New Nanomaterial Catches Sun Invisible Rays

By Roland Piquepaille

Researchers at the University of Toronto (U of T) have designed an infrared-sensitive material made of nanocrystals so small they were able to tune them to catch the Sun's invisible rays. In "Nanotechnologists' new plastic can see in the dark," you'll discover that it's the first time that a light-sensitive material works in the invisible light spectrum. This opens the way to a broad range of applications, from clothing to digital cameras that work in the dark. But the real breakthrough is that it will permit to catch five more times energy from the Sun, up to 30 percent from the 6 percent achieved today by the best plastic solar cells. Hats off to these researchers...

Here is the somewhat lyrical opening paragraph of the U of T news release.

Imagine a home with "smart" walls responsive to the environment in the room, a digital camera sensitive enough to work in the dark, or clothing with the capacity to turn the sun's power into electrical energy. Researchers at the University of Toronto have invented an infrared-sensitive material that could shortly turn these possibilities into realities.

Professor Ted Sargent, from Nortel Networks and U of T, explains the process.

"We made particles from semiconductor crystals which were exactly two, three or four nanometres in size. The nanoparticles were so small they remained dispersed in everyday solvents just like the particles in paint," explains Sargent. Then, they tuned the tiny nanocrystals to catch light at very short wavelengths. The result -- a sprayable infrared detector.

Existing technology has given us solution-processible, light-sensitive materials that have made large, low-cost solar cells, displays, and sensors possible, but these materials have so far only worked in the visible light spectrum, says Sargent. "These same functions are needed in the infrared for many imaging applications in the medical field and for fibre optic communications," he says.

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But in my mind, the best consequence from this discovery is the potential to vastly improve our capacity to recover one of the sources of renewable energy, the solar one.

Professor Peter Peumans of Stanford University, who has reviewed the U of T team's research, also acknowledges the groundbreaking nature of the work. "Our calculations show that, with further improvements in efficiency, combining infrared and visible photovoltaics could allow up to 30 per cent of the sun's radiant energy to be harnessed, compared to six per cent in today's best plastic solar cells."

The research work has been published by Nature Materials as an Advance Online Publication on January 9, 2005. The article is called "Solution-processed PbS quantum dot infrared photodetectors and photovoltaics". For your convenience, here is the text of the abstract.

In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage1. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the infrared for telecommunications (1,300-1,600 nm), thermal imaging (1,500 nm and beyond), biological imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800-2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond 800 nm (refs 2,3). Sensitizing conjugated polymers with infrared-active nanocrystal quantum dots provides a spectrally tunable means of accessing the infrared while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the infrared. We achieve, in a solution-processed device and with sensitivity far beyond 800 nm, harvesting of infrared-photogenerated carriers and the demonstration of an infrared photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the infrared spectrum.

I would never have guessed from this abstract that this new infrared-sensitive material could allow us one day to capture 5 more times energy from the sun.

But after all, the writer of this news release, Sonnet L'Abbé, is a renowned poet and the author of a collection of poems, "A Strange Relief." Please read this other U of T news release from December 24, 2004, Rhyme and reason, if you're interested in her poems.

Sources: University of Toronto news release, via EurekAlert!, January 9, 2005; and various websites

Related stories can be found in the following categories.

• Energy
  
• Materials
  
• Nanotechnology
  
• Optics