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Smart windows get darker to filter out the sun’s rays on bright days, and turn clear on cloudy days to let more light in. This feature can help control indoor temperatures and offers some privacy without resorting to aids such as mini-blinds.Now scientists report a new development in this growing niche: solar smart windows that can turn opaque on demand and even power other devices. The study appears in ACS Photonics (“Electrically Controllable Light Trapping for Self-Powered Switchable Solar Windows”).

 

Smart windows get darker to filter out the sun’s rays on bright days, and turn clear on cloudy days to let more light in. This feature can help control indoor temperatures and offers some privacy without resorting to mini-blinds. Now scientists report a new development in this growing niche: solar smart windows that can turn opaque on demand and even power other devices.

Most existing solar-powered smart windows are designed to respond automatically to changing conditions, such as light or heat. But this means that on cool or cloudy days, consumers can’t flip a switch and tint the windows for privacy.
Also, these devices often operate on a mere fraction of the light energy they are exposed to while the rest gets absorbed by the windows. This heats them up, which can add warmth to a room that the windows are supposed to help keep cool. Jeremy Munday and colleagues wanted to address these limitations.
The researchers created a new smart window by sandwiching a polymer matrix containing microdroplets of liquid crystal materials, and an amorphous silicon layer — the type often used in solar cells — between two glass panes. 

When the window is “off,” the liquid crystals scatter light, making the glass opaque. The silicon layer absorbs the light and provides the low power needed to align the crystals so light can pass through and make the window transparent when the window is turned “on” by the user.

The extra energy that doesn’t go toward operating the window is harvested and could be redirected to power other devices, such as lights, TVs or smartphones, the researchers say.
Source: American Chemical Society
efficiently-photo-charging-lithium-ion-batteries-by-perovskite-solar-cell-1“By combining these two cells, the perovskite cell and the silicon cell, we are able to make much better use of the solar energy and achieve higher efficiencies than either cell on its own.”

Researchers at The Australian National University (ANU) have found a new way to fabricate high efficiency semi-transparent perovskite solar cells in a breakthrough that could lead to more efficient and cheaper solar electricity (Advanced Energy Materials, “Efficient Indium-Doped TiOxElectron Transport Layers for High-Performance Perovskite Solar Cells and Perovskite-Silicon Tandems”).

 

Dr Tom White from the ANU Research School of Engineering said the new fabrication method significantly improved the performance of perovskite solar cells, which can combine with conventional silicon solar cells to produce more efficient solar electricity. 
ANU Ph.D. student The Duong, Dr.Tom White and Ph.D. student Jun Peng
ANU Ph.D. student The Duong, Dr.Tom White and Ph.D. student Jun Peng.
He said perovskite solar cells were extremely good at making electricity from visible light – blue, green and red – while conventional silicon solar cells were more efficient at converting infrared light into electricity.
“The prospect of adding a few additional processing steps at the end of a silicon cell production line to make perovskite cells is very exciting and could boost solar efficiency from 25 per cent to 30 per cent,” Dr White said.
“By combining these two cells, the perovskite cell and the silicon cell, we are able to make much better use of the solar energy and achieve higher efficiencies than either cell on its own.”
While perovskite cells can improve efficiency, they are not yet stable enough to be used on rooftops. Dr White said the new fabrication technique could help develop more reliable perovskite cells.
The new fabrication method involves adding a small amount of the element indium into one of the cell layers during fabrication. That could increase the cell’s power output by as much as 25 per cent.
“We have been able to achieve a record efficiency of 16.6 per cent for a semi-transparent perovskite cell, and 24.5 per cent for a perovskite-silicon tandem, which is one of the highest efficiencies reported for this type of cell,” said Dr White.
Dr White said the research placed ANU in a small group of labs around the world with the capability to improve silicon solar cell efficiency using perovskites.
The development builds on the state-of-the-art silicon cell research at ANU and is part of a $12.2 million “High-efficiency silicon/perovskite solar cells” project led by University of New South Wales and supported by $3.6 million of funding from the Australian Renewable Energy Agency.
Research partners include Monash University, Arizona State University, Suntech R&D Australia Pty Ltd and Trina Solar.
Source: The Australian National University

Rectenna Naval Optical 150928122542_1_540x360

 

A new kind of nanoscale rectenna (half antenna and half rectifier) can convert solar and infrared into electricity, plus be tuned to nearly any other frequency as a detector.

Right now efficiency is only one percent, but professor Baratunde Cola and colleagues at the Georgia Institute of Technology (Georgia Tech, Atlanta) convincingly argue that they can achieve 40 percent broad spectrum efficiency (double that of silicon and more even than multi-junction gallium arsenide) at a one-tenth of the cost of conventional solar cells (and with an upper limit of 90 percent efficiency for single wavelength conversion).

It is well suited for mass production, according to Cola. It works by growing fields of carbon nanotubes vertically, the length of which roughly matches the wavelength of the energy source (one micron for solar), capping the carbon nanotubes with an insulating dielectric (aluminum oxide on the tethered end of the nanotube bundles), then growing a low-work function metal (calcium/aluminum) on the dielectric and voila–a rectenna with a two electron-volt potential that collects sunlight and converts it to direct current (DC).

“Our process uses three simple steps: grow a large array of nanotube bundles vertically; coat one end with dielectric; then deposit another layer of metal,” Cola told EE Times. “In effect we are using one end of the nanotube as a part of a super-fast metal-insulator-metal tunnel diode, making mass production potentially very inexpensive up to 10-times cheaper than crystalline silicon cells.”

For commercialization, billions or even trillions of carbon-nanotube bundles could be grown side-by-side, ramping up the power output into the megaWatt range, after optimization for higher efficiency.

“We still have a lot of work to do to lower contact resistance which will improve the impedance match between the antenna and diode, thus raising efficiency,” Cola told us.”Our proof-of-concept was tuned to the near-infrared. We used infrared-, solar- and green laser-light and got efficiencies of less than one percent, but what was key to our demo was we showed our computer model matched our experimental results, giving us the confidence that we can improve the efficiency up to 40 percent in just a few years.”

For the future, Cola’s group has a three tiered goal–first develop sensor applications that don’t require high efficiencies, second to get the efficiency to 20 percent for harvesting waste heat in the infrared spectrum, then start replacing standard solar cells with 40 percent efficient panels in the visible spectrum. The team is also seeking suitable flexible substrates for applications that require bending.

Rectenna Naval Optical 150928122542_1_540x360

 

Schematic of the components making up the optical rectenna–carbon nanotubes capped with a metal-oxide-metal tunneling diode. (Credit: Thomas Bougher)
(Source: Georgia Tech)

 

Nature Nanotechnology – A carbon nanotube optical rectenna

An optical rectenna—a device that directly converts free-propagating electromagnetic waves at optical frequencies to direct current—was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 pHz (switching speed on the order of 1 fs).

Diodes operating at these frequencies are feasible if their capacitance is on the order of a few attofarads but they remain extremely difficult to fabricate and to reliably couple to a nanoscale antenna. Here we demonstrate an optical rectenna by engineering metal–insulator–metal tunnel diodes, with a junction capacitance of ∼2 aF, at the tip of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna. Upon irradiation with visible and infrared light, we measure a d.c. open-circuit voltage and a short-circuit current that appear to be due to a rectification process (we account for a very small but quantifiable contribution from thermal effects). In contrast to recent reports of photodetection based on hot electron decay in a plasmonic nanoscale antenna a coherent optical antenna field appears to be rectified directly in our devices, consistent with rectenna theory. Finally, power rectification is observed under simulated solar illumination, and there is no detectable change in diode performance after numerous current–voltage scans between 5 and 77 °C, indicating a potential for robust operation.

 

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A team of scientists at Royal Melbourne Institute of Technology in Australia has announced the development of a nanostructure material made of what they are calling nanocones—it is a type of nanomaterial that can be added to boost the efficiency of photovoltaics by increasing their light absorbing abilities. In their paper published in the journal Science Advances, the team describes the new material, how it works, and their hopes for its use in a wide variety of photovoltaic applications.

The new cone structured material’s positive attributes come about due to an ultrahigh refractive index—each cone is made of a type of material that acts inside as an insulator and outside as a conductor—under a microscope the material looks like a mass of bullets stood up on end atop a flat base. It, like other topological insulators, exploits oscillations that occur as a result of changes in the concentration of electrons that come about when the material is struck by photons. Each cone has a metal shell coating and a core that is based on a dielectric—a material made with them would be able to provide superior light absorption properties, making it ideal for not just solar cells, but a wide variety of ranging from optical fibers to waveguides and even lenses. The researchers suggest that if such a material were to be used as part of a traditional thin-film solar cell, it could increase light absorption by up to 15 percent in both the visible and ultraviolet range.

In interviews with the press, the researchers pointed out that theirs is the first time that such a nanocone structure has been created and perhaps just as importantly, noted that creating them would not require any new fabrication techniques. Also, they suggested that because of the better properties of the new material, “both the short circuit current and photoelectric conversion efficiency could be enhanced.”16-CNT Dye Solar Cells figure1

The researchers also note that unlike other nanostructures the oscillations generated by the nanocones are polarization insensitive, which means they do not have to be directionally perpendicular to nanoslits making them more useful in a wider array of applications because they can be directly integrated into current hardware. They add that they next plan to shift their efforts towards focusing on plasmonics that occur in other sorts of structures with different types of shapes.

Explore further: Nanocones could be key to making inexpensive solar cells

More information: Z. Yue et al. Intrinsically core-shell plasmonic dielectric nanostructures with ultrahigh refractive index, Science Advances (2016). DOI: 10.1126/sciadv.1501536

Abstract
Topological insulators are a new class of quantum materials with metallic (edge) surface states and insulating bulk states. They demonstrate a variety of novel electronic and optical properties, which make them highly promising electronic, spintronic, and optoelectronic materials. We report on a novel conic plasmonic nanostructure that is made of bulk-insulating topological insulators and has an intrinsic core-shell formation. The insulating (dielectric) core of the nanocone displays an ultrahigh refractive index of up to 5.5 in the near-infrared frequency range. On the metallic shell, plasmonic response and strong backward light scattering were observed in the visible frequency range. Through integrating the nanocone arrays into a-Si thin film solar cells, up to 15% enhancement of light absorption was predicted in the ultraviolet and visible ranges. With these unique features, the intrinsically core-shell plasmonic nanostructure paves a new way for designing low-loss and high-performance visible to infrared optical devices.

 

 

 

The researchers also note that

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

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PolyU’ invention of semitransparent perovskite solar cells with graphene electrodes, the power conversion efficiencies (PCEs) are around 12% which is much higher than existing semitransparent solar cells.
Credit: Image courtesy of The Hong Kong Polytechnic University

Developing transparent or semitransparent solar cells with high efficiency and low cost to replace the existing opaque and expensive silicon-based solar panels has become increasingly important due to the increasing demands of the building integrated photovoltaics (BIPVs) systems. The Department of Applied Physics of The Hong Kong Polytechnic University (PolyU) has successfully developed efficient and low-cost semitransparent perovskite solar cells with graphene electrodes. The power conversion efficiencies (PCEs) of this novel invention are around 12% when they are illuminated from Fluorine-doped Tin Oxide bottom electrodes (FTO) or the graphene top electrodes, compared with 7% of conventional semitransparent solar cells. Its potential low cost of less than HK$0.5/Watt, more than 50% reduction compared with the existing cost of Silicon solar cells, will enable it to be widely used in the future.

Reshape Solar 150727180231_1_540x360Photographs of upconversion in a cuvette containing cadmium selenide/rubrene mixture. The yellow spot is emission from the rubrene originating from (a) an unfocused continuous wave 800 nm laser with an intensity of 300 W/cm2. (b) a focused continuous wave 980 nm laser with an intensity of 2000 W/cm2. The photographs, taken with an iPhone 5, were not modified in any way.

When it comes to installing solar cells, labor cost and the cost of the land to house them constitute the bulk of the expense. The solar cells — made often of silicon or cadmium telluride — rarely cost more than 20 percent of the total cost. Solar energy could be made cheaper if less land had to be purchased to accommodate solar panels, best achieved if each solar cell could be coaxed to generate more power.

A huge gain in this direction has now been made by a team of chemists at the University of California, Riverside that has found an ingenious way to make solar energy conversion more efficient. The researchers report in Nano Letters that by combining inorganic semiconductor nanocrystals with organic molecules, they have succeeded in “upconverting” photons in the visible and near-infrared regions of the solar spectrum.

“The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today’s solar cells,” explained Christopher Bardeen, a professor of chemistry. The research was a collaborative effort between him and Ming Lee Tang, an assistant professor of chemistry. “This is energy lost, no matter how good your solar cell. The hybrid material we have come up with first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted.”

Bardeen added that these materials are essentially “reshaping the solar spectrum” so that it better matches the photovoltaic materials used today in solar cells. The ability to utilize the infrared portion of the solar spectrum could boost solar photovoltaic efficiencies by 30 percent or more.

In their experiments, Bardeen and Tang worked with cadmium selenide and lead selenide semiconductor nanocrystals. The organic compounds they used to prepare the hybrids were diphenylanthracene and rubrene. The cadmium selenide nanocrystals could convert visible wavelengths to ultraviolet photons, while the lead selenide nanocrystals could convert near-infrared photons to visible photons.

In lab experiments, the researchers directed 980-nanometer infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nanometer light, almost doubling the energy of the incoming photons. The researchers were able to boost the upconversion process by up to three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands, providing a route to higher efficiencies.

“This 550 — nanometer light can be absorbed by any solar cell material,” Bardeen said. “The key to this research is the hybrid composite material — combining inorganic semiconductor nanoparticles with organic compounds. Organic compounds cannot absorb in the infrared but are good at combining two lower energy photons to a higher energy photon. By using a hybrid material, the inorganic component absorbs two photons and passes their energy on to the organic component for combination. The organic compounds then produce one high-energy photon. Put simply, the inorganics in the composite material take light in; the organics get light out.”

Besides solar energy, the ability to upconvert two low energy photons into one high energy photon has potential applications in biological imaging, data storage and organic light-emitting diodes. Bardeen emphasized that the research could have wide-ranging implications.

“The ability to move light energy from one wavelength to another, more useful region, for example, from red to blue, can impact any technology that involves photons as inputs or outputs,” he said.

The research was supported by grants from the National Science Foundation and the US Army.

The research was conducted also by the following coauthors on the research paper: Zhiyuan Huang (first author), Xin Li, Melika Mahboub, Kerry M. Hanson, Valerie M. Nichols and Hoang Le.

Tang’s group helped design the experiments and provided the nanocrystals.


Story Source:

The above post is reprinted from materials provided by University of California – Riverside. Note: Materials may be edited for content and length.

Large Solar panels University of Adelaide chemistry researchers are studying energy loss at the molecular level of new ‘plastic’ materials as a step towards the development of highly efficient, low-cost and flexible solar energy cells.

One aim is to be able to “tune” the molecules to make them more energy efficient, thereby reducing energy loss, and able to harvest more photons from the sun.

They hope one day this research could lead to applications such as whole buildings being covered with a semi-transparent, flexible window tint that would act as a giant solar cell, trapping energy from the sun and generating electricity to power the building.

“Traditional used in the solar panels we have on our roof-tops are made from silicon which require a large amount of energy to produce and are expensive,” says Patrick Tapping, PhD candidate in the School of Physical Sciences.

“There is a whole category of new ‘plastic’ , called organic semi-conductors and, like normal plastics, they are made from hydrocarbon chains or polymers. But unlike normal plastics, they can conduct electricity.

“These materials are flexible and cheap to manufacture – they can be printed out as giant sheets. But at the moment they are not currently very good at turning absorbed light into harvestable electricity. They don’t transport electrons to electrodes as efficiently as they should.

“Our research is giving us a better understanding of how these materials behave at molecular level.”

The researchers in the University’s Department of Chemistry ─ including Mr Tapping and his supervisors Dr Tak Kee and Dr David Huang ─ are using ultra-fast laser spectroscopy and computer modelling to “watch” the reactions occurring inside the polymer-based solar cells.

Spectroscopy allows the study of matter via its interaction with light. Ultra-fast laser spectroscopy uses extremely short pulses of light, measuring the interactions at the with an electronic detector.

“We’re conducting experiments and using computer simulations to look at the arrangements of the polymer chains to see how they affect the electricity-generating properties of the materials,” says Mr Tapping.

“By the speed and intensity of the changes to light-absorption, we can gain insight to where potential sources of may be occurring. This can then guide changes in order to better harvest the sun for more efficient energy generation.”

Explore further: Creating low-cost solar energy

All Star Nano Crystals 061615 id40257Ames Laboratory scientists discovered semiconducting nanocrystals that function not only as stellar light-to-energy converters but also as stable light emitters (“Shape Evolution and Single Particle Luminescence of Organometal Halide Perovskite Nanocrystals”).
The Impact
Honing methods to fine-tune optimal characteristics of materials that convert light to energy may lead to more efficient materials, as performance depends critically on composition, crystallinity, and morphology. These perovskites could be used in the construction of new solar cell architectures, as well as for light-emitting devices and single particle imaging and tracking.

Perovskite nanowires have been found to function as shape-correlated stable light emitters

Perovskite nanowires have been found to function as shape-correlated stable light emitters. (Image courtesy of The Ames Laboratory)

Summary

Perovskite materials, such as CH3NH3PbX3 (X = I, Br), are known to display intriguing electronic, light-emitting, and chemical properties.
Researchers at the Ames Laboratory synthesized a series of perovskite nanocrystals with different morphologies (i.e., dots, rods, wires, plates, and sheets) by using different solvents and capping ligands. The Ames Laboratory team tested the nanocrystals to explore their morphology, growth, properties, and stability under various conditions. Characterization studies of photoluminescence, like that seen with glow-in-the-dark paint, found that the rods and wires showed higher photoluminescence and longer photoluminescence lifetimes compared to other shapes. Perovskite nanocrystals with bromine were found to be particularly unstable when exposed to an electron beam during transmission electron microscopy analysis, “melting” to form smaller dot-like particles of unknown composition.
Further optical studies revealed that the nanocrystals with iodine are shape-correlated stable light emitters at room temperature.
Source: U.S. Department of Energy, Office of Science

Read more: All-star nanocrystals


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