Regulating H‐Aggregation via a Thiazole‐Modified Acceptor‐Based Packing Reshaper Toward Highly Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) based on non‐fullerene acceptors have achieved power conversion efficiencies (PCEs) exceeding 20%. However, their long‐term stability remains limited by uncontrolled molecular aggregation and morphological degradation in bulk heterojunction (BHJ) films. Here, we report a thiazole‐based acceptor (BDTTz) as a molecular packing reshaper that selectively regulates acceptor H‐aggregation to enable efficient and stable OSCs. Theoretical calculations reveal stronger interactions between BDTTz and L8‐BO than between BDTTz and PM6, enabling targeted reinforcement of acceptor packing without disrupting donor organization. Incorporating BDTTz into PM6:L8‐BO blends extends the acceptor crystallization window from 32 to 48 ms, increases the coherence length from 50.21 to 57.17 Å, and raises the glass‐transition temperature from 104°C to 142°C, thereby suppressing molecular diffusion and stabilizing the blend morphology. The optimized morphology facilitates charge transport, suppresses the nonradiative recombination, and mitigates the electron–phonon coupling, affording a champion PCE of 20.66% with an extended T 80 lifetime of 1200 h under continuous illumination and heating conditions. Moreover, BDTTz incorporation into D18:L8‐BO blends yields a champion PCE of 21.08%. This work establishes a molecular‐level packing‐reshaping strategy for developing high‐efficiency and long‐term‐stable OSCs.