Optimizing Double‐Fibril Network Morphology via Solid Additive Strategy Enables Binary All‐Polymer Solar Cells with 19.50% Efficiency

Abstract Double‐fibril network morphology (DFNM), in which the donor and the acceptor can self‐assemble into a double‐fibril structure, is beneficial for exciton dissociation and charge transport in organic solar cells. Herein, it is demonstrated that such DFNM can be constructed and optimized in all‐polymer solar cells (all‐PSCs) with the assistance of 2‐alkoxynaphthalene volatile solid additives. It is revealed that the incorporation of 2‐alkoxynaphthalene can induce a stepwise regulation in the aggregation of donor and acceptor molecules during film casting and thermal annealing processes. Through altering the alkoxy of 2‐alkoxynaphthalene solid additives, both the intermolecular interactions and molecular miscibility with the host materials can be precisely tuned, which allows for the optimization of the molecular aggregation process and facilitation of molecular self‐assembly, and thus leading to reinforced molecular packing and optimized DFNM. As a result, an unprecedented efficiency of 19.50% (certified as 19.1%) is obtained for 2‐ethoxynaphthalene‐processed PM6:PY‐DT‐X all‐PSCs with excellent photostability (T₈₀ = 1750 h). This work reveals that the optimization of DFNM via solid additive strategy is a promising avenue to boosting the performance of all‐PSCs.