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A cheap, long-lasting, sustainable battery for grid energy storage

The zinc-ion battery consists of a water-based electrolyte, a pillared vanadium oxide positive electrode (right), and an inexpensive metallic zinc negative electrode (left). The battery generates electricity through a reversible process called intercalation, where positively-charged zinc ions are oxidized from the zinc metal negative electrode, travel through the electrolyte and insert between the layers of vanadium oxide nanosheets in the positive electrode. This drives the flow of electrons in the external circuit, creating an electrical current. The reverse process occurs on charge. (credit: Dipan Kundu et al./Nature Energy)

University of Waterloo chemists have developed a long-lasting, safe, zinc-ion battery that costs half the price of current lithium-ion batteries. It could help communities shift from traditional power plants to renewable solar and wind energy production, where electricity storage overnight is needed.

The battery is water-based and uses cheap but safe, non-flammable, non-toxic materials, compared to expensive, flammable, organic electrolytes in lithium-ion batteries, which are used in the exploding Samsung Galaxy Note 7 smartphones reported last week and in previously reported exploding hoverboards.

Where cost, safety, and life cycle are vital, not size

Lithium-ion batteries have much higher energy density (energy that can be stored per unit volume) than water-based batteries (making lithium-ion batteries attractive for smartphones and other compact devices), but water-based zinc-ion batteries are more feasible for grid-scale applications, where cost, safety, and life cycle are important, not size.

The cell design satisfies four vital criteria: high reversibility, rate, capacity, and no zinc dendrite formation. It provides more than 1,000 cycles. Lithium-ion batteries also operate by intercalation (of lithium ions) but they typically use expensive, flammable, organic electrolytes.

The bonus for manufacturers is they can produce this zinc battery at low cost because its fabrication does not require special conditions, such as ultra-low humidity or the handling of flammable materials needed for lithium ion batteries, the chemists say.

“The focus used to be on minimizing size and weight for the portable electronics market and cars,” said Dipan Kundu, a University of Waterloo postdoctoral fellow and the paper’s first author. “Grid storage needs a different kind of battery and that’s given us license to look into different materials.”

The discovery appears in the journal Nature Energy.

Abstract of A high-capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode

Although non-aqueous Li-ion batteries possess significantly higher energy density than their aqueous counterparts, the latter can be more feasible for grid-scale applications when cost, safety and cycle life are taken into consideration. Moreover, aqueous Zn-ion batteries have an energy storage advantage over alkali-based batteries as they can employ Zn metal as the negative electrode, dramatically increasing energy density. However, their development is plagued by a limited choice of positive electrodes, which often show poor rate capability and inadequate cycle life. Here we report a vanadium oxide bronze pillared by interlayer Zn2+ ions and water (Zn0.25V2O5⋅nH2O), as the positive electrode for a Zn cell. A reversible Zn2+ ion (de)intercalation storage process at fast rates, with more than one Zn2+ per formula unit (a capacity up to 300 mAh g−1), is characterized. The Zn cell offers an energy density of ∼450 Wh l−1 and exhibits a capacity retention of more than 80% over 1,000 cycles, with no dendrite formation at the Zn electrode.

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A cheap, long-lasting, sustainable battery for grid energy storage

The zinc-ion battery consists of a water-based electrolyte, a pillared vanadium oxide positive electrode (right), and an inexpensive metallic zinc negative electrode (left). The battery generates electricity through a reversible process called intercalation, where positively-charged zinc ions are oxidized from the zinc metal negative electrode, travel through the electrolyte and insert between the layers of vanadium oxide nanosheets in the positive electrode. This drives the flow of electrons in the external circuit, creating an electrical current. The reverse process occurs on charge. (credit: Dipan Kundu et al./Nature Energy)

University of Waterloo chemists have developed a long-lasting, safe, zinc-ion battery that costs half the price of current lithium-ion batteries. It could help communities shift from traditional power plants to renewable solar and wind energy production, where electricity storage overnight is needed.

The battery is water-based and uses cheap but safe, non-flammable, non-toxic materials, compared to expensive, flammable, organic electrolytes in lithium-ion batteries, which are used in the exploding Samsung Galaxy Note 7 smartphones reported last week and in previously reported exploding hoverboards.

Where cost, safety, and life cycle are vital, not size

Lithium-ion batteries have much higher energy density (energy that can be stored per unit volume) than water-based batteries (making lithium-ion batteries attractive for smartphones and other compact devices), but water-based zinc-ion batteries are more feasible for grid-scale applications, where cost, safety, and life cycle are important, not size.

The cell design satisfies four vital criteria: high reversibility, rate, capacity, and no zinc dendrite formation. It provides more than 1,000 cycles. Lithium-ion batteries also operate by intercalation (of lithium ions) but they typically use expensive, flammable, organic electrolytes.

The bonus for manufacturers is they can produce this zinc battery at low cost because its fabrication does not require special conditions, such as ultra-low humidity or the handling of flammable materials needed for lithium ion batteries, the chemists say.

“The focus used to be on minimizing size and weight for the portable electronics market and cars,” said Dipan Kundu, a University of Waterloo postdoctoral fellow and the paper’s first author. “Grid storage needs a different kind of battery and that’s given us license to look into different materials.”

The discovery appears in the journal Nature Energy.

Abstract of A high-capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode

Although non-aqueous Li-ion batteries possess significantly higher energy density than their aqueous counterparts, the latter can be more feasible for grid-scale applications when cost, safety and cycle life are taken into consideration. Moreover, aqueous Zn-ion batteries have an energy storage advantage over alkali-based batteries as they can employ Zn metal as the negative electrode, dramatically increasing energy density. However, their development is plagued by a limited choice of positive electrodes, which often show poor rate capability and inadequate cycle life. Here we report a vanadium oxide bronze pillared by interlayer Zn2+ ions and water (Zn0.25V2O5⋅nH2O), as the positive electrode for a Zn cell. A reversible Zn2+ ion (de)intercalation storage process at fast rates, with more than one Zn2+ per formula unit (a capacity up to 300 mAh g−1), is characterized. The Zn cell offers an energy density of ∼450 Wh l−1 and exhibits a capacity retention of more than 80% over 1,000 cycles, with no dendrite formation at the Zn electrode.

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‘Star in a jar’ could lead to limitless fusion energy

A test cell for the National Spherical Torus Experiment Upgrade with tokamak in the center. (credit: Elle Starkman/PPPL Office of Communications)

Physicists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL)* are building a “star in a jar” — a miniature version of the how our Sun creates energy through fusion. It could provide humankind with near limitless energy, ending dependence on fossil fuels for generating electricity — without contributing greenhouse gases that warm the Earth, and with no long-term radioactive waste.

But that requires a “jar” that can contain superhot plasma — and is low-cost enough to be built around the world. A model for such a “jar,” or fusion device, already exists in experimental form: the tokamak, or fusion reactor. Invented in the 1950s by Soviet physicists, it’s a device that uses a powerful magnetic field to confine plasma (superhot charged gas) in the shape of a torus.

Diagram of magnetic confinement in a tokamak; interactive version: http://www.ccfe.ac.uk/tokamak.aspx (credit: EUROfusion)

There are many experimental tokamaks currently in operation, but they all face physics challenges, the PPPL physicists explain. “For example, they must control the turbulence that arises when superhot plasma particles are subjected to powerful electromagnetic fields. They must also carefully control how the plasma particles interact with the surrounding walls to avoid possible disruptions that can halt fusion reactions if the plasma becomes too dense or impure.”

Spherical tokamak: a new, compact “jar”

Spherical torus/tokamak design for a fusion nuclear science facility showing magnets and other systems and structures (credit: J.E. Menard et al./Nucl. Fusion)

So researchers at PPPL and in and Culham, England  are looking at ways of solving these challenges for the next generation of fusion devices, based on compact spherical tokamaks. They suggest that these could provide the design for possible next steps in fusion energy: a Fusion Nuclear Science Facility (FNSF) that would develop reactor components and also produce electricity as a pilot plant for a commercial fusion power station.

The detailed proposal for such a “jar” is described in a paper published in August 2016 in the journal Nuclear Fusion. “We are opening up new options for future plants,” said lead author Jonathan Menard, program director for the recently completed National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL. The $94-million upgrade of the NSTX, financed by the U.S. Department of Energy’s Office of Science, began operating last year.

Spherical tokamaks are compact devices shaped like cored apples, compared with the bulkier doughnut-like shape of conventional tokamaks. The increased power of the upgraded PPPL machine and the soon-to-be completed MAST Upgrade device “will push the physics frontier, expand our knowledge of high temperature plasmas, and, if successful, lay the scientific foundation for fusion development paths based on more compact designs,” said PPPL Director Stewart Prager.

Next steps in fusion energy

ITER design (credit: ITER Organization)

The spherical design produces high-pressure plasmas  — the superhot charged gas (also known as the “fourth state of matter”) that fuels fusion reactions, using relatively low, inexpensive magnetic fields. This unique capability points the way to a possible next generation of fusion experiments to complement ITER, the international tokamak that 35 nations including the U.S. are building in France to demonstrate the feasibility of fusion power. ITER will be the largest in the world when completed within the next decade.

But ITER is a doughnut-shaped tokamak. “The main reason we research spherical tokamaks is to find a way to produce fusion at much less cost than conventional tokamaks require,” said Ian Chapman, the newly appointed chief executive of the United Kingdom Atomic Energy Authority and leader of the UK’s magnetic confinement fusion research programme at the Culham Science Centre.

The 43-page Nuclear Fusion paper describes how the spherical design can provide the next steps in fusion energy. A key issue is the size of the hole in the center of the tokamak, which holds and shapes the plasma. In spherical tokamaks, this hole can be half the size of the hole in conventional tokamaks, and that enables control of the plasma with relatively low magnetic fields.

The smaller hole could also be compatible with a blanket system for the FNSF that would breed tritium, a rare isotope of hydrogen. Tritium will fuse with deuterium, another isotope of hydrogen, to produce fusion reactions in next-step tokamaks.

Superconducting magnets for pilot plants

For pilot FNSF plants, the authors call for superconducting magnets to replace the primary copper magnets in the FNSF. Superconducting magnets can be operated far more efficiently than copper magnets, but require thicker shielding. However, recent advances in high-temperature superconductors could lead to much thinner superconducting magnets that would require less space and reduce considerably the size and cost of the machine.

Included in the paper is a description of a device called a “neutral beam injector” that will start and sustain plasma current without relying on a heating coil in the center of the tokamak. Such a coil is not suitable for continuous long-term operation. The neutral beam injector will pump fast-moving neutral atoms into the plasma and will help optimize the magnetic field that confines and controls the superhot gas.

The researchers believe the upgraded NSTX and MAST facilities will provide crucial data for determining the best path for ultimately generating electricity from fusion. The research is funded by the U.S. Department of Energy.

* PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the U.S.

Abstract of Fusion nuclear science facilities and pilot plants based on the spherical tokamak

A fusion nuclear science facility (FNSF) could play an important role in the development of fusion energy by providing the nuclear environment needed to develop fusion materials and components. The spherical torus/tokamak (ST) is a leading candidate for an FNSF due to its potentially high neutron wall loading and modular configuration. A key consideration for the choice of FNSF configuration is the range of achievable missions as a function of device size. Possible missions include: providing high neutron wall loading and fluence, demonstrating tritium self-sufficiency, and demonstrating electrical self-sufficiency. All of these missions must also be compatible with a viable divertor, first-wall, and blanket solution. ST-FNSF configurations have been developed simultaneously incorporating for the first time: (1) a blanket system capable of tritium breeding ratio TBR  ≈  1, (2) a poloidal field coil set supporting high elongation and triangularity for a range of internal inductance and normalized beta values consistent with NSTX/NSTX-U previous/planned operation, (3) a long-legged divertor analogous to the MAST-U divertor which substantially reduces projected peak divertor heat-flux and has all outboard poloidal field coils outside the vacuum chamber and superconducting to reduce power consumption, and (4) a vertical maintenance scheme in which blanket structures and the centerstack can be removed independently. Progress in these ST-FNSF missions versus configuration studies including dependence on plasma major radius

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Musk’s new master plan for Tesla

Tesla Autopilot (credit: Tesla Motors)

Elon Musk revealed his new master plan for Tesla today (July 20) in a blog post published on Tesla’s website:

  • Create stunning solar roofs with seamlessly integrated battery storage.
  • Expand the electric vehicle product line to address all major segments.
  • Develop a self-driving capability that is 10X safer than manual via massive fleet learning.
  • Enable your car to make money for you when you aren’t using it.

Increasing safety: “morally reprehensible to delay”

In the context of the recent Autopilot problem, Musk clarified why Tesla is deploying partial autonomy now, rather than waiting until some point in the future: “When used correctly, it is already significantly safer than a person driving by themselves and it would therefore be morally reprehensible to delay release simply for fear of bad press or some mercantile calculation of legal liability.

“According to the recently released 2015 NHTSA report, automotive fatalities increased by 8% to one death every 89 million miles. Autopilot miles will soon exceed twice that number and the system gets better every day. It would no more make sense to disable Tesla’s Autopilot, as some have called for, than it would to disable autopilot in aircraft, after which our system is named.”

Another way to increase safety, he says, is new heavy-duty trucks and high passenger-density urban transport, both planned for unveiling next year. “With the advent of autonomy, it will probably make sense to shrink the size of buses and transition the role of bus driver to that of fleet manager. … Traffic congestion would improve due to increased passenger areal density by eliminating the center aisle and putting seats where there are currently entryways, and matching acceleration and braking to other vehicles, thus avoiding the inertial impedance to smooth traffic flow of traditional heavy buses. It would also take people all the way to their destination.”

Lowering the cost of an autonomous car

Musk said that when true self-driving is approved by regulators, “it will mean that you will be able to summon your Tesla from pretty much anywhere. Once it picks you up, you will be able to sleep, read, or do anything else enroute to your destination.

“You will also be able to add your car to the Tesla shared fleet just by tapping a button on the Tesla phone app and have it generate income for you while you’re at work or on vacation, significantly offsetting and at times potentially exceeding the monthly loan or lease cost. This dramatically lowers the true cost of ownership to the point where almost anyone could own a Tesla. Since most cars are only in use by their owner for 5% to 10% of the day, the fundamental economic utility of a true self-driving car is likely to be several times that of a car which is not.”

Musk said that in cities where demand exceeds the supply of customer-owned cars, “Tesla will operate its own fleet, ensuring you can always hail a ride from us no matter where you are.”

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US has potential to produce more than a billion tons of biomass annually by 2040


Oak Ridge National Laboratory | 2016 Billion-Ton Report

The U.S. has the potential to sustainably produce at least 1 billion dry tons of nonfood biomass resources annually by 2040, according to the 2016 Billion-Ton Report, jointly released by the U.S. Department of Energy and Oak Ridge National Laboratory. That amount would substantially decrease greenhouse gas emissions in the utility and transportation sectors and (as the domestic bioeconomy grows) reduce U.S. dependence on imported oil, the scientists project.

These renewable resources include agricultural, forestry and algal biomass, as well as waste. They encompass the current and future potential of biomass, from currently available logging and crop residues to future available algae and dedicated energy crops — all useable for the production of biofuel, biopower and bioproducts.

Current feedstock, sector consumption, and final product distribution, in million dry tons per year. Biomass resources are shown on the left and their allocations are shown on the right. The size of the flow is representative of the amount of biomass allocated to that end use. For this figure, contributions from landfill gas are represented as tons of biomass equivalent by applying a conversion factor of 0.2665 lb/scf (credit: U.S. DOE)

The report findings show that under a base-case scenario, the United States could increase its use of dry biomass resources from a current 400 million tons to 1.57 billion tons under a high-yield scenario.

The analysis was led by ORNL with contributions from 65 experts from federal agencies, national laboratories, universities (the University of Tennessee, North Carolina State University, South Dakota State University and Oregon State University), and private companies (Energetics, Inc. and Allegheny Science and Technology).

Proposed future feedstock supply system for transforming raw biomass into stable, tradeable
commodities suitable for long-distance transport and handling in existing infrastructure (credit: Idaho National Laboratory)

New to the 2016 report are assessments of potential biomass supplies from algae, from new energy crops (miscanthus, energy cane, eucalyptus), and from municipal solid waste. For the first time, the report also considers how the cost of pre-processing and transporting biomass to the biorefinery may impact feedstock availability.

Interactive tools available through the Bioenergy Knowledge Discovery Framework allow users to visualize biomass availability scenarios and  tailor the data by factors such as geographic area, biomass source and price. Researchers and decision makers can use these features to better inform national bioenergy policies and research, development and deployment strategies. Each diagram and map in the report is available in an interactive interface on the Bioenergy Knowledge Discovery Framework.

Volume 2 of the report, set for release later this year, will consist of a collection of analyses on the potential environmental sustainability effects of a subset of agricultural and forestry biomass production scenarios presented in volume 1. Volume 2 will also discuss algae sustainability, land use, and land management changes, and strategies to enhance environmental sustainability.

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The top 10 emerging technologies of 2016

(credit: WEF)

The World Economic Forum’s annual list of this year’s breakthrough technologies, published today, includes “socially aware” openAI, grid-scale energy storage, perovskite solar cells, and other technologies with the potential to “transform industries, improve lives, and safeguard the planet.” The WEF’s specific interest is to “close gaps in investment and regulation.”

“Horizon scanning for emerging technologies is crucial to staying abreast of developments that can radically transform our world, enabling timely expert analysis in preparation for these disruptors. The global community needs to come together and agree on common principles if our society is to reap the benefits and hedge the risks of these technologies,” said Bernard Meyerson, PhD, Chief Innovation Officer of IBM and Chair of the WEF’s Meta-Council on Emerging Technologies.

The list also provides an opportunity to debate human, societal, economic or environmental risks and concerns that the technologies may pose — prior to widespread adoption.

One of the criteria used by council members during their deliberations was the likelihood that 2016 represents a tipping point in the deployment of each technology. So the list includes some technologies that have been known for a number of years, but are only now reaching a level of maturity where their impact can be meaningfully felt.

The top 10 technologies that make this year’s list are:

  1. Nanosensors and the Internet of Nanothings  — With the Internet of Things expected to comprise 30 billion connected devices by 2020, one of the most exciting areas of focus today is now on nanosensors capable of circulating in the human body or being embedded in construction materials. They could use DNA and proteins to recognize specific chemical targets, store a few bits of information, and then report their status by changing color or emitting some other easily detectable signal.
  2. Next-Generation Batteries — One of the greatest obstacles holding renewable energy back is matching supply with demand, but recent advances in energy storage using sodium, aluminum, and zinc based batteries makes mini-grids feasible that can provide clean, reliable, around-the-clock energy sources to entire villages.
  3. The Blockchain — With venture investment related to the online currency Bitcoin exceeding $1 billion in 2015 alone, the economic and social impact of blockchain’s potential to fundamentally change the way markets and governments work is only now emerging.
  4. 2D Materials — Plummeting production costs mean that 2D materials like graphene are emerging in a wide range of applications, from air and water filters to new generations of wearables and batteries.
  5. Autonomous Vehicles — The potential of self-driving vehicles for saving lives, cutting pollution, boosting economies, and improving quality of life for the elderly and other segments of society has led to rapid deployment of key technology forerunners along the way to full autonomy.
  6. Organs-on-chips — Miniature models of human organs could revolutionize medical research and drug discovery by allowing researchers to see biological mechanism behaviors in ways never before possible.
  7. Perovskite Solar Cells — This new photovoltaic material offers three improvements over the classic silicon solar cell: it is easier to make, can be used virtually anywhere and, to date, keeps on generating power more efficiently.
  8. Open AI Ecosystem — Shared advances in natural language processing and social awareness algorithms, coupled with an unprecedented availability of data, will soon allow smart digital assistants to help with a vast range of tasks, from keeping track of one’s finances and health to advising on wardrobe choice.
  9. Optogenetics — Recent developments mean light can now be delivered deeper into brain tissue, something that could lead to better treatment for people with brain disorders.
  10. Systems Metabolic Engineering — Advances in synthetic biology, systems biology, and evolutionary engineering mean that the list of building block chemicals that can be manufactured better and more cheaply by using plants rather than fossil fuels is growing every year.

To compile this list, the World Economic Forum’s Meta-Council on Emerging Technologies, a panel of global experts, “drew on the collective expertise of the Forum’s communities to identify the most important recent technological trends. By doing so, the Meta-Council aims to raise awareness of their potential and contribute to closing gaps in investment, regulation and public understanding that so often thwart progress.”

You can read 10 expert views on these technologies here or download the series as a PDF.

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China’s Sunway TaihuLight tops world supercomputer ratings

Sunway TaihuLight System (credit: National Supercomputing Center)

Chinese supercomputers maintained their No. 1 ranking on the 47th edition of the TOP500 list of the world’s top supercomputers, announced today (June 20). The new Sunway TaihuLight supercomputer operates at 93 petaflop/s (quadrillions of calculations per second) Rmax on the LINPACK benchmark — twice as fast and three times as efficient as China’s Tianhe-2 (at 33.86 petaflop/s), now in the #2 spot.

The new supercomputer was developed by the National Research Center of Parallel Computer Engineering & Technology (NRCPC) and installed at the National Supercomputing Center in Wuxi in China’s Jiangsu province. The complete system has a theoretical peak performance of 125.4 Pflop/s, with 10,649,600 cores and 1.31 PB of primary memory, according to a report by Top500 co-compiler Jack Dongarra of the University of Tennessee.

The newest edition of the semiannual TOP500 list was announced today at the 2016 International Supercomputer Conference in Frankfurt.

China now leads with largest number of supercomputers

The latest list marks the first time since the inception of the TOP500 that the U.S is not home to the largest number of systems. China now leads with 167 systems and the U.S. is second with 165. China also leads the performance category, thanks to the No. 1 and No. 2 systems. Titan, a Cray XK7 system installed at the Department of Energy’s (DOE) Oak Ridge National Laboratory, is now the No. 3 system, at 17.59 petaflop/s.

Sunway TaihuLight was also built entirely using processors designed and made in China (Tianhe-2 was built with Intel processors).

U.S. primacy on the Top500 list has slipped for a number of reasons, including lower government support, private-sector investing now focused on cloud-computing centers, and the U.S. policy of blocking the sale of a number of advanced microprocessors to China, possibly accelerating development of China’s own technology, the New York Times reports. (Last year, the Obama administration began a new effort to develop an“exascale” supercomputer; it would be more than 10 times faster than the Sunway TaihuLight.)

However, because of funding shortages and technology challenges, “there has been a delay in getting the exascale launched in the U.S., and as a result, we’re further behind than we should be,” Dongarra told the Times, noting that the Chinese government is committed to reaching the exascale goal by the end of this decade.

Cray continues to be the leader in the TOP500 list in total installed performance share, with 19.9 percent (down from 25 percent). Thanks to the Sunway TaihuLight system, the National Research Center of Parallel Computer Engineering & Technology takes the second spot with 16.4 percent of the total performance — with just one machine. IBM takes the third spot with 10.7 percent share, down from 14.9 percent six months ago.

Energy-efficiency ratings

For the first time, the data collection and curation of the Green500 project, which ranks supercomputers by energy efficiency, is now integrated with the TOP500 project. The most energy-efficient system and No. 1 on the Green500 is Shoubu, a PEZY Computing/Exascaler ZettaScaler-1.6 System achieving  6.67 GFfops/Watt at the Advanced Center for Computing and Communication at RIKEN in Japan.

Other highlights from the Top 500 list:

  • Total combined performance of all 500 systems has grown to 566.7 petaflop/s, compared to 420 petaflop/s six months ago and 363 petaflop/s one year ago.
  • There are 95 systems with performance greater than a petaflop/s on the list, up from 81 six months ago.
  • Intel continues to provide the processors for the largest share – 455 systems or 91 percent – of the TOP500 systems. The share of IBM Power processors is now at 23 systems, down from 26 systems six month ago. The AMD Opteron family is used in 13 systems (2.6 percent), down from 4.2 percent on the previous list.
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Ultra-flexible solar cells thin enough to wrap around a glass stirring rod

Ultra-thin solar cells flexible enough to bend around small objects, such as this 6-mm-diameter glass rod (credit: Juho Kim, et al./APL)

Scientists in South Korea have designed ultra-thin photovoltaics that are flexible enough to wrap around a thin glass rod. The new solar cells could power wearable electronics like smart watches and fitness trackers.

“Our photovoltaic is about 1 micrometer thick” (the thinnest human hair is about 17 micrometers), said Jongho Lee, an engineer at the Gwangju Institute of Science and Technology in South Korea. Standard photovoltaics are usually hundreds of times thicker, and most other thin photovoltaics are 2 to 4 times thicker, he explained.

Fabrication procedure of the flexible vertical ultra-thin gallium-arsenide solar microcells.* (credit: Juho Kim, et al./APL)

The researchers made the ultra-thin solar cells from the semiconductor gallium arsenide. They stamped the cells directly onto a flexible substrate without using an adhesive (which would add to the material’s thickness).

The cells were then “cold welded” to the electrode on the substrate by applying pressure at 170 degrees Celsius and melting a top layer of material called photoresist, which acted as a temporary adhesive. The photoresist was later peeled away, leaving the direct metal-to-metal bond.

The metal bottom electrode layer also serves as a reflector to direct stray light back to the solar cells (to increase current output). The researchers tested the efficiency of the device at converting sunlight to electricity and found that it was comparable to thicker photovoltaics.

The team performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters. They also performed numerical analysis of the cells, finding that they experience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick.

A few other groups have reported solar cells with thicknesses of around 1 micrometer, but have produced the cells in different ways, for example, by removing the whole subtrate by etching. By transfer-printing instead of etching, the new method developed by Lee and his colleagues could be used to make very flexible photovoltaics with a smaller amount of materials, according to Lee.

The thin cells can also be integrated onto glasses frames or fabric and might power the next wave of wearable electronics, Lee said.

The researchers report the results in an open-access paper in the journal Applied Physics Letters, from AIP Publishing.

* (a) Schematic illustration of a film stamp with vertical gallium-arsenide microcells fabricated and isolated from the epitaxially grown source wafers. The photoresist (PR) temporarily holds the solar microcells on the source wafers. (b) The bottom electrode, which also serves as a back reflector, is deposited onto the backside of the ultra-thin vertical GaAs microcells. (c) After the film stamp is brought into contact with the receiver substrate, heat (∼170 °C) and pressure (∼80 kPa) are applied to melt the PR to serve as an adhesive. (d) Cross-sectional scanning electron microscope (SEM) image of the microcell covered with the adhesive (PR) on the receiver substrate after the printing process. The bottom electrode is in direct contact with the Au layer on the receiver substrate. (e) Peeling the film stamp leaves the vertical ultra-thin solar microcells on the receiver substrate. (h) An optical image of the microcell wrapped on a glass slide with a radius of 1 mm. The microcell is encapsulated with a thin epoxy layer (thickness ∼2 μm).