As-made SWCNTs naturally contain a mixture of semiconducting nanotubes with varying diameters and chiralities. Carbon nanotubes can be thought of as rolled up sheets of carbon, and just as sheets of paper can be rolled up in many ways, so can nanotubes. The direction in which they are rolled can be described by indices known as n and m.

SWCNTs are interesting in that small changes in their (n, m) type dramatically changes their optical properties, explains team leader Benjamin Flavel. “As such, different (n,m) species absorb light at different wavelengths and it is exactly this property that we have measured in our new work.”

Indeed, each (n,m) species used in Flavel and colleagues’ experiments have unique so-called first and second optical transitions at light wavelengths of around 900-1350 nm and 500–850 nm, respectively. So by selecting the appropriate combination of SWCNTs, it is possible, in principle, to fabricate a solar cell capable of harvesting light not only in the visible part of the electromagnetic spectrum but also in the infrared – a region usually inaccessible to traditional silicon solar cells.

Better understanding photocurrent generation

The researchers obtained their results by first building arrays of five different (n,m) carbon nanotube species, each capable of absorbing light and generating photocurrent at a different wavelength. Each tube was then integrated into a two terminal electronic device interfaced with metallic contacts. A focused light beam with a variable wavelength was scanned across each carbon nanotube film and the resulting photocurrent it produced measured.

“Scientists still not do not really know how carbon nanotubes generate photocurrent, with some research groups reporting a photothermoelectric effect and others a photovoltaic one,” says Flavel. “In truth, it is likely a combination of the two, but we now hope to help settle this debate,” he tells nanotechweb.org.

To this end, the team adds that it also plans to look at first-order optical transitions in these nanostructures – something that has never been done before.

The current work is detailed in ACS Nano.