Forschungszentrum Jülich GmbH

Free charge carriers in perovskite solar cells likely have a special form of protection from recombination, researchers have discovered. This may be key to the high efficiency of this cell type, which has been increased to over 25 % within a decade. Free charge carriers in perovskite solar cells likely have a special form of protection from recombination, researchers at Forschungszentrum Jülich have discovered by means of innovative photoluminescence measurements. Highly efficient and relatively inexpensive to produce -- perovskite solar cells have been the subject of repeated surprises in recent years. Scientists at Forschungszentrum Jülich have now discovered another special feature of the cells using a new photoluminescence measurement technique. They found that the loss of charge carriers in this type of cell follows different physical laws than those known for most semiconductors. This may be one of the main reasons for their high level of efficiency. The results were presented in the journal Nature Materials. Perovskite solar cells are regarded as highly promising for photovoltaics, even if their stability leaves much to be desired. Cells of this type are inexpensive to print and very efficient. In the last decade, their efficiency has doubled to over 25 % and is therefore currently on a par with conventional solar cells made of silicon. Further improvements also appear to be possible in the future. "An important factor here is the question of how long excited charge carriers remain in the material, in other words their lifetime," explains Thomas Kirchartz. "Understanding the processes is crucial to further improving the efficiency of perovskite-based solar cells." The electrical engineer is the head of a working group on organic and hybrid solar cells at Forschungszentrum Jülich's Institute of Energy and Climate Research (IEK-5). It's the lifetime that counts In a solar cell, electrons are dislodged by photons and raised to a higher energy level from the valence band to the conduction band. Only then can they move more freely and flow through an external circuit. They can only contribute to electrical energy generation if their lifetime is long enough for them to pass through the absorber material to the electrical contact. An excited electron also leaves a hole in the underlying valence band -- a mobile vacancy that can be moved through the material like a positive charge carrier. It is mainly defects in the crystal lattice which ensure that excited electrons quickly fall back down to lower energy levels again. The electrons affected are then no longer able to contribute to the current flow. "This mechanism is also known as recombination and is the main loss process of every solar cell," says Kirchartz. Recombination crucial for efficiency No solar cell is perfect on an atomic level; each one has different types of defects due to the manufacturing process. These defects or foreign atoms in the lattice structure are the collection points where electrons and holes tend to come together. The electrons then fall back into the valence band and become worthless in terms of electricity generation. "It had previously been assumed that recombination is predominantly triggered by defects that are energetically located in the middle between the valence and conduction bands. This is because these deep defects are similarly accessible to excited electrons and their counterparts, the holes," says Kirchartz. Indeed, this is likely true for most types of solar cells. Shallow defects dominate However, Kirchartz and his team have now disproved this assumption for perovskite solar cells and shown that the shallow defects are ultimately decisive in terms of their final efficiency. Unlike the deep defects, they are not located in the middle of the band gap, but very close to the valence or conduction band. "The cause of this unusual behavior has not yet been fully clarified," Kirchartz adds. "It is reasonable to assume that deep defects simply cannot exist in these materials. This restriction may also be one of the reasons for the particularly high efficiency of the cells." New HDR measurement technique with extended dynamic range The observation was only made possible by innovative transient photoluminescence measurements. In previous measurements, it was not possible to distinguish loss processes caused by shallow defects from those caused by other factors. The new measuring method developed by Thomas Kirchartz and his team at Forschungszentrum Jülich delivers data with a significantly increased dynamic range compared to conventional technology, i.e. data over a larger measuring range and with better fine gradation. The process is based on a similar principle to HDR image in high dynamic range quality. The dynamic range of the camera is increased by superimposing different images or measurements -- in this case signals with different levels of amplification -- to create a data set.

A recent advance could enable more efficient compact fusion reactors that are easier to repair and maintain. University of Wisconsin-Madison engineers have used a spray coating technology to produce a new workhorse material that can withstand the harsh conditions inside a fusion reactor. The advance, detailed in a paper published recently in the journal Physica Scripta, could enable more efficient compact fusion reactors that are easier to repair and maintain. "The fusion community is urgently looking for new manufacturing approaches to economically produce large plasma-facing components in fusion reactors," says Mykola Ialovega, a postdoctoral researcher in nuclear engineering and engineering physics at UW-Madison and lead author on the paper. "Our technology shows considerable improvements over current approaches. With this research, we are the first to demonstrate the benefits of using cold spray coating technology for fusion applications." The researchers used a cold spray process to deposit a coating of tantalum, a metal that can withstand high temperatures, on stainless steel. They tested their cold spray tantalum coating in the extreme conditions relevant to a fusion reactor and found that it performed very well. Importantly, they discovered the material is exceptionally good at trapping hydrogen particles, which is beneficial for compact fusion devices. "We discovered that the cold spray tantalum coating absorbs much more hydrogen than bulk tantalum because of the unique microstructure of the coating," says Kumar Sridharan, a professor of nuclear engineering and engineering physics and materials science and engineering. Over the last decade, Sridharan's research group has introduced cold spray technology to the nuclear energy community by implementing it for multiple applications related to fission reactors. "The simplicity of the cold spray process makes it very practical for applications," Sridharan says. In fusion devices, plasma -- an ionized hydrogen gas -- is heated to extremely high temperatures, and atomic nuclei in the plasma collide and fuse. That fusion process produces energy. However, some hydrogen ions may get neutralized and escape from the plasma. "These hydrogen neutral particles cause power losses in the plasma, which makes it very challenging to sustain a hot plasma and have an effective small fusion reactor," says Ialovega, who works in the research group of Oliver Schmitz, a professor of nuclear engineering and engineering physics. That's why the researchers set out to create a new surface for plasma-facing reactor walls that could trap hydrogen particles as they collide with the walls. Tantalum is inherently good at absorbing hydrogen -- and the researchers suspected that creating a tantalum coating using a cold spray process would boost its hydrogen-trapping abilities even more. Creating a cold sprayed coating is somewhat like using a can of spray paint. It consists of propelling particles of the coating material at supersonic velocities onto a surface. Upon impact, the particles flatten like pancakes and coat the entire surface, while preserving nanoscale boundaries between the coating particles. The researchers discovered that those tiny boundaries facilitate trapping of hydrogen particles. Ialovega conducted experiments on the coated material at facilities at Aix Marseille University in France and Forschungszentrum Jülich GmbH in Germany. During these experiments, he found that when he heated the material to a higher temperature, it expelled the trapped hydrogen particles without modifying the coatings -- a process that essentially regenerates the material so it can be used again. "Another big benefit of the cold spray method is that it allows us to repair reactor components on site by applying a new coating," Ialovega says. "Currently, damaged reactor components often need to be removed and replaced with a completely new part, which is costly and time consuming." The researchers plan to use their new material in the Wisconsin HTS Axisymmetric Mirror (WHAM). The experimental device is under construction near Madison, Wis., and will serve as a prototype for a future next-generation fusion power plant that UW-Madison spinoff Realta Fusion aims to develop. Housed in the Physical Sciences Laboratory, the WHAM experiment is a partnership between UW-Madison, Massachusetts Institute of Technology and Commonwealth Fusion Systems. "Creating a refractory metal composite with these features of well-controlled hydrogen handling combined with erosion resistance and general material resilience is a breakthrough for the design of plasma devices and fusion energy systems," Schmitz says. "The prospect of changing the alloy and including other refractory metals to enhance the composite for nuclear applications is particularly exciting." The researchers are patenting their technology through the Wisconsin Alumni Research Foundation. Oliver Schmitz is the Thomas and Suzanne Werner Professor, associate dean for research innovation in the UW-Madison College of Engineering, and director of the Grainger Institute of Engineering. Sridharan is a Grainger Professor. Additional co-authors on the paper from UW-Madison include: Tyler Dabney, Marcos Navarro Gonzalez, Hwasung Yeom, Danah Velez, Evan Willing, Jay Anderson, and Cary Forest. Thierry Angot and Régis Bisson from Aix Marseille University in France, and Arkadi Kreter from Forschungszentrum Jülich GmbH in Germany are also co-authors on the paper. Seed funding for this research came in part from UW-Madison's Research Forward program. This research was also supported by grants from the Advanced Research Projects Agency-Energy (ARPA-E) and the U.S. Department of Energy.

What does the inside of a carbon atom's nucleus look like? A new study provides a comprehensive answer to this question. In the study, the researchers simulated all known energy states of the nucleus. These include the puzzling Hoyle state. If it did not exist, carbon and oxygen would only be present in the universe in tiny traces. Ultimately, we therefore also owe it our own existence. What does the inside of a carbon atom's nucleus look like? A new study by Forschungszentrum Jülich, Michigan State University (USA) and the University of Bonn provides the first comprehensive answer to this question. In the study, the researchers simulated all known energy states of the nucleus. These include the puzzling Hoyle state. If it did not exist, carbon and oxygen would only be present in the universe in tiny traces. Ultimately, we therefore also owe it our own existence. The study has now been published in the journal Nature Communications. The nucleus of a carbon atom normally consists of six protons and six neutrons. But how exactly are they arranged? And how does their configuration change when the nucleus is bombarded with high-energy radiation? For decades, science has been searching for answers to these questions. Not least because they could provide the key to a mystery that has long puzzled physicists: Why is there a significant amount of carbon in space at all -- an atom without which there would be no life on Earth? After all, shortly after the Big Bang, there was only hydrogen and helium. The hydrogen nucleus consists of a single proton, that of helium of two protons and two neutrons. All heavier elements were only created many billions of years later by aging stars. In them, helium nuclei fused into carbon nuclei at immense pressure and extremely high temperatures. This requires three helium nuclei to fuse together. "But it's actually very unlikely for this to happen," explains Prof. Dr. Ulf Meißner of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn and the Institute for Advanced Simulation at Forschungszentrum Jülich. The reason: The helium nuclei together have a much higher energy than a carbon nucleus. However, this does not mean that they fuse particularly readily -- on the contrary: It is as if three people wanted to jump onto a merry-go-round. But since they run much faster than the merry-go-round turns, they do not succeed. Simulation on the supercomputer As early as the 1950s, the British astronomer Fred Hoyle therefore postulated that the three helium nuclei first come together to form a kind of transition state. This "Hoyle state" has a very similar energy to the helium nuclei. To stay in the picture: It is a faster-spinning version of the merry-go-round, which the three passengers can therefore easily jump onto. When that happens, the carousel slows down to its normal speed. "Only by taking a detour via the Hoyle state can stars create carbon at all in any appreciable quantity," says Meißner, who is also a member of the Transdisciplinary Research Areas "Modeling" and "Matter" of the University of Bonn. About ten years ago, together with colleagues from the USA, Forschungszentrum Jülich and Ruhr-Universität Bochum, he succeeded in simulating this Hoyle state for the first time. "We already had an idea then of how the protons and neutrons of the carbon nucleus are arranged in this state," he explains. "However, we were not able to prove with certainty that this assumption was true." With the help of an advanced method, the researchers have now succeeded. This is essentially based on confinement: In reality, the protons and neutrons -- the nucleons -- can be located anywhere in space. For their calculations, however, the team restricted this freedom: "We arranged our nuclear particles on the nodes of a three-dimensional lattice," Meißner explains. "So we allowed them only certain strictly defined positions." Computing time: five million processor hours Thanks to this restriction, it was possible to calculate the motion of nucleons. Since nuclear particles affect each other differently depending on their distance from each other, this task is very complex. The researchers also ran their simulation several million times with slightly different starting conditions. This allowed them to see where the protons and neutrons were most likely to be. "We performed these calculations for all known energy states of the carbon nucleus," Meißner says. The calculations were performed on the JEWELS supercomputer at Forschungszentrum Jülich. They required a total of about five million processor hours, with many thousands of processors working simultaneously. The results effectively provide images from the carbon nucleus. They prove that the nuclear particles do not exist independently of each other. "Instead, they are clustered into groups of two neutrons and two protons each," the physicist explains. This means that the three helium nuclei can still be detected after they have fused to form the carbon nucleus. Depending on the energy state, they are present in different spatial formations -- either arranged into an isosceles triangle or like a slightly bent arm, with the shoulder, elbow joint and wrist each occupied by a cluster. The study not only allows researchers to better understand the physics of the carbon nucleus. Meißner: "The methods we have developed can easily be used to simulate other nuclei and will certainly lead to entirely new insights." Participating institutions and funding: Forschungszentrum Jülich, Michigan State University (USA), the China Academy of Engineering Physics and the University of Bonn were involved in the study. The work was made possible by funding from the German Research Foundation, the National Natural Science Foundation of China, the Chinese Academy of Sciences (CAS), the Volkswagen Foundation, the European Research Council (ERC), the U.S. Department of Energy, the Nuclear Computational Low-Energy Initiative (NUCLEI), and the Gauss Center for Supercomputing e.V.