UC Berkeley Labs: A Semiconductor That Can Beat the Heat

Berkeley Lab, UC Berkeley scientists discover unique thermoelectric properties in cesium tin iodide

JULY 31, 2017

A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity – a rare pairing that scientists say could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

A team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered these exotic traits in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.

These interrelated thermal and electrical (or “thermoelectric”) properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure.

Image – Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). (Credit: Berkeley Lab/UC Berkeley)

This so-called single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

“Its properties originate from the crystal structure itself. It’s an atomic sort of phenomenon,” said Woochul Lee, a postdoctoral researcher at Berkeley Lab who was the lead author of the study, published the week of July 31 in the Proceedings of the National Academy of Sciences journal. These are the first published results relating to the thermoelectric performance of this single crystal material.

Researchers earlier thought that the material’s thermal properties were the product of “caged” atoms rattling around within the material’s crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material.

“We initially thought it was atoms of cesium, a heavy element, moving around in the material,” said Peidong Yang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division who led the study.

Jeffrey Grossman, a researcher at the Massachusetts Institute of Technology, then performed some theory work and computerized simulations that helped to explain what the team had observed. 

Researchers also used Berkeley Lab’s Molecular Foundry, which specializes in nanoscale research, in the study.

“We believe there is essentially a rattling mechanism, not just with the cesium. It’s the overall structure that’s rattling; it’s a collective rattling,” Yang said. “The rattling mechanism is associated with the crystal structure itself,” and is not the product of a collection of tiny crystal cages. “It is group atomic motion,” he added.

Within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.

But because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. Picture its electrical conductivity is like a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

Yang said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion, he said.

A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

Cesium tin iodide was first discovered as a semiconductor material decades ago, and only in recent years has it been rediscovered for its other unique traits, Yang said. “It turns out to be an amazing gold mine of physical properties,” he noted.

SEM images of suspended micro-island devices. Individual AIHP NW is suspended between two membranes. (Credit: Berkeley Lab/UC Berkeley)

To measure the thermal conductivity of the material, researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. The nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. 
Researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.

They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.

“A next step is to alloy this (cesium tin iodide) material,” Lee said. “This may improve the thermoelectric properties.”

Also, just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties – a process known as “doping” – scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials, Yang said.

The research team also included other scientists from Berkeley Lab’s Materials Sciences Division and the Molecular Foundry, the Kavli Energy NanoScience Institute at UC Berkeley and Berkeley Lab, and UC Berkeley’s Department of Chemistry.

The Molecular Foundry is a DOE Office of Science User Facility that provides free access to state-of-the-art equipment and multidisciplinary expertise in nanoscale science to visiting scientists from all over the world.

This work was supported by the Department of Energy’s Office of Basic Energy Sciences.
More information about Peidong Yang’s research group: http://nanowires.berkeley.edu/.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit http://www.lbl.gov.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Researchers develop blue-, yellow-, and red-emitting Graphene Quantum Dots

Graphene QD's China id47457_1

PL spectra of GQDs (a), PEI1800 GQDs (b), and PEI600 GQDs (c) at different excitation wavelengths. Inset: photograph of aqueous solution of these three GQDs under room light (left) and 365 nm UV irradiation lamp (right). UV-vis absorption spectra (d) of GQDs, PEI1800 GQDs, and PEI600 GQDs dispersed in water. (© ACS) 

Graphene quantum dots (GQDs) show great potential in the fields of photoelectronics, photovoltaics, biosensing, and bioimaging owing to their unique photoluminescence (PL) properties, including excellent biocompatibility, low toxicity, and high stability against photobleaching and photoblinking.

However, further development of GQDs is limited by their synthetic methodology and unclear PL mechanism. Therefore, it is urgent to find efficient and universal methods for the synthesis of GQDs with high stability, controllable surface properties, and tunable PL emission wavelength.In new work reported in ACS Applied Materials & Interfaces (“Red, Yellow, and Blue Luminescence by Graphene Quantum Dots: Syntheses, Mechanism, and Cellular Imaging”), researchers in China have synthesized PL-tunable GQDs with blue, yellow, and red emission colors by coating with polyethyleneimine (PEI) of different molecular weights.

photoluminescence spectra of graphene quantum dotsPL spectra of GQDs (a), PEI1800 GQDs (b), and PEI600 GQDs (c) at different excitation wavelengths. Inset: photograph of aqueous solution of these three GQDs under room light (left) and 365 nm UV irradiation lamp (right). UV-vis absorption spectra (d) of GQDs, PEI1800 GQDs, and PEI600 GQDs dispersed in water. (© ACS) (click on image to enlarge)

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(continued from above)

The team employed TEM, AFM, XRD, FTIR, XPS, DLS, and zeta potential to characterize the structures of the as-prepared GQDs and they stufied the PL mechanism by theoretical calculations.The average sizes of uncoated yellow-emitting GQDs, blue-emitting PEI1800 GQDs, and red-emitting PEI600 GQDs were 2.37, 6.05, and 57.31 nm, respectively. The yellow-emitting and blue-emitting GQDs were monolayer structures, whereas the red-emitting GQDs were multilayer structures. The red-emitting GQDs possessed a big PEI cage with multiple GQDs inside, whereas the blue-emitting PEI-coated GQDs had a single GQD core.The scientists found that carboxyl groups were changed to amide groups on the surface of GQDs and that this amidation reaction was crucial for PL change. By analyzing the molecular orbital and charge density, it was found that amide bonds decreased the conjugation and increased the energy gap thus inducing the blue shift of the PL.For the red-emitting GQDs, the conjugation area was enlarged by the interaction of GQDs in the PEI cage; thus, the PL peak exhibited a red shift.Remarkably, as the team points out, all GQDs exhibited good stability at high ionic strength and resisted photobleaching. Cell viability after treatment with the as-prepared GQDs indicated that GQDs had quite low cytotoxicity.”The GQDs could be used for bioimaging and are expected to be widely applied in multicolor imaging and bioanalysis applications,” the authors cocnlude their report. “We hope that this work will inspire the design of even better GQDs with tunable PL properties.”

Primary Story Contributed by Micheal Berger Nanowerk

Quantum Dot Transistor Simulates Synaptic Responses and Functions of Neurons


QD Transistor id47090

This research demonstrates a nanoscaled memdevice able to act as an electronic analogue of tipping buckets that allows reducing the dimensionality and complexity of a sensing problem by transforming it into a counting problem. The device offers a well adjustable, tunable, and reliable periodic reset that is controlled by the amounts of transferred quantum dot charges per gate voltage sweep. When subjected to periodic voltage sweeps, the quantum dot (bucket) may require up to several sweeps before a rapid full discharge occurs thus displaying period doubling, period tripling, and so on between self-governing reset operations. (© ACS)

A transistor that simulates some of the functions of neurons has been invented based on experiments and models developed by researchers at the Federal University of São Carlos (UFSCar) in São Paulo State, Brazil, Würzburg University in Germany, and the University of South Carolina in the United States.The device, which has micrometric as well as nanometric parts, can see light, count, and store information in its own structure, dispensing with the need for a complementary memory unit.It is described in an article in the journal Nano Letters (“Nanoscale tipping bucket effect in a quantum dot transistor-based counter”

“In this article, we show that transistors based on quantum dots can perform complex operations directly in memory. This can lead to the development of new kinds of device and computer circuit in which memory units are combined with logical processing units, economizing space, time, and power consumption,” said Victor Lopez Richard, a professor in UFSCar’s Physics Department and one of the coordinators of the study.
The transistor was produced by a technique called epitaxial growth, which consists of coating a crystal substrate with thin film. On this microscopic substrate, nanoscopic droplets of indium arsenide act as quantum dots, confining electrons in quantized states. Memory functionality is derived from the dynamics of electrical charging and discharging of the quantum dots, creating current patterns with periodicities that are modulated by the voltage applied to the transistor’s gates or the light absorbed by the quantum dots.
“The key feature of our device is its intrinsic memory stored as an electric charge inside the quantum dots,” Richard said. “The challenge is to control the dynamics of these charges so that the transistor can manifest different states. Its functionality consists of the ability to count, memorize, and perform the simple arithmetic operations normally done by calculators, but using incomparably less space, time, and power.”
According to Richard, the transistor is not likely to be used in quantum computing because this requires other quantum effects. However, it could lead to the development of a platform for use in equipment such as counters or calculators, with memory intrinsically linked to the transistor itself and all functions available in the same system at the nanometric scale, with no need for a separate space for storage.
“Moreover, you could say the transistor can see light because quantum dots are sensitive to photons,” Richard said, “and just like electric voltage, the dynamics of the charging and discharging of quantum dots can be controlled via the absorption of photons, simulating synaptic responses and some functions of neurons.”
Further research will be necessary before the transistor can be used as a technological resource. For now, it works only at extremely low temperatures – approximately 4 Kelvin, the temperature of liquid helium.
“Our goal is to make it functional at higher temperatures and even at room temperature. To do that, we’ll have to find a way to separate the electronic spaces of the system sufficiently to prevent them from being affected by temperature. We need more refined control of synthesis and material growth techniques in order to fine-tune the charging and discharging channels. And the states stored in the quantum dots have to be quantized,” Richard said.
Source: Fundação de Amparo à Pesquisa do Estado de São Paulo

Read more: Quantum dot transistor simulates functions of neurons

Advanced (SWIR) Quantum Dots Offer Solution for Tagging and Imaging the Biological Processes in LIVE Animals

nanocrystalsFluorescent quantum dots are valuable tools used to tag and image biological processes in live animals. However, precise fluorescent imaging at the cellular and molecular levels has not been possible because of non-specific fluorescence and light scattering by surrounding tissues.

Now researchers have created short wave infrared (SWIR) quantum dots that resolve many of these problems. The system was used in live mice to image working organs, take metabolic measurements, and track microvascular blood flow in normal brain and brain tumors.

“Quantum dots are small (nanoscale) particles that can be engineered to emit light at different wavelengths,” explains Behrouz Shabestari, Ph.D., director of the Optical Imaging Program at NIH’s National Institute of Biomedical Imaging and Bioengineering, which co-funded the research. “When they are injected into a live animal, the emitted fluorescent light can be seen with special cameras. By engineering the dots to bind to specific tissues of interest, researchers can use them to study biological processes in real-time.” qdot_tech_note_graph
An international group of investigators led by Moungi G. Bawendi, Ph.D., the Lester Wolfe Professor in Chemistry at the Massachusetts Institute of Technology, collaborated to create what Bawendi calls the “next-generation,” of quantum dots.
Said Bawendi, “We took advantage of the special qualities of short wave infrared light, which is essentially the ability to give a clear bright signal emitted from the tissue of interest that is not blocked or scattered by the surrounding tissues. The system allows us to view biological processes in living, moving animals with great clarity and detail.”
The work is described in the April issue of the journal Nature Biomedical Engineering (“Next-generation in vivo optical imaging with short-wave infrared quantum dots”).
experimental set-up with composite SWIR quantum dots injected into the circulation and then imaged through a cranial window in the mouse brain
The top outlines the experimental set-up with composite SWIR quantum dots injected into the circulation and then imaged through a cranial window in the mouse brain. The bottom shows the resulting fluorescent image with healthy arteries in red, veins in blue, and the disorganized blood vessels of a brain tumor in green.

Engineering SWIR quantum dots to target tissues of interest

While the inner core of a SWIR quantum dot (SWIR-QDs) generates the unique fluorescent properties of short wave infrared light, the other critical component of the dot is the outer surface, which must be engineered to target a tissue of interest. The researchers call this “functionalization,” which means making them useful for studying specific tissues and biological processes. Bawendi and colleagues engineered three distinct types of SWIR quantum dots to demonstrate their use in studying different biological processes.
The first type of SWIR-QDs were engineered with phospholipid micelle surface coatings. Micelles are small particles that have a hydrophilic (water-loving) outer shell and a hydrophobic (water repelling) inner layer. The micelle-embedded SWIR-QDs dissolved and circulated through the bloodstream for an extended period, allowing the researchers to study heart and respiration rates in awake mice.
The advantage of these SWIR-QDs is the ability to image physiological processes that occur too rapidly to be detected by common imaging methods such as MRI or PET. This ability would allow unobtrusive monitoring of animals in their normal environment for changes in heartbeat and breathing rates during various exercise tests or in response to drug candidates for conditions such as cardiac arrhythmia.
The second type of SWIR-QDs created were embedded in chylomicrons. Chylomicrons are lipoprotein particles that consist of triglycerides, phospholipids, cholesterol, and proteins and are known to transport dietary lipids from the intestines to other locations.

These SWIR-QDs were used to study the movement and metabolism of lipids between brown adipose tissue, blood, and liver in real-time. The researchers explained that lipid-coated SWIR-QDs could be used to assess the immediate effects of medications designed to affect lipid metabolism—for example, to increase the liver’s uptake of lipids from the bloodstream of an individual with high cholesterol.

SWIR quantum dot imaging
a) Experimental set-up with lipid micelle SWIR quantum dots injected into the circulation and whole body scan with SWIR camera. b) Resulting fluorescent image shows the accumulation of the lipid micelle SWIR quantum dots in the liver (blue circle) and heart (red circle).
The third type of SWIR-QDs were composites, containing multiples QDs, and coated with PEG, which allows them to dissolve in blood. This third type was used to measure blood flow in the vasculature of the mouse brain by tracking individual SWIR-QD composite particles as they moved through the blood vessels. The researchers could view the dramatic differences between blood flow in healthy vasculature and in vessels at the margin of a brain tumor.
These SWIR-QDs would make it possible to measure blood flow in the brain before and after a stroke, and changes in response to experimental stroke medications.
“In addition to the ability to test much-needed new medications to treat stroke, the potential application to difficult-to-treat tumors is one that we are also very excited about,” said Bawendi. “We can potentially use SWIR-QDs to study how the blood flow pattern in a tumor changes over time. We could monitor disease progression or regression in response to drug treatment.

This opens a new way to assess experimental treatments for both stroke and brain cancer that have not been possible with other imaging methods.”

Source: National Institute of Biomedical Imaging and Bioengineering

Read more: Advanced quantum dots shed bright light on biological processes

Quantum Dots (LE) Create Carbon-Bonds as effectively as the rare-metal catalysts = Low Cost Synthesis of Pharmaceuticals, Fine and Argo-Chemicals

lightemittinA quantum dot has the chemical and photo stability of minerals, but has a layer of organic molecules on the outside that “allows it to be manipulated just as you would manipulate small molecules in solution. You can spray them, you can coat …more

At one time you could wander through the labs of pharmaceutical companies and hardly ever see light being used to mediate chemical reactions. Now “photoredox catalysis” has become an essential way to synthesize novel organic compounds.

This type of chemistry may soon be used even more widely—and less expensively— thanks to University of Rochester researchers.

In a paper published recently in the Journal of the American Chemical Society, the labs of Todd Krauss and Daniel Weix demonstrate for the first time how light emitting can be used as photoredox catalysts to create .

Moreover, the researchers— including Jill Caputo ’16 (PhD) and Norman Zhao ’17 from Weix’s lab and Leah Frenette ’14 (MS) and Kelly Sowers ’16 (PhD) from Krauss’s group—showed that quantum dots create these bonds just as effectively as the rare-metal catalysts now used in photoredox chemistry, such as ruthenium and iridium.Efficient-and-Limiting-Reactions-in-Aqueous_14_jacs_Palomares_Llobet

“The potential impact could be great,” says Weix, an associate professor in the Department of Chemistry. Carbon-carbon bonds are the basic building blocks for numerous molecular forms, many of them essential to biological functions.

The quantum dots have potential applications in the synthesis of pharmaceuticals, fine chemicals, and agro-chemicals. “These are markets where people are most actively searching for chemical compounds with new properties,” Weix says.

Quantum dots are tiny semiconductor crystals. Containing some thousands of atoms, they “live in a world between bulk minerals—like a chunk of rock, with billions upon billions of atoms—and a single molecule with only 10 or 20 atoms,” says Krauss, a professor of chemistry and chair of the department. But, he adds, “quantum dots have properties of both the molecular and the macroscopic world.”

For example, a quantum dot has the chemical and photo stability of minerals, but has a layer of organic molecules on the outside that “allows it to be manipulated just as you would manipulate small molecules in solution. You can spray them, you can coat them on surfaces, you can mix them, and do all different chemistries with them,” Krauss says.

Until now, most chemists have studied quantum dots for their basic properties, with applications primarily limited to displays such as televisions. This particular discovery originated in prior work at Rochester that demonstrated quantum dots could be excellent catalysts for creating hydrogen-hydrogen bonds for solar fuel applications.

NPs Applications 353d9d7

For this study, Krauss and Weix tested the effectiveness of cadmium/selenium (CdSe) quantum dots in creating carbon-carbon bonds by using five well-known photoredox reactions. They found that a single-sized, easily made CdSe quantum dot could replace several different catalysts now used, with equal or greater efficiency.

“The chemistry ranged from more simple reactions, where the quantum dot served as the sole redox mediator [sole agent transferring an electron], to reactions involving one or more cocatalysts, with a lot of reagents in the flask,” Weix says. “There was a concern in the beginning whether the dots would survive in this chemical stew, but they did.”

Weix cautions that paper represents only a “first step towards showing you could use to replace other catalysts.” The dots may need to be further refined to be suitable for industrial applications.

But he’s excited about their potential, and momentum appears to be building. He notes that concurrent with their work, colleagues at Northwestern made important strides toward improving quantum dot catalysts. Weix further pointed to related photochemical work with nanocrystaline titanium dioxide (TiO2) from researchers at the University of Ottawa and the University of Wisconsin.

“We, and others, have so far looked at how quantum dots would perform in reactions that were reasonably well studied, because this is a new and we wanted to compare it to what came before,” Weix says. “The next step is to look at what these things do that nothing else can do. That’s the promise of the future.”

Explore further: Finding needles in chemical haystacks

More information: Jill A. Caputo et al. General and Efficient C–C Bond Forming Photoredox Catalysis with Semiconductor Quantum Dots, Journal of the American Chemical Society (2017). DOI: 10.1021/jacs.6b13379


An EV Battery That Charges Fully In 5 Minutes? Commercialization Step-Up Could Come Soon

storedot-ev-battery-21-889x592 (1)

Electric vehicles now comprise a substantial part of the automotive market. But the fact remains that despite the increasing number of charging stations, it is still inconvenient to charge a car in comparison to getting a tank full of gas.

StoreDot, an Israeli startup, might have the solution to the woes of electric vehicle (EV) owners, with a new battery it claims can fully charge in five minutes and drive the EV 300 miles on a single charge.

StoreDot aa8b81a83f20b19b089ceb4e4a25e036


Read About the Company: Enabling the Future of Charging

The battery is made of nano-materials in a layered structure, made of special organic compounds manufactured by the company. This, the company said, is a massive improvement over traditional lithium-ion battery.

The company first demonstrated the technology at Microsoft Think Next in 2015. The company says the batteries are in the “advanced stages of development” and might be integrated into electric vehicles in the next three years. It also says that its chemical compound is not flammable and has a higher level of combustion, reducing the level of resistance in the batteries making it safe for use in cars.

The batteries won’t be too difficult to manufacture either — the company estimates that 80 percent of the manufacturing process is the same as regular lithium-ion batteries.

StoreDot specializes in battery technology. Last year, it showcased a smartphone battery capable of fully charging within 30 seconds. The EV battery is a scaled up version of this battery which has multi-function electrodes, a combination of polymer and metal oxide.

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An electric car battery that could charge in just five minutes ~ Where is the Israeli Start-Up “+StoreDot” One Year Later? +Video

storedot-ev-battery-21-889x592 (1)

Perovskite Nanocrystals: Bright – Cheap – Stable: Discovery Illuminates Path to Highly Efficient Perovskite based Quantum Dots Photovoltaics

Perovskite Nanocrystals id46560

Digital picture of colloidal solution in toluene taken under UV-light (λ = 365 nm) and crystal structure of Formamidinium lead-halide perovskite. (Image: Friedrich-Alexander-Universität Erlangen-Nürnberg)

The team reports facile and rapid room temperature synthesis of cubic and platelet-like colloidal nanocrystals (NCs) of Formamidinium Lead Halide Perovskite FAPbX3 (X=Cl, Br, I, or mixed Cl/Br and Br/I) by ligand-assisted re-precipitation method (LARP).
The obtained NCs are 15-25 nm in size and exhibit a remarkably high photoluminescence quantum yield of up to 85% as well as colloidal and chemical stability.
The cubic and platelet-like NCs with their emission in the range of 415-740 nm, full width at half maximum of 20-44 nm and radiative lifetimes of 5-166 ns, allow precise band gap tuning by halide composition as well as by tailoring their dimensions.
Notably, for the first time they have demonstrate thermodynamically stable FAPbI3 NCs in the black cubic α-phase without transition to the yellow hexagonal δ-phase even after 150 days of storage. This is in strong contrast to polycrystalline films and single crystals which convert within hours.
This fact paves the way to highly efficient perovskite based quantum dots photovoltaics, which is underpinned by demonstrating FAPbI3 NCs based photodetector.
To highlight the potential of FAPbX3 NCs as a promising candidate for optoelectronic and luminescent applications, the scientists modified the surface with polyhedral oligomeric silsesquioxane. This modification protects the brightly luminescent FAPbX3 NCs from decomposition even after storage in water for more than 2 months.
Source: Friedrich-Alexander-Universität Erlangen-Nürnberg


DOE: One small change makes Quantum Dot solar cells more efficient

The quest for more efficient solar cells has led to the search of new materials. For years, scientists have explored using tiny drops of designer materials, called quantum dots.

Now, we know that adding small amounts of manganese decreases the ability of quantum dots to absorb light but increases the current produced by an average of 300%. Under certain conditions, the current produced increased by 700%.

The enhancement is due to the faster rate that the electrons move from the quantum dot to the balance of the solar cell (what the scientists call the electron tunneling rate) in the presence of the manganese atoms at the interface.

Importantly, this observation is confirmed by theory, opening up possibilities for applying this approach to other systems (Applied Physics Letters, “Giant photocurrent enhancement by transition metal doping in quantum dot sensitized solar cells”).

The power conversion efficiency of quantum dot solar cells has reached about 12%. However, the overall efficiency of quantum dot solar cells is relatively low compared to photovoltaic systems in use today that are based on silicon. In addition, quantum dot solar cells are not as efficient as emerging next-generation solar cells.

The results obtained in this work point to a surprisingly straightforward alternative route. Scientists can significantly improve the performance of this family of solar cells by adding small amounts of alternate metals.

In the quest to replace more traditional solar materials, such as silicon, with more efficient and high-performing options, scientists have been studying quantum dot solar cells as an alternative to harvest sunlight for conversion to electricity.

In this solar cell design, quantum dots are used as the material that absorbs sunlight and converts it to electricity. Quantum dots are very small, nanometer-sized, particles, whose solar conversion properties, in this case a characteristic gap in the energy levels of the electrons called the “bandgap,” are tunable by changing the size or chemical composition.

This is in contrast to bulk materials whose bandgap is fixed by the chemical composition or choice of material(s) alone. This size dependence of bandgap makes quantum dots attractive for multi-junction solar cells, whose efficiency is enhanced by using a variety of materials that absorb different parts of the “rainbow” of wavelengths of light found in the solar spectrum.

This research team discovered that adding small amounts of the transition metal manganese (Mn), or “doping,” resulted in a huge enhancement in the efficiency rate of changing light to electricity for lead sulfide (PbS) quantum dot sensitized solar cells.

Relatively small concentrations of Mn (4 atomic percent) cause the current to increase by an average of 300% with a maximum increase of up to 700%.

Moreover, the mechanism by which this occurs cannot be explained by the light absorption alone because both the experimental and theoretical absorption spectra demonstrate a several times decrease in the absorption coefficient on the addition of Mn.

The team proposes that the dramatic increase is due to a mechanism of increased electron tunneling through the atom pairs at the quantum dot interface with the next layer of the solar cell.

The team used ab initio calculations, which is a computational approach that can describe new phenomena without the need to fit or extrapolate experimental data, to confirm this mechanism.

While typical doping approaches focus on improving exciton lifetime and light absorption channels, results obtained in this study provide an alternative route for significant improvement on the efficiency of quantum dot sensitized solar cells.

Source: U.S. Department of Energy, Office of Science

Squashed quantum dots solve a multi-faceted problem – Optoelectronics

Squashed QD cqds.jpg

Oblate-shaped quantum dots

Quantum dots have revolutionized the field of optoelectronics due to their atom-like electronic structure. However, the prospect of colloidal quantum-dot lasers has long been deemed impractical due to the high energies required to induce optical gain. But recent work published in Nature and led by Ted Sargent of the University of Toronto shows that the lasing threshold in cadmium selenide (CdSe)/cadmium sulphide (CdS) core-shell colloidal quantum dots can be lowered by squashing the CdSe core via a clever ligand exchange process.

To instigate optical gain in a semiconductor laser, the difference between the lowest electron level and the highest hole level must be wider than the band gap so that the light emitted when they recombine can stimulate emission in neighbouring nanocrystals. Colloidal quantum dots (CQDs) then, should make ideal candidates for lasing applications, as their atom-like electronic structure means that the electron and hole energy levels are easier to separate.Squashed QD cqds

In practice, however, the energies required to trigger optical gain in CQDs are so high that they can heat up to the point of burning. While electrons tend to occupy one energy state upon excitation, the hole that they leave behind in the valence band can populate one of eight closely spaced states. This degeneracy pushes the hole Fermi level into the band gap and increases the amount of energy required to instigate optical gain.

To overcome this issue, the researchers took advantage of the fact that CdS imposes a strain on CdSe due to a slight lattice mismatch of 3.9%. By growing an asymmetric CdS shell around a “squashed” oblate CdSe core, they were able to induce a biaxial strain that affected the heavy and light holes of the valence band to different extents, thus lifting the degeneracy.

To produce these asymmetric CQDs the group invented a technique called facet-selective epitaxy, making use of ligands that interact differently with the surfaces of CdSe. One of these ligands, trioctylphosphine sulphide, or TOPS, binds weakly to the (0001) facet of CdSe and not at all to the (0001), while octanethiol interacts similarly with all CdSe surfaces. Therefore, by growing CdS on the (0001) facet with TOPS and then replacing with octanethiol to stimulate epitaxial growth, oblate-shaped CQDs could be made throughout the entire particle ensemble with remarkable uniformity.

The resulting lasers had an unprecedentedly high performance, exhibiting a low lasing threshold of 6.4–8.4 kW cm–2, a seven-fold reduction compared with previous attempts. They also emitted light over a narrow energy range of just 36 meV. Both of these properties can be attributed to the enhanced splitting of the valence band levels that arises due to the oblate CQD shape.

The international team of researchers has certainly proved that continuous-wave CQD lasers are possible, yet there are still some obstacles to be overcome before they are seen on the market. Most importantly, the next step will be exciting the CQDs via electrical rather than optical means, as in standard commercial lasers. Nevertheless, facet-selective epitaxy opens up a whole host of other CQD materials for lasing applications and beyond.

NREL & Colorado School of Mines Researchers Capture Excess Photon Energy to Produce Solar Fuels

Photo shows a lead sulfide quantum dot solar cell. A lead sulfide quantum dot solar cell developed by researchers at NREL. Photo by Dennis Schroeder.

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a proof-of-principle photoelectrochemical cell capable of capturing excess photon energy normally lost to generating heat.

Using quantum dots (QD) and a process called Multiple Exciton Generation (MEG), the NREL researchers
were able to push the peak external quantum efficiency for hydrogen generation to 114 percent.

The advancement could significantly boost the production of hydrogen from sunlight by using the cell to split water at a higher efficiency and lower cost than current photoelectrochemical approaches.

Details of the research are outlined in the Nature Energy paper Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%, co-authored by Matthew Beard, Yong Yan, Ryan Crisp, Jing Gu, Boris Chernomordik, Gregory Pach, Ashley Marshall, and John Turner.

All are from NREL; Crisp also is affiliated with the Colorado School of Mines, and Pach and Marshall are affiliated with the University of Colorado, Boulder.

Beard and other NREL scientists in 2011 published a paper in Science that showed for the first time how MEG allowed a solar cell to exceed 100 percent quantum efficiency by producing more electrons in the electrical current than the amount of photons entering the solar cell.

“The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current,” Beard said.

“We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds.”

The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption bandedge lost to heat.

The MEG process takes advantages of the additional photon energy to generate more electrons and thus additional chemical or electrical potential, rather than generating heat. QDs, which are spherical semiconductor nanocrystals (2-10 nm in diameter), enhance the MEG process.

In current report, the multiple electrons, or charge carriers, that are generated through the MEG process within the QDs are captured and stored within the chemical bonds of a H2 molecule.

NREL researchers devised a cell based upon a lead sulfide (PbS) QD photoanode. The photoanode involves a layer of PbS quantum dots deposited on top of a titanium dioxide/fluorine-doped tin oxide dielectric stack.

The chemical reaction driven by the extra electrons demonstrated a new direction in exploring high-efficiency approaches for solar fuels.

Funds for the research came from the Department of Energy’s Office of Science.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

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