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The rapid increase in the power conversion efficiency of organic solar cells (OSCs) during the last few years has been achieved through the development of non-fullerene small-molecule acceptors (NF-SMAs). Stability is now becoming a pressing concern. The presentation will discuss how intermolecular interactions govern morphological stability, specifically, how the diffusion of an NF-SMA exhibits Arrhenius behavior with an activation energy that scales linearly with the enthalpic Flory-Huggins interaction parameter. Consequently, the thermodynamically most unstable systems (high interaction parameter) are the most kinetically stabilized. In short, unfavorable interactions can enable stability. The activation energy is shown to scale with the glass transition temperature (Tg) of the NF-SMA and mechanical characteristics of the polymer (elastic modulus) of the polymers [1]. This allows predicting relative diffusion properties and thus morphological stability from simple analytical measurements or molecular dynamic simulations. Unfortunately, the star acceptor Y6 and its analogs have low glass transition temperatures, are highly diffusive and thus yield unstable morphologies. In contrast, IEICO-4F is a stable, high Tg material. The relationship of the Tg to the chemical structure is not yet understood except in the most general terms within a homologous series. The side-chains needed to provide solubility are the enemy of stability. Shorter side-chains are better for stability but often results in difficulties in processing. The impact of the molecular core on the Tg is not yet understood. A new approach to molecular design or novel stabilization strategies are needed if devices are to have high performance and high stability at the same time.
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