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Theoretically, two layers are better than one for solar-cell efficiency

Solar cells have come a long way, but inexpensive, thin film solar cells are still far behind more expensive, crystalline solar cells in efficiency. Now, a team of researchers suggests that using two thin films of different materials may be the way to go to create affordable, thin film cells with about 34% efficiency.

“Ten years ago I knew very little about solar cells, but it became clear to me they were very important,” said Akhlesh Lakhtakia, Evan Pugh University Professor and Charles Godfrey Binder Professor of Engineering Science and Mechanics, Penn State.

Investigating the field, he found that researchers approached solar cells from two sides, the optical side — looking on how the sun’s light is collected — and the electrical side — looking at how the collected sunlight is converted into electricity. Optical researchers strive to optimize light capture, while electrical researchers strive to optimize conversion to electricity, both sides simplifying the other.

“I decided to create a model in which both electrical and optical aspects will be treated equally,” said Lakhtakia. “We needed to increase actual efficiency, because if the efficiency of a cell is less than 30% it isn’t going to make a difference.” The researchers report their results in a recent issue of Applied Physics Letters.

Lakhtakia is a theoretician. He does not make thin films in a laboratory, but creates mathematical models to test the possibilities of configurations and materials so that others can test the results. The problem, he said, was that the mathematical structure of optimizing the optical and the electrical are very different.

Solar cells appear to be simple devices, he explained. A clear top layer allows sunlight to fall on an energy conversion layer. The material chosen to convert the energy, absorbs the light and produces streams of negatively charged electrons and positively charged holes moving in opposite directions. The differently charged particles get transferred to a top contact layer and a bottom contact layer that channel the electricity out of the cell for use. The amount of energy a cell can produce depends on the amount of sunlight collected and the ability of the conversion layer. Different materials react to and convert different wavelengths of light.

“I realized that to increase efficiency we had to absorb more light,” said Lakhtakia. “To do that we had to make the absorbent layer nonhomogeneous in a special way.”

That special way was to use two different absorbent materials in two different thin films. The researchers chose commercially available CIGS — copper indium gallium diselenide — and CZTSSe — copper zinc tin sulfur selenide — for the layers. By itself, CIGS’s efficiency is about 20% and CZTSSe’s is about 11%.

These two materials work in a solar cell because the structure of both materials is the same. They have roughly the same lattice structure, so they can be grown one on top of the other, and they absorb different frequencies of the spectrum so they should increase efficiency, according to Lakhtakia.

“It was amazing,” said Lakhtakia. “Together they produced a solar cell with 34% efficiency. This creates a new solar cell architecture — layer upon layer. Others who can actually make solar cells can find other formulations of layers and perhaps do better.”

According to the researchers, the next step is to create these experimentally and see what the options are to get the final, best answers.

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Bioactive nano-capsules to hijack cell behavior

Many diseases are caused by defects in signaling pathways of body cells. In the future, bioactive nanocapsules could become a valuable tool for medicine to control these pathways. Researchers from the University of Basel have taken an important step in this direction: They succeed in having several different nanocapsules work in tandem to amplify a natural signaling cascade and influence cell behavior.

Cells constantly communicate with each other and have ways to pick up signals and process them — similar to humans who need ears to hear sounds and knowledge of language to process their meaning. Controlling the cell’s own signaling pathways is of great interest for medicine in order to treat various diseases.

A research team of the Department of Chemistry at the University of Basel and the NCCR Molecular Systems Engineering develops bioactive materials that could be suitable for this purpose. To achieve this, the researchers led by Professor Cornelia Palivan combine nanomaterials with natural molecules and cells.

In the journal ACS Nano, they now report how enzyme loaded nano-capsules can enter cells and be integrated into their native signaling processes. By functionally coupling several nano-capsules, they are able to amplify a natural signaling pathway.

Protecting the cargo

In order to protect the enzymes from degradation in a cellular environment the research team loaded them into polymeric small capsules. Molecules can enter the compartment through biological pores specifically inserted in its synthetic wall and react with the enzymes inside.

The researchers conducted experiments with nano-capsules harboring different enzymes that worked in tandem: the product of the first enzymatic reaction entered a second capsule and started the second reaction inside. These nano-capsuled could stay operative for days and actively participated in natural reactions in mammalian cells.

Tiny loudspeakers and ears

One of the many signals that cells receive and process is nitric oxide (NO). It is a well-studied cellular mechanism since defects in the NO signaling pathway are involved in the emergence of cardiovascular diseases, but also in muscular and retinal dystrophies. The pathway encompasses the production of NO by an enzyme family called nitric oxide synthases (NOS). The NO can then diffuse to other cells where it is sensed by another enzyme named soluble guanylate cyclase (sGC). The activation of sGC starts a cascade reaction, regulating a plethora of different processes such as smooth muscle relaxation and the processing of light by sensory cells, among others.

The researchers lead by Palivan produced capsules harboring NOS and sGC, which are naturally present in cells, but at much lower concentrations: the NOS-capsules, producing NO, act similarly to loudspeakers, “shouting” their signal loud and clear; the sGC-capsules, act as “ears,” sensing and processing the signal to amplify the response.

Using the intracellular concentration of calcium, which depends on the action of sGC, as an indicator, the scientists showed that the combination of both NOS and sGC loaded capsules makes the cells much more reactive, with an 8-fold increase in the intracellular calcium level.

A new strategy for enzyme replacement therapy

“It’s a new strategy to stimulate such changes in cellular physiology by combining nanoscience with biomolecules,” comments Dr. Andrea Belluati, the first author of the study. “We just had to incubate our enzyme-loaded capsules with the cells, and they were ready to act at a moment’s notice.”

“This proof of concept is an important step in the field of enzyme replacement therapy for diseases where biochemical pathways malfunction, such as cardiovascular diseases or several dystrophies,” adds Cornelia Palivan.

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High-precision electrochemistry: The new gold standard in fuel cell catalyst development

Vehicles powered by polymer electrolyte membrane fuel cells (PEMFCs) are energy-efficient and eco-friendly, but despite increasing public interest in PEMFC-powered transportation, current performance of materials that are used in fuel cells limits their widespread commercialization.

Scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory led a team to investigate reactions in PEMFCs, and their discoveries informed the creation of technology that could bring fuel cells one step closer to realizing their full market potential.

PEMFCs rely on hydrogen as a fuel, which is oxidized on the cell’s anode side through a hydrogen oxidation reaction, while oxygen from the air is used for an oxygen reduction reaction (ORR) at the cathode. Through these processes, fuel cells produce electricity to power electric motors in vehicles and other applications, emitting water as the only by-product.

Platinum-based, nano-sized particles are the most effective materials for promoting reactions in fuel cells, including the ORR in the cathode. However, in addition to their high cost, platinum nanoparticles suffer from gradual degradation, especially in the cathode, which limits catalytic performance and reduces the lifetime of the fuel cell.

The research team, which included DOE’s Oak Ridge National Laboratory and several university partners, used a novel approach to examine dissolution processes of platinum at the atomic and molecular level. The investigation enabled them to identify the degradation mechanism during the cathodic ORR, and the insights guided the design of a nanocatalyst that uses gold to eliminate platinum dissolution.

“The dissolution of platinum occurs at the atomic and molecular scale during exposure to the highly corrosive environment in fuel cells,” said Vojislav Stamenkovic, a senior scientist and group leader for the Energy Conversion and Storage group in Argonne’s Materials Science Division (MSD). “This material degradation affects the fuel cell’s long-term operations, presenting an obstacle for fuel cell implementation in transportation, specifically in heavy duty applications such as long-haul trucks.”

Starting small

The scientists used a range of customized characterization tools to investigate the dissolution of well-defined platinum structures in single-crystal surfaces, thin films and nanoparticles.

“We have developed capabilities to observe processes at the atomic scale to understand the mechanisms responsible for dissolution and to identify the conditions under which it occurs,” said Pietro Papa Lopes, a scientist in Argonne’s MSD and first author on the study. “Then we implemented this knowledge into material design to mitigate dissolution and increase durability.”

The team studied the nature of dissolution at the fundamental level using surface-specific tools, electrochemical methods, inductively coupled plasma mass spectrometry, computational modeling and atomic force, scanning tunneling and high-resolution transmission microscopies.

In addition, the scientists relied on a high-precision synthesis approach to create structures with well-defined physical and chemical properties, ensuring that the relationships between structure and stability discovered from studying 2D surfaces were carried over to the 3D nanoparticles they produced.

“We performed these studies — from single crystals, to thin films, to nanoparticles — which showed us how to synthesize platinum catalysts to increase durability,” said Lopes, “and by looking at these different materials, we also identified strategies for using gold to protect the platinum.”

Going for gold

As the scientists uncovered the fundamental nature of dissolution by observing its occurrence in several testbed scenarios, the team used the knowledge to mitigate dissolution with the addition of gold.

The researchers used transmission electron microscopy capabilities at Argonne’s Center for Nanoscale Materials and at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory — both DOE Office of Science User Facilities — to image platinum nanoparticles after synthesis and before and after operation. This technique allowed the scientists to compare the stability of the nanoparticles with and without incorporated gold.

The team found that controlled placement of gold in the core promotes the arrangement of platinum in an optimal surface structure that grants high stability. In addition, gold was selectively deposited on the surface to protect specific sites that the team identified as particularly vulnerable for dissolution. This strategy eliminates dissolution of platinum from even the smallest nanoparticles used in this study by keeping platinum atoms attached to the sites where they can still effectively catalyze the ORR.

Atomic-level understanding

Understanding the mechanisms behind dissolution at the atomic level is essential to uncovering the correlation between platinum loss, surface structure and size and ratio of platinum nanoparticles, and determining how these relationships affect long-term operation.

“The novel part of this research is resolving the mechanisms and fully mitigating platinum dissolution by material design at different scales, from single crystals and thin films to nanoparticles,” said Stamenkovic. “It’s the insights we gained in conjunction with the design and synthesis of a nanomaterial that addresses durability issues in fuel cells, as well as the ability to delineate and quantify dissolution of platinum catalyst from other processes that contribute to fuel cell performance decay.”

The team is also developing a predictive aging algorithm to assess the long-term durability of the platinum-based nanoparticles and found a 30-fold improvement in durability compared to nanoparticles without gold.

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Designed antiviral proteins inhibit SARS-CoV-2 in the lab

Computer-designed small proteins have now been shown to protect lab-grown human cells from SARS-CoV-2, the coronavirus that causes COVID-19.

The findings are reported today, Sept. 9, in Science

In the experiments, the lead antiviral candidate, named LCB1, rivaled the best-known SARS-CoV-2 neutralizing antibodies in its protective actions. LCB1 is currently being evaluated in rodents.

Coronaviruses are studded with so-called Spike proteins. These latch onto human cells to enable the virus to break in and infect them. The development of drugs that interfere with this entry mechanism could lead to treatment of or even prevention of infection.

Institute for Protein Design researchers at the University of Washington School of Medicine used computers to originate new proteins that bind tightly to SARS-CoV-2 Spike protein and obstruct it from infecting cells.

Beginning in January, more than two million candidate Spike-binding proteins were designed on the computer. Over 118,000 were then produced and tested in the lab.

“Although extensive clinical testing is still needed, we believe the best of these computer-generated antivirals are quite promising,” said lead author Longxing Cao, a postdoctoral scholar at the Institute for Protein Design.

“They appear to block SARS-CoV-2 infection at least as well as monoclonal antibodies, but are much easier to produce and far more stable, potentially eliminating the need for refrigeration,” he added.

The researchers created antiviral proteins through two approaches. First, a segment of the ACE2 receptor, which SARS-CoV-2 naturally binds to on the surface of human cells, was incorporated into a series of small protein scaffolds.

Second, completely synthetic proteins were designed from scratch. The latter method produced the most potent antivirals, including LCB1, which is roughly six times more potent on a per mass basis than the most effective monoclonal antibodies reported thus far.

Scientists from the University of Washington School of Medicine in Seattle and Washington University School of Medicine in St. Louis collaborated on this work.

“Our success in designing high-affinity antiviral proteins from scratch is further proof that computational protein design can be used to create promising drug candidates,” said senior author and Howard Hughes Medical Institute Investigator David Baker, professor of biochemistry at the UW School of Medicine and head of the Institute for Protein Design. In 2019, Baker gave a TED talk on how protein design might be used to stop viruses.

To confirm that the new antiviral proteins attached to the coronavirus Spike protein as intended, the team collected snapshots of the two molecules interacting by using cryo-electron microscopy. These experiments were performed by researchers in the laboratories of David Veesler, assistant professor of biochemistry at the UW School of Medicine, and Michael S. Diamond, the Herbert S. Gasser Professor in the Division of Infectious Diseases at Washington University School of Medicine in St. Louis.

“The hyperstable minibinders provide promising starting points for new SARS-CoV-2 therapeutics,” the antiviral research team wrote in their study pre-print, “and illustrate the power of computational protein design for rapidly generating potential therapeutic candidates against pandemic threats.”

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Terahertz receiver for 6G wireless communications

Future wireless networks of the 6th generation (6G) will consist of a multitude of small radio cells that need to be connected by broadband communication links. In this context, wireless transmission at THz frequencies represents a particularly attractive and flexible solution. Researchers at Karlsruhe Institute of Technology (KIT) have now developed a novel concept for low-cost terahertz receivers that consist of a single diode in combination with a dedicated signal processing technique. In a proof-of-concept experiment, the team demonstrated transmission at a data rate of 115 Gbit/s and a carrier frequency of 0.3 THz over a distance of 110 meters. The results are reported in Nature Photonics.

5G will be followed by 6G: The 6th generation of mobile communications promises even higher data rates, shorter latency, and strongly increased densities of terminal devices, while exploiting Artificial Intelligence (AI) to control devices or autonomous vehicles in the Internet-of-Things era. “To simultaneously serve as many users as possible and to transmit data at utmost speed, future wireless networks will consist of a large number of small radio cells,” explains Professor Christian Koos, who works on 6G technologies at KIT together with his colleague Professor Sebastian Randel. In these radio cells, distances are short such that high data rates can be transmitted with minimum energy consumption and low electromagnetic immission. The associated base stations will be compact and can easily be mounted to building facades or street lights.

To form a powerful and flexible network, these base stations need to be connected by high-speed wireless links that offer data rates of tens or even hundreds of gigabits per second (Gbit/s). This may be accomplished by terahertz carrier waves, which occupy the frequency range between microwaves and infrared light waves. However, terahertz receivers are still rather complex and expensive and often represent the bandwidht bottleneck of the entire link. In cooperation with Virginia Diodes (VDI) in Charlottesville, USA, researchers of KIT’s Institute of Photonics and Quantum Electronics (IPQ), Institute of Microstructure Technology (IMT), and Institute for Beam Physics and Technology (IBPT) have now demonstrated a particularly simple inexpensive receiver for terahertz signals. 

Highest Data Rate Demonstrated So Far for Wireless THz Communications over More Than 100 Meters

“At its core, the receiver consists a single diode, which rectifies the terahertz signal,” says Dr. Tobias Harter, who carried out the demonstration together with his colleague Christoph Füllner in the framework of his doctoral thesis. The diode is a so-called Schottky barrier diode, that offers large bandwidth and that is used as an envelope detector to recover the amplitude of the terahertz signal. Correct decoding of the data, however, additionally requires the time-dependent phase of the terahertz wave that is usually lost during rectification. To overcome this problem, researchers use digital signal processing techniques in combination with a special class of data signals, for which the phase can be reconstructed from the amplitude via the so-called Kramers-Kronig relations. The Kramers-Kronig relation describe a mathematical relationship between the real part and the imaginary part of an analytic signal. Using their receiver concept, the scientists achieved a transmission rate of 115 Gbit/s at a carrier frequency of 0.3 THz over a distance of 110 m. “This is the highest data rate so far demonstrated for wireless terahertz transmission over more than 100 m,” Füllner says. The terahertz receiver developed by KIT stands out due to its technical simplicity and lends itself to cost-efficient mass production.

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Giant nanomachine aids the immune system

Cells that are infected by a virus or carry a carcinogenic mutation, for example, produce proteins foreign to the body. Antigenic peptides resulting from the degradation of these exogenous proteins inside the cell are loaded by the peptide-loading complex onto so-called major histocompatibility complex molecules (MHC for short) and presented on the cell surface. There, they are specifically identified by T-killer cells, which ultimately leads to the elimination of the infected cells. This is how our immune system defends us against pathogens.

Machine operates with atomic precision

The peptide-loading complex ensures that the MHC molecules are correctly loaded with antigens. “The peptide-loading complex is a biological nanomachine that has to work with atomic precision in order to efficiently protect us against pathogens that cause disease,” says Professor Lars Schäfer, Head of the Molecular Simulation research group at the Centre for Theoretical Chemistry at RUB.

In previous studies, other teams successfully determined the structure of the peptide-loading complex using cryo-electron microscopy, but only with a resolution of about 0.6 to 1.0 nanometres, i.e. not in atomic detail. Based on these experimental data, Schäfer’s research team in collaboration with Professor Gunnar Schröder from Forschungszentrum Jülich has now succeeded in creating an atomic structure of the peptide-loading complex.

Exploring structure and dynamics

“The experimental structure is impressive. But only with our computer-based methods were we able to extract the maximum information content contained in the experimental data,” explains Schröder. The atomic model enabled the researchers to perform detailed molecular dynamics computer simulations of the peptide-loading complex and thus to study not only the structure but also the dynamics of the biological nanomachine.

Since the simulated system is extremely large with its 1.6 million atoms, the computing time at the Leibnitz Supercomputing Centre in Munich aided this task considerably. “Using the high-performance computer, we were able to push into the microsecond time scale in our simulations. This revealed the role of sugar groups bound to the protein for the mechanism of peptide loading, which had previously only been incompletely understood,” outlines Dr. Olivier Fisette, postdoc researcher at the Molecular Simulation research group.

Direct intervention in immune processes

The atomic model of the peptide-loading complex now facilitates further studies. For example, some viruses try to cheat our immune system by selectively switching off certain elements of the peptide-loading complex. “One feasible objective we’d like to pursue is the targeted intervention in these processes,” concludes Schäfer.

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Fuel cells for hydrogen vehicles are becoming longer lasting

Fuel cells are gaining in importance as an alternative to battery-operated electromobility in heavy traffic, especially since hydrogen is a CO2-neutral energy carrier if it is obtained from renewable sources. For efficient operation, fuel cells need an electrocatalyst that improves the electrochemical reaction in which electricity is generated. The platinum-cobalt nanoparticle catalysts used as standard today have good catalytic properties and require only as little as necessary rare and expensive platinum. In order for the catalyst to be used in the fuel cell, it must have a surface with very small platinum-cobalt particles in the nanometer range, which is applied to a conductive carbon carrier material. Since the small particles and also the carbon in the fuel cell are exposed to corrosion, the cell loses efficiency and stability over time.

An international team led by Professor Matthias Arenz from the Department of Chemistry and Biochemistry (DCB) at the University of Bern has now succeeded in using a special process to produce an electrocatalyst without a carbon carrier, which, unlike existing catalysts, consists of a thin metal network and is therefore more durable. “The catalyst we have developed achieves high performance and promises stable fuel cell operation even at higher temperatures and high current density,” says Matthias Arenz. The results have been published in Nature Materials. The study is an international collaboration between the DCB and, among others, the University of Copenhagen and the Leibniz Institute for Plasma Science and Technology, which also used the Swiss Light Source (SLS) infrastructure at the Paul Scherrer Institute.

The fuel cell — direct power generation without combustion

In a hydrogen fuel cell, hydrogen atoms are split to generate electrical power directly from them. For this purpose, hydrogen is fed to an electrode, where it is split into positively charged protons and negatively charged electrons. The electrons flow off via the electrode and generate electric current outside the cell, which drives a vehicle engine, for example. The protons pass through a membrane that is only permeable to protons and react on the other side on a second electrode coated with a catalyst (here from a platinum-cobalt alloy network) with oxygen from the air, thus producing water vapor. This is discharged via the “exhaust.”

The important role of the electrocatalyst

For the fuel cell to produce electricity, both electrodes must be coated with a catalyst. Without a catalyst, the chemical reactions would proceed very slowly. This applies in particular to the second electrode, the oxygen electrode. However, the platinum-cobalt nanoparticles of the catalyst can “melt together” during operation in a vehicle. This reduces the surface of the catalyst and therefore the efficiency of the cell. In addition, the carbon normally used to fix the catalyst can corrode when used in road traffic. This affects the service life of the fuel cell and consequently the vehicle. “Our motivation was therefore to produce an electrocatalyst without a carbon carrier that is nevertheless powerful,” explains Matthias Arenz. Previous, similar catalysts without a carrier material always only had a reduced surface area. Since the size of the surface area is crucial for the catalyst’s activity and hence its performance, these were less suitable for industrial use.

Industrially applicable technology

The researchers were able to turn the idea into reality thanks to a special process called cathode sputtering. With this method, a material’s individual (here platinum or cobalt) are dissolved (atomized) by bombardment with ions. The released gaseous atoms then condense as an adhesive layer. “With the special sputtering process and subsequent treatment, a very porous structure can be achieved, which gives the catalyst a high surface area and is self-supporting at the same time. A carbon carrier is therefore superfluous,” says Dr. Gustav Sievers, lead author of the study from the Leibniz Institute for Plasma Science and Technology.

“This technology is industrially scalable and can therefore also be used for larger production volumes, for example in the automotive industry,” says Matthias Arenz. This process allows the hydrogen fuel cell to be further optimized for use in road traffic. “Our findings are consequently of importance for the further development of sustainable energy use, especially in view of the current developments in the mobility sector for heavy goods vehicles,” says Arenz.

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Bio-based communication networks could control cells in the body to treat conditions

Like electronic devices, biological cells send and receive messages, but they communicate through very different mechanisms. Now, scientists report progress on tiny communication networks that overcome this language barrier, allowing electronics to eavesdrop on cells and alter their behavior — and vice versa. These systems could enable applications including a wearable device that could diagnose and treat a bacterial infection or a capsule that could be swallowed to track blood sugar and make insulin when needed.

The researchers will present their results today at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo.

“We want to expand electronic information processing to include biology,” says principal investigator William E. Bentley, Ph.D. “Our goal is to incorporate biological cells in the computational decision-making process.”

The new technology Bentley’s team developed relies on redox mediators, which move electrons around cells. These small molecules carry out cellular activities by accepting or giving up electrons through reduction or oxidation reactions. Because they can also exchange electrons with electrodes, thereby producing a current, redox mediators can bridge the gap between hardware and living tissue. In ongoing work, the team, which includes co-principal investigator Gregory F. Payne, Ph.D., is developing interfaces to enable this information exchange, opening the way for electronic control of cellular behavior, as well as cellular feedback that could operate electronics.

“In one project that we are reporting on at the meeting, we engineered cells to receive electronically generated information and transmit it as molecular cues,” says Eric VanArsdale, a graduate student in Bentley’s lab at the University of Maryland, who is presenting the latest results at the meeting. The cells were designed to detect and respond to hydrogen peroxide. When placed near a charged electrode that generated this redox mediator, the cells produced a corresponding amount of a quorum sensing molecule that bacteria use to signal to each other and modulate behavior by altering gene expression.

In another recent project, the team engineered two types of cells to receive molecular information from the pathogenic bacteria Pseudomonas aeruginosa and convert it into an electronic signal for diagnostic and other applications. One group of cells produced the amino acid tyrosine, and another group made tyrosinase, which converts tyrosine into a molecule called L-DOPA. The cells were engineered so this redox mediator would be produced only if the bacteria released both a quorum sensing molecule and a toxin associated with a virulent stage of P. aeruginosa growth. The size of the resulting current generated by L-DOPA indicated the amount of bacteria and toxin present in a sample. If used in a blood test, the technique could reveal an infection and also gauge its severity. Because this information would be in electronic form, it could be wirelessly transmitted to a doctor’s office and a patient’s cell phone to inform them about the infection, Bentley says. “Ultimately, we could engineer it so that a wearable device would be triggered to give the patient a therapeutic after an infection is detected.”

The researchers envision eventually integrating the communication networks into autonomous systems in the body. For instance, a diabetes patient could swallow a capsule containing cells that monitor blood sugar. The device would store this blood sugar data and periodically send it to a cell phone, which would interpret the data and send back an electronic signal directing other cells in the capsule to make insulin as needed. As a step toward this goal, VanArsdale developed a biological analog of computer memory that uses the natural pigment melanin to store information and direct cellular signaling.

In other work, Bentley’s team and collaborators including Reza Ghodssi, Ph.D., recently designed a system to monitor conditions inside industrial bioreactors that hold thousands of gallons of cell culture for drug production. Currently, manufacturers track oxygen levels, which are vital to cells’ productivity, with a single probe in the side of each vessel. That probe can’t confirm conditions are uniform everywhere in the bioreactor, so the researchers developed “smart marbles” that will circulate throughout the vessel measuring oxygen. The marbles transmit data via Bluetooth to a cell phone that could adjust operating conditions. In the future, these smart marbles could serve as a communication interface to detect chemical signals within a bioreactor, send that information to a computer, and then transmit electronic signals to direct the behavior of engineered cells in the bioreactor. The team is working with instrument makers interested in commercializing the design, which could be adapted for environmental monitoring and other uses.

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For solar boom, scrap silicon for this promising mineral

When it comes to the future of solar energy cells, say farewell to silicon and hello to calcium titanium oxide — the compound mineral better known as perovskite.

Cornell University engineers have found that photovoltaic wafers in solar panels with all-perovskite structures outperform photovoltaic cells made from state-of-the-art crystalline silicon, as well as perovskite-silicon tandem cells, which are stacked pancake-style cells that absorb light better.

In addition to offering a faster return on the initial energy investment than silicon-based solar panels, all-perovskite solar cells mitigate climate change because they consume less energy in the manufacturing process, according to Cornell research published in Science Advances.

“Layered tandem cells for solar panels offer more efficiency, so this is a promising route to widespread deployment of photovoltaics,” said Fengqi You, Professor in Energy Systems Engineering at Cornell.

The paper, “Life Cycle Energy Use and Environmental Implications of High-Performance Perovskite Tandem Solar Cells,” compares energy and life-cycle environmental impacts of modern tandem solar cells made of silicon and perovskites.

Perovskite needs less processing, and much less of the heat or pressure, during the fabrication of solar panels, You said.

Silicon photovoltaics require an expensive initial energy outlay, and the best ones take about 18 months to get a return on that investment. A solar cell wafer with an all-perovskite tandem configuration, according to the researchers, offers an energy payback on the investment in just four months. “That’s a reduction by a factor of 4.5, and that’s very substantial,” You said.

But solar panels don’t last forever. After decades of service, silicon solar panels become less efficient and must be retired. And as in the manufacturing phase, breaking down silicon panels for recycling is energy intensive. Perovskite cells can be recycled more easily.

“When silicon-based solar panels have reached the end of their efficiency lifecycle, the panels must be replaced,” You said. “For silicon, it’s like replacing the entire automobile at the end of its useful life,” while replacing perovskite solar panels is akin to installing a new battery.

Adopting materials and processing steps to make perovskite solar cell manufacturing scalable is also critical to developing sustainable tandem solar cells, You said.

“Perovskite cells are promising, with a great potential to become cheaper, more energy-efficient, scalable and longer lasting,” You said. “Solar energy’s future needs to be sustainable.”

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Understanding the love-hate relationship of halide perovskites with the sun

Solar cells made of perovskite are at the center of much recent solar research. The material is cheap, easy to produce and almost as efficient as silicon, the material traditionally used in solar cells. However, perovskite cells have a love-hate-relationship with the sun. The light that they need to generate electricity, also impairs the quality of the cells, severely limiting their efficiency and stability over time. Research by scientists at the Eindhoven University of Technology and universities in China and the US now sheds new light on the causes of this degradation and paves the way for designing new perovskite compositions for the ultimate stable solar cells.

Perovskite is an attractive alternative to silicon, because it’s abundant and easy to produce. What’s more, over the past decade, the performance of perovskite solar cells has improved dramatically, with efficiency rates reaching more than 25 percent, which is close to the state-of-art for silicon solar cells.

The new research focuses on perovskite solar cells made from formamidinium-cesium lead iodide, a halide compound that has become increasingly popular as it combines high efficiency and reasonable heat resistance with low manufacturing costs.

Love-hate

However, solar panels made of this particular compound have a rather ambivalent relationship with sunlight, a problem that is well-known in the field, but barely understood. While the light of the sun feeds it with the much-wanted energy to convert into electricity, it also impairs the stability of the cells. Over time this affects their performance.

To understand why this is the case, the researchers at TU/e, Peking University and University of California San Diego did both practical experiments — monitoring the photovoltaic performance of the panels over 600 hours of exposure and characterizing the degraded perovskites — and theoretical analysis.

From this they conclude that sunlight generates charged particles in the perovskite, which tend to flow to places in the solar panel where the band gap (the minimum amount of energy needed for generating the free electrons) is lowest, in this case the formamidinium perovskite. The resulting energy differences make the mixed compounds that worked together so well to make the cell efficient, fall apart into separate clusters. It appears that especially the cesium-heavy clusters (the green dots in the image) are photoinactive and current-blocking, limiting the performance of the device.

Solutions

According to Shuxia Tao, who together with PhD candidate Zehua Chen and her colleague Geert Brocks was responsible for the TU/e part of the research, the new findings are one step further to finding the way to possible solutions.

“By combining macroscopic tests, microscopic materials characterization and atomistic modelling, we were able to thoroughly understand the instability of halide perovskites that are intrinsic to device operation. This opens the possibility for designing new perovskite compositions for the ultimate stable solar cells.”

Possible strategies include using additives to enhance the chemical interaction inside the materials in the panels, tuning the band gaps by using other elements like bromide and rubidium instead of iodide and cesium, or modifying the energy levels to extract photo-carriers more efficiently.

Tao stresses that more research is needed to see what solution works best. In addition, separation of halide compounds is not the only cause for perovskite degradation. These additional causes require separate analysis.

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