Materials Science + Policy = Sustainability?
Symposium EQ03 Plasmonics, Nanophotonics and Metamaterials—From Design to Applications

Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

Plenary 2_270x180Jenny Nelson, Imperial College London

Optimizing Solar Energy Conversion in Molecular Electronic Materials

Written by Rahul Rao

Today, the solar energy world is ruled by crystalline silicon. But recent advances in materials have spawned a host of alternative materials seeking to challenge silicon’s dominance. One class of challengers are molecular electronic materials: carbon-based organic semiconductors that can be excited by visible light.

At Monday’s Plenary Session, Jenny Nelson spoke about the titular materials, how they’ve rapidly developed, and how her group’s work has contributed to bring these organic materials closer to viable solar cells.

Just several decades ago, these materials barely registered on solar researchers’ radars. Most of them had been fashioned from fullerenes, limiting their variety. Additionally, such materials tend to be disordered and suffer from a poor dielectric constant, hurting their prowess as semiconductors.

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But by the 1990s, researchers had learned how to circumvent those restrictions by interfacing two different disordered materials—one to donate electrons, another to accept them. Since then, the materials themselves have diversified from fullerenes into a wide range of other materials that researchers could easily process and readily customize. As a bonus, researchers have found that their coupled molecular electronic materials’ properties are easy to computationally predict.

The result: solar cells made from molecular electronic materials have blossomed in efficiency, from less than 2.5% in 2002 to 11% in 2016 to over 19% in 2022.

Still, any organic solar cells have a long way to go before they can match their crystalline silicon counterparts, many of which now clear the 30% mark. Nelson said that researchers need to quantify and understand the loss in materials.

One way of doing that is to take a solar cell and run it in reverse, like an LED—pushing electrons in and watching the light that comes out. Because a physical relationship exists between that emitted light and the light absorbed when the device is operating as designed, researchers can study how close their device’s efficiency is to its ideal limit.

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Through this method, by 2015, Nelson’s group had helped demonstrate that organic solar cells shed much of their energy through non-radiative loss—something that researchers could address by improving their materials. Recent efforts to do just that have helped drive much of the rapid efficiency jumps these solar cells experienced from the late 2010s.

Their efficiency could rise even farther if researchers can understand which molecular properties lead to which changes in a solar cell’s performance. Nelson’s group probed this computationally, outfitting a model of a solar cell device with a molecular model. They found that varying just four molecular parameters controlled a device’s voltage, current density, and fill factor.

And molecular electronic materials may not need separate donor and acceptor components forever. A smattering of recent research has suggested that single-molecule materials—for instance, polymers built with donor and acceptor subunits—have already reached at least 11% efficiency. Rearranging and reshaping polymers might boost that even further.

Finally, Nelson mentioned that optimal solar cell materials already exist in nature: in the photosystems of photosynthesizing plants. If materials scientists studied them, she said, their findings could light possible paths into a bright future of bountiful organic solar cells.

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

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