Molecular Architecture of Non‑Fullerene Acceptors Balances Homo‐ and Hetero‐Intermolecular Interactions to Enhance Thermal Stability of Organic Photovoltaics

High‐efficiency organic solar cells require optimal donor‐acceptor heterojunctions to facilitate efficient charge generation, separation, and collection. These bulk‐heterojunction (BHJ) blends are typically kinetically quenched in nonequilibrium morphologies, contributing to device degradation over time. Yet how acceptor molecules govern homo‐ and hetero‐intermolecular interactions to confer thermal stability to BHJ blends remains elusive. Herein, we present experimental and theoretical studies of 28 distinct non‐fullerene acceptors (NFAs) to elucidate the relationships between molecular structure, intermolecular interaction, and morphological stability of BHJ blends, covering the well‐known indacenodithieno[3,2‐ b ]thiophene‐based (ITIC‐series) acceptors, indacenodithiophene‐based (IEICO‐series) acceptors, and state‐of‐the‐art Y‐series and dimer/trimer acceptors. We demonstrate that structural modifications of NFAs not only modulate the homo‐interaction energies between acceptor molecules but also fine‐tune their hetero‐interaction energies with donors, thereby governing the stability of BHJ nanostructures. Thermally stable BHJ blends necessitate strong homo‐interactions while minimizing hetero‐interactions to suppress NFA diffusion within the blends. The proposed intermolecular interaction effect on thermal stability is further validated by 13 literature‐reported NFA blends, demonstrating the broad applicability. These findings underscore the pivotal role of NFA structural engineering in regulating intermolecular interactions and offer valuable insights into balancing homo‐ and hetero‐interactions for the rational molecular design of thermally stable organic semiconductor blends.