Near-infrared nonfullerene acceptors with halogenated terminated fused tris(thienothiophene) for efficient polymer solar cells

The fused-ring electron acceptors (FREAs) with A-D-A structures have experienced rapid development recent years. However, these FREAs still suffer the relative limited absorption in the near-infrared (NIR) region thus leading to low utilization of the solar light. Herein, two FREAs with strong absorption in NIR region, namely 6TIC-2Cl and 6TIC-2Br, were designed and synthesized by employing the fused tris(thienothiophene) (3TT) unit as the “D” along with 2-(5(6)-chloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (DCI-Cl) and 2-(5(6)-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (DCI-Br) as the end-groups, respectively. The strong electron-donating and molecular packing ability of 3TT and the enhanced electron-withdrawing ability of the halogenated terminal groups facilitated the intramolecular charge transfer thus bathochromic-shifting the absorption edges of 6TIC-2Cl and 6TIC-2Br to 940 nm and 934 nm, respectively. Consequently, the best-performing organic solar cells (OSCs) based on PBDB-T:6TIC-2Cl and PBDB-T:6TIC-2Br achieved the impressive short-circuit current density (JSC) of 23.83 mA cm−2 and 22.74 mA cm−2. In addition, the results showed that brominated terminal group was more beneficial to obtain a higher open-circuit voltage (VOC) and fill factor (FF) than the chlorinated end-group, which contributed to a decent PCE of 11.77%. This work revealed the impact of halogen atoms in the terminal group with 3TT units as core on the organic photovoltaic performance and provided guidance for designing SMAs with strong absorption in the visible-NIR region.