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A bromine‐substituted [2‐(9H‐Carbazol‐9‐yl) ethyl] phosphonic acid, 1Br‐2PACz, is designed as hole‐selective self‐assembled monolayers (SAMs), contributing to an outstanding power conversion efficiency (PCE) of 19.35% for binary bulk‐heterojunction (BHJ) based organic solar cells (OSCs). As compared to the previous high‐performance 2Br‐2PACz SAMs, 1Br‐2PACz molecules can effectively reduce the interaction of the SAM with the BTP‐eC9 nonfullerene acceptors with a decreased binding energy, resulting in the suppressed vertical self‐aggregation of BTP‐eC9 small molecules as the bottom side of PM6:BTP‐eC9 BHJ during the solidification process. There is decreased energetic disorder within photoactive layer together with more efficient charge transfer and suppressed non‐radiative recombination. Furthermore, five additional binary BHJ systems are applied in 1Br‐2PACz SAM‐based OSCs, exhibiting continuously superior performance as compared to the reference cells with conventional PEDOT:PSS hole transport layer. This work underscores the potential of advancing OSCs by fine‐tuning SAMs through halogenation strategies to improve active layer morphology and overall device performance. [ABSTRACT FROM AUTHOR]

Achieving high‐performance in all‐small‐molecule organic solar cells (ASM‐OSCs) significantly relies on precise nanoscale phase separation through domain size manipulation in the active layer. Nonetheless, for ASM‐OSC systems, forging a clear connection between the tuning of domain size and the intricacies of phase separation proves to be a formidable challenge. This study investigates the intricate interplay between domain size adjustment and the creation of optimal phase separation morphology, crucial for ASM‐OSCs' performance. It is demonstrated that exceptional phase separation in ASM‐OSCs' active layer is achieved by meticulously controlling the continuity and uniformity of domains via re‐packing process. A series of halogen‐substituted solvents (Fluorobenzene, Chlorobenzene, Bromobenzene, and Iodobenzene) is adopted to tune the re‐packing kinetics, the ASM‐OSCs treated with CB exhibited an impressive 16.2% power conversion efficiency (PCE). The PCE enhancement can be attributed to the gradual crystallization process, promoting a smoothly interconnected and uniformly distributed domain size. This, in turn, leads to a favorable phase separation morphology, enhanced charge transfer, extended carrier lifetime, and consequently, reduced recombination of free charges. The findings emphasize the pivotal role of re‐packing kinetics in achieving optimal phase separation in ASM‐OSCs, offering valuable insights for designing high‐performance ASM‐OSCs fabrication strategies. [ABSTRACT FROM AUTHOR]