Researchers at the University of Kansas have uncovered an extraordinary new phenomenon that may help to usher in a revolution in solar energy technology. In a groundbreaking paper featured in Advanced Materials, the group describes how an interplay of entropy increases the efficiency of organic cells to the point that they might someday soon rival the efficiency of their silicon counterparts.
Silicon, the staple material for solar panels, has a reputation for efficiency and rigidity. However, this comes at a cost, literally. There are huge drawbacks, including the cost of this material and its inflexibility for use on curved or otherwise non-rigid surfaces. Because of these shortcomings, scientists are pursuing alternative materials, with organic semiconductors having shown considerable promise. These easily accessible and cheap carbon-based materials are also environmentally friendly.
“They can potentially lower the production cost for solar panels because these materials can be coated on arbitrary surfaces using solution-based methods—just like how we paint a wall,” explained Wai-Lun Chan, associate professor of physics and astronomy at the University of Kansas. He added that these organic materials can be customized to absorb light at specific wavelengths, paving the way for transparent or colored solar panels, ideal for sustainable building integration.
Despite that promise, organic semiconductors have so far been much less efficient than silicon, with a 12% conversion of light into electrical energy versus the 25% for silicon. However, the discovery of a new class of materials called non-fullerene acceptors, or NFAs, has suddenly closed that gap. Now, organic solar cells using NFAs are near efficiencies of 20%.
The task was facilitated by a surprising enabler, entropy. Usually, excited electrons lose their energy to the medium around them, much as a hot coffee cools down. Yet Chan’s team has shown that, under certain conditions, the excited electrons gained energy from their environment. This seemingly paradoxical effect is made possible by an electron-bouncing act, in which the electron surfs on more than one molecule at once, a quantum effect. This combines with the Second Law of Thermodynamics, which says that every physical process increases total entropy.
“In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in the total entropy,” said Kushal Rijal, the study’s lead author. “But we found for organic molecules arranged in a specific nanoscale structure, the typical direction of the heat flow is reversed for the total entropy to increase. This reversed heat flow allows neutral excitons to gain heat from the environment and dissociate into a pair of positive and negative charges. These free charges can in turn produce electrical current.”
This entropy-driven mechanism enhances efficiency not only for organic solar cells but also for other applications, e.g., a photocatalyst toward solar-fuel production. This sunlight-driven, carbon dioxide-to-organic-fuel technology is poised to play a pivotal role in the transition from energy sources to renewables.
Results from this University of Kansas team, supported by the Department of Energy’s Office of Basic Energy Sciences, open up new avenues for designing nanostructures that leverage entropy to direct energy flow on the nanoscale. “Understanding the underlying charge separation mechanism will allow researchers to design new nanostructures to take advantage of entropy to direct heat, or energy, flow on the nanoscale,” Rijal said.
This step has huge implications as the world strives for sustainable energy solutions, changing the landscape for solar energy technology.