Perovskites in the spotlight
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Energy Sector Analysis
Perovskites’ extraordinary properties could revolutionize solar energy
Perovskites in the spotlight By Prachi Patel Feature Editor David Mitzi
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olar power is gaining mass appeal as photovoltaic panel prices fall and efficiencies improve. A watt of power generated from sunlight now costs less than $0.70, within shot of conventional electricity prices. Slashing that number by half could be a gamechanger for solar. An upstart photovoltaic material with astonishing properties provides a potential pathway to that end, and it has set the solar research world ablaze. Perovskites, materials with the crystal structure of calcium titanate, have only been intensely studied for solar cells since 2012. The progress in two years has been astounding: light-toelectricity power conversion efficiency has soared from a few percent to a certified 17.9%. “That’s never happened before,” said Michael McGehee, a Stanford University materials science and engineering professor. “Normally a doubling of efficiency takes 10 years, if not 20.” The best crystalline silicon solar cells—which form most of today’s commercial panels—have efficiencies around 25%. Established thin-film devices made of cadmium telluride and copper indium gallium selenide are around 20% efficient. Gallium arsenide yields thin-film devices with efficiencies over 28%, but the material is expensive to process. Perovskite solar cells could combine the high efficiency of silicon and GaAs with the low cost, easy processing of thin-film technologies. Making crystalline perovskites requires gently heating inexpensive precursor salt solutions deposited on a substrate. This solution processing opens up the possibility of printing them on flexible substrates using inexpensive roll-to-roll processes. Perovskites are extraordinary light absorbers, so solar cells can be made with layers only 300–500 nm thick, which also lowers cost. Tweaking the materials’ chemical composition gives a range of optical and electronic properties. For example, the type and ratio of elements can be tuned to engineer the bandgap, which determines the swath of the light spectrum the material can absorb. Perovskite cells also have a high open-circuit voltage—necessary for high efficiency—of over 1 V. Crystalline silicon solar cells, by comparison, have an open-circuit voltage of 0.6 V and are typically 500 times thicker. The original perovskite was a mineral composed mainly of calcium titanate. But the term now refers to any compound with an ABX3 composition and a three-dimensional network of corner-sharing BX6 octahedra, where A is a larger inorganic or organic cation, B a metal, and X is generally a halogen or oxygen. Methylammonium, lead, and iodide have been the most common
picks for the three respective components in hybrid perovskites used for PV. Tsutomu Miyasaka and his colleagues at Toin University of Yokohama were the first to use perovskites in a dye-sensitized solar cell (DSSC) configuration in 2009. In traditional DSSCs, a nanoporous titania film coated with a li
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