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The amount of installed solar-capacity has been doubling roughly every three years. At the same time, researchers have found ways to make the cells better at absorbing the energy in sunlight. Modern solar panels operate with efficiency rates of 22-24%—a massive increase from the 6% achieved when the first practical solar cells were invented in the 1950s at Bell Labs in New Jersey, and were so expensive they mostly powered satellites.
Yet most processes have their limits. The maximum theoretical efficiency of a silicon solar cell—the amount of energy in sunlight that is turned into electricity—is around 29%. The rest of the solar energy is lost as heat. This theoretical maximum is only possible in laboratory conditions. In addition, when cells are packed together into solar panels, the total efficiency of the panel is unlikely to get above 26%. This is partly because the spaces between cells and other parts of the panel, such as the frame, do not contribute to making electricity. There are also inevitable losses of energy in the wires connecting the cells.
The future of solar power, however, could lie in a new, more efficient, type of solar cell that has just gone into production. Made with a family of crystalline materials called perovskites, they are capable of delivering panels with practical efficiency rates well above 30%.
Traditional solar cells typically contain two layers of ultra-pure silicon, both doped with an additive to make them semiconducting (ie, the ability to work as either a conductor or insulator). As they absorb light, electrons receive enough energy to jump across the junction between the layers, producing an electric current. Although other semiconductors can do the same, none rivals the affordability of silicon, which is produced cheaply from sand.
The original perovskite is a mineral called calcium titanium oxide. It was discovered in 1839 and named after Count Lev Perovski, a Russian mineralogist. The name has since become a generic term for substances with a similar crystalline structure. One of the things that makes perovskites so attractive to researchers as an alternative to silicon is that, in addition to being efficient at absorbing the energy in sunlight, they can also be made cheaply from easily obtainable materials, including a number of metals and halogens, like chlorine, bromide and iodine.
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