Improved organic solar cell efficiency based on the regulation of an alkyl chain on chlorinated non-fullerene acceptors

Non-fullerene acceptors (NFAs) have attracted significant research attention as they enable high-performance organic solar cells (OSCs). Recently, a new NFA named Y6 was reported to yield highly efficient OSCs with efficiency up to 15.7%. Then, the research interests regarding the structure–property relationship and further structural optimization of Y6 have risen sharply. In this study, we report a new NFA, N3-4Cl, using IC-2Cl as the end group and replacing the alkyl side chains on the N-position of the pyrrole unit with branched side chains on the third position. The combined substitution on Y6 yields electronic effects on energy levels and thus the open-circuit voltage (Voc) of the corresponding OSCs. The lowest unoccupied molecular orbital (LUMO) level of N3-4Cl is up-shifted compared to that of Y6-4Cl; thus, a higher Voc of 0.85 V is obtained. In addition, PM6: an N3-4Cl blend film can also maintain an optimal blend morphology with reasonably high and balanced charge mobility, which leads to a high fill factor (FF) of up to 74.9%. As a result, PM6:N3-4Cl-based device reached a PCE of 16.5% in binary devices. The results suggest that tuning the side chains of NFAs is an effective method for optimizing energy levels and aggregation properties, thus enhancing the OSC performances.