Microfluidic device generates passive hydraulic power, may be used to make small robots move.

MIT Tree on Chip Web
MIT engineers have designed a microfluidic device they call a “tree-on-a-chip,” which mimics the pumping mechanism of trees and other plants.

Trees and other plants, from towering redwoods to diminutive daisies, are nature’s hydraulic pumps. They are constantly pulling water up from their roots to the topmost leaves, and pumping sugars produced by their leaves back down to the roots. This constant stream of nutrients is shuttled through a system of tissues called xylem and phloem, which are packed together in woody, parallel conduits.

Now engineers at MIT and their collaborators have designed a microfluidic device they call a “tree-on-a-chip,” which mimics the pumping mechanism of trees and plants. Like its natural counterparts, the chip operates passively, requiring no moving parts or external pumps. It is able to pump water and sugars through the chip at a steady flow rate for several days. The results are published this week in Nature Plants.

Anette “Peko" Hosoi, professor and associate department head for operations in MIT’s Department of Mechanical Engineering, says the chip’s passive pumping may be leveraged as a simple hydraulic actuator for small robots. Engineers have found it difficult and expensive to make tiny, movable parts and pumps to power complex movements in small robots. The team’s new pumping mechanism may enable robots whose motions are propelled by inexpensive, sugar-powered pumps.

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Jennifer Chu | MIT News Office
March 20, 2017

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Novel laminated nanostructure gives steel bone-like resistance to fracturing under repeated stress.

MIT Nanostructure Tasan Web
Researchers at MIT and in Japan and Germany, have developed a type of steel with three characteristics that help it resist microcracks that lead to fatigue failure –: a layered nanostructure, a mixture of microstructural phases with different degrees of hardness, and a metastable composition. They compared samples of metal with just one or two of these key attributes (top left, top right, and bottom left) and with all three (bottom right). The metal alloy with all three attributes outperformed all the others in crack resistance. Images, Courtesy of the researchers

Metal fatigue can lead to abrupt and sometimes catastrophic failures in parts that undergo repeated loading, or stress. It’s a major cause of failure in structural components of everything from aircraft and spacecraft to bridges and powerplants. As a result, such structures are typically built with wide safety margins that add to costs.

Now, a team of researchers at MIT and in Japan and Germany has found a way to greatly reduce the effects of fatigue by incorporating a laminated nanostructure into the steel. The layered structuring gives the steel a kind of bone-like resilience, allowing it to deform without allowing the spread of microcracks that can lead to fatigue failure.

The findings are described in a paper in the journal Science by C. Cem Tasan, the Thomas B. King Career Development Professor of Metallurgy at MIT; Meimei Wang, a postdoc in his group; and six others at Kyushu University in Japan and the Max Planck Institute in Germany.

“Loads on structural components tend to be cyclic,” Tasan says.

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David L. Chandler | MIT News Office
March 9, 2017

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The Materials Processing Center and the Center for Materials Science and Engineering have selected 12 outstanding undergraduates to conduct graduate-level research on the MIT campus in Cambridge, Mass., from June 15, 2017 to August 5, 2017.

Njie Graphene CloseUp 6498 Web
Suna Njie, a 2016 Summer Scholar, soaks graphene in water to remove an etching solution. She worked in the Strano Lab at MIT under postdoc Pingwei Liu. Photo, Denis Paiste, Materials Processing Center

This year’s Summer Scholars are:

• Alejandro Aponte, University of Puerto Rico, Mayaguez
• Stephanie Bauman, University of South Florida
• Lucia Brunel, Northwestern University
• Richard Canty, University of Virginia
• Amrita Duggal, California State University, Channel Islands
• Kaila Holloway, Howard University
• Saleem Iqbal, University of New Mexico
• Gaetana Michelet, University of Puerto Rico, Mayaguez
• Grace Noel, The Pennsylvania State University
• Alexandra Oliveira, University of Connecticut
• Kirill Shmilovich, University of Wisconsin - Milwaukee
• Luke Soule, New Mexico Institute of Mining and Technology

Interns select their own projects from faculty presentations given during the first few days of the program. MPC and the CMSE sponsor the eight-week National Science Foundation Research Experience for Undergraduates internships with support from NSF’s Materials Research Science and Engineering Centers program [grant DMR-14-19807].

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A bio-inspired gel material developed at MIT could help engineers control movements of soft robots.

A new material that naturally adapts to changing environments was inspired by the strength, stability, and mechanical performance of the jaw of a marine worm. The protein material, which was designed and modeled by researchers from the Laboratory for Atomistic and Molecular Mechanics (LAMM) in the Department of Civil and Environmental Engineering (CEE), and synthesized in collaboration with the Air Force Research Lab (AFRL) at Wright-Patterson Air Force Base, Ohio, expands and contracts based on changing pH levels and ion concentrations. It was developed by studying how the jaw of Nereis virens, a sand worm, forms and adapts in different environments.

The resulting pH- and ion-sensitive material is able to respond and react to its environment. Understanding this naturally-occurring process can be particularly helpful for active control of the motion or deformation of actuators for soft robotics and sensors without using external power supply or complex electronic controlling devices. It could also be used to build autonomous structures.

“The ability of dramatically altering the material properties, by changing its hierarchical structure starting at the chemical level, offers exciting new opportunities to tune the material, and to build upon the natural material design towards new engineering applications,” wrote Markus J. Buehler, the McAfee Professor of Engineering, head of CEE, and senior author of the paper.

The research, recently published in ACS Nano, shows that depending on the ions and pH levels in the environment, the protein material expands and contracts into different geometric patterns.

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Read more at the MIT News Office.

Carolyn Schmitt | Department of Civil and Environmental Engineering
March 20, 2017 

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Graduate students, postdocs must apply by March 31, 2017, for two-week program featuring leaders in Quantum Computing science and technology.

Modular Quantum Computer Edwards JQI Web
Ion qubit modules (small cubes on left) can be linked using photonic interconnects. A switch, depicted here as the large cube on right, would contain microelectromechanical mirrors to change connections between any arbitrary pair of modules. Illustration, Emily Edwards, Joint Quantum Institute and University of Maryland.

The Quantum Science Summer School (QS3), a program jointly organized by MIT, Johns Hopkins University, Cornell University and Pennsylvania State University, kicks off this summer with its initial two-week session focused on “Fundamentals and Applications of Quantum Computing.”

The program will be held from June 5 to 16, 2017, on the Johns Hopkins Homewood campus in Baltimore, Md. Jointly supported by the National Science Foundation and the Department of Energy, QS3 is open to qualified graduate students and postdoctoral associates with a maximum of 40 participants. Accepted participants will be awarded round-trip travel within the U.S. and attendance expenses. The application deadline is March 31, 2017.

“A summer school is a great opportunity for students to get in-depth exposure to new concepts and also to have extended interactions with leaders in the field as well as their peers,” says MIT Assistant Professor of Physics Joseph G. Checkelsky, who is one of the program organizers. “It’s an opportunity to develop a solid foundation in both the theoretical and experimental understanding in a setting that bridges the gap between classroom teaching and a research seminar.”

Just as the understanding of quantum mechanics in the early 1900s unleashed a wave of new science and technology from the laser to the transistor, science now appears poised for a second quantum revolution with breakthroughs in computing, communications, energy and other fields. “A critical component of this development is the education of the next generation of scientists in the principles, methods, and goals of this rapidly expanding field. There is a challenge here given the breadth of disciplines brought together in this area – reaching across mathematics, chemistry, physics, materials science, and engineering. An educational program that helps to address this challenge is a step towards the work in computing, telecommunications, and new energy technologies that will shape the 21st century,” Checkelsky says. Checkelsky previously served as co-organizer for the Boulder Condensed Matter and Materials Physics Summer School.

QS3 presentations are geared towards building up participants’ basic knowledge towards the cutting edge. “We’ve tried to prepare this school with all the necessary pieces,” he says. “Some aspects will touch on a student’s particular expertise, but they will undoubtedly also have broader exposure and a chance to learn something about the other sub-disciplines that are related to quantum science and quantum computing. That can be an invigorating educational experience, particularly as part of a focused summer school.”

APPLY

The lecture-based program will include talks by:

Jason Alicea, professor of theoretical physics, Caltech.
Alán Aspuru-Guzik, professor of chemistry and chemical biology, Harvard.
Danna Freedman, assistant professor of chemistry, Northwestern.
Steven Girvin, deputy provost for science and technology and Eugene Higgins professor of physics and applied physics, Yale.
John M. Martinis, Quantum Artificial Intelligence team, Google, & professor of condensed matter experimental physics, University of California, Santa Barbara.
Douglas T. McClure, research staff member, experimental quantum computing, IBM Thomas J. Watson Research Center.
Christopher Monroe, distinguished university professor and Bice Seci-Zorn professor of physics, University of Maryland.
Scott D. Pakin, scientist, Programming Models team, Los Alamos National Laboratory.
David S. Weiss, associate head for research and professor of physics, Pennsylvania State University.

The NSF/DOE Quantum Science Summer School is dedicated to educating graduate students and postdocs in the fields of condensed matter physics, materials science, and quantum science and their applications to new technologies in academic and industrial contexts.

Program organizers are: Joe Checkelsky, assistant professor of physics, MIT; Natalia Drichko, associate research scientist, physics and astronomy, Johns Hopkins University; Lawrence C. (1944) and Sarah W. Biedenharn Career Development Assistant Professor Liang Fu, MIT; Kyle Shen, associate professor of physics, Cornell Universit y; and Jun Zhu, associate professor of physics, Pennsylvania State University. The site of future summer programs will rotate.

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March 21, 2017

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Study sheds light on interactions that change the way heat and electricity move through microchips.

MIT Measuring Dislocation Web
Atomic irregularities in crystals, known as dislocations, are common defects, and they affect how heat dissipates through a silicon microchip or how well current flows through a silicon solar panel. New MIT research offers insights into how these crystal dislocations affect electrical and heat transport through crystals, at a microscopic, quantum mechanical level. The MIT team has found a new mathematical approach using a new quasiparticle they formulated called a “dislon,” which is a quantized version of a dislocation.

New research offers insights into how crystal dislocations — a common type of defect in materials — can affect electrical and heat transport through crystals, at a microscopic, quantum mechanical level.

Dislocations in crystals are places where the orderly three-dimensional structure of a crystal lattice — whose arrangement of atoms repeats with exactly the same spacing — is disrupted. The effect is as if a knife had sliced through the crystal and then the pieces were stuck back together, askew from their original positions. These defects have a strong effect on phonons, the modes of lattice vibration that play a role in the thermal and electrical properties of the crystals through which they travel. But a precise understanding of the mechanism of the dislocation-phonon interaction has been elusive and controversial, which has slowed progress toward using dislocations to tailor the thermal properties of materials.

A team at MIT has been able to learn important details about how those interactions work, which could inform future efforts to develop thermoelectric devices and other electronic systems. The findings are reported in the journal Nano Letters, in a paper co-authored by postdoc Mingda Li, Department of Mechanical Engineering head Professor Gang Chen, the late Institute Professor Emerita Mildred Dresselhaus, and five others.

Dislocations — which Li describes as “atomic irregularities in a regular crystal” — are very common defects in crystals, and they affect, for example, how heat dissipates through a silicon microchip or how well current flows through a silicon solar panel.

Read more at the MIT News Office.

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David L. Chandler | MIT News Office
March 14, 2017

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Researchers have discovered a new way to tune electronic energy levels in some 2-D materials.

MIT Valleytronics 1 Laser Web
Researchers at MIT have discovered a new way of using laser light to tune electronic energy levels in two-dimensional films of crystal. The discovery could ultimately pave the way for the development of so-called “valleytronic” devices, which harness the way electrons gather around two equal energy states, known as valleys. Photo, Jose-Luis Olivares, MIT.

Faster, more efficient data storage and computer logic systems could be on the horizon thanks to a new way of tuning electronic energy levels in two-dimensional films of crystal, discovered by researchers at MIT.

The discovery could ultimately pave the way for the development of so-called “valleytronic” devices, which harness the way electrons gather around two equal energy states, known as valleys.

Engineers have for some time warned that we are reaching the limits of how small we can build conventional electronic transistors, which are based on electrons’ electrical charge.

As a result, researchers have been investigating the utility of a property of electrons known as spin, to store and manipulate data; such technologies are known as spintronics.

But as well as their charge and spin, electrons in materials also have another “degree of freedom,” known as the valley index. This is so-called because plotting the energy of electrons relative to their momentum results in a graph consisting of a curve with two valleys, which are populated by electrons as they move through a material.

Harnessing this degree of freedom could allow information to be stored and processed in some materials by selectively populating the two valleys with electrons.

However, developing such valleytronic devices requires a system to selectively control the electrons within the two valleys, which has so far proven very difficult to achieve.

In a paper published March 9 in the journal Science, researchers led by Nuh Gedik, an associate professor of physics at MIT, describe a new way of using laser light to control the electrons in both valleys independently, within atomically thin crystals of tungsten disulfide.

Read more at the MIT News Office.

Helen Knight | MIT News correspondent
March 9, 2017

Published in Daily News

Researchers discover that chaos makes carbon materials lighter and stronger.

 

In the quest for more efficient vehicles, engineers are using harder and lower density carbon materials, such as carbon fibers, which can be manufactured sustainably by "baking" naturally occurring soft hydrocarbons in the absence of oxygen. However the optimal "baking" temperature for these hardened, charcoal-like carbon materials remained a mystery since the 1950s when British scientist Rosalind Franklin, who is perhaps better known for providing critical evidence of DNA’s double helix structure, discovered how the carbon atoms in sugar, coal and similar hydrocarbons, react to temperatures approaching 3,000 degrees Celsius (5,432 F) in oxygen-free processing. Confusion over whether disorder makes these graphite-like materials stronger, or weaker, prevented identifying the ideal "baking" temperature for more than 40 years.

Fewer, more chaotically arranged carbon atoms produce higher strength materials, MIT researchers report in the journal Carbon. They find a tangible link between the random ordering of carbon atoms within a phenol-formaldehyde resin, which was “baked” at high temperatures, and the strength and density of the resulting graphite-like carbon material. Phenol-formaldehyde resin is a hydrocarbon commonly known as “SU-8” in the electronics industry. Additionally, by comparing the performance of the “baked” carbon material, the MIT researchers identified a “sweet spot” manufacturing temperature: 1,000 degrees Celsius (1,832 F).

“These materials we’re working with, which are commonly found in SU-8 and other hydrocarbons that can be hardened using ultraviolet [UV] light, are really promising for making strong and light lattices of beams and struts on the nanoscale, which only recently became possible due to advances in 3D printing,” says MIT postdoc Itai Stein [SM ’13, PhD ’16]. “But up to now, nobody really knew what happens when you’re changing the manufacturing temperature, that is, how the structure affects the properties. There was a lot of work on structure and a lot of work on properties, but there was no connection between the two. ... We hope that our study will help to shed some light on the governing physical mechanisms that are at play.”

Stein, who is the lead author of the paper published in Carbon, led a team under Professor of Aeronautics and Astronautics Brian L. Wardle, consisting of junior Chlöe V. Sackier, alums Mackenzie E. Devoe [SB ’15] and Hanna M. Vincent [SB, ’14], and undergraduate Summer Scholars Alexander J. Constable [2015] and Naomi Morales-Medina [2014].

“Our investigations into this carbon material as a matrix for nanocomposites kept leading to more questions making this topic increasingly interesting in and of itself. Through a series of contributions, notably from MIT undergraduate researchers and Summer Scholars, a sustained investigation of several years resulted, allowing some paradoxical results in the extant literature to be resolved,” says Prof. Wardle.

By “baking” the resin at high temperature in inert gas, a process commonly known as pyrolysis, the researchers formed a type of disordered graphite-like carbon material that is often called glassy carbon. Stein and Wardle showed that when it is processed at temperatures higher than 1,000 degrees Celsius, the material becomes more ordered but weaker. They estimated the strength of their glassy carbon by applying a local force and measuring their material's ability to resist deformation. This type of measurement, which is known to engineers as the Vickers hardness test, is a highly versatile technique that can be used to study a wide variety of materials, such as metals, glasses, and plastics, and enabled the researchers to compare their findings to many well-known engineering materials that include diamond, carbon fiber composites, and metal carbides.

Some disorder is good

The carbon atoms within the MIT researchers' material were more chaotically organized than is typical for graphite, and this was because phenol-formaldehyde with which they started is a complicated mix of carbon-rich compounds. “Because the hydrocarbon was disordered to begin with, a lot of the disorder remains in your crystallites, at least at this temperature,” Stein explains. In fact, the presence of more complex carbon compounds in the material strengthens it by leading to three-dimensional connections that are hard to break. “Basically you get pinned at the crystallite interface, and that leads to enhanced performance,” he says.

These high-temperature baked materials have only one carbon atom in their structure for every three in a diamond structure. “When you’re using these materials to make nanolattices, you can make the overall lattice even less dense. Future studies should be able to show how to make lighter and cheaper materials,” Stein suggests. Hydrocarbons similar to the phenol-formaldehyde studied here can also be sourced in an environmentally friendly way, he says.

“Up until now there wasn’t really consensus about whether having a low density was good or bad, and we’re showing in this work, that having a low density is actually good,” Stein says. That’s because low density in these crystallites means more molecular connections in three dimensions, which helps the material resist shearing, or sliding apart. Because of its low density, this material compares favorably to diamond and boron nitrides for aerospace uses. “Essentially you can use a lot more of this material and still end up saving weight overall,” Stein says.

“This study represents sound materials science – connecting all three facets of synthesis, structure, and property – toward elucidating poorly understood scaling laws for mechanical performance of pyrolytic carbon,” says Dr. Eric Meshot, a staff scientist at Lawrence Livermore National Laboratory, who was not involved in this research. “It is remarkable that by employing routinely available characterization tools, the researchers pieced together both the molecular and nanoscale structural pictures and deciphered this counterintuitive result that more graphitization does not necessarily equal a harder material. It is an intriguing concept in and of itself that a little structural disorder can enhance the hardness.”

“Their structural characterization proves how and why they achieve high hardness at relatively low synthesis temperatures,” Meshot adds. “This could be impactful for industries seeking to scale up production of these types of materials since heating is a seriously costly step.” The study also points to new directions for making low-density composite structures with truly transformative properties, he suggests. “For example, by incorporating the starting SU-8 resin in, on, or around other structures (such as nanotubes as the authors suggest), can we synthesize materials that are even harder or more resistant to sheer? Or composites that possibly embed additional functionality, such as sensing?” Meshot asks.

The new research has particular relevance now because a group of German researchers showed last year in a Nature Materials paper how these materials can form highly structured nanolattices that are strong, lightweight and are outperformed only by diamond. Those researchers processed their material at 900 degrees Celsius, Stein notes. “You can do a lot more optimization, knowing what the scaling is of the mechanical properties with the structure, then you can go ahead and tune the structure accordingly, and that’s where we believe there is broad implication for our work in this study,” he says.

Summer Scholars role

Constable [Penn State, ’16] and Morales-Medina [University of Puerto Rico-Rio Piedras, ’15] participated in Wardle’s lab through the annual Summer Scholars program of the Materials Processing Center and the Center for Materials Science and Engineering at MIT, which is a National Science Foundation Research Experience for Undergraduates program [grants DMR-08-19762 and DMR-1419807]. “They were only here for a very short amount of time, and they got a lot of data collected,” Stein says. Morales-Medina, who is currently a PhD student at the Medical Physics Program at Duke University, developed techniques for the in situ X-Ray Diffraction used to evaluate how this pyrolytic carbon evolves as a function of temperature, and Constable, who is currently an M.Sc. Candidate in Engineering Mechanics at The University of Texas at Austin, collected data that was published in the current paper. They also quantified the crystal sizes and surface chemistries using shared experimental facilities at the Center for Materials Science and Engineering.

This work was partly supported by MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium members Airbus Group, Boeing, Embraer, Lockheed Martin, Saab AB, ANSYS, Hexcel, and TohoTenax. Stein was supported in part by a National Defense Science & Engineering Graduate Fellowship.
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Denis Paiste, Materials Processing Center
Updated March 17, 2017

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MIT mechanical engineers are leading an effort to detect defects that reduce efficiency.

 

As the world transitions to a low-carbon energy future, near-term, large-scale deployment of solar power will be critical to mitigating climate change by midcentury. Climate scientists estimate that the world will need 10 terawatts (TW) or more of solar power by 2030 — at least 50 times the level deployed today. At the MIT Photovoltaics Research Laboratory (PVLab), teams are working both to define what’s needed to get there and to help make it happen. “Our job is to figure out how to reach a minimum of 10 TW in an economically and environmentally sustainable way through technology innovation,” says Tonio Buonassisi, associate professor of mechanical engineering and lab director.

Their analyses outline a daunting challenge. First they calculated the growth rate of solar required to achieve 10 TW by 2030 and the minimum sustainable price that would elicit that growth without help from subsidies. Current technology is clearly not up to the task. “It would take between $1 trillion and $4 trillion of additional debt to just push current technology into the marketplace to do the job, and that’d be hard,” says Buonassisi. So what needs to change?

Using models that combine techno­logical and economic variables, the researchers determined that three changes are required: reduce the cost of modules by 50 percent, increase the conversion efficiency of modules (the fraction of solar energy they convert into electricity) by 50 percent, and decrease the cost of building new factories by 70 percent.

Making strides on efficiency

Major gains are already being made on the conversion efficiency front — both at the MIT PVLab and around the world. One especially promising technology is the passivated emitter and rear cell (PERC), which is based on low-cost crystalline silicon but has a special “architecture” that captures more of the sun’s energy than conventional silicon cells do. While costs must be brought down, the technology promises to bring a 7 percent increase in efficiency, and many experts predict its widespread adoption.

But there’s been a problem. In field tests, some modules containing PERC solar cells have degraded in the sun, with conversion efficiency dropping by fully 10 percent in the first three months. “These modules are supposed to last 25 years, and within just weeks to months they’re generating only 90 percent as much electricity as they’re designed for,” says Ashley Morishige, postdoc in mechanical engineering. That behavior is perplexing because manufacturers thoroughly test the efficiency of their products before releasing them. In addition, not all modules exhibit the problem, and not all companies encounter it. Interestingly, it took up to a few years before individual companies realized that other companies were having the same problem. Manufacturers came up with a variety of engineering solutions to deal with it, but its exact cause remained unknown, prompting concern that it could recur at any time and could affect next-generation cell architectures.

To Buonassisi, it seemed like an opportunity. His lab generally focuses on basic materials problems at the wafer and cell level, but the researchers could equally well apply their equipment and expertise to modules and systems. By defining the problem, they could support the adoption of this energy-efficient technology, helping to bring down materials and labor costs for each watt of power generated.

Working closely with an industrial solar cell manufacturer, the MIT team undertook a “root-cause analysis” to define the source of the problem. The company had come to them for help with the unexpected degradation of their PERC modules and reported some odd trends. PERC modules stored in sunlight for 60 days with their wires connected into a closed loop lost no more efficiency than conventional solar cells typically do during their break-in period. But modules stored in sunlight with open circuits degraded significantly more. In addition, modules made from different silicon ingots displayed different power-loss behavior. And, as shown in Figure 1 in the slideshow above, the drop in efficiency was markedly higher in modules made with cells that had been fabricated at a peak temperature of 960 degrees Celsius than in those containing cells fired at 860 C.

Subatomic misbehavior

Understanding how defects can affect conversion efficiency requires understanding how solar cells work at a fundamental level. Within a photoreactive material such as silicon, electrons exist at two distinct energy levels. At the lower level, they’re in the “valence band” and can’t flow; at the higher level, they’re in the “conduction band” and are free to move. When solar radiation shines onto the material, electrons can absorb enough energy to jump from the valance band to the conduction band, leaving behind vacancies called holes. If all is well, before the electrons lose that extra energy and drop back to the valence band, they travel through an external circuit as electric current.

Generally, an electron or hole has to gain or lose a set amount of energy to move from one band to the other. (Although holes are defined as the absence of electrons, physicists view both electrons and holes as “moving” within semiconductors.) But sometimes a metal impurity or a structural flaw in the silicon provides an energy “state” between the valence and conduction bands, enabling electrons and holes to jump to that intermediate energy level — a move achieved with less energy gain or loss. If an electron and hole both make the move, they can recombine, and the electron is no longer available to pass through the external circuit. Power output is lost.

The PVLab researchers quantify that behavior using a measure called lifetime — the average time an electron remains in an excited state before it recombines with a hole. Lifetime critically affects the energy conversion efficiency of a solar cell, and it is “exquisitely sensitive to the presence of defects,” says Buonassisi.

To measure lifetime, the team — led by Morishige and mechanical engineering graduate student Mallory Jensen — uses a technique called lifetime spectroscopy. It involves shining light on a sample or heating it up and monitoring electrical conductivity during and immediately afterward. When current flow goes up, electrons excited by the added energy have jumped into the conduction band. When current drops, they’ve lost that extra energy and fallen back into the valence band. Changes in conductivity over time thus indicate the average lifetime of electrons in the sample.

Locating and characterizing the defect

To address the performance problems with PERC solar cells, the researchers first needed to figure out where in the modules the primary defects were located. Possibilities included the silicon surface, the aluminum backing, and various interfaces between materials. But the MIT team thought it was likely to be in the bulk silicon itself.

To test that assumption, they used partially fabricated solar cells that had been fired at 750 C or at 950 C and — in each category — one that had been exposed to light and one that had been kept in the dark. They chemically removed the top and bottom layers from each cell, leaving only the bare silicon wafer. They then measured the electron lifetime of all the samples. As shown in Figure 2 in the slideshow above, with the low-temperature pair, lifetime is about the same in the light-exposed and unexposed samples. But with the high-temperature pair, lifetime in the exposed sample is significantly lower than that in the unexposed sample.

Those findings confirm that the observed degradation is largely attributable to defects that are present in the bulk silicon and — when exposed to light — affect lifetime, thus conversion efficiency, in cells that have been fired at higher temperatures. In follow-up tests, the researchers found that by reheating the degraded samples at 200 C for just an hour, they could bring the lifetime back up — but it dropped back down with re-exposure to light.

So how do those defects interfere with conversion efficiency, and what types of contaminants might be involved in their formation? Two characteristics of the defects would help the researchers answer those questions. First is the energy level of the defect — where it falls between the valence and conduction bands. Second is the “capture cross section,” that is, the area over which a defect at a particular location can capture electrons and holes. (The area might be different for electrons than for holes.)

While those characteristics can’t easily be measured directly in the samples, the researchers could use a standard set of equations to infer them based on lifetime measurements taken at different illumination intensities and test temperatures. Using samples that had been fired at 950 C and then exposed to light, they ran lifetime spectroscopy experiments under varying test conditions. With the gathered data, they calculated the energy level and capture cross section of the primary defect causing recombination in their samples. They then consulted the literature to see what elements are known to exhibit those characteristics, making them likely candidates for causing the drop in conversion efficiency observed in their samples.

According to Morishige, the team has narrowed down the list of candidates to a handful of possibilities. “And at least one of them is consistent with much of what we’ve observed,” she says. In this case, a metal contaminant creates defects in the crystal lattice of the silicon during fabrication. Hydrogen atoms that are present combine with those metal atoms, making them electrically neutral so they don’t serve as sites for electron-hole recombination. But under some conditions — notably, when the density of electrons is high — the hydrogen atoms dissociate from the metal, and the defects become very recombination-active.

That explanation fits with the com­pany’s initial reports on their modules. Cells fired at higher temperatures would be more susceptible to light-induced damage because the silicon in them typically contains more impurities and less hydrogen. And performance would vary from ingot to ingot because different batches of silicon contain different concentrations of contaminants as well as hydrogen. Finally, baking the silicon at 200 C — as the researchers did — could cause the hydrogen atoms to recombine with the metal, neutralizing the defects.

Based on that possible mechanism, the researchers offer manufacturers two recommendations. First, try to adjust their manufacturing processes so that they can perform the firing step at a lower temperature. And second, make sure that their silicon has sufficiently low concentrations of certain metals that the researchers have pinpointed as likely sources of the problem.

Unintended consequences

The bottom line, observes Buonassisi, is that the very feature that makes the PERC technology efficient — the special architecture designed to capture solar energy efficiently — is what reveals a problem inherent in the fabricated material. “The cell people did everything right,” he says. “It’s the quintessential law of unintended consequences.” And if the problem is the higher density of excited electrons interacting with defects in the silicon wafer, then developing effective strategies for dealing with it will only get more important because next-generation device designs and decreasing wafer thicknesses will bring even higher electron densities.

To Buonassisi, this work demonstrates the importance of talking across boundaries. He advocates communication among all participants in the solar community — both private companies and research organizations — as well as collaboration among experts in every area — from feedstock materials to wafers, cells, and modules to system integration and module installation. “Our laboratory is taking active steps to bring together a community of stakeholders and create a vertically integrated R&D platform that I hope will enable us to more quickly address the technical challenges and help lead to 10 TW of PV by 2030,” he says.

This research was funded by the National Science Foundation, the U.S. Department of Energy, and the National Research Foundation Singapore through the Singapore-MIT Alliance for Research and Technology.

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Nancy W. Stauffer | MIT Energy Initiative
February 17, 2017

Published in Newsletter Articles

Faculty from four MIT departments among 126 selected from across the U.S. and Canada.

 

MIT Physics 02212017 Sloan Fakhri Perez Web
MIT Assistant Professors of Physics Nikta Fakhri [left] and Kerstin Perez are among the 126 American and Canadian researchers awarded 2017 Sloan Research Fellowships, the Alfred P. Sloan Foundation announced Feb. 21, 2017.

Seven MIT researchers from four departments are among the 126 American and Canadian researchers awarded 2017 Sloan Research Fellowships, the Alfred P. Sloan Foundation announced Feb. 21, 2017.

New MIT-affiliated Sloan Research Fellows are: Mohammad Alizadeh, the TIBCO Career Development Assistant Professor in Electrical Engineering and Computer Science and a member of the Computer Science and Artificial Intelligence Laboratory (CSAIL); Semyon Dyatlov, an assistant professor of mathematics; Nikta Fakhri, an assistant professor of physics; Kerstin Perez, an assistant professor of physics; Aaron Pixton, an assistant professor of mathematics; Caroline Uhler, an assistant professor of electrical engineering and computer science, and a member of the Institute for Data, Systems, and Society and of the Laboratory for Information and Decision Systems; and Alexander Wolitzky, an associate professor of economics.

The new fellows also includes new faculty member Virginia Vassilevska Williams, the Steven and Renee Finn Career Development Associate Professor of Electrical Engineering and Computer Science and a member of CSAIL, who is being honored for work done at Stanford University, before she joined MIT in January 2017.

Read more at the MIT News Office.

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MIT News Office
February 21, 2017

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