The first efficient tandem solar cell based on colloidal quantum dots (CQD) was reported at the University of Toronto, which said it may pave the way to inexpensive coatings that convert the sun's rays to electricity. 

"The U of T device is a stack of two light-absorbing layers — one tuned to capture the sun's visible rays, the other engineered to harvest the half of the sun's power that lies in the infrared," said Dr. Xihua Wang, lead author of a paper on a work.

"We needed a breakthrough in architecting the interface between the visible and infrared junction,” said Ted Sargent, a professor of electrical and computer cngineering at U of T, who is also the Canada Research Chair in Nanotechnology. "The team engineered a cascade — really a waterfall — of nanometers-thick materials to shuttle electrons between the visible and infrared layers."

Doctoral student Ghada Koleilat said, "We needed a new strategy — which we call the graded recombination layer — so that our visible and infrared light harvesters could be linked together efficiently, without any compromise to either layer."

The team pioneered solar cells made using CQD, nanoscale materials that can readily be tuned to respond to specific wavelengths of the visible and invisible spectrum. By capturing such a broad range of light waves — wider-than-normal solar cells — tandem CQD solar cells can, in principle, reach up to 42 percent efficiencies. The best single-junction solar cells are constrained to a maximum of 31 percent efficiency. In reality, solar cells that are on the roofs of houses and in consumer products have a 14 to 18 percent efficiency. The work expands the Toronto team's world-leading 5.6-percent-efficient CQD solar cells.

"Building efficient, cost-effective solar cells is a grand global challenge. The University of Toronto is extremely proud of its world-class leadership in the field," said Farid Najm, professor and chairman of the Edward S. Rogers Sr. Department of Electrical & Computer Engineering.

Sargent hopes that, in five years, solar cells using the graded recombination layer will be integrated into building materials, mobile devices and automobile parts.

"The solar community — and the world — needs a solar cell that is over 10 percent efficient and that dramatically improves on today's photovoltaic module price points," Sargent said. "This advance lights up a practical path to engineering high-efficiency solar cells that make the best use of the diverse photons making up the sun's broad palette."
 
The work appears in the journal Nature Photonics.

For more information, visit: www.utoronto.ca