Power/Performance Bits: May 31

Solar thermophotovoltaics; graphene tunnel transistors; silver nanowire touchscreens.

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Solar thermophotovoltaics

A team of MIT researchers demonstrated a device based on a method that enables solar cells to break through a theoretically predicted ceiling on how much sunlight they can convert into electricity.

Since 1961 it has been known that there is an absolute theoretical limit, called the Shockley-Queisser Limit, to how efficient traditional solar cells can be in their energy conversion. For a single-layer cell made of silicon, that upper limit is about 32%. But it has also been known that there are some possible avenues to increase that overall efficiency, such as by using multiple layers of cells or by converting the sunlight first to heat before generating electrical power. It is the latter method, using devices known as solar thermophotovoltaics, or STPVs, that caught the team’s interest.

“We believe that this new work is an exciting advancement in the field,” said Evelyn Wang, a professor at MIT, “as we have demonstrated, for the first time, an STPV device that has a higher solar-to-electrical conversion efficiency compared to that of the underlying PV cell.” In the demonstration, the team used a relatively low-efficiency PV cell, so the overall efficiency of the system was only 6.8%, but it clearly showed, in direct comparisons, the improvement enabled by the STPV system.

(Source: MIT)

(Source: MIT)

The key was using nanophotonic crystals, which can be made to emit precisely determined wavelengths of light when heated. In this test, the nanophotonic crystals were integrated into a system with vertically aligned carbon nanotubes, and operate at a high temperature of 1,000 degrees Celsius. Once heated, the nanophotonic crystals continue to emit a narrow band of wavelengths of light that precisely matches the band that an adjacent photovoltaic cell can capture and convert to an electric current.

In operation, this approach would use a conventional solar-concentrating system, with lenses or mirrors that focus the sunlight, to maintain the high temperature. An additional component, an advanced optical filter, lets through all the desired wavelengths of light to the PV cell, while reflecting back any unwanted wavelengths, since even this advanced material is not perfect in limiting its emissions. The reflected wavelengths then get re-absorbed, helping to maintain the heat of the photonic crystal.

Next steps for STPVs include finding ways to make larger versions of the small, laboratory-scale experimental unit, and developing ways of manufacturing such systems economically.

Graphene tunnel transistors

Scientists from the Moscow Institute of Physics and Technology (MIPT), the Institute of Physics and Technology RAS, and Tohoku University proposed a new design for a tunnel transistor based on bilayer graphene, and using modelling, demonstrated that the material is an ideal platform for low-voltage electronics.

Building transistors that are capable of operating below 0.5V supply voltage is a great challenge of modern electronics. Tunnel transistors (TFETs) are a promising candidate: unlike in conventional transistors, where electrons jump through the energy barrier, in TFETs the electrons “filter” through the barrier due to the quantum tunneling effect. However, in most semiconductors the tunneling current is very small and this prevents transistors that are based on these materials from being used in real circuits.

Bands of bilayer graphene are in the shape of what the researchers are calling a “Mexican hat,” compared to the bands of most semiconductors which form a parabolic shape. It turns out that the density of electrons that can occupy spaces close to the edges of the “Mexican hat” tends to infinity, an effect called a van Hove singularity. With the application of even a very small voltage to the gate of a transistor, a huge number of electrons at the edges of the “hat” begin to tunnel at the same time. This causes a sharp change in current from the application of a small voltage, and this low voltage is the reason for the record low power consumption.

(A) Electron spectrum E(p) in bilayer graphene (left) and energy dependence of its density of states, DoS (right). At energy levels corresponding to the edge of the "Mexican hat" the DoS tends to infinity. (B) The red areas show the states of electrons that participate in tunneling in bilayer graphene (left) and in a conventional semiconductor with "ordinary" parabolic bands (right). Electrons that are capable of tunneling at low voltages are found in the ring in graphene, but in the semiconductor they are only found at the single point. The dotted lines indicate the tunneling transitions. The red lines indicate the trajectories of the tunneling electrons in the valence band. (Source: MIPT)

(A) Electron spectrum E(p) in bilayer graphene (left) and energy dependence of its density of states, DoS (right). At energy levels corresponding to the edge of the “Mexican hat” the DoS tends to infinity. (B) The red areas show the states of electrons that participate in tunneling in bilayer graphene (left) and in a conventional semiconductor with “ordinary” parabolic bands (right). Electrons that are capable of tunneling at low voltages are found in the ring in graphene, but in the semiconductor they are only found at the single point. The dotted lines indicate the tunneling transitions. The red lines indicate the trajectories of the tunneling electrons in the valence band. (Source: MIPT)

“The point is not so much about saving electricity – we have plenty of electrical energy,” said Dmitry Svintsov, the head of MIPT’s Laboratory of Optoelectronics and Two-Dimensional Materials. “This means that the transistor requires less energy for switching, chips will require less energy, less heat will be generated, less powerful cooling systems will be needed, and clock speeds can be increased without the worry that the excess heat will destroy the chip.”

An important feature of the proposed transistor is the use of “electrical doping” (the field effect) to create a tunneling p-n junction. The complex process of chemical doping, which is required when building transistors on three-dimensional semiconductors, is not needed (and can even be damaging) for bilayer graphene. In electrical doping, additional electrons (or holes) occur in graphene due to the attraction towards closely positioned doping gates.

Under optimum conditions, a graphene transistor can change the current in a circuit ten thousand times with a gate voltage swing of only 150 millivolts.

Silver nanowire touchscreens

Researchers from the University of Surrey in collaboration with M-SOLV Ltd, a touch-sensor manufacturer based in Oxford, investigated the suitability of silver nanowires for flexible, touch-screen technologies. Currently, touch screen devices mainly rely on electrodes made from indium tin oxide (ITO), a material that is expensive to source, expensive to process and very brittle.

“Our research hasn’t just identified silver nanowires as a viable replacement touchscreen material, but has gone one step further in showing how a process called ‘ultrasonication’ can allow us to tailor performance capabilities,” said Matthew Large, the first author on the research published in Scientific Reports. “By applying high frequency sound energy to the material we can manipulate how long the nanosized ‘rods’ of silver are. This allows us to tune how transparent or how conductive our films are, which is vital for optimising these materials for future technologies like flexible solar cells and roll-able electronic displays.”

The team also showed how silver nanowires can be processed using the same laser ablation technique commonly used to manufacture ITO devices, in a paper published in Materials Today Communications. Using this technique, the team produced a fully operating five inch multi-touch sensor, identical to those typically used in smartphone technology. They found it performed comparably to one based on ITO but used significantly less energy to produce.

(Source: Maria Cann et al./Materials Today Communications)

(Source: Maria Cann et al./Materials Today Communications)

Now based at the University of Sussex, the team is looking to develop the scalability of the process to make it more industrially viable. One limiting factor is the current cost of silver nanowires. Funded by Innovate UK and EPSRC, the team are collaborating with M-SOLV and a graphene supplier Thomas Swan to use a nanowire and graphene combination in the electrodes to markedly reduce the cost.