Promising solar energy material gets curious boost from entropy, researchers show

Solar energy is crucial to a clean energy future. Traditionally, solar energy has been harvested using silicon, the same semiconductor material used in everyday electronic devices. But silicon solar panels have drawbacks, including being expensive and difficult to mount on curved surfaces.

Researchers have developed alternative materials for harvesting solar energy to overcome such shortcomings. Among the most promising of these are so-called “organic” semiconductors, carbon-based semiconductors that are found everywhere on Earth, are cheaper, and more environmentally friendly.

“They have the potential to reduce the cost of manufacturing solar panels because these materials can be applied to arbitrary surfaces using solution-based methods, much like we paint a wall,” said Wai-Lun Chan, an associate professor of physics and astronomy at the University of Kansas.

“These organic materials can be tuned to absorb light at selected wavelengths, which can be used to create transparent solar panels or panels with different colors. These properties make organic solar panels particularly suitable for use in next-generation green and sustainable buildings.”

Although organic semiconductors are already used in the display panels of consumer electronics such as mobile phones, TVs and virtual reality headsets, they have not yet been widely used in commercial solar panels. One shortcoming of organic solar cells is their low light-to-electric conversion efficiency, about 12% compared to monocrystalline silicon solar cells which have an efficiency of 25%.

According to Chan, electrons in organic semiconductors typically bind to their positive counterparts, known as “holes.” In this way, light absorbed by organic semiconductors often produces electrically neutral quasi-particles known as “excitons.”

But the recent development of a new class of organic semiconductors, known as non-fullerene acceptors (NFAs), changed this paradigm. Organic solar cells made with NFAs can achieve efficiencies closer to 20%.

Despite their excellent performance, it is still unclear to the scientific community why this new class of NFAs significantly outperforms other organic semiconductors.

In a groundbreaking study conducted in Advanced materialsChan and his team, including doctoral students Kushal Rijal (lead author), Neno Fuller and Fatimah Rudayni of the Department of Physics and Astronomy, and in collaboration with Cindy Berrie, professor of chemistry at KU, have discovered a microscopic mechanism that partly explains the excellent performance of an NFA.

The key to this discovery were measurements performed by lead author Rijal using an experimental technique called time-resolved two photon photoemission spectroscopy, or TR-TPPE. This method allowed the team to track the energy of excited electrons with sub-picosecond time resolution (less than a trillionth of a second).

“In these measurements, Kushal [Rijal] noted that some of the optically excited electrons in the NFA can gain energy from the environment instead of losing energy to the environment,” Chan said. “This observation is counterintuitive because excited electrons typically lose their energy to the environment, just as a cup of hot coffee loses its heat to the environment.”

The team believes that this unusual process occurs on a microscopic scale due to the quantum behavior of electrons, which allows an excited electron to appear on multiple molecules at once. This quantum strangeness goes hand in hand with the second law of thermodynamics, which states that any physical process will lead to an increase in total entropy (often known as “disorder”) to produce the unusual energy-gaining process.

“In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in total entropy,” Rijal said. “But we found that for organic molecules arranged in a specific nanoscale structure, the typical direction of heat flow is reversed to increase total entropy. This reversed heat flow allows neutral excitons to pull heat from the surroundings and dissociate into a pair of positive and negative charges. These free charges can in turn produce electric current.”

Based on their experimental findings, the team proposes that this entropy-driven charge separation mechanism allows organic solar cells made with NFAs to achieve much higher efficiencies.

“Understanding the underlying charge separation mechanism allows researchers to design new nanostructures that use entropy to direct heat, or energy, at the nanoscale,” Rijal said. “Despite entropy being a well-known concept in physics and chemistry, it has rarely been actively exploited to improve the performance of energy conversion devices.”

And that’s not all: The KU team believes the mechanism discovered in this study could be used to produce more efficient solar cells. They also think it could help researchers design more efficient photocatalysts for solar fuel production, a photochemical process that uses sunlight to convert carbon dioxide into organic fuels.