Thermodynamic Phase Transition of Three‐Dimensional Solid Additives Guiding Molecular Assembly for Efficient Organic Solar Cells

Abstract Fine‐tuning the thermodynamic self‐assembly of molecules via volatile solid additives has emerged to be an effective way to construct high‐performance organic solar cells. Here, three‐dimensional structured solid molecules have been designed and applied to facilitate the formation of organized molecular assembly in the active layer. By means of systematic theory analyses and film‐morphology characterizations based on four solid candidates, we preselected the optimal one, 4‐fluoro‐ N,N ‐diphenylaniline (FPA), which possesses good volatility and strong charge polarization. The three‐dimensional solids can induce molecular packing in active layers via strong intermolecular interactions and subsequently provide sufficient space for the self‐reassembly of active layers during the thermodynamic transition process. Benefitting from the optimized morphology with improved charge transport and reduced energy disorder in the FPA‐processed devices, high efficiencies of over 19 % were achieved. The strategy of three‐dimensional additives inducing ordered self‐assembly structure represents a practical approach for rational morphology control in highly efficient devices, contributing to deeper insights into the structural design of efficient volatile solid additives.