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Efficient ternary polymer solar cells (PSCs) are prepared with poly‐[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′] dithiophene‐co‐3fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th):COi8DFIC as host system and medium bandgap material BDTThIT‐4F as the third component. The power conversion efficiency of PSCs can be increased from 11.47% to 13.08% by incorporating 20 wt% BDTThIT‐4F in acceptors, along with the simultaneously improved three key photovoltaic parameters. The absorption edge of ternary blend films can be tilted up in long wavelength range by incorporating appropriate BDTThIT‐4F, although the bandgap of BDTThIT‐4F is wider than that of COi8DFIC, leading to the extended external quantum efficiency spectra of ternary PSCs. The tilted up absorption edge of blend films should be attributed to the variation of COi8DFIC molecular arrangement, which can be well demonstrated from the transient and steady absorption spectra of blend films with different donors and acceptors. A new ground state bleach signal can be clearly observed in transient absorption spectra of the optimized ternary blend films, which may be due to the varied COi8DFIC molecular energy levels by incorporating BDTThIT‐4F. Meanwhile, the lifetimes on excited states are increased in the ternary blend films, which is beneficial to exciton dissociation for improving the performance of ternary PSCs.

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A medium-bandgap nonfullerene acceptor IBCT was developed, delivering 11.26% and 15.25% power conversion efficiencies in single-junction and tandem solar cells, respectively.

Phenazine copolymer donors demonstrate an efficiency of 15.14% in nonfullerene organic solar cells.

The in-plane and out-of-plane alkyl chains significantly affect the acceptors’ solubility and photovoltaic performance. NNFA[6, 6] and NNFA[12, 3] with medium solubility present suitable miscibility with the donor material, affording more favorable morphology and higher performance than other NNFAs.