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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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The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells
Abstract The relation of phase morphology and solid‐state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase‐pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device‐relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk‐heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology–function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high‐performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.
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- Award ID(s):
- 1729737
- PAR ID:
- 10455723
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials
- Volume:
- 32
- Issue:
- 47
- ISSN:
- 0935-9648
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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