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


Perovskite GT 190207142218_1_540x360
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.

Structure-of-perovskite-solar-cells-a-Device-architecture-and-b-energy-band-diagram

“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.”

Story Source:

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

Advertisements

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 Hybrid solar cells harness energy from … raindrops?


Renewable energy is the cleanest and inexhaustible source of energy. They are a great alternative to fossil fuels.

Renewable energy doesn’t emit any greenhouse gases in the environment. They are environment-friendly and help us tackle the most important concern of the 21st Century – Climate Change.

Solar is one of the most important forms of renewable energy. Sun is an inexhaustible source of energy and solar cells help capture that clean energy for both commercial and domestic purposes. Despite all these advantages, Solar cells are not efficient when it comes to producing energy during rainy seasons. Since the input energy gets reduced, solar cells become practically useless when rain clouds are overhead.

But what if we could overcome this problem?  What if we could actually generate energy from raindrops?

Scientists from the University of Soochow, China have overcome the design flaw of solar cells by allowing them to generate energy both in the sunny and rainy season.

This technology holds the potential of revolutionizing renewable energy completely.

The key part of this new Hybrid solar technology is the triboelectric nanogenerator or TENG. A device capable of producing an electric charge from the friction of two materials rubbing together.

How Hybrid solar cells work?

These new hybrid solar cells works using a material called Graphene. It has the ability to produce energy from raindrops.

Like any other solar panel, these hybrid solar cells also generate electricity during a normal sunny day using the current technology, but when cloud gathers and raindrop falls, this solar panels system switch to its graphene system.

Graphene, in its liquid form, can produce electricity due to the presence of delocalized electrons that help us create a pseudocapacitor framework. This pseudo framework helps us generate electricity.

When raindrops fall on hybrid solar panels, they get separated as positive ions and negative ions.

These positive ions are mainly salt-related ions, like sodium and calcium which accumulates on the surface of graphene. These positive ions interact with the loosely associated negative ions in graphene and create a system that acts like a pseudocapacitor.

The difference in potential between these ions produces current and voltage.

Although, it is important to mention that this is not a first attempt to invent all-weathered Solar panels. Earlier, researchers created a solar panel with triboelectric nanogenerator on top, an insulating layer in the middle and solar panel at the bottom. But this system possessed too much electrical resistance and sunlight was not able to reach the solar cells due to the opaque nature of insulators.

The newly designed hybrid solar panel is an efficient device, where the triboelectric nanogenerator and the solar panel share a common and transparent electrode. There are special grooves incorporated in the material which increases the efficiency of both raindrops and sunlight captured.

According to the researchers, the idea of special grooves was derived from commercial DVD’s. DVD’s come pre-etched with parallel grooves just hundreds of nanometer across. Designing the device with this grooves helps to boost the surface interaction of raindrops and sunlight that would be otherwise lost to reflection.

Benefits of Solar Hybrid Panels  

Until now solar cells have this drawback of producing energy only in the presence of sunlight, making it impossible to harness energy during the rainy season. Countries in the northern hemisphere were not able to switch to solar energy due to the presence of low-intensity sunlight.

With hybrid solar panels, anyone in the world could harness solar power. Researchers expect that in a few years, these panels will be efficient enough to provide electricity for homes and businesses and thus ending our dependency on fossil fuels.

They will also save a lot of money on daily electricity bills. Even though the initial setup costs are higher, countries with good exposure to both sunlight and rain can expect a good ROI.

Hurdles in Solar hybrid panels    

The current designs are not efficient enough to be used commercially. The device was tested in various simulated weather conditions, in sunlight, the device was able to produce around 13% efficiency and simulated raindrops had an efficiency of around 6%.

Currently used commercial solar cells gives an efficiency of around 15%, thus the new design is a viable option for presently used solar panels. However, the efficiency of triboelectric nanogenerators was not reported.

Conclusion

With continuous depletion of non-renewable sources and the disastrous climate change occurring due to fossil fuels, many countries are moving towards eco-friendly alternatives. Solar energy is one of the cleanest energy available. With the advent of new technology like the hybrid solar panels, we can hope to achieve a viable method of electricity generation.

Researchers are continuously trying to improve the efficiency of hybrid solar cells in order to make it commercially available. This will boost our efforts of producing energy in all-weather condition, which is not possible with the currently available technology. With the expansion of solar energy projects worldwide, researchers of hybrid solar cells are expecting to roll out commercial designs in next five years.

Researchers at china are even trying to integrate this new technology into mobile and electronic device such as electronic clothing.     

How a ‘solar battery’ could bring electricity to rural areas – A ‘solar flow’ battery could “Harvest (energy) in the Daytime and Provide Electricity in the Evening


New solar flow battery with a 14.1 percent efficiency. Photo: David Tenenbaum, UW-Madison

Solar energy is becoming more and more popular as prices drop, yet a home powered by the Sun isn’t free from the grid because solar panels don’t store energy for later. Now, researchers have refined a device that can both harvest and store solar energy, and they hope it will one day bring electricity to rural and underdeveloped areas.

The problem of energy storage has led to many creative solutions, like giant batteries. For a paper published today in the journal Chem, scientists trying to improve the solar cells themselves developed an integrated battery that works in three different ways.

It can work like a normal solar cell by converting sunlight to electricity immediately, explains study author Song Jin, a chemist at the University of Wisconsin at Madison. It can store the solar energy, or it can simply be charged like a normal battery.

“IT COULD HARVEST IN THE DAYTIME, PROVIDE ELECTRICITY IN THE EVENING.”

It’s a combination of two existing technologies: solar cells that harvest light, and a so-called flow battery.

The most commonly used batteries, lithium-ion, store energy in solid materials, like various metals. Flow batteries, on the other hand, store energy in external liquid tanks.

What is A ‘Flow Battery’

This means they are very easy to scale for large projects. Scaling up all the components of a lithium-ion battery might throw off the engineering, but for flow batteries, “you just make the tank bigger,” says Timothy Cook, a University at Buffalo chemist and flow battery expert not involved in the study.

“You really simplify how to make the battery grow in capacity,” he adds. “We’re not making flow batteries to power a cell phone, we’re thinking about buildings or industrial sites.

Jin and his team were the first to combine the two features. They have been working on the battery for years, and have now reached 14.1 percent efficiency.

Jin calls this “round-trip efficiency” — as in, the efficiency from taking that energy, storing it, and discharging it. “We can probably get to 20 percent efficiency in the next few years, and I think 25 percent round-trip is not out of the question,” Jin says.

Apart from improving efficiency, Jin and his team want to develop a better design that can use cheaper materials.

The invention is still at proof-of-concept stage, but he thinks it could have a large impact in less-developed areas without power grids and proper infrastructure. “There, you could have a medium-scale device like this operate by itself,” he says. “It could harvest in the daytime, provide electricity in the evening.” In many areas, Jin adds, having electricity is a game changer, because it can help people be more connected or enable more clinics to be open and therefore improve health care.

And Cook notes that if the solar flow battery can be scaled, it can still be helpful in the US.

The United States might have plenty of power infrastructure, but with such a device, “you can disconnect and have personalized energy where you’re storing and using what you need locally,” he says. And that could help us be less dependent on forms of energy that harm the environment.

NREL, University of Washington Scientists Elevate Quantum Dot Solar Cell World Record to 13.4 Percent



Researchers at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) established a new world efficiency record for quantum dot solar cells, at 13.4 percent.

Colloidal quantum dots are electronic materials and because of their astonishingly small size (typically 3-20 nanometers in dimension) they possess fascinating optical properties. 


Quantum dot solar cells emerged in 2010 as the newest technology on an NREL chart that tracks research efforts to convert sunlight to electricity with increasing efficiency. 

The initial lead sulfide quantum dot solar cells had an efficiency of 2.9 percent. Since then, improvements have pushed that number into double digits for lead sulfide reaching a record of 12 percent set last year by the University of Toronto. 

The improvement from the initial efficiency to the previous record came from better understanding of the connectivity between individual quantum dots, better overall device structures and reducing defects in quantum dots.


 NREL scientists Joey Luther and Erin Sanehira are part of a team that has helped NREL set an efficiency record of 13.4% for a quantum dot solar cell.

The latest development in quantum dot solar cells comes from a completely different quantum dot material. The new quantum dot leader is cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. 

In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.

“This voltage, coupled with the material’s bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell,” said Joseph Luther, a senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. 

The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. 
Tandem cells can deliver a higher efficiency than conventional silicon solar panels that dominate today’s solar market.

This latest advance, titled “Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells,” is published in Science Advances. The paper was co-authored by Erin Sanehira, Ashley Marshall, Jeffrey Christians, Steven Harvey, Peter Ciesielski, Lance Wheeler, Philip Schulz, and Matthew Beard, all from NREL; and Lih Lin from the University of Washington.

The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. 
The quantum dot perovskite materials developed by Luther and the NREL/University of Washington team could be paired with cheap thin-film perovskite materials to achieve similar high efficiency as demonstrated for space solar cells, but built at even lower costs than silicon technology–making them an ideal technology for both terrestrial and space applications.

“Often, the materials used in space and rooftop applications are totally different. It is exciting to see possible configurations that could be used for both situations,” said Erin Sanehira a doctoral student at the University of Washington who conducted research at NREL.

The NREL research was funded by DOE’s Office of Science, while Sanehira and Lin acknowledge a NASA space technology fellowship.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

Breaking through the sunlight-to-electricity conversion limit



Solar-excited “hot” electrons are usually wasted as heat in conventional silicon solar cells. In a new type of solar cell, known as a hybrid organic-inorganic perovskite cell, scientists found these “hot” electrons last longer. These hot electrons have lifetimes more than a 1000 times longer than those formed in silicon cells. 

The rotation of oppositely charged ions plays a key role in protecting “hot” electrons from adverse energy-depleting interactions (Science, “Screening in crystalline liquids protects energetic carriers in hybrid perovskites”).


In the illustration of a perovskite structure, a “hot” electron is located at the center of the image. Positive molecules (red and blue dumbbells) surround the “hot” electron. The distortion of the crystal structure and the liquid-like environment of the positive molecules (blurred dumbbells at the periphery of the image) screen (yellow circle, partially shown) the “hot” electron. The “shield” protects the hot electron and allows it to survive 1000 times longer than it would in conventional silicon solar cells. (Image: Xiaoyang Zhu, Columbia University)

This research identified a possible route to dramatically increase the efficiency of solar cells. By slowing the cooling of excited “hot” electrons, scientists could produce more electricity. They could devise cells that function above the predicted efficiency limit, around 33 percent, for conventional solar cells.

Hybrid organic-inorganic lead halide perovskites (HOIP) are promising new materials for use in low-cost solar cells. HOIPs have already been demonstrated in solar cells with solar-to-electricity conversion efficiency exceeding 20 percent, which is on par with the best crystalline silicon solar cells.

Research is ongoing to discover why HOIPs work so well for solar energy harvesting and to determine their efficiency limit. A team led by Columbia University has discovered that electrons in HOIPs acquire protective shields that make them nearly invisible to defects and other electrons, which allows the electrons to avoid losing energy. The mechanism of protection is dynamic screening correlated with liquid-like molecular motions in the crystal structure.

Moreover, the researchers discovered that the protection mechanism works for electrons with excess energy (with energy greater than the semiconductor band gap); as a result, these so-called “hot” electrons are very long-lived in HOIPs. In a conventional solar cell, such as the silicon cell widely in use today, only part of the solar spectrum is used, and the energy of the “hot” electrons is wasted. Excess electron energy generated initially from the absorption of high-energy photons in the solar spectrum is lost as heat before the electron is harvested for electricity production.

For conventional solar cells, this loss is partially responsible for the theoretical efficiency limit of around 33 percent, called the Shockley-Queisser limit. However, the long lifetime of “hot” electrons in HOIPs makes it possible to harvest the “hot” electrons to produce electricity, thus increasing the efficiency of HOIP solar cells beyond the conventional limit.

Source: U.S. Department of Energy, Office of Science

Better photovoltaic efficiency grows from enormous solar crystals: MH Perovskites 


In-depth analysis of the mechanisms that generate floating crystals from hot liquids could lead to large-scale, printable solar cells


New evidence of surface-initiated crystallization may improve the efficiency of printable photovoltaic materials.

In the race to replace silicon in low-cost solar cells, semiconductors known as metal halide perovskites are favored because they can be solution-processed into thin films with excellent photovoltaic efficiency. 

A collaboration between King Abdullah University of Science and Technology (KAUST) and Oxford University researchers has now uncovered a strategy that grows perovskites into centimeter-scale, highly pure crystals thanks to the effect of surface tension (ACS Energy Letters, “The role of surface tension in the crystallization of metal halide perovskites”).

In their natural state, perovskites have difficultly moving solar-generated electricity because they crystallize with randomly oriented grains. 

Osman Bakr from KAUST’s Solar Center and coworkers are working on ways to dramatically speed up the flow of these charge carriers using inverse temperature crystallization (ITC). This technique uses special organic liquids and thermal energy to force perovskites to solidify into structures resembling single crystals—the optimal arrangements for device purposes.

While ITC produces high-quality perovskites far faster than conventional chemical methods, the curious mechanisms that initiate crystallization in hot organic liquids are poorly understood. Ayan Zhumekenov, a PhD student in Bakr’s group, recalls spotting a key piece of evidence during efforts to adapt ITC toward large-scale manufacturing. “At some point, we realized that when crystals appeared, it was usually at the solution’s surface,” he says. “And this was particularly true when we used concentrated solutions.”

The KAUST team partnered with Oxford theoreticians to identify how interfaces influence perovskite growth in ITC. They propose that metal halides and solvent molecules initially cling together in tight complexes that begin to stretch and weaken at higher temperatures. With sufficient thermal energy, the complex breaks and perovskites begin to crystallize.

But interestingly, the researchers found that complexes located at the solution surface can experience additional forces due to surface tension—the strong cohesive forces that enable certain insects to stride over lakes and ponds. The extra pull provided by the surface makes it much easier to separate the solvent-perovskite complexes and nucleate crystals that float on top of the liquid.

Exploiting this knowledge helped the team produce centimeter-sized, ultrathin single crystals and prototype a photodetector with characteristics comparable to state-of-the-art devices. Although the single crystals are currently fragile and difficult to handle due to their microscale thicknesses, Zhumekenov explains that this method could help direct the perovskite growth onto specific substrates.

“Taking into account the roles of interfaces and surface tension could have a fundamental impact,” he says, “we can get large-area growth, and it’s not limited to specific metal cations—you could have a library of materials with perovskite structures.”

Source: King Abdullah University of Science and Technology

NREL, Swiss Scientists Power Past Solar Efficiency Records


NREL scientist Adele Tamboli, co-author of a recent article on silicon-based multijunction solar cells, stands in front of an array of solar panels. Credit: Dennis Schroeder

August 25, 2017




Second collaborative effort proves silicon-based multijunction cells that reach nearly 36% efficiency

Collaboration between researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), the Swiss Center for Electronics and Microtechnology (CSEM), and the École Polytechnique Fédérale de Lausanne (EPFL) shows the high potential of silicon-based multijunction solar cells.

The research groups created tandem solar cells with record efficiencies of converting sunlight into electricity under 1-sun illumination. The resulting paper, “Raising the One-Sun Conversion Efficiency of III–V/Si Solar Cells to 32.8% for Two Junctions and 35.9% for Three Junctions,” appears in the new issue of Nature Energy. Solar cells made solely from materials in Groups III and V of the Periodic Table have shown high efficiencies, but are more expensive.

Stephanie Essig, a former NREL post-doctoral researcher now working at EPFL in Switzerland, is lead author of the newly published research that details the steps taken to improve the efficiency of the multijunction cell. While at NREL, Essig co-authored “Realization of GaInP/Si Dual-Junction Solar Cells with 29.8% 1-Sun Efficiency,” which was published in the IEEE Journal of Photovoltaics a year ago.

In addition to Essig, authors of the new research paper are Timothy Remo, John F. Geisz, Myles A. Steiner, David L. Young, Kelsey Horowitz, Michael Woodhouse, and Adele Tamboli, all with NREL; and Christophe Allebe, Loris Barraud, Antoine Descoeudres, Matthieu Despeisse, and Christophe Ballif, all from CSEM.

“This achievement is significant because it shows, for the first time, that silicon-based tandem cells can provide efficiencies competing with more expensive multijunction cells consisting entirely of III-V materials,” Tamboli said. “It opens the door to develop entirely new multijunction solar cell materials and architectures.”

In testing silicon-based multijunction solar cells, the researchers found that the highest dual-junction efficiency (32.8%) came from a tandem cell that stacked a layer of gallium arsenide (GaAs) developed by NREL atop a film of crystalline silicon developed by CSEM. An efficiency of 32.5% was achieved using a gallium indium phosphide (GaInP) top cell, which is a similar structure to the previous record efficiency of 29.8% announced in January 2016. 

A third cell, consisting of a GaInP/GaAs tandem cell stacked on a silicon bottom cell, reached a triple-junction efficiency of 35.9%—just 2% below the overall triple-junction record.

The existing photovoltaics market is dominated by modules made of single-junction silicon solar cells, with efficiencies between 17% and 24%. 

The researchers noted in the report that making the transition from a silicon single-junction cell to a silicon-based dual-junction solar cell will enable manufacturers to push efficiencies past 30% while still benefiting from their expertise in making silicon solar cells.

The obstacle to the adoption of these multijunction silicon-based solar cells, at least in the near term, is the cost. Assuming 30% efficiency, the researchers estimated the GaInP-based cell would cost $4.85 per watt and the GaAs-based cell would cost $7.15 per watt. 

But as manufacturing ramps up and the efficiencies of these types of cells climbs to 35%, the researchers predict the cost per watt could fall to 66 cents for a GaInP-based cell and to 85 cents for the GaAs-based cell. 

The scientists noted that such a precipitous price drop is not unprecedented; for instance, the cost of Chinese-made photovoltaic modules fell from $4.50 per watt in 2006 to $1 per watt in 2011.

The cost of a solar module in the United States accounts for 20% to 40% of the price of a photovoltaic system. Increasing cell efficiency to 35%, the researchers estimated, could reduce the system cost by as much as 45 cents per watt for commercial installations. 

However, if the costs of a III-V cell cannot be reduced to the levels of the researchers’ long-term scenario, then the use of cheaper, high-efficiency materials for the top cell will be needed to make them cost-competitive in general power markets.

The funding for the research came from the Energy Department’s SunShot Initiative—which aims to make solar energy a low-cost electricity source for all Americans through research and development efforts in collaboration with public and private partners—and from the Swiss Confederation and the Nano-Tera.ch initiative.


NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

Harnessing the Functionality and ‘Power’ of Perovskites for Better Solar and LED’s



Originally a mineral, the perovskite used in today’s technology is quite different from the rock found in the Earth mantle. 

A “perovskite structure” uses a different combination of atoms but keep the general 3-dimensional structure originally observed in the mineral, which possesses superb optoelectronic properties such as strong light absorption and facilitated charge transport. These advantages qualify the perovskite structure as particularly suited for the design of electronic devices, from solar cells to lights.

The accelerating progress in perovskite technology over the past few years suggest new perovskite-based devices will soon outperform current technology in the energy sector. 

The Energy Materials and Surface Sciences Unit at OIST led by Prof. Yabing Qi is at the forefront of this development, with now two new scientific publications focusing on the improvement of perovskite solar cells and a cheaper and smarter way to produce emerging perovskite-based LED lights.

An extra layer in a solar cell “sandwich”

Perovskite-based solar cells is a rising technology forecast to replace the classic photovoltaic cells currently dominating the industry. 




In just seven years of development, the efficiency of perovskite solar cells increased to almost rival – and is expected to soon overtake – commercial photovoltaic cells, but the perovskite structure still plagued by a short lifespan due to stability issues. 


OIST
scientists have made constant baby steps in improving the cells stability, identifying the degradations factors and providing solutions towards better solar cell architecture.

The new finding, reported in the Journal of Physical Chemistry B (“Engineering Interface Structure to Improve Efficiency and Stability of Organometal Halide Perovskite Solar Cells”), suggests interactions between components of the solar cell itself are responsible for the rapid degradation of the device. 

More precisely, the titanium oxide layer extracting electrons made available through solar energy – effectively creating an electric current – causes unwanted deterioration of the neighboring perovskite layer. 

Imagine the solar cell as a multi-layered club sandwich: if not properly assembled, fresh and juicy vegetables in contact with the bread slices will make the bread very soggy in a matter of hours. 

But if you add a layer of ham or turkey between the vegetables and the bread, then your sandwich stays crisp all day in the lunchroom refrigerator.


A perovskite-based layer includes many layers, including for example the electrodes on both sides, and the perovskite in the middle. The addition of a polystyrene layer in-between prevents the titanium oxide layer to deteriorate the perovskite, but does not affect the overall power conversion efficiency. (© American Chemical Society)

This is exactly what the OIST researchers achieved: they inserted in the solar cell an additional layer made from a polymer to prevent direct contact between the titanium oxide and the perovskite layers. 
This polymer layer is insulating but very thin, which means it lets the electron current tunnel through yet does not diminish the overall efficiency of the solar cell, while efficiently protecting the perovskite structure.

“We added a very thin sheet, only a few nanometers wide, of polystyrene between the perovskite layer and the titanium oxide layer,” explained Dr. Longbin Qiu. 

“Electrons can still tunnel cross this new layer and it does not affect the light absorption of the cell. This way, we were able to extend the lifetime of the cell four-fold without loss in energy conversion efficiency”.

The lifespan of the new perovskite device was extended to over 250 hours – still not enough to compete with commercial photovoltaic cells regarding stability, but an important step forward toward fully functional perovskite solar cells.

Manufacturing LED lights from gasses

The bipolar electronic properties of the perovskite structure not only confer them the ability to generate electricity from solar energy but also can convert electricity into vivid light. Light-Emitting Diode – LED – technology, omnipresent in our daily life from laptop and smartphone screens to car lights and ceiling tubes, currently relies on semi-conductors that are difficult and expensive to manufacture. Perovskite LEDs are envisaged to become the new industry standard in the near future due to the lower cost and their efficiency to convert power into light. Moreover, by changing the atomic composition in the perovskite structure, perovskite LED can be easily tuned to emit specific colors.

The manufacturing of these perovskite LEDs is currently based on dipping or covering the targeted surface with liquid chemicals, a process which is difficult to setup, limited to small areas and with low consistency between samples. To overcome this issue, OIST researchers reported in the Journal of Physical Chemistry Letters (“Methylammonium Lead Bromide Perovskite Light-Emitting Diodes by Chemical Vapor Deposition”) the first perovskite LED assembled with gasses, a process called chemical vapor deposition or CVD.

“Chemical vapor deposition is already compatible with the industry, so in principle it would be easy to use this technology to produce LEDs,” commented Prof. Yabing Qi. “The second advantage in using CVD is a much lower variation from batch to batch compared to liquid-based techniques. Finally, the last point is scalability: CVD can achieve a uniform surface over very large areas”.

Like the solar cell, the perovskite LED also comprises many layers working in synergy. First, an indium tin oxide glass sheet and a polymer layer allow electrons into the LED. The chemicals required for the perovskite layer – lead bromide and methylammonium bromide – are then successively bound to the sample using CVD, in which the sample is exposed to gasses in order to convert to perovskite instead of typically solution-coating processes with liquid. In this process, the perovskite layer is composed of nanometer-small grains, whose sizes play a critical role in the efficiency of the device. Finally, the last step involves the deposition of two additional layers and a gold electrode, forming a complete LED structure. The LED can even form specific patterns using lithography during the manufacturing process.

Perovskite LED fabrication


Top: the perovskite LED sits in a furnace, where the Methylammonium Bromide (MABr) in gaseous form will be introduced into the system and deposit on the LED surface. Bottom left: a glass-based LED, glowing green when electricity is applied. Bottom right: size and shapes of the perovskite grains on the surface of the LED. (© American Chemical Society)

“With large grains, the surface of the LED is rough and less efficient in emitting light. The smaller the grain size, the higher the efficiency and the brighter the light,” explained Dr. Lingqiang Meng. “By changing the assembly temperature, we can now control the growth process and the size of the grains for the best efficiency”.

Controlling the grain size is not the only challenge for this first-of-its-kind assembling technique of LED lights.

“Perovskite is great, but the choice in the adjacent layers is really important too,” added Dr. Luis K. Ono. “To achieve high electricity-to-light conversion rates, every layer should be working in harmony with the others.”

The result is a flexible, thick film-like LED with a customizable pattern. The luminance, or brightness, currently reaches 560 cd/m2, while a typical computer screen emits 100 to 1000 cd/m2 and a ceiling fluorescent tube around 12,000 cd/m2.

Perovskite-LED


This large perovskite-LED was produced using chemical vapor deposition and connect to a 5V current, illuminating through an OIST pattern etched on the surface. (© American Chemical Society)

“Our next step is to improve the luminance a thousand-fold or more,” concluded Dr. Meng. “In addition, we have achieved a CVD-based LED emitting green light but we are now trying to repeat the process with different combinations of perovskite to obtain a vivid blue or red light”.

Source: By Wilko Duprez, Okinawa Institute of Technology

MIT team creates flexible, transparent solar cells with graphene electrodes



Researchers at the Massachusetts Institute of Technology (MIT) have developed flexible and transparent graphene-based solar cells, which can be mounted on various surfaces ranging from glass to plastic to paper and tape. The graphene devices exhibited optical transmittance of 61% across the whole visible regime and up to 69% at 550 nanometers. The power conversion efficiency of the graphene solar cells ranged from 2.8% to 4.1%.

MIT team’s flexible, transparent solar cell with graphene electrodes image

A common challenge in making transparent solar cells with graphene is getting the two electrodes to stick together and to the substrate, as well as ensuring that electrons only flow out of one of the graphene layers. Using heat or glue can damage the material and reduce its conductivity, so the MIT team developed a new technique to tackle this issue. Rather than applying an adhesive between the graphene and the substrate, they sprayed a thin layer of ethylene-vinyl acetate (EVA) over the top, sticking them together like tape instead of glue.

The MIT team compared their graphene electrode solar cells against others made from standard materials like aluminum and indium tin oxide (ITO), built on rigid glass and flexible substrates. The power conversion efficiency (PCE) of the graphene solar cells was far lower than regular solar panels, but much better than previous transparent solar cells. This is a positive advancement, obviously.
Samples of solar cells using electrodes of different materials for testing image


Efficiency is often a trade-off from the graphene solar cells being flexible and transparent. In that regard the cells performed well, transmitting almost 70% of the light in the middle of the human range of vision. Hopefully the numbers will continue to improve. According to the researchers’ calculations, the efficiency of these graphene solar cells could be pushed as high as 10% without losing any transparency, and doing just that is the next step in the project. The researchers are also working on ways to scale up the system to cover windows and walls.
Source:  newatlas