Bromination: An Alternative Strategy for Non-Fullerene Small Molecule Acceptors

The concept of bromination for organic solar cells has received little attention. However, the electron withdrawing ability and noncovalent interactions of bromine are similar to those of fluorine and chlorine atoms. A tetra-brominated non-fullerene acceptor, designated as BTIC-4Br, has been recently developed by introducing bromine atoms onto the end-capping group of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile and displayed a high power conversion efficiency (PCE) of 12%. To further improve its photovoltaic performance, the acceptor is optimized either by introducing a longer alkyl chain to the core or by modulating the numbers of bromine substituents. After changing each end-group to a single bromine, the BTIC-2Br-m-based devices exhibit an outstanding PCE of 16.11% with an elevated open-circuit voltage of V-oc = 0.88 V, one of the highest PCEs reported among brominated non-fullerene acceptors. This significant improvement can be attributed to the higher light harvesting efficiency, optimized morphology, and higher exciton quenching efficiencies of the di-brominated acceptor. These results demonstrate that the substitution of bromine onto the terminal group of non-fullerene acceptors results in high-efficiency organic semiconductors, and promotes the use of the halogen-substituted strategy for polymer solar cell applications.