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 go-to material for photovoltaic panels. However, silicon-based solar cells are rigid and expensive to produce, limiting their potential for curved surfaces.
Organic semiconductors, these carbon-based materials offer a viable alternative at a lower cost and with greater flexibility. “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 & 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 plethora 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 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 proved to be a significant obstacle to widespread adoption.
Unlocking efficiency
Recent developments have rejuvenated the excitement around organic semiconductors. A new class of materials called non-fullerene acceptors (NFAs) pushed organic solar cell efficiency 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: in certain circumstances, excited electrons in NFAs can gain energy from their surroundings instead of losing it.
This finding flies in the face of conventional wisdom. “This observation is counterintuitive because excited electrons typically lose their energy to the environment like a cup of hot coffee losing its heat to the surrounding,” 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 to less than a trillionth of a second.
An unlikely ally
The researchers believe this unusual energy gain stems from a combination of quantum mechanics and thermodynamics. At the quantum level, excited electrons can appear to exist on multiple molecules simultaneously.
Coupled 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 the heat flow is reversed for the total entropy to increase,” Rijal explained in a press release. “This reversed heat flow allows neutral excitons to gain heat from the environment and dissociates into a pair of positive and negative charges. These free charges can in turn produce electrical current.”
Beyond solar cells
Beyond improving solar cells, the team believes that their findings are applicable in other areas of renewable energy research. They think the discovered mechanism will lead to more efficient photocatalysts to convert carbon dioxide into organic fuels.
“Despite entropy being a well-known concept in physics and chemistry, it’s rarely been actively utilized to improve the performance of energy conversion devices,” emphasized Rijal.
The team’s findings were published in the journal Advanced Materials.
ABOUT THE EDITOR
Amal Jos Chacko Amal writes code on a typical business day and dreams of clicking pictures of cool buildings and reading a book curled by the fire. He loves anything tech, consumer electronics, photography, cars, chess, football, and F1.
Dr. Sarah Adams is a scientist and science communicator who makes complex topics accessible to all. Her articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.
