Effects of Halogenated End Groups on the Performance of Nonfullerene Acceptors.

The end groups' halogenations among the nonfullerene acceptors (NFAs) were a very useful method to fabricate high-performance NFAs-based organic solar cells (OSCs). We report three high-performance NFAs, , , and . They all have a fused benzothiadiazole as the core unit and different dihalogenated end groups (IC-2F, IC-2Cl, and IC-2Br) as the terminal unit. Thanks to the improved intramolecular charge-transfer ability of the brominated NFAs, bromination is more effective than fluorination and chlorination in lowering the energy levels and red-shifting the absorption spectra of the resulting NFAs. When compared with the chlorinated and fluorinated counterparts, the blend films exhibit lower roughness, better phase separation size, and stronger face-on stacking. When blended with poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-:4,5-']-dithiophene-2,6-diyl]--[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4,8-benzo[1,2-:4,5-']dithiophene-1,3-diyl]]} (PBDB-TF) as the polymer donor material, the -based OSCs exhibit the highest power conversion efficiency (12.41%), with a higher current density and a higher open-circuit voltage than the -based OSCs (11.29%) and -based OSCs (10.64%). These results show that the bromination of the NFAs' electron-withdrawing end groups can also be very effective in constructing high-performance photovoltaic materials.