SAN FRANCISCO， Oct. 23 (Xinhua) -- A new design for solar cells that uses inexpensive， commonly available materials， including tin and other abundant elements， could rival and even outperform conventional cells made of silicon.
The resulting new type of solar cell， which replaces silicon with a crystal called perovskite， converts sunlight to electricity at efficiencies similar to current technology but at much lower cost， said researchers from Stanford University in the United States and Oxford University in Britain.
Perovskite is a photovoltaic crystalline material that is thinner， more flexible and easier to manufacture than silicon crystals. The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass.
"Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost，" said Michael McGehee， a professor of materials science and engineering at Stanford and co-authored on a study published in the recent issue of the journal Science. "We have designed a robust， all-perovskite device that converts sunlight into electricity with an efficiency of 20.3 percent， a rate comparable to silicon solar cells on the market today."
A conventional silicon solar panel begins by converting silica rock into silicon crystals through a process that involves temperatures above 3，000 degrees Fahrenheit， or 1，600 degrees Celsius， noted co-lead author Tomas Leijtens， a postdoctoral scholar at Stanford. "Perovskite cells can be processed in a laboratory from common materials like lead， tin and bromine， then printed on glass at room temperature."
Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. And a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build， the authors said. However， building an all-perovskite tandem device has been a difficult challenge. The main problem is creating stable perovskite materials capable of capturing enough energy from the sun to produce a decent voltage.
A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons can cause electrons in the perovskite crystal to jump across an "energy gap" and create an electric current. A solar cell with a small energy gap can absorb most photons but produces a very low voltage. A cell with a larger energy gap generates a higher voltage， but lower-energy photons pass right through it.
An efficient tandem device would consist of two ideally matched cells， co-lead author Giles Eperon， an Oxford postdoctoral scholar currently at the University of Washington， was quoted as saying in a news release from Stanford. "The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage. The cell with the smaller energy gap can harvest photons that aren't collected by the first cell and still produce a voltage."
The smaller gap has proven to be the bigger challenge for researchers. Working together， Eperon and Leijtens used a unique combination of tin， lead， cesium， iodine and organic materials to create an efficient cell with a small energy gap. "We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8 percent conversion efficiency，" Eperon said. "We then combined it with a perovskite cell composed of similar materials but with a larger energy gap."
The result: A tandem device consisting of two perovskite cells with a combined efficiency of 20.3 percent.
"This is just the beginning，" said Henry Snaith， a professor of physics at Oxford and co-author on the study， adding that "the all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30 percent efficiency."
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