A 2 -A 1 -D-A 1 -A 2 -Type Nonfullerene Acceptors.

The A 2 -A 1 -D-A 1 -A 2 -type molecules consist of one electron-donating (D) core flanked by two electron-accepting units (A 1 and A 2 ) and have emerged as an essential branch of nonfullerene acceptors (NFAs). These molecules generally possess higher molecular energy levels and wider optical bandgaps compared with those of the classic A-D-A- and A-DA'D-A-type NFAs, owing to the attenuated intramolecular charge transfer effect. These characteristics make them compelling choices for the fabrication of high-voltage organic photovoltaics (OPVs), ternary OPVs, and indoor OPVs. Herein, the recent progress in the A 2 -A 1 -D-A 1 -A 2 -type NFAs are reviewed, including the molecular engineering, structure-property relationships, voltage loss (V loss ), device stability, and photovoltaic performance of binary, ternary, and indoor OPVs. Finally, the challenges and provided prospects are discussed for the further development of this type of NFAs.