A research team from the University of Kansas has discovered that organic semiconductors, known as non-fullerene acceptors, exhibit high solar cell efficiency due to reverse heat flow.
A team of researchers at the University of Kansas has studied a counterintuitive effect in organic semiconductors that could cause the efficiency of solar cells to rival that of traditional silicon solar panels. The research is published in Advanced materials.
Researchers around the world are actively testing alternative materials to silicon for the production of solar cells. While silicon offers strong efficiency and durability, there are other, more abundant materials that could serve as cheaper alternatives. Silicon is also stiff, with some photovoltaic materials having shown the ability to be flexibly deposited in thin layers on uneven surfaces.
One type of material researchers are developing is called “organic” semiconductors. These carbon-based semiconductors are plentiful on Earth, cheap and potentially more environmentally friendly.
“They could potentially reduce production costs for solar panels because these materials can be coated onto arbitrary surfaces using solution-oriented methods – just like we paint a wall,” said Wai-Lun Chan, associate professor of physics and astronomy at the university. of Kansas.
Chan said these organic materials can be tuned to absorb light at specific wavelengths. The materials can be used to make transparent solar panels or panels with specific colors, making them a good fit for integration into sustainable buildings.
Organic semiconductors are already used in displays of consumer electronics such as mobile phones and TVs. They have not yet been commercialized in PV, as their light-to-electricity conversion efficiency is about 12%, about half as powerful as traditional silicon solar panels.
However, the use of a new class of materials called non-fullerene acceptors (NFA) can help bridge the gap in efficiency. Organic solar cells made with NFAs have shown efficiency closer to 20%.
The significant performance improvement of NFAs came from a counterintuitive effect. The team found that some of the excited electrons in the material were gaining energy from the environment, rather than losing it through entropy.
“This observation is counterintuitive because excited electrons typically lose their energy to the environment, much like a cup of hot coffee loses its heat to the environment,” says Chan.
The researchers think the energy gain may be due to a quantum effect in electrons, where an electron can appear on multiple molecules at the same time. This quantum effect, combined with the Second Law of Thermodynamics, which states that any physical process will lead to an increase in total entropy, leads to the unexpected energy gain.
“In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in total entropy,” says Rijal. “But we found that for organic molecules arranged in a specific nanoscale structure, the typical direction of heat flow is reversed, increasing the overall entropy. This reverse heat flow allows neutral excitons to draw heat from the environment and dissociate into a pair of positive and negative charges. These free charges can in turn produce electric current.”
The researchers said the mechanism could be used to make more efficient solar cells. They also believe it could be useful if applied to photocatalysts for the production of solar fuel, a photochemical process that uses sunlight to convert carbon dioxide into organic fuels.
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