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, BTIC-4EO-4F, BTIC-4EO-4Cl, and BTIC-4EO-4Br. 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 BTIC-4EO-4Br 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-b:4,5-b']-dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3-diyl]]} (PBDB-TF) as the polymer donor material, the BTIC-4EO-4Br-based OSCs exhibit the highest power conversion efficiency (12.41%), with a higher current density and a higher open-circuit voltage than the BTIC-4EO-4Cl-based OSCs (11.29%) and BTIC-4EO-4F-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.