Elucidating the Key Role of the Cyano (−C≡N) Group to Construct Environmentally Friendly Fused-Ring Electron Acceptors

Abstract

In recent years, advances in nonfullerene acceptors, especially fused-ring electron acceptors (FREAs), have enabled the power conversion efficiencies of organic solar cells to exceed 16%. FREAs typically consist of a planar fused-ring core connecting two strongly electron-withdrawing end groups, which are usually 1,1-dicyanomethylene-3-indanone and its derivatives. However, 1,1-dicyanomethylene-3-indanone and its derivatives are cyanides, which need to introduce a highly toxic substance, malononitrile, during the synthesis process. Therefore, most of the FREAs investigated currently are not environmentally friendly during the synthesis process. In this work, we answered two questions through systematic studies: (1) what is the role of the toxic cyano (−C≡N) group in FREAs? (2) Can any other groups replace −C≡N to construct high-performance environmentally friendly FREAs? Results show that it is the electron-withdrawing ability of −C≡N that determines the indispensable ability of FREAs to accept electrons. Using other electron-withdrawing groups (−CF3, −SO3H, and −NO2) to replace the −C≡N group in the high-performance electron acceptor Y6 shows that the acceptor based on the −NO2 group exhibits similar electron acceptance, stronger absorption, smaller hole–electron Coulomb attraction, and higher electron mobility than Y6. These results suggest that we can use the −NO2 group to replace the–C≡N group for constructing environmentally friendly FREAs with a high performance.