Power/Performance Bits: Nov. 10

Singing to your storage; record efficiency in perovskite/silicon tandem solar cell; add hydrogen for enhanced graphene electrodes.


Singing to your storage

Existing research on ‘racetrack memory’, which uses tiny magnetic wires, each one hundreds of times thinner than a human hair, down which magnetic bits of data run like racing cars around a track, has focused on using either magnetic fields or electric currents to move the data bits down the wires. However, both these options create heat and reduce power efficiency.

However, a team from the University of Sheffield and the University of Leeds came up with a completely new solution: passing sound waves across the surface on which the wires are fixed. They also found that the direction of data flow depends on the pitch of the sound generated – in effect they “sang” to the data to move it.

The sound they used is in the form of surface acoustic waves. Although already harnessed for use in electronics and other areas of engineering, this is the first time surface acoustic waves have been applied to a data storage system.

Dr. Tom Hayward, from Sheffield’s Faculty of Engineering, said: “The key advantage of surface acoustic waves in this application is their ability to travel up to several centimeters without decaying, which at the nano-scale is a huge distance. Because of this, we think a single sound wave could be used to ‘sing’ to large numbers of nanowires simultaneously, enabling us to move a lot of data using very little power. We’re now aiming to create prototype devices in which this concept can be fully tested.”

Record efficiency in perovskite/silicon tandem solar cell

Teams from the Helmholtz-Zentrum Berlin (HZB) and École Polytechnique Fédérale de Lausanne (EPFL) have successfully combined a silicon heterojunction solar cell with a perovskite solar cell monolithically into a tandem device.

Because perovskite layers absorb light in the blue region of the spectrum very efficiently, it is useful to combine these with silicon layers that primarily convert long-wavelength red and near-infrared light. Nevertheless, the construction of these kinds of tandem cells in a monolithic stack of deposited layers has been difficult. This is because for high efficiency perovskite cells, it is usually required to coat the perovskite onto titanium dioxide layers that must be previously sintered at about 500 degrees Celsius. However, at such high temperatures, the amorphous silicon layers that cover the crystalline silicon wafer in silicon heterojunction degrades.

To create this kind of monolithic tandem cell, the researchers deposited a layer of tin dioxide at low temperatures to replace the usually used titanium dioxide. A thin layer of perovskite could then be spin-coated onto this intermediate layer and covered with hole-conductor material.

perovskite silicon tandem cell sem HZB EPFL nov10

A cross section through the tandem cell is shown in a SEM-image. (Source: HZB)

With an efficiency of 18%, the tandem cell surpassed the efficiency level of individual cells by nearly 20%, and is the highest currently reported value for its type of device architecture. The cell’s open-circuit voltage was 1.78 volts. “At that voltage level, this combination of materials could even be used for the generation of hydrogen from sunlight,” says Dr. Steve Albrecht, lead author of the paper.

“This perovskite-silicon tandem cell is presently still being fabricated on a polished silicon wafer. By texturing this wafer with light-trapping features, such as random pyramids, the efficiency might be increased further to 25 or even 30 percent,” says Dr. Lars Korte of HZB.

Add hydrogen for enhanced graphene electrodes

The growing demand for energy storage emphasizes the urgent need for higher-performance batteries. Several key characteristics of lithium ion battery performance – capacity, voltage and energy density – are ultimately determined by the binding between lithium ions and the electrode material. Subtle changes in the structure, chemistry and shape of an electrode can significantly affect how strongly lithium ions bond to it.

Commercial applications of graphene materials for energy storage devices, including lithium ion batteries and supercapacitors, hinge critically on the ability to produce these materials in large quantities and at low cost. However, the chemical synthesis methods frequently used leave behind significant amounts of atomic hydrogen, whose effect on the electrochemical performance of graphene derivatives is difficult to determine.

However, according to scientists at the Lawrence Livermore National Laboratory, that hydrogen might be a very good thing: deliberate low-temperature treatment of defect-rich graphene with hydrogen can actually improve rate capacity. Hydrogen interacts with the defects in the graphene and opens small gaps to facilitate easier lithium penetration, which improves the transport. Additional reversible capacity is provided by enhanced lithium binding near edges, where hydrogen is most likely to bind.

“We found a drastically improved rate capacity in graphene nanofoam electrodes after hydrogen treatment. By combining the experimental results with detailed simulations, we were able to trace the improvements to subtle interactions between defects and dissociated hydrogen. This results in some small changes to the graphene chemistry and morphology that turn out to have a surprisingly huge effect on performance,” said LLNL scientist Brandon Wood, who directed the theory effort on the paper.

To study the involvement of hydrogen and hydrogenated defects in the lithium storage ability of graphene, the team applied various heat treatment conditions combined with hydrogen exposure and looked into the electrochemical performance of 3-D graphene nanofoam electrodes, which are comprised chiefly of defective graphene. The team used 3-D graphene nanofoams due to their numerous potential applications, including hydrogen storage, catalysis, filtration, insulation, energy sorbents, capacitive desalination, supercapacitors and lithium ion batteries.