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.