Rational Terminal‐Group Engineering of Guest Acceptors for 20.7% Efficient Organic Solar Cells

ABSTRACT Achieving higher efficiency in organic solar cells requires simultaneous control over energy‐level alignment, acceptor aggregation, and active‐layer crystallization. Herein, two guest acceptors, S‐SMA and AS‐SMA, were designed by stepwise replacing the fluorinated terminal group of L8‐BO with a weaker electron‐withdrawing 2‐(3‐ethyl‐4‐oxothiazolidin‐2‐ylidene)malononitrile (CN‐Rh) unit. This terminal‐group engineering upshifts the acceptor energy levels, enables cascade the lowest unoccupied molecular energy levels among D18, the guest acceptor, and L8‐BO, and regulates blend morphology. In situ spectroscopic analysis reveals that both guest acceptors accelerate the liquid‐to‐solid transition during film formation and promote tighter molecular packing. Compared with symmetric S‐SMA, asymmetric AS‐SMA enables a more favorable morphology and device operation, as reflected by suitable domain sizes, suppressed trap‐assisted recombination, and more balanced charge transport. Consequently, the ternary D18:L8‐BO:AS‐SMA device delivers the best overall photovoltaic performance, achieving a power conversion efficiency (PCE) of 20.71% with an open‐circuit voltage ( V OC ) of 0.930 V and a fill factor (FF) of 80.62%. These results demonstrate that asymmetric terminal‐group engineering is an effective strategy for coordinating energy‐level tuning, crystallization kinetics, and morphology optimization in high‐performance ternary organic solar cells.