Solvent‐Induced Polymorphism in Non‐Fullerene‐Based Organic Solar Cells

Organic photovoltaics have achieved breakthroughs in power conversion efficiency due to the superior aggregation and packing nature of non‐fullerene acceptors (NFAs). Solution processing and various treatments would tend to form distinct packing motifs for state‐of‐the‐art NFAs. Herein, the solvent‐induced polymorphism for 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophne) (ITIC) prepared by chloroform (CF) and chlorobenzene (CB) is revealed. The packing motif of ITIC exhibits dense π–π stacking from CF induction, which presents red‐shifted absorption and reversible high‐temperature crystallization and melting. Meanwhile, strong lamellar stacking and π–π stacking can be formed in the CB solution with unstable low‐temperature crystallization and melting. Combining in situ absorption spectra and interaction calculation, the stronger preaggregation of ITIC in the CF solution was found to be the main reason for forming a different packing motif from in the CB solution. The packing and thermodynamic features are retained in the PBDB‐T:ITIC blends, though good miscibility weakens characteristic features. Benefiting from the polymorph structure, CB‐processed devices denote more favorable performance but less thermal stability. This research indicates the significant effect of solvent induction for manipulating and optimizing the morphology of organic solar cell devices.