Asymmetrical Nanotubes Found to be Photovoltaic with no P-N Junction

July 02, 2019 by Scott McMahan

A team of physicists discovered a novel kind of nanotube that can generate current in the presence of light. Devices including infrared imaging chips and optical sensors are potential applications for the technology. In the future, if the effect can be magnified and the technology scaled up, it could lead to a new kind of high-efficiency solar power devices.

Working with an international team of physicists, University of Tokyo Professor Yoshihiro Iwasa was investigating possible functions of a unique semiconductor nanotube.

He decided to expose the nanotube to light from a laser. Iwasa found that particular wavelengths and intensities of light generated a current in the sample.

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"Essentially our research material generates electricity like solar panels, but in a different way," said Iwasa. "Together with Dr. 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-generating material is fabricated from a special semiconductor material based on tungsten disulfide (WS2). Uniquely, the material does not produce a current in the presence of light unless it is rolled into tubes. It is especially interesting because of how it differs from existing photovoltaic materials.

Generally, photovoltaic solar cells employ a particular arrangement of materials called a p-n junction. This critical structure is where two different kinds of materials (p-type and n-type) are attached, which by themselves do not produce a current in the presence of light.

However, when they are place together they do. While p-n junction-based photovoltaics have improved in efficiency over the 80 years or so since their discovery, some kinds of the photovoltaics are approaching their theoretical limits in terms of efficiency.

WS2 nanotubes do not rely on a junction between materials to gain the photovoltaic effect. When exposed to light, WS2 nanotubes produce a current throughout their entire structure.

This bulk photovoltaic effect (BPVE) occurs only because the WS2 nanotube is not symmetrical. If it were symmetrical, the current generated would not have a preferred direction and, therefore, would not flow.

So other symmetrical nanotubes including the famous 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 WS2 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, the researchers hope this kind of material could allow fabrication of more efficient solar panels.

The foreseeable size constraints in the near term, make its use more likely in other applications such as more sensitive and higher-fidelity optical or infrared sensors.

Other potential applications have include 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."

Reference Material

Zang, Y. J., Ideue, T., Onga, M., Qin, F., Suzuki, R., Zak, A., Tenne, R., Smet, J. H., and Iwasa, Y. "Enhanced intrinsic photovoltaic effect in tungsten disulfide nanotubes," Nature: June 19, 2019, doi:10.1038/s41586-019-1303-3.