TECH SPECIAL .................Crystal can make solar power more efficient

TECH SPECIAL  Crystal can make  solar power more efficient  Engineers have shone new light on an emerging family of solar-absorbing materials that could clear the way for cheaper and more efficient solar panels and LEDs. The materials, called perovskites, are particularly good at absorbing visible light, but had never been thoroughly studied in their purest form: as perfect single crystals. Using a new technique, researchers grew large, pure perovskite crystals and studied how electrons move through the material as light is converted to electricity. Led by Professor Ted Sargent from the University of Toronto and Professor Osman Bakr of the King Abdullah University of Science and Technology (KAUST), the team used a combination of laser-based  techniques  to measure selected properties of the perovskite crystals. By tracking down the rapid motion of electrons in the material, they have  been able to determine the diffusion length ­ how far electrons can travel  without getting trapped by imperfections in the material ­ as well as mobility ­  how fast the electrons can move through the material. Their work was  published this week in the journal Science. “Our work identifies the bar for the ultimate solar energy-harvesting  potential of perovskites,“ says Riccardo Comin, a post-doctoral fellow  with the Sargent Group. “With these materials it's been a race to try to  get record efficiencies, and our results indicate that progress is slated to continue without slowing down..“ In recent years, perovskite effi ciency has soared to certified efficiencies  of just over 20 per cent, beginning to approach the present-day performance of commercial-grade silicon-based solar panels mounted in Spanish  deserts and on Californian roofs. “In their efficiency, perovskites are closely approaching  conventional materials that have already been commercialised,“  says Valerio Adinolfi,  co-first author on the paper.“They have the potential to offer  further progress on reducing the cost of solar electricity in light  of their convenient manufacturability from a liquid chemical precursor.“ The study has obvious implications for green energy, but may also  enable innovations in lighting.Think of a solar panel made of  perovskite crystals as a fancy slab of glass: light hits the crystal  surface and gets absorbed, exciting electrons in the material.  Those electrons travel easily through the crystal to electrical   contacts on its underside, where they are collected in the form of electric current. Now imagine the sequence in reverse ­  power the slab with electricity, inject electrons, and release energy as light. A more efficient electricity-to-light conversion means perovskites  could open new frontiers for energy-efficient LEDs. Parallel work in the Sargent Group focuses on improving  nanoengineered solar-absorbing particles called colloidal quantum  dots. “Perovskites are great visible-light harvesters, and quantum  dots are great for infrared,“ says Sargent. “The materials are highly complementary in solar energy  harvesting in  view of the sun's broad visible and infrared  power spectrum.“ “In future, we will explore the opportunities for stacking  together complementary absorbent materials,“ says Comin. “There are very promising prospects for combining  perovskite work and quantum  dot work for further  boosting the efficiency.“

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