New Kind of Nanotubes Generate Electric Current When Exposed to Light

By Teodor Teofilov

An international team of researchers led by the University of Tokyo has discovered a new material which, when rolled into a nanotube, generates an electric current if exposed to light. If magnified and scaled up, say the scientists, the technology could be used in future high-efficiency solar devices.

University of Tokyo Professor Yoshihiro Iwasa was exploring possible functions of a special semiconductor nanotube together with an international team of physicists, when he had a lightbulb moment. He shone the light (a laser) on the nanotube and discovered something enlightening — certain wavelengths and intensities of light induced a current in the sample. This is called the photovoltaic (photovoltaic simply means they convert sunlight into electricity) effect and there are several materials with this property, but the nature and behavior of this nanotube is the cause for excitement.

While exploring the various functions of the semiconductor nanotube made from tungsten disulfide, the team discovered that it exhibited the photovoltaic effect at a far greater efficiency than other materials currently known to have the phenomenon. The photovoltaic effect happens when a current is generated through the entire structure of a material rather than relying on a junction between materials.

The study by the international team was published in the scientific journal Nature.

“Essentially our research material generates electricity like solar panels, but in a different way,” said Iwasa in a statement. “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.”

Transmission electron microscopy image of a hollow core nanotube. The white scale bar corresponds to 10 nanometers (left). Rendering of a nanotube (right). Image: © 2019 Iwasa et al.Enter a caption

Tungsten disulfide only exhibits the photovoltaic effect when it is rolled-up into nanotubes. This is an emergent behavior that is not intrinsic to the material unless it is modified. The effect occurs because the nanotube isn’t symmetrical and the current generated has a direction it prefers to flow in. If it were symmetrical, the current wouldn’t have a preferred direction and would not flow.

There have been other materials that have a similar “broken inversion symmetry” structure that exhibit the photovoltaic effect, but the researchers found that the tungsten disulfide nanotubes have a conversion efficiency much higher than previously observed.

How We Harness Solar Energy

Solar cells are what converts sunlight into electricity. The cells are made of materials that can carry electrical current and help capture light. When the photons of sunlight hit a photovoltaic cell, they are either reflected, pass through or absorbed and only the absorbed ones provide energy that can be used for electricity.

The photons knock electrons free from atoms and generate a flow of electricity in the solar cells. A solar panel is comprised of many of these photovoltaic cells that are linked together. Each cell is basically made of two slices of semiconducting material, usually silicon.

To be able to work, the photovoltaic cells need to establish an electric field and much like a magnetic one, which happens because of the opposite poles, the electric field happens when opposite charges are separated. For this to happen, manufacturers need to infuse silicon with other materials, giving each side a negative or positive charge.

The top layer of silicon is seeded with phosphorus, which adds extra electrons and gives a negative charge to the layer, while the bottom is dosed with boron, which results in fewer electrons and a positive charge. This all creates the electric field in the junction between the silicon layers. When a photon knocks an electron free, the electric field pushes it out of the silicon junction. Then metal conductive plates on the sides of the cell collect the electrons and transfer them to wires, where the electrons flow like any other source of electricity.

Encyclopedia Britannica explains the effect in a solar cell using an analogy to a child on a slide:

“Initially, both the electron and the child are in their respective “ground states.” Next, the electron is lifted up to its excited state by consuming energy received from the incoming light, just as the child is lifted up to an “excited state” at the top of the slide by consuming chemical energy stored in his body. In both cases there is now energy available in the excited state that can be expended. In the absence of junction-forming materials, there is no incentive for excited, free electrons to move along a specific direction; they eventually fall back to the ground state. On the other hand, whenever two different materials are placed in contact, an electric field is generated along the contact. This is the so-called built-in field, and it exerts a force on free electrons, effectively “tilting” the electron states and forcing the excited free electrons into an external electrical load where their excess energy can be dissipated. The external load can be a simple resistor, or it can be any of a myriad of electrical or electronic devices ranging from motors to radios. Correspondingly, the child moves to the slide because of his desire for excitement. It is on the slide that the child dissipates his excess energy. Finally, when the excess energy is expended, both the electron and the child are back in the ground state, where they can begin the whole process over again. The motion of the electron, like that of the child, is in one direction, as can be seen from the figure. In short, the photovoltaic effect produces a direct current (DC)—one that flows constantly in only a single direction.”

A More Efficient Solar Panel

Solar panels generally make use of this type of arrangement of materials, which is called a p-n junction. The two different kinds of materials (p-type and n-type) are attached and together generate a current in the presence of light, whereas alone they don’t. This type of junction based solar cells have improved in efficiency over the past 80 years since their discovery.

However, these cells are getting near their theoretical limit (called the Shockley Queisser Efficiency Limit) of 33 percent, which was first calculated by William Shockley and Hans Queisser in 1961. In 2012, the best silicon cell efficiency was 24 percent.

The discovery by the researchers of the tungsten disulfide nanotubes is great news because it doesn’t rely on a junction to have the photovoltaic effect, as previously mentioned, because the nanotube isn’t symmetrical. This allows it to exhibit the bulk photovoltaic effect (BPVE), which isn’t present in symmetrical nanotubes, such as carbon nanotubes (which are great electrical conductors).

“Our research shows an entire order of magnitude improvement in efficiency of BPVE compared to its presence in other materials,” said 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.”

The researchers hope that his kind of material can allow for the creation of more efficient solar panels in the long run. However, with size constraints in the near future, it is more likely there will be other applications, such as creating more sensitive and more accurate optical or infrared sensors. These can even have further applications in monitoring devices, sensor-filled self-driving cars or even sensors for imaging for astronomical telescopes.

“My colleagues from around the world and I eagerly explore the potential of this unprecedented technology,” said Iwasa. “For me, the idea of creating new materials beyond anything nature could provide is a fascinating reward in its own right.”

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