Effects of Halogenation on the Benzotriazole Unit of Non-Fullerene Acceptors in Organic Solar Cells with High Voltages.

Non-fullerene acceptors (NFAs) can be simply divided into three categories: A-D-A, A-DA'D-A, and A-A-D-A-A according to their chemical structures. Benefiting from the easily modified 1,1-dicyanomethylene-3-indanone end groups, the halogenation on the first two types of materials has been proved to be very effective to modulate their optoelectronic properties and improve their photovoltaic performance. Hence, in this work, we systematically investigate the effect of halogenation on the classic NFA molecule of , which has the linear A-A-D-A-A-type backbone. After fluorination and chlorination, and have similar optical band gaps but lower energy levels than . When blending with a linear copolymer composed of benzodifuran and chlorinated benzotriazole (BTA) according to "Same-A-Strategy", the corresponding of the halogenated NFAs gradually decreases (1.13 V for and 1.09 V for ), compared with that of the -based device ( = 1.22 V). This tendency mainly comes from the lower lowest unoccupied molecular orbital energy levels due to the strong electron-withdrawing ability of halogen atoms and the larger nonradiative energy loss. However, the power conversion efficiencies of the halogenated materials are slightly improved, from 9.08% for to 10.45% for and 10.75% for , with the nonhalogenated solvent tetrahydrofuran as the processing solvent. The improved photovoltaic performance of and should come from the higher carrier mobility, weaker bimolecular recombination, and higher fluorescence quenching rate. This study illustrates that halogenation on the A unit is a promising strategy for developing novel and effective A-A-D-A-A-type NFAs.