A basic organic solar cell consists of a thin film of organic semiconductors sandwiched between two electrodes which extract charges generated in the organic semiconductor layer to the external circuit. It has long been assumed that 100 percent of the surface of each electrode should be electrically conductive to maximise the efficiency of charge extraction.
Scientists at the University of Warwick have discovered that the electrodes in organic solar cells actually only need just one percent of their surface area to be electrically conductive to be fully effective, which opens the door to using a range of composite materials at the interface between the electrodes and the light harvesting organic semiconductor layers to improve device performance and reduce cost.
“It’s widely assumed that if you want to optimise the performance of organic solar cells you need to maximize the area of the interface between the electrodes and the organic semiconductors. We asked whether that was really true,” said Dr Ross Hatton from the University’s Department of Chemistry.
The researchers developed a model electrode where they could systematically change the surface area and found that when as much as 99% of its surface was electrically insulating the electrode still performs as well as if 100% of the surface was conducting, provided the conducting regions aren’t too far apart.
“What we’ve done is to demonstrate a design rule for this type of solar cell, which opens up much greater possibilities for materials choice in the device and so could help to enable their realisation commercially,” he said. “There is a fast growing need for solar cells that can be supported on flexible substrates that are lightweight and colour-tuneable. Conventional silicon solar cells are fantastic for large scale electricity generation in solar farms and on the roofs of buildings, but they are poorly matched to the needs of electric vehicles and for integration into windows on buildings, which are no longer niche applications.”
“This discovery may help enable these new types of flexible solar cells to become a commercial reality sooner because it will give the designers of this class of solar cells more choice in the materials they can use,” he said.
A high performance organic solar cell has additional transparent layers at the interfaces between the electrodes and the light harvesting organic semiconductor layer that are essential for optimising the light distribution in the device and improving its stability, but must also be able to conduct charges to the electrodes. Not many materials meet all of these requirements.
Organic solar cells are potentially very environmentally friendly, because they contain no toxic elements and can be processed at low temperature using roll-to-roll manufacutring techniques.
“This means composites of insulators and conducting nano-particles such as carbon nanotubes, graphene fragments or metal nanoparticles, could have great potential for this purpose, offering enhanced device performance or lower cost,” said researcher Dr Dinesha Dabera. “Organic solar cells are very close to being commercialised but they’re not quite there yet, so anything that allows you to further reduce cost whilst also improving performance is going to help enable that.”
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