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In a report published in October, scientists from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) used single-walled carbon nanotubes (SWNCTs) to advance the thermoelectric performance of organic semiconductors. The carbon nanotube thin films, they said, could ultimately be integrated into fabrics to convert waste heat into electricity or serve as a small power source.

In organic thermoelectric materials, carbon nanotubes are often an electrically conductive “filler” – one part of a polymer-based composite. The NREL researchers believe that carbon nanotubes could be a thermoelectric material in their own right, and a primary material for efficient thermoelectric generators.

The discovery is outlined in the new Energy & Environmental Science paper, Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films.

Lead authors of the report included Jeffrey Blackburn and Andrew Ferguson, two senior scientists in NREL’s Chemical and Materials Science and Technology center.

New Reports

The latest research follows two previous NREL papers addressing SWCNTs. The first report, featured in Nature Energy, demonstrated the carbon nanotubes’ effectiveness in thermoelectric applications; the films prepared in the study, however, retained a large amount of insulating polymer.

The second paper, in ACS Energy Letters, revealed that removing the polymer from a SWNCT thin film improved thermoelectric properties.

The new report, released in October, asserts that eliminating polymers from all SWCNT starting materials improved how charge carriers move through the semiconductor. The polymer is removed with a solution-based acid.

“By removing these polymers, you actually get something like a factor of two improvement in the thermoelectric performance,” Ferguson told Tech Briefs.

Taking Charge: P-Type vs. N-Type

Most metals conduct electricity due to the flow of electrons, or negative charge carriers. "N-type” semiconductors possess negative charge carriers, and “p-type” semiconductors have positive charge carriers, otherwise known as “holes.”

Organic semiconducting polymers typically produce n-type materials that are more unstable and perform less effectively than their p-type counterparts.

In contrast to many semiconducting polymers, the energy laboratory’s semiconducting SWCNTs (s-SWCNTs) represent unique one-dimensional organic semiconductors. The semiconductors feature chemical and physical properties that facilitate equivalent transport of electrons and holes, or n-type and p-type carriers respectively.

NREL scientists Andrew Ferguson, left, and Jeffrey Blackburn stand in front of a screen displaying single-walled carbon nanotubes. (Image Credit: Dennis Schroeder/NREL)

The NREL researchers demonstrated that the same SWCNT thin films achieve equivalent thermoelectric performance when doped with either positive or negative charge carriers – an important finding, says Ferguson.

The identical performance, he said, suggests that carbon nanotube networks have the potential to be used for both the p-type and n-type legs in a thermoelectric device. P-type and n-type legs can be made from the same SWCNT material, inherently balancing the electrical current in each and simplifying device manufacturing.

“That opens up the possibility of fabricating a device that is essentially a single semiconductor material, and then creating p- and n-type regions in that semiconductor,” said Ferguson.

The same cannot be said of almost all inorganic semiconductor materials, said the senior scientist, which are typically n-type or p-type, but rarely both.

Generating New Applications

According to the team’s report, NREL’s combination of ink chemistry, solid-state polymer removal, and charge-transfer doping strategies enable n-type and p-type TE power factors, in the range of 700 μW m−1 K−2 at 298 K, for the thin films containing 100% s-SWCNTs.

“Our results indicate that the TE performance of s-SWCNT-only material systems is approaching that of traditional inorganic semiconductors, paving the way for these materials to be used as the primary components for efficient, all-organic TE generators,” said the authors in their Energy & Environmental Science abstract.

The team will next attempt to build prototype devices and demonstrate that the measured thin-film performance can be translated to a working thermoelectric generator. Although Ferguson acknowledges that applications are years away, the NREL scientist envisions the carbon nanotubes being initially employed in low-power tasks, like harvesting waste heat to support smart clothing or sensors.

“These could essentially be flexible fabric or devices that could act as a power source,” said Ferguson.

Other NREL authors of the Energy & Environmental Science paper included Bradley MacLeod, Rachelle Ihly, Zbyslaw Owczarczyk, and Katherine Hurst. The NREL researchers also teamed with collaborators from the University of Denver and partners at International Thermodyne, Inc., based in Charlotte, N.C.

What do you think? Will SWNCTs support sensors and other low-power applications? Share your thoughts below.

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