MIT: Some catalysts contribute their own oxygen for reactions ~ Crucial for Chemical energy storage, Water splitting, & Electrochemical carbon dioxide reduction


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New research shows that when metal oxide (flat array of atoms at bottom) is used as a catalyst for splitting water molecules, some of the oxygen produced comes out of the metal oxide itself, not just from the surrounding water. This was proved by first using water with a heavier isotope of oxygen (oxygen 18, shown in white), and later switching to ordinary water (made with oxygen 16, shown in red). The detection of the heavier oxygen 18 in the resulting gas proves that this came out of the catalyst.

Chemical reactions that release oxygen in the presence of a catalyst, known as oxygen-evolution reactions, are a crucial part of chemical energy storage processes, including water splitting, electrochemical carbon dioxide reduction, and ammonia production. The kinetics of this type of reaction are generally slow, but compounds called metal oxides can have catalytic activities that vary over several orders of magnitude, with some exhibiting the highest such rates reported to date. The physical origins of these observed catalytic activities is not well-understood.

Now, a team at MIT has shown that in some of these catalysts oxygen doesn’t come only from the water molecules surrounding the catalyst material; some of it comes from within the crystal lattice of the catalyst material itself. The new findings are being reported this week in the journal Nature Chemistry, in a paper by recent MIT graduate Binghong Han PhD ’16, postdoc Alexis Grimaud, Yang Shao-Horn, the W.M. Keck Professor of Energy, and six others.

The research was aimed at studying how water molecules are split to generate oxygen molecules and what factors limit the reaction rate, Grimaud says. Increasing those reaction rates could lead to more efficient energy storage and retrieval, for example, so determining just where the bottlenecks may be in the reaction is an important step toward such improvements.

The catalysts used to foster the reactions are typically metal oxides, and the team wanted “to be able to explain the activity of the sites [on the surface of the catalyst] that split the water,” Grimaud says.

The question of whether some oxygen gets stored within the crystal structure of the catalyst and then contributes to the overall oxygen output has been debated before, but previous work had never been able to resolve the issue. Most researchers had assumed that only the active sites on the surface of the material were taking any part in the reaction. But this team found a way of directly quantifying the contribution that might be coming from within the bulk of the catalyst material, and showed clearly that this was an important part of the reaction.

They used a special “labeled” form of oxygen, the isotope oxygen-18, which makes up only a tiny fraction of the oxygen in ordinary water. By collaborating with Oscar Diaz-Morales and Marc T. Koper at Leiden University in the Netherlands, they first exposed the catalyst to water made almost entirely of oxygen-18, and then placed the catalyst in normal water (which contains the more common oxygen-16).

Upon testing the oxygen output from the reaction, using a mass spectrometer that can directly measure the different isotopes based on their atomic weight, they showed that a substantial amount of oxygen-18, which cannot be accounted for by a surface-only mechanism, was indeed being released. The measurements were tricky to carry out, so the work has taken some time to complete. Diaz-Morales “did many experiments using the mass spectrometer to detect the kind of oxygen that was evolved from the water,” says Shao-Horn, who has joint appointments in the departments of Mechanical Engineering and Materials Science and Engineering, and is a co-director of the MIT Energy Initiative’s Center for Energy Storage.

With that knowledge and with detailed theoretical calculations showing how the reaction takes place, the researchers say they can now explore ways of tuning the electronic structure of these metal-oxide materials to increase the reaction rate.

The amount of oxygen contributed by the catalyst material varies considerably depending on the exact chemistry or electronic structure of the catalyst, the team found. Oxides of different metal ions on the perovskite structure showed greater or lesser effects, or even none at all. In terms of the amount of oxygen output that is coming from within the bulk of the catalyst, “you observe a well-defined signal of the labeled oxygen,” Shao-Horn says.

One unexpected finding was that varying the acidity or alkalinity of the water made a big difference to the reaction kinetics. Increasing the water’s pH enhances the rate of oxygen evolution in the catalytic process, Han says.

These two previously unidentified effects, the participation of the bulk material in the reaction, and the influence of the pH level on the reaction rate, which were found only for oxides with record high catalytic activity, “cannot be explained by the traditional mechanism” used to explain oxygen evolution reaction kinetics, Diaz-Morales says. “We have proposed different mechanisms to account for these effects, which requires further experimental and computational studies.”

“I find it very interesting that the lattice oxygen can take part in the oxygen evolution reactions,” says Ib Chorkendorff, a professor of physics at the Technical University of Denmark, who was not involved in this work. “We used to think that all these basic electrochemical reactions, related to proton membrane fuel cells and electrolyzers, are all taking place at the surface,” but this work shows that “the oxygen sitting inside the catalyst is also taking part in the reaction.”

These findings, he says, “challenge the common way of thinking and may lead us down new alleys, finding new and more efficient catalysts.”

The team also included Wesley Hong PhD ’16, former postdoc Yueh-Lin Lee, research scientist Livia Giordano in the Department of Mechanical Engineering, Kelsey Stoerzinger PhD ’16, and Marc Koper of the Leiden Institute of Chemistry, in the Netherlands. The work was supported by the Skoltech Center for Electrochemical Energy, the Singapore-MIT Alliance for Research and Technology, the Department of Energy, and the National Energy Technology Laboratory.

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NREL: Nanoscale confinement leads to new all-inorganic perovskite with exceptional solar cell properties – Using Quantum Dots to Create Increased Solar Cell Efficiency: Colorado School of Mines


confinement-for-qdots-100816-nanoscaleconAshley Marshall, Erin Sanehira and Joey Luther with solutions of all-inorganic perovskite quantum dots, showing intense photoluminescence when illuminated with UV light. Credit: National Renewable Energy Laboratory

Scientists with the Energy Department’s National Renewable Energy Laboratory (NREL) for the first time discovered how to make perovskite solar cells out of quantum dots and used the new material to convert sunlight to electricity with 10.77 percent efficiency.

The research, Quantum dot-induced phase stabilization of a-CsPbI3 perovskite for high-efficiency photovoltaics, appears in the journal Science. The authors are Abhishek Swarnkar, Ashley Marshall, Erin Sanehira, Boris Chernomordik, David Moore, Jeffrey Christians, and Joseph Luther from NREL. Tamoghna Chakrabarti from the Colorado School of Mines also is a co-author.co-school-of-mines-222925_original

In addition to developing quantum dot , the researchers discovered a method to stabilize a crystal structure in an all-inorganic perovskite material at room temperature that was previously only favorable at high temperatures. The crystal phase of the inorganic material is more stable in .

Most research into perovskites has centered on a hybrid organic-inorganic structure. Since research into perovskites for photovoltaics began in 2009, their efficiency of converting sunlight into electricity has climbed steadily and now shows greater than 22 percent power conversion efficiency. However, the organic component hasn’t been durable enough for the long-term use of perovskites as a solar cell.

NREL scientists turned to quantum dots-which are essentially nanocrystals-of cesium lead iodide (CsPbI3) to remove the unstable and open the door to high-efficiency quantum dot optoelectronics that can be used in LED lights and photovoltaics. NREL 20140609_buildings_26954_hp

The nanocrystals of CsPbI3 were synthesized through the addition of a Cs-oleate solution to a flask containing PbI2 precursor. The NREL researchers purified the nanocrystals using methyl acetate as an anti-solvent that removed excess unreacted precursors. This step turned out to be critical to increasing their stability.

Contrary to the bulk version of CsPbI3, the nanocrystals were found to be stable not only at temperatures exceeding 600 degrees Fahrenheit but also at room temperatures and at hundreds of degrees below zero. The bulk version of this material is unstable at , where photovoltaics normally operate and convert very quickly to an undesired crystal structure.

NREL scientists were able to transform the nanocrystals into a thin film by repeatedly dipping them into a methyl acetate solution, yielding a thickness between 100 and 400 nanometers. Used in a solar cell, the CsPbI3 nanocrystal film proved efficient at converting 10.77 percent of sunlight into electricity at an extraordinary high open circuit voltage. The efficiency is similar to record quantum dot solar cells of other materials and surpasses other reported all-inorganic perovskite solar cells.

Explore further: Rubidium pushes perovskite solar cells to 21.6 percent efficiency

More information: A. Swarnkar et al. Quantum dot-induced phase stabilization of -CsPbI3 perovskite for high-efficiency photovoltaics, Science (2016). DOI: 10.1126/science.aag2700

 

World’s most efficient nanowire lasers: Benefit to Fiber Optics Communications


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Perovskite-based nanowire lasers are the most efficient known. A topological image of a nanowire is shown (left insert). Room temperature emission images above the lasing threshold for two nanowires composed of different halides, iodide (red in center) and bromide (green on the right), are shown in top inserts.
Credit: Image courtesy of Xiaoyang Zhu, Columbia University

Known for their low cost, simple processing and high efficiency, perovskites are popular materials in solar panel research. Now, researchers demonstrated that nanowires made from lead halide perovskite are the most efficient nanowire lasers known.

Efficient nanowire lasers could benefit fiber optic communications, pollution characterization, and other applications. The challenge is getting the right material. These ultra-compact wires have a superior ability to emit light, can be tuned to emit different colors, and are relatively easy to synthesize. The development of these perovskite wires parallels the rapid development of the same materials for efficient solar cells.

Semiconductor nanowire lasers, due to their ultra-compact physical sizes, highly localized coherent output, and efficiency, are promising components for use in fully integrated nanoscale photonic and optoelectronic devices. Lasing requires a minimum (threshold) excitation density, below which little light is emitted.

A high “lasing threshold” not only makes critical technical advances difficult, but also imposes fundamental limits on laser performance due to the onset of other losses. In searching for an ideal material for nanowire lasing, researchers at Columbia University and the University of Wisconsin-Madison investigated a new class of hybrid organic-inorganic semiconductors, methyl ammonium lead halide perovskites (CH3NH3PbX3), which is emerging as a leading material for high-efficiency photovoltaic solar cells due to low cost, simple processing and high efficiencies.

The exceptional solar cell performance in these materials can be attributed to the long lifetimes of the carriers that move energy through the systems (electrons and holes) and carrier diffusion lengths.

These properties, along with other attributes such as high fluorescence yield and wavelength tunability, also make them ideal for lasing applications. Room temperature lasing in these nanowires was demonstrated with:

  • The lowest lasing thresholds and the highest quality factors reported to date
  • Near 100% quantum yield (ratio of the number of photons emitted to those absorbed)
  • Broad tunability of emissions covering the near infrared to visible wavelength region.

Specifically, the laser emission shifts from near infrared to blue with decreasing atomic number of the halides (X=I, Br, Cl) in the nanowires. These nanowires could advance applications in nanophotonics and optoelectronic devices. In particular, lasers that operate in the near infrared region could benefit fiber optic communications and advance pollution characterization from space.

This work was supported by the DOE Office of Science (Office of Basic Energy Sciences) and the National Science Foundation.


Story Source:

The above post is reprinted from materials provided byDepartment of Energy, Office of Science. Note: Materials may be edited for content and length.


Journal Reference:

  1. Haiming Zhu, Yongping Fu, Fei Meng, Xiaoxi Wu, Zizhou Gong, Qi Ding, Martin V. Gustafsson, M. Tuan Trinh, Song Jin, X-Y. Zhu. Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. Nature Materials, 2015; 14 (6): 636 DOI: 10.1038/NMAT4271

A Chemical Switch-Flip Helps Perovskite Solar Cells Beat the Heat


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Thin films of crystalline materials called perovskites provide a promising new way of making inexpensive and efficient solar cells. Now, an international team of researchers has shown a way of flipping a chemical switch that converts one type of perovskite into another — a type that has better thermal stability and is a better light absorber.

The study, by researchers from Brown University, the National Renewable Energy Laboratory (NREL) and the Chinese Academy of Sciences’ Qingdao Institute of Bioenergy and Bioprocess Technology published in the Journal of the American Chemical Society, could be one more step toward bringing perovskite solar cells to the mass market.

“We’ve demonstrated a new procedure for making solar cells that can be more stable at moderate temperatures than the perovskite solar cells that most people are making currently,” said Nitin Padture, professor in Brown’s School of Engineering, director of Brown’s Institute for Molecular and Nanoscale Innovation, and the senior co-author of the new paper. “The technique is simple and has the potential to be scaled up, which overcomes a real bottleneck in perovskite research at the moment.”

Perovskites have emerged in recent years as a hot topic in the solar energy world. The efficiency with which they convert sunlight into electricity rivals that of traditional silicon solar cells, but perovskites are potentially much cheaper to produce. These new solar cells can also be made partially transparent for use in windows and skylights that can produce electricity, or to boost the efficiency of silicon solar cells by using the two in tandem.

Despite the promise, perovskite technology has several hurdles to clear — one of which deals with thermal stability. Most of the perovskite solar cells produced today are made with of a type of perovskite called methylammonium lead triiodide (MAPbI3). The problem is that MAPbI3 tends to degrade at moderate temperatures.

“Solar cells need to operate at temperatures up to 85 degrees Celsius,” said Yuanyuan Zhou, a graduate student at Brown who led the new research. “MAPbI3 degrades quite easily at those temperatures.”Flip Chem Switch 042716 solarenergy

That’s not ideal for solar panels that must last for many years. As a result, there’s a growing interest in solar cells that use a type of perovskite called formamidinium lead triiodide (FAPbI3) instead. Research suggests that solar cells based on FAPbI3 can be more efficient and more thermally stable than MAPbI3. However, thin films of FAPbI3 perovskites are harder to make than MAPbI3 even at laboratory scale, Padture says, let alone making them large enough for commercial applications.

(Right) Thin films of crystalline materials called perovskites provide a promising new way of making inexpensive and efficient solar cells. Now, an international team of researchers has shown a way of flipping a chemical switch that converts one type of perovskite into another — a type that has better thermal stability and is a better light absorber. Credit: Padture Lab / Brown University

Part of the problem is that formamidinium has a different molecular shape than methylammonium. So as FAPbI3 crystals grow, they often lose the perovskite structure that is critical to absorbing light efficiently.

This latest research shows a simple way around that problem. The team started by making high-quality MAPbI3 thin films using techniques they had developed previously. They then exposed those MAPbI3 thin films to formamidine gas at 150 degrees Celsius. The material instantly converted from MAPbI3 to FAPbI3 while preserving the all-important microstructure and morphology of the original thin film.

“It’s like flipping a switch,” Padture said. “The gas pulls out the methylammonium from the crystal structure and stuffs in the formamidinium, and it does so without changing the morphology. We’re taking advantage of a lot of experience in making excellent quality MAPbI3 thin films and simply converting them to FAPbI3 thin films while maintaining that excellent quality.”

This latest research builds on the work this international team of researchers has been doing over the past year using gas-based techniques to make perovskites. The gas-based methods have the potential of improving the quality of the solar cells when scaled up to commercial proportions. The ability to switch from MAPbI3 to FAPbI3 marks another potentially useful step toward commercialization, the researchers say.

“The simplicity and the potential scalability of this method was inspired by our previous work on gas-based processing of MAPbI3 thin films, and now we can make high-efficiency FAPbI3-based perovskite solar cells that can be thermally more stable,” Zhou said. “That’s important for bringing perovskite solar cells to the market.”

Laboratory scale perovskite solar cells made using this new method showed efficiency of around 18 percent — not far off the 20 to 25 percent achieved by silicon solar cells.

“We plan to continue to work with the method in order to further improve the efficiency of the cells,” said Kai Zhu, senior scientist at NREL and co-author of the new paper. “But this initial work demonstrates a promising new fabrication route.”

Will New Method of Making Perovskites Solar Cells Make Solar Energy More Efficient – Less Costly?


Perovsjite 1-solarcellPerovskites, substances that perfectly absorb light, are the future of solar energy. The opportunity for their rapid dissemination has just increased thanks to a cheap and environmentally safe method of production of these materials, developed by chemists from Warsaw, Poland. Rather than in solutions at a high temperature, perovskites can now be synthesized by solid-state mechanochemical processes: by grinding powders.
We associate the milling of chemicals less often with progress than with old-fashioned pharmacies and their inherent attributes: the pestle and mortar. It’s time to change this! Recent research findings show that by the use of mechanical force, effective chemical transformations take place in solid state. Mechanochemical reactions have been under investigation for many years by the teams of Prof. Janusz Lewinski from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Faculty of Chemistry of Warsaw University of Technology.
In their latest publication (“Mechanosynthesis of the hybrid perovskite CH3NH3PbI3: characterization and the corresponding solar cell efficiency”), the Warsaw researchers describe a surprisingly simple and effective method of obtaining perovskites – futuristic photovoltaic materials with a spatially complex crystal structure.
perovskite powders
A simple, fast and safe method of obtaining perovskites has been discovered by scientists from IPC PAS in Warsaw, Poland. The perovskite (a black powder) is milled from two powders: a white one, methylammonium iodide, and a yellow one, lead iodide.

 

“With the aid of mechanochemistry we are able to synthesize a variety of hybrid inorganic-organic functional materials with a potentially great significance for the energy sector. Our youngest ‘offspring’ are high quality perovskites. These compounds can be used to produce thin light-sensitive layers for high efficiency solar cells,” says Prof. Lewinski.
Perovskites are a large group of materials, characterized by a defined spatial crystalline structure. In nature, the perovskite naturally occurring as a mineral is calcium titanium(IV) oxide CaTiO3. Here the calcium atoms are arranged in the corners of the cube, in the middle of each wall there is an oxygen atom and at the centre of the cube lies a titanium atom. In other types of perovskite the same crystalline structure can be constructed of various organic and inorganic compounds, which means titanium can be replaced by, for example, lead, tin or germanium. As a result, the properties of the perovskite can be adjusted so as to best fit the specific application, for example, in photovoltaics or catalysis, but also in the construction of superconducting electromagnets, high voltage transformers, magnetic refrigerators, magnetic field sensors, or RAM memories.
At first glance, the method of production of perovskites using mechanical force, developed at the IPC PAS, looks a little like magic.
“Two powders are poured into the ball mill: a white one, methylammonium iodide CH3NH3I, and a yellow one, lead iodide PbI2. After several minutes of milling no trace is left of the substrates. Inside the mill there is only a homogeneous black powder: the perovskite CH3NH3PbI3,” explains doctoral student Anna Maria Cieslak (IPC PAS).
“Hour after hour of waiting for the reaction product? Solvents? High temperatures? In our method, all this turns out to be unnecessary! We produce chemical compounds by reactions occurring only in solids at room temperature,” stresses Dr. Daniel Prochowicz (IPC PAS).
The mechanochemically manufactured perovskites were sent to the team of Prof. Michael Graetzel from the Ecole Polytechnique de Lausanne in Switzerland, where they were used to build a new laboratory solar cell. The performance of the cell containing the perovskite with a mechanochemical pedigree proved to be more than 10% greater than a cell’s performance with the same construction, but containing an analogous perovskite obtained by the traditional method, involving solvents.
“The mechanochemical method of synthesis of perovskites is the most environmentally friendly method of producing this class of materials. Simple, efficient and fast, it is ideal for industrial applications. With full responsibility we can state: perovskites are the materials of the future, and mechanochemistry is the future of perovskites,” concludes Prof. Lewinski.
The described research will be developed within GOTSolar collaborative project funded by the European Commission under the Horizon 2020 Future and Emerging Technologies action.
Perovskites are not the only group of three-dimensional materials that has been produced mechanochemically by Prof. Lewinski’s team. In a recent publication the Warsaw researchers showed that by using the milling technique they can also synthesize inorganic-organic microporous MOF (Metal-Organic Framework) materials. The free space inside these materials is the perfect place to store different chemicals, including hydrogen.
Source: Institute of Physical Chemistry of the Polish Academy of Sciences

 

University of Utah: Uncovering the secrets of super solar power perovskites


Perovskite II 031615 uncoveringthThe best hope for cheap, super-efficient solar power is a remarkable family of crystalline materials called hybrid perovskites. In just five years of development, hybrid perovskite solar cells have attained power conversion efficiencies that took decades to achieve with the top-performing conventional materials used to generate electricity from sunlight.

Now researchers at the University of Utah, in collaboration with the University of Texas at Dallas, have uncovered some of the secrets behind the amazing material’s performance. The findings, published today in the journal Nature Physics, help fill a deep void in hybrid perovskite solar cell research. Scientists and engineers have lacked a clear understanding of the precise goings on at the molecular level.

Among the practical results of the new study is proof of a way to rapidly test the performance of different prototypes of hybrid perovskite materials using magnetic fields, according to lead author Charlie Zhang, a post-doctoral research fellow, and senior author Z. Valy Vardeny, a distinguished professor of physics at the University of Utah.

“Our group has unique expertise in effects,” Vardeny says. “We wanted to see if magnetic field effects would tell us why the efficiency is so high.”

Probing electronic properties

Applying a magnetic field makes it possible to glean clues about the behavior of electrons and “holes” in semiconductor compounds. In , molecules absorb incoming photons of sunlight. Each absorbed photon can generate an exciton, the pairing of an electron and a corresponding electron hole. These pairings are short-lived and split into free, charge-carrying particles that drive an electric current.

Perovskite II 031615 uncoveringth

Schematic presentation of the obtained magnetic field effect of photocarriers in photovoltaic cells and injected carriers in light emitting diodes based on hybrid organic/inorganic perovskite semiconductors, which originates from different …more

Electrons and holes have a magnetic-related property called ‘spin’, a form of angular momentum; and the torque of a magnetic field can alter the spin direction. Spin can’t be observed directly, but spin properties can be inferred by looking at readily measurable properties, such as changes in the electrical conductivity of a material, or changes in photoluminescence – its tendency to emit light after absorbing photons – when it is subjected to a magnetic field.

Zhang and colleagues measured magnetic field-induced changes in these properties in an assortment of fabricated hybrid having different solar power conversion efficiencies. They used a typical hybrid perovskite material, methylammonium lead iodide, or MAPbI3. (Hybrid perovskites follow the naming convention MAPbX3, with MA denoting the organic methylammonium group that is combined with an inorganic group made of lead (Pb) and either chloride, bromide, or iodide (X)). Contrary to conventional wisdom in the field, the Utah scientists found pronounced magnetic field effects. The magnetic properties of the heavy atoms of lead and iodine were thought to minimize magnetic field effects in hybrid perovskite .

How it works

The researchers proposed a mechanism to explain the effects based on how a magnetic field changes the spin configuration of electron-hole pairs. The spin configuration affects the rate at which electron-hole pairs split apart or recombine, which in turn respectively changes the electrical conductivity and photoluminescence of the perovskite. They dubbed this effect the ‘delta-g mechanism’, with g being a factor that describes the magnetic moment of an electron in the material. Delta-g is the difference between the g-factors of an electron and hole, a difference that becomes crucial in how hybrid perovskite materials perform.

They verified this mechanism by measuring delta-g directly using a technique called field-induced circular polarized emission. It proved to be much larger than delta-g in ordinary , as would be expected if the delta-g mechanism were correct. For further confirmation, the researchers used a spectroscopy technique to measure the fleeting lifetimes – in trillionths of a second – of electron-hole pairs created by light absorption in the hybrid perovskite solar cells. The results also fit the delta-g mechanism.

Answering key questions

The findings point to an answer to a critical question: whether hybrid perovskite devices behave more like silicon solar cells or like so-called excitonic solar cells made of organic polymers. Vardeny said the magnetic field effects nailed down by his group are telling. “This material is not excitonic. If it were, we would not see this effect. It is not like organic photovoltaic materials.”

The efficiency of converting sunlight to electric power has a theoretical limit of 33 percent. The hybrid perovskite photovoltaic devices are pushing 20 percent, not as good as the 26 percent of the best silicon cells, but closing in – and the hybrid perovskites can be produced at a fraction of the cost. The new findings provide more detailed understanding of the underlying physics that should help researchers to fully optimize hybrid perovskite solar cells.

Harnessing solar energy using photovoltaic cells has become more accessible with the addition of the hybrid perovskite ‘miracle materials’, Vardeny says. “This is important since the gasoline price at the pumps would not stay that low forever.”

Explore further: Team develops new technique for growing high-efficiency perovskite solar cells

Perovskites Provide BIG BOOST in Silicon Solar Cells


Perovskite_solar_cell_generic_structureStacking perovskites, a crystalline material, onto a conventional silicon solar cell dramatically improves the overall efficiency of the cell, according to a new study led by Stanford Univ. scientists.

The researchers describe their novel perovskite-silicon solar cell in Energy & Environmental Science.

“We’ve been looking for ways to make solar panels that are more efficient and lower cost,” said study co-author Michael McGehee, a professor of materials science and engineering at Stanford. “Right now, silicon solar cells dominate the world market, but the power conversion efficiency of silicon photovoltaics has been stuck at 25% for 15 years.”

One cost-effective way to improve efficiency is to build a tandem device made of silicon and another inexpensive photovoltaic material, he said.

“Making low-cost tandems is very desirable,” McGehee said. “You simply put one solar cell on top of the other, and you get more efficiency than either could do by itself. From a commercial standpoint, it makes a lot of sense to use silicon for the bottom cell. Until recently, we didn’t have a good material for the top cell, then perovskites came along.”

Perovskite is a crystalline material that is inexpensive and easy to produce in the lab. In 2009, scientists showed that perovskites made of lead, iodide and methylammonium could convert sunlight into electricity with an efficiency of 3.8%. Since then, researchers have achieved perovskite efficiencies above 20%, rivaling commercially available silicon solar cells and spawning widespread interest among silicon manufacturers. Stanford_University_seal_2003_svg

“Our goal is to leverage the silicon factories that already exist around the world,” said Stanford graduate student Colin Bailie, co-lead author of the study. “With tandem solar cells, you don’t need a billion-dollar capital expenditure to build a new factory. Instead, you can start with a silicon module and add a layer of perovskite at relatively low cost.”

Sunlight to electricity


Solar cells work by converting photons of sunlight into an electric current that moves between two electrodes. Silicon solar cells generate electricity by absorbing photons of visible and infrared light, while perovskite cells harvest only the visible part of the solar spectrum where the photons have more energy.

“Absorbing the high-energy part of the spectrum allows perovskite solar cells to generate more power per photon of visible light than silicon cells,” Bailie said.

A key roadblock to building an efficient perovskite-silicon tandem has been a lack of transparency.

“Colin had to figure out how to put a transparent electrode on the top so that some photons could penetrate the perovskite layer and be absorbed by the silicon at the bottom,” McGehee said. “No one had ever made a perovskite solar cell with two transparent electrodes.”

Perovskites are easily damaged by heat and readily dissolve in water. This inherent instability ruled out virtually all of the conventional techniques for applying electrodes onto the perovskite solar cell, so Bailie did it manually.

“We used a sheet of plastic with silver nanowires on it,” he said. “Then we built a tool that uses pressure to transfer the nanowires onto the perovskite cell, kind of like a temporary tattoo. You just need to rub it to transfer the film.”

Remarkable efficiency


For the experiment, the Stanford team stacked a perovskite solar cell with an efficiency of 12.7% on top of a low-quality silicon cell with an efficiency of just 11.4%.

“By combining two cells with approximately the same efficiency, you can get a very large efficiency boost,” Bailie said.

The results were impressive.

“We improved the 11.4% silicon cell to 17% as a tandem, a remarkable relative efficiency increase of nearly 50%,” McGehee said. “Such a drastic improvement in efficiency has the potential to redefine the commercial viability of low-quality silicon.”

In another experiment, the research team replaced the silicon solar cell with a cell made of copper indium gallium diselenide (CIGS). The researchers stacked a 12.7% efficiency perovskite cell onto a CIGS cell with a 17% efficiency. The resulting tandem achieved an overall conversion efficiency of 18.6%.

“Since most, if not all, of the layers in a perovskite cell can be deposited from solution, it might be possible to upgrade conventional solar cells into higher-performing tandems with little increase in cost,” the authors wrote.

A big unanswered question is the long-term stability of perovskites, McGehee added.

“Silicon is a rock,” he said. “You can heat it to about 600 degrees Fahrenheit, shine light on it for 25 years, and nothing will happen. But if you expose perovskite to water or light, it likely will degrade. We have a ways to go to show that perovskite solar cells are stable enough to last 25 years. My vision is that some day we’ll be able to get low-cost tandems that are 25% efficient. That’s what companies are excited about. In five to 10 years, we could even reach 30% efficiency.”

Source: Stanford Univ.

For More In-Depth Reading About Perovskite Go Here:

 Growing perovskite into polymers for easy-processable optoelectronic devices:

http://www.nature.com/srep/2015/150112/srep07725/full/srep07725.html