Exploring Nitration Effects on the Two-Dimensionally Extended Central Unit of Acceptors for Organic Photovoltaics

Nitration on small molecular acceptors (SMAs) could enlarge their dipole moments and relative dielectric constants without damaging the molecular planarity. However, the existing strategy of nitration on end groups will inevitably downshift the lowest unoccupied molecular orbitals (LUMOs) of SMAs and decrease the open-circuit voltages of organic solar cells (OSCs). Herein, benefiting from the additional active sites on central units of CH-series acceptors, two SMAs of CH1NO and CH2NO are constructed successfully by stepwise nitration on the central unit rather than end groups. The central nitration leads to the large relative dielectric constant, excellent molecular packing, and fiber-like morphology in active layers, especially for CH1NO; however, it only exerts little effect on the LUMO energy levels of SMAs. Consequently, CH1NO-based binary OSC reaches a better efficiency of 18.57% compared to that of 15.58% for the CH2NO-based one, mainly due to its broader photoelectric response range and improved charge transfer/transport dynamics. This work highlights the potential of nitration groups in developing high-performance SMAs due to their unique delocalized π system and planar conformation.