Journal Article organic solar cells fused ring electron acceptors nonfullerene acceptors charge-carrier diffusion length charge-carrier recombination extending donor units fluorination fullerene fullerene liquid crystals fused-ring electron acceptors high-efficiency thick-film devices isomers layer-by-layer techniques nonfullerene nonfullerene solar cells perovskites photophysics polymer solar cells solar cells terpolymers trap density vertical phase separation wide bandgap wide-bandgap polymer donors