Power/Performance Bits: Feb. 8

Transparent sensor; graphene-enhanced heat pipes; large perovskite solar cells.


Transparent sensor
Researchers at Osaka University created a thin, flexible, transparent sensor using silver nanowire networks. High-resolution printing was used to fabricate the centimeter-scale cross-aligned silver nanowire arrays, with reproducible feature sizes from 20 to 250 micrometers. As a proof-of-concept for functionality, they used their arrays to detect electrophysiological signals from plants.

A patterned polymer surface was created to define the subsequent nanowire feature size. Using a glass rod to sweep silver nanowires across the pattern led to either parallel or cross-aligned nanowire networks.

“The sheet resistance of patterns less than 100 micrometers ranged from 25 to 170 ohms per square, and the visible light transmittance at 550 nanometers was 96% to 99%,” said Teppei Araki, an assistant professor at Osaka University. “These values are well-suited for transparent electronics.”

To test the sensor sheet, it was used to monitor the electric potential of Brazilian waterweed leaves. Because the nanowire arrays are transparent, the researchers were able to keep the leaf under visual observation while acquiring data over long periods of time. A 2- to 3-micrometer-thick device conformed to the surface of a leaf without causing damage.

“Our microelectrodes-based organic field-effect transistors exhibited excellent multi-functionality,” says Tsuyoshi Sekitani, a professor at Osaka University. “For example, transparency of 90%, the on-off ratio was ~106, and the leakage current remained stable upon bending at a radius of 8 millimeters.”

The researchers plan on making further technical improvements, such as incorporating graphene onto the nanowire’s surface to improve the uniformity of the microelectrodes’ sheet resistance. Ultimately, they hope the technology will help minimize the raw material input of electronics and exceed the functionality of conventional non-transparent electronics.

Graphene-enhanced heat pipes
Researchers at Chalmers University, Shanghai University, SHT Smart High Tech, Marche Polytechnic University, and Fudan University used graphene-enhanced heat pipes to cool electronics and power systems.

“Heat pipes are one of the most efficient tools for this purpose, because of their high efficiency and unique ability to transfer heat over a large distance,” says Johan Liu, Professor of Electronics Production, at the Department of Microtechnology and Nanoscience at Chalmers.

Typically, heat pipes are made of copper, aluminum, or their alloys. However, these materials have relatively high density and limited heat transmission capacity. So the team turned to graphene-enhanced pipes, which exhibited a specific thermal transfer coefficient which is about 3.5 times better than that of copper-based heat pipe.

Graphene-enhanced heat pipes can efficiently cool power electronics. Credit: Ya Liu and Johan Liu / Chalmers University of Technology

The new graphene-enhanced heat pipes are made of high thermal conductivity graphene assembled films assisted with carbon fiber wicker enhanced inner surfaces. The researchers tested pipes of 6mm outer diameter and 150mm length. According to the researchers, they show great advantages and potential for cooling of a variety of electronics and power systems, especially where low weight and high corrosion resistance are required.

“The condenser section, the cold part of the graphene-enhanced heat pipe, can be substituted by a heat sink or a fan to make the cooling even more efficient when applied in a real case,” explains Ya Liu, PhD Student at the Electronics Materials and Systems Laboratory at Chalmers.

The team said the new heat pipes could be used in lightweight and large capacity cooling applications, such as avionics, automotive electronics, laptop computers, handsets, data centers as well as space electronics.

Large perovskite solar cells
Researchers at Stanford University and the National Renewable Energy Laboratory developed a way to make large scale perovskite solar cells that is quick and cost-effective.

“Perovskite solar technology is at a crossroads between commercialization and flimflammery,” said Nick Rolston, a postdoctoral scholar at Stanford. “Millions of dollars are being poured into startups. But I strongly believe that in the next three years, if there isn’t a breakthrough that extends cell lifetimes, that money will start to dry up.”

While offering promising efficiencies, perovskite solar cells suffer from instability and early degradation when exposed to heat and moisture. Plus, most studies focus on creating small test cells – and have trouble scaling them up to commercial sizes.

“Most work done on perovskites involves really tiny areas of active, usable solar cell. They’re typically a fraction of the size of your pinky fingernail,” said Rolston. Attempts to make bigger cells have produced defects and pinholes that significantly decrease cell efficiency.

“You can make a small demonstration device in the lab,” added Reinhold Dauskardt, a professor in the Stanford School of Engineering. “But conventional perovskite processing isn’t scalable for fast, efficient manufacturing.”

To address this, the team used a technique called rapid-spray plasma processing. This technology uses a robotic device with two nozzles to quickly produce thin films of perovskite. One nozzle spray-coats a liquid solution of perovskite chemical precursors onto a pane of glass, while the other releases a burst of highly reactive ionized gas.

“Conventional processing requires you to bake the perovskite solution for about half an hour,” Rolston said. “Our innovation is to use a plasma high-energy source to rapidly convert liquid perovskite into a thin-film solar cell in a single step.”

A perovskite solar module produced by rapid-spray plasma processing. Stanford Prof. Reinhold Dauskardt’s lab has shown that perovskite modules can be produced cheaper and four times faster than conventional silicon panels. (Credit: Nick Rolston / Stanford University)

Using their method, the researchers were able to produce 40 feet (12 meters) of perovskite film per minute, a rate about four times faster than it takes to manufacture a silicon cell. Plus, the new perovskite cells achieved a power conversion efficiency of 18%. They also created perovskite modules that continued to operate at 15.5% efficiency after being left on the shelf for five months.

“We want to make this process as applicable and broadly useful as possible,” Rolston said. “A plasma treatment system might sound fancy, but it’s something you can buy commercially for a very reasonable cost.”

The team estimated that their perovskite modules can be manufactured for about 25 cents per square foot, less than the $2.50 or so per square foot needed to produce a typical silicon module. Next, they are exploring new encapsulation technologies and other ways to significantly improve durability, such as a weatherproof layer that could keep out moisture for at least a decade.

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