Semiconductor Value Chain

Over the Horizon: Graphene Photoelectric Converter, Nanotech Germanium, Photonic Topological Insulator, Peel-and-Stick Solar Cells

30 April 2013

Every month, Electronics360 Contributing Editor Abe Michelen surveys academic and technical journals to uncover promising research that will impact the development of “over the horizon” commercial products.

A revolutionary flexible, wearable photoelectric converter

Researchers at University of Exeter in the U.K. have combined the unique properties of graphene, the thinnest conductor material, and graphexeter, a lighter and more bendable transparent conductor material, to create a device that converts light into electrical signals with the same or increased efficiency as photoelectric devices known today.

Dr. Saverio Russo, a professor of physics at the university, and his team created an ultra-light, flexible, transparent and efficient photoelectric converter. The difference between the Exeter device and existing devices is that Saverio’s device does not contain any metallic components.

“This new flexible and transparent photosensitive device uses graphene and graphexeter to convert light into electrical signals with efficiency comparable to that found in opaque devices based on graphene and metals,” Saverio said.

Because of its transparency and flexibility, the device can be incorporated into clothing, thereby converting clothes into solar panels that can be used to charge phones and other devices while being worn. The researchers believe these photosensitive materials can be used to coat glass windows to collect energy, while retaining the window’s transparency.

Back to the future: Germanium faster than silicon

Silicon has been our ‘daily bread’ since the early days of transistor manufacturing. Germanium was the material used to build the first transistors more than 65 years ago, but became second to silicon for many reasons. One of which was the high speed which electrons move through silicon, also referred to as “carrier mobility.”

This month, researchers at Ohio State University used nanotechnology techniques to produce one-atom-thick germanium sheets that could replace silicon as the material of choice to manufacture electronic devices. This new material, researchers have found, conducts carriers ten times faster than silicon and five times faster than conventional germanium.

Dr. Joshua Goldberger and his fellow researchers think that germanium — not graphene — will be the material to replace silicon. “Most people think of graphene as the electronic material of the future,” Goldberger said, “but silicon and germanium are still the materials of the present. Sixty years’ worth of brainpower has gone into developing techniques to make chips out of them. So we’ve been searching for unique forms of silicon and germanium with advantageous properties, to get the benefits of a new material but with less cost and using existing technology.”

The chemical structure of this new material, called germanane, is similar to graphene and has similar electrical properties. Germanane is chemically more stable than silicon and it does not oxidize in air or water, which makes it easier to work with in electronics manufacturing.

Invention improves the quality of optical transmission in small devices.

A team of researchers from the Technion-Israel Institute of Technology and the Friedrich-Schiller University in Jena, Germany, have developed a device that will protect the transport and quality of light in photonic systems. The device is called photonic topological insulator. A description of the first such devices is published in the current issue of Nature, as well as the Jewish Business News.

This invention comes at the right time for the photonics and electronics industries. Photonics plays an important role in the telecommunication industry, and will become vital in the future of computers. As nodes in semiconductors become smaller and the density of chips becomes larger, there is a need to create better, faster and smaller communication links. However, when devices get smaller, many imperfections are formed during the fabrication process, causing optical links to ‘misbehave’ when carrying light. In this situation, light travels irregularly, scattering from any defect found in the link. The industry has been looking into new techniques to overcome this problem.

Professor Mordechai Segev’s group at the Technion, in collaboration with the team of researchers led by Professor Alex Szameit at the Friedrich-Schiller University, have developed just such a technique. The researchers found that by creating a special (helical) lattice of ’waveguides,’ arranged in a honeycomb lattice, light can be transmitted regularly even in the presence of deformities in the transmission medium. This phenomenon is called ‘topological protection.’ The light is topologically protected because it can flow uninterrupted even if the path is full of defects. Topological protection was achieved earlier for electrons flowing in solid materials. The two teams found out how to bring topological protection to photonic systems.

"Photonic topological insulators have the potential to provide an entirely new platform for probing and understanding topological protection," explained Dr. Mikael Rechtsman from the Technion. "For example, all sorts of experiments that would be difficult or impossible to carry out in solid-state materials can now be accessed using light."

"This discovery is another step in the progress towards optical and quantum computing," said Julia Zeuner, a graduate student at Friedrich-Schiller University, who fabricated the sophisticated photonic structure and did part of the experiments.

"We have discovered a completely novel phenomena," added Segev, "and new phenomena are destined to find applications in directions that we can't even imagine.

Peel-and-stick solar cells

A new method of fabricating solar cells may allow us to charge our smart phones or power small devices just by sticking on a thin-film solar cell that is flexible and cheap to produce. A team of Stanford University researchers working with a team of researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) developed a method for the fabrication of thin film solar cells (TFSCs) on substrates other than silicon or glass. This is an important achievement because the conventional fabrication process is not suitable for all types of substrates. A scientific paper published in the online version of Scientific Reports – a subsidiary of the British journal Nature – accounts for the method.

The researchers developed the peel-and-stick method by creating the thin-film solar cells (less than a micron thick) using conventional methods on silicon substrate. Then the solar cells are removed (peeled) from the silicon substrate by dipping them in water at room temperature. After exposure to heat at about 90°C for a few seconds, the cells can be attached to almost any surface. The peel-and-stick method, also known as water-assisted transfer printing (WTP), was developed at Stanford where it has been used for electronics based on nanowires. "We were able to peel it off nicely and test the cell both before and after. We found almost no degradation in performance due to the peel-off," said Xiaolin Zheng, a researcher at Stanford,

The cells can be attached to almost any surface because the method does not require any fabrication steps when attached to the final substrate.

“The cells' ability to adhere to a universal substrate is unusual; most thin-film cells must be affixed to a special substrate,” said NREL Principal Scientist Qi Wang.

“The peel-and-stick approach allows the use of flexible polymer substrates and high processing temperatures. The resulting flexible, lightweight, and transparent devices then can be integrated onto curved surfaces such as military helmets, portable electronics and sensors,” he said.

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