A Paradigm Shift Toward Quasi‐Thermodynamically Stable Bulk Heterojunction Morphology Enabled by Controlled Crystallization Kinetics of High Quadrupole Moment Nonfullerene Acceptors

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ABSTRACT Recent advances in high quadrupole moment () non‐fullerene acceptors (NFAs) have improved exciton diffusion and charge separation in organic photovoltaics (OPVs). However, conventional bulk heterojunctions (BHJs) with a balanced donor‐to‐acceptor ratio often exhibit amorphous mixed domains, introducing energetic disorder and limiting the optoelectronic potential and stability of high‐ NFAs. Here, a diffusion‐driven 3D crystallization strategy is demonstrated using an NFA‐rich BHJ model (Non‐fullerene Matrix‐BHJ, NM‐BHJ) that suppresses electron trap states and charge localization, thereby reducing energy losses driven by static disorder. Notably, the laterally oriented 3D superstructures significantly enhance thermodynamic stability and improve resistance to morphological changes under thermal stress. OPVs incorporating the optimized NM‐BHJ retain 88.2% of their initial efficiency after 720 h at 65 °C and over 99% after 2400 h at room‐temperature. The obtained T 90 of 644 h is, to the best of our knowledge, one of the longest experimentally verified lifetimes reported among Y‐series‐based BHJ systems in the literature, including those employing chemically modified oligomeric derivatives. Combined with its generality across different NFA systems, this study suggests that the NM‐BHJ strategy opens a new era for highly efficient and thermally robust OPV devices by morphologically overcoming the inherent thermal limitations of high‐ NFAs.

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