Large energy loss is one of the main limiting factors for power conversion efficiencies (PCEs) of organic solar cells (OSCs). To this effect, the chemical modifications of the famous Y‐series nonfullerene acceptor (NFA) BTP‐4Cl‐BO with a new end group, TPC‐Cl, whose π‐conjugation is extended through the fusing of 3‐(dicyanomethylene)indanone (IC) group with a chlorinated thiophene ring, to synthesize two novel NFAs, BTP‐T‐2Cl and BTP‐T‐3Cl are performed. For BTP‐T‐2Cl with two TPC‐Cl groups, the resulting OSC exhibits a modest PCE of 14.89% but an extraordinary low energy loss of 0.49 eV, because its superior electroluminescence quantum efficiency of 0.0606% mitigates significantly the nonradiative loss (0.191 eV). For BTP‐T‐3Cl with one TPC‐Cl group, the corresponding device shows a higher PCE of 17.61% accompanied by a slightly bigger energy loss of 0.51 eV, which can be ascribed to the optimized morphology and/or efficient charge generation. Furthermore, the ternary OSC adopting two NFAs of BTP‐T‐3Cl and BTP‐4Cl‐BO achieves an impressive PCE of 18.21% (certified value of 17.9%), which is among the highest values for OSCs to date. The above results demonstrate that expanding end groups of NFAs with electron‐donating rings is an effective strategy to realize lower energy losses for OSCs.

Developing donor:acceptor (D:A) blends that work efficiently with small charge separation driving forces (or energetic offset) is highly desired for reducing the voltage loss and unveiling the working mechanism of organic photovoltaics (OPVs). Herein, by incorporating the weak electron-withdrawing acceptor unit of benzobis-(thiazole), we design and synthesize two wide-bandgap (∼2 eV) polymer donors, PBTz-TF and PBTz-TCl, with deep highest occupied molecular orbital (HOMO) levels approaching that of the typical electron acceptor Y6, forming negligible HOMO offsets of 0.04 and 0 eV for PBTz-TF:Y6 and PBTz-TCl:Y6 blends, respectively. Interestingly, efficient charge separation is observed in both blends, while the PBTz-TCl:Y6 blend possesses multiple superiorities, including higher mobilities, longer exciton lifetimes, purer domains, and more dominant face-on orientation. These lead to a better efficiency of 15.06% at 0 eV HOMO offset in the PBTz-TCl:Y6-based OPV, which is superior compared to the PBTz-TF:Y6-based OPVs (14.73%) with a HOMO offset of 0.04 eV. This work explores the properties of low driving force D:A blends and should have implications in the fundamental understanding of the carrier dynamics in OPVs with a flat energetic offset.