The role of isomeric alkyl chains of polymer donors on charge-transfer processes in non-fullerene organic photovoltaics: a theoretical investigation

To investigate the effects of polymer donors with isomeric alkyl chains on the interfacial charge-transfer (CT) processes, the rates of exciton dissociation (k_ED) and charge recombination (k_CR) have been calculated for complexes PM6/Y6 and PM6i/Y6, where isomer PM6i was designed by shifting the side chains in the electron-withdrawing units to the thiophene-bridges based on PM6. For complex PM6/Y6, k_ED of PM6 and Y6 exciton are calculated to be 10¹⁰–10¹² s⁻¹ and 10¹²–10¹³ s⁻¹, and k_CR is 10¹¹–10¹² s⁻¹ for recombination of the lowest CT (CT₀) excitons into the lowest triplet states of Y6, which are consistent with the experimental results. When going to complex PM6i/Y6, k_ED of PM6i exciton is higher than that of PM6 exciton, and of Y6 exciton is decreased to 10¹¹–10¹² s⁻¹, but k_CR is decreased to 10¹⁰–10¹¹ s⁻¹. The calculated results reveal that isomeric alkyl chains of polymer donors can enhance the polymer exciton dissociation and suppress the charge-recombination processes, underlining the important role of isomeric alkyl chains in design of polymer donors for high performance non-fullerene organic photovoltaics.