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

Solar power is critical to a clean energy future. Traditionally, solar energy is harvested using silicon – the same semiconductor material used in everyday electronic devices. But silicon solar panels have drawbacks: for example, they are expensive and difficult to mount on curved surfaces.

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

“They could 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,” said Wai-Lun Chan, 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 characteristics make organic solar panels particularly suitable for use in in next-generation green and sustainable buildings.”

While organic semiconductors are already used in the display panels of consumer electronics such as mobile phones, televisions and virtual reality headsets, they have yet to be widely used in commercial solar panels. A shortcoming of organic solar cells has been their low light-to-electricity conversion efficiency of about 12% versus monocrystalline silicon solar cells that perform at an efficiency of 25%.

According to Chan, electrons in organic semiconductors typically bond with their positive counterparts known as “holes.” In this way, light absorbed by organic semiconductors often produces neutral electric quasiparticles 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 NFA can achieve efficiencies closer to the 20% mark.

Despite their remarkable performance, it has remained unclear to the scientific community why this new class of NFAs significantly outperforms other organic semiconductors.

In a revealing study appearing in Advanced MaterialsChan and his team, including graduate students Kushal Rijal (lead author), Neno Fuller and Fatimah Rudayni from the Department of Physics and Astronomy, and in collaboration with Cindy Berrie, professor of chemistry at KU, have discovered a microscopic mechanism that solves in part of the outstanding performance achieved by an AKU.

Key to this discovery were measurements taken 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 the excited electrons with a subpicosecond time resolution (less than a trillionth of a second).

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

The team believes that this unusual process occurs on a microscopic scale thanks to the quantum behavior of electrons, which allow an excited electron to appear simultaneously in several molecules. This quantum weirdness is coupled 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 process of energy gain.

“In most cases, a hot object transfers heat to its cold surroundings because heat transfer leads to an increase in total entropy,” Rijal said. “But we have found for organic molecules arranged in a specific nano-scale structure, the typical direction of heat flow is reversed so that the total entropy increases. This reversed heat flow allows neutral excitons to gain heat from the environment and distributed to a positive pair.and negative charges these free charges can 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 better efficiency.

“Understanding the mechanism of charge separation will allow researchers to design new nanostructures to take advantage of entropy to direct the flow of heat or energy at the nanoscale,” Rijal said. “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.”

Not only that: While the KU team believes the mechanism discovered in this work can be used to make more efficient solar cells, they also think it could help researchers create more efficient photocatalysts for producing solar fuel, a photochemical process using sunlight to convert carbon. dioxide in organic fuels.