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 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. 

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.


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

EPPL Creates a low-cost system for splitting carbon dioxide – Turning Renewable Energy into Fuel

Ball-and-stick model of carbon dioxide. Credit: Wikipedia

Using Earth-abundant materials, EPFL scientists have built the first low-cost system for splitting CO2 into CO, a reaction necessary for turning renewable energy into fuel.
The future of clean energy depends on our ability to efficiently store energy from renewable sources and use it later. 

A popular way to do this is to electrolyze carbon dioxide to carbon monoxide, which is then mixed with hydrogen to produce liquid hydrocarbons like gasoline or kerosene that can be used as fuel. 

However, we currently lack efficient and Earth-abundant catalysts for the initial splitting of CO2 into CO and oxygen, which makes the move into renewable energy expensive and prohibitive. 

EPFL scientists have now developed an Earth-abundant catalyst based on copper-oxide nanowires modified with tin oxide. 
The system can split CO2 with an efficiency of 13.4%. 
The work is published in Nature Energy, and can help worldwide efforts to synthetically produce carbon-based fuels from CO2 and water.

The research was carried out by the lab of Michael Grätzel at EPFL. Grätzel is known worldwide for the invention of the first ever dye-sensitized solar cells (or “Grätzel cells”). 

The catalyst, developed by PhD student Marcel Schreier and postdoc Jingshan Luo, is made by depositing an atomic layer of tin oxide on copper oxide nanowires

By using such Earth-abundant materials, the design keeps the cost of the catalyst low while significantly increasing the yield of CO, as opposed to the other products that are generated from CO2 electrocatalysis.
The catalyst was integrated into a CO2 electrolysis system and linked to a triple-junction solar cell (GaInP/GaInAs/Ge) to make a CO2 electrolyzer. 

The system uses the catalyst as a bifunctional electrode that both reduces CO2 into CO and produces oxygen through what is known as the “oxygen evolution” reaction. The two products are separated with a bipolar membrane.

Using solar energy, the system was able to selectively convert CO2 to CO with an efficiency of 13.4%, and do so with a Faradaic efficiency up to 90%—this describes how efficiently electrical charge is transferred in a electrocatalysis system like the one developed here. “The work sets a new benchmark for solar-driven CO2 reduction,” says Luo.

“This is the first time that such a bi-functional and low-cost catalyst is demonstrated,” adds Schreier. “Very few catalysts—except expensive ones, like gold and silver—can selectively transform CO2 to CO in water, which is crucial for industrial applications.”

More information: Marcel Schreier, Florent Héroguel, Ludmilla Steier, Shahzada Ahmad, Jeremy S. Luterbacher, Matthew T. Mayer, Jingshan Luo, Michael Grätzel. Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO. Nature Energy 2, 17087 (05 June 2017).
Provided by: Ecole Polytechnique Federale de Lausanne

U of Minnesota: Discovery of new transparent thin film material – Less Costly than Indium – Could lead to smaller, faster, more powerful electronics, improve solar cells

U of Minn ThinFilm Solar 5-discoveryofnA team of researchers, led by the University of Minnesota, have discovered a new nano-scale thin film material with the highest-ever conductivity in its class.  Credit: University of Minnesota

A team of researchers, led by the University of Minnesota, have discovered a new nano-scale thin film material with the highest-ever conductivity in its class. The new material could lead to smaller, faster, and more powerful electronics, as well as more efficient solar cells.

The discovery is being published today in Nature Communications, an open access journal that publishes high-quality research from all areas of the natural sciences.

Researchers say that what makes this new material so unique is that it has a high conductivity, which helps electronics conduct more electricity and become more powerful. But the material also has a wide bandgap, which means light can easily pass through the material making it optically transparent. In most cases, materials with wide bandgap, usually have either low conductivity or poor transparency.

“The high conductivity and wide bandgap make this an ideal material for making optically transparent conducting films which could be used in a wide variety of electronic devices, including , electronic displays, touchscreens and even in which light needs to pass through the device,” said Bharat Jalan, a University of Minnesota chemical engineering and materials science professor and the lead researcher on the study.

Currently, most of the in our electronics use a chemical element called indium. The price of indium has gone up tremendously in the past few years significantly adding to the cost of current display technology. As a result, there has been tremendous effort to find alternative materials that work as well, or even better, than indium-based transparent conductors.

In this study, researchers found a solution. They developed a new transparent conducting thin film using a novel synthesis method, in which they grew a BaSnO3 thin film (a combination of barium, tin and oxygen, called barium stannate), but replaced elemental tin source with a chemical precursor of tin. The chemical precursor of tin has unique, radical properties that enhanced the chemical reactivity and greatly improved the metal oxide formation process. Both barium and tin are significantly cheaper than indium and are abundantly available.

“We were quite surprised at how well this unconventional approach worked the very first time we used the tin chemical precursor,” said University of Minnesota engineering and materials science graduate student Abhinav Prakash, the first author of the paper. “It was a big risk, but it was quite a big breakthrough for us.”

Jalan and Prakash said this new process allowed them to create this material with unprecedented control over thickness, composition, and defect concentration and that this process should be highly suitable for a number of other material systems where the element is hard to oxidize. The new process is also reproducible and scalable.

They further added that it was the structurally superior quality with improved defect concentration that allowed them to discover high conductivity in the material. They said the next step is to continue to reduce the defects at the atomic scale.

“Even though this material has the highest within the same class, there is much room for improvement in addition, to the outstanding potential for discovering new physics if we decrease the defects. That’s our next goal,” Jalan said.

Explore further: See-through circuitry: New and cheap alternative for transparent electronics

More information: Abhinav Prakash et al, Wide bandgap BaSnO3 films with room temperature conductivity exceeding 104 S cm−1, Nature Communications (2017). DOI: 10.1038/ncomms15167


Los Alamos National Laboratory Studies Perovskites for Efficient Optoelectronics: Video

Los Alamos III 13785853973_eee18af4fc_b

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:…

And the research paper in Science:…

What is up with the U.S. ‘Solar Industry’? Is There and Impending US Solar Energy Crash?

Solar Crash I solar-and-wind-energy

After nice stretch of sunny weather, the last few months have clouded over for big solar. Declining prices for photovoltaic cells are hurting panel manufacturers and stressing solar installation businesses.

This situation was in sharp relief this week in Tesla’s (TSLA Tesla Motors Inc TSLA 307.19 -0.38%) earnings, as its solar installation business, SolarCity, disclosed a big slowdown in builds. SolarCity commands 41 percent of the residential solar installation market, according to GTM. In its latest earnings, the firm revealed that it had installed 150 MW of panels in the first quarter, down nearly 39 percent y/y.

“Rather than prioritizing the growth of MW of solar deployed at any cost, we are selectively deploying projects that have higher margin and generate cash up front. Consequently, solar energy generation deployments in Q1 2017 declined year-over-year, but had better financial results,” said the earnings release.

The Curious Logic of the Solar Market

Industry body Solar Energy Industries Association (SEIA) reports that installations for the past year actually went up. In 2016, the U.S. saw 14.8GW solar capacity installed with a new installation taking place every 84 seconds.

There are companies that are doing well. First Solar (FSLR First Solar In FSLR 35.15 +1.77%) just reported strong earnings while Vivint Solar (VSLR Vivint Solar Inc VSLR 3.00+1.70%) announced is expansion into Rhode Island and is expected to announce financial results next week. However, the list of struggling companies in the sector is longer.

SunPower Corp. (SPWR) reported its sixth consecutive quarter of losses and laid off 25 percent of its workforce. Verengo Solar filed for bankruptcy last year, while Sungevity and Suninva did the same earlier this year.

But if solar energy is seeing such high demand, why are the companies feeling the heat?

The Price Is Not Right

The cost of the production and installation of solar panels has dropped dramatically and that is driving demand. According to SEIA, the cost to install solar capacity dropped 29 percent in the final quarter of 2016, compared to the same period last year. Over the past 10 years, installation costs have come down by nearly 60 percent.

There is more than one reason for price suppression in the solar industry.

“Driving the cost reductions were lower module and inverter prices, increased competition, lower installer and developer overheads, improved labor productivity, and optimized system configurations,” a National Renewable Energy Laboratory report states.

At home, the government tried to promote solar energy to consumers by making it affordable. One such initiative was the Solar Investment Tax Credit for residential and business solar installations, adopted in 2006 and extended in 2015.

In the international arena, U.S. solar companies blame declining panel prices on foreign imports, especially from countries like China, Mexico and Canada. Suniva recently implored President Trump for protectionist policies for the sector.

However, as the big ones struggled, someone made hay as the sun shone. According to GTM research’s U.S. Residential Solar Update 2017, many of the larger firms struggled to do well while smaller, local companies thrived.

More Insights: Investopedia

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

NREL & Colorado School of Mines Researchers Capture Excess Photon Energy to Produce Solar Fuels

Photo shows a lead sulfide quantum dot solar cell. A lead sulfide quantum dot solar cell developed by researchers at NREL. Photo by Dennis Schroeder.

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a proof-of-principle photoelectrochemical cell capable of capturing excess photon energy normally lost to generating heat.

Using quantum dots (QD) and a process called Multiple Exciton Generation (MEG), the NREL researchers
were able to push the peak external quantum efficiency for hydrogen generation to 114 percent.

The advancement could significantly boost the production of hydrogen from sunlight by using the cell to split water at a higher efficiency and lower cost than current photoelectrochemical approaches.

Details of the research are outlined in the Nature Energy paper Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%, co-authored by Matthew Beard, Yong Yan, Ryan Crisp, Jing Gu, Boris Chernomordik, Gregory Pach, Ashley Marshall, and John Turner.

All are from NREL; Crisp also is affiliated with the Colorado School of Mines, and Pach and Marshall are affiliated with the University of Colorado, Boulder.

Beard and other NREL scientists in 2011 published a paper in Science that showed for the first time how MEG allowed a solar cell to exceed 100 percent quantum efficiency by producing more electrons in the electrical current than the amount of photons entering the solar cell.

“The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current,” Beard said.

“We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds.”

The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption bandedge lost to heat.

The MEG process takes advantages of the additional photon energy to generate more electrons and thus additional chemical or electrical potential, rather than generating heat. QDs, which are spherical semiconductor nanocrystals (2-10 nm in diameter), enhance the MEG process.

In current report, the multiple electrons, or charge carriers, that are generated through the MEG process within the QDs are captured and stored within the chemical bonds of a H2 molecule.

NREL researchers devised a cell based upon a lead sulfide (PbS) QD photoanode. The photoanode involves a layer of PbS quantum dots deposited on top of a titanium dioxide/fluorine-doped tin oxide dielectric stack.

The chemical reaction driven by the extra electrons demonstrated a new direction in exploring high-efficiency approaches for solar fuels.

Funds for the research came from the Department of Energy’s Office of Science.

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.