Nanotubes light up solar cells

From Optics.org

Researchers in the UK have made a new type of hybrid electrode from multiwalled carbon nanotubes and indium-tin oxide that could be used for solar cell applications.

The nanotubes are directly grown on indium-tin oxide coated glass, a transparent electrode commonly employed in organic optoelectronic devices, such as solar cells and light-emitting diodes. The nanotube electrodes are highly transparent at longer wavelengths, making them ideal for harnessing light from the Sun.

Organic solar cells are cheaper and easier to make than their inorganic counterparts but their efficiencies and lifetimes remain an issue. This is because charge carrier mobilities - especially those of holes - are low, which limits the current flowing through the device. One way of overcoming this problem is to reduce the thickness of the active layer, but doing this also reduces the solar cell's ability to absorb light.

An alternative approach, adopted by Ravi Silva and colleagues at the University of Surrey, is to use an electrode that penetrates into the organic layer and which extracts charge carriers so that they are free to move through the device.


Silva's team has shown that it can grow multiwalled carbon nanotubes directly onto indium-tin oxide (ITO) coated glass using chemical vapour deposition (CVD) techniques. Multiwalled carbon nanotubes are very good electrical and thermal conductors and can therefore function as part of the underlying electrode. The scientists demonstrated that the nanotubes act as interpenetrating electrodes over large surface areas and efficiently help to extract positive charge carriers, or holes, from the active layer in the organic devices. "Crucially, these electrodes have high transparency, particularly at longer wavelengths, making them well matched to the solar spectrum," Silva told nanotechweb.org.

The researchers also showed that they were able to carefully control the growth of the nanotubes on the ITO substrate. Controlling the growth of the nanotubes is critical if nanotextured electrodes of this kind are to be used in thin film optoelectronics devices because nanotubes that are too long will short a device. Silva and co-workers discovered that the carbon nanotubes grow much more slowly on ITO than on other materials, such as silicon and glass, which allows the growth rate to be controlled. Moreover, the nanotubes are directly bonded to the ITO substrate, which improves the mechanical strength of the device and ensures that all the nanotubes are in direct contact with the substrate.

The Surrey team tested its solar cells using a "solar simulator" that provides light with a power of 100 mW/cm2. It found that the same number of photogenerated electrons are extracted, despite around a 30% reduction in the amount of light entering the cell, with an overall efficiency as high as around 1%. "Since the CVD growth method is scalable and relatively low-cost, we envisage the widespread utility of multiwalled carbon nanotube electrodes of this type in organic solar cells," said Silva. "The electrodes will facilitate efficient charge carrier extraction upon improved matching of the photoactive matrix with the solar spectrum".

The team reported its work in Appl. Phys. Lett.
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High density optical storage - on a single photon

From Optics.org

New technique developed at the University of Rochester stores and retrieves an entire digital image from a single photon.

Researchers at the University of Rochester, NY, US, say they have made an optics breakthrough that allows them to encode an entire image's worth of data into a single photon, slow the image down for storage, and then retrieve the image intact. While the initial test image consists of only a few hundred pixels, a tremendous amount of information can be stored with the new technique.



The image - "UR" for the University of Rochester - was made using a single pulse of light and the team can fit as many as 100 of these pulses at once into a tiny, 100 mm cell. The researchers say that squeezing that much information into so small a space and retrieving it intact opens the door to optical buffering - storing information as light.

"It almost sounds impossible, but instead of storing just ones and zeros, we're storing an entire image," said John Howell, associate professor of physics and leader of the team that created the device, which is revealed in Physical Review Letters. "It's analogous to the difference between snapping a picture with a single pixel and doing it with a camera--this is like a 6-megapixel camera."


"You can have a tremendous amount of information in a pulse of light, but normally if you try to buffer it, you can lose much of that information," said Ryan Camacho, Howell's student and lead author on the PRL article. "We're showing it's possible to pull out an enormous amount of information with an extremely high signal-to-noise ratio even at low light levels."



Optical buffering is currently a particularly hot field of research because engineers are trying to speed up computer processing and network speeds using light, but their systemsare slowed down when they have to convert light signals to electronic signals to store information.

Howell's group employed a new approach that preserves all the properties of the pulse. The buffered pulse is essentially a perfect original; there is almost no distortion, no additional diffraction, and the phase and amplitude of the original signal are all preserved. Howell is also working to demonstrate that quantum entanglement remains unscathed.

To produce the UR image, Howell shone a beam of light through a stencil with the U and R etched out. Anyone who has made shadow puppets knows how this works, but Howell turned down the light so much that a single photon was all that passed through the stencil.

Quantum mechanics dictates some strange things at that scale, so that bit of light could be thought of as both a particle and a wave. As a wave, it passed through all parts of the stencil at once, carrying the "shadow" of the UR logo with it. The pulse of light then entered a 100 mm cell of cesium gas at a warm 100°C, where it was slowed and compressed, allowing many pulses to fit inside the small tube at the same time.

"The parallel amount of information Howell has sent all at once in an image is enormous in comparison to what anyone else has done before," said Alan Willner, professor of electrical engineering at the University of Southern California and president of the IEEE Lasers and Optical Society. "To do that and be able to maintain the integrity of the signal is a wonderful achievement."



Howell has so far been able to delay light pulses by 100 ns and compress them to 1% of their original length. He is now working toward delaying dozens of pulses for as long as "several milliseconds", and as many as 10,000 pulses for up to 1 ns.

"Now I want to see if we can delay something almost permanently, even at the single photon level," said Howell. "If we can do that, we're looking at storing incredible amounts of information in just a few photons."

About the author

Matthew Peach is a contributing editor to optics.org and Optics & Laser Europe.
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