Scientists with the Energy Department’s National Renewable Energy Laboratory (NREL) for the first time discovered how to make perovskite solar cells out of quantum dots and used the new material to convert sunlight to electricity with 10.77 percent efficiency.
The research, Quantum dot-induced phase stabilization of a-CsPbI3 perovskite for high-efficiency photovoltaics, appears in the journal Science. The authors are Abhishek Swarnkar, Ashley Marshall, Erin Sanehira, Boris Chernomordik, David Moore, Jeffrey Christians, and Joseph Luther from NREL. Tamoghna Chakrabarti from the Colorado School of Mines also is a co-author.
In addition to developing quantum dot perovskite solar cells, the researchers discovered a method to stabilize a crystal structure in an all-inorganic perovskite material at room temperature that was previously only favorable at high temperatures. The crystal phase of the inorganic material is more stable in quantum dots.
Most research into perovskites has centered on a hybrid organic-inorganic structure. Since research into perovskites for photovoltaics began in 2009, their efficiency of converting sunlight into electricity has climbed steadily and now shows greater than 22 percent power conversion efficiency. However, the organic component hasn’t been durable enough for the long-term use of perovskites as a solar cell.
NREL scientists turned to quantum dots-which are essentially nanocrystals-of cesium lead iodide (CsPbI3) to remove the unstable organic component and open the door to high-efficiency quantum dot optoelectronics that can be used in LED lights and photovoltaics.
The nanocrystals of CsPbI3 were synthesized through the addition of a Cs-oleate solution to a flask containing PbI2 precursor. The NREL researchers purified the nanocrystals using methyl acetate as an anti-solvent that removed excess unreacted precursors. This step turned out to be critical to increasing their stability.
Contrary to the bulk version of CsPbI3, the nanocrystals were found to be stable not only at temperatures exceeding 600 degrees Fahrenheit but also at room temperatures and at hundreds of degrees below zero. The bulk version of this material is unstable at room temperature, where photovoltaics normally operate and convert very quickly to an undesired crystal structure.
NREL scientists were able to transform the nanocrystals into a thin film by repeatedly dipping them into a methyl acetate solution, yielding a thickness between 100 and 400 nanometers. Used in a solar cell, the CsPbI3 nanocrystal film proved efficient at converting 10.77 percent of sunlight into electricity at an extraordinary high open circuit voltage. The efficiency is similar to record quantum dot solar cells of other materials and surpasses other reported all-inorganic perovskite solar cells.
Explore further: Rubidium pushes perovskite solar cells to 21.6 percent efficiency
More information: A. Swarnkar et al. Quantum dot-induced phase stabilization of -CsPbI3 perovskite for high-efficiency photovoltaics, Science (2016). DOI: 10.1126/science.aag2700
17 August 2016
The US Department of Energy will invest $16 million over the next four years to accelerate the design of new materials through use of supercomputers.
Two four-year projects—one team led by DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), the other teamled by DOE’s Oak Ridge National Laboratory (ORNL)—will leverage the labs’ expertise in materials and take advantage of lab supercomputers to develop software for designing fundamentally new functional materials destined to revolutionize applications in alternative and renewable energy, electronics, and a wide range of other fields. The research teams include experts from universities and other national labs.
The new grants—part of DOE’s Computational Materials Sciences (CMS) program launched in 2015 as part of the US Materials Genome Initiative—reflect the enormous recent growth in computing power and the increasing capability of high-performance computers to model and simulate the behavior of matter at the atomic and molecular scales.
The teams are expected to develop sophisticated and user-friendly open-source software that captures the essential physics of relevant systems and can be used by the broader research community and by industry to accelerate the design of new functional materials.
The Berkeley Lab team will be led by Steven G. Louie, an internationally recognized expert in materials science and condensed matter physics. A longtime user of NERSC supercomputers, Louie has a dual appointment as Senior Faculty Scientist in the Materials Sciences Division at Berkeley Lab and Professor of Physics at the University of California, Berkeley. Other team members are Jack Deslippe, Jeffrey B. Neaton, Eran Rabani, Feng Wang, Lin-Wang Wang and Chao Yang, Lawrence Berkeley National Laboratory; and partners Daniel Neuhauser, University of California at Los Angeles, and James R. Chelikowsky, University of Texas, Austin.
This investment in the study of excited-state phenomena in energy materials will, in addition to pushing the frontiers of science, have wide-ranging applications in areas such as electronics, photovoltaics, light-emitting diodes, information storage and energy storage. We expect this work to spur major advances in how we produce cleaner energy, how we store it for use, and to improve the efficiency of devices that use energy.—Steven Louie
ORNL researchers will partner with scientists from national labs and universities to develop software to accurately predict the properties of quantum materials with novel magnetism, optical properties and exotic quantum phases that make them well-suited to energy applications, said Paul Kent of ORNL, director of the Center for Predictive Simulation of Functional Materials, which includes partners from Argonne, Lawrence Livermore, Oak Ridge and Sandia National Laboratories and North Carolina State University and the University of California–Berkeley.
Our simulations will rely on current petascale and future exascale capabilities at DOE supercomputing centers. To validate the predictions about material behavior, we’ll conduct experiments and use the facilities of the Advanced Photon Source, Spallation Neutron Source and the Nanoscale Science Research Centers.—Paul Kent
At Argonne, our expertise in combining state-of-the-art, oxide molecular beam epitaxy growth of new materials with characterization at the Advanced Photon Source and the Center for Nanoscale Materials will enable us to offer new and precise insight into the complex properties important to materials design. We are excited to bring our particular capabilities in materials, as well as expertise in software, to the center so that the labs can comprehensively tackle this challenge.—Olle Heinonen, Argonne materials scientist
Researchers are expected to make use of the 30-petaflop/s Cori supercomputer now being installed at the National Energy Research Scientific Computing center (NERSC) at Berkeley Lab, the 27-petaflop/s Titan computer at the Oak Ridge Leadership Computing Facility (OLCF) and the 10-petaflop/s Mira computer at Argonne Leadership Computing Facility (ALCF). OLCF, ALCF, and NERSC are all DOE Office of Science User Facilities. One petaflop/s is 1015 or a million times a billion floating-point operations per second.
In addition, a new generation of machines is scheduled for deployment between 2016 and 2019 that will take peak performance as high as 200 petaflops. Ultimately the software produced by these projects is expected to evolve to run on exascale machines, capable of 1000 petaflops and projected for deployment in the mid-2020s.
Research will combine theory and software development with experimental validation, drawing on the resources of multiple DOE Office of Science User Facilities, including the Molecular Foundry and Advanced Light Source at Berkeley Lab, the Center for Nanoscale Materials and the Advanced Photon Source at Argonne National Laboratory (ANL), and the Center for Nanophase Materials Sciences and the Spallation Neutron Source at ORNL, as well as the other Nanoscience Research Centers across the DOE national laboratory complex.
The new research projects will begin in Fiscal Year 2016. Subsequent annual funding will be contingent on available appropriations and project performance.
The two new projects expand the ongoing CMS research effort, which began in FY 2015 with three initial projects, led respectively by ANL, Brookhaven National Laboratory and the University of Southern California.
Combining quantum dots and organic molecules can enable solar cells to capture more of the sun’s light.
Light from the sun is our most abundant source of renewable energy, and learning how best to harvest this radiation is key for the world’s future power needs. Researchers at KAUST have discovered that the efficiency of solar cells can be boosted by combining inorganic semiconductor nanocrystals with organic molecules.
Quantum dots are nano-crystals that only measure roughly 10 nanometers across. An electron trapped by the dot has quite different properties from those of an electron free to move through a larger material.
“One of the greatest advantages of quantum dots for solar cell technologies is their optical properties’ tunability,” explained KAUST Assistant Professor of Chemical Science Omar Mohammed. “They can be controlled by varying the size of the quantum dot.”
Mohammed and his colleagues are developing lead sulfide quantum dots for optical energy harvesting; these tend to be larger than dots made from other materials. Accordingly, lead sulfide quantum dots can absorb light over a wider range of frequencies. This means they can absorb a greater proportion of the light from the sun when compared to other smaller dots.
To make a fully functioning solar cell, electrons must be able to move away from the quantum dot absorption region and flow toward an electrode. Ironically, the property of large lead sulfide quantum dots that makes them useful for broadband absorption—a smaller electron energy bandgap—also hinders this energy harvesting process. Previously, efficient electron transfer had only been achieved for lead sulfide quantum dots smaller than 4.3 nanometers across, which caused a cut-off in the frequency of light converted.
The innovation by Mohammed and the team was to mix lead sulfide quantum dots of various sizes with molecules from a family known as porphyrins. The researchers showed that by changing the porphyrin used, it is possible to control the charge transfer from large lead sulfide dots; while one molecule switched off charge transfer altogether, another one enabled transfer at a rate faster than 120 femtoseconds.
The team believe this improvement in energy harvesting ability is due to the interfacial electrostatic interactions between the negatively charged quantum dot surface and the positively charged porphyrin.
“With this approach, we can now extend the quantum dot size for efficient charge transfer to include most of the near-infrared spectral region, reaching beyond the previously reported cut-off,” stated Mohammed. “We hope next to implement this idea in solar-cells with different architectures to optimize efficiency.”
Explore further: Quantum dots with built-in charge boost solar cell efficiency by 50%
More information: Ala’a O. El-Ballouli et al. Overcoming the Cut-Off Charge Transfer Bandgaps at the PbS Quantum Dot Interface, Advanced Functional Materials (2015). DOI: 10.1002/adfm.201504035
University of Vermont: Building the Electron Superhighway: Scientists Invent New Approach in quest for Organic Solar Panels and Flexible Electronics
genesisnanotech.wordpress.com – But the basic science of how to get electrons to move quickly and easily in these organic materials remains murky. To help, Furis and a team of UVM materials scientists have invented a new way to c…
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The technology, which is described online in the American Chemical Society journal Nano Letters, relies on a configuration of light-activated gold nanoparticles that harvest sunlight and transfer solar energy to highly excited electrons, which scientists sometimes refer to as “hot electrons.”
“Hot electrons have the potential to drive very useful chemical reactions, but they decay very rapidly, and people have struggled to harness their energy,” said lead researcher Isabell Thomann, assistant professor of electrical and computer engineering and of chemistry and materials science and nanoengineering at Rice. “For example, most of the energy losses in today’s best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat.”
Capturing these high-energy electrons before they cool could allow solar-energy providers to significantly increase their solar-to-electric power-conversion efficiencies and meet a national goal of reducing the cost of solar electricity.
In the light-activated nanoparticles studied by Thomann and colleagues at Rice’s Laboratory for Nanophotonics (LANP), light is captured and converted into plasmons, waves of electrons that flow like a fluid across the metal surface of the nanoparticles. Plasmons are high-energy states that are short-lived, but researchers at Rice and elsewhere have found ways to capture plasmonic energy and convert it into useful heat or light. Plasmonic nanoparticles also offer one of the most promising means of harnessing the power of hot electrons, and LANP researchers have made progress toward that goal in several recent studies.
Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules. Credit: I. Thomann/Rice University
Thomann and her team, graduate students Hossein Robatjazi, Shah Mohammad Bahauddin and Chloe Doiron, created a system that uses the energy from hot electrons to split molecules of water into oxygen and hydrogen. That’s important because oxygen and hydrogen are the feedstocks for fuel cells, electrochemical devices that produce electricity cleanly and efficiently.
To use the hot electrons, Thomann’s team first had to find a way to separate them from their corresponding “electron holes,” the low-energy states that the hot electrons vacated when they received their plasmonic jolt of energy. One reason hot electrons are so short-lived is that they have a strong tendency to release their newfound energy and revert to their low-energy state. The only way to avoid this is to engineer a system where the hot electrons and electron holes are rapidly separated from one another. The standard way for electrical engineers to do this is to drive the hot electrons over an energy barrier that acts like a one-way valve. Thomann said this approach has inherent inefficiencies, but it is attractive to engineers because it uses well-understood technology called Schottky barriers, a tried-and-true component of electrical engineering.
Quantum dots made of pure selenium can be made by simply firing a laser beam at selenium powder mixed into a glass of water. The easy and inexpensive process was developed by researchers at the University of Texas at San Antonio and Northeastern University in the US, and unlike other techniques, does not involve potentially toxic chemicals. The high-quality nanostructures could be used in two very different applications: as antibacterial agents and as light harvesters in solar cells.
Quantum dots are tiny pieces of semiconductor – such as selenium – that are typically tens of nanometres across. The size of a quantum dot dictates how their charge-carrying electrons and holes interact with light. As a result, they are of great interest to researchers trying to develop photonic technologies and especially solar cells. However, growing quantum dots that are pure and all the same size can be a challenge.
Green and easy
The researchers, led by Gregory Guisbiers in San Antonio, created their pure selenium quantum dots using a technique called pulsed laser ablation in liquids (PLAL), which involves simply firing a pulsed laser beam at a target – in this case selenium powder in water. “Our method is ‘green’ because it does not involve any dangerous solvents, only water, and there are no toxic adducts or by-products, like those often encountered in many wet chemistry processes,” explains Guisbiers. “It is also cheap and easy because we do not need a vacuum chamber or clean room – everything is done in a beaker of water.” The pure nanoparticles produced are also easy to collect and store because they are directly synthesized in solution, he adds.
This is the first time that selenium quantum dots have been synthesized using PLAL at ultraviolet and visible wavelengths, he says. These wavelengths are particularly interesting because they are better at reducing the size of particles compared with light at near-infrared wavelengths. Guisbiers and colleagues also showed that the crystallinity of the nanoparticles created by this technique depends on their size – that is, the smallest particles are crystalline while the largest ones are amorphous.
Antibacterial and anti-cancer
Selenium nanoparticles have antibacterial and anti-cancer properties, and could be used in medicine because the material is biocompatible and already exists in our bodies. However, nanoparticles need to be free of surface contaminants if they are to be employed in a biomedical setting – something that has proved difficult to achieve in the past.
The team, which has already tested its nanoparticles on E. coli, is now looking to see if they are efficient at killing other types of bacteria. “We are particularly interested in other bacteria involved in nosocomial diseases, like the methicillin-resistant Staphylococcus aureus,” Guisbiers says. “I’m told that [hospital-acquired infections] cause roughly 100,000 deaths every year in the US alone because bacteria are becoming more and more resistant to existing antibiotics. What’s more, these so-called super-germs are spreading worldwide, making this a major international health concern.”
The researchers will report their work in an upcoming issue of Laser Physics Letters. The team is also planning to incorporate the pure selenium quantum dots that they made into third-generation solar cells. “Indeed, since the element itself is a p-type semiconductor, when combined with an n-type semiconductor, we can build p–n junctions (the building blocks of all modern-day electronics) at the nanoscale,” adds Guisbiers.
No solar cell material ever has enjoyed such a rapid trajectory of improvements — nor the subsequent attention from researchers, industry, and media — as perovskite. This material, known for decades but whose ability to convert sunlight wasn’t appreciated until the past few years, has suddenly gained popularity with a velocity proportional to the flood of performance improvements coming out months and even weeks apart: from barely 3 percent conversion efficiency in 2009 to 10 percent in 2012, 16 percent in 2013, and as high as 19 percent according to recent conference reports. (A combination of c-Si cells and perovskite is thought to be able to achieve 32 percent efficiency.) That’s tantalizingly alongside the performance of mainstream conventional silicon-based PV, but with the potential for far simpler and cheaper processes and manufacturing.
Here’s why perovskite is attracting so much attention:
• It can better absorb sunlight, meaning thinner films can be used, which means less materials (bringing costs down)
• Perovskite requires only familiar wet chemistry techniques and simple benchtop processes — manufacturing is vastly simpler than other solar technologies, from c-Si to liquid DSC
• It’s not liquid, which also makes for much simpler manufacturing especially at larger scale, and also means better lifetime stability
• It offers significantly higher voltages because less energy is lost from activation and regeneration
NanoMarkets sees very important ramifications from perovskite’s unprecedented trajectory for one specific market segment: dye-sensitized solar cells (DSC). Several key DSC firms already have been at the forefront of the perovskite solar revolution:
• The École Polytechnique Fédéral de Lausanne (EPFL) in Switzerland, a pioneer of modern-day DSC technology, touched 15 percent efficiency a year ago, and more recently behind closed doors has shown north of 17 percent efficiency, sources tell NanoMarkets. EPFL’s DSC uses an inorganic-organic composite material with a perovskite, such as a dye and a hole-transporting material consisting of organic materials in place of electrolyte. (Specifically, the structure is a solid-state DSC of glass-FTO-TiO2-CH3NH3PbI3-HTM-Au.) Performance fluctuations have been resolved by tightening the perovskite material’s particle diameters, through a two-step deposition (Pbl2 accumulated on TiO2, then immersed in a CH3NH3I solution),resulting in a DSC with relatively high power conversion efficiency and high reproducibility.
• Another DSC-perovskite pioneer is Oxford Photovoltaics in the U.K., which is developing DSC modules with ambitious cost targets of $0.32/W for its technology which emphasizes transparency and color ideal for BIPV applications on building glass facades and rooftops. The company, a spinoff from the U. of Oxford (with 13 exclusive patents), believes it can achieve 20 percent conversion efficiencies with perovskite DSC panels. In fact, the company tells NanoMarkets that it’s entirely shifting its strategy and future business away from DSC to focus on perovskite thin-film solar cells, targeting the same BIPV markets with broader reach while also exploring the aforementioned hybrid silicon-perovskite cells.
• Dyesol, a longtime leader in DSC and organic PV, is transitioning from expensive liquid-based materials to relatively cheaper solid-state materials, including perovskite sensitizers and the Spiro solid-state electrolyte — a significant move considering the higher efficiency, lower cost, and better scalability prospects for solid-state DSCs. Dyesol also has a long association with EPFL.
The flip side of perovskite’s promise is its disruption to suppliers. A major reason why it’s received so much research attention is because as mentioned above its simplicity is utterly the opposite of a barrier to entry. According to some back-of-the-napkin math from one DSC materials provider, a 1 MW system with 100,000 sq. m of surface area would require up to 25 lbs of film layers but just 1.5 kg of perovskite material at a market value of $100.
All this excitement about perovskite material is, of course, happening at small-scale R&D. NanoMarkets understands much more work is needed before commercial viability can happen, most importantly in understanding the mechanics behind their degradation, making them stable and reliable over years of lifetime, improving their sensitivity to humidity, improving and simplifying how to properly package them, and all the while wringing costs out. A newer area of focus is replacing the tiny amounts of lead in current perovskite with other substances such as tin; the Pb amounts are tiny but could be subject to penalties of toxicity, not to mention negative marketplace perceptions. We reiterate that all of this hoopla about perovskite is still several years away from translating into commercial-ready large-size (e.g. 1 square meter) modules with 10 percent conversion efficiencies.
NanoMarkets believes, though, that perovskite’s rapid trajectory of efficiency improvement, with potential pairing of DSC’s features, could very well attract heightened interest from investors, strategically from within the industry and/or from capital markets, which could accelerate innovation and product development efforts and shorten the combined technologies’ runway toward commercialization.