Researchers invent new triple-junction tandem solar cells with world-record efficiency

Scientists from the National University of Singapore (NUS) have developed a novel triple-junction perovskite/Si tandem solar cell that can achieve a certified world-record power conversion efficiency of 27.1 per cent across a solar energy absorption area of 1 sq cm, representing the best-performing triple-junction perovskite/Si tandem solar cell thus far. To achieve this, the team engineered a new cyanate-integrated perovskite solar cell that is stable and energy efficient.

Solar cells can be fabricated in more than two layers and assembled to form multi-junction solar cells to increase efficiency. Each layer is made of different photovoltaic materials and absorbs solar energy within a different range. However, current multi-junction solar cell technologies pose many issues, such as energy loss which leads to low voltage and instability of the device during operation.

To overcome these challenges, Assistant Professor Hou Yi led a team of scientists from NUS College of Design and Engineering (CDE) and Solar Energy Research Institute of Singapore (SERIS) to demonstrate, for the first time, the successful integration of cyanate into a perovskite solar cell to develop a cutting-edge triple-junction perovskite/Si tandem solar cell that surpasses the performance of other similar multi-junction solar cells. Asst Prof Hou is a Presidential Young Professor at the Department of Chemical and Biomolecular Engineering under CDE as well as a Group Leader at SERIS, a university-level research institute in NUS.

"Remarkably, after 15 years of ongoing research in the field of perovskite-based solar cells, this work constitutes the first experimental evidence for the inclusion of cyanate into perovskites to boost the stability of its structure and improve power conversion efficiency," said Asst Prof Hou.

The experimental process that led to this ground-breaking discovery was published in Nature on 4 March 2024.

Fabricating energy-efficient solar cell technology

The interactions between the components of the perovskite structure determine the energy range that it can reach. Adjusting the proportion of these components or finding a direct substitute can help modify the perovskite's energy range. However, prior research has yet to produce a perovskite recipe with an ultrawide energy range and high efficiency.

In this recently published work, the NUS team experimented on cyanate, a novel pseudohalide, as a substitute for bromide -- an ion from the halide group that is commonly used in perovskites. Dr Liu Shunchang, Research Fellow in Asst Prof Hou's team, employed various analytical methods to confirm the successful integration of cyanate into the perovskite structure, and fabricated a cyanate-integrated perovskite solar cell.

Further analysis of the new perovskite's atomic structure provided -- for the first time -- experimental evidence that incorporating cyanate helped to stabilise its structure and form key interactions within the perovskite, demonstrating how it is a viable substitute for halides in perovskite-based solar cells.

When assessing performance, the NUS scientists found that perovskite solar cells incorporated with cyanate can achieve a higher voltage of 1.422 volts compared to 1.357 volts for conventional perovskite solar cells, with a significant reduction in energy loss.

The researchers also tested the newly engineered perovskite solar cell by continuously operating it at maximum power for 300 hours under controlled conditions. After the test period, the solar cell remained stable and functioned above 96 per cent capacity.

Encouraged by the impressive performance of the cyanate-integrated perovskite solar cells, the NUS team took their ground-breaking discovery to the next step by using it to assemble a triple-junction perovskite/Si tandem solar cell. The researchers stacked a perovskite solar cell and a silicon solar cell to create a dual-junction half-cell, providing an ideal base for the attachment of the cyanate-integrated perovskite solar cell.

Once assembled, the researchers demonstrated that despite the complexity of the triple-junction perovskite/Si tandem solar cell structure, it remained stable and attained a certified world-record efficiency of 27.1 per cent from an accredited independent photovoltaic calibration laboratory.

"Collectively, these advancements offer ground-breaking insights into mitigating energy loss in perovskite solar cells and set a new course for the further development of perovskite-based triple junction solar technology," said Asst Prof Hou.

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Record 19.31% efficiency with organic solar cells

Researchers from The Hong Kong Polytechnic University (PolyU) have achieved a breakthrough power-conversion efficiency (PCE) of 19.31% with organic solar cells (OSCs), also known as polymer solar cells. This remarkable binary OSC efficiency will help enhance applications of these advanced solar energy devices.

The PCE (Power-conversion efficiency), a measure of the power generated from a given solar irradiation, is considered a significant benchmark for the performance of photovoltaics (PVs), or solar panels, in power generation. The improved efficiency of over 19% that was achieved by the PolyU researchers constitutes a record for binary OSCs, which have one donor and one acceptor in the photo-active layer.

Led by Prof. LI Gang, Chair Professor of Energy Conversion Technology and Sir Sze-Yen Chung Endowed Professor in Renewable Energy at PolyU, the research team invented a novel OSC morphology-regulating technique by using 1,3,5-trichlorobenzene as a crystallisation regulator. This new technique boosts OSC efficiency and stability.

The team developed a non-monotonic intermediated state manipulation (ISM) strategy to manipulate the bulk-heterojunction (BHJ) OSC morphology, which simultaneously optimises crystallisation dynamics and energy loss of non-fullerene OSCs. Unlike the strategy of using traditional solvent additives, which is based on excessive molecular aggregation in films, the ISM strategy promotes the formation of more ordered molecular stacking and favourable molecular aggregation. As a result, the PCE was considerably increased and the undesirable non-radiative recombination loss was reduced. Notably, non-radiative recombination lowers the light generation efficiency and increases the heat loss.

The research team's findings are described in the study "19.3% Binary Organic Solar Cell and Low Non-Radiative Recombination Enabled by Non-Monotonic Intermediate State Transition" published in Nature Communications . The conversion of solar energy to electricity is an essential technology for achieving a sustainable environment. Although OSCs are promising devices that harness solar energy cost-effectively, their efficiency must be improved if they are to be used widely in practical applications.

Non-fullerene acceptors based organic solar cells represent the frontier of research in the field of organic photovoltaics due to both the materials and morphology manipulation innovations. Nevertheless, non-radiative recombination loss suppress and performance boosting are in the centre of organic cell research.

Prof. Li said, "Challenges in research came from the existing additive-based benchmark morphology control methods, which suffer from non-radiative recombination loss, thus lowering the open-circuit voltage due to excessive aggregation." The research team took about two years to devise a non-monotonic ISM strategy for increasing the OSC efficiency and lowering the non-radiative recombination loss. The publication of the study promises to galvanise OSC research.

Prof. Li said, "The new finding will make OSC research an exciting field, and this will likely create tremendous opportunities in applications like portable electronics and building-integrated PVs." The new door will open when low cost single-junction OSCs can achieve a PCE of over 20%, along with more stable performance and other unique advantages such as flexibility, transparency, stretchability, low weight and tuneable colour.

Prof. Li has been recognised as a Highly Cited Researcher 9 years in a row since 2014, which testifies to his significant impact on global research. His pioneering contributions to research on polymer solar cells since 2005 have brought sustainable influence on printable solar energy development with global recognition.

Underpinning the research on OPV field, Prof LI's study titled, "High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends," was published on Nature Materials in 2005. This represented OPV's first generation research breakthrough which has fuelled solar technology from this frontier study.

In 2010, Prof LI's study titled "For the Bright Future -- Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%" was published on Advanced Materials.

Prof. Li said, "The latest study shows a record low non-radiative recombination loss of 0.168 eV in a binary OSC with a PCE of over 19%. This is a very encouraging result for the long-standing research on OSCs that I have conducted over the past two decades. We have already achieved better OSC efficiency, and this will subsequently help accelerate the applications of solar energy."

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Why do Earth's hemispheres look equally bright when viewed from space?

When seen from space, Earth's hemispheres -- northern and southern -- appear equally bright. This is particularly unexpected because the Southern Hemisphere is mostly covered with dark oceans, whereas the Northern Hemisphere has a vast land area that is much brighter than these oceans. For years, the brightness symmetry between hemispheres remained a mystery. In a new study, published in the Proceedings of the National Academy of Sciences (PNAS), Weizmann Institute of Science researchers and their collaborators reveal a strong correlation between storm intensity, cloudiness and the solar energy reflection rate in each hemisphere. They offer a solution to the mystery, alongside an assessment of how climate change might alter the reflection rate in the future.

As early as the 1970s, when scientists analyzed data from the first meteorological satellites, they were surprised to find out that the two hemispheres reflect the same amount of solar radiation. Reflectivity of solar radiation is known in scientific lingo as "albedo." To better comprehend what albedo is, think about driving at night: It is easy to spot the intermittent white lines, which reflect light from the car's headlights well, but difficult to discern the dark asphalt. The same is true when observing Earth from space: The ratio of the solar energy hitting the Earth to the energy reflected by each region is determined by various factors. One of them is the ratio of dark oceans to bright land, which differ in reflectivity, just like asphalt and intermittent white lines. The land area of the Northern Hemisphere is about twice as large as that of the Southern, and indeed when measuring near the surface of the Earth, when the skies are clear, there is more than a 10 percent difference in albedo. Still, both hemispheres appear to be equally bright from space.

In this study, the team of researchers, led by Prof. Yohai Kaspi and Or Hadas of Weizmann's Earth and Planetary Sciences Department, focused on another factor influencing albedo, one located in high altitudes and reflecting solar radiation -- clouds. The team analyzed data derived from the world's most advanced databases, including cloud data collected via NASA satellites (CERES), as well as data from ERA5, which is a global weather database containing information collected using a variety of sources in the air and on the ground, dating back to 1950. ERA5 data was utilized to complete cloud data and to cross-correlate 50 years of this data with information on the intensity of cyclones and anticyclones.

Next, the scientists classified storms of the last 50 years into three categories, according to intensity. They discovered a direct link between storm intensity and the number of clouds forming around the storm. While Northern Hemisphere and land areas in general are characterized by weaker storms, above oceans in the Southern Hemisphere, moderate and strong storms prevail. Data analysis showed that the link between storm intensity and cloudiness accounts for the difference in cloudiness between the hemispheres. "Cloud albedo arising from strong storms above the Southern Hemisphere was found to be a high-precision offsetting agent to the large land area in the Northern Hemisphere, and thus symmetry is preserved," says Hadas, adding: "This suggests that storms are the linking factor between the brightness of Earth's surface and that of clouds, solving the symmetry mystery."

Could climate change make one of the hemispheres darker?

Earth has been undergoing rapid change in recent years, owing to climate change. To examine whether and how this could affect hemispheric albedo symmetry, the scientists used CMIP6, a set of models run by climate modeling centers around the world to simulate climate change. One of these models' major shortcomings is their limited ability to predict the degree of cloudiness. Nevertheless, the relation found in this study between storm intensity and cloudiness enables scientists to assess future cloud amounts, based on storm predictions.

Models predict global warming will result in a decreased frequency of all storms above the Northern Hemisphere and of weak and moderate storms above the Southern Hemisphere. However, the strongest storms of the Southern Hemisphere will intensify. The cause of these predicted differences is "Arctic amplification," a phenomenon in which the North Pole warms twice as fast as Earth's mean warming rate. One might speculate that this difference should break hemispheric albedo symmetry. However, the research shows that a further increase in storm intensity might not change the degree of cloudiness in the Southern Hemisphere because cloud amounts reach saturation in very strong storms. Thus, symmetry might be preserved.

"It is not yet possible to determine with certainty whether the symmetry will break in the face of global warming," says Kaspi. "However, the new research solves a basic scientific question and deepens our understanding of Earth's radiation balance and its effectors. As global warming continues, geoengineered solutions will become vital for human life to carry on alongside it. I hope that a better understanding of basic climate phenomena, such as the hemispheric albedo symmetry, will help in developing these solutions."

Read more at Science Daily

How photoelectrodes change in contact with water

Every green leaf is able to convert solar energy into chemical energy, storing it in chemical compounds. However, an important sub-process of photosynthesis can already be technically imitated -- solar hydrogen production: Sunlight generates a current in a so-called photoelectrode that can be used to split water molecules. This produces hydrogen, a versatile fuel that stores solar energy in chemical form and can release it when needed.

Photoelectrodes with many talents

At the HZB Institute for Solar Fuels, many teams are working on this vision. The focus of their research is on producing efficient photoelectrodes. These are semiconductors that remain stable in aqueous solutions and are highly active: Not only can they convert sunlight into electrical current, but they may also act as catalysts to accelerate the splitting of water. Among the best candidates for inexpensive and efficient photoelectrodes is bismuth vanadate (BiVO4).

What happens when in water?

"Basically, we know that just by immersing bismuth vanadate in the aqueous solution the chemical composition of the surface changes," says Dr. David Starr of the HZB Institute for Solar Fuels. And his colleague Dr. Marco Favaro adds: "Although there are a great many studies on BiVO4, it has not been clear until now exactly what implications this has on the surface electronic properties once they come into contact with the water molecules." In this work, they have now investigated this question.

Doped BiVO4 under water vapor

They studied single crystals of BiVO4 doped with molybdenum under water vapor with resonant ambient pressure photoemission spectroscopy at the Advanced Light Source at Lawrence Berkeley National Laboratory. A team led by Giulia Galli at the University of Chicago then performed density functional theory calculations to help interpret the data and to untangle the contributions of individual elements and electron orbitals to the electronic states.

Polarons on the surface detected

"In situ resonant photoemission has allowed us to understand how the electronic properties of our BiVO4 crystals changed upon water adsorption," Favaro says. The combination of measurements and calculations showed that due to excess charge, generated by either doping or defects on certain surfaces of the crystal, so-called polarons may form: negatively charged localized states, where water molecules can easily attach and then dissociate. The hydroxyl groups formed via water dissociation help to stabilize further polaron formation. "The excess electrons are localized as polarons at VO4 units on the surface," Starr summarizes the results.

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Solar harvesting system has potential to generate solar power 24/7

The great inventor Thomas Edison once said, "So long as the sun shines, man will be able to develop power in abundance." His wasn't the first great mind to marvel at the notion of harnessing the power of the sun; for centuries inventors have been pondering and perfecting the way to harvest solar energy.

They've done an amazing job with photovoltaic cells which convert sunlight directly into energy. And still, with all the research, history and science behind it, there are limits to how much solar power can be harvested and used -- as its generation is restricted only to the daytime.

A University of Houston professor is continuing the historic quest, reporting on a new type of solar energy harvesting system that breaks the efficiency record of all existing technologies. And no less important, it clears the way to use solar power 24/7.

"With our architecture, the solar energy harvesting efficiency can be improved to the thermodynamic limit," reports Bo Zhao, Kalsi Assistant Professor of mechanical engineering and his doctoral student Sina Jafari Ghalekohneh in the journal Physical Review Applied. The thermodynamic limit is the absolute maximum theoretically possible conversion efficiency of sunlight into electricity.

Finding more efficient ways to harness solar energy is critical to transitioning to a carbon-free electric grid. According to a recent study by the U.S. Department of Energy Solar Energy Technologies Office and the National Renewable Energy Laboratory, solar could account for as much as 40% of the nation's electricity supply by 2035 and 45% by 2050, pending aggressive cost reductions, supportive policies and large-scale electrification.

How Does it Work?

Traditional solar thermophotovoltaics (STPV) rely on an intermediate layer to tailor sunlight for better efficiency. The front side of the intermediate layer (the side facing the sun) is designed to absorb all photons coming from the sun. In this way, solar energy is converted to thermal energy of the intermediate layer and elevates the temperature of the intermediate layer.

But the thermodynamic efficiency limit of STPVs, which has long been understood to be the blackbody limit (85.4%), is still far lower than the Landsberg limit (93.3%), the ultimate efficiency limit for solar energy harvesting.

"In this work, we show that the efficiency deficit is caused by the inevitable back emission of the intermediate layer towards the sun resulting from the reciprocity of the system. We propose nonreciprocal STPV systems that utilize an intermediate layer with nonreciprocal radiative properties," said Zhao. "Such a nonreciprocal intermediate layer can substantially suppress its back emission to the sun and funnel more photon flux towards the cell.

We show that, with such improvement, the nonreciprocal STPV system can reach the Landsberg limit, and practical STPV systems with single-junction photovoltaic cells can also experience a significant efficiency boost."

Besides improved efficiency, STPVs promise compactness and dispatchability (electricity that can be programmed on demand based on market needs).

In one important application scenario, STPVs can be coupled with an economical thermal energy storage unit to generate electricity 24/7.

Read more at Science Daily

Converting solar energy to electricity on demand

The researchers behind an energy system that makes it possible to capture solar energy, store it for up to eighteen years and release it when and where it is needed have now taken the system a step further. After previously demonstrating how the energy can be extracted as heat, they have now succeeded in getting the system to produce electricity, by connecting it to a thermoelectric generator. Eventually, the research -- developed at Chalmers University of Technology, Sweden -- could lead to self-charging electronics using stored solar energy on demand.

"This is a radically new way of generating electricity from solar energy. It means that we can use solar energy to produce electricity regardless of weather, time of day, season, or geographical location. It is a closed system that can operate without causing carbon dioxide emissions," says research leader Kasper Moth-Poulsen, Professor at the Department of Chemistry and Chemical Engineering at Chalmers.

The new technology is based on the solar energy system MOST -- Molecular Solar Thermal Energy Storage Systems, developed at Chalmers University of Technology. Very simply, the technology is based on a specially designed molecule that changes shape when it comes into contact with sunlight. The research has already attracted great interest worldwide when it has been presented at earlier stages.

The new study, published in Cell Reports Physical Science and carried out in collaboration with researchers in Shanghai, takes the solar energy system a step further, detailing how it can be combined with a compact thermoelectric generator to convert solar energy into electricity.

Ultra-thin chip converts heat into electricity

The Swedish researchers sent their specially designed molecule, loaded with solar energy, to colleagues Tao Li and Zhiyu Hu at Shanghai Jiao Tong University, where the energy was released and converted into electricity using the generator they developed there. Essentially, Swedish sunshine was sent to the other side of the world and converted into electricity in China.

"The generator is an ultra-thin chip that could be integrated into electronics such as headphones, smart watches and telephones. So far, we have only generated small amounts of electricity, but the new results show that the concept really works. It looks very promising," says researcher Zhihang Wang from Chalmers University of Technology.

Fossil free, emissions free

The research has great potential for renewable and emissions-free energy production. But a lot of research and development remains before we will be able to charge our technical gadgets or heat our homes with the system's stored solar energy.

"Together with the various research groups included in the project, we are now working to streamline the system. The amount of electricity or heat it can extract needs to be increased. Even if the energy system is based on simple basic materials, it needs to be adapted to be sufficiently cost-effective to produce, and thus possible to launch more broadly," says Kasper Moth-Poulsen.

More about the Most technology

Molecular Solar Thermal Energy Storage Systems, Most, is a closed energy system based on a specially designed molecule of carbon, hydrogen and nitrogen, which when hit by sunlight changes shape into an energy-rich isomer -- a molecule made up of the same atoms but arranged together in a different way. The isomer can then be stored in liquid form for later use when needed, such as at night or in winter. The researchers have refined the system to the point that it is now possible to store the energy for up to 18 years. A specially designed catalyst releases the saved energy as heat while returning the molecule to its original shape, so it can then be reused in the heating system. Now, in combination with a micrometer-thin thermoelectric generator, the energy system can also generate electricity to order.

Read more at Science Daily

Breakthrough in converting carbon dioxide into fuel using solar energy

A research team led by Lund University in Sweden has shown how solar power can convert carbon dioxide into fuel, by using advanced materials and ultra-fast laser spectroscopy. The breakthrough could be an important piece of the puzzle in reducing the levels of greenhouse gases in the atmosphere in the future. The study is published in Nature Communications.

The sunlight that hits Earth during one hour corresponds roughly to humanity's total energy consumption for an entire year. Our global carbon dioxide emissions are also increasing. Using the sun's energy to capture greenhouse gases and converting it into fuel or another useful chemical, is a research focus for many today. However, there is still no satisfactory solution, but an international research team has now revealed a possible way forward.

"The study uses a combination of materials that absorb sunlight and use its energy to convert carbon dioxide. With the help of ultra-fast laser spectroscopy, we have mapped exactly what happens in that process," says Tönu Pullerits, chemistry researcher at Lund University.

The researchers have studied a porous organic material called COF -- covalent organic framework. The material is known for absorbing sunlight very efficiently. By adding a so-called catalytic complex to COF, they succeeded, without any additional energy, in converting carbon dioxide to carbon monoxide.

"The conversion to carbon monoxide requires two electrons. When we discovered that photons with blue light create long-lived electrons with high energy levels, we could simply charge COF with electrons and complete a reaction," says Kaibo Zheng, chemistry researcher at Lund University.

How can these results be useful? Tönu Pullerits and Kaibo Zheng hope that in the future the discovery can be used to develop larger units that can be used on a global level to, with the help of the sun, absorb carbon dioxide from the atmosphere and convert it into fuel or chemicals. That could be one of many solutions to overcome the climate crisis we are facing.

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An anode-free zinc battery that could someday store renewable energy

 Renewable energy sources, such as wind and solar power, could help decrease the world's reliance on fossil fuels. But first, power companies need a safe, cost-effective way to store the energy for later use. Massive lithium-ion batteries can do the job, but they suffer from safety issues and limited lithium availability. Now, researchers reporting in ACS' Nano Letters have made a prototype of an anode-free, zinc-based battery that uses low-cost, naturally abundant materials.

Aqueous zinc-based batteries have been previously explored for grid-scale energy storage because of their safety and high energy density. In addition, the materials used to make them are naturally abundant. However, the rechargeable zinc batteries developed so far have required thick zinc metal anodes, which contain a large excess of zinc that increases cost. Also, the anodes are prone to forming dendrites -- crystalline projections of zinc metal that deposit on the anode during charging -- that can short-circuit the battery. Yunpei Zhu, Yi Cui and Husam Alshareef wondered whether a zinc anode was truly needed. Drawing inspiration from previous explorations of "anode-free" lithium and sodium-metal batteries, the researchers decided to make a battery in which a zinc-rich cathode is the sole source for zinc plating onto a copper current collector.

In their battery, the researchers used a manganese dioxide cathode that they pre-intercalated with zinc ions, an aqueous zinc trifluoromethanesulfonate electrolyte solution and a copper foil current collector. During charging, zinc metal gets plated onto the copper foil, and during discharging the metal is stripped off, releasing electrons that power the battery. To prevent dendrites from forming, the researchers coated the copper current collector with a layer of carbon nanodiscs. This layer promoted uniform zinc plating, thereby preventing dendrites, and increased the efficiency of zinc plating and stripping. The battery showed high efficiency, energy density and stability, retaining 62.8% of its storage capacity after 80 charging and discharging cycles. The anode-free battery design opens new directions for using aqueous zinc-based batteries in energy storage systems, the researchers say.

From Science Daily

On the road to invisible solar panels: How tomorrow's windows will generate electricity

 Five years after the Paris climate agreement, all eyes are on the world's progress on the road to a carbon-free future. A crucial part of this goal involves the energy transition from fossil fuels to renewable sources, such as sun, water, wind and wave energy. Among those, solar energy has always held the highest hope in the scientific community, as the most reliable and abundant energy source on Earth. In recent decades, solar cells have become cheaper, more efficient, and environment friendly. However, current solar cells tend to be opaque, which prevents their wider use and integration into everyday materials, constrained to being lined up on roofs and in remote solar farms.

But what if next-generation solar panels could be integrated to windows, buildings, or even mobile phone screens? That is the hope of Professor Joondong Kim from the Department of Electrical Engineering at Incheon National University, Korea. In a recent study published in Journal of Power Sources, he and his colleagues detail their latest invention: a fully transparent solar cell. "The unique features of transparent photovoltaic cells could have various applications in human technology," says Prof. Kim.

The idea of transparent solar cells is well known, but this novel application where scientists have been able to translate this idea into practice is a crucial new finding. At present, the materials making the solar cell opaque are the semiconductor layers, those responsible for capturing light and translating it into an electrical current. Hence, Prof. Kim and his colleagues looked at two potential semiconductor materials, identified by previous researchers for their desirable properties.

The first is titanium dioxide (TiO2), a well-known semiconductor already widely used to make solar cells. On top of its excellent electrical properties, TiO2 is also an environment-friendly and non-toxic material. This material absorbs UV light (a part of the light spectrum invisible to the naked eye) while letting through most of the visible light range. The second material investigated to make this junction was nickel oxide (NiO), another semiconductor known to have high optical transparency. As nickel is one of the mist abundant elements on Earth, and its oxide can easily be manufactured at low industrial temperatures, NiO is also a great material to make eco-friendly cells.

The solar cell prepared by the researchers was composed of a glass substrate and a metal oxide electrode, on top of which they deposited thin layers of the semiconductors (TiO2 first, then NiO) and a final coating of silver nanowires, acting as the other electrode in the cell. They ran several tests to evaluate the device's absorbance and transmittance of light, as well as its effectiveness as a solar cell.

Their findings were encouraging; with a power conversion efficiency of 2.1%, the cell's performance was quite good, given that it targets only a small part of the light spectrum. The cell was also highly responsive and worked in low light conditions. Furthermore, more than 57% of visible light was transmitted through the cell's layers, giving the cell this transparent aspect. In the final part of their experiment, the researchers demonstrated how their device could be used to power a small motor. "While this innovative solar cell is still very much in its infancy, our results strongly suggest that further improvement is possible for transparent photovoltaics by optimizing the cell's optical and electrical properties," suggests Prof. Kim.

Read more at Science Daily

Even short-lived solar panels can be economically viable

A new study shows that, contrary to widespread belief within the solar power industry, new kinds of solar cells and panels don't necessarily have to last for 25 to 30 years in order to be economically viable in today's market.

Rather, solar panels with initial lifetimes of as little as 10 years can sometimes make economic sense, even for grid-scale installations -- thus potentially opening the door to promising new solar photovoltaic technologies that have been considered insufficiently durable for widespread use.

The new findings are described in a paper in the journal Joule, by Joel Jean, a former MIT postdoc and CEO of startup company Swift Solar; Vladimir Bulović, professor of electrical engineering and computer science and director of MIT.nano; and Michael Woodhouse of the National Renewable Energy Laboratory (NREL) in Colorado.

"When you talk to people in the solar field, they say any new solar panel has to last 25 years," Jean says. "If someone comes up with a new technology with a 10-year lifetime, no one is going to look at it. That's considered common knowledge in the field, and it's kind of crippling."

Jean adds that "that's a huge barrier, because you can't prove a 25-year lifetime in a year or two, or even 10." That presumption, he says, has left many promising new technologies stuck on the sidelines, as conventional crystalline silicon technologies overwhelmingly dominate the commercial solar marketplace. But, the researchers found, that does not need to be the case.

"We have to remember that ultimately what people care about is not the cost of the panel; it's the levelized cost of electricity," he says. In other words, it's the actual cost per kilowatt-hour delivered over the system's useful lifetime, including the cost of the panels, inverters, racking, wiring, land, installation labor, permitting, grid interconnection, and other system components, along with ongoing maintenance costs.

Part of the reason that the economics of the solar industry look different today than in the past is that the cost of the panels (also known as modules) has plummeted so far that now, the "balance of system" costs -- that is, everything except the panels themselves -- exceeds that of the panels. That means that, as long as newer solar panels are electrically and physically compatible with the racking and electrical systems, it can make economic sense to replace the panels with newer, better ones as they become available, while reusing the rest of the system.

"Most of the technology is in the panel, but most of the cost is in the system," Jean says. "Instead of having a system where you install it and then replace everything after 30 years, what if you replace the panels earlier and leave everything else the same? One of the reasons that might work economically is if you're replacing them with more efficient panels," which is likely to be the case as a wide variety of more efficient and lower-cost technologies are being explored around the world.

He says that what the team found in their analysis is that "with some caveats about financing, you can, in theory, get to a competitive cost, because your new panels are getting better, with a lifetime as short as 15 or even 10 years."

Although the costs of solar cells have come down year by year, Bulović says, "the expectation that one had to demonstrate a 25-year lifetime for any new solar panel technology has stayed as a tautology. In this study we show that as the solar panels get less expensive and more efficient, the cost balance significantly changes."

He says that one aim of the new paper is to alert the researchers that their new solar inventions can be cost-effective even if relatively short lived, and hence may be adopted and deployed more rapidly than expected. At the same time, he says, investors should know that they stand to make bigger profits by opting for efficient solar technologies that may not have been proven to last as long, knowing that periodically the panels can be replaced by newer, more efficient ones.

"Historical trends show that solar panel technology keeps getting more efficient year after year, and these improvements are bound to continue for years to come," says Bulović. Perovskite-based solar cells, for example, when first developed less than a decade ago, had efficiencies of only a few percent. But recently their record performance exceeded 25 percent efficiency, compared to 27 percent for the record silicon cell and about 20 percent for today's standard silicon modules, according to Bulović. Importantly, in novel device designs, a perovskite solar cell can be stacked on top of another perovskite, silicon, or thin-film cell, to raise the maximum achievable efficiency limit to over 40 percent, which is well above the 30 percent fundamental limit of today's silicon solar technologies. But perovskites have issues with longevity of operation and have not yet been shown to be able to come close to meeting the 25-year standard.

Bulović hopes the study will "shift the paradigm of what has been accepted as a global truth." Up to now, he says, "many promising technologies never even got a start, because the bar is set too high" on the need for durability.

For their analysis, the team looked at three different kinds of solar installations: a typical 6-kilowatt residential system, a 200-kilowatt commercial system, and a large 100-megawatt utility-scale system with solar tracking. They used NREL benchmark parameters for U.S. solar systems and a variety of assumptions about future progress in solar technology development, financing, and the disposal of the initial panels after replacement, including recycling of the used modules. The models were validated using four independent tools for calculating the levelized cost of electricity (LCOE), a standard metric for comparing the economic viability of different sources of electricity.

In all three installation types, they found, depending on the particulars of local conditions, replacement with new modules after 10 to 15 years could in many cases provide economic advantages while maintaining the many environmental and emissions-reduction benefits of solar power. The basic requirement for cost-competitiveness is that any new solar technology that is to be installed in the U.S should start with a module efficiency of at least 20 percent, a cost of no more than 30 cents per watt, and a lifetime of at least 10 years, with the potential to improve on all three.

Jean points out that the solar technologies that are considered standard today, mostly silicon-based but also thin-film variants such as cadmium telluride, "were not very stable in the early years. The reason they last 25 to 30 years today is that they have been developed for many decades." The new analysis may now open the door for some of the promising newer technologies to be deployed at sufficient scale to build up similar levels of experience and improvement over time and to make an impact on climate change earlier than they could without module replacement, he says.

Read more at Science Daily

Brilliant iron molecule could provide cheaper solar energy

The new molecule.

For the first time, researchers have succeeded in creating an iron molecule that can function both as a photocatalyst to produce fuel and in solar cells to produce electricity. The results indicate that the iron molecule could replace the more expensive and rarer metals used today.

Some photocatalysts and solar cells are based on a technology that involves molecules containing metals, known as metal complexes. The task of the metal complexes in this context is to absorb solar rays and utilise their energy. The metals in these molecules pose a major problem, however, as they are rare and expensive metals, such as the noble metals ruthenium, osmium and iridium.

"Our results now show that by using advanced molecule design, it is possible to replace the rare metals with iron, which is common in the Earth's crust and therefore cheap," says Chemistry Professor Kenneth Wärnmark of Lund University in Sweden.

Together with colleagues, Kenneth Wärnmark has for a long time worked to find alternatives to the expensive metals. The researchers focused on iron which, with its six per cent prevalence in the Earth's crust, is significantly easier to source. The researchers have produced their own iron-based molecules whose potential for use in solar energy applications has been proven in previous studies.

In this new study, the researchers have moved one step further and developed a new iron-based molecule with the ability to capture and utilise the energy of solar light for a sufficiently long time for it to react with another molecule. The new iron molecule also has the ability to glow long enough to enable researchers to see iron-based light with the naked eye at room temperature for the first time.

"The good result depends on the fact that we have optimised the molecular structure around the iron atom," explains colleague Petter Persson of Lund University.

The study is now published in the journal Science. According to the researchers, the iron molecule in question could be used in new types of photocatalysts for the production of solar fuel, either as hydrogen through water splitting or as methanol from carbon dioxide. Furthermore, the new findings open up other potential areas of application for iron molecules, e.g. as materials in light diodes (LEDs).

What surprised the Lund researchers is that they arrived at good results so quickly. In just over five years, they succeeded in making iron interesting for photochemical applications, with properties largely as good as those of the best noble metals.

"We believed it would take at least ten years," says Kenneth Wärnmark.

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