Colloidal quantum dots offer the possibility of cheap and efficient solar cells

Colloidal quantum dot solar cells, University of Toronto.

What?

Colloidal quantum dots (CQD) are semiconductor nanoscale materials that can be tuned to respond to certain wavelengths of light. They offer the possibility of making a solar cell that responds to both visible and infrared light, resulting in increased efficiency when compared to standard (silicon) cells that absorb only visible wavelengths. A way of using CQD to make efficient solar cells is making a visible semiconductor that is then stacked with an infrared semiconductor. The problem “is how to do that stacking, how to make a layer that connects the bottom visible cell and the top infrared cell in a way that is efficient and it doesn’t produce any damage to any of those layers,” as explained by Ted Sargent, lead researcher of this study. The solution discovered, he continues, “is called a graded recombination layer [GRL], which is a new combination of materials, a new device concept that allows us to make a tandem, and ultimately multi-junction, solar cell based on colloidal quantum dots.”[1]

Why?

Quantum dots (…) have been seen as a promising route to low-cost solar cells because the particles can be sprayed onto surfaces much like paint. But cells based on this technology have been too inefficient to be practical. By discovering a way to combine two different types of quantum dots in a solar cell, the researchers could open the way to making such cells much more efficient.[2]

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 per cent efficiencies. The best single-junction solar cells are constrained to a maximum of 31 per cent efficiency. In reality, solar cells that are on the roofs of houses and in consumer products have 14 to 18 per cent efficiency.[1]

The GRL concept can be used to build photovoltaic devices with three or more junctions. Such many-junction devices provide a route to overcoming the charge carrier transportation limitations in CQD films today (…).[3]

WhoWhereWhen?

The new concept has come from a team of engineers in the Department of Electrical and Computer Engineering of the University of Toronto (U of T) in Canada. The lab where the research took place (the Sargent Group, after the lead researcher) is involved in projects in the areas of photovoltaics, photodetectors, and nanobiosensors. The work is described in papers published in ACS Nano and, most recently, in Nature Photonics. The latest publication was based in part on work supported by King Abdullah University of Science and Technology (KAUST), the Ontario Research Fund Research ExcellenceProgram, the Natural Sciences and Engineering Research Council of Canada, and equipment donated by Angstrom Engineering and Innovative Technology.

How?

In the Toronto researchers’ cell, one layer of quantum dots is tuned to capture visible light and the other to capture infrared light. The researchers also found a way to reduce electrical resistance between the layers, a problem that can limit the power output of a two-layer cell. They introduced a transition layer, made up of four films of different metal oxides, that keeps resistance ‘nice and low,’ says Ted Sargent (…). The researchers chose transparent oxides for this layer, allowing light to pass through them to the bottom cell.[2]

We needed a breakthrough in architecting the interface between the visible and infrared junction,” said Sargent (…) “The team engineered a cascade – really a waterfall – of nanometers-thick materials to shuttle electrons between the visible and infrared layers.[1]

But?

While in theory tandem solar cells can reach efficiencies of 42% (or 49% in the case of triple-junction cells), the devices now developed have a solar power conversion efficiency of up to 4.2% only. Researchers hope to keep improving the fabrication of the CQD solar cells and stacking so that efficiency exceeds 10% within five years.

John Asbury, a professor of chemistry at Penn State University, says that by opening up the ability to make multilayer cells from quantum dots, the U of T team has boosted the theoretical efficiency of the technology from 30 percent to almost 50 percent. But getting anywhere near those kinds of efficiencies will require a lot of work to eliminate ‘trapped states’—places within the quantum-dot material where electrons can become stuck. ‘The problem with quantum dots is that electrons have a high probability of not making it to the electrodes where they can be collected, so that has limited their efficiency,’ he says. ‘To really have an impact means developing strategies to control those trapped states.’[2]

References

  1. U of T Engineers Crack Solar Challenge, 27 June 2011.
  2. Spray-on Solar Goes Double-decker, Technology Review, MIT, 1 July 2011.
  3. Xihua Wang, Ghada I. Koleilat, Jiang Tang, Huan Liu, Illan J. Kramer, Ratan Debnath, Lukasz Brzozowski, D. Aaron R. Barkhouse, Larissa Levina, Sjoerd Hoogland, and Edward H. Sargent, Tandem colloidal quantum dot solar cells employing a graded recombination layer, Nature Photon., p. 480–484, 2011.

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