Stanford and Oxford scientists report New Perovskite low cost solar cell design could outperform existing commercial technologies: Video

Researchers have created a new type of solar cell that replaces silicon with a crystal called perovskite. This design converts sunlight to electricity at efficiencies similar to current technology but at much lower cost.

A new design for solar cells that uses inexpensive, commonly available materials could rival and even outperform conventional cells made of silicon.

Stanford and Oxford have created novel solar cells from crystalline perovskite that could outperform existing silicon cells on the market today. This design converts sunlight to electricity at efficiencies of 20 percent, similar to current technology but at much lower cost.

 

Writing in the Oct. 21 edition of Science, researchers from Stanford and Oxford describe using tin and other abundant elements to create novel forms of perovskite – a photovoltaic crystalline material that’s thinner, more flexible and easier to manufacture than silicon crystals.

Video: Stanford and Oxford scientists have created novel solar cells from crystalline perovskite that could rival and even outperform existing silicon cells on the market today. The new design converts sunlight to electricity at efficiencies of 20 percent, similar to current technology but at much lower cost.

In the video, Professor Michael McGehee and postdoctoral scholar Tomas Leijtens of Stanford describe the discovery, which could lead to thin-film solar cells with a record-setting 30% efficiency.

“Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost,” said study co-author Michael McGehee, a professor of materials science and engineering at Stanford. “We have designed a robust, all-perovskite device that converts sunlight into electricity with an efficiency of 20.3 percent, a rate comparable to silicon solar cells on the market today.”

The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic, McGehee added.

“The all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30 percent efficiency,” said co-author Henry Snaith, a professor of physics at Oxford. “This is just the beginning.”

Tandem technology

Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. But a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build, the authors said.

Stanford post-doctoral scholar Tomas Leijtens and Professor Mike McGehee examine perovskite tandem solar cells. (Image credit: L.A. Cicero)

“A silicon solar panel begins by converting silica rock into silicon crystals through a process that involves temperatures above 3,000 degrees Fahrenheit (1,600 degrees Celsius),” said co-lead author Tomas Leijtens, a postdoctoral scholar at Stanford. “Perovskite cells can be processed in a laboratory from common materials like lead, tin and bromine, then printed on glass at room temperature.”

But building an all-perovskite tandem device has been a difficult challenge. The main problem is creating stable perovskite materials capable of capturing enough energy from the sun to produce a decent voltage.

A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons can cause electrons in the perovskite crystal to jump across an “energy gap” and create an electric current.

A solar cell with a small energy gap can absorb most photons but produces a very low voltage. A cell with a larger energy gap generates a higher voltage, but lower-energy photons pass right through it.

An efficient tandem device would consist of two ideally matched cells, said co-lead author Giles Eperon, an Oxford postdoctoral scholar currently at the University of Washington.

“The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage,” Eperon said. “The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage.”

Cross-section of a new tandem solar cell designed by Stanford and Oxford scientists. The brown upper layer of perovskite captures low-energy lightwaves, and the red perovskite layer captures high-energy waves. (Image credit: Scanning electron microscopy image by Rebecca Belisle and Giles Eperon)

The smaller gap has proven to be the bigger challenge for scientists. Working together, Eperon and Leijtens used a unique combination of tin, lead, cesium, iodine and organic materials to create an efficient cell with a small energy gap.

“We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8 percent conversion efficiency,” Eperon said. “We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”

The result: A tandem device consisting of two perovskite cells with a combined efficiency of 20.3 percent.

“There are thousands of possible compounds for perovskites,” Leijtens added, “but this one works very well, quite a bit better than anything before it.”

Seeking stability

One concern with perovskites is stability. Rooftop solar panels made of silicon typically last 25 years or more. But some perovskites degrade quickly when exposed to moisture or light. In previous experiments, perovskites made with tin were found to be particularly unstable.

To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees Fahrenheit (100 degrees Celsius) for four days.

“Crucially, we found that our cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites,” the authors wrote.

“The efficiency of our tandem device is already far in excess of the best tandem solar cells made with other low-cost semiconductors, such as organic small molecules and microcrystalline silicon,” McGehee said. “Those who see the potential realize that these results are amazing.”

The next step is to optimize the composition of the materials to absorb more light and generate an even higher current, Snaith said.

“The versatility of perovskites, the low cost of materials and manufacturing, now coupled with the potential to achieve very high efficiencies, will be transformative to the photovoltaic industry once manufacturability and acceptable stability are also proven,” he said.

Co-author Stacey Bent, a professor of chemical engineering at Stanford, provided key insights on tandem-fabrication techniques. Other Stanford coauthors are Kevin Bush, Rohit Prasanna, Richard May, Axel Palmstrom, Daniel J. Slotcavage and Rebecca Belisle. Oxford co-authors are Thomas Green, Jacob Tse-Wei Wang, David McMeekin, George Volonakis, Rebecca Milot, Jay Patel, Elizabeth S. Parrott, Rebecca Sutton, Laura Herz, Michael Johnston and Henry Snaith. Other co-authors are Bert Conings, Aslihan Babayigit and Hans-Gerd Boyen of Hasselt University in Belgium, and Wen Ma and Farhad Moghadam of SunPreme Inc.

Funding was provided by the Graphene Flagship, The Leverhulme Trust, U.K. Engineering and Physical Sciences Research Council, European Union Seventh Framework Programme, Horizon 2020, U.S. Office of Naval Research and the Global Climate and Energy Project at Stanford.

 

Nanoparticles Could Help Deliver a Killer Blow to Cancer

Nanotherapy is showing promise as a means to target chemotherapy, kill tumour cells by heating or enhance the effectiveness of radiotherapy.

A hundred years ago, the outbreak of the first world war saw Europe’s industrial superpowers embark on a technological arms race with increasingly lethal consequences. Over the following four years this would not only revolutionise warfare but have far-reaching consequences for communication, transport and – perhaps most surprisingly – medicine.

In 1943, American pharmacologists Louis Goodman and Alfred Gilman were investigating nitrogen mustard, a descendent of the mustard gas produced on a large scale basis by the German army in 1916, which in addition to being a blister agent had devastating effects on the immune system. Their studies found this chemical agent had the potential to prevent the replication of cancer cells, leading to the very first cancer chemotherapy regimes in the 1950s.

The limitation of chemotherapy has always been that has the potential to harm every cell in the body, rather than merely impacting the rogue tissue. However, over the past decade, the fledgling field of nanotechnology has provided researchers with a technique that may soon allow them to deliver these drugs directly to tumours.

Electron micrograph of a breast cancer cell. Iron oxide nanoparticles could be injected into a tumour and heated in an alternating magnetic field. Photograph: Rex

“The reason chemotherapy doesn’t always work is because you can’t give enough of it without exposing the body to too many toxins,” explains Jack Hoopes of the Norris Cotton Cancer Centre in New Hampshire. “So you can’t get enough drug into the tumour to be effective. I think what nanotechnology offers is the ability to target things to individual cancer cells and that’s the future of cancer therapy.”

Nanoparticles are typically between 3 and 200 nanometres across, allowing them to be injected directly into the tumour for more accessible cancers, or injected in close proximity in combination with antibodies that target cancer cells.

The unique architecture of tumours’ blood supply makes it easy for them to absorb nanoparticles. There are “fenestrations” or gaps in the walls of blood vessels that opened up when the tumours formed, says Helen Townley of the Department of Engineering Science at Oxford University.

“Instead of having a nice continuous sheet of cells as you see in normal blood vessels, the arrangement is very rapid, chaotic and disorganised. These gaps are up to 300nm, so as long as our nanoparticles are smaller than that, they’re going to leave the blood vessel and enter the tumour.”

Once the nanoparticles are inside the tumour they’re likely to stay there, she says. Normal tissue is drained by lymph vessels, but tumour tissue lacks this efficient drainage system

The main aim has been to use nanoparticles to increase chemotherapy doses but researchers have been increasingly looking at additional means of destroying tumours or slowing their growth. Hoopes’s group uses iron oxide nanoparticles coated with biocompatible substances. Once inside the tumour, the iron oxide nanoparticles can be heated using an alternating magnetic field, killing it with little damage to the surrounding tissue.

“We started out primarily with breast cancer tumours in mice and we’ve been able to make this work with lots of different types of tumours,” he said. “Our most recent study has targeted mouth tumours in dogs. This cancer tends to be terminal for the dogs about five months after diagnosis so it’s highly malignant, but after inserting nanoparticles in the tumours and exposing them to the magnetic field, we’ve had quite good success.” The research has yet to be published, but Hoopes says that after just two weeks of therapy the tumours had almost disappeared.

Within the next six months he hopes to begin a phase I breast cancer trial in humans. Patients who sign up will be scheduled to have a mastectomy, enabling the researchers to examine the tissue after magnetic nanotherapy to evaluate its effectiveness.

If that proves successful then phase II will involve using a combination of magnetic nanotherapy and radiotherapy on women undergoing lumpectomy, with a future goal of being able to eradicate the tumours without any need for an operation.

Breast cancer has proved an ideal target for magnetic nanotherapy for several reasons. The tumours are accessible, easily imaged and localised, and often spread to lymph nodes, where nanoparticles tend to accumulate when injected into the bloodstream.

It is far easier to apply the magnetic field to a limb or accessible organ rather than to one that’s deep inside the body, Hoopes says. “We can be more effective and safer. For treating something like a liver tumour we have a lot of research to do on that to find ways to safely get these fields in the body.”

Researchers at Oxford University have been looking at titanium nanoparticles as a means of enhancing the effectiveness of radiotherapy. When the particles are excited by x-rays they generate high concentrations of reactive oxygen species – chemically reactive molecules that cause significant damage to cell structures leading to cell death.

“Essentially you can treat any tumour with radiotherapy if you can turn [the power] up high enough,” Townley says. “But you’ve got this therapeutic index which is the balance between damaging healthy tissue and damaging the tumour tissue. If we can get the nanoparticles into tumours where normally you wouldn’t be able to crank the radiation up high enough without severely harming the patient, we have the potential for a lower dose or a shorter course of treatment.”

Her group has also developed microparticles for another technique called chemoembolisation. The microparticles – which are about a thousand times bigger than nanoparticles – block the blood supply carrying food and oxygen to the tumour, causing the cancerous tissue to die. Nanoparticles attached to the bigger particles also deliver chemotherapy to the site of the tumour.

So far the researchers have applied their techniques in preclinical studies looking at lung cancer in mice. While lung cancer survival rates in humans have almost doubled over the past 20 years, the disease remains one of the deadliest common cancers in the UK, with five-year survival rates of less than 10% for both men and women.

“There’s a huge need for novel treatments,” Townley says. “It’s one of most untreatable cancers but in our [mice] study we found that our techniques stopped the growth of the tumour completely in its tracks while with conventional radiotherapy it still grew two and a half times in volume.”

She adds that if they can halt the growth of tough tumours like these that are resistant to radiotherapy, they can probably shrink tumours that are more sensitive it.

In the past, most cancer nanotherapy research has been aimed at enhancing the effectiveness of existing methods such as chemotherapy or radiotherapy, but in future they may be developed as standalone treatments.

“Nanotechnology is still an early science, and right now it’s most effective in human cancer patients when used in combination with other therapies, especially in these preliminary trials,” Hoopes says. “But as we improve the targeting and identify safe ways of penetrating deeper into the body, other avenues will open up.”

Perovskite: New Wonder Material to make Cheaper & Easier to Manufacture LED’s

 

 

Colourful LEDs made from a material known as perovskite could lead to LED displays which are both cheaper and easier to manufacture in future. 

A hybrid form of perovskite – the same type of material which has recently been found to make highly efficient solar cells that could one day replace silicon – has been used to make low-cost, easily manufactured LEDs, potentially opening up a wide range of commercial applications in future, such as flexible colour displays.

This particular class of semiconducting perovskites have generated excitement in the solar cell field over the past several years, after Professor Henry Snaith’s group at Oxford University found them to be remarkably efficient at converting light to electricity. In just two short years, perovskite-based solar cells have reached efficiencies of nearly 20%, a level which took conventional silicon-based solar cells 20 years to reach.

Now, researchers from the University of Cambridge, University of Oxford and the Ludwig-Maximilians-Universität in Munich have demonstrated a new application for perovskite materials, using them to make high-brightness LEDs. The results are published in the journal Nature Nanotechnology.

Perovskite is a general term used to describe a group of materials that have a distinctive crystal structure of cuboid and diamond shapes. They have long been of interest for their superconducting and ferroelectric properties. But in the past several years, their efficiency at converting light into electrical energy has opened up a wide range of potential applications.

The perovskites that were used to make the LEDs are known as organometal halide perovskites, and contain a mixture of lead, carbon-based ions and halogen ions known as halides. These materials dissolve well in common solvents, and assemble to form perovskite crystals when dried, making them cheap and simple to make.

“These organometal halide perovskites are remarkable semiconductors,” said Zhi-Kuang Tan, a PhD student at the University of Cambridge’s Cavendish Laboratory and the paper’s lead author. “We have designed the diode structure to confine electrical charges into a very thin layer of the perovskite, which sets up conditions for the electron-hole capture process to produce light emission.”

The perovskite LEDs are made using a simple and scalable process in which a perovskite solution is prepared and spin-coated onto the substrate. This process does not require high temperature heating steps or a high vacuum, and is therefore cheap to manufacture in a large scale. In contrast, conventional methods for manufacturing LEDs make the cost prohibitive for many large-area display applications.

“The big surprise to the semiconductor community is to find that such simple process methods still produce very clean semiconductor properties, without the need for the complex purification procedures required for traditional semiconductors such as silicon,” said Professor Sir Richard Friend of the Cavendish Laboratory, who has led this programme in Cambridge.

“It’s remarkable that this material can be easily tuned to emit light in a variety of colours, which makes it extremely useful for colour displays, lighting and optical communication applications,” said Tan. “This technology could provide a lot of value to the ever growing flat-panel display industry.”

The team is now looking to increase the efficiency of the LEDs and to use them for diode lasers, which are used in a range of scientific, medical and industrial applications, such as materials processing and medical equipment. The first commercially-available LED based on perovskite could be available within five years.

Explore further: Scientists develop pioneering new spray-on solar cells

More information: Nature Nanotechnology, www.nature.com/nnano/journal/v… /nnano.2014.149.html

Read more at: http://phys.org/news/2014-08-material-perovskite.html#jCp

‘Nano-pixels’ promise thin, flexible, high resolution displays

A new discovery will make it possible to create pixels just a few hundred nanometres across that could pave the way for extremely high-resolution and low-energy thin, flexible displays for applications such as ‘smart’ glasses, synthetic retinas, and foldable screens.

A team led by Oxford University scientists explored the link between the electrical and optical properties of phase change materials (materials that can change from an amorphous to a crystalline state). They found that by sandwiching a seven nanometre thick layer of a phase change material (GST) between two layers of a transparent electrode they could use a tiny current to ‘draw’ images within the sandwich ‘stack’.

Initially still images were created using an atomic force microscope but the team went on to demonstrate that such tiny ‘stacks’ can be turned into prototype pixel-like devices. These ‘nano-pixels’ — just 300 by 300 nanometres in size — can be electrically switched ‘on and off’ at will, creating the coloured dots that would form the building blocks of an extremely high-resolution display technology.

A report of the research is published in this week’s Nature.

‘We didn’t set out to invent a new kind of display,’ said Professor Harish Bhaskaran of Oxford University’s Department of Materials, who led the research. ‘We were exploring the relationship between the electrical and optical properties of phase change materials and then had the idea of creating this GST ‘sandwich’ made up of layers just a few nanometres thick. We found that not only were we able to create images in the stack but, to our surprise, thinner layers of GST actually gave us better contrast. We also discovered that altering the size of the bottom electrode layer enabled us to change the colour of the image.’

Oxford University technology can draw images 70 micrometers across, each image is smaller than the width of a human hair. The researchers have shown that using this technology they can create ‘nano-pixels’ just 100 nanometers in size that could pave the way for extremely high-resolution and low-energy thin, flexible displays for applications such as ‘smart’ glasses, synthetic retinas, and foldable screens.

 

Whilst the work is still in its early stages, realising its potential, the Oxford team has filed a patent on the discovery with the help of Isis Innovation, Oxford University’s technology commercialisation company. Isis is now discussing the displays with companies who are interested in assessing the technology, and with investors.

The layers of the GST sandwich are created using a sputtering technique where a target is bombarded with high energy particles so that atoms from the target are deposited onto another material as a thin film.

‘Because the layers that make up our devices can be deposited as thin films they can be incorporated into very thin flexible materials — we have already demonstrated that the technique works on flexible Mylar sheets around 200 nanometres thick,’ said Professor Bhaskaran. ‘This makes them potentially useful for ‘smart’ glasses, foldable screens, windshield displays, and even synthetic retinas that mimic the abilities of photoreceptor cells in the human eye.’

Peiman Hosseini of Oxford University’s Department of Materials, first author of the paper, said: ‘Our models are so good at predicting the experiment that we can tune our prototype ‘pixels’ to create any colour we want — including the primary colours needed for a display. One of the advantages of our design is that, unlike most conventional LCD screens, there would be no need to constantly refresh all pixels, you would only have to refresh those pixels that actually change (static pixels remain as they were). This means that any display based on this technology would have extremely low energy consumption.’

The research suggests that flexible paper-thin displays based on the technology could have the capacity to switch between a power-saving ‘colour e-reader mode’, and a backlit display capable of showing video. Such displays could be created using cheap materials and, because they would be solid-state, promise to be reliable and easy to manufacture. The tiny ‘nano-pixels’ make it ideal for applications, such as smart glasses, where an image would be projected at a larger size as, even enlarged, they would offer very high-resolution.

Professor David Wright of the Department of Engineering at the University of Exeter, co-author of the paper, said: ‘Along with many other researchers around the world we have been looking into the use of these GST materials for memory applications for many years, but no one before thought of combining their electrical and optical functionality to provide entirely new kinds of non-volatile, high-resolution, electronic colour displays — so our work is a real breakthrough.’

The phase change material used was the alloy Ge2Sb2Te5 (Germanium-Antimony-Tellurium or GST) sandwiched between electrode layers made of indium tin oxide (ITO).

(Solar) Cell a Million?

SOLAR cells were once a bespoke product, reserved for satellites and military use. In 1977 a watt of solar generating capacity cost $77. That has now come down to about 80 cents, and solar power is beginning to compete with the more expensive sort of conventionally generated electricity. If the price came down further, though, solar might really hit the big time—and that is the hope of Henry Snaith, of Oxford University, and his colleagues. As he described recently in Science, Dr Snaith plans to replace silicon, the material used to make most solar cells, with a substance called a perovskite. This, he believes, could cut the cost of a watt of solar generating capacity by three-quarters.

When light falls on a solar cell, it knocks electrons away from the cell’s material and leaves behind empty spaces called holes. Electrons and holes then flow in opposite directions and the result is an electric current.

 

The more electrons and holes there are, and the faster they flow, the bigger the current will be. Electrons, however, often get captured by holes while still inside the cell, and cannot therefore contribute to the current. The average distance an electron travels in a material before it gets captured is known as that material’s diffusion length. The larger the diffusion length, the more efficient the cell.

The silicon used in commercial solar cells has a diffusion length of ten nanometres (billionth of a metre), which is not much. Partly for this reason a silicon cell’s efficiency at converting incident light into electricity is less than 10%. Dr Snaith’s perovskite does better. It has a diffusion length of 1,000 nanometres, giving it an efficiency of 15%. And this, Dr Snaith says, has been achieved without much tweaking of the material. The implication is that it could be made more efficient still.

Perovskites are substances composed of what are known as cubo-octahedral crystals—in other words, cubes with the corners cut off. They thus have six octagonal faces and eight triangular ones. Perovskite itself is a natually occuring mineral, calcium titanium oxide, but lots of other elemental combinations adopt the same shape, and tinkering with the mix changes the frequency of the light the crystal absorbs best.

Dr Snaith’s perovskite is a particularly sophisticated one. It has an organic part, made of carbon, hydrogen and nitrogen, and an inorganic part, made of lead, iodine and chlorine. The organic part acts as a dye, absorbing lots of sunlight. The inorganic part helps conduct the electrons thus released.

It is also cheap to make. Purifying silicon requires high (and therefore costly) temperatures. Dr Snaith’s perovskite can be blended at room temperature. Laboratory versions of cells made from it cost about 40 cents per watt (ie, about half the cost of commercial silicon-based solar cells). At an industrial scale, Dr Snaith expects, that will halve again.

There are caveats, of course. The new perovskite is such a recent invention that its durability has not been properly tested. Many otherwise-promising materials fail to survive constant exposure to the sun, a sine qua non of being a solar cell. And the process of converting a laboratory-made cell into a mass-manufactured one is not always straight forward.

If it leaps these hurdles, though, Dr Snaith’s material will be a strong challenger for silicon. As solar power-generation becomes a mainstream technology over the next few years, the once-strange word “perovskite” may enter everyday language.