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What Makes for Better CdTe Solar Cells

Cadmium-telluride (CdTe) solar cell materials have had a bumpy ride ever since they were first introduced as an alternative to silicon-based photovoltaics. They were never quite as efficient at converting sunlight into electricity as silicon.

As a result, back in 2002, British Petroleum, which at the time was billing itself as the world’s biggest solar company, terminated U.S. production of CdTe and amorphous silicon cells. However, the fortunes of CdTe started to turnaround back in 2010 when General Electric announced its plans to enter the business. Since then, GE has been announcing ever-increasing conversion efficiency numbers, with a 19.6-percent conversion efficiency reached last year. First Solar Inc. recently broke this record by achieving a conversion efficiency of 20.4 percent.

As good as these numbers are, they still fall short of the 18 to 21 percent conversion efficiency of conventional silicon. (Recently, Panasonic announced that it had achieved a conversion efficiency of 25.6% for its silicon-based solar cells, a new record.)

To see if the latest conversion efficiency numbers of CdTe solar cells could be improved upon, and to find out what was behind the escalating numbers of the recent past, researchers at the Department of Energy’s Oak Ridge National Laboratory along with colleagues from the University of Toledo and DOE’s National Renewable Energy Laboratory used electron microscopy to peer into cadmium-telluride solar cell materials to see what made them tick.

Specifically, the researchers wanted to examine CdTe solar cell materials that had been treated with cadmium-chloride, which had been improving the efficiency numbers of the cadmium-based solar cells since the 1980s, though no one knows why.

“We knew that chlorine was responsible for this magical effect, but we needed to find out where it went in the material’s structure,” said ORNL’s Chen Li in a press release. “Only by understanding the structure can we understand what’s wrong in this solar cell—why the efficiency is not high enough, and how can we push it further.”

In research published in the journal Physical Review Letters (“Grain-Boundary-Enhanced Carrier Collection in CdTe Solar Cells”), the research team discovered atom-scale grain boundaries were involved in the enhanced performance. Grain boundaries are essentially tiny defects, which, in the case of solar cells, typically result in reduced efficiency numbers.

Using electron microscopy, the researchers saw that chlorine atoms were replacing tellurium atoms within these grain boundaries. The substitution was creating local electric fields at the grain boundaries that were improving the photovoltaic performance rather than worsening it.

The researchers believe that this understanding could lead future research into CdTe solar cells that could push their conversion efficiency closer to their theoretical maximum of 32 percent.

“We think that if all the grain boundaries in a thin film material could be aligned in same direction, it could improve cell efficiency even further,” Li added.

Graphene and Carbon Nanotubes Join Forces to Tackle Supercapacitors

Graphene and carbon nanotubes have been competing for many of the same applications for years, especially in the broad area of electronics. The jockeying for supremacy between these two carbon materials has been fierce in energy storage applications as well. In fact, both carbon nanotubes and graphene have been proposed as a replacement material for activated carbon on the electrodes of supercapacitors.

Now, following a newly developing trend where graphene and carbon nanotubes join forces to create an even better material than they could on their own, researchers at George Washington University have combined the two materials to create a supercapacitor that is claimed to be both low cost and high performance.

In research published in the Journal of Applied Physics ("Paper-based ultracapacitors with carbon nanotubes-graphene composites"), the GWU researchers mixed graphene flakes with single-walled carbon nanotubes through an arc discharge under various magnetic conditions.

The resulting combination takes advantage of the high-surface area and good in-plane conductivity of graphene flakes while the carbon nanotubes connect all the structures to make a uniform network. The device’s specific capacitance—its ability to store a charge—was reported as 100 Farads per gram (F/g), three times higher than the specific capacitance of a supercapacitor made by carbon nanotubes alone.

“In our lab we developed an approach by which we can obtain both single-walled carbon nanotubes and graphene, so we came up with the idea to take advantage of the two promising carbon nanomaterials together," said Michael Keidar, a professor at GWU and director of the Micro-propulsion and Nanotechnology Laboratory, in a press release.

Supercapacitors, also known as ultracapcitors or electrochemical double-layer capacitors (EDLCs), have held out the promise that they could store as much energy as an electrochemical battery like a lithium-ion battery, but charge up in a matter of seconds and provide quick bursts of a large amount of power as they do now for applications such as powering cranes or buses.

This potential has fueled the hope that supercapacitors could be used to power all-electrical vehicles, providing as much range as a lithium-ion battery does but charge up faster than the time it takes to fill up a car with gasoline. The interest in applying nanomaterials to these devices has become so intense that the lines between batteries and supercapacitors are becoming blurred as new materials are proposed.

In the race to practical—and potentially lucrative—applications, a promising approach in giving supercapacitors the same storage capacity as an electrochemical battery is increasing the surface area of the electrodes. More surface area translates into more ions being stored on the electrodes and the greater specific capacitance. While much is made of graphene’s theoretical surface area of 2630 squared meters per gram, so far the largest surface area anyone has produced with graphene has been 1520 squared meters per gram, which is pretty typically found in today’s activated carbon made from crushed coconuts.

So, the jury is still out on whether graphene or carbon nanotubes are viable alternatives to activated carbon for today's supercapacitor applications, even if you lower the cost of the material (it’s hard to compete with crushed coconuts).

Nanometer-Scale Magnet Makes Tiny, Powerful MRI

The trend in making more powerful magnetic resonance imagining (MRI) devices has been to produce larger magnets. A European consortium, for example, is building what will be the most powerful MRI, capable of producing a field of 11.75 teslas using a superconducting magnet strong enough to lift a 60-metric-ton battle tank.

However, researchers at Harvard University have gone in the opposite direction and built a device with a magnet only 20 nanometers across, or approximately 1/300th the size of a red blood cell. Despite its small size, the researchers claim that the magnet can produce a magnetic field gradient 100 000 times larger than even the most powerful conventional systems.

The trick is that this nanoscale magnet can be brought within nanometers of the object being imaged to produce a spatial resolution down to the nanoscale. Most hospital MRI scanners can only reach a spatial resolution of 1 millimeter. With this capability, the Harvard researchers someday hope to produce detailed images of individual molecules.

“What we’ve done, essentially, is to take a conventional MRI and miniaturize it,” said Amir Yacoby, professor of physics in a press release. “Functionally, it operates in the same way, but in doing that, we’ve had to change some of the components, and that has enabled us to achieve far greater resolution than conventional systems.”

In research published in the journal Nature Nanotechnology (“Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins”), Yacoby and his colleagues used a combination of their nanoscale magnet with a bit of quantum computing.

First, the quantum computing part: The Harvard team milled lab-grown diamonds into super fine tips and embedded an impurity into each, called a nitrogen vacancy (NV). This impurity acted as a quantum bit, or qubit, which is the key to the operation of quantum computers.

When the tip is scanned over the surface of a diamond crystal, the qubit interacts with the electrons on the surface of the crystal. It is these interactions that serve as the basis for images of the electrons spins. While making a quantum bit magnetometer sensitive enough to detect the spin of individual electrons was groundbreaking work in its own right, the distance between the qubit sensor and the object being imaged limited the system's spatial resolution.

To overcome this limitation, Yacoby and his colleagues brought the nanoscale magnet close to both the qubit sensor and the sample being examined. With this combination, the team was able to detect distribution of spins surrounding the sensor so that they were able to image the three-dimensional landscape of electronic spins at the diamond surface and achieve a spatial resolution of  0.8 nm laterally and 1.5 nm vertically.

“This is really a game of bringing both the magnet very close to generate large gradients, and bringing the detector very close to get larger signals,” Yacoby said. “It’s that combination that gives us both the spatial resolution and the detectability."

The researchers are looking to push the technique beyond the ability to image the individual spin of electrons in 3-D and make it capable of imaging components within a molecule, such as the nuclear spins of the atoms making up the molecule.

"This is by no means an easy task, since the nuclear spin generates a signal that is 1/1000th that of the electron spin … but that’s where we’re headed,” said Yacoby.

Graphene You Can Whip Up In A Blender

The first graphene was made by pulling layers off of graphite using Scotch tape. Now, in keeping with the low-tech origins of the material, a team at Trinity College Dublin has found that it should be possible to make large quantities of the stuff by mixing up some graphite and stabilizing detergent with a blender.

The graphene produced in this manner isn't anything like the wafer-scale sheets of single-layer graphene that are being grown by Samsung, IBM and others for high-performance electronics. Instead, the blender-made variety consists of small flakes that are exfoliated off of bits of graphite and then separated out by centrifuge. But small-scale graphene has its place, the researchers say. Solutions of the stuff could be used in printed electronics and conductive coatings. The flakes could also be used as filler to boost the mechanical, thermal, or electrical properties of composite materials.

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Photoluminescent Nanoparticles Kill Cancer

In a case of serendipity, researchers at the University of Texas at Arlington started out trying to develop new security-related radiation detection and stumbled upon a potential breakthrough in cancer treatment.

In the research, which will be published in the August edition of the Journal of Biomedical Nanotechnology (“A New X-Ray Activated Nanoparticle Photosensitizer for Cancer Treatment”), Wei Chen, professor of physics at the UT Arlington, was exposing copper-cysteamine (Cu-Cy) nanoparticles to X-rays when he noticed an unusual luminescence over a time-lapse exposure. When he investigated the cause of the Cu-Cy nanoparticle luminescence, he realized that the particles were losing energy as they emitted singlet oxygen, a toxic byproduct that also happens to be used to in photodynamic cancer therapy to damage cancer cells.

“This new idea is simpler and better than previous photodynamic therapy methods. You don’t need as many steps. This material alone can do the job,” Chen, who is also leading federally-funded cancer research, said in a press release. “It is the most promising thing we have found in these cancer studies and we’ve been looking at this for a long time.”

Photodynamic therapy (PDT) is a technique in which some photosensitizer is introduced into tumor tissue. When exposed to light, the photosensizer produces singlet oxygen that kills the cancer cells. Two years ago, this blog covered research from Rice University related to a similar PDT technique in which gold nanoparticles were introduced near a tumor and then subjected to light. This light exposure to the cluster of nanoparticles created bubbles that burst, temporarily ripping open small pores in the cell membranes that allow drugs to penetrate. 

The problem with PDT techniques is that the light cannot penetrate very far into human tissue to activate the nanoparticles. Chen and his colleagues have discovered that Cu-Cy the nanoparticle's sensitivity to X-rays and the ability of X-rays to penetrate deeply into tissue means that the tumor can be deep inside tissue and still be effective. Another advantage of the Cu-Cy nanoparticle in combination with the X-rays is that no other photosensitizer needs to be used, making it convenient, efficient and cost-effective, according to the researchers.

Like some of the “theranostic” nanoparticles that are being developed, in which both therapeutic and diagnostic functions are combined into one nanoparticle, the Cu-Cy nanoparticle can serve both as treatment for cancer as well as for cell imaging.

In experiments carried out thus far, Chen and his team have used the Cu-Cy nanoparticle and X-ray combination to treat a tumor. The results over a 13-day period showed that the tumor had not grown at all with the treatment while a tumor that had not been treated grew three times in size.

The UT Arlington has applied for a patent on the technology. Meanwhile Chen and his team are working on ways to shrink the size of the Cu-Cy nanoparticles, which currently are around 250 nanometers, so that the tumor tissue can more easily absorb them. If they can reduce the nanoparticles down to dimensions below 200nm, this should improve cell uptake. Ideally, they would like to bring those dimensions down to around 50 to 100nm.

Nanomaterials Keep Pushing Lithium-Sulfur Battery Capabilities for EVs

Researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) have developed a nanomaterial powder that can be added to the cathode of lithium-sulfur batteries to capture problematic polysulfides that usually cause them to fail after a few charges.

The nanomaterial powder is a metal organic framework (MOF) in which metal ions are coordinated with rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional. The research paper was published in the journal Nano Letters.

"Lithium-sulfur batteries have the potential to power tomorrow's electric vehicles, but they need to last longer after each charge and be able to be repeatedly recharged," said materials chemist Jie Xiao at PNNL in a press release. "Our metal organic framework may offer a new way to make that happen."

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Nanostructures Could Bridge Gap Between Optics and Electronics

Researchers at the University of California, Santa Barbara (UCSB) have developed a recipe for creating a nearly perfect compound semiconductor that could lead to more efficient photovoltaics, safer and higher resolution biological imaging, and transmitting massive amounts of data at higher speeds.

The researchers took the rare earth element, erbium (Er), along with the element antimony (Sb) and made a compound of the two into semimetallic nanowires or nanoparticles. Then they embedded those nanostructures into the semiconducting matrix of gallium antimonide (GaSb). Because the arrangement of atoms within the ErSb nanostructures matches the pattern of the surrounding matrix, the compound semiconductor forms an uninterrupted crystal lattice capable of manipulating light energy in the mid-infrared range.

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IBM Combines Light Emission and Detection in Single Nanowire

Researchers at IBM Research Zurich and the Norwegian University of Science (NTNU) and Technology have demonstrated for the first time that both efficient light emission and detection functionalities can be achieved in the very same nanowire by applying mechanical strain.

In optical communications, III-V element semiconductor materials are typically used for light emission and silicon or germanium for light detection. By combining both these functions into the same material it may be possible to drastically reduce the complexity of nanophotonic chips.

The researchers, who published their findings in the journal Nature Communications (“Inducing a direct-to-pseudodirect bandgap transition in wurtzite GaAs nanowires with uniaxial stress”), discovered that gallium arsenide can be tuned with a small strain to function efficiently as a single light-emitting diode or a photodetector because of a hexagonal crystal structure, referred to as wurtzite. In wurtzite semiconductors, the atoms are located in very specific positions along the nanowire. That leads to electron and hole wave-functions overlapping strongly but optical transitions between these states being impaired by symmetry. If you change that symmetry with a strain, you can switch between direct or pseudo-direct bandgaps.

"When you pull the nanowire along its length, the nanowire is in a state that we call “direct bandgap” and it can emit light very efficiently; when instead you compress the length of the wire, its electronic properties change and the material stops emitting light,” said IBM scientist Giorgio Signorello in an IBM release. “We call this state “pseudo-direct”: the III-V material behaves similarly to silicon or germanium and becomes a good light detector."

Optical communications are not the only potential applications for this research. "It also gives us a much better understanding, allowing us to design the nanowires with a built-in compressive stress, for example to make them more effective in a solar cell,” said Helge Weman, a professor at NTNU in another release. “This can for instance be used to develop different pressure sensors, or to harvest electric energy when the nanowires are bent.”

Graphene and Carbon Nanotubes: Two Great Materials Even Better Together

James Tour at Rice University has a history of finding links between carbon nanotubes and graphene, which are often regarded merely as rivals for a host of electronic applications. A few years back, Tour developed a process for “unzipping” carbon nanotubes so that they transformed into graphene. Now Tour and his colleagues have used that unzipping technique to develop a method in which carbon nanotubes are used as a kind of reinforcing rebar for graphene, protecting it during the manufacturing process.

Why is this useful? Because to produce high-quality graphene for electronic applications (a promising one is replacing indium tin oxide as a transparent conductor in displays for controlling pixels), a manufacturing process known as chemical vapor deposition (CVD) is used, and CVD has an Achilles heel: while it is possible to grow large sheets of graphene on a copper substrate in a furnace, when you try to remove the graphene sheets from the copper, you find that it is difficult to do so without breaking the graphene. A reinforcement polymer is usually laid over the graphene to keep it from breaking during its removal, but this polymer leaves impurities.

Solving this manufacturing issue has led to some intriguing new methods, including work late last year out of the National University of Singapore, where researchers developed a method in which the copper is sandwiched between a silicon layer and graphene layer so that when the copper is etched away the graphene and silicon remain attached to each other.

The Rice University researchers have taken another approach. First, they coat the copper with both single-walled and multi-walled carbon nanotubes and then heat and cool the material. These carbon nanotubes serve as carbon source without the need of adding any other carbon to the process. When heated, the carbon nanotubes both decompose into graphene and—voilà!—unzip to form covalent junctions with the new graphene layer. The researchers have dubbed the resulting material "rebar graphene."

In a paper published in the journal ACS Nano, the Rice team describes how the rebar graphene could be transferred onto target substrates without needing a polymer coating due to the reinforcement effect. The resulting rebar graphene also exhibits better electrical conductivity than graphene produced through other CVD processes.

"Normally you grow graphene on a metal, but you can’t just dissolve away the metal," Tour said in a press release. “You put a polymer on top of the graphene to reinforce it, and then dissolve the metal. Then you have polymer stuck to the graphene. When you dissolve the polymer, you’re left with residues, trace impurities that limit graphene’s effectiveness for high-speed electronics and biological devices. By taking away the polymer support step, we greatly expand the potential for this material."

Tour believes that this rebar graphene could be a competitive alternative for the replacement of indium tin oxide in displays, an application that could potentially add flexibility to them. Before that happens, however, the researchers will have to show that their new manufacturing process can scale up enough and lower production costs to make it truly competitive with ITO.

Molecular Gears Turn Under Pressure

In collaborative research between Georgia Tech and the University of Toledo, both computer modeling and experiments have demonstrated that when pressure is applied to a superlattice structure the bonds that link the nanoparticles making up the structure behave as though they were gear-like, molecular-scale machines.

Superlattices form when an almost atomically thin layer of one material is laid down over another in an alternating pattern, creating numerous interfaces. IBM has used graphene-based superlattices to build experimental photodetectors.The researchers in this latest resarch believe that this type of superlattice could prove useful for developing molecular-scale switching, sensing and even energy absorption applications.

The appearance of gear-like movement in the structure is the result of the hydrogen bonds that connect organic molecules surrounding clusters of silver nanoparticles in the superlattice. Under pressure these bonds move like a hinge keeping the nanostructure from breaking.

While this hinge-like movement was unexpected, the pressure itself also had an unexpected consequence of softening the superlattice so that that subsequent compression of the structure could be done with less force. The researchers discovered that after the structure had been compressed by about six percent of its volume, the pressure required for additional compression suddenly dropped significantly. This drop occurred as the nanocrystal components rotated, layer-by-layer, in opposite directions.

“As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change," said Uzi Landman, a professor in the School of Physics at the Georgia Institute of Technology, in a press release. "When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed."

 In the video below you can see the structure of the superlattice, which consists of clusters with cores of 44 silver atoms each. Thirty molecules of an organic material known as mercaptobenzoic acid (p-MBA) protect the silver clusters. The organic molecules are attached to the silver by sulfur atoms. Again, the parts that move, the “hinges”, are the hydrogen bonds that attach the organic molecules.

The research, which was published in the journal Nature Materials ("Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice"), described the production of the superlattice structure through self assembly. In a solution, clusters of the silver nanoparticles and organic molecules assemble themselves into the larger superlattice, guided by the hydrogen bonds, which can only form between the p-MBA molecules at certain angles.

As if producing some of the tiniest mechanical objects ever weren't enough, the researchers believe there work also represents the largest solid ever mapped in detail using a combined X-ray and computational techniques.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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