New findings regarding the high efficiency of perovskite solar cells
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.
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HDR PHOTOLUMINESCENCE MEASURING STATION: DR. GENGHUA YAN WAS RESPONSIBLE FOR A LARGE PART OF THE MEASUREMENTS.
view moreCREDIT: FORSCHUNGSZENTRUM JÜLICH/RALF-UWE LIMBACH
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.
JOURNAL
Nature Materials
ARTICLE TITLE
Shallow defects and variable photoluminescence decay times up to 280 µs in triple-cation perovskites
ARTICLE PUBLICATION DATE
9-Jan-2024
How black silicon, a prized material used in solar cells, gets its dark, rough edge
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PPPL POSTDOCTORAL RESEARCH ASSOCIATE YURI BARSUKOV
view moreCREDIT: MICHAEL LIVINGSTON, PPPL
Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have developed a new theoretical model explaining one way to make black silicon, an important material used in solar cells, light sensors, antibacterial surfaces and many other applications.
Black silicon is made when the surface of regular silicon is etched to produce tiny nanoscale pits on the surface. These pits change the color of the silicon from gray to black and, critically, trap more light, an essential feature of efficient solar cells.
While there are many ways to make black silicon, including some that use the charged, fourth state of matter known as plasma, the new model focuses on a process that uses only fluorine gas. PPPL Postdoctoral Research Associate Yuri Barsukov said the choice to focus on fluorine was intentional: the team at PPPL wanted to fill a gap in publicly available research. While some papers have been published about the role of charged particles called ions in the production of black silicon, not much has been published about the role of neutral substances, such as fluorine gas.
“We now know — with great specificity — the mechanisms that cause these pits to form when fluorine gas is used,” said Barsukov, one of the authors of a new paper about the work. “This kind of information, published publicly and openly available, benefits us all, whether we pursue further knowledge into the basic knowledge that underlines such processes or we seek to improve manufacturing processes.”
Model reveals bonds break based on atom orientation at the surface
The new etching model precisely explains how fluorine gas breaks certain bonds in the silicon more often than others, depending on the orientation of the bond at the surface. As silicon is a crystalline material, atoms bond in a rigid pattern. These bonds can be characterized based on the way they are oriented in the pattern, with each type of orientation, or plane, identified by a bracketed number, such as (100), (110) or (111).
“If you etch silicon using fluorine gas, the etching proceeds along (100) and (110) crystal planes but does not etch (111), resulting in a rough surface after the etching,” explained Barsukov. As the gas etches away at the silicon unevenly, pits are created on the surface of the silicon. The rougher the surface, the more light it can absorb, making rough black silicon ideal for solar cells. Smooth silicon, in contrast, is an ideal surface for creating the atomic-scale patterns necessary for computer chips.
“If you want to etch silicon while leaving a smooth surface, you should use another reactant than fluorine. It should be a reactant that etches uniformly all crystalline planes,” Barsukov said.
PPPL expands its expertise into quantum chemistry
The research is also notable because it represents an early success in one of PPPL’s newest research areas.
“The Lab is diversifying,” said Igor Kaganovich, principal research physicist and co-author of the paper, which was published in the Journal of Vacuum Science & Technology A. “This is a first for PPPL to do this kind of quantum chemistry work.”
Quantum chemistry is a branch of science investigating the structure and reactivity of molecules using quantum mechanics: the laws of physics governing very small and very light objects, such as electrons and nuclei.
Other researchers who contributed to the paper include Joseph Vella, associate research physicist; Sierra Jubin, a graduate student at Princeton University; and former research assistant at PPPL Omesh Dhar Dwivedi.
This research was supported by the PPPL Laboratory Directed Research and Development funding for novel, innovative processes for highly selective and self-limiting etching relevant to nanofabrication of microelectronics and quantum device materials.
PPPL is mastering the art of using plasma — the fourth state of matter — to solve some of the world's toughest science and technology challenges. Nestled on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, our research ignites innovation in a range of applications including fusion energy, nanoscale fabrication, quantum materials and devices, and sustainability science. The University manages the Laboratory for the U.S. Department of Energy’s Office of Science, which is the nation’s single largest supporter of basic research in the physical sciences. Feel the heat at https://energy.gov/science and http://www.pppl.gov.
JOURNAL
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
ARTICLE TITLE
Orientation-dependent etching of silicon by fluorine molecules: A quantum chemistry computational study
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