May 24th, 2012 by James Ayre
A new solar cell that is cheap, environmentally friendly, and durable has been developed by researchers from Northwestern University.
The solar cell is the first to solve the problem of the Grätzel cell, a low-cost, environmentally friendly solar cell that has one significant disadvantage — it leaks and causes corrosion of the solar cell itself.
Grätzel cells use a molecular dye to absorb sunlight and then convert it to electricity, in a similar way to chlorophyll in plants. The cells generally don’t last longer than 18 months though, and they are quite inefficient, making them commercially nonviable. People have been looking for a solution to this problem for two decades.
The researchers’ solution was to use a new material for the electrolyte that starts as a liquid but ends up as a solid, eliminating the problem of leakage.
“The Grätzel cell is like having the concept for the light bulb but not having the tungsten wire or carbon material,” said Mercouri Kanatzidis of Northwestern University, a professor of chemistry and co-author of the paper. “We created a robust novel material that makes the Grätzel cell concept work better. Our material is solid, not liquid, so it should not leak or corrode.”
The cell is made up of a thin film of cesium, tin, and iodine, called CsSnI3, and it replaces the entire liquid electrolyte of the old Grätzel cell design. Kanatzidis “knew that scientists at IBM and elsewhere had been developing good solid electrical semiconductors for years,” ScienceNOW reports. “So he teamed up with Northwestern colleague Robert P. H. Chang, a materials scientist, to try” one of these compounds, the compound above.
“This is the first demonstration of an all solid-state dye-sensitized solar cell system that promises to exceed the performance of the Grätzel cell,” Chang said in the Northwestern news release. “Our work opens up the possibility of these materials becoming state of the art with much higher efficiencies than we’ve seen so far.”
The researchers’ solar cell exhibits the highest conversion efficiency seen in a solid-state system equipped with a dye-sensitizer, 10.2 percent (previously, such cells were hitting an efficiency of up to 6 percent). This 10.2 efficiency is close to the highest reported for a Grätzel cell, 11-12 percent. 10 percent is considered by many to be a benchmark efficiency needed for commercial viability. Conventional solar cells made from highly purified silicon can convert roughly 20 percent of incoming sunlight, but they are more costly to produce and often come with some toxicity concerns.
Different from the Grätzel cell, the bew cell “uses both n-type and p-type semiconductors and a monolayer dye molecule serving as the junction between the two. Each nearly spherical nanoparticle, made of titanium dioxide, is an n-type semiconductor. The CsSnI3 thin-film material is a new kind of soluble p-type semiconductor.”
“Our inexpensive solar cell uses nanotechnology to the hilt,” Chang said. “We have millions and millions of nanoparticles, which gives us a huge effective surface area, and we coat all the particles with light-absorbing dye.”
In the new solar cell, the nanoparticles are packed in, and the new CsSnI3 material is then poured in flowing around the nanoparticles. Like with paint, the solvent then evaporates and a solid mass remains. Leaving the sunlight-absorbing dye right between the two semiconductors.
“This is only the beginning,” Chang said. “Our concept is applicable to many types of solar cells. There is a lot of room to grow.”
Image Credits: solar cell diagram via Chung et al., Nature, 485 (24 May) and solar panel via Shutterstock
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