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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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Materials provided by Penn State. Original written by A’ndrea Elyse Messer. Note: Content may be edited for style and length.

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Egg-based coating preserves fresh produce

Eggs that would otherwise be wasted can be used as the base of an inexpensive coating to protect fruits and vegetables, according to Rice University researchers.

The Brown School of Engineering lab of materials scientist Pulickel Ajayan and colleagues have developed a micron-thick coating that solves problems both for the produce and its consumers, as well as for the environment.

When the coating was applied to produce by spraying or dipping, it showed a remarkable ability to resist rotting for an extended period comparable to standard coatings like wax but without some of the inherent problems.

The work by Rice undergraduate students Seohui (Sylvia) Jung and Yufei (Nancy) Cui is detailed in Advanced Materials.

The coating relies on eggs that never reach the market. As the United States produces more than 7 billion eggs a year and manufacturers reject 3% of them, the researchers estimate more than 200 million eggs end up in landfills.

Even before the impact of the new coronavirus, the world wasted a third of the food produced around the globe, the researchers wrote.

“Reducing food shortages in ways that don’t involve genetic modification, inedible coatings or chemical additives is important for sustainable living,” Ajayan said. “The work is a remarkable combination of interdisciplinary efforts involving materials engineers, chemists and biotechnologists from multiple universities across the U.S.”

Along with being edible, the multifunctional coating retards dehydration, provides antimicrobial protection and is largely impermeable both to water vapor to retard dehydration and to gas to prevent premature ripening. The coating is all-natural and washes off with water.

“If anyone is sensitive to the coating or has an egg allergy, they can easily eliminate it,” Jung said.

Egg whites (aka albumen) and yolks account for nearly 70 percent of the coating. Most of the rest consists of nanoscale cellulose extracted from wood, which serves as a barrier to water and keeps produce from shriveling, a small amount of curcumin for its antimicrobial powers and a splash of glycerol to add elasticity.

Lab tests on dip-coated strawberries, avocadoes, bananas and other fruit showed they maintained their freshness far longer than uncoated produce. Compression tests showed coated fruit were significantly stiffer and more firm than uncoated and demonstrated the coating’s ability to keep water in the produce, slowing the ripening process.

An analysis of freestanding films of the coating showed it to be extremely flexible and able to resist cracking, allowing better protection of the produce. Tests of the film’s tensile properties showed it to be just as tough as other products, including synthetic films used in produce packaging. Further tests proved the coating to be nontoxic, and solubility tests showed a thicker-than-usual film is washable.

Rinsing in water for a couple of minutes can completely disintegrate it, Ajayan said.

The researchers continue to refine the coating’s composition and are considering other source materials. “We chose egg proteins because there are lots of eggs wasted, but it doesn’t mean we can’t use others,” said co-corresponding author Muhammad Rahman, a research scientist in Ajayan’s Rice lab, who mentored and led the team.

Jung noted the team is testing proteins that could be extracted from plants rather than animal produce to make coatings.

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‘Deceptively simple’ process could boost plastics recycling

Plastics are a victim of their own success, so inexpensive, easy to use and versatile that the world is awash in plastic waste. Now researchers from the University of Houston have reported a new method of producing polyolefins — made from hydrocarbons and the most common building block of plastics — structured to address one of the biggest stumbling blocks to plastics recycling.

The process also would allow plastics to be produced from food oils and other natural substances.

Eva Harth, director of the Welch-UH Center for Excellence in Polymer Chemistry, said the process addresses a long-standing need for industrial plastics producers, without requiring a new catalyst or expensive additives. “It’s a very simple process,” she said.

Harth is a corresponding author for a paper describing the discovery, published in the German journal Angewandte Chemie. Co-authors include co-corresponding author Glen R. Jones, a post-doctoral researcher with the Welch-UH Center, and first author Hatice E. Basburg Alhan, a graduate student at UH.

Polyolefins, and derivative products such as polyethylene and polypropylene, are used for everything from grocery bags to industrial pipes. The qualities of the plastics — rigidity vs. flexibility, for example — are determined in part by a chemical process known as branching: Jones said highly branched polyolefins are used in products that require softness or flexibility, such as grocery bags, while low branching is used to produce rigid plastics.

Traditionally, different catalysts have been required to spark differing levels of branching, Harth said, meaning that only one type of plastic could be produced at a time. “You have to be specific about what material you are after, what type of branching you need,” she said.

The new method allows branching to be modulated using a palladium catalyst with varying amounts of added aluminum chloride, which functioned as a Lewis acid; the aluminum chloride — an abundant and inexpensive substance — can be added at different points in the process, allowing the resulting polyolefin to contain differing branching properties.

The new process could address two growing issues faced by plastics producers — how to dispose of plastic waste in an environmentally friendly way, and how to reduce the use of oil and natural gas by instead using food oils and other natural substances.

Current polymers used in everyday materials — grocery bags, milk jugs, toys and medical equipment, for example — won’t readily mix when they are melted down for chemical recycling. “These new polymers could sit at the boundary,” Jones said, allowing plastics with disparate properties to be more easily recycled.

The process will work with a variety of molecules to produce a polymer, Harth said, suggesting that the concept provides a new platform to produce plastics.

And that platform, she said, could lend itself to producing a variety of functional plastics from natural oils and other molecular sources. “This has exciting sustainability possibilities for the industry.”

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Metal-organic frameworks can separate gases despite the presence of water

Metal-organic frameworks (MOFs) are promising materials for inexpensive and less energy-intensive gas separation even in the presence of impurities such as water.

Experimental analyses of the performance of metal-organic frameworks (MOFs) for the separation of propane and propene under real-world conditions revealed that the most commonly used theory to predict the selectivity does not yield accurate estimates, and also that water as an impurity does not have a detrimental effect on the material’s performance.

Short chain hydrocarbons are produced in mixtures after treatment of crude oil in refineries and need to be separated in order to be industrially useful. For example, propane is used as a fuel and propene as a raw material for chemical synthesis such as the production of polymers. However, the separation process usually requires high temperatures and pressures, and additionally the removal of other impurities such as water makes the process costly and energy-consuming.

The structure of the studied MOF offers a long-lived, adaptable, and most importantly efficient separation alternative at ambient conditions. They build on the fact that unsaturated molecules such as propene can be complexed with the material’s exposed metal atoms, while saturated ones such as propane fail to do so. While research has focused on developing different metal-organic frameworks for different separation processes, the feasibility of using these materials on industrial-scale applications is commonly only gauged by relying on a theory that makes many idealizing assumptions on both the material and the purity of the gasses. Thus, it has not been clear whether these predictions hold under more complicated but also more realistic conditions.

A team of Hokkaido University researchers around Professor Shin-ichiro Noro in collaboration with Professor Roland A. Fischer’s group at the Technical University of Munich conducted a series of measurements on the performance of a prototypical MOF to ascertain the material’s real-world selectivity, for both completely dry frameworks and ones pre-exposed to water.

Their results recently published in ACS Applied Materials & Interfaces show that the predicted selectivities of the material are too high compared to the real-world results. It also demonstrated that water does not drastically decrease the selectivity, although it does reduce the material’s capacity to adsorb gas. The team then performed quantum-chemical computations to understand why and realized that the water molecules themselves offer new binding sites to unsaturated hydrocarbons, such as propene (but not propane), thus retaining the material’s functionality.

The researchers state: “We showed the power of multi-component adsorption experiments to analyze the feasibility of using an MOF system.” They thus want to raise awareness of the shortcomings of commonly used theories and motivate other groups to also use a combination of different real-world measurements.

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New tech takes radiation out of cancer screening

Researchers have developed a new, inexpensive technology that could save lives and money by routinely screening women for breast cancer without exposure to radiation.

The system, developed by researchers at the University of Waterloo, uses harmless microwaves and artificial intelligence (AI) software to detect even small, early-stage tumors within minutes.

“Our top priorities were to make this detection-based modality fast and inexpensive,” said Omar Ramahi, a professor of electrical and computer engineering at Waterloo. “We have incredibly encouraging results and we believe that is because of its simplicity.”

A prototype device — the culmination of 15 years of work on the use of microwaves for tumor detection, not imaging — cost less than $5,000 to build.

It consists of a small sensor in an adjustable box about 15 centimetres square that is situated under an opening in a padded examination table.

Patients lie face-down on the table so that one breast at a time is positioned in the box. The sensor emits microwaves that bounce back and are then processed by AI software on a laptop computer.

By comparing the tissue composition of one breast with the other, the system is sensitive enough to detect anomalies less than one centimeter in diameter.

Ramahi said a negative result could quickly rule out cancer, while a positive result would trigger referral for more expensive tests using mammography or magnetic resonance imaging (MRI).

“If women were screened regularly with this, potential problems would be caught much sooner — in the early stages of cancer,” he said. “Our system can complement existing technology, reserving much more expensive options for when they’re really needed.

“We need a mixture, a combination of technologies. When our device sent up a red flag, it would mean more investigation was warranted.”

In addition to reducing patient wait times and enabling earlier diagnosis, Ramahi said, the device would eliminate radiation exposure, improve patient comfort and work on particularly dense breasts, a problem with mammograms.

It would also save health-care systems enormous amounts of money and, because of its low cost and ease of use, dramatically increase access to screening in the developing world.

Researchers have applied for a patent and started a company, Wave Intelligence Inc. of Waterloo, to commercialize the system and hope to begin trials on patients within six months. Three rounds of preliminary testing included the use of artificial human torsos known as phantoms.

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Simple test could prevent fluoride-related disease

Northwestern University synthetic biologists developed a simple, inexpensive new test that can detect dangerous levels of fluoride in drinking water.

Costing just pennies to make, the system only needs a drip and a flick: Drip a tiny water droplet into a prepared test tube, flick the tube once to mix it and wait. If the water turns yellow, then an excessive amount of fluoride — exceeding the EPA’s most stringent regulatory standards — is present.

This method is starkly different from current tests, which cost hundreds of dollars and often require scientific expertise to use.

The researchers tested the system both in the laboratory at Northwestern and in the field in Costa Rica, where fluoride is naturally abundant near the Irazu volcano. When consumed in high amounts over long periods of time, fluoride can cause skeletal fluorosis, a painful condition that hardens bones and joints.

Americans tend to think of the health benefits of small doses of fluoride that strengthen teeth. But elsewhere in the world, specifically across parts of Africa, Asia and Central America, fluoride naturally occurs at levels that are dangerous to consume.

“In the United States, we hear about fluoride all the time because it’s in toothpaste and the municipal water supply,” said Northwestern’s Julius Lucks, who led the project. “It makes calcium fluoride, which is very hard, so it strengthens our tooth enamel. But above a certain level, fluoride also hardens joints. This mostly isn’t an issue in the U.S. But it can be a debilitating problem in other countries if not identified and addressed.”

The research was published online last week (Dec. 13) in the journal ACS Synthetic Biology.

Lucks is an associate professor of chemical and biological engineering in the McCormick School of Engineering and a member of Northwestern’s Center for Synthetic Biology. The work was performed in collaboration with Michael Jewett, professor of chemical and biological engineering in McCormick and director of the Center for Synthetic Biology. Graduate students Walter Thavarajah, Adam Silverman and Matthew Verosloff spearheaded the research.

Field test success

Fluoride is a naturally occurring element, which can seep out of bedrock into groundwater. Also found in volcanic ash, fluoride is particularly abundant in regions surrounding volcanoes.

Home to three volcanic range systems, Costa Rica seemed like a natural place to test the device in the field. Matthew Verosloff, a Ph.D. candidate in Lucks’ laboratory, traveled to Costa Rica and sampled various water samples — from mud puddles, ponds and ditches.

“Every test on these field samples worked,” Lucks said. “It’s exciting that it works in the lab, but it’s much more important to know that it works in the field. We want it to be an easy, practical solution for people who have the greatest need. Our goal is to empower individuals to monitor the presence of fluoride in their own water.”

How it works

Although the device is simple to use, the prepared test tube houses a sophisticated synthetic biology reaction. Lucks has spent years working to understand RNA folding mechanisms. In his new test, he puts this folding mechanism to work.

“RNA folds into a little pocket and waits for a fluoride ion,” he explained. “The ion can fit perfectly into that pocket. If the ion shows up, then RNA expresses a gene that turns the water yellow. If the ion doesn’t show up, then RNA changes shape and stops the process. It’s literally a switch.”

According to Lucks, organisms already perform this function in nature. “Fluoride is toxic to bacteria,” he said. “They use RNA to sense fluoride in the cell, then they make a protein to pump it out and detoxify.”

Lucks’ system works in the same way. But instead of producing a protein pump, his test produces a protein enzyme that makes a yellow pigment, so people can see the results with a simple glance.

Lucks’ team freeze-dried the RNA reaction, which looks like a tiny cotton ball, and put it into a test tube. In this form, the reaction is safe and shelf-stable. A small pipette accompanies the test tube. When placed in water, the pipette absorbs exactly 20 microliters — just the small drop that’s needed to rehydrate the reaction. From there, it takes two hours to get a result, which Lucks intends to accelerate in future iterations.

“We’re currently limited to testing for fluoride,” said Thavarajah, the paper’s first author. “But we’re trying to engineer other RNAs to detect all sorts of targets.”

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Device generates light from the cold night sky

An inexpensive thermoelectric device harnesses the cold of space without active heat input, generating electricity that powers an LED at night, researchers report September 12 in the journal Joule.

“Remarkably, the device is able to generate electricity at night, when solar cells don’t work,” says lead author Aaswath Raman (@aaraman), an assistant professor of materials science and engineering at the University of California, Los Angeles. “Beyond lighting, we believe this could be a broadly enabling approach to power generation suitable for remote locations, and anywhere where power generation at night is needed.”

While solar cells are an efficient source of renewable energy during the day, there is currently no similar renewable approach to generating power at night. Solar lights can be outfitted with batteries to store energy produced in daylight hours for night-time use, but the addition drives up costs.

The device developed by Raman and Stanford University scientists Wei Li and Shanhui Fan sidesteps the limitations of solar power by taking advantage of radiative cooling, in which a sky-facing surface passes its heat to the atmosphere as thermal radiation, losing some heat to space and reaching a cooler temperature than the surrounding air. This phenomenon explains how frost forms on grass during above-freezing nights, and the same principle can be used to generate electricity, harnessing temperature differences to produce renewable electricity at night, when lighting demand peaks.

Raman and colleagues tested their low-cost thermoelectric generator on a rooftop in Stanford, California, under a clear December sky. The device, which consists of a polystyrene enclosure covered in aluminized mylar to minimize thermal radiation and protected by an infrared-transparent wind cover, sat on a table one meter above roof level, drawing heat from the surrounding air and releasing it into the night sky through a simple black emitter. When the thermoelectric module was connected to a voltage boost convertor and a white LED, the researchers observed that it passively powered the light. They further measured its power output over six hours, finding that it generated as much as 25 milliwatts of energy per square meter.

Since the radiative cooler consists of a simple aluminum disk coated in paint, and all other components can be purchased off the shelf, Raman and the team believe the device can be easily scaled for practical use. The amount of electricity it generates per unit area remains relatively small, limiting its widespread applications for now, but the researchers predict it can be made twenty times more powerful with improved engineering — such as by suppressing heat gain in the radiative cooling component to increase heat-exchange efficiency — and operation in a hotter, drier climate.

“Our work highlights the many remaining opportunities for energy by taking advantage of the cold of outer space as a renewable energy resource,” says Raman. “We think this forms the basis of a complementary technology to solar. While the power output will always be substantially lower, it can operate at hours when solar cells cannot.”

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Lollies, vitamins and fish-shaped sauce containers hit the MRI mark

For children fearful of undergoing MRI scans, an inexpensive everyday item used as a marker, such as a jelly baby lolly or a plastic, fish-shaped soy sauce container, might make the process a little less intimidating.

And thanks to a QUT study, these and other relatively cheap, common items have been shown to be visible, effective MRI markers, when placed on a patient’s skin, to pinpoint specific anatomical areas or pathologies being scanned.

The study findings have been published in the British Medical Journal’s BMJ Open.

Senior Research Fellow Dr Paige Little, from the QUT Biomechanics and Spine Research Group, said MRI uses strong magnetic fields to generate images of organs, bone and tissue inside the body and the loud noises made by the equipment adds to the challenges radiographers face to perform MRI scans successfully on children.

She said the impetus for looking at alternatives to commercial markers was the group’s collaborative sleep postures research project.

“Single-use commercial markers cost between $6 and $10 each, and for our sleep posture study we had 50 participants, and we needed 50 markers for each participant, which made the cost prohibitive,” Dr Little said.

“We needed to find a marker that was small, inexpensive and easily sourced, which showed up clearly on MRI and was easily distinguishable from bone and soft tissue.

“While makeshift markers of various types, including fish oil capsules, have been anecdotally trialled in clinical radiology departments over the years, we couldn’t find a study in the scientific literature, so we did our own trial.

“We tested 17 items, including different lollies, a coffee bean, vitamin tablets and capsules, and the capped fish-shaped soy sauce containers that you can get when you buy sushi.”

Dr Little said all the alternative markers, plus a commercial product for comparison, were tested on the thigh of a member of the research team, and scanned at Mater Medical Imaging using the five most commonly ordered MRI sequences.

“The visibility of some of the items, like the coffee bean, was poor, but overall we found the vitamin D capsule proved to be the best substitute for a commercial marker for all tests, and particularly good for smaller areas of the body like fingers and toes,” Dr Little said.

“The jelly baby lolly, fish-shaped sauce container, and the fish oil capsule were also viable alternatives for some, although not all, of the MRI sequences.

“Our conclusion was that depending on the reason for imaging and the sequence selected, these four items were cheap and reliable alternatives to a commercial marker.

“Single-use markers are a significant component of an imaging department’s costs, and also for researchers, so we thought this study was a practical examination of viable alternatives.

“And if using something familiar would help make the imaging process less frightening for children, particularly those children who have experienced many medical procedures, then that is a terrific bonus.”

Dr Little said while the study didn’t test at what point a vitamin capsule or sauce container might rupture and spill its contents, the research group has since used vitamin D capsules routinely in spinal studies, and in its Sealy of Australia-supported ‘Science of Sleep’ postures project, requiring multiple markers under body weight throughout extended MRI scanning sessions, and none had ruptured or degraded.

Co-authors with Dr Little of the BMJ Open study are Mrs Maree Izatt, Dr Caroline Grant, and Dr Deborah Lees from the QUT Biomechanics and Spine Research Group, and Ms Susan Mills from Mater Medical Imaging.

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A SimpliSafe Security System Can Be Bypassed with a $2 Device

Security systems like SimpliSafe promise to make it simple, easy, and inexpensive to keep your home safe. These, along with many similar systems, utilize a number of wireless sensors that can be positioned at entry points in your home. Those simple magnetic sensors can be placed on doors or windows, and send a signal to the base station when the entry point is breached. The base station, in turn, can sound an alarm, call the police, or send a notification to your phone. But as YouTuber LockPickingLawyer demonstrates, burglars can bypass the alarm using a readily-available $2 device.

This vulnerability relies on the fact that the SimpliSafe security system, along with other budget systems, has wireless sensors that transmit at 433.92 MHz. That frequency band is used by a huge variety of wireless home products, and is also part of the amateur radio range. It’s inexpensive for SimpliSafe to manufacture and sell sensors that utilize that frequency, but it isn’t ideal from a security perspective. That’s because it can be easily overcome by interference, preventing the system from registering that a door or window has been opened.

As LockPickingLawyer explains, that can be done using just about any transmitter that works in the same frequency. A simple $25 handheld ham radio works well and has a lot of power, but even a $2 wireless remote can cause enough interference to stop the system from registering a sensor signal. The SimpliSafe system does have the ability to detect high levels of interference, but won’t call the police when it does — there would just be too many false alarms triggered by nearby devices. But you can set it to send you a notification when interference is detected, which is probably prudent if you own one of these systems.

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Author: Cameron Coward