New tomographic AFM imaging technique reveals that microstructural defects, generally thought to be detrimental, actually improve conductivity in cadmium telluride solar cells
A team of scientists studying solar cells made from cadmium telluride, a promising alternative to silicon, has discovered that microscopic “fault lines” within and between crystals of the material act as conductive pathways that ease the flow of electric current. This research—conducted at the University of Connecticut and the U.S. Department of Energy’s Brookhaven National Laboratory, and described in the journal Nature Energy—may help explain how a common processing technique turns cadmium telluride into an excellent material for transforming sunlight into electricity, and suggests a strategy for engineering more efficient solar devices that surpass the performance of silicon.
“If you look at semiconductors like silicon, defects in the crystals are usually bad,” said co-author Eric Stach, a physicist at Brookhaven Lab’s Center for Functional Nanomaterials (CFN). As Stach explained, misplaced atoms or slight shifts in their alignment often act as traps for the particles that carry electric current—negatively charged electrons or the positively charged “holes” left behind when electrons are knocked loose by photons of sunlight, making them more mobile. The idea behind solar cells is to separate the positive and negative charges and run them through a circuit so the current can be used to power houses, satellites, or even cities. Defects interrupt this flow of charges and keep the solar cell from being as efficient as it could be.
But in the case of cadmium telluride, the scientists found that boundaries between individual crystals and “planar defects”—fault-like misalignments in the arrangement of atoms—create pathways for conductivity, not traps.
These CTAFM images show a cadmium telluride solar cell from the top (above) and side profile (bottom) with bright spots representing areas of higher electron conductivity. The images reveal that the conductive pathways coincide with crystal grain boundaries. Credit: University of Connecticut.
Members of Bryan Huey’s group at the Institute of Materials Science at the University of Connecticut were the first to notice the surprising connection. In an effort to understand the effects of a chloride solution treatment that greatly enhances cadmium telluride’s conductive properties, Justin Luria and Yasemin Kutes studied solar cells before and after treatment. But they did so in a unique way.
Several groups around the world had looked at the surfaces of such solar cells before, often with a tool known as a conducting atomic force microscope. The microscope has a fine probe many times sharper than the head of a pin that scans across the material’s surface to track the topographic features—the hills and valleys of the surface structure—while simultaneously measuring location-specific conductivity. Scientists use this technique to explore how the surface features relate to solar cell performance at the nanoscale.
But no one had devised a way to make measurements beneath the surface, the most important part of the solar cell. This is where the UConn team made an important breakthrough. They used an approach developed and perfected by Kutes and Luria over the last two years to acquire hundreds of sequential images, each time intentionally removing a nanoscale layer of the material, so they could scan through the entire thickness of the sample. They then used these layer-by-layer images to build up a three-dimensional, high-resolution ‘tomographic’ map of the solar cell—somewhat like a computed tomography (CT) brain scan.
“Everyone using these microscopes basically takes pictures of the ‘ground,’ and interprets what is beneath,” Huey said. “It may look like there’s a cave, or a rock shelf, or a building foundation down there. But we can only really know once we carefully dig, like archeologists, keeping track of exactly what we find every step of the way—though, of course, at a much, much smaller scale.”
The resulting CT-AFM maps uniquely revealed current flowing most freely along the crystal boundaries and fault-like defects in the cadmium telluride solar cells. The samples that had been treated with the chloride solution had more defects overall, a higher density of these defects, and what appeared to be a high degree of connectivity among them, while the untreated samples had few defects, no evidence of connectivity, and much lower conductivity.
Huey’s team suspected that the defects were so-called planar defects, usually caused by shifts in atomic alignments or stacking arrangements within the crystals. But the CTAFM system is not designed to reveal such atomic-scale structural details. To get that information, the UConn team turned to Stach, head of the electron microscopy group at the CFN, a DOE Office of Science User Facility.
“Having previously shared ideas with Eric, it was a natural extension of our discovery to work with his group,” Huey said.
Said Stach, “This is the exact type of problem the CFN is set up to handle, providing expertise and equipment that university researchers may not have to help drive science from hypothesis to discovery.”
CFN staff physicist Lihua Zhang used a transmission electron microscope (TEM) and UConn’s results as a guide to meticulously study how atomic scale features of chloride-treated cadmium telluride related to the conductivity maps. The TEM images revealed the atomic structure of the defects, confirming that they were due to specific changes in the stacking sequence of atoms in the material. The images also showed clearly that these planar defects connected different grains in the crystal, leading to high-conductivity pathways for the movement of electrons and holes.
“When we looked at the regions with good conductivity, the planar defects linked from one crystal grain to another, forming continuous pathways of conductance through the entire thickness of the material,” said Zhang. “So the regions that had the best conductivity were the ones that had a high degree of connectivity among these defects.”
These transmission electron microscopy images taken at Brookhaven’s CFN reveals how the stacking pattern of individual atoms (bright spots) shifts. The images confirmed that the bright spots of high conductivity observed with CTAFM imaging at UConn occurred at the interfaces between two different atomic alignments (left) and that these “planar defects” were continuous between individual crystals, creating pathways of conductivity (right). The labels WZ and ZB refer to the two atomic stacking sequences “wurtzite” and “zinc blende,” which are the two types of crystal structures cadmium telluride can form.
The authors say it’s possible that the chloride treatment helps to create the connectivity, not just more defects, but that more research is needed to definitively determine the most significant effects of the chloride solution treatment.
In any case, Stach says that combining the CTAFM technique and electron microscopy, yields a “clear winner” in the search for more efficient, cost-competitive alternatives to silicon solar cells, which have nearly reached their limit for efficiency.
“There is already a billion-dollar-a-year industry making cadmium telluride solar cells, and lots of work exploring other alternatives to silicon. But all of these alternatives, because of their crystal structure, have a higher tendency to form defects,” he said. “This work gives us a systematic method we can use to understand if the defects are good or bad in terms of conductivity. It can also be used to explore the effects of different processing methods or chemicals to control how defects form. In the case of cadmium telluride, we may want to find ways to make more of these defects, or look for other materials in which defects improve performance.”
An international group of top biologists led by UConn ecologist Mark Urban is calling for a coordinated effort to gather important species information that is urgently needed to improve predictions for the impact of climate change on future biodiversity.
We need to pull on our boots, grab our binoculars, and go back into the field to gather more detailed information if we are going to make realistic predictions. — Mark Urban
Current prediction models fail to account for important biological factors like species competition and movement that can have a profound influence on whether a plant or animal survives changes to its environment, the scientists say in the Sept. 9 issue of the journal Science. While more sophisticated forecasting models exist, much of the detailed species information that is needed to improve predictions is lacking.
“Right now, we’re treating a mouse the same way as an elephant or a fish or a tree. Yet we know that those are all very different organisms and they are going to respond to their environment in different ways,” says Urban, the Science article’s lead author. “We need to pull on our boots, grab our binoculars, and go back into the field to gather more detailed information if we are going to make realistic predictions.”
The 22 biologists affiliated with the article identify six key types of biological information: life history, physiology, genetic variation, species interactions, dispersal, and response to environmental changes that will significantly improve prediction outcomes for individual species. Obtaining that information will not only help the scientific community better identify the most at-risk populations and ecosystems, the scientists say, it will also allow for a more targeted distribution of resources as global temperatures continue to rise at a record pace.
Current climate change predictions for biodiversity draw on broad statistical correlations and can vary widely, making it difficult for policymakers and others to respond accordingly. Many of those predictions tend not to hold up over time if they fail to account for the full range of biological factors that can influence an organism’s survival rate: species demographics, competition from other organisms, species mobility, and the capacity to adapt and evolve.
“We haven’t been able to sufficiently determine what species composition future ecosystems will have,” says co-author Karin Johst of the Helmholtz Centre for Environmental Research and the German Centre for Integrative Biodiversity Research. “This is because current ecological models often do not include important biological processes and mechanisms: so far only 23 percent of the reviewed studies have taken into account biological mechanisms.”
Generating more accurate predictions is essential for global conservation efforts. Many species are already moving to higher ground or toward the poles to seek cooler temperatures as global temperatures rise. But the capacity of different organisms to survive varies greatly. Some species of frog, for instance, can traverse their terrain for miles to remain in a habitable environment. Other species, such as some types of salamander, are less mobile and are capable of moving only a few meters over generations.
“New Zealand’s strong foundation in ecological research will help,” explains study co-author William Godsoe, a Lincoln University lecturer and member of New Zealand’s Bio-Protection Research Centre. “One of our hopes is to build on these strengths and highlight new opportunities to improve predictions by explicitly considering evolution, interactions among species, and dispersal.” This will aid in the development of strategies to manage impacts on species and ecosystems before they become critical.
With more than 8.7 million species worldwide, gathering the necessary biological information to improve predictions is a daunting task. Even a sampling of key species would be beneficial, the authors say, as the more sophisticated models will allow scientists to extrapolate their predictions and apply them to multiple species with similar traits.
The biologists are calling for the launch of a global campaign to be spearheaded by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services or IPBES. The IPBES operates under the auspices of four United Nations entities and is dedicated to providing scientific information to policymakers worldwide. One thousand scientists from all over the world currently contribute to the work of IPBES on a voluntary basis. The scientists are also encouraging conservation strategies to support biodiversity such as maintaining dispersal corridors, and preserving existing natural habitats and genetic diversity.
“Our biggest challenge is pinpointing which species to concentrate on and which regions we need to allocate resources,” says Urban, an associate professor of ecology and evolutionary biology at UConn. In an earlier study in Science, Urban predicted that as many as one in six species internationally could be wiped out by climate change. “We are at a triage stage at this point. We have limited resources and patients lined up at the door.”
The University of Connecticut (UConn) is a public research university in the US State of Connecticut.
Known as a Public Ivy, UConn was founded in 1881 and is a Land Grant and Sea Grant college & member of the Space Grant Consortium. The university serves more than 30,000 students on its six campuses, including more than 8,000 graduate students in multiple programs.
UConn’s main campus is located in Storrs, Connecticut. The university’s president is Susan Herbst.
UConn is one of the founding institutions of the Hartford, Connecticut/Springfield, Massachusetts regional economic and cultural partnership alliance known as New England’s Knowledge Corridor. UConn is a member of Universitas 21, a global network of 24 research-intensive universities, who work together to foster global citizenship and institutional innovation through research-inspired teaching and learning, student mobility, connecting students and staff, and promote advocacy for internationalisation. UConn is accredited by the New England Association of Schools and Colleges.
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