Power/Performance Bits: Nov. 15

Another record-breaking tandem perovskite solar cell; invisibility for photonic devices; gold-plated DNA nanowires.

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Another record-breaking tandem perovskite solar cell

University of California, Berkeley, and Lawrence Berkeley National Laboratory scientists report a new design for perovskite solar cells that achieves an average steady-state efficiency of 18.4%, with a high of 21.7% and a peak efficiency of 26%.

“This has a great potential to be the cheapest photovoltaic on the market, plugging into any home solar system,” said Onur Ergen, a UC Berkeley physics graduate student.

The work is based on a new way to combine two perovskite solar cell materials – each tuned to absorb a different wavelength or color of sunlight – into one “graded bandgap” solar cell that absorbs nearly the entire spectrum of visible light.

The key to mating the two materials into a tandem solar cell is a single-atom thick layer of hexagonal boron nitride. In this case, the perovskite materials are made of the organic molecules methyl and ammonia, but one contains the metals tin and iodine, while the other contains lead and iodine doped with bromine. The former is tuned to preferentially absorb light with an energy of 1 electron volt (eV) – infrared, or heat energy – while the latter absorbs photons of energy 2 eV, or an amber color.

The perovskite/boron nitride sandwich is placed atop a lightweight aerogel of graphene that promotes the growth of finer-grained perovskite crystals, serves as a moisture barrier and helps stabilize charge transport though the solar cell. Moisture makes perovskite fall apart.

Cross section of the new solar cell, showing the two perovskite layers (beige and red) separated by a single-atom layer of boron nitride and the thicker graphene aerogel (dark gray), which prevents moisture from destroying the perovskite. Gallium nitride (blue) and gold (yellow) electrodes channel the electrons generated when light hits the solar cell. (Source: UC Berkeley)

Cross section of the new solar cell, showing the two perovskite layers (beige and red) separated by a single-atom layer of boron nitride and the thicker graphene aerogel (dark gray), which prevents moisture from destroying the perovskite. Gallium nitride (blue) and gold (yellow) electrodes channel the electrons generated when light hits the solar cell. (Source: UC Berkeley)

The whole thing is capped at the bottom with a gold electrode and at the top by a gallium nitride layer that collects the electrons that are generated within the cell. The active layer of the thin-film solar cell is about 400 nanometers thick.

It is possible to add even more layers of perovskite separated by hexagonal boron nitride, though this may not be necessary, given the broad-spectrum efficiency they’ve already obtained, the researchers said.

“People have had this idea of easy-to-make, roll-to-roll photovoltaics, where you pull plastic off a roll, spray on the solar material, and roll it back up,” said Alex Zettl, a UC Berkeley professor of physics and senior faculty member at Berkeley Lab. “With this new material, we are in the regime of roll-to-roll mass production; it’s really almost like spray painting.”

Invisibility for photonic devices

Engineers at the University of Utah developed a cloaking device for photonic integrated devices in an effort to make future chips smaller, faster and consume much less power.

While photonic chips hold advantages, if two of these photonic devices are too close to each other, they will not work because the light leakage between them will cause crosstalk much like radio interference. If they are spaced far apart to solve this problem, the chip that is much too large.

The team decided to address this by placing a special nanopatterened silicon-based barrier in between two of the photonic devices, which acts like a “cloak” and tricks one device from not seeing the other.

“The principle we are using is similar to that of the Harry Potter invisibility cloak,” said Rajesh Menon, associate professor of electrical and computer engineering at University of Utah. “Any light that comes to one device is redirected back as if to mimic the situation of not having a neighboring device. It’s like a barrier — it pushes the light back into the original device. It is being fooled into thinking there is nothing on the other side.”

Menon believes the most immediate application for this technology and for photonic chips in general will be for data centers. According to a study from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, data centers just in the U.S. consumed 70 billion kilowatt hours in 2014, or about 1.8 percent of total U.S. electricity consumption. And that power usage is expected to rise another 4% by 2020.

Gold-plated DNA nanowires

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and Paderborn University constructed a gold-plated, DNA-based interconnect capable of conducting a current.

In order to produce the nanowires, the researchers combined a long single strand of genetic material with shorter DNA segments through the base pairs to form a stable double strand. Using this method, the structures independently take on the desired form.

“With the help of this approach, which resembles the Japanese paper folding technique origami and is therefore referred to as DNA-origami, we can create tiny patterns,” said Artur Erbe, HZDR researcher. “Extremely small circuits made of molecules and atoms are also conceivable here.”

There is, however, a problem: “Genetic matter doesn’t conduct a current particularly well,” said Erbe. So, the team placed gold-plated nanoparticles on the DNA wires using chemical bonds. Using electron beam lithography they subsequently make contact with the individual wires electronically. “This connection between the substantially larger electrodes and the individual DNA structures have come up against technical difficulties until now. By combining the two methods, we can resolve this issue. We could thus very precisely determine the charge transport through individual wires for the first time,” added Erbe.

“We are actually still in the basic research phase, which is why we are using gold rather than a more cost-efficient metal. We have, nevertheless, made an important stride, which could make electronic devices based on DNA possible in the future.”

In order to improve the conduction, the team aims to incorporate conductive polymers between the gold particles. They also believe the metallization process could be improved.