Tuning Solid Additive Side‐Chain Length to Optimize Acceptor Stacking Morphology for Over 20% Efficiency in Organic Solar Cells ^†

Comprehensive Summary The incorporation of solid additive in photoactive layer as an effective strategy has been successfully employed to optimize the formation of a bi‐continuous interpenetrating network morphology in blend bulk heterojunction, which is a critical determinant of photovoltaic performance in organic solar cells (OSCs). However, the influence of additive side‐chain length on the morphological evolution remains insufficiently understood. In this work, we propose two novel solid additives, 1,3,5‐tribromobenzene (TBB) and 1,3,5‐tris(bromomethyl)benzene (TBMB) with different side‐chain lengths. Theoretical calculations reveal that TBMB, featuring longer side‐chain length, demonstrates stronger non‐covalent intermolecular interaction with donors and acceptors compared to TBB, thereby favoring optimized molecular aggregation and crystallization behavior during film formation. As a result, the TBMB‐treated device achieves a champion power conversion efficiency (PCE) of 17.92% in PM6:Y6 system, outperforming the TBB‐treated counterpart (17.20%). Remarkably, TBMB exhibits universal effectiveness across other systems, achieving an exceptional efficiency of 20.04% in D18:L8‐BO‐based device. This work provides deep insights into the potential working mechanism of solid additives with precise side‐chain length modulation, establishing a valuable additive side‐chain effects for future research on morphology regulation in OSCs.