Asymmetric Alkyl Chain Engineering of Non-fullerene Acceptors for Efficient Organic Solar Cells

The asymmetric molecular design strategy, with advantages in modulating the molecular dipole moment and intermolecular interactions and achieving more favorable molecular packing and orientation, has been an effective approach for designing high-performance non-fullerene acceptors (NFAs). Herein, two asymmetric NFAs, Y-CN-2F and Y-CN-2Cl, were designed and synthesized by introducing a linear alkyl chain terminated with the 4-cyanobiphenyl group, a well-known mesogenic unit, at one of the inner pyrrole positions instead of the normal 2-butyloctyl branched alkyl chain. The difference between Y-CN-2F and Y-CN-2Cl is the terminated IC-groups, which was modified with F and Cl halogens, respectively. Both NFAs displayed strong absorption in the near-infrared to visible-light range, which is complementary to that of typical medium-bandgap donor polymers. After optimization with D18 donor in organic solar cells (OSCs), Y-CN-2F and Y-CN-2Cl provided comparable power conversion efficiencies (PCEs) of 15.33% and 15.88%. While the D18:Y-CN-2F based devices displayed higher fill factors (FFs), those based on D18:Y-CN-2Cl exhibited higher current densities and open-circuit voltages. The Y-CN-2Cl film showed longer light absorption than Y-CN-2F, which is beneficial for more light harvesting. Moreover, D18:Y-CN-2Cl displayed a lower fluorescence lifetime and faster carrier transfer processes, which could be attributed to its higher mobility. For the D18:Y-CN-2F blended film, a more pronounced fiber network structure and balanced carrier mobility were observed, which contributed to the higher FFs values. This work presents new efforts to develop more asymmetric NFAs with specific functional segments for efficient organic electronics.