By combining designer quantum dot light-emitters with spectrally matched photonic mirrors, a team of scientists with Berkeley Lab and the University of Illinois created solar cells that collect blue photons at 30 times the concentration of conventional solar cells, the highest luminescent concentration factor ever recorded. This breakthrough paves the way for the future development of low-cost solar cells that efficiently utilize the high-energy part of the solar spectrum.
“We’ve achieved a luminescent concentration ratio greater than 30 with an optical efficiency of 82-percent for blue photons,” says Berkeley Lab director Paul Alivisatos, who is also the Samsung Distinguished Professor of Nanoscience and Nanotechnology at the University of California Berkeley, and director of the Kavli Energy Nanoscience Institute (ENSI), was the co-leader of this research. “To the best of our knowledge, this is the highest luminescent concentration factor in literature to date.”
Luminescent solar concentrators featuring quantum dots and photonic mirrors suffer far less parasitic loss of photons than LSCs using molecular dyes as lumophores.
Alivisatos and Ralph Nuzzo of the University of Illinois are the corresponding authors of a paper in ACS Photonics describing this research entitled “Quantum Dot Luminescent Concentrator Cavity Exhibiting 30-fold Concentration”. Noah Bronstein, a member of Alivisatos’s research group, is one of three lead authors along with Yuan Yao and Lu Xu. Other co-authors are Erin O’Brien, Alexander Powers and Vivian Ferry.
The solar energy industry in the United States is soaring with the number of photovoltaic installations having grown from generating 1.2 gigawatts of electricity in 2008 to generating 20-plus gigawatts today, according to the U.S. Department of Energy (DOE). Still, nearly 70-percent of the electricity generated in this country continues to come from fossil fuels. Low-cost alternatives to today’s photovoltaic solar panels are needed for the immense advantages of solar power to be fully realized. One promising alternative has been luminescent solar concentrators (LSCs).
Unlike conventional solar cells that directly absorb sunlight and convert it into electricity, an LSC absorbs the light on a plate embedded with highly efficient light-emitters called “lumophores” that then re-emit the absorbed light at longer wavelengths, a process known as the Stokes shift. This re-emitted light is directed to a micro-solar cell for conversion to electricity. Because the plate is much larger than the micro-solar cell, the solar energy hitting the cell is highly concentrated.
With a sufficient concentration factor, only small amounts of expensive III-V photovoltaic materials are needed to collect light from an inexpensive luminescent waveguide. However, the concentration factor and collection efficiency of the molecular dyes that up until now have been used as lumophores are limited by parasitic losses, including non-unity quantum yields of the lumophores, imperfect light trapping within the waveguide, and reabsorption and scattering of propagating photons.
“We replaced the molecular dyes in previous LSC systems with core/shell nanoparticles composed of cadmium selenide (CdSe) cores and cadmium sulfide (CdS) shells that increase the Stokes shift while reducing photon re-absorption,” says Bronstein.
“The CdSe/CdS nanoparticles enabled us to decouple absorption from emission energy and volume, which in turn allowed us to balance absorption and scattering to obtain the optimum nanoparticle,” he says. “Our use of photonic mirrors that are carefully matched to the narrow bandwidth of our quantum dot lumophores allowed us to achieve waveguide efficiency exceeding the limit imposed by total internal reflection.”
In their ACS Photonics paper, the collaborators express confidence that future LSC devices will achieve even higher concentration ratios through improvements to the luminescence quantum yield, waveguide geometry, and photonic mirror design.
The success of this CdSe/CdS nanoparticle-based LSC system led to a partnership between Berkeley Lab, the University of Illinois, Caltech and the National Renewable Energy Lab (NREL) on a new solar concentrator project. At the recent Clean Energy Summit held in Las Vegas, President Obama and Energy Secretary Ernest Moniz announced this partnership will receive a $3 million grant for the development of a micro-optical tandem LCS under MOSAIC, the newest program from DOE’s ARPA-E. MOSAIC stands for Micro-scale Optimized Solar-cell Arrays with Integrated Concentration.
The LCS work reported in this story was carried out through the U.S. Department of Energy’s Energy Frontier Research Center program and the National Science Foundation.
Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots
Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future.
Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices.
Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture.
Here, we use quantum dots of ternary I–III–VI2 semiconductors to realize the first large-area quantum dot–luminescent solar concentrators free of toxic elements, with reduced reabsorption and extended coverage of the solar spectrum.
By incorporating CuInSexS2–x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colourless slabs that introduce no distortion to perceived colours and are thus well suited for the realization of photovoltaic windows.
Thanks to the suppressed reabsorption and high emission efficiencies of the quantum dots, we achieve an optical power efficiency of 3.2%.Ultrafast spectroscopy studies suggest that the Stokes-shifted emission involves a conduction-band electron and a hole residing in an intragap state associated with a native defect.
A luminescent solar concentrator is an emerging sunlight harvesting technology that has the potential to disrupt the way we think about energy; It could turn any window into a daytime power source.
“In these devices, a fraction of light transmitted through the window is absorbed by nanosized particles (semiconductor quantum dots) dispersed in a glass window, re-emitted at the infrared wavelength invisible to the human eye, and wave-guided to a solar cell at the edge of the window,” said Victor Klimov, lead researcher on the project at the Department of Energy’s Los Alamos National Laboratory. “Using this design, a nearly transparent window becomes an electrical generator, one that can power your room’s air conditioner on a hot day or a heater on a cold one.”
The luminescent solar concentrator could turn any window into a daytime power source.
In April 2014, using special composite quantum dots, the American-Italian collaboration demonstrated the first example of large-area luminescent solar concentrators free from reabsorption losses of the guided light by the nanoparticles. This represented a fundamental advancement with respect to the earlier technology, which was based on organic emitters that allowed for the realization of concentrators of only a few centimeters in size.
However, the quantum dots used in previous proof-of-principle devices were still unsuitable for real-world applications, as they were based on the toxic heavy metal cadmium and were capable of absorbing only a small portion of the solar light. This resulted in limited light-harvesting efficiency and strong yellow/red coloring of the concentrators, which complicated their application in residential environments.
Klimov, CASP’s director, explained how the updated approach solves the coloring problem: “Our new devices use quantum dots of a complex composition which includes copper (Cu), indium (In), selenium (Se) and sulfur (S). This composition is often abbreviated as CISeS. Importantly, these particles do not contain any toxic metals that are typically present in previously demonstrated LSCs.”
“Furthermore,” Klimov noted, “the CISeS quantum dots provide a uniform coverage of the solar spectrum, thus adding only a neutral tint to a window without introducing any distortion to perceived colors. In addition, their near-infrared emission is invisible to a human eye, but at the same time is ideally suited for most common solar cells based on silicon.”
Francesco Meinardi, professor of Physics at UNIMIB, described the emerging work, noting, “In order for this technology to leave the research laboratories and reach its full potential in sustainable architecture, it is necessary to realize non-toxic concentrators capable of harvesting the whole solar spectrum.”
“We must still preserve the key ability to transmit the guided luminescence without reabsorption losses, though, so as to complement high photovoltaic efficiency with dimensions compatible with real windows. The aesthetic factor is also of critical importance for the desirability of an emerging technology,” Meinardi said.
Hunter McDaniel, formerly a Los Alamos CASP postdoctoral fellow and presently a quantum dot entrepreneur (UbiQD founder and president), added, “with a new class of low-cost, low-hazard quantum dots composed of CISeS, we have overcome some of the biggest roadblocks to commercial deployment of this technology.”
“One of the remaining problems to tackle is reducing cost, but already this material is significantly less expensive to manufacture than alternative quantum dots used in previous LSC demonstrations,” McDaniel said.
A key element of this work is a procedure comparable to the cell casting industrial method used for fabricating high optical quality polymer windows. It involves a new UNIMIB protocol for encapsulating quantum dots into a high-optical quality transparent polymer matrix. The polymer used in this study is a cross-linked polylaurylmethacrylate, which belongs to the family of acrylate polymers. Its long side-chains prevent agglomeration of the quantum dots and provide them with the “friendly” local environment, which is similar to that of the original colloidal suspension. This allows one to preserve light emission properties of the quantum dots upon encapsulation into the polymer.
Sergio Brovelli, the lead researcher on the Italian team, concluded: “Quantum dot solar window technology, of which we had demonstrated the feasibility just one year ago, now becomes a reality that can be transferred to the industry in the short to medium term, allowing us to convert not only rooftops, as we do now, but the whole body of urban buildings, including windows, into solar energy generators.”
“This is especially important in densely populated urban area where the rooftop surfaces are too small for collecting all the energy required for the building operations,” he said. He proposes that the team’s estimations indicate that by replacing the passive glazing of a skyscraper such as the One World Trade Center in NYC (72,000 square meters divided into 12,000 windows) with our technology, it would be possible to generate the equivalent of the energy need of over 350 apartments.
“Add to these remarkable figures, the energy that would be saved by the reduced need for air conditioning thanks to the filtering effect by the LSC, which lowers the heating of indoor spaces by sunlight, and you have a potentially game-changing technology towards “net-zero” energy cities,” Brovelli said.
The triplet state lifetime varies with the distance and the strength of binding between the porphyrin and the surface of the quantum dot.
Nanotechnology could improve the efficiency of organic photovoltaic technology, researchers at King Abdullah University of Science and Technology (KAUST) have demonstrated1.
In general, solar cells made from organic materials offer a cheap, simple and sustainable approach to harvesting light from the sun. But there is an urgent need to improve the efficiency of these organic cells.
The performance of these devices is limited by the re-emission of light that has been absorbed, thus detracting energy that should be converted to electricity. When an organic material absorbs light, it can create an exciton — an electron paired to a positively charged equivalent called a hole. This exciton exists for a very short period before recombining radiatively or non-radiatively. So, for a useful current to be produced, the electron and hole must separate before they recombine.
Research by Omar Mohammed and his colleagues from the KAUST Solar and Photovoltaics Engineering Research Center show how the lifetime of excitons in an organic material can be extended by using quantum dots.
Quantum dots are nanometer scale particles. Their advantage in solar-cell technologies is their tunability: the optical properties, such as absorption wavelength, can be changed by varying the size of the dot. Additional molecules attached to the surface of the nanostructure can tailor the functionality of the dots even further.
Mohammed’s team investigated a family of organic compounds (commonly used in solar applications) known as porphyrins. The electron-hole pair generated in porphyrin by light absorption forms a high-energy exciton, which then relaxes to one of two different lower-energy excitons know as a singlet and a triplet.
“The photo-generated singlet excitons exhibit very short lifetimes and consequently they have short diffusion lengths, which is one of the greatest challenges for achieving high power-conversion efficiencies in solar-cell devices,” explains Mohammed. “Triplet excitons with their long lifetimes are an alternative way to overcome this problem.”
The researchers showed that cadmium telluride quantum dots can improve not only the path from excited exciton to triplet exciton — so-called intersystem crossing, but also the elongation of the triplet exciton lifetime. They were able to tune the intersystem crossing and the triplet state lifetime by changing the size of the quantum dots in the solution.
“We are currently testing other absorber materials and other semiconductor quantum dots,” says Mohammed. “In addition, we are planning to fabricate solar cell devices from these nano-assemblies.”
Ahmed, G. H., Aly, S. M., Usman, A., Eita, M. S., Melnikov, V. A. & Mohammed, O. F. Quantum confinement-tunable intersystem crossing and the triplet state lifetime of cationic porphyrin–CdTe quantum dot nano-assemblies. Chemical Communications 51, 8010—8013 (2015). | article
*** For More on Quantum Dots and Solar Energy ‘Search’ Quantum Dots on our Blog. ***
EPFL scientists have created the first perovskite nanowire-graphene hybrid phototransistors. Even at room temperature, the devices are highly sensitive to light, making them outstanding photodetectors.
The lead-containing perovskite materials can turn light into electricity with high efficiency, which is why they have revolutionized solar cell technologies. On the other hand, graphene is known for its super-strength as well as its excellent electrical conductivity. Combining the two materials, EPFL scientists have created the first ever class of hybrid transistors that turn light into electricity with high sensitivity and at room temperature. The work is published in Small.
The lab of László Forró at EPFL, where the chemical activity is led by Endre Horváth, used its expertise in microengineering to create nanowires of the perovskite methylammonium lead iodide. This highly non-trivial route for the synthesis of nanowires was developed by him in 2014 and called slip-coating method. The advantage of nanowires is their consistency, while their manufacturing can be controlled to modify their architecture and explore different designs.
Making a device by depositing the perovskite nanowires onto graphene has increased the efficiency in converting light to electrical current at room temperature. “Such a device shows almost 750,000 times higher photoresponse compared to detectors made only with perovskite nanowires,” added Massimo Spina who fabricated the miniature photodetectors. Because of this exceptionally high sensitivity, the graphene/perovskite nanowire hybrid device is considered to be a superb candidate for even a single-photon detection.
This work was founded by the Swiss National Science Foundation. The hybrid devices were fabricated in part at EPFL’s Center for Micro/Nanotechnology.
New research from Rice University could make it easier for engineers to harness the power of light-capturing nanomaterials to boost the efficiency and reduce the costs of photovoltaic solar cells.
Although the domestic solar-energy industry grew by 34 percent in 2014, fundamental technical breakthroughs are needed if the U.S. is to meet its national goal of reducing the cost of solar electricity to 6 cents per kilowatt-hour.
In a study published July 13 in Nature Communications, scientists from Rice’s Laboratory for Nanophotonics (LANP) describe a new method that solar-panel designers could use to incorporate light-capturing nanomaterials into future designs. By applying an innovative theoretical analysis to observations from a first-of-its-kind experimental setup, LANP graduate student Bob Zheng and postdoctoral research associate Alejandro Manjavacas created a methodology that solar engineers can use to determine the electricity-producing potential for any arrangement of metallic nanoparticles.
LANP researchers study light-capturing nanomaterials, including metallic nanoparticles that convert light into plasmons, waves of electrons that flow like a fluid across the particles’ surface. For example, recent LANP plasmonic research has led to breakthroughs in color-display technology, solar-powered steam production and color sensors that mimic the eye.
“One of the interesting phenomena that occurs when you shine light on a metallic nanoparticle or nanostructure is that you can excite some subset of electrons in the metal to a much higher energy level,” said Zheng, who works with LANP Director and study co-author Naomi Halas. “Scientists call these ‘hot carriers’ or ‘hot electrons.'”
Halas, Rice’s Stanley C. Moore Professor of Electrical and Computer Engineering and professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering, said hot electrons are particularly interesting for solar-energy applications because they can be used to create devices that produce direct current or to drive chemical reactions on otherwise inert metal surfaces.
Today’s most efficient photovoltaic cells use a combination of semiconductors that are made from rare and expensive elements like gallium and indium. Halas said one way to lower manufacturing costs would be to incorporate high-efficiency light-gathering plasmonic nanostructures with low-cost semiconductors like metal oxides. In addition to being less expensive to make, the plasmonic nanostructures have optical properties that can be precisely controlled by modifying their shape.
“We can tune plasmonic structures to capture light across the entire solar spectrum,” Halas said. “The efficiency of semiconductor-based solar cells can never be extended in this way because of the inherent optical properties of the semiconductors.”
The plasmonic approach has been tried before but with little success.
Zheng said, “Plasmonic-based photovoltaics have typically had low efficiencies, and it hasn’t been entirely clear whether those arose from fundamental physical limitations or from less-than-optimal designs.”
He and Halas said Manjavacas, a theoretical physicist in the group of LANP researcher Peter Nordlander, conducted work in the new study that offers a fundamental insight into the underlying physics of hot-electron-production in plasmonic-based devices.
Manjavacas said, “To make use of the photon’s energy, it must be absorbed rather than scattered back out. For this reason, much previous theoretical work had focused on understanding the total absorption of the plasmonic system.”
He said a recent example of such work comes from a pioneering experiment by another Rice graduate student, Ali Sobhani, where the absorption was concentrated near a metal semiconductor interface.
“From this perspective, one can determine the total number of electrons produced, but it provides no way of determining how many of those electrons are actually useful, high-energy, hot electrons,” Manjavacas said.
He said Zheng’s data allowed a deeper analysis because his experimental setup selectively filtered high-energy hot electrons from their less-energetic counterparts. To accomplish this, Zheng created two types of plasmonic devices. Each consisted of a plasmonic gold nanowire atop a semiconducting layer of titanium dioxide. In the first setup, the gold sat directly on the semiconductor, and in the second, a thin layer of pure titanium was placed between the gold and the titanium dioxide. The first setup created a microelectronic structure called a Schottky barrier and allowed only hot electrons to pass from the gold to the semiconductor. The second setup allowed all electrons to pass.
“The experiment clearly showed that some electrons are hotter than others, and it allowed us to correlate those with certain properties of the system,” Manjavacas said. “In particular, we found that hot electrons were not correlated with total absorption. They were driven by a different, plasmonic mechanism known as field-intensity enhancement.”
LANP researchers and others have spent years developing techniques to bolster the field-intensity enhancement of photonic structures for single-molecule sensing and other applications. Zheng and Manjavacas said they are conducting further tests to modify their system to optimize the output of hot electrons.
Halas said, “This is an important step toward the realization of plasmonic technologies for solar photovoltaics. This research provides a route to increasing the efficiency of plasmonic hot-carrier devices and shows that they can be useful for converting sunlight into usable electricity.”
Additional co-authors include Hangqi Zhao and Michael McClain, both of Rice. The research was supported by the Welch Foundation, the Office of Naval Research and the Air Force Office of Science and Research.
Photographs 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.
Credit: Zhiyuan Huang, UC Riverside.
Solar energy could be made cheaper if solar cells could be coaxed to generate more power.
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.
By Michael Berger – Nanowerk. During 2002 and 2003, Nobel laureate Richard E. Smalley developed a list of the Top Ten Problems Facing Humanity over the next 50 years. The Richard E. Smalley Institute for Nanoscale Science and Technology at Rice University (which in May 2015 has been merged with the Rice Quantum Institute into a new entity: the Smalley-Curl Institute) has identified 5 of these problems as society’s Grand Challenges – and energy tops the list.
Since then, researchers around the world have demonstrated the potential for nanotechnology to be a key technology on the path to a sustainable energy future. Against the double-whammy backdrop of an energy challenge – the world’s appetite for energy keeps growing1 – plus a climate challenge – climate goals (2°C target) require substantial reduction in greenhouse gases (see: Climate change: Action, trends and implications for business. pdf) – it is the role of innovative energy technologies to provide socially acceptable solutions through energy savings; efficiency gains; and decarbonization.
Why is nanotechnology relevant here? Many effects important for energy happen at the nanoscale: In solar cells, for instance, photons can free electrons from a material, which can then flow as an electric current; the chemical reactions inside a battery or fuel cell release electrons which then move through an external circuit; or the role of catalysts in a plethora of chemical reactions. These are just a few examples where nanoscale engineering can significantly improve the efficiency of the underlying processes. The working principle of a solar cell. (Image: University of Massachusetts Amherst) Nanotechnologies are not tied exclusively to renewable energy technologies. While researchers are exploring ways in which nanotechnology could help us to develop energy sources, they also develop techniques to access and use fossil fuels much more efficiently. Corrosion resistant nanocoatings, nanostructured catalysts, and nanomembranes have been used in the extraction and processing of fossil fuels and in nuclear power. There is no silver bullet – nanotechnology applications for energy are extremely varied, reflecting the complexity of the energy sector, with a number of different markets along its value chain, including energy generation, transformation, distribution, storage, and usage. Nanotechnology has the potential to have a positive impact on all of these – albeit with varying effects.
Nanomaterials could lead to energy savings through weight reduction or through optimized function:
In the future, novel, nano-technologically optimized materials, for example plastics or metals with carbon nanotubes (CNTs), will make airplanes and vehicles lighter and therefore help reduce fuel consumption;
Novel lighting materials (OLED: organic light-emitting diodes) with nanoscale layers of plastic and organic pigments are being developed; their conversion rate from energy to light can apparently reach 50 % (compared with traditional light bulbs = 5%);
Nanoscale carbon black has been added to modern automobile tires for some time now to reinforce the material and reduce rolling resistance, which leads to fuel savings of up to 10%;
Self-cleaning or “easy-to-clean”-coatings, for example on glass, can help save energy and water in facility cleaning because such surfaces are easier to clean or need not be cleaned so often;
Nanotribological wear protection products as fuel or motor oil additives could reduce fuel consumption of vehicles and extend engine life;
Nanoparticles as flow agents allow plastics to be melted and cast at lower temperatures;
Nanoporous insulating materials in the construction business can help reduce the energy needed to heat and cool buildings.
Nanomaterials could improve energy generation and energy efficiencies:
Various nanomaterials can improve the efficiency of photovoltaic facilities;
Dye solar cells (‘Grätzel cells’) with nanoscale semiconductor materials mimic natural photosynthesis in green plants;
Plastics with carbon nanotubes as coatings on the rotor blades of wind turbines make these lighter and increase the energy yield;
Nano optimized lithium-ion batteries have an improved storage capacity as well as an increased lifespan and find use in electric vehicles for example;
Fuel cells with nanoscale ceramic materials for energy production require less energy and resources during manufacturing;
The effectiveness of catalytic converters in vehicles can be increased by applying catalytically active precious metals in the nanoscale size range.
We have compiled an overview of Nanotechnology in Energy that shows how nanotechnology innovations could impact each part of the value-added chain in the energy sector – energy sources; energy conversion; energy distribution; energy storage; and energy usage. The European GENNESYS project identified a range of nanomaterial application and requirements for future energy applications3. (click on image to enlarge) In the short term, energy nanotechnology is likely to have the greatest impact in the areas of efficiency of photovoltaics (among renewables, solar has by far the biggest global energy potential) and energy storage where it can help overcome current performance barriers and substantially improve the collection and conversion of solar energy. Nanotechnology for Solar Energy Collection and Conversion is one of the five Signature Initiatives funded by the U.S. National Nanotechnology Initiative. The goals are to enhance understanding of conversion and storage phenomena at the nanoscale, improve nanoscale characterization of electronic properties, and help enable economical nanomanufacturing of robust devices. The initiative has three major thrust areas:
– improve photovoltaic solar electricity generation;
– improve solar thermal energy generation and conversion; and
– improve solar-to-fuel conversions.
The thermodynamic limit of 80% efficiency is well beyond the capabilities of current photovoltaic technologies, whose laboratory performance currently approaches only 43%2. Nanomaterials even make it possible to raise light yield of traditional crystalline silicon solar cells. By using cheaper, nanoscale materials than the current dominant technology (single-crystal silicon, which uses a large amount of fossil fuels for production), the cost of solar cells could be brought down. Numerous research labs are working on nanotechnology-enabled batteries to increase their efficiencies for electric vehicles, home, or grid storage systems. Improving the efficiency/storage capacity of batteries and supercapacitors with nanomaterials will have a substantial economical impact.
Against the double-whammy backdrop of an energy challenge and a climate challenge it is the role of innovative energy technologies to provide socially acceptable solutions through energy savings; efficiency gains; and decarbonization.
So where does that leave ‘nanotechnology’? It may not be the silver bullet, but nanomaterials and nanoscale applications will have an important role to play.
Notes 1) Energy demand grows by 37% to 2040 on planned policies, an average rate of growth of 1.1%. World electricity demand increases by almost 80% over the period 2012-2040. 1.6bn people still without access to electricity, thereof 950 million in sub-Saharan Africa. (Source: IEA World Energy Outlook 2014) 2) Source: NSI Solar White Paper (pdf) 3) Source: GENNESYS White paper
Solar panels are an investment—not only in terms of money, but also energy. It takes energy to mine, process and purify raw materials, and then to manufacture and install the final product.
Silicon-based panels, which dominate the market for solar power, usually need about two years to return this energy investment. But for technology made with perovskites—a class of materials causing quite a buzz in the solar research community—the energy payback time could be as quick as two to three months.
By this metric, perovskite modules are better than any solar technology that is commercially available today.
These are the findings of a study by scientists at Northwestern University and the U.S. Department of Energy’s Argonne National Laboratory. The study took a broad perspective in evaluating solar technology: In what’s called a cradle-to-grave life cycle assessment, scientists traced a product from the mining of its raw materials until its retirement in a landfill. They determined the ecological impacts of making a solar panel and calculated how long it would take to recover the energy invested.
Perovskite technology has yet to be commercialized, but researchers everywhere are excited about the materials. Most projects, however, have been narrowly focused on conversion efficiency—how effectively the technology transforms sunlight into useable energy.
“People see 11 percent efficiency and assume it’s a better product than something that’s 9 percent efficient,” said Fengqi You, corresponding author on the paper and assistant professor of chemical and biological engineering at Northwestern. “But that’s not necessarily true.”
A more comprehensive way to compare solar technology is the energy payback time, which also considers the energy that went into creating the product.
This study looked at the energy inputs and outputs of two perovskite modules. A solar panel consists of many parts, and the module is the piece directly involved in converting energy from one form into another—sunlight into electricity.
Perovskites lag behind silicon in conversion efficiency, but they require much less energy to be made into a solar module. So perovskite modules pull ahead with a substantially shorter energy payback time—the shortest, in fact, among existing options for solar power.
“Appreciating energy payback times is important if we want to move perovskites from the world of scientific curiosity to the world of relevant commercial technology,” said Seth Darling, an Argonne scientist and co-author on the paper.
To get a complete picture of the environmental impacts a perovskite panel could have, the researchers also analyzed metals used for electrodes and other parts of the device.
One of the modules tested includes lead and gold, among other metals. Many perovskite models have lead in their active layer, which absorbs sunlight and plays a leading role in conversion efficiency. People in the research community have expressed concern because everyone knows lead can be toxic, Darling said.
Surprisingly, the team’s assessment showed that gold was much more problematic.
Gold isn’t typically perceived as hazardous, but the process of mining the precious metal is extremely damaging to the environment. The module in this study uses gold in its positive electrode, where charges are collected in the process of generating electricity.
The harmful effects of gold mining, an indirect impact of this particular perovskite technology, is something that could only be uncovered by a cradle-to-grave investigation, said Jian Gong, the study’s first author and a PhD student in You’s research group at Northwestern.
The team hopes that future projects use this same zoomed-out approach to identify the best materials and manufacturing processes for the next generation of solar technology—products that will have to be environmentally sustainable and commercially viable.
“Soon, we’re going to need to produce an extremely high number of solar panels,” You said. “We don’t have time for trial-and-error in finding the ideal design. We need a more rigorous approach, a method that systematically considers all variables.”
While this paper featured a thorough environmental assessment of different solar power options, further studies are needed to factor in economic costs. Before putting a perovskite panel on the market, scientists will likely have to replace gold and other unsustainable materials, for both environmental and economic reasons, Darling said.
In addition, extending the lifetime of perovskite modules will be important in order to make sure they are stable enough for long-term commercial use, You said. Despite a few necessary improvements, he said perovskite technology could be commercialized within two years if researchers use comprehensive analysis to optimize the selection of raw materials and manufacturing.
One of the motivations for this study, according to the authors, was the need to improve technology so that solar energy can be scaled up in a big way.
Global energy demand is expected to nearly double by 2050, and Darling said there’s no question that solar power must contribute a significant fraction.
The real question, Darling said, is “How quickly do we have to get a technology to market to save the planet? And how can we make that happen?”
I know, I actually love all that stuff as much as the rest of you — it’s what I read, edit, & write about every day(!) — but it’s basically all general history and trends we know all about. But then JB dropped the awesome-bomb:
“I think we’re at the beginning of a new cost-decline curve, and, you know, this is something where there’s a lot of similarities to what happened with photovoltaics. Almost no one [would have predicted] that photovoltaic prices would have dropped as fast as they have, and storage is right at the cliff, heading down that price curve. It’s soon going to be cheaper to drive a car on electricity — a pure EV on electricity — than it is to drive a gasoline car. And as soon as we see that kind of shift in the actual cost of operation in a car that you can actually use for your daily driver, you know, from all manufacturers I believe we’re going to see electric vehicles come to dominate the whole transportation fleet.
“Also, that same battery cost decrease is going to drive batteries in the grid. There’s going to be much faster growth of grid energy storage than I think most people expected. You suddenly get to have energy that’s 100% firm and buffered from photovoltaics that’s cheaper than fossil energy. And we’re within sort of grasping distance of that goal, which is very, very exciting.
“Because once we get to that, and there really is no going back, it will make sense to do this economically without any environmental consideration whatsoever. So that’s the amazing tipping point that’s going to happen within I’m quite certain the next 10 years.”
The next 10 years!
(Though, he didn’t actually drop the mic.)
You can watch the highlights via Intersolar on YouTube — video below:
Intersolar North America Conference Opening 2015
Another great quote, however, was this gem: “It’s not going to be many years before Tesla will have a million cars, or 70 gigawatt-hours of storage.” (That quote wasn’t in the highlight video above for some reason!)
One million cars. Can you imagine? Will you have a Tesla by then?