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Solar energy is free and almost unlimited. For human beings whose “energy is lifeâ€, there is no reason to ignore it and let it be wasted. However, even the most efficient silicon solar cell, which is currently the most efficient in photoelectric conversion, can only utilize one quarter of the light energy. According to an article published online by the British "Economist" magazine, although crystalline silicon solar cells still have significant advantages such as low prices and mature technologies, scientists are increasingly approaching the crisis caused by the depletion of fossil fuels. Has begun to look for more efficient alternatives.
One of them is Dr. John Rogge of the University of Illinois at Urbana-Champaign. He developed a new silicon wafer jointly developed with Semprius, a concentrator PV module manufacturer in the United States, and demonstrated excellent performance in the latest test projects. In fact, as early as 2012, Semprius company has announced that its PV module conversion efficiency created a world record at the time, reaching 33.9% (effective area), and confirmed by verification and outdoor inspection. This time, at the 2014 American Association for the Advancement of Science, Rogge announced that its photoelectric conversion efficiency reached 42.5%. Even if packaged into the panel, it can be kept at 35%, and after proper debugging, it can reach a staggering 50%.
What is the secret of this new type of battery?
The answer is that its construction is not a traditional one. Instead, four wafers are stacked on each panel.
In general, solar cells are made of semiconductor materials. Each type of semiconductor has a property called a bandgap, and the difference in bandgap distinguishes the semiconductors from each other. The band gap determines the longest wavelength of light that a semiconductor can absorb, and it also controls the upper limit of energy that can be obtained from photons of shorter wavelengths.
Under normal circumstances, it is precisely because of the existence of a band gap that it is difficult for a conventional solar cell to capture photons in long-wave light, and it is impossible to fully utilize short-wave light photons.
In order to overcome this drawback, John Rogge uses different materials when making four stacked solar cells. Its purpose is to match the lowest bandgap of the upper cell with the highest value of the bandgap of the next cell, and so on. In this way, each cell can absorb several wavelengths in the spectrum and convert it to electrical energy, while allowing other wavelengths to be “leaked†to the next layer for ultimate use.
As with other previous laboratory results, the biggest problem facing this new type of battery is to reduce the cost of rare metals such as arsenic, antimony, and indium. John Roger's method is to reduce the coating area of ​​semiconductor materials on the surface of solar cells. Traditional cells require full coating, but Rogge's new product requires only 0.1% of the surface area. The semiconductor material is spread over the surface of the entire cell in the form of dots of only 1 square millimeter each—this means that on a 125 square meter battery panel, there will be 1 million such material dots. Above these points, cheap glass lenses are installed to gather sunlight into the battery stack to ensure that all incident light has material points corresponding to it.
Even more noteworthy is the coating process of this new type of battery - the tiny cells are grown on gallium arsenide wafers, which are then quickly stripped and then used to make more cells. The specific approach is to place a temporary coating on the original gallium arsenide substrate and then place the substrate of the new solar cell on this coating. Once the wafer begins processing, the transfer coating process transfers the battery from the gallium arsenide substrate to the inserted silicon wafer.
Currently, Semprius's products are being tested in 14 locations around the world. As for the cost of entering large-scale production, it is still difficult to estimate. However, John Rogge himself hinted that Siemens has announced that it has the ability to make the cost of photovoltaic power generation lower than thermal power generation. Although photovoltaic power generation cannot fully replace the status of petrochemical energy before it solves the problem of energy storage, it can still contribute in certain aspects in terms of current cost levels, including subsidies.
The new panel also has aesthetic advantages that can look pleasing to the eye. At least, it is definitely more beautiful than a coal-fired power station. (Reporter Zhang Mengran)
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