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Abstract Organic solar cells incorporating non‐fullerene acceptors (NFAs) have reached remarkable power conversion efficiencies of over 18%. Unlike fullerene derivatives, NFAs tend to crystallize from solutions, resulting in bulk heterojunctions that include a crystalline acceptor phase. This must be considered in any morphology‐function models. Here, it is confirmed that high‐performing solution‐processed indacenodithienothiophene‐based NFAs, i.e., ITIC and its derivatives ITIC‐M, ITIC‐2F, and ITIC‐Th, exhibit at least two crystalline forms. In addition to highly ordered polymorphs that form at high temperatures, NFAs arrange into a low‐temperature metastable phase that is readily promoted via solution processing and leads to the highest device efficiencies. Intriguingly, the low‐temperature forms seem to feature a continuous network that favors charge transport despite of a poorly order along the π–π stacking direction. As the optical absorption of the structurally more disordered low‐temperature phase can surpass that of the more ordered polymorphs while displaying comparable—or even higher—charge transport properties, it is argued that such a packing structure is an important feature for reaching highest device efficiencies, thus, providing guidelines for future materials design and crystal engineering activities.
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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.
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- Award ID(s):
- 1905790
- PAR ID:
- 10385021
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Solar RRL
- Volume:
- 6
- Issue:
- 12
- ISSN:
- 2367-198X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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