Currently solar panels come in two main forms, photovoltaic panels to harvest visible light for electricity production and thermal panels to harvest the infrared spectrum for heat.  In a paper published in Nature Photonics, University of Toronto Engineering (UTE) researchers report a new solar cell that may pave the way to inexpensive coatings that convert the 100% of the sun’s solar energy rays to electricity. *Note that’s not efficiency.*

So far the two forms of panels have been pretty much exclusive from each other.  Infrared is a great way to make heat, leaving out the rest of the spectrum doesn’t seem so important.  But for photovoltaic the visible light spectrum isn’t consumer motive enough without lots of economic stimulation, patience and considerable disposable income to get going.  Adding any bit of the infrared to electricity production could score big on cutting down on those high and long investments and payback periods.

The UTE team pioneered solar cells capture such a broad range of light waves – wider than normal solar cells –that can in principle reach up to 42% efficiencies. Keep in mind the important part, that’s 42% of the visible and infrared spectrum.  Meanwhile the best visible light or single-junction solar cells are holding to a maximum of 31% efficiency.

For the market reality check, solar cells selling today that are on the roofs of houses and in consumer products range from 14 to 18% efficiency.

The UTE engineers, led by Professor Ted Sargent, a Professor of Electrical and Computer Engineering at the University of Toronto, who is also the Canada Research Chair in Nanotechnology, are reporting the first efficient tandem solar cell based on colloidal quantum dots (CQD). Sargent explains the drive behind the research, “We needed a breakthrough in architecting the interface between the visible and infrared junction. The team engineered a cascade  – really a waterfall – of nanometers-thick materials to shuttle electrons between the visible and infrared layers.”

Ghada Koleilat, doctoral student and lead coauthor expands, “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.”

Lead coauthor Dr. Xihua Wang said, “The UTE 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.”

Tandem CQD solar cell cross-sectional SEM image. See the paper's Supplemental Information for more details. Click image for the largest view.

The team pioneered solar cells made using the CQDs, which are 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% efficiencies. The latest work expands the Toronto team’s world-leading 5.6 % efficient colloidal quantum dot solar cells.

Professor Farid Najm, Chair of The Edward S. Rogers Sr. Department of Electrical & Computer Engineering tossed in a little well deserved chest beating with, “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.”  Just how far to the extremes of the short visible spectrum and the long infrared the CQDs can catch will be interesting.  That’s when the incoming energy would be known and calculations can be compared to electrical output.

An optimist, Sargent is hopeful that in five years solar cells using the graded recombination layer explained in the paper will be integrated into building materials, automobiles and mobile devices.  “The solar community – and the world – needs a solar cell that is over 10 per cent efficient, and that dramatically improves on today’s photovoltaic module price points. 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,” said Sargent.

It seems the funding is primarily from Canada’s own sources. Canada being so far north is at something of a disadvantage in gathering solar energy both in overall solar time and the solar angle.  The work’s support comes from an award made by the King Abdullah University of Science and Technology, the Ontario Research Fund Research Excellence Program, and by the Natural Sciences and Engineering Research Council of Canada. Equipment from Angstrom Engineering and Innovative Technology enabled the research.

This is just the first step. But the CQD idea surely has some legs if scale can be achieved.  The blends of the CQD tuning might offer a very different set of choices to consumers and may offer a far wider range where solar power could be applicable.

It’s good innovation and development work.  Still, it doesn’t seem strong enough that an average home’s roof can power an average household.  But if they get to 42% – it’s getting close.


3 Comments so far

  1. Benjamin Cole on June 28, 2011 11:59 AM

    This is fascinating. I hope solar panels are made that harvest more of the spectrum. With that approach, they might become viable, commercially speaking.

  2. William de Bruyn on September 6, 2011 8:12 PM

    An efficiency of over 40% would be pretty good, for a household generating capacity of say 5-6 KW peak around 20 sq. M. of panel area normal to the sun would be required, the initial cost and pay back period of the embodied energy in manufacture is an issue and of course storage becomes a critical requirement, hopefully battery technology and cost will also improve. Low cost storage coupled with a many grid connected systems may well reduce the need for very large generating plant.

  3. Solar Distributor Toronto on September 23, 2011 5:37 AM

    Thank you for posting this hopefully it will make others more aware of the potential of solar power

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