Re-entrant Phase Behavior of Organic Semiconductors: A Thermodynamic Framework for Designing Stable Non-Fullerene Organic Solar Cells
The stability of organic solar cells (OSCs) is a critical bottleneck for their commercialization. This study introduces a low-free-energy, two-state (LF-TSB) model to describe the re-entrant phase behavior observed in blends of small molecule acceptors (SMAs) and semiconducting polymers. The model integrates molecular rigidity (glass transition temperature, T_g), side-chain architecture (effective monomeric volume), and molecular symmetry (configurational entropy) to predict phase diagrams. Experimental validation using SMA:polymer blends demonstrates that suppressed driving force for phase separation, achieved by modulating these molecular parameters, leads to superior thermal stability. For instance, blends with reduced molecular rigidity and optimized side-chain volume exhibit stable morphology over extended thermal stress. The model also explains the re-entrant phase transition, where a homogeneous blend becomes unstable at intermediate temperatures but re-stabilizes at higher temperatures, enabling multi-step annealing strategies to trap beneficial morphological states. While the LF-TSB model shows predictive potential, challenges remain in determining new parameters (e.g., effective monomeric volume, flexing energy) a priori for novel systems, and its applicability to other blend types (polymer-polymer, small molecule-small molecule, hydrogels) requires further exploration. This work provides new guidelines for designing stable OSCs by rationally tailoring molecular parameters to achieve desired phase morphology.