Halogenated thiophenes serve as solvent additives in mediating morphology and achieving efficient organic solar cells

Incorporating solvent additives in donor and acceptor solutions is an effective strategy for achieving efficient organic solar cells. As the state-of-the-art photovoltaic materials have thiophene sequences along their conjugated backbones, in this contribution, thiophene analogues 2,5-dibromothiophene (HBrT) and 2,5-dibromo-3,4-difluoro-thiophene (FBrT) have been developed as solvent additives. Results show that FBrT has a high boiling point and the ability to dissolve donors and acceptors readily due to the "like dissolve like" principle, providing an effective tool in prolonging the crystallization and phase separation kinetic process of photovoltaic materials. Due to the concurrent formation of increased crystallinity, favorable phase separation, and P-i-N-like heterojunction, the FBrT-treated solar cells demonstrated improved exciton dissociation, charge transportation, extraction, and suppressed carrier recombination. Therefore, the PM6:Y6-based solar cells with FBrT additive reached an efficiency of 17.9% and fill factor of 78.6%, which are one of the highest values reached for the PM6:Y6-based binary solar cells. More strikingly, the FBrT solvent additive possesses general applicability in L8-BO, Y6-BO, and BTP-eC9-based solar cells and achieves significantly enhanced efficiencies of 18.6 similar to 18.7% in devices. These findings make it clear that the thiophene derivatives are effective solvent additives for the further optimization of organic solar cells.