Light-induced charge separation in a P3HT/PC₇₀BM composite as studied by out-of-phase electron spin echo spectroscopy

A composite material of semiconducting polymer P3HT and fullerene derivative PC₇₀BM was studied by means of electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal was observed under laser irradiation of the composite at low temperature. We assume that during the charge separation process firstly the spin-correlated radical pairs in the singlet-polarized spin state are formed, and then the net polarization of radical pairs arises due to spin evolution. Both types of polarizations contribute to the out-of-phase ESE signal in the case of non-ideal microwave pulses. Analytical calculation of the echo shape for both types of initial polarization revealed that the contribution of the net polarization becomes zero after averaging over the whole EPR spectrum of the radical pair. This behavior was experimentally confirmed; thus the analysis of the out-of-phase ESE signal was simplified. Interspin distance distributions in the charge transfer state were obtained by modeling the out-of-phase ESE envelope modulation measured at different delays after laser flash T_DAF from 300 ns to 3.3 μs at a temperature of 65 K. Due to geminate recombination and diffusion of the radicals from the interface the distribution becomes significantly broader with larger distances prevailing at longer T_DAF values. The average distance between charges increases from 3.5 nm to 5.6 nm with an increase in T_DAF.