Nonfullerene acceptors with cyano-modified terminal groups for organic solar cells: Effect of substitution position on photovoltaic properties

Two kinds of nonfullerene acceptors based on cyano-modified 1,1-dicyanomethylene-3-indanone (IC) as end groups and the dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (BTP) unit as a core are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption with the maximum absorption peak at 832 nm, and the absorption edge extends to 953 nm, corresponding to the narrow bandgap down to 1.30 eV. Comparing with the electron acceptor with cyano groups at mixed gamma/delta position, the acceptor with cyano substituted at the beta position shows more twisted structure and slightly higher energy level. When a wide bandgap polymer PM6 is adopted as the donor, the blend film with beta-cyano substituted acceptor shows more suitable phase separation morphology and more balanced charge transport. The device based on the beta-cyano substituted acceptor achieves a larger open-circuit voltage (V-OC) of 0.88 V than the acceptor with cyano groups at mixed gamma/delta position (V-OC = 0.78 V), and the energy loss was calculated to be 0.45 eV, leading to a higher power conversion efficiency (PCE) of 12.41% comparing with the devices with the gamma/delta-cyano substituted acceptor (PCE = 11.08%).