Efficient modulation of end groups for the asymmetric small molecule acceptors enabling organic solar cells with over 15% efficiency

Non-fullerene organic solar cells (OSCs) have attracted tremendous interest and made an impressive breakthrough, largely due to advances in high-performance small molecule acceptors (SMAs). The relationship between short-circuit current density (J_SC) and open-circuit voltage (V_OC) is usually shown as one falls, the other rises. Controlling the trade-off between J_SC and V_OC to harvest high power conversion efficiencies (PCEs) still remains as a challenge. Herein, dithieno[3,2-b:2′,3′-d]pyrrole (DTP) based asymmetric SMAs with different chlorinated dicyanoindanone-based end groups, named TPIC, TPIC-2Cl and TPIC-4Cl, are designed and synthesized. These asymmetric acceptors exhibit a remarkable red-shifted absorption profile, while energy levels are simultaneously down-shifted when the numbers of chlorine atoms alter from 0, 1 to 2, due to the gradually improved electronegativity. As a result, PM7:TPIC-4Cl based OSCs achieved a champion PCE of 15.31%, which is the highest PCE for non-fullerene binary OSCs based on asymmetric SMAs. The superiority of the PM7:TPIC-4Cl system consists of the balanced charge transport, favorable phase separation, efficient exciton dissociation and extraction, coupled with the remarkable π-π stacking and crystallinity of the SMAs. Our results highlight the important strategy of asymmetric molecular design to optimize the trade-off between V_OC and J_SC, reaching a high PCE.