the steady-state saturation method. The polaron diffusion along and between polymer chains as well as the rotational diffusion of fullerene ion-radicals accelerates electron relaxation in both spin ensembles. This allowed to determine separately all these dynamics parameters. The dynamics of both the photoinduced charge carriers was interpreted in terms of their activation hopping over energy barriers E_b.

The analysis showed that at the increase of photon energy from 1.88 eV up to 2.22 eV and then up to 2.75 eV the value of E_b of polarons photoinduced in P3DDT/PCBM composite first decreases from 65 meV down to 44 meV and then increases up to 95 meV. In the same time this value of fullerene molecules decreases monotonically from 24 meV down to 9 meV and then down to 6 meV. These energetic parameters obtained for P3HT/PCBM composite are characterized by extremal dependence on the photon energy near 2.0 and 3.1 eV. This can be a result of the formation of charge carriers in heterogeneous domains of this composite. Inhomogeneous ordering of such domains originates a variety of their band gap and, hence, sensitivity to photons with defined but different energy. The temperature annealing of P3HT/PCBM enhances its dimensionality (crystallinity) due to the formation of polymer crystallites and fullerene clusters that improves main electronic properties of plastic solar cells. The strategy of a creation of plastic solar-cells with optimal energetic is discussed.