In a new study, an international team of researchers created phase diagrams for organic solar cells based on a composite of a polymeric semiconductor and a “small molecule acceptor” (SMA). The phase diagrams show that the mixing behavior of these composites can have an unexpected dependence on the temperature, indicating that researchers should consider additional parameters when trying to predict material performance. The work could accelerate the development of improved materials for use in high-efficiency solar cells.

“Polymer:SMA blends offer high solar cell efficiencies and stability, but only if their mixing behavior is precisely tuned,” says Harald Ade, Goodnight Innovation Distinguished Professor of Physics at North Carolina State University and co-corresponding author of the study. “We show that their mixing behavior is far more complex than observed for traditional commodity polymers. However, prior to this study there hasn’t been much work done in understanding the phase behavior of these ‘solar composites.’”

The research team determined the binary phase diagrams of over 50 polymer:SMA composites. A binary phase diagram shows how temperature determines whether two materials want to mix or separate. Since the operation of the solar cell critically depends on the mixing behavior, such diagrams are pivotal in predicting device stability and performance.

Polymer:SMA composites were thought to mix better by increasing the temperature, like most materials do. However, for 50% of the blends the team investigated, the components separated when temperature increased and mixed when temperature decreased, giving “re-entrant” phase diagrams.

Re-entrance is a phenomenon where, as the temperature changes, a material goes through two or more phase transitions before returning to its initial state.

“The fact that organic semiconductors have a much richer phase behavior than traditional materials relates to their molecular complexity,” says Jasper Michels, staff scientist at the Max Planck Institute for Polymer Research and co-corresponding author of the work. “Classical models for polymer blends need to be extended with additional parameters to capture the implications of that complexity for their behavior in a composite.”

Specifically, the researchers focused on the free volume within the composites – which makes a material expand or shrink when heated or cooled, as well as the glass transition – the temperature at which a material “freezes” into a non-crystalline solid.

“There is a rough relationship between the glass transition temperature and the shape of the phase diagram,” Michels says. “The approach to the glassy state has so far not been considered when explaining the mixing behavior of organic semiconductors but including it in the model seems to give a more complete picture as it qualitatively reproduces the experimental observations.”

The team hopes their findings inform the future design and development of new materials for solar cells.

“Our traditional understanding of mixing is that it is dominated by two contributions: disorder and interaction,” Ade says. “But organic semiconductors have additional properties, leading to a complex phase behavior. My hope is that this work will aid our understanding of how efficiency and stability depend on molecular interaction at smaller length scales.”

The work appears in Nature Materials and is supported in part by the Office of Naval Research (grant N000142012155) and by the Max Planck Society. Former NC State Ph.D. student Zhengxing Peng is first author. Former NC State postdoctoral researcher Masoud Ghasemi also contributed to the work.

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