Materials engineers at the University of Wisconsin-Madison (US) have developed the first thin-film solar cell that takes advantage of a previously unexploited property of carbon nanotubes — their ability to absorb light. Using nanotubes as the principle light absorbing material, the proof-of-concept device converts more than 75% of the light it absorbs into electricity.
Carbon nanotubes could be highly attractive candidates for the primary light-absorbing component in a solar cell because they are strong light absorbers in addition to having high charge transport mobility and solution-processability. Nanotubes are also more air stable than most other semiconducting conjugated carbon materials.
Other research groups have previously used carbon nanotubes in photovoltaic devices, but mostly as secondary materials such as electrode materials or charge-collection or charge-transport aids rather than for their ability to absorb light. “We estimate more than 60% of the current in our cell comes from the nanotube component,” says Michael Arnold, an assistant professor of materials science and engineering at UW-Madison. “We achieve these results using highly pure semiconducting carbon nanotubes with band gap in the near-infrared and very few metallic nanotube impurities and tuning the optical interference effects within the device.” The cell has a bilayer structure with a planar heterojunction between the carbon nanotubes and fullerene-C60. The carbon nanotubes absorb light, generating bound electron-hole pairs — or excitons — in the nanotube film. The electrons are extracted by the fullerene-C60 layer and transported to the silver top electrode, where they are extracted from the device.
“We believe that carbon nanotubes stand to be a major player in materials science for future technologies, not just in solar cells but also in transistors and even medical sensor applications,” Arnold, a pioneer in developing carbon nanotube-based materials for solar energy applications, says. As he explains, nanotubes combine the advantages of inorganic semiconductors (crystallinity, high charge transport mobility, high chemical and thermal stability) with the advantages of semiconducting polymers (carbon based and solution processable).
UW-Madison’s cell is 1.02% efficient at 1.5 suns, which may be the first report of a >1% efficient solar cell using nanotubes as the principle light absorbing material. “What is impressive about this cell is that the internal quantum efficiency for light to electron conversion by the nanotubes is > 75%,” Arnold says. He further points out that the photoabsorbing film is only a few nanometres thick and nearly transparent, and optimised future designs with thicker layers of nanotubes with controlled orientation should absorb many times more light, while maintaining the high internal efficiency.
Increasing the thickness of the solar cell will not be the only step required before a technology like this can be commercially viable. “Ultimately, we would like to pair the light-absorbing nanotubes with more stable inorganic electron-accepting materials other than fullerene buckyballs,” Arnold says, adding that further advances in the scale and economics of semiconducting nanotube synthesis and processing will be needed as well. He does believe, however, that carbon nanotubes very desirable materials for manufacturing, because, like polymers, they are solution-processable and could be fabricated via roll-to-roll processing or inkjet printing.
Carbon nanotubes absorb strongly in tunable spectral bands, so they could enable new types of applications, e.g. they could be used as very sensitive photodetectors. “By optimising near-infrared and ultraviolet absorptivity while minimising visible absorptivity, new classes of transparent solar cells should become possible,” Arnold predicts. “We have only scratched the surface of what is possible using carbon nanotubes as light absorbers in solar cells, and there are many ways by which it will be possible to drastically improve their efficiency, moving forward,” the expert says in conclusion.
The paper “1% solar cells derived from ultrathin carbon nanotube photoabsorbing films,” published in the journal Applied Physics Letters, details the development of the cell. The first author is PhD student Matthew Shea. Furthermore, Arnold credits his graduate students Dominick Bindl with being “key in developing the foundation that led up to our recent Applied Physics Letters paper over the last 5 years.”
Written by Sandra Henderson, Research Editor, Solar Novus Today
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