University of Toronto Engineering researchers have discovered a dose threshold that greatly increases the delivery of cancer-fighting drugs into a tumor.
Determining this threshold provides a potentially universal method for gauging nanoparticle dosage and could help advance a new generation of cancer therapy, imaging and diagnostics.
“It’s a very simple solution, adjusting the dosage, but the results are very powerful,” says MD/Ph.D. candidate Ben Ouyang, who led the research under the supervision of Professor Warren Chan.
Their findings were published today in Nature Materials, providing solutions to a drug-delivery problem previously raised by Chan and researchers four years ago in Nature Reviews Materials.
Nanotechnology carriers are used to deliver drugs to cancer sites, which in turn can help a patient’s response to treatment and reduce adverse side effects, such as hair loss and vomiting. However, in practice, few injected particles reach the tumor site.
In the Nature Reviews Materials paper, the team surveyed literature from the past decade and found that on median, only 0.7 percent of the chemotherapeutic nanoparticles make it into a targeted tumor.
“The promise of emerging therapeutics is dependent upon our ability to deliver them to the target site,” explains Chan. “We have discovered a new principle of enhancing the delivery process. This could be important for nanotechnology, genome editors, immunotherapy, and other technologies.”
Chan’s team saw the liver, which filters the blood, as the biggest barrier to nanoparticle drug delivery. They hypothesized that the liver would have an uptake rate threshold—in other words, once the organ becomes saturated with nanoparticles, it wouldn’t be able to keep up with higher doses. Their solution was to manipulate the dose to overwhelm the organ’s filtering Kupffer cells, which line the liver channels.
The researchers discovered that injecting a baseline of 1 trillion nanoparticles in mice, in vivo, was enough to overwhelm the cells so that they couldn’t take up particles quick enough to keep up with the increased doses. The result is a 12 percent delivery efficiency to the tumor.
“There’s still lots of work to do to increase the 12 percent but it’s a big step from 0.7,” says Ouyang. The researchers also extensively tested whether overwhelming Kupffer cells led to any risk of toxicity in the liver, heart or blood.
“We tested gold, silica, and liposomes,” says Ouyang. “In all of our studies, no matter how high we pushed the dosage, we never saw any signs of toxicity.”
The team used this threshold principle to improve the effectiveness of a clinically used and chemotherapy-loaded nanoparticle called Caelyx. Their strategy shrank tumors 60 percent more when compared to Caelyx on its own at a set dose of the chemotherapy drug, doxorubicin.
Because the researchers’ solution is a simple one, they hope to see the threshold having positive implications in even current nanoparticle-dosing conventions for human clinical trials. They calculate that the human threshold would be about 1.5 quadrillion nanoparticles.
“There’s a simplicity to this method and reveals that we don’t have to redesign the nanoparticles to improve delivery,” says Chan. “This could overcome a major delivery problem.”
Quantum dots have revolutionized the field of optoelectronics due to their atom-like electronic structure. However, the prospect of colloidal quantum-dot lasers has long been deemed impractical due to the high energies required to induce optical gain. But recent work published in Nature and led by Ted Sargent of the University of Toronto shows that the lasing threshold in cadmium selenide (CdSe)/cadmium sulphide (CdS) core-shell colloidal quantum dots can be lowered by squashing the CdSe core via a clever ligand exchange process.
To instigate optical gain in a semiconductor laser, the difference between the lowest electron level and the highest hole level must be wider than the band gap so that the light emitted when they recombine can stimulate emission in neighbouring nanocrystals. Colloidal quantum dots (CQDs) then, should make ideal candidates for lasing applications, as their atom-like electronic structure means that the electron and hole energy levels are easier to separate.
In practice, however, the energies required to trigger optical gain in CQDs are so high that they can heat up to the point of burning. While electrons tend to occupy one energy state upon excitation, the hole that they leave behind in the valence band can populate one of eight closely spaced states. This degeneracy pushes the hole Fermi level into the band gap and increases the amount of energy required to instigate optical gain.
To overcome this issue, the researchers took advantage of the fact that CdS imposes a strain on CdSe due to a slight lattice mismatch of 3.9%. By growing an asymmetric CdS shell around a “squashed” oblate CdSe core, they were able to induce a biaxial strain that affected the heavy and light holes of the valence band to different extents, thus lifting the degeneracy.
To produce these asymmetric CQDs the group invented a technique called facet-selective epitaxy, making use of ligands that interact differently with the surfaces of CdSe. One of these ligands, trioctylphosphine sulphide, or TOPS, binds weakly to the (0001) facet of CdSe and not at all to the (0001), while octanethiol interacts similarly with all CdSe surfaces. Therefore, by growing CdS on the (0001) facet with TOPS and then replacing with octanethiol to stimulate epitaxial growth, oblate-shaped CQDs could be made throughout the entire particle ensemble with remarkable uniformity.
The resulting lasers had an unprecedentedly high performance, exhibiting a low lasing threshold of 6.4–8.4 kW cm–2, a seven-fold reduction compared with previous attempts. They also emitted light over a narrow energy range of just 36 meV. Both of these properties can be attributed to the enhanced splitting of the valence band levels that arises due to the oblate CQD shape.
The international team of researchers has certainly proved that continuous-wave CQD lasers are possible, yet there are still some obstacles to be overcome before they are seen on the market. Most importantly, the next step will be exciting the CQDs via electrical rather than optical means, as in standard commercial lasers. Nevertheless, facet-selective epitaxy opens up a whole host of other CQD materials for lasing applications and beyond.
A University of Toronto Engineering innovation could make printing solar cells as easy and inexpensive as printing a newspaper.
Dr. Hairen Tan and his team have cleared a critical manufacturing hurdle in the development of a relatively new class of solar devices called perovskite solar cells. This alternative solar technology could lead to low-cost, printable solar panels capable of turning nearly any surface into a power generator.
“Economies of scale have greatly reduced the cost of silicon manufacturing,” said Professor Ted Sargent, an expert in emerging solar technologies and the Canada Research Chair in Nanotechnology. “Perovskite solar cells can enable us to use techniques already established in the printing industry to produce solar cells at very low cost. Potentially, perovskites and silicon cells can be married to improve efficiency further, but only with advances in low-temperature processes.”
The new perovskite solar cells have achieved an efficiency of 20.1 per cent and can be manufactured at low temperatures, which reduces the cost and expands the number of possible applications. (Image: Kevin Soobrian)
Today, virtually all commercial solar cells are made from thin slices of crystalline silicon which must be processed to a very high purity. It’s an energy-intensive process, requiring temperatures higher than 1,000 degrees Celsius and large amounts of hazardous solvents.
In contrast, perovskite solar cells depend on a layer of tiny crystals — each about 1,000 times smaller than the width of a human hair — made of low-cost, light-sensitive materials. Because the perovskite raw materials can be mixed into a liquid to form a kind of ‘solar ink’, they could be printed onto glass, plastic or other materials using a simple inkjet printing process.
But, until now, there’s been a catch: in order to generate electricity, electrons excited by solar energy must be extracted from the crystals so they can flow through a circuit. That extraction happens in a special layer called the electron selective layer, or ESL. The difficulty of manufacturing a good ESL has been one of the key challenges holding back the development of perovskite solar cell devices.
“The most effective materials for making ESLs start as a powder and have to be baked at high temperatures, above 500 degrees Celsius,” said Tan. “You can’t put that on top of a sheet of flexible plastic or on a fully fabricated silicon cell — it will just melt.”
Tan and his colleagues developed a new chemical reaction than enables them to grow an ESL made of nanoparticles in solution, directly on top of the electrode. While heat is still required, the process always stays below 150 degrees C, much lower than the melting point of many plastics.
The new nanoparticles are coated with a layer of chlorine atoms, which helps them bind to the perovskite layer on top — this strong binding allows for efficient extraction of electrons. In a paper recently published in Science (“Efficient and stable solution-processed planar perovskite solar cells via contact passivation”), Tan and his colleagues report the efficiency of solar cells made using the new method at 20.1 per cent.
“This is the best ever reported for low-temperature processing techniques,” said Tan. He adds that perovskite solar cells using the older, high-temperature method are only marginally better at 22.1 per cent, and even the best silicon solar cells can only reach 26.3 per cent.
Another advantage is stability. Many perovskite solar cells experience a severe drop in performance after only a few hours, but Tan’s cells retained more than 90 per cent of their efficiency even after 500 hours of use. “I think our new technique paves the way toward solving this problem,” said Tan, who undertook this work as part of a Rubicon Fellowship.
“The Toronto team’s computational studies beautifully explain the role of the newly developed electron-selective layer. The work illustrates the rapidly-advancing contribution that computational materials science is making towards rational, next-generation energy devices,” said Professor Alan Aspuru-Guzik, an expert on computational materials science in the Department of Chemistry and Chemical Biology at Harvard University, who was not involved in the work.
“To augment the best silicon solar cells, next-generation thin-film technologies need to be process-compatible with a finished cell. This entails modest processing temperatures such as those in the Toronto group’s advance reported in Science,” said Professor Luping Yu of the University of Chicago’s Department of Chemistry. Yu is an expert on solution-processed solar cells and was not involved in the work.
Keeping cool during the manufacturing process opens up a world of possibilities for applications of perovskite solar cells, from smartphone covers that provide charging capabilities to solar-active tinted windows that offset building energy use. In the nearer term, Tan’s technology could be used in tandem with conventional solar cells.
“With our low-temperature process, we could coat our perovskite cells directly on top of silicon without damaging the underlying material,” said Tan. “If a hybrid perovskite-silicon cell can push the efficiency up to 30 per cent or higher, it makes solar power a much better economic proposition.”
Gains are derived from nanowires and light trapping for better energy conversion.
Sajeev John is a University Professor at the University of Toronto and Government of Canada Research Chair holder. He received his bachelor’s degree in physics in 1979 from the Massachusetts Institute of Technology and his PhD in physics at Harvard in 1984. His PhD work introduced the theory of classical wave localization and in particular the localization of light in three-dimensional strongly scattering dielectrics.
His groundbreaking work in the field of light localization that enables light to be controlled at the microscopic level has earned him an international reputation. He is a pioneering theoretician in photonic band gap (PBG) materials. This new class of optical materials presents exciting possibilities in the fields of physics, chemistry, engineering and medicine. PBG materials could eventually be used for optical communications/information processing, clinical medicine, lighting and solar energy harvesting.
John has received numerous awards, including the King Faisal International Prize in Science (2001), the IEEE Nanotechnology Pioneer Award (2008), and the Killam Prize in Natural Sciences (2014) from the Canada Council for the Arts. He is a Fellow of the American Physical Society, OSA, the Royal Society of Canada, and a member of the Max-Planck Society of Germany.
Professor Warren Chan (IBBME, ChemE, MSE) has spent the last decade figuring out how to deliver chemotherapy drugs into tumours — and nowhere else. Now his lab has designed a set of nanoparticles attached to strands of DNA that can change …more
Chemotherapy isn’t supposed to make your hair fall out—it’s supposed to kill cancer cells. A new molecular delivery system created at U of T could help ensure that chemotherapy drugs get to their target while minimizing collateral damage.
Many cancer drugs target fast-growing cells. Injected into a patient, they swirl around in the bloodstream acting on fast-growing cells wherever they find them. That includes tumours, but unfortunately also hair follicles, the lining of your digestive system, and your skin.
Professor Warren Chan (IBBME, ChemE, MSE) has spent the last decade figuring out how to deliver chemotherapy drugs into tumours—and nowhere else. Now his lab has designed a set of nanoparticles attached to strands of DNA that can change shape to gain access to diseased tissue.
“Your body is basically a series of compartments,” says Chan. “Think of it as a giant house with rooms inside. We’re trying to figure out how to get something that’s outside, into one specific room. One has to develop a map and a system that can move through the house where each path to the final room may have different restrictions such as height and width.”
One thing we know about cancer: no two tumours are identical. Early-stage breast cancer, for example, may react differently to a given treatment than pancreatic cancer, or even breast cancer at a more advanced stage. Which particles can get inside which tumours depends on multiple factors such as the particle’s size, shape and surface chemistry.
Chan and his research group have studied how these factors dictate the delivery of small molecules and nanotechnologies to tumours, and have now designed a targeted molecular delivery system that uses modular nanoparticles whose shape, size and chemistry can be altered by the presence of specific DNA sequences.
“We’re making shape-changing nanoparticles,” says Chan. “They’re a series of building blocks, kind of like a LEGO set.” The component pieces can be built into many shapes, with binding sites exposed or hidden. They are designed to respond to biological molecules by changing shape, like a key fitting into a lock.
These shape-shifters are made of minuscule chunks of metal with strands of DNA attached to them. Chan envisions that the nanoparticles will float around harmlessly in the blood stream, until a DNA strand binds to a sequence of DNA known to be a marker for cancer. When this happens, the particle changes shape, then carries out its function: it can target the cancer cells, expose a drug molecule to the cancerous cell, tag the cancerous cells with a signal molecule, or whatever task Chan’s team has designed the nanoparticle to carry out.
Their work was published this week in two key studies in the Proceedings of the National Academy of Sciences and the leading journal Science.
“We were inspired by the ability of proteins to alter their conformation—they somehow figure out how to alleviate all these delivery issues inside the body,” says Chan. “Using this idea, we thought, ‘Can we engineer a nanoparticle to function like a protein, but one that can be programmed outside the body with medical capabilities?'”
Applying nanotechnology and materials science to medicine, and particularly to targeted drug delivery, is still a relatively new concept, but one Chan sees as full of promise. The real problem is how to deliver enough of the nanoparticles directly to the cancer to produce an effective treatment.
“Here’s how we look at these problems: it’s like you’re going to Vancouver from Toronto, but no one tells you how to get there, no one gives you a map, or a plane ticket, or a car—that’s where we are in this field,” he says. “The idea of targeting drugs to tumours is like figuring out how to go to Vancouver. It’s a simple concept, but to get there isn’t simple if not enough information is provided.”
“We’ve only scratched the surface of how nanotechnology ‘delivery’ works in the body, so now we’re continuing to explore different details of why and how tumours and other organs allow or block certain things from getting in,” adds Chan.
He and his group plan to apply the delivery system they’ve designed toward personalized nanomedicine—further tailoring their particles to deliver drugs to your precise type oftumour, and nowhere else.
More information: Edward A. Sykes et al. Tailoring nanoparticle designs to target cancer based on tumor pathophysiology, Proceedings of the National Academy of Sciences(2016). DOI: 10.1073/pnas.1521265113
Funding from KAUST is helping to bring innovative solar power technology to fruition with startup company QD Solar.
Enabling researchers to take their ideas from the earliest research stages right through to the development and commercialization of a final product is a foundation goal at King Abdullah University of Science and Technology (KAUST). One route is for KAUST to provide active support to startup companies that have strong connections with the university’s research activities.
Nicola Bettio, manager of the funding scheme, says seed funding and early-stage capital are made available to startup companies through the KAUST Innovation Fund. “We invest in companies launched by our faculty, researchers and students, alongside international startups that are willing to move their operations to KAUST and Saudi Arabia,” Bettio explains. “By encouraging the transition of science into innovation and new ventures we can help to establish a fertile ecosystem for early-stage technology-based companies in Saudi Arabia.”
Efficient and cost-effective solar power generation will bring significant economic benefits and drastically reduce the carbon footprint of Saudi Arabia and the rest of the world. For the past several years, KAUST has been supporting research into new technologies using ‘colloidal quantum dots’ – a potential alternative to the established design of solar cells.
With an ideal design for solar panels the paint made from quantum dots is highly flexible.
Colloidal quantum dots are tiny semiconductor particles with the capacity to harvest energy from both the visible spectrum and the previously-untapped near-infrared portion of the sun’s light. Edward Sargent, a senior researcher at the University of Toronto, has been working on the development and application of colloidal quantum dots for the past ten years, and helped found QD Solar, a Canadian startup company aiming to commercialize the new technology.
“When KAUST was founded back in 2009, there was a very strong emphasis on advancing solar technology that led to the creation of KAUST’s Solar and Photovoltaics Engineering Research Center (SPERC),” explains Sargent. “The university employed top professors in the field who have been a key asset during the development of the new quantum dot technique I have been working on. I was fortunate enough to be awarded a KAUST Investigators grant around that time, enabling me to pursue my research in collaboration with these top scientists.”
In 2009, Sargent’s team at the University of Toronto received a grant from KAUST to advance their research into colloidal quantum dots specifically for solar power applications. Since then, the team’s advances have evolved in leaps and bounds. “Our latest design incorporates the dots into a solution, or ‘paint’, which can be applied to flexible surfaces very easily and cheaply,” Sargent explains. “We have also vastly improved the power conversion efficiency of the dots over the past six years, thanks in part to the funding from KAUST.”
Sargent and his team, in collaboration with KAUST researchers, are now investigating ways of combining the dots with existing silicon-based solar cells to create new hybrid structure solar panels. This will allow them to harness a far greater fraction of the sun’s energy than ever before, improving the panels’ efficiency and capacity to generate more power.
Marc Vermeersch, the SPERC managing director, points out that current solar cell technologies are reaching their physical limits. He explains that a key advantage of creating QD Solar’s hybrid system is that existing solar panel producers will be able to modify their manufacturing practices relatively easily to incorporate the new design, rather than starting from scratch. Manufacturers will be able to achieve significant economic and efficiency improvements using QD Solar’s quantum dot-based solar products, Bettio adds.
KAUST anticipates welcoming a QD Solar presence to Saudi Arabia in the near future, as Bettio explains: “QD Solar is working with the KAUST Innovation Fund to raise capital to establish a significant development facility in our Research & Technology Park. Ultimately, this may lead to the establishment of a Saudi player in the photovoltaic industry and the creation of a hybrid solar panel manufacturing facility in the Kingdom.”
Researchers at the University of Toronto have unveiled a cheap, fast spray-on solar cell process that could enable the creation of solar arrays using the most modest of manufacturing methods. Even better, with the cells ‘printed’ onto flexible material, they could turn anything from airplane wings to your patio furniture into a solar power plant.
The research was led by University of Toronto researcher Illan Kramer, who calls the system sprayLD. Much as with many previous spray-on solar designs, spray LD utilizes colloidal quantum dots (CQD). CQD are tiny, light-sensitive dots—invisible to the naked eye—that can act as an absorbing photovoltaic material. The technology has long held promise, but the process of incorporating light-sensitive CQD’s onto surfaces has been expensive, slow and laborious.
What Kramer and his colleagues have done is to invent an entirely new, cheap, efficient technique for creating solar cells using CQD. They based the technique on a newspaper-printing process, and explain “SprayLD blasts a liquid containing CQDs directly onto flexible surfaces, such as film or plastic… by applying ink onto a roll of paper. This roll-to-roll coating method makes incorporating solar cells into existing manufacturing processes much simpler.”
Watch the Video:
As for their prototype, the team admits that it may look “more like junkyard wars than high-tech,” but that’s because it was made entirely from affordable, readily available parts; U of T explains “[Kramer] sourced a spray nozzle used in steel mills to cool steel with a fine mist of water, and a few regular air brushes from an art store.”
Which perhaps makes it all the more surprising that this technique does not appear to cause any major drop offs in efficiency. However, efficiency is still a major consideration with spray-on solar. The sprayLD system converts about 8.1% of sunlight to power, as opposed to 15-20% for standard rooftop solar arrays. But, speaking to Co.Exist, Kramer explained that while he is working on boosting sprayLD’s efficiency “we think of ourselves as operating in a slightly different paradigm, where we don’t have to be quite as efficient because we’re so much less expensive.”
And in the long run, Kramer explains: “My dream is that one day you’ll have two technicians with Ghostbusters backpacks come to your house and spray your roof!”
Every electrical device, from a simple lightbulb to the latest microchips, is enabled by the movement of electrical charge, or current. The nascent field of ‘spintronics’ taps into a different electronic attribute, an intrinsic quantum property known as spin, and may yield devices that operate on the basis of spin-transport.
Atom-optical lattice systems offer a clean, well-controlled way to study the manipulation and movement of spins because researchers can create particle configurations analogous to crystalline order in materials. JQI/CMTC* theorists Xiaopeng Li, Stefan Natu, and Sankar Das Sarma, in collaboration with Arun Paramekanti from the University of Toronto, have been developing a model for what happens when ultracold atomic spins are trapped in an optical lattice structure with a “double-valley” feature, where the repeating unit resembles the letter “W”. This new theory result, recently published in the multidisciplinary journal Nature Communications, opens up a novel path for generating what’s known as the spin Hall effect, an important example of spin-transport.
Behavior in double-valley lattices has previously been studied for a collection of atoms that all have the same value of spin. In this new work, theorists consider two-component atoms–here, the spin state each atom has can vary between “spin-up” and “spin-down” states.
The spontaneous chiral spin superfluid. When loaded into a hexagonal lattice, chiral spin currents are expected to develop. Here, the orange represents spin-up particles, and green spin-down. Arrows are used to guide the eye along the path of movement. Credit S. Kelley/JQI – See more at: http://jqi.umd.edu/news/restoring-order#sthash.pjtKa0oy.dpuf
Atomic spins in a lattice can be thought of as an array of tiny bar magnets. In the single-component case, the atom-magnets are all oriented the same direction in the lattice. In this case, the magnets can have a tendency to favor only one of the wells of the double-valley. In the two-component system studied here, each atom-magnet can have its’ north-pole pointed either up or down, with respect to a particular magnetic field.
Adding this kind of freedom to the model leads to some very curious behavior – the atoms spontaneously separate, with the spin-up atoms collecting into one well of the double-valley, and spin-down atoms in the other. Theorists have dubbed this new state a “spontaneous chiral spin superfluid” (for further explanation of what a superfluid is, see here).
This sort of spin-dependent organization is of great interest to researchers, who could employ it to study the spin Hall effect, analogous to the Hall effect for electrons. Normally, this effect is seen as a result of spin-orbit coupling, or the association of an atom’s spin with its motion. In fact, this has long been the approach for producing a spin Hall effect – apply a current to material with spin-orbit coupling, and the spins will gather at the edges according to their orientation.
Ian Spielman’s group at the JQI has pioneered laser-based methods for realizing both spin-orbit coupling and the spin Hall effect phenomenon in ultracold atomic gases. In contrast, for the superfluid studied here, spin-sorting is not the result of an applied field or asymmetric feature of the system, but rather emerges spontaneously. It turns out this behavior is driven by tiny, random quantum fluctuations, in a paradoxical phenomenon known as quantum order by disorder.
Quantum order by disorder
Generally speaking, the transition from disorder to order is a familiar one. Consider water condensing into ice: this is a disordered system, a liquid, transitioning to a more ordered one, a solid. This phase transformation happens because the molecules become limited in their degrees of freedom, or the ability to move in different ways. Conversely, adding noise to a system, such as heating an ice cube, generally leads to a more disordered state, a pool of water. Amazingly, noise or fluctuations in a system can sometimes drive a system into a more ordered state.
The theory team showed that this is indeed the case for certain kinds of atoms loaded into a double-valley optical lattice. While this system is a quiet, mostly non-thermal environment, noise still lurks in the form of quantum mechanical fluctuations. In this system, the spin-up and spin-down atoms can potentially be configured in four different, but energetically identical arrangements. This is known as degeneracy and can be indicative of the amount of order in a system–the more equal energy states, the more disordered a system. It turns out that these arrangements have different amounts of quantum noise and these fluctuations play a crucial role. Surprisingly, the quantum fluctuations will break up the degeneracy, thus restoring order.
What’s the upshot? In this system, the resulting lowest energy configuration–a chiral spin superfluid– is preferred independent of the type of double-valley lattice geometry, indicating a type of universal behavior. With this in mind, the theorists examine a number of lattice structures where this phenomenon might be realized. For instance, if the fluid is placed into a hexagonal lattice configuration, similar to the structure of graphene, they expect the characteristic spin currents of the spin Hall effect to emerge, as depicted in the graphic, above.
In the publication, the team points out that optical lattice systems are a flexible, pristine platform for examining the effect of these tiny variations in quantum fluctuations, which are often masked in real materials. Outside of exploring novel forms of matter like the one found here, research into spin and atom manipulation has applications in emerging electronic-like technologies, such as spintronics, valleytronics and atomtronics.
*JQI (Joint Quantum Institute) and CMTC (Condensed Matter Theory Center)
*** “Throw-Back Thursdays” – Maybe not so much! Today’s Post(s) are a compilation of some of 2014’s LATEST advances in making Solar Renewable Energy an Affordable, Efficient and Commercially Viable Energy Source.Quote, “Solar-cell technology has advanced rapidly, as hundreds of groups around the world pursue more than two dozen approaches using different materials, technologies, and approaches to improve efficiency and reduce costs.”
Universities and Organizations such as the University of California, University of Sheffield, University of Electro-Communications, Okinawa Institute of Science and Technology Graduate University, MIT, University of Toronto, NREL, NIST, NRL … ALL are seeking new discoveries, new methods of application-processes … to achieve the actualization and implementation of “Abundant, Cheap, Renewable Energy”. In this Compilation:
Singlet Fission – For Increased Efficiencies – 2014-07
Spray-On Solar Cells – 2014-08
Pervoskites – A New (LOW COST) Semiconductor Material for Solar Cells – 2014-09
‘Dark’ Spin-Triplet Excitons – 2014-10
Self-Organized Indium Arsenide Quantum Dots for Solar Cells – 2014-09
A New Breed of “Quantum Dot” Solar Cells – 2014-05
Solar Energy Boost: ‘Singlet Fission’ Could Increase Efficiency by as much as 30%
Jul 08, 2014
A perspective article published last month by University of California, Riverside chemists in the Journal of Physical Chemistry Letters was selected as an Editors Choice—an honor only a handful of research papers receive. The perspective reviews the chemists’ work on “singlet fission,” a process in which a single photon generates a pair of excited states. This 1->2 conversion process, as it is known, has the potential to boost solar cell efficiency by as much as 30 percent.
Applications of the research include more energy-efficient lighting and photodetectors with 200 percent efficiency that can be used for night vision. Biology may use singlet fission to deal with high-energy solar photons without generating excess heat, as a protective mechanism.
Currently, solar cells work by absorbing a photon, which generates an exciton, which subsequently separates into an electron-hole pair. It is these electrons that become solar electricity. The efficiency of these solar cells is limited to about 32 percent, however, by what is called the “Shockley-Queisser Limit.” Future solar cells, also known as “Third Generation” solar cells, will have to surpass this limit while remaining inexpensive, requiring the use of new physical processes. Singlet fission is an example of such a process.
“Our research got its launch about ten years ago when we started thinking about solar energy and what new types of photophysics this might require,” said Christopher Bardeen, a professor of chemistry, whose lab led the research. “Global warming concerns and energy security have made solar energy conversion an important subject from society’s point-of-view. More efficient solar cells would lead to wider use of this clean energy source.”
Research details
When a photon is absorbed, its energy takes the form of an exciton inside the material. Bardeen explained that excitons come in two “flavors,” defined by the electron spins in them. One flavor is singlet, where all spins are paired. The other flavor is triplet, where two electrons are unpaired. In organic semiconductors, these two types of excitons have different energies.
“If a triplet exciton has half the energy of a singlet, then it is possible for one singlet exciton, generated by one photon, to split into two triplet excitons,” Bardeen said. “Thus, you could have a 200 percent yield of excitons—and hopefully, electrons—per absorbed photon.”
He explained that the Shockley-Queisser Limit involves photon absorption to create an exciton, which is basically a bound electron (- charge) and hole (+ charge) pair. In order to get useful electron flow (photocurrent), these excitons must be dissociated. Ideally, one exciton produces one electron (hole) and thus current to run, say, a light bulb.
“To absorb a photon, the photon energy has to be greater than the bandgap of the semiconductor, so you already miss part of the solar spectrum,” Bardeen said. “But if you absorb a photon with energy higher than the bandgap, it has too much energy, and that excess energy is usually wasted as heat. The trick is to take that high energy exciton and split the energy into two excitons, rather than dissipating it as heat.”
Bardeen explained that the singlet exciton spontaneously splits into the two triplets, through a mechanism that is still under active investigation.
“The exact mechanism is unknown, but it does happen quickly—at the sub-nanosecond timescale—and with high efficiency,” he said. “Our work has shown that it is very sensitive to the alignment and position of the two molecules—at least two are required, since we have two excitons—involved in singlet fission. Recent work at MIT has already demonstrated an organic photovoltaic cell with more than 100 percent external quantum efficiency based on this effect. It may be possible to integrate this effect with inorganic semiconductors and use it to raise their efficiencies.”
Next, Bardeen’s lab will look for new materials that exhibit singlet fission, figure out how to take the triplet excitons and turn them into photocurrent efficiently, and look at how the spin properties of the electrons affect the exciton dynamics.
Explore further:Two for one in solar power
Scientists develop pioneering new spray-on solar cells
Aug 01, 2014
A team of scientists at the University of Sheffield are the first to fabricate perovskite solar cells using a spray-painting process – a discovery that could help cut the cost of solar electricity.
Published on Mar 4, 2013
A robot spray-coating glass with the polymer to create a solar cell. The technology could one day be used on glass in buildings and cars. For more information about our solar cell research visit http://www.sheffield.ac.uk/physics
Experts from the University’s Department of Physics and Astronomy and Department of Chemical and Biological Engineering have previously used the spray-painting method to produce solar cells using organic semiconductors – but using perovskite is a major step forward.
Efficient organometal halide perovskite based photovoltaics were first demonstrated in 2012. They are now a very promising new material for solar cells as they combine high efficiency with low materials costs.
The spray-painting process wastes very little of the perovskite material and can be scaled to high volume manufacturing – similar to applying paint to cars and graphic printing.
Lead researcher Professor David Lidzey said: “There is a lot of excitement around perovskite based photovoltaics.
“Remarkably, this class of material offers the potential to combine the high performance of mature solar cell technologies with the low embedded energy costs of production of organic photovoltaics.”
While most solar cells are manufactured using energy intensive materials like silicon, perovskites, by comparison, requires much less energy to make. By spray-painting the perovskite layer in air the team hope the overall energy used to make a solar cell can be reduced further.
Professor Lidzey said: “The best certified efficiencies from organic solar cells are around 10 per cent. “Perovskite cells now have efficiencies of up to 19 per cent. This is not so far behind that of silicon at 25 per cent – the material that dominates the world-wide solar market.”
He added: “The perovskite devices we have created still use similar structures to organic cells. What we have done is replace the key light absorbing layer – the organic layer – with a spray-painted perovskite. “Using a perovskite absorber instead of an organic absorber gives a significant boost in terms of efficiency.”
The Sheffield team found that by spray-painting the perovskite they could make prototype solar cells with efficiency of up to 11 per cent.
Professor Lidzey said: “This study advances existing work where the perovskite layer has been deposited from solution using laboratory scale techniques. It’s a significant step towards efficient, low-cost solar cell devices made using high volume roll-to-roll processing methods.”
Solar power is becoming an increasingly important component of the world-wide renewables energy market and continues to grow at a remarkable rate despite the difficult economic environment.
Professor Lidzey said: “I believe that new thin-film photovoltaic technologies are going to have an important role to play in driving the uptake of solar-energy, and that perovskite based cells are emerging as likely thin-film candidates. “
One of the most common complaints about solar power is solar panels are still too expensive to be worth the investment. Many researchers have responded by making solar cells, the tile-like components of solar panels that absorb and transfer energy, more efficient and longer lasting. But even the longest living solar cells that most effectively convert sunlight to energy will not become common if they are prohibitively expensive.
Therefore, Professor Yabing Qi, the head of the Energy Materials and Surface Sciences Unit at the Okinawa Institute of Science and Technology Graduate University, has a different idea: make solar cells using a type of semiconductor called perovskite materials, which are, in Qi’s words, “dirt cheap.” If solar cells are cheap enough, Qi reasons that people will want to use them for the immediate payback in energy savings.
Now Professor Qi and members of his research unit have developed a new method for making perovskite solar cells worthy of attention, and The Royal Society of Chemistry published their findings September 5, 2014 in their journal, Energy & Environmental Science.
Qi’s new method uses what he calls hybrid deposition to create perovskite solar cells, made from a mixture of inexpensive organic and inorganic raw materials. In addition, his solar cell is about a thousand times thinner than a silicon solar cell, and therefore uses far less material.
Qi estimates that for the same price, he could either buy raw materials to build 1000 square meters of his solar cell, or he could buy about 20 wafers of crystallized silicon, to build 0.16 square meters of traditional solar panels. “Silicon is not rare,” Qi explains, “but processing silicon requires expensive equipment and sophisticated steps demanding high temperature, vacuum, or high pressure, and that makes crystallized silicon very expensive.”
In contrast, the hybrid deposition process uses less energy to produce a solar cell at a far lower temperature. In fact, Qi envisions manufacturing the new solar cells using a low-cost printing process. The process would deposit the materials onto thin sheets of PET plastic very quickly to make large quantities of cheap solar cells. Qi does not yet know the limits of his hybrid cells, but optimists in his field hope that they could reach 20% efficiency. This means that that the solar cells will convert 20% of the energy they absorb from the sun into usable energy, which is comparable to the best silicon solar panels on the market.
The extremely thin perovskite cell that Qi and his lab designed measures merely 135 nanometers and reaches an efficiency of 9.9%. Because these films are semitransparent, Qi hopes to use them on windows, as a sort of lightweight set of blinds. “It will be a window and at the same time it will be a solar cell,” he says. “Some of the light could go through and the rest will be absorbed. Then, a certain percentage of the absorbed light will be converted to electricity.”
If solar cells are cheap enough, consumers will reap almost immediate benefits even if the solar cells are not the most efficient, because their savings on air conditioning and electricity will offset the expense. “If it’s so cheap that it is like wallpaper, then you might as well use it,” said Qi. “It’s like a free gift. It’s an investment with a lot of payback.”
Hybrid Materials could Smash the Solar Efficiency Ceiling
October 9, 2014
A new method for transferring energy from organic to inorganic semiconductors could boost the efficiency of widely used inorganic solar cells.
Researchers have developed a new method for harvesting the energy carried by particles known as ‘dark’ spin-triplet excitons with close to 100% efficiency, clearing the way for hybrid solar cells which could far surpass current efficiency limits.
The team, from the University of Cambridge, have successfully harvested the energy of triplet excitons, an excited electron state whose energy in harvested in solar cells, and transferred it from organic to inorganic semiconductors. To date, this type of energy transfer had only been shown for spin-singlet excitons. The results are published in the journal Nature Materials.
In the natural world, excitons are a key part of photosynthesis: light photons are absorbed by pigments and generate excitons, which then carry the associated energy throughout the plant. The same process is at work in a solar cell.
In conventional semiconductors such as silicon, when one photon is absorbed it leads to the formation of one free electron that can be extracted as current. However, in pentacene, a type of organic semiconductor, the absorption of a photon leads to the formation of two electrons. But these electrons are not free and they are difficult to pin down, as they are bound up within ‘dark’ triplet exciton states.
Excitons come in two ‘flavours’: spin-singlet and spin-triplet. Spin-singlet excitons are ‘bright’ and their energy is relatively straightforward to harvest in solar cells. Triplet-spin excitons, in contrast, are ‘dark’, and the way in which the electrons spin makes it difficult to harvest the energy they carry.
“The key to making a better solar cell is to be able to extract the electrons from these dark triplet excitons,” said Maxim Tabachnyk of the University’s Cavendish Laboratory, the paper’s lead author. “If we can combine materials like pentacene with conventional semiconductors like silicon, it would allow us to break through the fundamental ceiling on the efficiency of solar cells.”
Using state-of-art femtosecond laser spectroscopy techniques, the team discovered that triplet excitons could be transferred directly into inorganic semiconductors, with a transfer efficiency of more than 95%. Once transferred to the inorganic material, the electrons from the triplets can be easily extracted.
“Combining the advantages of organic semiconductors, which are low cost and easily processable, with highly efficient inorganic semiconductors, could enable us to further push the efficiency of inorganic solar cells, like those made of silicon,” said Dr Akshay Rao, who lead the team behind the work.
The team is now investigating how the discovered energy transfer of spin-triplet excitons can be extended to other organic/inorganic systems and are developing a cheap organic coating that could be used to boost the power conversion efficiency of silicon solar cells.
Self-Organized Indium Arsenide Quantum Dots for Solar Cells
Sep 25, 2014
Kouichi Yamaguchi is internationally recognized for his pioneering research on the fabrication and applications of ‘semiconducting quantum dots’ (QDs). “We exploit the ‘self-organization’ of semiconducting nanocrystals by the ‘Stranski-Krasnov (SK) mode of crystal growth for producing ordered, highly dense, and highly uniform quantum dots,” explains Yamaguchi. “Our ‘bottom-up’ approach yields much better results than the conventional photolithographic or ‘top-down’ methods widely used for the fabrication of nano-structures.”
Notably, electrons in quantum dot structures are confined inside nanometer sized three dimension boxes. Novel applications of ‘quantum dots‘—including lasers, biological markers, qubits for quantum computing, and photovoltaic devices—arise from the unique opto-electronic properties of the QDs when irradiated with light or under external electromagnetic fields.
“Our main interest in QDs is for the fabrication of high efficiency solar cells,” says Yamaguchi. “Step by step we have pushed the limits of ‘self-organization’ based growth of QDs and succeeded in producing highly ordered, ultra-high densities of QDs.”
The realization of an unprecedented QDs density of 5 x 1011 cm-2 in 2011 was one of the major milestones in the development of ‘self-organization‘ based semiconducting QDs for solar cells by Yamaguchi and his colleagues at the University of Electro-Communications (UEC). “This density was one of the critical advances for achieving high efficiency quantum dot based photo-voltaic devices,” says Yamaguchi.
Specifically, Yamaguchi and his group used molecular beam epitaxy (MBE) to grow a layer of InAs QDs with a density of 5 x 1011 cm-2 on GaAsSb/GaAs (100) substrates. Importantly, the breakthrough that yielded this high density of highly ordered QDs was the discovery that InAs growth at a relatively low substrate temperature of 470 degrees Celsius on Sb-irradiated GaAs layers suppressed coalescence or ‘ripening’ of InAs QDs that was observed at higher temperatures. Thus the combination of the Sb surfactant effect and lower growth temperature yielded InAs QDs with an average height of 2.02.5 nm.
InAs QD density: 1.0×1012 cm-2
The potential for photovoltaic device applications was examined by sandwiching a single layer of InAs QDs in a pin-GaAs cell structure. The resulting external quantum efficiency of these solar cell structures in the 900 to 1150 nm wavelength range was higher than devices with the QD layer.
“Theoretical studies suggest QDs solar cells could yield conversion efficiencies over 50%,” explains Yamaguchi. “This is a very challenging target but we hope that our innovative approach will be an effective means of producing such QD based high performance solar cells. We have recently achieved InAs QDs with a density of 1 x 1012 cm-2.”
Variation of power conversion efficiency with quantum dot density (calculated results). Enlarge
Quantum Dot Photovoltaics: A New Breed of Solar Cells: Setting New Records for Efficiency
May 28, 2014
Solar-cell technology has advanced rapidly, as hundreds of groups around the world pursue more than two dozen approaches using different materials, technologies, and approaches to improve efficiency and reduce costs.
Now a team at MIT has set a new record for the most efficient quantum-dot cells—a type of solar cell that is seen as especially promising because of its inherently low cost, versatility, and light weight.
While the overall efficiency of this cell is still low compared to other types—about 9 percent of the energy of sunlight is converted to electricity—the rate of improvement of this technology is one of the most rapid seen for a solar technology. The development is described in a paper, published in the journal Nature Materials, by MIT professors Moungi Bawendi and Vladimir Bulović and graduate students Chia-Hao Chuang and Patrick Brown.
The new process is an extension of work by Bawendi, the Lester Wolfe Professor of Chemistry, to produce quantum dots with precisely controllable characteristics—and as uniform thin coatings that can be applied to other materials. These minuscule particles are very effective at turning light into electricity, and vice versa. Since the first progress toward the use of quantum dots to make solar cells, Bawendi says, “The community, in the last few years, has started to understand better how these cells operate, and what the limitations are.”
The new work represents a significant leap in overcoming those limitations, increasing the current flow in the cells and thus boosting their overall efficiency in converting sunlight into electricity.
Many approaches to creating low-cost, large-area flexible and lightweight solar cells suffer from serious limitations—such as short operating lifetimes when exposed to air, or the need for high temperatures and vacuum chambers during production.
By contrast, the new process does not require an inert atmosphere or high temperatures to grow the active device layers, and the resulting cells show no degradation after more than five months of storage in air.
Bulović, the Fariborz Maseeh Professor of Emerging Technology and associate dean for innovation in MIT’s School of Engineering, explains that thin coatings of quantum dots “allow them to do what they do as individuals—to absorb light very well—but also work as a group, to transport charges.” This allows those charges to be collected at the edge of the film, where they can be harnessed to provide an electric current.
The new work brings together developments from several fields to push the technology to unprecedented efficiency for a quantum-dot based system: The paper’s four co-authors come from MIT’s departments of physics, chemistry, materials science and engineering, and electrical engineering and computer science. The solar cell produced by the team has now been added to the National Renewable Energy Laboratories’ listing of record-high efficiencies for each kind of solar-cell technology.
The overall efficiency of the cell is still lower than for most other types of solar cells. But Bulović points out, “Silicon had six decades to get where it is today, and even silicon hasn’t reached the theoretical limit yet. You can’t hope to have an entirely new technology beat an incumbent in just four years of development.” And the new technology has important advantages, notably a manufacturing process that is far less energy-intensive than other types.
Chuang adds, “Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented.”
The system is so new that it also has potential as a tool for basic research. “There’s a lot to learn about why it is so stable. There’s a lot more to be done, to use it as a testbed for physics, to see why the results are sometimes better than we expect,” Bulović says.
A companion paper, written by three members of the same team along with MIT’s Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering, and three others, appears this month in the journal ACS Nano, explaining in greater detail the science behind the strategy employed to reach this efficiency breakthrough.
The new work represents a turnaround for Bawendi, who had spent much of his career working with quantum dots. “I was somewhat of a skeptic four years ago,” he says. But his team’s research since then has clearly demonstrated quantum dots‘ potential in solar cells, he adds.
Arthur Nozik, a research professor in chemistry at the University of Colorado who was not involved in this research, says, “This result represents a significant advance for the applications of quantum-dot films and the technology of low-temperature, solution-processed, quantum-dot photovoltaic cells. … There is still a long way to go before quantum-dot solar cells are commercially viable, but this latest development is a nice step toward this ultimate goal.”
The global race to harvest the sun’s energy has researchers all over the world competing to develop the new technologies that they hope will end our reliance on fossil fuels.
One of the latest announcements is from a team at Michigan State University that’s developed a clear pane of plastic that could be used on windows to capture sunlight. ScienceDaily, an online publication of research news, reports that the luminescent solar concentrator, as it’s called, has the potential to revolutionize commercial or industrial window design by turning windows into large-scale collectors of solar energy.
But even while science writers enthusiastically describe the promise of this latest solar technology, researchers immersed in the field tend to greet the news more cautiously.
Professor Ted Sargent, a leader internationally in the development of “paintable” solar cells, which has spawned related research around the world, says “what’s clearly different and clearly interesting” is that the Michigan team’s work allows them to take a significant part of the sun’s rainbow spectrum, convert it into a single colour and “put it off to the edges” of the plastic pane.
Professor Sargent’s lab members, Zhijun Ning and Oleksandr Vozny, examining a new paint-on solar chip. (Ted Sargent/University of Toronto)
‘If this development can go all the way,” says Sargent, it offers the possibility of yet another venue for deploying solar cells. Instead of just putting solar panels on the roof of a house, says Sargent, these transparent cells could be placed on windows and windshields.
But Sargent says that in the search for the solar technology breakthrough that will transform the world, “this one’s not there yet.”
“They kind of get the light ready in an appealing way,” says Sargent. Problem is, the mostly transparent material is “nice for a window, but solar cells have to absorb light, so it’s not good for collecting light.”
The paint-on solar chip, about the size of a postage stamp. (Ted Sargent/University of Toronto )
Many new developments in the solar field are plagued by those kinds of trade-offs — between cost of production and efficiency. In the case of a luminescent solar concentrator for windows, Sargent says if it can be manufactured cheaply enough it has the potential for wide-spread application by turning windows and window walls around the world into solar harvestors, but at the cost of inefficiently capturing the full potential of the sun’s energy.
Sargent’s own work in nanotechnology led to an ink-based solar cell — essentially suspending the ingredients of solar cells in liquid that could be “painted’ onto any surface.
In a Tedx Toronto talk, Sargent describes the incredible potential of solar energy: “Every hour enough energy reaches the earth from the sun to power the earth for an entire year.”
It’s all about the technology,” says Sargent. “Cheap and efficient is the holy grail.”
Led by Sargent, a researchers at U of T are re-imagining the solar cell. Conventional cells use silicon, “a beautiful semi-conductor material”, limited by what Sargent calls “a quality of rigid perfection,” accompanied by equally rigid costs associated with the manufacturing process and installation.
Sargent’s team has developed a solar cell that he says is more akin to printing a newspaper – a semi-conductor ink printed onto a flexible backing. Unlike silicon-based cells, in this application solar cells are painted on a light-weight flexible carpet that can be spread out on a roof, a road or any other surface exposed to the sun.
“You can literally paint surfaces with this ink” – a cheap way to make a solar cell, says Sargent. “And it’s very black – the perfect absorber of visible light and of infrared light.”
Solar conference coming
Ted Sargent is hosting a conference in Toronto next October with world leaders in solar technologies. There will be a public lecture by Connaught Symposium guest of honour Professor Sir Richard Friend (Cambridge University) on Wednesday, October 8, 2014 at 8 p.m. at the Royal Conservatory of Music. It is open to all, with $10 tickets going on sale on September 3. Professor Sir Richard Friend is the world’s most renowned scholar in the field of energy harvesting.See more here.
“This group of researchers has come together around the idea that nature offers us ideas and inspirations to develop cost-effective solar technologies. It’s such an important topic for scientists, politicians and public and it impacts the future of our planet,” said Sargent.
“Sir Richard Friend is one of the leading researchers. He’s also a truly engaging speaker from Cambridge who can speak to the big picture about what we can learn from nature and potential for solar-energy harvesting.”
That capacity to absorb infrared light is especially powerful because the spectrum includes as much infrared as visible light. But, says Sargent, the infrared portion of the sun’s spectrum is often ignored And yet, when the sun beats down on our face, much of the heat we feel is infrared light.
Sargent’s group led the world in making most efficient solar cells in the world based on ink-based colloidal quantum dots. A finished solar cell — about the size of a postage stamp – has the ability to both absorb the sun’s energy and to extract it as electricity.
In an interview with Metro Morning‘s Matt Galloway, Sargent said that at this point, the technological breakthrough that will revolutionize the world’s energy production is “years, not decades away.
But it will require teams of researchers working in many different disciplines to get there, “from chemistry to physics to material science.”
In Sargent’s case, It’s led to a collaboration with Saudi Arabia’s KAUST (King Abdullah University of Science and Technology), a new but well-funded university that has made the development of cheap, efficient solar energy one of its main research goals.
The collaboration carries intriguing strategic possibilities.
Ted Sargent is a professor at University of Toronto. (Ted Sargent/University of Toronto)
Historically, Saudi Arabia’s major export is fossil fuels but in the future, says Sargent, Saudi Arabia intends to be a world leader in exporting technology associated with solar energy.
It’s supported by that other natural resource that Saudi Arabia has in such abundance — sunshine.
And nature itself is teaching researchers to think differently. In the international race to develop cheap, efficient solar technology, Sargent is increasingly turning to nature to learn its processes.
“It’s remarkable what nature does,” says Sargent. “It’s the epitome of ‘efficient-enough’ solar capture. How much does it cost for grass to grow or for a leaf to grow on a tree? Nature has its own systems.”
Sargent speaks with the kind of respect that can only come from a lifetime of trying to crack one of the world’s most urgent environmental challenges. Nature, says Sargent, proves that it’s possible to cover the earth with solar harvestors. “It’s an inspiration and challenge to researchers to do the same.”
Credit: CC0 Public Domain
A University of Toronto Engineering innovation could make printing solar cells as easy and inexpensive as printing a newspaper.
Gains are derived from nanowires and light trapping for better energy conversion.
Genesis Nanotechnologyo and l o 
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