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Turning Heat into Power, Flexibly

Photonics Spectra
Feb 2005
Besides making the man, clothing could eventually make the machine -- or, at least, power it. Researchers at the University of Toronto have demonstrated infrared detectors and photovoltaics that have the potential to be woven into cloth.

The new materials are conjugated polymers filled with 1- to 10-nm-diameter lead-sulfide nanocrystal quantum dots. Conjugated and conductive polymers photosensitive to wavelengths shorter than 800 nm have been produced before, but they can’t be used for such applications as telecommunications, thermal imaging or thermal photovoltaics, which require a response to infrared radiation. The researchers have developed a method to create polymer/nanocrystal hybrids that are photosensitive in this spectral band.

Any spectral region between 800 and 2000 nm, and perhaps even broader, is accessible by tuning the diameter of the dots with little further modification, said Edward H. Sargent, who holds the Nortel Networks-Canada research chair in emerging technologies at the university and is a co-author of a Jan. 9 Nature Materials Advance Online Publication paper on this research.

In their work, the researchers added PbS quantum dot nanocrystals to the conjugated polymer MEH-PPV by attaching a linear organic molecule, a ligand, to the nanoparticles. Finding the right ligand length was a matter of trial and error made all the more difficult, Sargent noted, because they weren’t completely free in their choice of chain length.

With the quantum dots incorporated into the polymer, the investigators used solution processing to create a sandwich of glass, indium tin oxide, the quantum dot/polymer blend and an upper magnesium contact. Because of the construction of the sandwich and the interaction of the radiation with the materials, the device was sensitive in the IR. By using quantum dots of different diameters, they created polymers with absorption peaks at 980, 1200 and 1355 nm. Absorption, Sargent said, began at those wavelengths but continued into the blue.

The photosensitive polymer can be deposited in a solution, so it can be used to create inexpensive, flexible, roller-processed devices. Commercial applications of the new materials are three to five years away.

Calculations by professor Peter Peumans of Stanford University in California indicate that combining visible- and IR-sensitive polymers might result in flexible solar cells with efficiencies as high as 30 percent, compared with the 6 percent currently achievable with plastic solar cells.

quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.
CommunicationsenergyLighter Sidephotosensitivequantum dotsSensors & Detectorssolar cellsUniversity of Toronto

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