Squashed quantum dots solve a multi-faceted problem – Optoelectronics


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Oblate-shaped quantum dots

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.Squashed QD cqds

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.

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U of Toronto: A Printable Solar Cell Closer to Commercial Reality


u-toronto-solar-cell-id45884A 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.”

 

Perovskite Solar Cell
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.”
Source: University of Toronto

 

Perovskite phosphor boosts visible light communication: Flashy nanocrystals help LEDs send data in the blink of an eye


Flashy NP Perovskite 1466097172999A green-emitting perovskite nanocrystal phosphor mixed with a red-emitting nitride phosphor looks yellow under ambient light (left). When excited by blue laser light, the phosphor combination produces white light (right).
Credit: Osman Bakr

 

Light-emitting diodes (LEDs) are increasingly used to illuminate homes and offices; soon, the same lights could also transmit data to your computer or smartphone in photon pulses so fast the eye can’t see them. But this form of visible light communication faces two key challenges: The light must flicker fast enough to carry sizeable amounts of data; and at the same time it should provide the warm, balanced color tones needed for pleasant ambient lighting.

 

Nanocrystals of cesium lead bromide (CsPbBr3) could help to solve both problems, according to a team led by Boon S. Ooi and Osman M. Bakr at King Abdullah University of Science & Technology (KAUST). They have found that LEDs coated with the material can reach high data transmission rates of 2 gigabits per second, comparable to the fastest Wi-Fi, while producing a quality of light that matches commercial white-light LEDs (ACS Photonics 2016, DOI: 10.1021/acsphotonics.6b00187).

 

Visible light communication, sometimes called Li-Fi, is already finding real-world applications. Last year, for example, Dutch company Phillips installed a smart LED system in a French supermarket that uses Li-Fi to transmit discount offers to shoppers’ cellphones, based on their location in the store. If data rates could be increased significantly, Li-Fi might add much-needed capacity to congested Wi-Fi networks that rely on radio waves.

 

And since the smart LEDs are doing double duty, by providing both lighting and communication, they offer an economical solution, says Bakr. Ooi adds that these systems do not even need a direct line of sight between LED and computer: “As long as your device can see light, you can detect a signal,” he says.

 

White-light LEDs typically contain a blue LED coated with phosphors that turn some of the light into green and red. But most phosphors take too long to recover between excitation and emission, pulsing no more than a few million times per second. Last year, other researchers showed that polymer semiconductors could reach more than 200 MHz (ACS Photonics2015, DOI: 10.1021/ph500451y).

 

The KAUST team instead turned to CsPbBr3, part of a family of materials known as perovskites that have become the darling of the photovoltaic research community. Perovskite solar cells have seen remarkable efficiency gains over the past seven years, and the materials are cheap and relatively easy to prepare in solution.

 

The team created nanocrystals of the perovskite, roughly 8 nm across, and found that their green emission faded in just seven nanoseconds. This allowed them to pulse reliably at almost 500 MHz, setting what the researchers believe is a new record for LED phosphors. “It is an extremely impressive and important achievement,” says Ted Sargent of the University of Toronto, who works on optoelectronic materials and has collaborated with the KAUST group in the past.

 

The rapid response is partly due to the size of the crystals, Bakr explains. When blue light excites an electron in the material, it forms an electron-hole pair called an exciton. The confines of the tiny crystal change the exciton’s energy levels, making the electron more likely to recombine with its hole and emit a photon.

 

When the researchers teamed the perovskite phosphor with a commercial red-emitting phosphor and a blue gallium nitride LED, the device produced a warm white light with a color rendering index of 89, as good as white LEDs already on the market (natural sunlight itself is rated at 100). “This quality makes this material ideal for low-power indoor illumination,” Sargent says.

 

Jakoah Brgoch of the University of Houston, who develops novel phosphors for LED lighting, says that it is relatively easy to fine-tune the chemistry of perovskites by substituting different halides or metal ions. “That means there’s a lot of potential to improve these properties,” he says.

Chemical & Engineering News ISSN 0009-2347 Copyright © 2016 American Chemical Society

University of Toronto: Two Great Photovoltaic Materials Brought Together Make Better LEDs


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Ted Sargent at the University of Toronto has built a reputation over the years as being a prominent advocate for the use of quantum dots in photovoltaics. Sargent has even penned a piece for IEEE Spectrum covering the topic, and this blog has covered his record breaking efforts at boosting the conversion efficiency of quantum dot-based photovoltaics a few times.

Earlier this year, however, Sargent started to take an interest in the hot material that has the photovoltaics community buzzing: perovskite. Now, he and his research team at the University of Toronto have combined perovskite and quantum dots  into a hybrid that they believe could transform LED technology.

Read the Full Article Here: Two Great Photovoltaic Materials Brought Together Make Better LEDs.

QD Solar Brings Quantum Dot – Cell Technology into the Light


QD Solar untitledFunding 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.

QD Solar untitled

With an ideal design for solar panels the paint made from quantum dots is highly flexible.

© 2015 KAUST

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.”

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