Researchers are exploring alternative materials to address these issues. One promising option is "organic" semiconductors-carbon-based materials that are abundant, cost-effective, and eco-friendly.
"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. "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 next-generation green and sustainable buildings."
While organic semiconductors are already used in display panels for electronics like cell phones and TVs, they have not yet been widely adopted in commercial solar panels due to their lower efficiency, around 12% compared to 25% for silicon cells.
Chan explained that electrons in organic semiconductors typically bind to positive counterparts called "holes," creating electrically neutral quasiparticles known as "excitons" when light is absorbed.
However, a new class of organic semiconductors called non-fullerene acceptors (NFAs) has changed this. Organic solar cells with NFAs can achieve nearly 20% efficiency.
Despite this progress, the reason for NFAs' superior performance remained unclear. In a study published in Advanced Materials, Chan and his team, including graduate students Kushal Rijal (lead author), Neno Fuller, and Fatimah Rudayni, along with chemistry professor Cindy Berrie, identified a microscopic mechanism contributing to the high efficiency of NFAs.
Lead author Rijal used an experimental technique called "time-resolved two photon photoemission spectroscopy" (TR-TPPE) to measure the energy of excited electrons with sub-picosecond resolution (less than a trillionth of a second).
"In these measurements, Kushal [Rijal] observed that some of the optically excited electrons in the NFA can gain energy from the environment instead of losing energy to the environment," said Chan. "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 surrounding."
Supported by the Department of Energy's Office of Basic Energy Sciences, the team believes this process occurs due to the quantum behavior of electrons, allowing an excited electron to exist on several molecules simultaneously. This phenomenon, coupled with the Second Law of Thermodynamics, which dictates that physical processes increase total entropy, leads to the energy gain observed.
"In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in the total entropy," said Rijal. "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."
The team suggests that this entropy-driven charge separation mechanism is key to the improved efficiency of NFA-based organic solar cells.
"Understanding the underlying charge separation mechanism will allow researchers to design new nanostructures to take advantage of entropy to direct heat, or energy, flow on the nanoscale," Rijal said. "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."
Moreover, the KU team believes their findings could help design more efficient photocatalysts for solar-fuel production, converting carbon dioxide into organic fuels using sunlight.
Research Report:Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction
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