Observing halogen-bond-assisted electron transport in high-performance polymer solar cells

It is a broad agreement that a balance of the electron/hole mobility ratio mainly affects the device performance of organic solar cells. However, this consensus is still a semi-empirical model, rather than a universal principle with strict proof. Here, we evaluate the effects of hole/electron mobility ratios to device performance from the literature and four typical bulk-heterojunction systems. The results indicate that balanced hole/electron mobilities may not be a necessary and sufficient condition to high-performance polymer solar cells statistically, although the device performance may also appear to exist with clear mobility-ratio dependence in many reports. We propose a clear correlation between the electron hopping distance and fill factor in solar cells, rather than the absolute mobility values. In the high-performance PM6:BTP-eC9 device, an electron hopping distance of 2.98 angstrom can be observed, which is even much shorter than the pi-pi stacking distance in such a film. A halogen-bond-assisted charge transport model is proposed and correlated with the device performance from molecular dynamics simulations, and it provides a perspective to understand the device physics and charge transport in organic solar cells.