The light-induced charge-transfer (CT) state in the composite of the conductive polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been studied by electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal corresponding to the spin-correlated radical pair P3HT(+)/PCBM(-) has been observed in this composite material. The time-domain ESE shape for different delays between the laser flash and the microwave pulse sequence has been analyzed. In order to explain the evolution of the out-of-phase ESE signal as a function of the delay between the microwave pulses, a model of the CT state is proposed. The hole is assumed to be delocalized on the P3HT chain over several thiophene subunits, while the point-dipole approximation is used to describe the interaction with the electron on PCBM. The distribution of distances between the positive and negative charges in the CT state has been evaluated.

A composite of conductive polythiophene P3HT and soluble fullerene derivative PC 70 BM (the material widely used as the active layer in organic photovoltaics devices) was studied by light-induced EPR (LEPR). In contrast to P3HT/PC 60 BM composite, LEPR signal in P3HT/PC 70 BM can be detected in a wide temperature range up to room temperature. This signal was attributed to charge carriers P3HT + and PC 70 BM − . The dependence of the intensity of LEPR signal on light intensity and the decay of LEPR signal upon switching light off are interpreted in frame of trap-limited bimolecular recombination model with finite rate of back electron transfer in [P3HT + PC 70 BM − ] encounter complex at polymer−fullerene interface. The apparent recombination order was found to be close to p = 3.5 for the temperature range from 100 K to room temperature. For temperatures above 150 K Arrhenius behavior of effective recombination rate constant was obtained with the activation energy E a = 0.16 ± 0.01 eV, which is larger than analogous values for P3HT/PC 60 BM reported previously. This difference is attributed to the change of polymer/fullerene interface induced by the replacement of PC 60 BM by PC 70 BM.

A composite material of semiconducting polymer P3HT and fullerene derivative PCBM 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 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 values. The average distance between charges increases from 3.5 nm to 5.6 nm with an increase in T.