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Tuesday, 17 January 2012

Nanotube bundles could make good solar cells

Jan 17, 2012

Photon hitting a bundle of carbon nanotubes

Bundles of carbon nanotubes could increase the efficiency of thin-film solar cells. So say researchers at the Los Alamos National Laboratory in the US who have used high-speed spectroscopy to show that the bundles can not only generate electron-hole pairs when exposed to sunlight but can separate these pairs of charge carriers too. This is the first time that these two crucial functions have been demonstrated in a single thin-film photovoltaic material.

Thin-film photovoltaic materials are better than conventional solar-cell materials, such as silicon, in that they are cheaper to make, are lighter and more flexible. They work by absorbing photons from sunlight and converting these into electron-hole pairs (or excitons). To generate electric current, an electron and hole must then be separated in the brief space of time it takes before the two particles come back together and are reabsorbed into the material. In solar cells, the exciton must quickly travel to another layer in the device (where the charge separation will occur), but it is normally reabsorbed too fast, something that ultimately leads to low light absorption efficiencies.

Semiconducting carbon nanotube bundles could come into their own here, say Jared Crochet and colleagues. Individual semiconducting nanotubes (which are tubules of the semi-metal graphene) suffer from the low efficiency mentioned above, but this problem can be overcome when the tubes are aggregated into bundles of tubes that have the same chirality. Chirality is the direction in which the graphene sheet has been twisted to form a tube – from left to right, or right to left.

Light absorption and charge separation
Such nanotube bundles respond to absorbed light in the same way as the parent material graphene, and charge separation can thus be very efficient. "This effect is promising for incorporating carbon nanotubes into photovoltaic devices as active layers where both light absorption and charge separation can occur," Crochet told

The materials used in these experiments were produced by centrifuging individual carbon nanotubes so that tubes of the same twist direction and diameter aggregated together. The researchers chose bundles with a diameter and twist that strongly absorb light at a wavelength of about 570 nm – ideal for exposing to sunlight.

High-speed spectroscopy
By exposing the samples to a brief flash of laser light and recording spectra every tens of femtosecond, Crochet's team was able to observe signals that are characteristic of excitons being formed, plus additional peaks that indicated the production of free electrons and holes. In samples made of non-bundled individual carbon nanotubes, only the peak corresponding to exciton creation was seen.

The team now plans to incorporate single chirality semiconducting carbon nanotube networks into real-world photovoltaic devices as active layers. "We would ideally like to see an all-carbon solar cell made of graphene, graphene oxide and carbon nanotubes," said Crochet.

The researchers are also busy trying to better understand exciton dissociation and charge transport in the nanotube bundles using the high-speed spectroscopy technique. "The advantage of having the material in a device is that we can investigate every step, from photon absorption to charge collection," concluded Crochet.

The work was reported in Physical Review Letters.



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