Influence of ultrasonic, microwave, and thermal effects on photoinduced charge transfer in poly(3-hexylthiophene)-Methanofullerene composites: EPR study

Radical pairs, polarons, and anion radicals of fullerenes that are induced in bulk heterojunctions of two composites, poly(3-hexylthiophene) with 6,6-phenyl-C61-butanoic acid methyl ester and with 6,6-phenyl-C62-butanoic acid bis(methyl ester), by photons with an energy of 1.98–2.73 eV at 77 K are studied via the method of photoinduced electron paramagnetic resonance. It is found that a part of the polarons and anion radicals of fullerenes are entrapped by energy traps of the polymer matrix, the quantity and depth of which are determined by ordering in the composites and the energy of exciting photons. A comparative study of the influence of composite treatment with ultrasonic, microwave, and thermal annealing on the formation and main resonance parameters of spin charge carriers in these bulk heterojunctions demonstrates a sharp gain in the number of charge carriers during illumination of the polymer-methanofullerene system, and the maximum effect is observed for photons with energies of 1.98 and 2.1 eV. This treatment procedure leads to the development of crystalline domains in the polymer matrix and increases the efficiency of light-energy conversion.