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The same kind of surface that helps to protect a raincoat against downpours or a pan against sticky foods can also boost the performance of solar cells, according to a new study from University of Nebraska-Lincoln (UNL) engineers.
The study showed that constructing a type of organic solar cell on a "non-wetting" plastic surface made it 1.5 times more efficient at converting sunlight to electricity. As the name implies, non-wetting surfaces cause liquid to bead and run off rather than spread and absorb on contact.
The researchers used the technique to grow polycrystalline cells, which are less expensive, faster and easier to produce than those made from only a single crystal. However, single-crystal cells have traditionally boasted better efficiency, partly because they feature far fewer grains than polycrystalline cells.
The barriers between these grains reduce efficiency by trapping and recombining negatively charged electrons with their positively charged "holes," which ideally produce electric current by migrating in opposite directions following their separation by photon-carrying sunlight.
Lead author Jinsong Huang and his team sought to reduce the number of these efficiency-draining barriers by increasing the size of the grains themselves. Though grain size is typically limited to the thickness of a solar cell, Huang's team found that a non-wetting surface allowed it to fabricate grains up to eight times larger than the cell is thick.
Huang and his colleagues discovered that this type of non-wetting surface acted on grain boundaries in a similar fashion, accelerating their movement and encouraging the formation of larger grains when subjected to heat.
Huang, an associate professor of mechanical and materials engineering at UNL, said: "We found that the difference is huge. When you have two small grains merge into a larger grain, what happens is that a boundary actually moves from (the middle of two grains) to the end of one or the other. How easily these boundaries move will determine how fast these grains can merge and grow.
"A non-wetting surface is slippery, like when you pour oil on a floor. It's easier for the grain boundary to move because we're reducing some of the drag force on it."
This “slip-and-slide quality” also limited the presence of particles known as nuclei, which act as seeds from which crystal grains sprout when a liquid compound solidifies. The report claimed that the relative scarcity of these nuclei naturally led to the growth of larger grains with fewer boundaries.
Using non-wetting surfaces as fabrication sites might also lead to improvements in other technology, the study reported, possibly in the form of faster transistors and more sensitive photodetectors.
Huang said: "When it comes to electronic properties, crystallinity and grain size determine a lot. This is a simple method with a lot of potential applications."