Journal Article organic solar cells fused-ring electron acceptors 3D stacking Fullerenes Polymers Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. bulk heterojunctions charge transport direct bandgap exciton diffusion fibril assembly fluorination fused ring electron acceptors fused-ring electron acceptor morphology nonfullerene nonfullerene acceptors nonfullerene solar cells organic solar cell organoboron perovskites photophysics polymer solar cells quasi-homojunction semiconductors side chain engineering ultrafast photoluminescence