Efficiency of polymer based solar cells is much higher when bulky heterojunctions of interpenetrating nets of electron donor and electron acceptor materials are used. The most promising compounds were found to be organic conjugated polymer - poly(3-hexylthiophene (P3HT), and fullerene derivative - [1-(3-methoxycarbonyl)-propyl-l-phenyl-(6,6)C₆₁] (PCBM). Photoinduced charge transfer in P3HT/PCBM bulky heterojunctions is accompanied by the formation of a radical pair - polaron and a fullerene radical anion with S = 1/2. Therefore, relaxation and dynamic properties of such paramagnetic centers can be investigated using light-induced ESR spectroscopy (LESR). The ordered chain structure of P3HT can perform as a center of nanostructurization of polymer molecules to two-dimensional layers via interchain alignment. It was shown [1] that self-organization of macromolecules upon regioregular P3HT temperature annealing improves charge carrier mobility by more than two orders of magnitude. However, nanostructurization can essentially be different in the starting polymer and in the P3HT/PCBM composite matrix because of the effect of fullerene molecules on self-organization of the polymer chains. The goal of this investigation was to study the effect of thermal modification of the P3HT/PCBM composite on dynamic and kinetic properties of photoinduced charge carriers.

The analysis of the magnetic, relaxation and dynamic parameters of polarons and fullerene radical anions induced in the bulky heterojunction of the starting P3HT/PCBM composite and the temperature annealing P3HT/PCBM composite by white light at different temperatures in a wide (77 - 300 K) range showed that charges carriers in a radical pair do not interact. Thus, they interact differently with their microenvironment This allowed us to separately determine magnetic, relaxation and dynamic parameters and identify the recombination and dynamics mechanism for both types of charge carriers in the P3HT/PCBM bulky heterojunction. It was shown that the interaction of polarons with the lattice is defined by its phonon energy, E_ph, while rotational diffusion of fullerene can be described in the frames of the Elliot model of activation electron transfer through energy barrier, E_b The calculation showed the E_ph and E_b values to be essentially lower after composite temperature annealing. The temperature dependences of diffusion coefficients were calculated using polaron movement along and across polymer chains and rotational diffusion of fullerene molecules. The decrease of anisotropy of polaron mobility after thermal modification of the composite was found. This is evidence of higher effective crystallinity of the polymer matrix. Composite heating provides nanostructurization of the bulky heterojunction in the P3HT/PCBM composite as a result of the formation of fullerene and polymer crystalline network and, hence, improves electronic properties of such plastic solar cells.

1. Sirringhaus H., Brown P.J. et.al, Nature, 1999, 401, p.685-688.