A Path to Cheaper Flexible Solar Cells -Researchers at Georgia IT and MIT are Developing the Potential Perovskite-Based Solar Cells


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A researcher at Georgia Tech holds a perovskite-based solar cell, which is flexible and lighter than silicon-based versions. Credit: Rob Felt, Georgia Tech

There’s a lot to like about perovskite-based solar cells. They are simple and cheap to produce, offer flexibility that could unlock a wide new range of installation methods and places, and in recent years have reached energy efficiencies approaching those of traditional silicon-based cells.

But figuring out how to produce perovskite-based energy devices that last longer than a couple of months has been a challenge.

Now researchers from Georgia Institute of Technology, University of California San Diego and Massachusetts Institute of Technology have reported new findings about perovskite solar cells that could lead the way to devices that perform better.

“Perovskite solar cells offer a lot of potential advantages because they are extremely lightweight and can be made with flexible plastic substrates,” said Juan-Pablo Correa-Baena, an assistant professor in the Georgia Tech School of Materials Science and Engineering. “To be able to compete in the marketplace with silicon-based solar cells, however, they need to be more efficient.”

In a study that was published February 8 in the journal Science and was sponsored by the U.S Department Energy and the National Science Foundation, the researchers described in greater detail the mechanisms of how adding alkali metal to the traditional perovskites leads to better performance. Perov SCs 091_main

“Perovskites could really change the game in solar,” said David Fenning, a professor of nanoengineering at the University of California San Diego. “They have the potential to reduce costs without giving up performance. But there’s still a lot to learn fundamentally about these materials.”

To understand perovskite crystals, it’s helpful to think of its crystalline structure as a triad. One part of the triad is typically formed from the element lead. The second is typically made up of an organic component such as methylammonium, and the third is often comprised of other halides such as bromine and iodine.

In recent years, researchers have focused on testing different recipes to achieve better efficiencies, such as adding iodine and bromine to the lead component of the structure. Later, they tried substituting cesium and rubidium to the part of the perovskite typically occupied by organic molecules.

“We knew from earlier work that adding cesium and rubidium to a mixed bromine and iodine lead perovskite leads to better stability and higher performance,” Correa-Baena said.

But little was known about why adding those alkali metals improved performance of the perovskites.

To understand exactly why that seemed to work, the researchers used high-intensity X-ray mapping to examine the perovskites at the nanoscale.

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“By looking at the composition within the perovskite material, we can see how each individual element plays a role in improving the performance of the device,” said Yanqi (Grace) Luo, a nanoengineering PhD student at UC San Diego.

They discovered that when the cesium and rubidium were added to the mixed bromine and iodine lead perovskite, it caused the bromine and iodine to mix together more homogeneously, resulting in up to 2 percent higher conversion efficiency than the materials without these additives.

“We found that uniformity in the chemistry and structure is what helps a perovskite solar cell operate at its fullest potential,” Fenning said. “Any heterogeneity in that backbone is like a weak link in the chain.”

Even so, the researchers also observed that while adding rubidium or cesium caused the bromine and iodine to become more homogenous, the halide metals themselves within their own cation remained fairly clustered, creating inactive “dead zones” in the solar cell that produce no current.

“This was surprising,” Fenning said. “Having these dead zones would typically kill a solar cell. In other materials, they act like black holes that suck in electrons from other regions and never let them go, so you lose current and voltage.

“But in these perovskites, we saw that the dead zones around rubidium and cesium weren’t too detrimental to solar cell performance, though there was some current loss,” Fenning said. “This shows how robust these materials are but also that there’s even more opportunity for improvement.”

The findings add to the understanding of how the perovskite-based devices work at the nanoscale and could lay the groundwork for future improvements.

“These materials promise to be very cost effective and high performing, which is pretty much what we need to make sure photovoltaic panels are deployed widely,” Correa-Baena said. “We want to try to offset issues of climate change, so the idea is to have photovoltaic cells that are as cheap as possible.”

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Materials provided by Georgia Institute of TechnologyNote: Content may be edited for style and length.

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MIT: Unleashing perovskites’ potential for solar cells


Solar cells made of perovskite have great promise, in part because they can easily be made on flexible substrates, like this experimental cell. Image: Ken Richardson

New results show how varying the recipe could bring these materials closer to commercialization.

Perovskites — a broad category of compounds that share a certain crystal structure — have attracted a great deal of attention as potential new solar-cell materials because of their low cost, flexibility, and relatively easy manufacturing process.

But much remains unknown about the details of their structure and the effects of substituting different metals or other elements within the material.

Conventional solar cells made of silicon must be processed at temperatures above 1,400 degrees Celsius, using expensive equipment that limits their potential for production scaleup.

In contrast, perovskites can be processed in a liquid solution at temperatures as low as 100 degrees, using inexpensive equipment. What’s more, perovskites can be deposited on a variety of substrates, including flexible plastics, enabling a variety of new uses that would be impossible with thicker, stiffer silicon wafers.

Now, researchers have been able to decipher a key aspect of the behavior of perovskites made with different formulations:

With certain additives there is a kind of “sweet spot” where greater amounts will enhance performance and beyond which further amounts begin to degrade it.

The findings are detailed this week in the journal Science, in a paper by former MIT postdoc Juan-Pablo Correa-Baena, MIT professors Tonio Buonassisi and Moungi Bawendi, and 18 others at MIT, the University of California at San Diego, and other institutions.

Perovskite solar cells are thought to have great potential, and new understanding of how changes in composition affect their behavior could help to make them practical. Image: Ken Richardson

Perovskites are a family of compounds that share a three-part crystal structure. Each part can be made from any of a number of different elements or compounds — leading to a very broad range of possible formulations. Buonassisi compares designing a new perovskite to ordering from a menu, picking one (or more) from each of column A, column B, and (by convention) column X.

“You can mix and match,” he says, but until now all the variations could only be studied by trial and error, since researchers had no basic understanding of what was going on in the material.

In previous research by a team from the Swiss École Polytechnique Fédérale de Lausanne, in which Correa-Baena participated, had found that adding certain alkali metals to the perovskite mix could improve the material’s efficiency at converting solar energy to electricity, from about 19 percent to about 22 percent.

But at the time there was no explanation for this improvement, and no understanding of exactly what these metals were doing inside the compound. “Very little was known about how the microstructure affects the performance,” Buonassisi says.

Now, detailed mapping using high-resolution synchrotron nano-X-ray fluorescence measurements, which can probe the material with a beam just one-thousandth the width of a hair, has revealed the details of the process, with potential clues for how to improve the material’s performance even further.

It turns out that adding these alkali metals, such as cesium or rubidium, to the perovskite compound helps some of the other constituents to mix together more smoothly. As the team describes it, these additives help to “homogenize” the mixture, making it conduct electricity more easily and thus improving its efficiency as a solar cell.

But, they found, that only works up to a certain point. Beyond a certain concentration, these added metals clump together, forming regions that interfere with the material’s conductivity and partly counteract the initial advantage. In between, for any given formulation of these complex compounds, is the sweet spot that provides the best performance, they found.

“It’s a big finding,” says Correa-Baena, who in January became an assistant professor of materials science and engineering at Georgia Tech.

What the researchers found, after about three years of work at MIT and with collaborators at UCSD, was “what happens when you add those alkali metals, and why the performance improves.” They were able to directly observe the changes in the composition of the material, and reveal, among other things, these countervailing effects of homogenizing and clumping.

“The idea is that, based on these findings, we now know we should be looking into similar systems, in terms of adding alkali metals or other metals,” or varying other parts of the recipe, Correa-Baena says.

While perovskites can have major benefits over conventional silicon solar cells, especially in terms of the low cost of setting up factories to produce them, they still require further work to boost their overall efficiency and improve their longevity, which lags significantly behind that of silicon cells.

Although the researchers have clarified the structural changes that take place in the perovskite material when adding different metals, and the resulting changes in performance, “we still don’t understand the chemistry behind this,” Correa-Baena says. That’s the subject of ongoing research by the team. The theoretical maximum efficiency of these perovskite solar cells is about 31 percent, according to Correa-Baena, and the best performance to date is around 23 percent, so there remains a significant margin for potential improvement.

Although it may take years for perovskites to realize their full potential, at least two companies are already in the process of setting up production lines, and they expect to begin selling their first modules within the next year or so. Some of these are small, transparent and colorful solar cells designed to be integrated into a building’s façade. “It’s already happening,” Correa-Baena says, “but there’s still work to do in making these more durable.”

Once issues of large-scale manufacturability, efficiency, and durability are addressed, Buonassisi says, perovskites could become a major player in the renewable energy industry. “If they succeed in making sustainable, high-efficiency modules while preserving the low cost of the manufacturing, that could be game-changing,” he says. “It could allow expansion of solar power much faster than we’ve seen.”

Perovskite solar cells “are now primary candidates for commercialization. Thus, providing deeper insights, as done in this work, contributes to future development,” says Michael Saliba, a senior researcher on the physics of soft matter at the University of Fribourg, Switzerland, who was not involved in this research.

Saliba adds, “This is great work that is shedding light on some of the most investigated materials. The use of synchrotron-based, novel techniques in combination with novel material engineering is of the highest quality, and is deserving of appearing in such a high-ranking journal.” He adds that work in this field “is rapidly progressing. Thus, having more detailed knowledge will be important for addressing future engineering challenges.”

The study, which included researchers at Purdue University and Argonne National Laboratory, in addition to those at MIT and UCSD, was supported by the U.S. Department of Energy, the National Science Foundation, the Skolkovo Institute of Science and Technology, and the California Energy Commission.

New Material For Splitting Water: Halide double Perovskites – “All the Right Properties” for creating Fuel Cells


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New Nanomaterial helps Store Solar Energy (as Hydrogen) Efficiently and Inexpensively


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Efficient storage technologies are necessary if solar and wind energy is to help satisfy increased energy demands.

One important approach is storage in the form of hydrogen extracted from water using solar or wind energy. This process takes place in a so-called electrolyser. Thanks to a new material developed by researchers at the Paul Scherrer Institute PSI and Empa, these devices are likely to become cheaper and more efficient in the future. The material in question works as a catalyst accelerating the splitting of water molecules: the first step in the production of hydrogen. Researchers also showed that this new material can be reliably produced in large quantities and demonstrated its performance capability within a technical electrolysis cell – the main component of an electrolyser. The results of their research have been published in the current edition of the scientific journal Nature Materials.

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The scientists Emiliana Fabbri and Thomas Schmidt in a lab at PSI where they conducted experiments to study the performance of the newly developed catalyst for electrolysers. (Photo: Paul Scherrer Institute/Mahir Dzambegovic.)

Since solar and wind energy is not always available, it will only contribute significantly to meeting energy demands once a reliable storage method has been developed. One promising approach to this problem is storage in the form of hydrogen. This process requires an electrolyser, which uses electricity generated by solar or wind energy to split water into hydrogen and oxygen. Hydrogen serves as an energy carrier. It can be stored in tanks and later transformed back into electrical energy with the help of fuel cells. This process can be carried out locally, in places where energy is needed such as domestic residences or fuel cell vehicles, enabling mobility without the emission of CO2.

Inexpensive and efficient

Researchers at the Paul Scherrer Institute PSI have now developed a new material that functions as a catalyst within an electrolyser and thus accelerates the splitting of water molecules: the first step in the production of hydrogen. “There are currently two types of electrolysers on the market: one is efficient but expensive because its catalysts contain noble metals such as iridium. The others are cheaper but less efficient”, explains Emiliana Fabbri, researcher at the Paul Scherrer Institute. “We wanted to develop an efficient but less expensive catalyst that worked without using noble metals.”

Exploring this procedure, researchers were able to use a material that had already been developed: an intricate compound of the elements barium, strontium, cobalt, iron and oxygen – a so-called perovskite. But they were the first to develop a technique enabling its production in the form of miniscule nanoparticles. This is the form required for it to function efficiently since a catalyst requires a large surface area on which many reactive centres are able to accelerate the electrochemical reaction. Once individual catalyst particles have been made as small as possible, their respective surfaces combine to create a much larger overall surface area.

Researchers used a so-called flame-spray device to produce this nanopowder: a device operated by Empa that sends the material’s constituent parts through a flame where they merge and quickly solidify into small particles once they leave the flame. “We had to find a way of operating the device that reliably guaranteed the solidifying of the atoms of the various elements in the right structure,” emphasizes Fabbri. “We were also able to vary the oxygen content where necessary, enabling the production of different material variants.”

Successful Field Tests

Researchers were able to show that these procedures work not only in the laboratory but also in practice. The production method delivers large quantities of the catalyst powder and can be made readily available for industrial use. “We were eager to test the catalyst in field conditions. Of course, we have test facilities at PSI capable of examining the material but its value ultimately depends upon its suitability for industrial electrolysis cells that are used in commercial electrolysers,” says Fabbri. Researchers tested the catalyst in cooperation with an electrolyser manufacturer in the US and were able to show that the device worked more reliably with the new PSI-produced perovskite than with a conventional iridium-oxide catalyst.

Examining in Milliseconds

Researchers were also able to carry out precise experiments that provided accurate information on what happens in the new material when it is active. This involved studying the material with X-rays at PSI’s Swiss Light Source SLS. This facility provides researchers with a unique measuring station capable of analysing the condition of a material over successive timespans of just 200 milliseconds. “This enables us to monitor changes in the catalyst during the catalytic reaction: we can observe changes in the electronic properties or the arrangement of atoms,” says Fabbri. At other facilities, each individual measurement takes about 15 minutes, providing only an averaged image at best.” These measurements also showed how the structures of particle surfaces change when active – parts of the material become amorphous which means that the atoms in individual areas are no longer uniformly arranged. Unexpectedly, this makes the material a better catalyst.

Use in the ESI Platform

Working on the development of technological solutions for Switzerland’s energy future is an essential aspect of the research carried out at PSI. To this end, PSI makes its ESI (Energy System Integration) experimental platform available to research and industry, enabling promising solutions to be tested in a variety of complex contexts. The new catalyst provides an important base for the development of a new generation of water electrolysers.

Los Alamos National Laboratory Studies Perovskites for Efficient Optoelectronics: Video


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In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are gaining an extra degree of freedom in designing and fabricating efficient optoelectronic devices based on 2D layered hybrid perovskites. Industrial applications could include low cost solar cells, LEDs, laser diodes, detectors, and other nano-optoelectronic devices.

Los Alamos Lab lanl-logo-footerThe 2D, near-single-crystalline “Ruddlesden-Popper” thin films have an out-of-plane orientation so that uninhibited charge transport occurs through the perovskite layers in planar devices. The new research finds the existence of “layer-edge-states” at the edges of the perovskite layers which are key to both high efficiency of solar cells (greater than 12 percent) and high fluorescence efficiency (a few tens of percent) for LEDs. The spontaneous conversion of excitons (bound electron-hole pairs) to free carriers via these layer-edge states appears to be the key to the improvement of the photovoltaic and light-emitting thin film layered materials.

Watch the Video

See the news release here:
http://www.lanl.gov/discover/news-rel…

And the research paper in Science:
http://science.sciencemag.org/content…

Perovskite Nanocrystals: Bright – Cheap – Stable: Discovery Illuminates Path to Highly Efficient Perovskite based Quantum Dots Photovoltaics


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Digital picture of colloidal solution in toluene taken under UV-light (λ = 365 nm) and crystal structure of Formamidinium lead-halide perovskite. (Image: Friedrich-Alexander-Universität Erlangen-Nürnberg)

The team reports facile and rapid room temperature synthesis of cubic and platelet-like colloidal nanocrystals (NCs) of Formamidinium Lead Halide Perovskite FAPbX3 (X=Cl, Br, I, or mixed Cl/Br and Br/I) by ligand-assisted re-precipitation method (LARP).
The obtained NCs are 15-25 nm in size and exhibit a remarkably high photoluminescence quantum yield of up to 85% as well as colloidal and chemical stability.
The cubic and platelet-like NCs with their emission in the range of 415-740 nm, full width at half maximum of 20-44 nm and radiative lifetimes of 5-166 ns, allow precise band gap tuning by halide composition as well as by tailoring their dimensions.
Notably, for the first time they have demonstrate thermodynamically stable FAPbI3 NCs in the black cubic α-phase without transition to the yellow hexagonal δ-phase even after 150 days of storage. This is in strong contrast to polycrystalline films and single crystals which convert within hours.
This fact paves the way to highly efficient perovskite based quantum dots photovoltaics, which is underpinned by demonstrating FAPbI3 NCs based photodetector.
To highlight the potential of FAPbX3 NCs as a promising candidate for optoelectronic and luminescent applications, the scientists modified the surface with polyhedral oligomeric silsesquioxane. This modification protects the brightly luminescent FAPbX3 NCs from decomposition even after storage in water for more than 2 months.
Source: Friedrich-Alexander-Universität Erlangen-Nürnberg

 

Improving Perovskites to Surpass Silicon Solar Cell Performance: Answers from ANSER


Perovskites Water id46564The perovskite device is made of different layers, each of which has a specific function. Together, the titanium dioxide and PC61BM layers protect the perovskite from heat and water. (Image: Rebecca Palmer, ANSER EFRC)

 

Harvesting sunlight and using it to power our homes and devices is a reality today. Generally, most commercial solar cells are made of silicon. However, as highlighted previously, a type of material called perovskite halides are a potential competitor of silicon. Unfortunately, most perovskite halides are sensitive to moisture and high temperatures such that exposure to either will quickly degrade these materials — rendering them useless. Researchers at the Argonne-Northwestern Solar Energy Research Center (ANSER) have developed a way to protect perovskites from water and stabilize them against heat. By carefully growing an ultrathin layer of metal oxide on a carbon coating, the researchers made a perovskite device that worked even after dousing the device with a stream of water (Nano Letters, “Liquid Water- and Heat-Resistant Hybrid Perovskite Photovoltaics via an Inverted ALD Oxide Electron Extraction Layer Design”).

Solar cells are made up of layers, each with a specific duty. The perovskite layer absorbs sunlight, which can excite an electron. The electron could go right back to where it started, unless it can be extracted out of the absorbing layer quickly. For this device, the researchers placed a layer of PC61BM, a carbon-based material, on top of the perovskite, which has two roles. First, PC61BM is good at extracting electrons once they are excited by sunlight. Second, the PC61BM layer protects the perovskite from water vapor, which is one of the reactants used for forming the final protective coating — titanium dioxide.
The titanium dioxide layer was grown using atomic layer deposition (ALD), a method that deposits alternating layers of titanium and oxygen atoms. The researchers demonstrated that depositing the titanium dioxide by ALD creates a barrier with no pinholes, effectively blocking moisture from entering the film. Only about 20 nanometers of titanium dioxide on the PC61BM were needed to protect the perovskite. This layer is around 1,000 times thinner than the thickness of a human hair.
On top of the titanium dioxide, aluminum electrodes were deposited and protected by a thin layer of gold. On the opposite side of the perovskite, the team placed a nickel oxide layer that is good at extracting the positively charged holes left by the electrons. Glass, coated with a conductive film, is used as a support that allows light to pass through and a circuit to be formed.
The device held up to pure water and a temperature of 100 °C (around 200 °F) thanks to the titanium dioxide layer. In Soo Kim, a postdoctoral fellow and lead researcher, explained that he was excited about this result. “The key challenge to commercialization of any halide perovskite-based devices is the environmental stability.”
Many people have been studying perovskite halides, but the stability under real-world environmental situations has been largely overlooked. Kim’s work is one of the first examples of protecting perovskite from liquid water with an ultrathin metal oxide layer. Alex Martinson, who directed the work, said, “It is surprising when something simple works so well.”
Martinson explained that perovskite solar cells have a lot of promise because they have the potential to be cheaper than the current commercial devices, such as silicon. The silicon manufacturing process is energy intensive, and silicon materials are required to be highly pure. In contrast, there are many pathways to make perovskites, and the performance of perovskite devices are less sensitive to impurities. Scientists at ANSER are excited to continue to explore what perovskites can do. Enabling these devices to withstand water and heat is a big step towards being able to buy a perovskite device at a local hardware store.
Source: By Rebecca Palmer, Energy Frontier Research Centers

Read more: Teaching perovskites to swim

Replacing Silicon in Solar Cells with Hybrid perovskite material could double efficiency



A new material has been shown to have the capability to double the efficiency of solar cells by researchers at Purdue University and the National Renewable Energy Laboratory.

Hybrid perovskite

The material, called a hybrid perovskite, has an inorganic crystal “cage” which contains an organic molecule, methyl-ammonium. (Image: Libai Huang)

Conventional solar cells are at most one-third efficient, a limit known to scientists as the Shockley-Queisser Limit. The new material, a crystalline structure that contains both inorganic materials (iodine and lead) and an organic material (methyl-ammonium), boosts the efficiency so that it can carry two-thirds of the energy from light without losing as much energy to heat.

In less technical terms, this material could double the amount of electricity produced without a significant cost increase.

Enough solar energy reaches the earth to supply all of the planet’s energy needs multiple times over, but capturing that energy has been difficult – as of 2013, only about 1 percent of the world’s grid electricity was produced from solar panels.
Libai Huang, assistant professor of chemistry at Purdue, says the new material, called a hybrid perovskites, would create solar cells thinner than conventional silicon solar cells, and is also flexible, cheap and easy to make.

“My graduate students learn how to make it in a few days,” she says.

The breakthrough is published this week in the journal Science (“Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy”).

The most common solar cells use silicon as a semiconductor, which can transmit only one-third of the energy because of the band gap, which is the amount of energy needed to boost an electron from a bound state to a conducting state, in which the electrons are able to move, creating electricity.

How electrons move in hybrid perovskite


Scientists at Purdue University and the National Renewable Energy Laboratory have discovered how electrons move in a new crystalline material and this discovery could lead to doubling the efficiency of solar cells. Ultrafast microscope images, such as these, show that the electrons in material is able to move over 200 nanometers with minimal energy loss to heat. (Image: Libai Huang) (click on image to enlarge)

Incoming photons can have more energy than the band gap, and for a very short time – so short it’s difficult to imagine – the electrons exist with extra energy. These electrons are called “hot carriers,” and in silicon they exist for only one picosecond (which is 10-12 seconds) and only travel a maximum distance of 10 nanometers. At this point the hot carrier electrons give up their energy as heat. This is one of the main reasons for the inefficiency of solar cells.

Huang and her colleagues have developed a new technique that can track the range of the motion and the speed of the hot carriers by using fast lasers and microscopes.

“The distance hot carriers need to migrate is at least the thickness of a solar cell, or about 200 nanometers, which this new perovskite material can achieve,” Huang says. “Also these carriers can live for about 100 picoseconds, two orders of magnitude longer than silicon.”

Kai Zhu, senior scientist at the National Renewable Energy Laboratory in Golden, Colorado, and one of the journal paper’s co-authors, says that these are critical factors for creating a commercial hot-carrier solar cell.

“This study demonstrated that hot carriers in a standard polycrystalline perovskite thin film can travel for a distance that is similar to or longer than the film thickness required to build an efficient perovskite solar cell,” he says. “This indicates that the potential for developing hot carrier perovskite solar cell is good.”

However, before a commercial product is developed, researchers are trying to use the same techniques developed at Purdue by replacing lead in the material with other, less toxic, metals.

“The next step is to find or develop suitable contact materials or structures with proper energy levels to extract these hot carriers to generate power in the external circuit,” Zhu says. “This may not be easy.”

Source: Purdue University 

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


stanford-oxfoed-perovskite_news-960x640Researchers 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.

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 new tandem solar cell

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.

 

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