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

Solar energy is essential to our clean energy future. Traditionally, solar energy is harvested using silicon – the same semiconducting material used in everyday electronics. But silicon solar panels have drawbacks, including being expensive and difficult to mount on curved surfaces.

Researchers have developed alternative materials, Solar Energy To address these shortcomings, carbon-harvesting technologies are being developed, with the most promising being carbon-based semiconductors called “organic” semiconductors, which are earth-abundant, cheap and environmentally friendly.

“It could reduce production costs. solar panel “That’s because these materials can be applied to any surface using solution-based methods, similar to how you would paint a wall,” says Wai-Lung Chang, an associate professor of physics and astronomy at the University of Kansas.

“These organic materials can be tuned to absorb specific wavelengths of light and can be used to create transparent solar panels or panels in a variety of colors. These properties make organic solar panels particularly suitable for use in the next generation of green, sustainable buildings.”

Organic semiconductors are already used in display panels for consumer electronics such as mobile phones, televisions and virtual reality headsets, but they are not yet widely used in commercial solar panels. Organic Solar Cells The downside is that their light-to-electricity conversion efficiency is low, at around 12%, compared to 25% for monocrystalline silicon solar cells.

According to Zhang, electrons in organic semiconductors typically combine with positive charges called “holes,” so light absorbed by organic semiconductors often creates electrically neutral quasiparticles called “excitons.”

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

Despite their superior performance, it remained unclear to the scientific community why this new class of NFAs is so significantly superior to other organic semiconductors.

According to a groundbreaking study: Advanced MaterialsZhang and his team, which included graduate students Khushal Rijal (first author), Neno Fuller, and Fatima Rudaini from the Department of Physics and Astronomy, as well as University of Kansas chemistry professor Cindy Berry, discovered a microscopic mechanism that partially solves the superior performance achieved by the NFA.

Key to the discovery were measurements that lead author Rijal made using an experimental technique called “time-resolved two-photon photoemission spectroscopy,” or TR-TPPE, which allowed the team to track the energy of the excited electrons with sub-picosecond time resolution (less than a trillionth of a second).

“These measurements show that [Rijal] “We observed that some of the photoexcited electrons in the NFA are able to gain energy from the environment rather than losing it to the environment,” Chang said. “This observation is counterintuitive because excited electrons typically lose energy to the environment in the same way that a hot coffee cup loses heat to its surroundings.”

The team believes that this unusual process occurs on a microscopic scale due to the quantum behavior of electrons, which allows excited electrons to appear in multiple molecules simultaneously. This quantum weirdness is due to the fact that not all molecules Physical Processes This leads to an increase in total entropy (often known as “disorder”) and creates abnormal energy-increasing processes.

“Most of the time, a hot object transfers heat to its cooler surroundings. Heat Transfer “This leads to an increase in the total entropy,” Rijal says, “but we find that for organic molecules arranged in certain nanoscale structures, the typical direction of heat flow is reversed, resulting in an increase in total entropy. This reverse heat flow causes neutral excitons to gain heat from the environment and dissociate into positive and negative exciton pairs. Negative chargeThese free charges can, in turn, produce an electric current.”

Based on their experimental results, the team proposes that this entropy-driven charge separation mechanism could significantly improve the efficiency of organic solar cells made with NFAs.

“Understanding the underlying charge separation mechanism will enable researchers to design new nanostructures that harness entropy to direct heat and energy flow at the nanoscale,” Rijal said. “Entropy is a well-known concept in physics and chemistry, but it has rarely been actively exploited to improve the performance of energy conversion devices.”

But that’s not all: the KU team believes that the mechanisms discovered in this study could be used to enable more efficient production. Solar CellThe researchers also believe it could help design more efficient photocatalysts for solar fuel production, a photochemical process that uses sunlight to convert carbon dioxide into organic fuels.