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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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The hole in the bucky: structure-property mapping of closed- vs. open-cage fullerene solar-cell blends via temperature/composition phase diagrams
The morphology development of polymer-based blends, such as those used in organic photovoltaic (OPV) systems, typically arrests in a state away from equilibrium – how far from equilibrium this is will depend on the materials chemistry and the selected assembly parameters/environment. As a consequence, small changes during the blend assembly alters the solid-structure development from solution and, in turn, the final device performance. Comparing an open-cage ketolactam fullerene with the prototypical [6,6]-phenyl-C₆₁-butyric acid methyl ester in blends with poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT), we demonstrate that experimentally established, non-equilibrium temperature/composition phase diagrams can be useful beyond rationalization of optimum blend composition for OPV device performance. Indeed, they can be exploited as tools for rapid, qualitative structure-property mapping, providing insights into why apparent similar donor:acceptor blends display different optoelectronic processes resulting from changes in the phase-morphology formation induced by the different chemistries of the fullerenes.
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- PAR ID:
- 10296337
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- ISSN:
- 2050-7526
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1TC03082E
