Achieving Simultaneous Improvements in Interfacial Properties and Active Layer Formation Kinetics via a π‑Extended SAM for Organic Solar Cells Approaching 21% Efficiency

Despite the efficacy of self‐assembled monolayers (SAMs) in ITO work function modulation, their impact on active layer crystallization kinetics, notably the donor/acceptor (D/A) nucleation mismatch, remains an underexplored efficiency‐limiting factor in organic solar cells (OSCs). Herein, we propose an asymmetric π‐extended molecule, CbzPh, whose extra benzene ring affords a larger dipole than 4PACz (ITO work function: 5.28 eV vs. PEDOT:PSS's 4.87 eV) and strengthens non‐covalent interactions with D/A molecules. The strong PM6–CbzPh interactions not only accelerate donor nucleation to promote balanced D/A film formation kinetics but also generate a donor‐rich region near ITO, thereby enhancing charge selection and hole collection. Furthermore, this strategy minimizes D/A crystallization interference, improves polycrystalline orientation and coherence length, and suppresses trap states and energetic disorder, lowering non‐radiative loss to 0.224 eV. With CbzPh replacing PEDOT:PSS, the PM6:L8‐BO device achieves a FF of 81.14%, a PCE of 20.10% (up from 18.59%), and T 80 > 420 h. The universality of the strategy is testified across different systems with PCEs of 19.98%, 20.09%, 20.24% (certified 20.02%), and 20.88% (certified 20.48%) for PM6:BTP‐eC9, PM6:L8‐BO‐C4, PM6:L8‐BO‐X, and D18:L8‐BO, respectively. This work reveals that asymmetric π‐extended CbzPh suppresses non‐radiative recombination via synchronous crystallization, providing new insights into SAM mechanisms for OSCs.