Binary solvent mixture boosting high efficiency of polymer solar cells

Tremendous progress of organic solar cells (OSCs) has been exemplified by the use of non-fullerene electron acceptors (NFAs) in the past few years. Compared with fullerene derivative acceptors, NFAs show a multitude of advantages including tunable energy levels, broad absorption spectrum and strong light absorption ability, as well as high carrier mobility. To further improve the efficiency of non-fullerene OSCs, fluorine (F) or chlorine (Cl) atoms have been introduced into the chemical structure of NFAs as an effective approach to modulate the HOMO and LUMO levels. With a small Van der Waals radius and large electronegativity, the F atom improves the molecular planarity and aggregation tendency of NFAs, as well as increasing their crystallization ability.