Photoinduced charge separation and transfer in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) with fullerene derivatives, [1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61] (PCBM) and bis-PCBM were first studied comparatively by the light induced EPR (LEPR) method at wide temperature and photon energy h?ph regions. An electron transfer from polymer chain to the fullerene globule is accompanied by the formation of paramagnetic long-living polaron and fullerene anion-radical. Some charge carriers are trapped in a polymer matrix. Non-interacting character of charge carriers allowed to determine separately their magnetic, relaxation and dynamics parameters. These parameters were shown to depend on the structure of a counter-anion (see Figure) and also on the photon energy and temperature. An activation energy of polaron 1D hopping in the P3HT/PCBM composite changes with the h?ph nonlinearly with extremas lying near 2.0 and 3.1 eV. Such effect appears due to a photoexcitation of charge carriers in heterogeneous domains of the polymer/fullerene composite. Inhomogeneous ordering of such domains originates a variety of their band gap and, hence, sensitivity to photons with different energy.

A decay of radical pairs at T < 200 K was interpreted to depend strongly on an intercharge spatial distance. This process becomes slower at the PCBM -> bis-PCBM replacement. The temperature annealing of the composite increases additionally its dimensionality (crystallinity) due to the formation of polymer crystallites and fullerene clusters that, by turn, increases the lifetime of the spin pairs and improves main electronic properties of such plastic solar cells. The strategy of a creation of plastic solar-cells with optimal energetic is discussed.