Effects of Halogenated End Groups on the Performance of Nonfullerene Acceptors
Daize Mo, Hui Chen, Yulin Zhu, Hsin‐Hsiang Huang, Pengjie Chao, Feng He
Abstract
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.