“The new cell is more efficient and has a simpler design that may be useful for a variety of new applications, such as highly area-constrained applications or low-radiation space applications,” said Myles Steiner, a senior scientist in NREL’s High-Efficiency Crystalline Photovoltaics (PV) Group and principal investigator on the project. He worked alongside NREL colleagues Ryan France, John Geisz, Tao Song, Waldo Olavarria, Michelle Young, and Alan Kibbler.

Details of the development are outlined in the paper “Triple-junction solar cells with 39.5% terrestrial and 34.2% space efficiency enabled by thick quantum well superlattices,” which appears in the May issue of the journal Joule.

NREL scientists previously set a record in 2020 with a 39.2% efficient six-junction solar cell using III-V materials.

Several of the best recent solar cells have been based on the inverted metamorphic multijunction (IMM) architecture that was invented at NREL. This newly enhanced triple-junction IMM solar cell has now been added to the Best Research-Cell Efficiency Chart. The chart, which shows the success of experimental solar cells, includes the previous three-junction IMM record of 37.9% established in 2013 by Sharp Corporation of Japan.

The improvement in efficiency followed research into “quantum well” solar cells, which utilize many very thin layers to modify solar cell properties. The scientists developed a quantum well solar cell with unprecedented performance and implemented it into a device with three junctions with different bandgaps, where each junction is tuned to capture and utilize a different slice of the solar spectrum.

The III-V materials, so named because of where they fall on the periodic table, span a wide range of energy bandgaps that allow them to target different parts of the solar spectrum. The top junction is made of gallium indium phosphide (GaInP), the middle of gallium arsenide (GaAs) with quantum wells, and the bottom of lattice-mismatched gallium indium arsenide (GaInAs). Each material has been highly optimized over decades of research.

“A key element is that while GaAs is an excellent material and generally used in III-V multijunction cells, it does not have quite the correct bandgap for a three-junction cell, meaning that the balance of photocurrents between the three cells is not optimal,” said France, senior scientist and cell designer. “Here, we have modified the bandgap while maintaining excellent material quality by using quantum wells, which enables this device and potentially other applications.”

The scientists used quantum wells in the middle layer to extend the bandgap of the GaAs cell and increase the amount of light that the cell can absorb. Importantly, they developed optically thick quantum well devices without major voltage loss. They also learned how to anneal the GaInP top cell during the growth process in order to improve its performance and how to minimize the threading dislocation density in lattice-mismatched GaInAs, discussed in separate publications. Altogether, these three materials inform the novel cell design.

III-V cells are known for their high efficiency, but the manufacturing process has traditionally been expensive. So far, III-V cells have been used to power applications such as space satellites, unmanned aerial vehicles, and other niche applications. Researchers at NREL have been working toward drastically reducing the manufacturing cost of III-V cells and providing alternate cell designs, which will make these cells economic for a variety of new applications.

The new III-V cell was also tested for how efficient it would be in space applications, especially for communications satellites, which are powered by solar cells and for which high cell efficiency is crucial, and came in at 34.2% for a beginning-of-life measurement. The present design of the cell is suitable for low-radiation environments, and higher-radiation applications may be enabled by further development of the cell structure.

Arrhythmogenic cardiomyopathy (ACM) is a familial heart disease in which heart muscle tissue is replaced by fat cells. It can lead to life threatening, irregular heartbeats. Currently, no therapy exists to cure arrhythmogenic cardiomyopathy and patients may ultimately need a heart transplantation. Therefore, the group of Eva van Rooij at the Hubrecht Institute collaborated with the UMC Utrecht with the aim to better understand the process of heart muscle degeneration in ACM, to ultimately contribute to the identification of new therapeutic targets to treat the disease.

Local differences

Previous studies on ACM used methods that take a snapshot of the gene expression in diseased tissue to aim at understanding what happens in the disease, but lacked the spatial resolution that is required to identify local differences within the heart. Therefore, the group of Eva van Rooij collaborated with the group of Alexander van Oudenaarden to use a technique called Tomo-Seq, that enabled the researchers to study gene expression within different areas of the heart. It turned out that knowing the location of the cells with changed gene activity was key to the discovery of novel areas of heart muscle degeneration.

Human hearts

The researchers used explanted hearts from ACM patients that received a heart transplantation. Heart disease is often studied in cultured cells or animal models, since the use of patient heart tissue is usually not possible. However, due to a collaboration with Aryan Vink from the Pathology Department of UMC Utrecht, the researchers had access to one of the largest and best documented heart tissue biobanks of the world. This gave them the unique opportunity to study the disease process in patient material. The researchers were able to closely look at the patient heart muscle cells that are degenerating, and identified new genes that may play an important role in this process. Among these genes is the transcription factor ZBTB11, a gene that was not previously known to be involved in arrhythmogenic cardiomyopathy.

Fat cells

The gene ZBTB11 is specifically expressed in the heart muscle cells that are close to the fat cells. The researchers showed that the activity of ZBTB11 induced degeneration of heart muscle cells, a process that is key in arrhythmogenic cardiomyopathy. This indicates that the fatty regions in the heart induce damage to the neighboring heart muscle cells. In the future, the researchers aim to study if heart muscle damage can be slowed down or even reversed in arrhythmogenic cardiomyopathy.

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Materials provided by Hubrecht Institute. Note: Content may be edited for style and length.

This research was recently published in Science Advances.

In some human-interactive electronics, such as temperature gauges or health sensors, polymers are used that are capable of changing color depending on stimuli. This phenomenon is referred to as stimuli-interactive structural colors because the material reacts and changes color due to environmental changes, such as a solvent or solution.

A material that has a one-dimensional periodic structure comprised of two (A and B) layers acts like a photonic crystal and can reflect light of a given wavelength (color) depending on the thickness of each layer. Stimuli-interactive structural color works by altering photonic crystals using external stimuli or forces. The thickness of each polymer layer affects the color of the light reflected: if all the layers in a material are of the same thickness, a single color will be reflected. If different parts of the material are composed of stacks of layers, each having a different thickness, each layer will reflect a different color and the material will appear like a normal metal material, reflecting all colors.

In some cases, a preferential solvent is used to swell one of the particular polymer layers, purposefully causing color changes. The researchers noticed that the expected layers were swelling in these materials. However, it was unclear how the solvent was seeping/crossing through layers that did not swell to those that were supposed to swell.

“Let’s say we put a solvent over multiple polymer A and B layers,” said Thomas. “The first A layer swells, the B layer doesn’t swell, but the next layer A will. How does the second layer of solvent A get through the B layer? We realized there must be something in the overall polymer structure that allows the passage of solvent to the other layers.”

To understand what was occurring within the polymers, the researchers used an electron beam to develop a tomogram — a reconstruction technique that takes very thin, two-dimensional images of sections of 3D objects to uncover what is inside.

“Suppose you had a loaf of bread, and you wanted to know if there was a hole somewhere within the loaf,” said Thomas. “If you sliced it thin, you’d eventually hit the hole. You keep slicing, and then the hole would disappear. If you looked at all the slices, you could understand exactly where the holes are. This process is similar to the idea of a tomograph.”

Using this method, the researchers found that within the polymer photonic crystal material, helicoidal screw dislocations (defects) were present, allowing the solvent to easily and rapidly cross through to different layers, causing the swelling and producing the stimuli-interactive structural color changes.

Typically, defects are associated with high energy and are singular (abruptly disrupting the periodicity occurring in one location). In contrast, the helicoidal defects are nonsingular and spontaneously formed — an advantage for the materials.

“This is a good kind of defect that helps properties and allows swift and efficient penetration into the material with solvent and rapid swelling. If these things didn’t exist, the only way the layers could sweat would be from the edges,” said Thomas.

Because stimuli-interactive structural color presents an excellent potential for devices such as health sensors and human-interactive electronics, controlling the lateral spacing or amount of helicoidal defects could be a critical factor in future applications.

“These defects currently produce a favorable effect, but it depends on the application,” he said. “Our next challenge is deciphering how to control the spacing and amount of these defects and, in turn, having more control over the time it takes for the fluid to move through the layers. Understanding these defects is key for increasing the number of applications this technology can be used in.”

The Hagler Institute Fellowship supported the research completed on this project at Texas A&M.

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Materials provided by Texas A&M University. Original written by Michelle Revels. Note: Content may be edited for style and length.