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Thursday, May 8, 2014

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Tin-Based Solar Cell The Photovoltaics of The Future

Using readily available equipment, researchers at from Northwestern University have developed a new type of solar cell based on tin instead of lead perovskite, according to a new report in Nature Photonics.
“This is a breakthrough in taking the lead out of a very promising type of solar cell, called a perovskite,” said study author Mercouri G. Kanatzidis, an inorganic chemist at Northwestern. “Tin is a very viable material, and we have shown the material does work as an efficient solar cell.”
When made with lead, the perovskite structure has about 15-percent efficiency in converting solar energy into electricity. The Northwestern researchers said their new tin-based cell should be able to eventually equal and potentially surpass that level of efficiency. Being hailed as the new frontier of solar energy technology, perovskite cells have been increasingly researched in recent years.
“Our tin-based perovskite layer acts as an efficient sunlight absorber that is sandwiched between two electric charge transport layers for conducting electricity to the outside world,” said study author Robert P. H. Chang, a professor of materials science and engineering at Northwestern.
The solid-state tin cell is comprised of five levels, with each level being a crucial component. The first layer of the novel cell is electrically-conductive glass, which permits sunlight to get into the cell. The next layer, Titanium dioxide, is placed onto the glass and the two first layers act as the front contact side of the solar cell.
Next, the light-absorbing tin perovskite layer is laid down. This is carried out in a protected environment called a nitrogen glove box that prevents oxidation.
The next level is the hole transport layer, which is necessary to close the circuit and achieve a functional cell. This layer demands exact chemistry to prevent destroying the tin below. This essential chemistry was determined by learning the reactivity of the perovskite structure. This level also is put down in the glove box. The nearly-completed cell is then enclosed and can now be exposed to the air.
A slim layer of gold is used to top off the solar cell and act as the back electrode. Once completed, the entire device is approximately one to two microns thick.
The scientists then analyzed the device under simulated full sunlight and documented an efficiency of 5.73 percent. They added that the tin cell can absorb the majority of the visible light spectrum and the perovskite salt can be dissolved – reforming upon solvent removal.
“Other scientists will see what we have done and improve on our methods,” Kanatzidis said. “There is no reason this new material can’t reach an efficiency better than 15 percent, which is what the lead perovskite solar cell offers. Tin and lead are in the same group in the periodic table, so we expect similar results.”
The researchers said their cell offers the possibility of both higher efficiency and lower fabrication costs while being environmentally friendly.
“Solar energy is free and is the only energy that is sustainable forever,” Kanatzidis said. “If we know how to harvest this energy in an efficient way we can raise our standard of living and help preserve the environment.”


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