Researchers at the University of Kansas have made a breakthrough in understanding organic semiconductors, hinting at more efficient and versatile solar cells.
For years, silicon has dominated the solar energy landscape. Its efficiency and durability have made it the most suitable material for photovoltaic panels. However, silicon-based solar cells are rigid and expensive to manufacture, limiting their potential for curved surfaces.
Organic semiconductors, these carbon-based materials offer a sustainable alternative at a lower cost and with greater flexibility. “They can potentially lower the manufacturing cost for solar panels because these materials can be coated onto 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 an ability to be tuned to absorb specific wavelengths of light, opening up a wealth of new possibilities. “These characteristics make organic solar panels particularly suitable for use in next-generation green and sustainable buildings,” Chan noted. Imagine transparent and colored solar panels seamlessly integrated into architectural designs.
For all these advantages, organic solar cells have struggled to match the efficiency of their silicon counterparts. While silicon panels can convert up to 25% of sunlight into electricity, organic cells have typically hovered around 12% efficiency. This gap has proven to be a significant barrier to widespread adoption.
Unlocking efficiency
Recent developments have renewed excitement about organic semiconductors. A new class of materials called non-fullerene acceptors (NFAs) pushed the efficiency of organic solar cells closer to 20%, narrowing the gap with silicon.
The Kansas research team sought 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 the environment like a hot cup of coffee loses its heat to the environment,” 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 track the energy of excited electrons in less than a trillionth of a second.
An unlikely ally
The researchers believe that this unusual energy gain stems from a combination of quantum mechanics and thermodynamics. At the quantum level, excited electrons can appear to exist in multiple molecules simultaneously.
Coupled with the second law of thermodynamics, this quantum behavior changes the direction of heat flow.
“For organic molecules arranged in a specific nanoscale structure, the typical direction of heat flow is reversed so that the total entropy increases,” Rijal explained in a press release. “This reverse heat flow allows neutral excitons to gain heat from the environment and dissipate into a pair of positive and negative charges. These free charges can generate electricity.”
Beyond solar cells
Beyond improving solar cells, the team believes their findings are applicable to other areas of renewable energy research. They think 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 has rarely been actively used to improve the performance of energy conversion devices,” Rijal pointed out.
The team’s findings were published in the journal Advanced Materials.
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Amal Jos Chacko Amal writes code on a typical workday and dreams of clicking pictures of cool buildings and reading a book by the fire. He likes all things technology, consumer electronics, photography, cars, chess, football and F1.