A University of Kansas research team has discovered a counterintuitive effect in organic semiconductors that could transform the future of solar energy. According to a statement, researchers describe the progress realized towards that goal, published in Advanced Materials, towards enabling organic solar cells to achieve efficiencies similar to the traditional silicon-based ones.
Although silicon has been the established material for so many years in making solar panels, the greatest advantage it seems to offer is that of efficiency and durability. Some serious drawbacks would be the price and, most importantly, rigidity. Being rigid makes installations on curved or uneven surfaces just a little more expensive. Within months, research teams will start actively working on replacement materials. Of those, organic semiconductors have lately been considered the most promising candidate material. These all-carbon materials are abundant, and inexpensive.
“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,” said Wai-Lun Chan, associate professor of physics and astronomy at the University of Kansas. They have the attribute of being tunable for light absorption at particular wavelengths that may enable completely transparent or any-colored performing solar panels to integrate well into facade-building solutions.
Probably even more promising than silicon, though, was the discovery of Organic Solar Cells. They are only about 12% of converting light into electricity. At best, this corresponds to approximately one-half of silicon cells for converting light into electricity. The game, though, may change with a new class of material known as non-fullerene acceptors, NFAs. It’s tackling up to 20 percent efficiencies in Organic Solar Cells, made out of NFAs.
An important bump in performance toward driving the NFAs came from a kind of “what just happened” phenomenon. Scientists had found out that some excited electrons inside the material were losing their energy to space, something that was not dissipated as the scientific community had assumed. “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 surroundings,” Chan said.
The quantum effect that is argued to make up for this energy gain is that an electron is allowed to exist on several molecules at one time. It is such a quantum feature that is at play with the Second Law of Thermodynamics that allows for unexpected gain. “In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in 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 dissociates into a pair of positive and negative charges. These free charges can in turn produce electrical current.”
Such a discovery promises improvements not only in solar cells but also in photocatalysts in the production of solar fuels, using light to turn carbon dioxide into organic fuels.
If the new entropy-driven mechanism behind charge separation is understood, it could mean the possibility of designing a completely new class of nanostructures where entropy would guide energy flows on the nanoscale. “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,” said Rijal.
The impact of the research is broad and paves the way toward a much more effective and much cheaper solar energy solution that may change the face of renewable energy.