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University of Kansas researchers have made a breakthrough in understanding organic semiconductors, which could potentially lead to more efficient and versatile solar cells.

Solar energy is essential for a sustainable energy future. For years, silicon—a semiconductor material that’s found in most electrical devices—has been the gold-standard material in the solar energy industry. Its high efficiency and durability make it ideal for photovoltaic panels. However, the rigidity of silicon panels restricts their application on curved surfaces, and their high production cost is another significant drawback.

Organic semiconductors

To overcome these challenges, researchers have created alternative materials for solar energy. The most promising of these materials include organic carbon-based semiconductors. They’re cheaper, more readily available, and less harmful to the environment.

“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.

Despite these advantages, organic solar cells have historically struggled with efficiency. While silicon panels can convert up to 25 % of sunlight into electricity, organic cells have typically only achieved around 12 % efficiency, hindering their widespread adoption.

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A leap in efficiency

Recent advancements have reignited interest in organic semiconductors. A new class of materials known as non-fullerene acceptors (NFAs) has pushed the efficiency of organic solar cells closer to 20 %, significantly narrowing the gap with silicon. The research team at the University of Kansas aimed to understand the reasons behind the superior performance of NFAs compared to other organic semiconductors.

Their investigation revealed a surprising discovery: in certain conditions, excited electrons in NFAs can gain energy from their surroundings instead of losing it. This counterintuitive finding contradicts conventional wisdom, where excited electrons typically lose their energy to the environment, similar to how a hot cup of coffee loses heat.

The researchers believe this unusual energy gain is due to a combination of quantum mechanics and thermodynamics. At the quantum level, excited electrons can 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," explained graduate student Kushal Rijal. This reversed heat flow allows neutral excitons to gain heat from the environment and dissociate into pairs of positive and negative charges, which can then produce electrical current.

Beyond solar cells

The implications of the Kansas team’s research extend beyond improving solar cells. They believe that their findings could enhance other areas of renewable energy research, such as developing 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 research by the University of Kansas team, funded by the Office of Basic Energy Sciences of the US Department of Energy, has been published in the journal Advanced Materials. Their work underscores the potential of organic semiconductors to revolutionize the solar energy industry and contribute to the development of more sustainable and efficient renewable energy technologies.

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