Researchers from The University of Osaka create mirror-image semiconducting polymers that improve solar cell performance

Osaka, Japan – Organic solar cells are made from conductive polymers, which makes them cheap, light, and flexible. However, one drawback is that their efficiency lags behind the best silicon devices – but this may soon change – as researchers from Japan have developed a new method to improve organic solar cell performance.

In an article set to be published in Angewandte Chemie International Edition, researchers from The University of Osaka describe new semiconducting polymers for organic solar cells to improve electron flow and efficiency. This was achieved by changing the type of symmetry of the acceptor molecules, which can improve the efficiency of organic solar cells.

In organic solar cells, electron donor molecules absorb light, and give electrons to electron acceptor molecules, leaving behind electron “holes”. To be able to generate electricity, electrons and holes must remain separated and be moved to an external circuit to prevent them from recombining. Organic semiconducting polymers have a much stronger attraction between electrons and holes than silicon semiconductors, meaning recombination is a major hurdle to achieving high efficiency in organic solar cells.

Preventing recombination can be influenced by symmetry. Molecules can have two symmetrical sides that are mirror images of each other, but asymmetric left to right molecules have left and right sides that are not mirror images.

Similarly, vertical asymmetry means that the top and bottom of the molecule are different, so there are two mirror-image versions of the whole molecule that cannot be superimposed, similar to hands. This is property is called chirality.

“We knew that using electron acceptor molecules that are asymmetric left to right could boost organic solar cell performance by preventing recombination,” says lead author, Shuang Li. “But chiral acceptors that have vertical asymmetry hadn’t been explored.”

Vertical asymmetry has several important benefits. The acceptor molecule has a greater difference in charge between the top and bottom. Meaning the molecules pack together and mix better with donor molecules, improving the flow of electrons. says corresponding author, Fumitaka Ishiwari.

“Most importantly, we thought that these acceptor molecules could show chirality-induced spin selectivity,” explains senior author, Akinori Saeki.
Chirality-induced spin selectivity arises from electrons having an up or down “spin” on its axis. Each mirror-image acceptor molecule preferentially conducts up- or down-spin electrons, giving a spin-polarized current in which one spin dominates.

“This effect is really interesting because spin-polarization separates electrons and holes better, so it decreases charge recombination, which is a major problem in organic solar cells.” Saeki adds.

The creation of a spin-polarized current from the mirror image molecules led the organic solar cells to show power conversion efficiencies of about 8%, which is three times higher than the non-mirror-image version.

Using mirror-image molecules to prevent recombination with spin-polarized currents is a new design strategy that could increase the efficiency of organic solar cells and potentially provide cheaper, cleaner, and greener electricity generation technology.
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The article, “Chiral Bifacial Non-Fullerene Acceptors with Chirality-Induced Spin Selectivity: A Homochiral Strategy to Improve Organic Solar Cell Performance,” appears in Angewandte Chemie International Edition at DOI: https://doi.org/10.1002/anie.202518505