Great things from small things

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IMAGEIMAGE: A surfactant template guides the self-assembly of functional polymer structures in an aqueous solution. view more

Credit: Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; image by Youngkyu Han and Renee Manning.

OAK RIDGE, Tenn., Oct. 5, 2015–The efficiency of solar cells depends on precise engineering of polymers that assemble into films 1,000 times thinner than a human hair.

Today, formation of that polymer assembly requires solvents that can harm the environment, but scientists at the Department of Energy’s Oak Ridge National Laboratory have found a “greener” way to control the assembly of photovoltaic polymers in water using a surfactant– a detergent-like molecule–as a template. Their findings are reported in Nanoscale, a journal of the Royal Society of Chemistry.

“Self-assembly of polymers using surfactants provides huge potential in fabricating nanostructures with molecular-level controllability,” said senior author Changwoo Do, a researcher at ORNL’s Spallation Neutron Source (SNS).

The researchers used three DOE Office of Science User Facilities–the Center for Nanophase Materials Sciences (CNMS) and SNS at ORNL and the Advanced Photon Source (APS) at Argonne National Laboratory–to synthesize and characterize the polymers.

“Scattering of neutrons and X-rays is a perfect method to investigate these structures,” said Do.

The study demonstrates the value of tracking molecular dynamics with both neutrons and optical probes.

“We would like to create very specific polymer stacking in solution and translate that into thin films where flawless, defect-free polymer assemblies would enable fast transport of electric charges for photovoltaic applications,” said Ilia Ivanov, a researcher at CNMS and a corresponding author with Do. “We demonstrated that this can be accomplished through understanding of kinetic and thermodynamic mechanisms controlling the polymer aggregation.”

The accomplishment creates molecular building blocks for the design of optoelectronic and sensory materials. It entailed design of a semiconducting polymer with a hydrophobic (“water-fearing”) backbone and hydrophilic (“water-loving”) side chains. The water-soluble side-chains could allow “green” processing if the effort produced a polymer that could self-assemble into an organic photovoltaic material. The researchers added the polymer to an aqueous solution containing a surfactant molecule that also has hydrophobic and hydrophilic ends. Depending on temperature and concentration, the surfactant self-assembles into different templates that guide the polymer to pack into different nanoscale shapes–hexagons, spherical micelles and sheets.

In the semiconducting polymer, atoms are organized to share electrons easily. The work provides insight into the different structural phases of the polymer system and the growth of assemblies of repeating shapes to form functional crystals. These crystals form the basis of the photovoltaic thin films that provide power in environments as demanding as deserts and outer space.

“Rationally encoding molecular interactions to rule the molecular geometry and inter-molecular packing order in a solution of conjugated polymers is long desired in optoelectronics and nanotechnology,” said the paper’s first author, postdoctoral fellow Jiahua Zhu. “The development is essentially hindered by the difficulty of in situ characterization.”

In situ, or “on site,” measurements are taken while a phenomenon (such as a change in molecular morphology) is occurring. They contrast with measurements taken after isolating the material from the system where the phenomenon was seen or changing the test conditions under which the phenomenon was first observed. The team developed a test chamber that allows them to use optical probes while changes occur.

Neutrons can probe structures in solutions

Expertise and equipment at SNS, which provides the most intense pulsed neutron beams in the world, made it possible to discover that a functional photovoltaic polymer could self-assemble in an environmentally benign solvent. The efficacy of the neutron scattering was enhanced, in turn, by a technique called selective deuteration, in which specific hydrogen atoms in the polymers are replaced by heavier atoms of deuterium–which has the effect of heightening contrasts in the structure. CNMS has a specialty in the latter technique.

“We needed to be able to see what’s happening to these molecules as they evolve in time from some solution state to some solid state,” author Bobby Sumpter of CNMS said. “This is very difficult to do, but for molecules like polymers and biomolecules, neutrons are some of the best probes you can imagine.” The information they provide guides design of advanced materials.

By combining expertise in topics including neutron scattering, high-throughput data analysis, theory, modeling and simulation, the scientists developed a test chamber for monitoring phase transitions as they happened. It tracks molecules under conditions of changing temperature, pressure, humidity, light, solvent composition and the like, allowing researchers to assess how working materials change over time and aiding efforts to improve their performance.

Scientists place a sample in the chamber and transport it to different instruments for measurements. The chamber has a transparent face to allow entry of laser beams to probe materials. Probing modes–including photons, electrical charge, magnetic spin and calculations aided by high-performance computing–can operate simultaneously to characterize matter under a broad range of conditions. The chamber is designed to make it possible, in the future, to use neutrons and X-rays as additional and complementary probes.

“Incorporation of in situ techniques brings information on kinetic and thermodynamic aspects of materials transformations in solutions and thin films in which structure is measured simultaneously with their changing optoelectronic functionality,” Ivanov said. “It also opens an opportunity to study fully assembled photovoltaic cells as well as metastable structures, which may lead to unique features of future functional materials.”

Whereas the current study examined phase transitions (i.e., metastable states and chemical reactions) at increasing temperatures, the next in situ diagnostics will characterize them at high pressure. Moreover, the researchers will implement neural networks to analyze complex nonlinear processes with multiple feedbacks.

The title of the Nanoscale paper is “Controlling molecular ordering in solution-state conjugated polymers.”


Zhu, Do and Ivanov led the study. Zhu, Ivanov and Youngkyu Han conducted synchrotron X-ray scattering and optical measurements. Sumpter, Rajeev Kumar and Sean Smith performed theory calculations. Youjun He and Kunlun Hong synthesized the water-soluble polymer. Peter Bonnesen conducted thermal nuclear magnetic resonance analysis on the water-soluble polymer. Do, Han and Greg Smith performed neutron measurement and analysis of the scattering results. This research was conducted at CNMS and SNS, which are DOE Office of Science User Facilities at ORNL. Moreover, the Advanced Photon Source, a DOE Office of Science User Facility at Argonne National Laboratory, was used to perform synchrotron X-ray scattering on the polymer solution. Laboratory Directed Research and Development funds partially supported the work.

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit

graphene-silicon-perovskite-solar-cell-id41503Silicon absorbers primarily convert the red portion of the solar spectrum very effectively into electrical energy, whereas the blue portions are partially lost as heat. To reduce this loss, the silicon cell can be combined with an additional solar cell that primarily converts the blue portions.

Teams at Helmholtz Zentrum Berlin (HZB) have already acquired extensive experience with these kinds of tandem cells. A particularly effective complement to conventional silicon is the hybrid material called perovskite. It has a band gap of 1.6 electron volts with organic as well as inorganic components. However, it is very difficult to provide the perovskite layer with a transparent front contact. While sputter deposition of indium tin oxide (ITO) is common practice for inorganic silicon solar cells, this technique destroys the organic components of a perovskite cell.

Graphene as transparent front contact
Now a group headed by Prof. Norbert Nickel has introduced a new solution. Dr. Marc Gluba and PhD student Felix Lang have developed a process to cover the perovskite layer evenly with graphene (“Perovskite Solar Cells with Large-Area CVD-Graphene for Tandem Solar Cells”). Graphene consists of carbon atoms that have arranged themselves into a two-dimensional honeycomb lattice forming an extremely thin film that is highly conductive and highly transparent.
silicon-perovskite tandem solar cell
The perovskite film (black, 200-300 nm) is covered by Spiro.OMeTAD, Graphene with gold contact at one edge, a glass substrate and an amorphous/crystalline silicon solar cell. (Image: F. Lang / HZB)
Fishing for graphene
As a first step, the scientists promote growth of the graphene onto copper foil from a methane atmosphere at about 1000 degrees Celsius. For the subsequent steps, they stabilise the fragile layer with a polymer that protects the graphene from cracking. In the following step, Felix Lang etches away the copper foil. This enables him to transfer the protected graphene film onto the perovskite.
“This is normally carried out in water. The graphene film floats on the surface and is fished out by the solar cell, so to speak. However, in this case this technique does not work, because the performance of the perovskite degrades with moisture. Therefore we had to find another liquid that does not attack perovskite, yet is as similar to water as possible”, explains Gluba.
Ideal front contact
Subsequent measurements showed that the graphene layer is an ideal front contact in several respects. Thanks to its high transparency, none of the sunlight’s energy is lost in this layer. But the main advantage is that there are no open-circuit voltage losses, that are commonly observed for sputtered ITO layers. This increases the overall conversion efficiency.
“This solution is comparatively simple and inexpensive to implement”, says Nickel. “For the first time, we have succeeded in implementing graphene in a perovskite solar cell. This enabled us to build a high-efficiency tandem device.”
Source: Helmholtz Zentrum Berlin
QD Brightness U of Illinois 100515 id41509Researchers at the University of Illinois at Urbana-Champaign have introduced a new class of light-emitting quantum dots (QDs) with tunable and equalized fluorescence brightness across a broad range of colors. This results in more accurate measurements of molecules in diseased tissue and improved quantitative imaging capabilities.
“In this work, we have made two major advances–the ability to precisely control the brightness of light-emitting particles called quantum dots, and the ability to make multiple colors equal in brightness,” explained Andrew M. Smith, an assistant professor of bioengineering at Illinois. “Previously light emission had an unknown correspondence with molecule number. Now it can be precisely tuned and calibrated to accurately count specific molecules. This will be particularly useful for understanding complex processes in neurons and cancer cells to help us unravel disease mechanisms, and for characterizing cells from diseased tissue of patients.”
Quantum Dots
Left: Conventional fluorescent materials like quantum dots and dyes have mismatched brightness between different colors. When these materials are administered to a tumor (shown below) to measure molecular concentrations, the signals are dominated by the brighter fluorophores. Right: New brightness-equalized quantum dots that have equal fluorescence brightness for different colors. When these are administered to tumors, the signals are evenly matched, allowing measurement of many molecules at the same time. (Image: University of Illinois)
“Fluorescent dyes have been used to label molecules in cells and tissues for nearly a century, and have molded our understanding of cellular structures and protein function. But it has always been challenging to extract quantitative information because the amount of light emitted from a single dye is unstable and often unpredictable. Also the brightness varies drastically between different colors, which complicates the use of multiple dye colors at the same time. These attributes obscure correlations between measured light intensity and concentrations of molecules,” stated Sung Jun Lim, a postdoctoral fellow and first author of the paper, “Brightness-Equalized Quantum Dots,” published this week in Nature Communications.
According to the researchers, these new materials will be especially important for imaging in complex tissues and living organisms where there is a major need for quantitative imaging tools, and can provide a consistent and tunable number of photons per tagged biomolecule. They are also expected to be used for precise color matching in light-emitting devices and displays, and for photon-on-demand encryption applications. The same principles should be applicable across a wide range of semiconducting materials.
“The capacity to independently tune the QD fluorescence brightness and color has never before been possible, and these BE-QDs now provide this capability,” said Lim. “We have developed new materials-engineering principles that we anticipate will provide a diverse range of new optical capabilities, allow quantitative multicolor imaging in biological tissue, and improve color tuning in light-emitting devices. In addition, BE-QDs maintain their equal brightness over time while whereas conventional QDs with mismatched brightness become further mismatched over time.

These attributes should lead to new LEDs and display devices not only with precisely matched colors–better color accuracy and brightness–but also with improved performance lifetime and improved ease of manufacturing.” QDs are already in use in display devices (e.g. Amazon Kindle and a new Samsung TV).

Source: University of Illinois College of Engineering

Rectenna Naval Optical 150928122542_1_540x360Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.

Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for photodetectors that would operate without the need for cooling, energy harvesters that would convert waste heat to electricity — and ultimately for a new way to efficiently capture solar energy.

In the new devices, developed by engineers at the Georgia Institute of Technology, the carbon nanotubes act as antennas to capture light from the sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.

Billions of rectennas in an array can produce significant current, though the efficiency of the devices demonstrated so far remains below one percent. The researchers hope to boost that output through optimization techniques, and believe that a rectenna with commercial potential may be available within a year.

“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”

The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), is scheduled to be reported September 28 in the journal Nature Nanotechnology.

Rectenna Naval Optical 150928122542_1_540x360

Optical rectenna schematic. This schematic shows the components of the optical rectenna developed at the Georgia Institute of Technology.
Credit: Thomas Bougher, Georgia Tech

Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten microns, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.

“The physics and the scientific concepts have been out there,” said Cola. “Now was the perfect time to try some new things and make a device work, thanks to advances in fabrication technology.”

Using metallic multiwall carbon nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha Sharma, Virendra Singh and Thomas Bougher constructed devices that utilize the wave nature of light rather than its particle nature. They also used a long series of tests — and more than a thousand devices — to verify measurements of both current and voltage to confirm the existence of rectenna functions that had been predicted theoretically. The devices operated at a range of temperatures from 5 to 77 degrees Celsius.

Fabricating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapor deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapor deposition is used to deposit optically-transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the carbon nanotube antennas when they are excited by light.

In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching on and off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometer diameter diode to operate at these exceptional frequencies.

“A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode,” Cola explained. “The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode — which is the structure we made.”

The rectennas fabricated by Cola’s group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.

Cola sees the rectennas built so far as simple proof of principle. He has ideas for how to improve the efficiency by changing the materials, opening the carbon nanotubes to allow multiple conduction channels, and reducing resistance in the structures.

“We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” he said. “Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency.”

Story Source:

The above post is reprinted from materials provided by Georgia Institute of Technology. Note: Materials may be edited for content and length.

Journal Reference:

  1. Asha Sharma, Virendra Singh, Thomas L. Bougher, Baratunde A. Cola. A carbon nanotube optical rectenna. Nature Nanotechnology, 2015; DOI: 10.1038/nnano.2015.220
QDOTS Cad Free 100115 id41477Quantum dots have made it possible to substantially increase color quality in LCD displays. However, these cadmium-based nanocrystals have proven to be harmful to the environment. Fraunhofer researchers are working together with an industry partner to develop a promising alternative: quantum dots based on indium phosphide.
The landscape is breathtaking. Because it is so real, you forget for a moment that the eagle circling the sky is not outside your window, but is instead on your television. Such deceptively realistic images are not only a result of the high resolution displays available on modern devices; the colors play a role as well, and they are becoming ever brighter and richer. This is possible thanks to tiny crystals known as quantum dots (QDs), which have a thickness of merely a few atoms. These nanoparticles located in the backlight units of QD LCD displays offer a cornucopia of colors, but also they possess another extraordinary characteristic.
“One big advantage of quantum dots is that their optical properties can be selectively modified by changing their size,” explains Dr. Armin Wedel of the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam, Germany. “This means you no longer have to manufacture three separate materials for the colors red, green and blue; now it is possible to do the job with just one.” This saves both time and money.

Quantum dots

Quantum dots make it possible to display any color in full brilliance. (Image: Fraunhofer IAP)

Over the last several years, Fraunhofer IAP researchers in Potsdam have been developing quantum dots for customers in a wide range of industry sectors. They manufacture the nanoparticles using chemical synthesis and customize them for each application. This initially results in very small particles that radiate blue light. At sizes above approximately 2 nanometers, the color changes to green. The largest of the quantum particles, at 7 nanometers in size, emit within the red spectral range.

Currently, Wedel and his team are developing quantum dots for display backlighting on behalf of Dutch company NDF Special Light Products B.V. These quantum dots will improve the color rendering and color realism of the displays. Here, the crystals are manufactured for the different emission colors and embedded in plastics. These plastics are subsequently processed into films and built into the display as a conversion film.
Alternative materials based on indium phosphide
With this task, researchers are facing a new challenge. The EU Commission is currently considering a ban on cadmium in consumer goods by 2017, because of its damaging effect on the environment. However, it is also considered to be the ideal material for manufacturing the crystals – cadmium-based quantum dots can achieve a narrowband spectrum sharpness of just 20 to 25 nanometers.
Display manufacturers around the world are now looking for suitable replacement materials with similar characteristics. Against this backdrop, Fraunhofer IAP looks to be on a promising path.
“We are testing quantum dots based on indium phosphide together with NDF Special Light Products,” says Wedel.
His team has already managed to achieve a spectral sharpness of 40 nanometers. At first glance, that does not seem too far away from the quality achievable with cadmium-based quantum dots, but the differences in color fidelity are still present.
“We see this as a good first milestone, but we are still striving for further improvement,” says Wedel.
This effort is set to pay off, as television manufacturers are not the only ones who covet these little color wonders. There is also great market potential for special applications such as medical or aeronautical equipment displays. Furthermore, quantum dots can also increase the efficiency of solar cells, or can be employed in bioanalytics. For such special cases, the optical characteristics of the quantum dots must be precisely configured to the specific application requirements.
“We’re in a good position thanks to our extensive experience in manufacturing quantum dots to meet specific customer requirements,” says Wedel.
Source: Fraunhofer Society

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