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University of Kansas researchers have made a breakthrough in understanding organic semiconductors, hinting at more efficient and flexible solar cells.

For years, silicon has dominated the solar energy landscape. Its efficiency and durability have made it a preferred material for photovoltaic panels. However, silicon-based solar cells are stiff and expensive to manufacture, which limits their potential for curved surfaces.

Organic semiconductors, these carbon-based materials offer a viable alternative at lower cost and with greater flexibility. “They can potentially reduce the manufacturing cost of solar panels because these materials can be coated on arbitrary surfaces using solution-based methods – just like we paint a wall,” explained Wai-Lun Chan, associate professor of physics and astronomy at the University of Kansas.

But these organic semiconductors aren’t just about cost savings. They boast the ability to be tuned to absorb specific wavelengths of light, opening up a host of new possibilities. “These characteristics make organic solar panels particularly suitable for use in next-generation green and sustainable buildings,” noted Chan. Imagine transparent and colorful solar panels seamlessly integrated into an architectural design.

Because of all these advantages, organic solar cells struggle to reach the efficiency of their silicon counterparts. While silicon panels can convert up to 25% of sunlight into electricity, organic cells typically hover around 12% efficiency. This gap has proven to be a significant barrier to widespread adoption.

Unlocking efficiency

Recent developments have rejuvenated the excitement surrounding organic semiconductors. A new class of materials called non-fullerene acceptors (NFAs) have brought the efficiency of organic solar cells closer to 20%, narrowing the gap with silicon.

The Kansas research team set out to understand why NFAs perform so much better than other organic semiconductors. Their investigation led to a surprising discovery: under certain circumstances, excited electrons in NFAs can gain energy from their surroundings instead of losing it.

This finding goes against conventional wisdom. “This observation is counterintuitive because excited electrons typically lose their energy to their surroundings, just as a hot cup of coffee loses its heat to its surroundings,” Chan explained.

Led by graduate student Kushal Rijal, the team experimented with a sophisticated technique called time-resolved two-photon photoemission spectroscopy. This method allowed them to trace the energy of the excited electrons to less than one trillionth of a second.

An unlikely ally

The researchers believe that this unusual energy gain arises from a combination of quantum mechanics and thermodynamics. At the quantum level, excited electrons can appear to exist on multiple molecules simultaneously.

Combined with the second law of thermodynamics, this quantum behavior reverses the direction of heat flow.

“For organic molecules arranged in a specific nanoscale structure, the typical direction of heat flow is reversed to increase the total entropy,” Rijal explained in a press release. “This reversed heat flow allows the 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 an electric current.

Beyond solar cells

Beyond improving solar cells, the team believes their findings are applicable to other areas of renewable energy research. They believe the discovered mechanism will lead to more efficient photocatalysts for converting carbon dioxide into organic fuels.

“Although entropy is a well-known concept in physics and chemistry, it is rarely actively used to improve the performance of energy conversion devices,” emphasized Rijal.

The team’s findings were published in the journal Advanced materials.