Photovoltaic Nanotubes

Transmission electron microscopy image of a hollow core nanotube and a rendering of this novel kind of nanotube. (Source: Iwasa et al.)

Physicists discovered a novel kind of nanotube that generates current in the presence of light. Devices such as optical sensors and infrared imaging chips are likely appli­cations, which could be useful in fields such as automated transport and astronomy. In future, if the effect can be magnified and the tech­nology scaled up, it could lead to high-effi­ciency solar power devices.

Working with an international team of physicists, Yoshihiro Iwasa from the University of Tokyo was exploring possible functions of a special semi­conductor nanotube when he had a lightbulb moment. He took this proverbial lightbulb and shone it on the nanotube to discover something enlightening. Certain wavelengths and inten­sities of light induced a current in the sample. There are several photo­voltaic materials, but the nature and behavior of this nanotube is cause for excitement.

“Essentially our research material generates elec­tricity like solar panels, but in a different way,” said Iwasa. “Together with Yijin Zhang from the Max Planck Institute for Solid State Research in Germany, we demonstrated for the first time nano­materials could overcome an obstacle that will soon limit current solar technology. For now solar panels are as good as they can be, but our tech­nology could improve upon that.”

The current-inducing nanotube is made from rolled-up sheets of a special semi­conductor material based on tungsten disulfide. The sheets do not induce a current in the presence of light unless rolled into tubes. This is an emergent behavior, one not intrinsic to the material until it’s modified. What is interesting is how it differs from existing photo­voltaic materials.

Generally, photo­voltaic solar panels make use of a certain arrangement of materials, a p-n junction. This is where p-type and n-type of materials are attached, which alone do not generate a current in the presence of light, but when placed together, do. P-n junction-based photo­voltaics have improved in efficiency over the 80 years or so since their discovery. However, they are getting close to their theoretical limits due in part to their need for the arrange­ment of multiple materials.

Tungsten disulfide nanotubes do not rely on a junction between materials to gain the photovoltaic effect. When exposed to light, they generate a current throughout their entire structure or bulk. This bulk photo­voltaic effect (BPVE) occurs as the nanotube is not symmetrical if you were to reverse it. If it were symmetrical, the current induced would not have a preferred direction and thus would not flow. So other symme­trical nanotubes – such as carbon nanotubes – don’t exhibit BPVE despite being great electrical conductors.

“Our research shows an entire order of magnitude improve­ment in efficiency of BPVE compared to its presence in other materials,” continued Iwasa. “But despite this huge gain, our tungsten disulfide nanotube cannot yet compare to the generating potential of p-n junction materials. This is because the device is nanoscopic and will be difficult to make larger. But it is possible and I hope chemists are inspired to take on that challenge.”

In the long term, researchers hope this kind of material could allow fabrication of more efficient solar panels. But given the foreseeable size constraints in the near term, it’s more likely to find use in other appli­cations. BVPE could be used to create more sensitive and higher-fidelity optical or infrared sensors. These have further appli­cations in embedded monitoring devices, sensor-laden self-driving cars or even in the imaging sensors for astro­nomical telescopes. “My colleagues from around the world and I eagerly explore the potential of this unpre­cedented technology,” concluded Iwasa. “For me, the idea of creating new materials beyond anything nature could provide is a fasci­nating reward in its own right.” (Source: U. Tokyo)

Reference: Y. J. Zhang et al.: Enhanced intrinsic photovoltaic effect in tungsten disulfide nanotubes, Nature 570, 349 (2019); DOI: 0.1038/s41586-019-1303-3

Link: Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo, Japan

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