Power conversion efficiency of OPV devices, based on bulk-heterojunction particularly, was improved significantly in recent years. However, the reason of such high efficiency is still unclear, despite numerous studies. Charge transfer in OPV systems leads to formation of so called charge-separated (CS) state, which, as it was established by time-resolved EPR, is spin-correlated radical (polaron) pair (SCRP). The mechanism, keeping this SCRP from recombination, is not well understood.

Spin state of radical pair is well known to affect its recombination rate significantly. Therefore, detailed knowledge of SCPR spin state evolution can, in principle, give insight into the problem of efficiency of charge separation in OPV systems.

In this work we present detailed study of temporal shape of electron spin echo (ESE) signal of photoinduced SCRP in composite of conductive polymer P3HT and substituted fullerene PCBM. ESE signals of radical pairs (RP) P3HT⁺:PCBM^- are calculated in realistic model, taking into account finite microwave pulse length. Inhomogeneous broadening of resonant lines and interradical distance distribution are included. We found that experimentally observed ESE shape contradicts predictions of SCRP theory, which would be valid in the case of very fast electron transfer. Thus, instantaneous formation of singlet SCRP is not the case for P3HT⁺:PCBM^- pair, and spin system has enough time to evolve coherently.

While it is impossible to reproduce experimental data within the SCRP theory framework, assumption of presence of additional — with respect to what is predicted by singlet SCRP theory — spin polarization gives convincing accordance with the experiment.

Exact amount of this additional polarization depends on kinetic constants of charge separation and recombination, which in turn determine spin evolution of RP at the very beginning of overall charge separation process, inaccessible to EPR methods directly.

However, pulsed ESE techniques, in conjunction with numerical simulations, can be employed to measure contribution amount of additional polarization and, thus, to determine kinetic constants mentioned above. This information is critical for development of detailed theory of charge separation in OPV devices.

The work was supported by the Russian Foundation for Basic Research (Grant 15-03-07628a), by the Ministry of Science and Education of Russian Federation, and by Alexander von Humboldt Foundation research group linkage project "Light induced processes and paramagnetic species in organic photovoltaics and photosynthesis".