1st November 2017
New efficiency record for quantum dot solar cells
Researchers at the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) have set a new world efficiency record for quantum dot solar cells, at 13.4 percent.
Colloidal quantum dots are electronic materials and because of their astonishingly small size (typically 3-20 nanometres in diameter) they possess fascinating optical properties. Quantum dot solar cells emerged in 2010 as the newest technology on an NREL chart that tracks research efforts to convert sunlight to electricity with increasing efficiency. The first lead sulfide quantum dots, illustrated below, had an efficiency of 2.9 percent. Since then, rapid improvements have pushed that number into double digits. Progress from the initial low efficiency came from better understanding of the connectivity between individual quantum dots, better overall device structures and the prevention of defects in each dot.
Atomistic model of colloidal lead sulfide (selenide) nanoparticle, also known as quantum dot. By Zherebetskyy [CC BY-SA 3.0], via Wikimedia Commons.
The latest development in quantum dot solar cells, however, comes from a completely different material. The new quantum dot leader is cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.
"This voltage, coupled with the material's bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell," explains Joseph Luther, senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. Tandem cells can deliver a higher efficiency than conventional silicon solar panels that dominate today's solar market.
NREL scientists Joey Luther and Erin Sanehira. Image courtesy of DOE/National Renewable Energy Laboratory
The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. The quantum dot perovskite materials developed by Luther and the NREL/University of Washington team could be paired with cheap thin-film perovskite materials to achieve similar high efficiency as demonstrated for space solar cells, but built at even lower costs than silicon technology – making them ideal for both terrestrial and space applications.
"Often, the materials used in space and rooftop applications are totally different. It is exciting to see possible configurations that could be used for both situations," said Erin Sanehira a doctoral student at the University of Washington who conducted research at NREL.
This latest advance, titled "Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells," is published in the journal Science Advances.
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