Category Archives: Applications

Researchers from Moscow Institute of Physics and Technology (MIPT), Skolkovo Institute of Science and Technology (Skoltech), the Technological Institute for Superhard and Novel Carbon Materials (TISNCM), the National University of Science and Technology MISiS (Russia), and Rice University (USA) used computer simulations to find how thin a slab of salt has to be in order for it to break up into graphene-like layers. Based on the computer simulation, they derived the equation for the number of layers in a crystal that will produce ultrathin films with applications in nanoelectronics. Their findings were in The Journal of Physical Chemistry Letters (which has an impact factor of 8.54).

Transition from a cubic arrangement into several hexagonal layers. Credit: Authors of the study

Transition from a cubic arrangement into several hexagonal layers. Credit:
Authors of the study

From 3D to 2D

Unique monoatomic thickness of graphene makes it an attractive and useful material. Its crystal lattice resembles a honeycombs, as the bonds between the constituent atoms form regular hexagons. Graphene is a single layer of a three-dimensional graphite crystal and its properties (as well as properties of any 2D crystal) are radically different from its 3D counterpart. Since the discovery of graphene, a large amount of research has been directed at new two-dimensional materials with intriguing properties. Ultrathin films have unusual properties that might be useful for applications such as nano- and microelectronics.

Previous theoretical studies suggested that films with a cubic structure and ionic bonding could spontaneously convert to a layered hexagonal graphitic structure in what is known as graphitisation. For some substances, this conversion has been experimentally observed. It was predicted that rock salt NaCl can be one of the compounds with graphitisation tendencies. Graphitisation of cubic compounds could produce new and promising structures for applications in nanoelectronics. However, no theory has been developed that would account for this process in the case of an arbitrary cubic compound and make predictions about its conversion into graphene-like salt layers.

For graphitisation to occur, the crystal layers need to be reduced along the main diagonal of the cubic structure. This will result in one crystal surface being made of sodium ions Na? and the other of chloride ions Cl?. It is important to note that positive and negative ions (i.e. Na? and Cl?)–and not neutral atoms–occupy the lattice points of the structure. This generates charges of opposite signs on the two surfaces. As long as the surfaces are remote from each other, all charges cancel out, and the salt slab shows a preference for a cubic structure. However, if the film is made sufficiently thin, this gives rise to a large dipole moment due to the opposite charges of the two crystal surfaces. The structure seeks to get rid of the dipole moment, which increases the energy of the system. To make the surfaces charge-neutral, the crystal undergoes a rearrangement of atoms.

Experiment vs model

To study how graphitisation tendencies vary depending on the compound, the researchers examined 16 binary compounds with the general formula AB, where A stands for one of the four alkali metals lithium Li, sodium Na, potassium K, and rubidium Rb. These are highly reactive elements found in Group 1 of the periodic table. The B in the formula stands for any of the four halogens fluorine F, chlorine Cl, bromine Br, and iodine I. These elements are in Group 17 of the periodic table and readily react with alkali metals.

All compounds in this study come in a number of different structures, also known as crystal lattices or phases. If atmospheric pressure is increased to 300,000 times its normal value, an another phase (B2) of NaCl (represented by the yellow portion of the diagram) becomes more stable, effecting a change in the crystal lattice. To test their choice of methods and parameters, the researchers simulated two crystal lattices and calculated the pressure that corresponds to the phase transition between them. Their predictions agree with experimental data.

Just how thin should it be?

The compounds within the scope of this study can all have a hexagonal, “graphitic”, G phase (the red in the diagram) that is unstable in 3D bulk but becomes the most stable structure for ultrathin (2D or quasi-2D) films. The researchers identified the relationship between the surface energy of a film and the number of layers in it for both cubic and hexagonal structures. They graphed this relationship by plotting two lines with different slopes for each of the compounds studied. Each pair of lines associated with one compound has a common point that corresponds to the critical slab thickness that makes conversion from a cubic to a hexagonal structure energetically favourable. For example, the critical number of layers was found to be close to 11 for all sodium salts and between 19 and 27 for lithium salts.

Based on this data, the researchers established a relationship between the critical number of layers and two parameters that determine the strength of the ionic bonds in various compounds. The first parameter indicates the size of an ion of a given metal–its ionic radius. The second parameter is called electronegativity and is a measure of the ? atom’s ability to attract the electrons of element B. Higher electronegativity means more powerful attraction of electrons by the atom, a more pronounced ionic nature of the bond, a larger surface dipole, and a lower critical slab thickness.

And there’s more

Pavel Sorokin, Dr. habil., is head of the Laboratory of New Materials Simulation at TISNCM. He explains the importance of the study, ‘This work has already attracted our colleagues from Israel and Japan. If they confirm our findings experimentally, this phenomenon [of graphitisation] will provide a viable route to the synthesis of ultrathin films with potential applications in nanoelectronics.’

The scientists intend to broaden the scope of their studies by examining other compounds. They believe that ultrathin films of different composition might also undergo spontaneous graphitisation, yielding new layered structures with properties that are even more intriguing.

STMicroelectronics (NYSE:STM) today announced that it has acquired ams’ (SIX: AMS) assets related to NFC1 and RFID2 reader business. ST has acquired intellectual property, technologies, products and business highly complementary to its secure microcontroller solutions serving mobile devices, wearables, banking, identification, industrial, automotive and IoT markets. Approximately 50 technical experts from ams have been transferred to ST.

The acquired assets, combined with ST’s secure microcontrollers, position ST for a significant growth opportunity, with a complete portfolio of technologies, products and competencies that comprehensively address the full range of the NFC and RFID markets for a wide customer base.

“Security and NFC connectivity are key prerequisites for the broad rollout of mobile and IoT devices anticipated in the coming years. This acquisition builds on our deep expertise in secure microcontrollers and gives ST all of the building blocks to create the next generation of highly-integrated secure NFC solutions for mobile and for a broad range of Internet of Things devices,” said Claude Dardanne, Executive Vice President and General Manager of STMicroelectronics’ Microcontroller and Digital ICs Group. “We welcome this highly competent team from ams into ST for the benefit of our customers.”

The first NFC controller, leveraging the acquired assets, is already sampling to lead customers, as well as a new high-performance, highly-integrated System-in-Package solution which combines this NFC controller with ST’s secure element.

ST acquired the ams assets in exchange for a (i) cash payment of $77.8 million (funded with available cash), and (ii) deferred earn-out contingent on future results for which ST currently estimates will be about $13 million but which in any case will not exceed $37 million.

Beyond its gloom, the MEMS industry is showing numerous emerging devices that hold promise for future growth. These innovative MEMS solutions were listed by the MEMS & Sensors team of Yole Développement (Yole) in the Status of the MEMS Industry 2016 report (Yole Développement, May 2016). Today, more than 100 businesses, startups and large companies are involved in exciting developments using MEMS technology. The MEMS approach can be defined as a transfer function: It lowers cost and improves integration and performance.

transfer function

“MEMS can be seen as a ‘transfer function’ using semiconductor and micromachining technologies to create devices replacing devices that are more complex, bulky or less sensitive,” explains Dr. Eric Mounier, Sr. Technology & Market Analyst at Yole. Yole has identified at least 5 criteria that determine the success of a MEMS device. They are: size reduction, potential cost reduction, “good enough” specifications, batch manufacturing compared to existing solutions, and reliability.
At least 10 to 15 years of development are required to achieve all the successful criteria.

“Based on this segmentation, and out of all the MEMS devices in development that could undergo significant growth in the future, we foresee ultrasonic and gas sensors as well as microspeaker as the next success for the MEMS industry,” details Dr. Mounier.

As Yole’s market forecast announces, the gas sensor market is showing a 7.3% CAGR for the 2014–2021 period. The market should reach US$920 million in 2021. Moreover Yole’s analysts highlight a potential upside market of almost US$65 million in 2021. This positive scenario might be possible if gas sensors are widely adopted in consumer products, analysts say (Source: Gas Sensor Technology & Market report, Yole Développement, February 2016).

Microspeakers could be part of the success story as well. Indeed a big transition is happening now: for the first time, silicon speakers are ready for volume production, enabling the creation of a brand-new multibillion-dollar market for MEMS manufacturers. Last month, Yole’s analysts had an interesting interview with USound, an Austrian company founded three years ago by several veterans of the MEMS industry.

“Prototypes of the first balanced-armature replacement and the first micro-tweeter are currently being sampled to selected customers,” USound asserted. “Pre-production will start at the end of the summer, along with internal qualification. The technology is ready for adoption and will revolutionize the personal-audio market, similar to what happened with the MEMS microphone.”

USound intends to evolve into an audio-system developer, offering complete solutions ranging from hardware to firmware, in order to simplify technology adoption and help our customers achieve optimum product performance. To read the full interview, click USound.

For the next few months, Yole will pursue its investigation into the MEMS world. Numerous technology and market reports will be released, and Yole’s MEMS & Sensors team will attend many key conferences to present its vision of the industry.

For example, in mid-September Yole will be part of two major events in Asia: MEMS & Sensors Conference Asia and Sensor Expo & Conference – China. At both conferences, Yole will present attendees with the status of the industry and its new virtuous cycle. Yole’s Speaker, Claire Troadec, MEMS & Semiconductor Manufacturing Analyst, will focus her presentation on the Chinese MEMS industry, which is steadily transforming from “Made in China” to “Created in China.” Claire will also review the Chinese MEMS players and the new virtuous cycle the MEMS industry.

It may be clammy and inconvenient, but human sweat has at least one positive characteristic – it can give insight to what’s happening inside your body. A new study published in the ECS Journal of Solid State Science and Technology aims to take advantage of sweat’s trove of medical information through the development of a sustainable, wearable sensor to detect lactate levels in your perspiration.

“When the human body undergoes strenuous exercise, there’s a point at which aerobic muscle function becomes anaerobic muscle function,” says Jenny Ulyanova, CFD Research Corporation (CFDRC) researcher and co-author of the paper. “At that point, lactate is produce at a faster rate than it is being consumed. When that happens, knowing what those levels are can be an indicator of potentially problematic conditions like muscle fatigue, stress, and dehydration.”

Utilizing green technology

Using sweat to track changes in the body is not a new concept. While there have been many developments in recent years to sense changes in the concentrations of the components of sweat, no purely biological green technology has been used for these devices. The team of CFDRC researchers, in collaboration with the University of New Mexico, developed an enzyme-based sensor powered by a biofuel cell – providing a safe, renewable power source.

Biofuel cells have become a promising technology in the field of energy storage, but still face many issues related to short active lifetimes, low power densities, and low efficiency levels. However, they have several attractive points, including their ability to use renewable fuels like glucose and implement affordable, renewable catalysts.

“The biofuel cell works in this particular case because the sensor is a low-power device,” Ulyanova says. “They’re very good at having high energy densities, but power densities are still a work in progress. But for low-power applications like this particular sensor, it works very well.”

In their research, entitled “Wearable Sensor System Powered by a Biofuel Cell for Detection of Lactate Levels in Sweat,” the team powered the biofuel cells with a fuel based on glucose. This same enzymatic technology, where the enzymes oxidize the fuel and generate energy, is used at the working electrode of the sensor which allows for the detection of lactate in your sweat.

Targeting lactate

While the use of the biofuel cell is a novel aspect of this work, what sets it apart from similar developments in the field is the use of electrochemical processes to very accurately detect a specific compound in a very complex medium like sweat.

“We’re doing it electrochemically, so we’re looking at applying a constant load to the sensor and generating a current response,” Ulyanova says, “which is directly proportional to the concentration of our target analyte.”

Practical applications

Originally, the sensor was developed to help detect and predict conditions related to lactate levels (i.e. fatigue and dehydration) for military personnel.

“The sensor was designed for a soldier in training at boot camp,” says Sergio Omar Garcia, CFDRC researcher and co-author of the paper, “but it could be applied to people that are active and anyone participating in strenuous activity.”

As for commercial applications, the researchers believe the device could be used as a training aid to monitor lactate changes in the same way that athletes use heart rate monitors to see how their heart rate changes during exercise.

On-body testing

The team is currently working to redesign the physical appearance of the patch to move from laboratory research to on-body tests. Once the scientists optimize how the sensor adheres to the skin, its sweat sample delivery/removal, and the systems electronic components, volunteers will test its capabilities while exercising.

“We had actually talked about this idea to some local high school football coaches,” Ulyanova says, “and they seem to really like it and are willing to put forth the use of their players to beta test the idea.”

After initial data is gathered, the team will be able to work with other groups to interpret the data and relate it to the physical condition of the person. With this, predictive models could be built to potentially help prevent conditions related to individual overexertion.

Future plans for the device include implementing wireless transmission of results and the development of a suite of sensors (a hybrid sensor) that can detect various other biomolecules, indicative of physical or physiological stressors.

Despite strong double-digit percentage increases in annual unit shipments, semiconductor sensor sales growth has become uncharacteristically lethargic because of steep price erosion in several major product categories. Strong unit demand is being fueled by new wearable systems, greater automation in vehicles, and the much-anticipated Internet of Things (IoT), but sharply falling average selling prices (ASPs) on accelerometers, gyroscope chips, and magnetic-field measuring devices are capping annual growth of total sensor revenues in the low- to mid-single digit range, based on data in IC Insights’ 2016 O-S-D Report—A Market Analysis and Forecast for Optoelectronics, Sensors/Actuators, and Discretes.

The 2016 O-S-D Report shows worldwide dollar-volume revenues for sensors rising by a compound annual growth rate (CAGR) of 5.3% between 2015 and 2020 compared to an 8.9% annual rate in the last five years. In contrast, total sensor unit shipments are expected to climb by a CAGR of 12.4% in the five-year forecast period compared to a blistering 20.5% rate of increase in the 2010-2015 period, when new sensing, navigation, and automated embedded control functions in smartphones drove up strong growth along with steady increases in automotive and industrial applications.

Despite recent years of weak sales growth—just 1% in 2015 to $6.4 billion—the sensor market is expected to end this decade with 10 consecutive years of record-high revenues and reach $8.3 billion in 2020 (Figure 1). Unit shipments of sensors have reached record high levels each year since the beginning of the last decade—even in the 2009 downturn year, when worldwide unit volume grew 9% while sensor revenues dropped 3%. Record sensor shipments are expected to continue for another five years, reaching 28.9 billion units in 2020, according to the 360-page 2016 O-S-D Report, which contains a detailed five-year forecast of sales, unit volume, and ASPs for more than 30 individual product types and device categories in optoelectronics, sensors/actuators, and discretes.

Figure 1

Figure 1

Competition between suppliers and requirements for low-cost sensors in new high-volume applications drove down ASPs from about $0.66 in 2010 to $0.40 in 2015.  The need to squeeze more sensing solutions into wearable systems, far-flung IoT-connected applications, and multi-sensor packages for increased accuracy and multi-dimensional measurements is exerting more pricing pressure in the market, concludes the 2016 O-S-D Report.   The report’s forecast shows sensor ASPs dropping by a CAGR of  6.3% in the next five years to only $0.29.

Total sensor sales are expected to grow by about 3% in 2016 to $6.6 billion with worldwide shipments rising 13% to nearly 18.2 billion units this year.  Sales of sensors made with microelectromechanical systems (MEMS) technology (i.e., accelerometers, gyroscope devices, and pressure sensors, including microphone chips)—are expected to grow by 4% in 2016 to $4.8 billion with unit shipments increasing 10% to 7.6 billion.  The 2016 O-S-D Report projects MEMS-based sensor sales rising by a CAGR of 5.5% in the next five years to $6.1 billion in 2020 with unit shipments growing by an annual rate of 11.9% to nearly 13.4 billion.  ASPs for MEMS-based sensors are expected to decline by a CAGR of -5.7% to $0.45 in 2020 from $0.61 in 2015, according to the annual O-S-D Report.

Although shipments of microelectromechanical systems (MEMS) sensors used in automotive applications grew 8.4 percent in 2015, revenues were flat compared to the previous year, reaching $2.7 billion. In contrast, the value of this market is expected to recover this year, rising 4.3 percent to reach $2.8 billion in 2016, according to IHS Markit (Nasdaq: INFO).

The automotive MEMS market is forecast to grow at a compound annual growth rate of 6.9 percent from 2015 to 2022, to reach $3.2 billion in 2022. Global shipments will exceed two billion units for the first time at the end of this period, according to the IHS Markit Automotive Sensor Intelligence Service.

“Just three types of MEMS devices used in the automotive industry account for more than 95 percent of market value: pressure sensors, accelerometers and gyroscopes,” said Richard Dixon, principal analyst, automotive sensors, IHS Markit. “The primary systems relying on these devices are electronic stability control systems, airbags, tire-pressure monitors and manifold absolute-pressure sensors, although IHS tracks 34 other automotive MEMS applications.”

While these markets will remain, by their nature, still relatively small by 2022, the fastest growing volume applications in the coming years will include the detection of pedestrians, air-intake humidity measurement, microphones for hands-free calling in infotainment systems and microbolometers for night-vision systems used in driver assistance. New sensor areas on the horizon include scanning mirrors for head-up displays and adaptive LED headlights.

Top 10 automotive MEMS sensor suppliers

For second-tier suppliers of automotive sensors, 2015 was a good year. However, significant devaluations of the Euro and Yen affected the businesses of several companies. Leading Germany-based sensor supplier Robert Bosch was among the companies hit by exchange rate weakness, but its business continues to soar in local currency and shipments.

Sensata followed Bosch in the second-ranked position, exhibiting subdued 2015 revenue growth, despite last year’s acquisition of CST, including the sensor business of Kavlico. Along with its strong position in powertrain pressure sensors, Sensata benefits from its high-profile acquisition of Schrader, which made it the leading supplier of tire pressure monitors.

A name new to the MEMS sensor business is NXP, whose acquisition of Freescale last year catapulted the company into third-ranked position. NXP is known for its automotive magnetic sensors, while pressure sensors and accelerometers are the key sensors brought to the company via the Freescale acquisition.

The remaining seven companies also showed subdued results, with Japanese companies like Denso (ranked fourth) and Panasonic (ranked sixth). Both companies were adversely affected by the continued softness of the Yen.

Top_MEMS_Suppliers_IHS

It looks like a small piece of transparent film with tiny engravings on it, and is flexible enough to be bent into a tube. Yet, this piece of “smart” plastic demonstrates excellent performance in terms of data storage and processing capabilities. This novel invention, developed by researchers from the National University of Singapore (NUS), hails a breakthrough in the flexible electronics revolution, and brings researchers a step closer towards making flexible, wearable electronics a reality in the near future.

Associate Professor Yang Hyunsoo from the National University of Singapore, who led a research team to successfully embed a powerful magnetic memory chip on a plastic material, demonstrating the flexibility of the memory chip. Credit: National University of Singapore

Associate Professor Yang Hyunsoo from the National University of Singapore, who led a research team to successfully embed a powerful magnetic memory chip on a plastic material, demonstrating the flexibility of the memory chip. Credit: National University of Singapore

The technological advancement is achieved in collaboration with researchers from Yonsei University, Ghent University and Singapore’s Institute of Materials Research and Engineering. The research team has successfully embedded a powerful magnetic memory chip on a flexible plastic material, and this malleable memory chip will be a critical component for the design and development of flexible and lightweight devices. Such devices have great potential in applications such as automotive, healthcare electronics, industrial motor control and robotics, industrial power and energy management, as well as military and avionics systems.

The research team, led by Associate Professor Yang Hyunsoo of the Department of Electrical and Computer Engineering at the NUS Faculty of Engineering, published their findings in the journal Advanced Materials on 6 July 2016.

Flexible, high-performance memory devices a key enabler for flexible electronics 

Flexible electronics has become the subject of active research in recent times. In particular, flexible magnetic memory devices have attracted a lot of attention as they are the fundamental component required for data storage and processing in wearable electronics and biomedical devices, which require various functions such as wireless communication, information storage and code processing.

Although a substantial amount of research has been conducted on different types of memory chips and materials, there are still significant challenges in fabricating high performance memory chips on soft substrates that are flexible, without sacrificing performance.

To address the current technological challenges, the research team, led by Assoc Prof Yang, developed a novel technique to implant a high-performance magnetic memory chip on a flexible plastic surface.

The novel device operates on magnetoresistive random access memory (MRAM), which uses a magnesium oxide (MgO)-based magnetic tunnel junction (MTJ) to store data. MRAM outperforms conventional random access memory (RAM) computer chips in many aspects, including the ability to retain data after a power supply is cut off, high processing speed, and low power consumption.

Novel technique to implant MRAM chip on a flexible plastic surface

The research team first grew the MgO-based MTJ on a silicon surface, and then etched away the underlying silicon. Using a transfer printing approach, the team implanted the magnetic memory chip on a ?exible plastic surface made of polyethylene terephthalate while controlling the amount of strain caused by placing the memory chip on the plastic surface.

Assoc Prof Yang said, “Our experiments showed that our device’s tunneling magnetoresistance could reach up to 300 per cent – it’s like a car having extraordinary levels of horsepower. We have also managed to achieve improved abruptness of switching. With all these enhanced features, the flexible magnetic chip is able to transfer data faster.”

Commenting on the significance of the breakthrough, Assoc Prof Yang said, “Flexible electronics will become the norm in the near future, and all new electronic components should be compatible with flexible electronics. We are the first team to fabricate magnetic memory on a flexible surface, and this significant milestone gives us the impetus to further enhance the performance of flexible memory devices and contribute towards the flexible electronics revolution.”

Assoc Prof Yang and his team were recently granted United States and South Korea patents for their technology. They are conducting experiments to improve the magnetoresistance of the device by fine-tuning the level of strain in its magnetic structure, and they are also planning to apply their technique in various other electronic components. The team is also interested to work with industry partners to explore further applications of this novel technology.

STMicroelectronics has introduced a new line up of world’s smallest single-chip motor drivers for the portable and wearable applications. With the combination of low power consumption and small form factor, the ST’s new motor driver plans to contribute to the battery powered IoT device adoption.

The 3mm by 3mm motor drivers will focus on combining logic and power components in a single chip while taking care of power budgets and tight space. The drivers will operate from a supply voltage of 1.8V with the standby current of less than 80nA for a zero-power state. The drivers can also be used in a wide range of applications including robotic positioning systems, printer motors, camera-autofocus mechanisms, toothbrush motors or syringe pumps.

The existence of portable devices in everyday life is becoming more significant with the considerable increase in the use of well-developed battery-powered equipment with extended run-time that becomes smaller day by day.

“Our latest STSPIN single-chip devices are proven to simplify precision motor control and cut time to market for new products,” said Domenico Arrigo, General Manager Industrial and Power Conversion Division, STMicroelectronics. “The ultra-low power consumption extends runtime in battery-operated applications and enables designers to enhance portable and mobile devices with high-added-value motorized functions.”

ST’s new STSPIN motor devices are in production and are priced from $0.75 and $0.96 for order of 1,000 pieces.

With an eye to the next generation of tech gadgetry, a team of physicists at The University of Texas at Austin has had the first-ever glimpse into what happens inside an atomically thin semiconductor device. In doing so, they discovered that an essential function for computing may be possible within a space so small that it’s effectively one-dimensional.

In a paper published July 18 in the Proceedings of the National Academy of Sciences, the researchers describe seeing the detailed inner workings of a new type of transistor that is two-dimensional.

Transistors act as the building blocks for computer chips, sending the electrons on and off switches required for computer processing. Future tech innovations will require finding a way to fit more transistors on computer chips, so experts have begun exploring new semiconducting materials including one called molybdenum disulfide (MoS2). Unlike today’s silicon-based devices, transistors made from the new material allow for on-off signaling on a single flat plane.

Keji Lai, an assistant professor of physics, and a team found that with this new material, the conductive signaling happens much differently than with silicon, in a way that could promote future energy savings in devices. Think of silicon transistors as light bulbs: The whole device is either turned on or off at once. With 2-D transistors, by contrast, Lai and the team found that electric currents move in a more phased way, beginning first at the edges before appearing in the interior. Lai says this suggests the same current could be sent with less power and in an even tinier space, using a one-dimensional edge instead of the two-dimensional plane.

“In physics, edge states often carry a lot of interesting phenomenon, and here, they are the first to turn on. In the future, if we can engineer this material very carefully, then these edges can carry the full current,” Lai says. “We don’t really need the entire thing, because the interior is useless. Just having the edges running to get a current working would substantially reduce the power loss.”

Researchers have been working to get a view into what happens inside a 2-D transistor for years to better understand both the potential and the limitations of the new materials. Getting 2-D transistors ready for commercial devices, such as paper-thin computers and cellphones, is expected to take several more years. Lai says scientists need more information about what interferes with performance in devices made from the new materials.

“These transistors are perfectly two-dimensional,” Lai says. “That means they don’t have some of the defects that occur in a silicon device. On the other hand, that doesn’t mean the new material is perfect.”

Lai and his team used a microscope that he invented and that points microwaves at the 2-D device. Using a tip only 100 nanometers wide, the microwave microscope allowed the scientists to see conductivity changes inside the transistor. Besides seeing the currents’ motion, the scientists found thread-like defects in the middle of the transistors. Lai says this suggests the new material will need to be made cleaner to function optimally.

“If we could make the material clean enough, the edges will be carrying even more current, and the interior won’t have as many defects,” Lai says.

The paper’s other authors are postdoctoral researchers Di Wu and Xiao Li; research scientist Lan Luan, and graduate students Xiaoyu Wu and Zhaodong Chu, and professor Qian Niu in UT Austin’s Department of Physics; and graduate student Wei Li, former graduate student Maruthi N. Yogeesh, postdoctoral researcher Rudresh Ghosh, and associate professor Deji Akinwande of UT Austin’s Department of Electrical and Computer Engineering.

Earlier this year, both Lai and Akinwande won Presidential Early Career Awards for Scientists and Engineers, the U.S. government’s highest honor for early-stage scientists and engineers.

A novel three-dimensional solar cell design developed at Georgia Tech will soon get its first testing in space aboard the International Space Station. An experimental module containing 18 test cells was launched to the ISS on July 18, and will be installed on the exterior of the station to study the cells’ performance and their ability to withstand the rigors of space.

solar6_1

In addition to testing the three-dimensional format, the module will also study a low-cost copper-zinc-tin-sulfide (CZTS) solar cell formulation. In all, the module launched to the ISS contains four types of PV devices: 3-D cells based on conventional cadmium telluride, 3-D cells based on CZTS materials, traditional planar solar cells produced at Georgia Tech, and planar cells based on CZTS.

The experiment was aboard SpaceX’s Falcon 9 rocket that blasted off at 12:45 a.m. EDT from Cape Canaveral Air Force Station in Florida.

“We want to see both the light-trapping performance of our 3-D solar cells and how they are going to respond to the harshness of space,” said Jud Ready, a principal research engineer at the Georgia Tech Research Institute (GTRI) and an adjunct professor in the Georgia Tech School of Materials Science and Engineering. “We will also measure performance against temperature, because temperature has an influence on the performance of a solar cell.”

Built by coating miniature carbon nanotube “towers” with a photo-absorber that captures sunlight from all angles, the 3-D cells developed by Ready’s lab could boost the amount of power obtained from the small surface areas many spacecraft have. The cells would absorb light from any direction, eliminating the need for mechanical devices to aim PV modules toward the sun.

The PV cell experiment will be installed on the NanoRacks External Platform (NREP), where robustness of the solar cells will be studied under harsh space conditions for six months. The project is sponsored by the Center for the Advancement of Science in Space (CASIS), and the Space Station opportunity was provided by NanoRacks via its Space Act Agreement with NASA’s U.S. National Labs.

“The CZTS photovoltaic arrays were built using the readily available elements copper, zinc, tin and sulfur to replace rarer CIGS – copper, indium, gallium and selenium – which are used in similar thin-film solar cells,” said Ready. “The CZTS approach produces an efficient photo-absorber using earth-abundant materials that cost around a thousand times less than rare-earth elements like indium, gallium and selenium.”

One virtue of CZTS photovoltaic material is its electron band structure, Ready explained. Like CIGS, CZTS is a direct band gap material. In semiconductor physics, this means incoming solar photons are able to emit current-producing electrons directly, rather than moving through power-robbing intermediate states as indirect band gap materials, like silicon, require.

Moreover, Ready said, direct band gap materials have good resistance to the powerful ionizing radiation encountered in space. That’s because direct band gaps are larger than indirect band gaps; it’s harder for radiation to damage these larger gaps so severely that functionality is seriously impaired.

The 3-D capability could prove especially valuable on the International Space Station, which is exposed daily to 15-16 sunrises and sunsets as it orbits Earth every 92 minutes at 17,150 m.p.h. The 3-D towers can exploit the sun’s rays for longer periods than conventional 2-D planar – or flat – designs, which work most efficiently only when the sun is directly overhead.

“With our 3-D design, as the sun’s angle increases more surface is exposed and there’s a growing chance that photons will enter,” Ready said. “Also, 3-D technology provides more opportunity for photons to bounce around between the towers, increasing the likelihood they will be converted to electron hole pairs and produce mobile charge carriers.”

As the ISS orbits, the 3-D arrays’ performance will be compared to a high quality commercial 2-D planar cell array installed nearby. If things go as expected, GTRI’s cells will provide relatively better performance than the other cells as they move away from high noon. The new CZTS 3-D arrays will also be tested in space against an older 3-D design made by GTRI using cadmium telluride.

One of the GTRI development team’s key achievements to date has been identifying the best ways to manufacture CZTS solar cells. The team has pinpointed techniques for successfully processing the four Earth-abundant elements into an efficient photo absorber.

“In manufacturing you have to heat these elements, and one major issue is that they evaporate at different rates,” Ready explained. “Getting them to blend in the desired ratios, so that the stoichiometry is retained and electron levels of the constituent elements match up as they should, has been a challenge.”

GTRI’s photovoltaic arrays will be encased in Lexan containers aboard the ISS. Lexan, a clear yet strong polymer, produces minimal interference with incoming solar rays but can protect the delicate arrays from astronauts and space debris – and also protect the crew from any pieces of the arrays that might separate.

After the six-month mission, the solar cells will be sent back to Earth via a cargo ship. The research team will assess the cells’ post-mission performance and look for damage from radiation and other space hazards.

“If it can survive in space, which is the harshest of environments from the standpoint of wide temperature swings, radiation and numerous other factors, then we can be confident it will work well down on Earth,” Ready said.