A new material gets the lead out of highly efficient solar cells opening many more possibilities

A new type of material for next-generation solar cells eliminates the need to use lead, which has been a major roadblock for this technology.

Solar cells, incorporating the mineral perovskite, have been the focus of attention since the material was first shown to work in 2009. Solar cells that are built using this material are more efficient than current solar panels. Current solar panels capture 15% to 18% of the solar energy on average, while perovskite solar cells have been found to be as much as 28% efficient.

But there are major obstacles to using these materials commercially: The materials are not stable, and they contain water-soluble lead, which is a health hazard.

Now a team of scientists and engineers led by Letian Dou, assistant professor of chemical engineering at Purdue University, have developed a sandwich-like material incorporating organic and inorganic materials to form a hybrid structure that doesn’t use lead and has much improved stability.

“These structures are very exciting,” Dou said. “The sandwich structures are like semiconductor quantum wells that are widely used today in many electronic and optoelectronic devices, but they are much easier to produce and more tolerant to defects,”

The research was published Monday (Nov. 11) in the journal Nature Chemistry.

In a paper published in the Journal of the American Chemical Society in September, the scientists had incorporated the material into an essential component of many electronic devices, a field effect transistor.

Yao Gao, lead author of both research papers and a postdoctoral fellow in Dou’s research group, said the new organic-inorganic hybrid perovskite materials are cheaper and perform better than a traditional inorganic semiconductor. Also, Gao said, the new material’s design strategy could serve as a blueprint for many other functional hybrid materials.

“Solar cells, as many people have demonstrated, can be highly efficient,” he said. “With our new technology, we can make the hybrid perovskite materials intrinsically more stable. By replacing the toxic lead, these new materials are better for the environment and can also be safely used for bioelectronics sensors on the body.”

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Next-generation solar cells mimic photosynthesis

Congcong Wu, associate research professor at Penn State, works with materials that can be inserted into next generation solar cells to improve their efficiency. Perovskite solar cells are an area of intense research due to their potential to offer a more efficient and less expensive alternative to traditional silicon-based solar technology. IMAGE: DAVID KUBAREK

Next-generation solar cells that mimic photosynthesis with biological material may give new meaning to the term “green technology.” Adding the protein bacteriorhodopsin (bR) to perovskite solar cells boosted the efficiency of the devices in a series of laboratory tests, according to an international team of researchers.

“These findings open the door for the development of a cheaper, more environmentally friendly bioperovskite solar cell technology,” said Shashank Priya, associate vice president for research and professor of materials science at Penn State. “In the future, we may essentially replace some expensive chemicals inside solar cells with relatively cheaper natural materials.”

Perovskite solar cells, named for their unique crystal structures that excel at absorbing visible light, are an area of intense research because they offer a more efficient and less expensive alternative to traditional silicon-based solar technology.

The most efficient perovskite solar cells can convert 22 to 23 percent of sunlight to electricity. The researchers found that adding the bR protein to perovskite solar cells improved the devices’ efficiency from 14.5 to 17 percent. They reported their findings in the American Chemical Society journal ACS Applied Materials and Interfaces.

The research represents the first time scientists have shown that biological materials added to perovskite solar cells can provide a high efficiency. Future research could result in even more efficient bioperovskite materials, the researchers said.

“Previous studies have achieved 8 or 9 percent efficiency by mixing certain proteins inside solar cell structures,” said Priya, a co-lead author of the study. “But nothing has come close to 17 percent. These findings are very significant.”

Commercial solar arrays consist of hundreds or thousands of individual solar cells, so even small improvements in efficiency can lead to real savings, according to the researchers.

Mimicking nature

Drawing on nature, the researchers sought to further improve the performance of perovskite solar cells through Förster Resonance Energy Transfer (FRET), a mechanism for energy transfer between a pair of photosensitive molecules.

“The FRET mechanism has been around for a long time,” said Renugopalakrishnan Venkatesan, professor at Northeastern University and Boston Children’s Hospital, Harvard University, and co-lead author on the study. “It seems to be the basis of photosynthesis and can be found in technologies like the wireless transfer of energy, and even in the animal world as a mechanism for communication. We are using this mechanism to try to create a world of bio-inspired systems that have the potential to surpass either inorganic or organic molecules.”

The bR proteins and perovskite materials have similar electrical properties, or band gaps. By aligning these gaps, the scientists hypothesized they could achieve a better performance in perovskite solar cells through the FRET mechanism.

“Solar cells work by absorbing light energy, or photon molecules and creating electron-hole pairs,” said Subhabrata Das, who participated in the research while a doctoral student at Columbia University. “By sending the electrons and holes in opposite directions, solar cells generate an electrical current that’s turned into electricity.”

However, a certain percent of electron-hole pairs recombine, reducing the amount of current produced. Mixing the bR protein into perovskite solar cells helped electron-hole pairs better move through the devices, reducing recombination losses and boosting efficiency, the scientists said.

The findings could potentially have larger consequences, leading to the design of other hybrid devices in which artificial and biological materials work together, according to the researchers.

Bernardo Barbiellini, professor at the University of Lappeenranta, Finland, also served as co-lead author of the study.

Also contributing from Penn State were Congcong Wu, associate research professor, and Yuchen Hou, graduate student in material science and engineering. Other researchers on the study were Zhaoning Song, University of Toledo; Rainer Koch, the University of Oldenburg, Germany; and Ponisseril Somasundaran, Columbia University.

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Short-lived solar panels can be economically viable

A new study shows that replacing new solar panels after just 10 or 15 years, using the existing mountings and control systems, can make economic sense, contrary to industry expectations that a 25-year lifetime is necessary.

Research shows that, contrary to accepted rule of thumb, a 10- or 15-year lifetime can be good enough.

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 Bulovi?, 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, Bulovi? 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 Bulovi?. 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 Bulovi?. 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.

Bulovi? 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.

“This could enable us to launch ideas that would have died on the vine” because of the perception that greater longevity was essential, Bulovi? says.

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Revolutionizing how light is harnessed for solar energy use

Magnetic field data that shows the formation and decay of the excitons generated by singlet fission. CREDIT A. Asadpoor Darvish, McCamey Lab

Researchers develop new design rule for generating excitons will help advance next-generation devices

Researchers at Columbia University have developed a way to harness more power from singlet fission to increase the efficiency of solar cells, providing a tool to help push forward the development of next-generation devices.

In a study published this month in Nature Chemistry, the team details the design of organic molecules that are capable of generating two excitons per photon of light, a process called singlet fission. The excitons are produced rapidly and can live for much longer than those generated from their inorganic counterparts, which leads to an amplification of electricity generated per photon that is absorbed by a solar cell.

“We have developed a new design rule for singlet fission materials,” said Luis Campos, an associate professor of chemistry and one of three principal investigators on the study. “This has led us to develop the most efficient and technologically useful intramolecular singlet fission materials to date. These improvements will open the door for more efficient solar cells.”

All modern solar panels operate by the same process – one photon of light generates one exciton, Campos explained. The exciton can then be converted into electric current. However, there are some molecules that can be implemented in solar cells that have the ability to generate two excitons from a single photon – a process called singlet fission. These solar cells form the basis for next-generation devices, which are still at infancy. One of the biggest challenges of working with such molecules, though, is that the two excitons “live” for very short periods of time (tens of nanoseconds), making it difficult to harvest them as a form of electricity.

In the current study, funded in part by the Office of Naval Research, Campos and colleagues designed organic molecules that can quickly generate two excitons that live much longer than the state-of-the-art systems. It is an advancement that can not only be used in next-generation solar energy production, but also in photocatalytic processes in chemistry, sensors, and imaging, Campos explained, as these excitons can be used to initiate chemical reactions, which can then be used by industry to make drugs, plastics, and many other types of consumer chemicals.

“Intramolecular singlet fission has been demonstrated by our group and others, but the resulting excitons were either generated very slowly, or they wouldn’t last very long,” Campos said. “This work is the first to show that singlet fission can rapidly generate two excitons that can live for a very long time. This opens the door to fundamentally study how these excitons behave as they sit on individual molecules, and also to understand how they can be efficiently put to work in devices that benefit from light-amplified signals.”

The team’s design strategy should also prove useful in separate areas of scientific study and have many other yet-unimaginable applications, he added.

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New method shows a dramatic increase in solar cell output

Diagram depicts the process of “singlet fission,” which is the first step toward producing two electrons from a single incoming photon of light.
Image courtesy of the researchers

Method for collecting two electrons from each photon could break through theoretical solar-cell efficiency limit.

In any conventional silicon-based solar cell, there is an absolute limit on overall efficiency, based partly on the fact that each photon of light can only knock loose a single electron, even if that photon carried twice the energy needed to do so. But now, researchers have demonstrated a method for getting high-energy photons striking silicon to kick out two electrons instead of one, opening the door for a new kind of solar cell with greater efficiency than was thought possible.

While conventional silicon cells have an absolute theoretical maximum efficiency of about 29.1 percent conversion of solar energy, the new approach, developed over the last several years by researchers at MIT and elsewhere, could bust through that limit, potentially adding several percentage points to that maximum output. The results are described today in the journal Nature, in a paper by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and at Princeton University.

The basic concept behind this new technology has been known for decades, and the first demonstration that the principle could work was carried out by some members of this team six years ago. But actually translating the method into a full, operational silicon solar cell took years of hard work, Baldo says.

That initial demonstration “was a good test platform” to show that the idea could work, explains Daniel Congreve PhD ’15, an alumnus now at the Rowland Institute at Harvard, who was the lead author in that prior report and is a co-author of the new paper. Now, with the new results, “we’ve done what we set out to do” in that project, he says.

The original study demonstrated the production of two electrons from one photon, but it did so in an organic photovoltaic cell, which is less efficient than a silicon solar cell. It turned out that transferring the two electrons from a top collecting layer made of tetracene into the silicon cell “was not straightforward,” Baldo says. Troy Van Voorhis, a professor of chemistry at MIT who was part of that original team, points out that the concept was first proposed back in the 1970s, and says wryly that turning that idea into a practical device “only took 40 years.”

The key to splitting the energy of one photon into two electrons lies in a class of materials that possess “excited states” called excitons, Baldo says: In these excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.” In this case, they were going through a process called singlet exciton fission, which is how the light’s energy gets split into two separate, independently moving packets of energy. The material first absorbs a photon, forming an exciton that rapidly undergoes fission into two excited states, each with half the energy of the original state.

But the tricky part was then coupling that energy over into the silicon, a material that is not excitonic. This coupling had never been accomplished before.

As an intermediate step, the team tried coupling the energy from the excitonic layer into a material called quantum dots. “They’re still excitonic, but they’re inorganic,” Baldo says. “That worked; it worked like a charm,” he says. By understanding the mechanism taking place in that material, he says, “we had no reason to think that silicon wouldn’t work.”

What that work showed, Van Voorhis says, is that the key to these energy transfers lies in the very surface of the material, not in its bulk. “So it was clear that the surface chemistry on silicon was going to be important. That was what was going to determine what kinds of surface states there were.” That focus on the surface chemistry may have been what allowed this team to succeed where others had not, he suggests.

The key was in a thin intermediate layer. “It turns out this tiny, tiny strip of material at the interface between these two systems [the silicon solar cell and the tetracene layer with its excitonic properties] ended up defining everything. It’s why other researchers couldn’t get this process to work, and why we finally did.” It was Einzinger “who finally cracked that nut,” he says, by using a layer of a material called hafnium oxynitride.

The layer is only a few atoms thick, or just 8 angstroms (ten-billionths of a meter), but it acted as a “nice bridge” for the excited states, Baldo says. That finally made it possible for the single high-energy photons to trigger the release of two electrons inside the silicon cell. That produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum. Overall, that could produce an increase in the power produced by the solar cell — from a theoretical maximum of 29.1 percent, up to a maximum of about 35 percent.

Actual silicon cells are not yet at their maximum, and neither is the new material, so more development needs to be done, but the crucial step of coupling the two materials efficiently has now been proven. “We still need to optimize the silicon cells for this process,” Baldo says. For one thing, with the new system those cells can be thinner than current versions. Work also needs to be done on stabilizing the materials for durability. Overall, commercial applications are probably still a few years off, the team says.

Other approaches to improving the efficiency of solar cells tend to involve adding another kind of cell, such as a perovskite layer, over the silicon. Baldo says “they’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells.”

The researchers have measured one special property of hafnium oxynitride that helps it transfer the excitonic energy. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” Einzinger says. So far, no other material they’ve tested can match its properties.

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