The Influence of Molecular Fluorination on the Electronic and Magnetic Properties of the Photovoltaic Composite PM6:PC61BM: DFT and EPR Studies

A comparative study by EPR and optical spectroscopy has been carried out, as well as calculations by density functional theory (DFT) of the electronic and magnetic properties of spin charge carriers excited by light in a photovoltaic composite PM6:PC61BM with fluorine atoms 19F in its monomers. The data obtained lead to the conclusion that there was a significant change in charge density and spin population when hydrogen atoms in PCE12 are replaced by fluorine ones of PM6 in the thiophene side chains of the polymer matrix of the composite. Such a substitution results in a redistribution of the electron charge density by about 27% on 19F atoms and their nearest carbon microenvironment, a slowdown in the recombination of mobile spin pairs, and a decrease in their g-factor due to greater spin delocalization along the polymer chain. At the same time, the effective band gap and the energy levels of polarons photoexcited in PCE12 and PM6 polymers are practically unchanged. The methods used in the work made it possible to estimate the charge and spin distribution on all atoms of organic photovoltaic cells.