Unravelling the effect of strand orientation on exciton migration in conjugated polymers

The study of the average distance that singlet excitons travel during their lifetime in conjugated polymers has attracted considerable attention during the past decade, because of its importance in the functioning of many polymer-based optoelectronic devices, like solar cells and photodetectors. Intriguingly, different values of exciton diffusion length have been extracted from experiments on seemingly identical conjugated polymers. Here we use computer simulations to show that the observed discrepancies in the reported values of the exciton diffusion length may arise from differences in the orientation of conjugated polymer strands relative to the substrate surface, a factor which has been mostly overlooked. Our results show that, on pristine polymer nanodomains with conjugated strands perpendicular to the substrate surface, exciton migration length is approximately 30% and 40% lower than on those with parallel and random strand orientation relative to that surface, respectively, resulting from the different contents of physical traps present in nanodomains with different strand orientation. This work underlines the importance of molecular arrangement on exciton migration, and provides a novel theoretical framework for estimating the dependence of the exciton diffusion length with the orientation of conjugated polymers strands within the nanodomains, as well as helping the design of more efficient polymer-based optical and optoelectronic devices, such as optical sensors, photodiodes, photovoltaic cells and white light-emitting diodes.