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ASU engineers' work brings solar panels step closer to cheaper, more accessible.
May 17, 2016

New inventions from ASU researchers may lead to cheaper, more efficient solar power

Companies making solar panels have faced the same choice for decades. Their sun-soaking panels could be efficient or cheap, but not both.

Time to start erasing that rule.

Researchers within the Ira A. Fulton Schools of Engineering have wedded two types of solar technologies, putting solar panels a step closer to being cheaper and more accessible.

Here’s the challenge researchers faced: solar panels made with silicon are expensive but more efficient than the cheaper, thin film solar cells, which are made with cadmium telluride.

The ASU team, led by electrical engineering professor Yong-Hang Zhang and assistant professor Zachary Holman, figured out how to add a little silicon to the thin film cells, combining the qualities of each type of panel.

Their invention broke an efficiency record for thin film cells and achieved the highest open-circuit voltage ever recorded for that type of cell. Their results were published this week in a paper in the journal Nature Energy.

Overcoming obstacles

Open-circuit voltage measures the potential for a solar cell to pump electricity around a circuit. High voltage is created when light is absorbed in a solar cell, exciting electrons by shaking them off their atoms. The electrons then build up on one side of the solar cell, like at the negative terminal of a battery.

ASU electrical engineering professor Yong-Hang Zhang

Engineering a solar cell with high voltage is challenging because the excited electrons can be lost within microseconds or even nanoseconds of sunlight hitting a solar cell. Thus, a goal of solar cell research is to extract the electricity before it dissipates, which is generally accomplished by adding conductive contacts to the top and bottom of a solar cell, according to Zhang (pictured left), who also is an associate dean for research in the Ira A. Fulton Schools of Engineering.

“The traditional contacts are made through introducing impurities in the solar cell absorbing layer,” said Zhang, “which can degrade the device performance dramatically.”

Zhang, Holman and their research teams added a separate contact layer of low-cost amorphous silicon instead of an impurity. In doing so, they created a solar cell with a voltage of 1.1 volt, an unimaginable feat even one year ago.

“Essentially, we’ve created a solar cell that allows for the maximum number of electrons possible to build up before extracting them quickly and efficiently out the ‘smart’ contact,” Zhang said.

The cells not only reached high voltage but also a 17 percent efficiency, breaking a record of 15.2 percent for thin film solar cells. While other types of solar cells, such as silicon, boast a best efficiency rating of around 25 percent, such a dramatic improvement in thin film efficiency shows promise for widespread use.

Zhang’s next goal is 20 percent efficiency or more.

Impact on industry

Materials science doctoral student Calli Campbell fabricates solar cells.

Materials science doctoral student Calli Campbell uses a molecular beam epitaxy machine to fabricate the solar cell wafers in Yong-Hang Zhang’s lab. Photo courtesy of Yong-Hang Zhang/ASU

 

“The important thing is that this material system has been proven cost effective, but (until now) never efficient enough in terms of energy production to take over the solar market,” said Zhang. “Many times with a paper such as this, the findings are either scientifically interesting or commercially applicable, but not both. However, these results are.”

While silicon solar panels dominate the market, there are about 10 gigawatts — enough to power 2.5 million homes — of thin film solar panels in use worldwide today. First Solar in Tempe, Arizona, is the world’s largest manufacturer of thin-film solar cells.

“These latest results further confirm our long-standing conviction that CdTe (cadmium telluride) is an ideal material choice for photovoltaic application,” said Markus Gloeckler, vice president of advanced research at First Solar. “Reaching an open-circuit voltage of 1.1 volts is a milestone for the technology and provides confidence that thin-film CdTe has not reached its limits.”

The first step in an ongoing collaboration

The success grew out of an unanticipated alliance between Zhang and Holman’s separate teams.

ASU assistant professor Zachary Holman

“It’s a unique collaboration, and one that happened in the best way possible: through student initiative,” said Holman (pictured left). “One of Yong’s students reached out to one of my post-docs, and things took off from there.”

Though Zhang and his group initially conceived the underlying concept of these breakthrough solar cells, it took them nearly two years to improve the materials needed to fabricate them. They also called upon Holman’s expertise in silicon solar cells to marry two very different semiconductors to achieve these unique and efficient properties.

Zhang and Holman look to use their respective expertise to collaborate in the future as well.

“We’re exploring the possibility of developing a tandem solar cell,” Holman said, “basically two complimentary solar cells that would stack on top of one another, further boosting efficiency.”

The two teams plan to continue collaboration and are slated to present their recent results at the upcoming Institute of Electrical and Electronics Engineers’ Photovoltaic Specialists Conference in Portland, Oregon, in June.

The research was mainly supported by funding from the U.S. Department of Energy’s Bay Area Photovoltaic Consortium (BAPVC), a collaboration between universities, industry and government dedicated to improving photovoltaic technology, jointly led by Stanford University and the University of California Berkeley. Additional funding was supplied through Quantum Energy and Sustainable Solar Technologies (QESST), one of the four National Science Foundation-funded Engineering Research Centers at the Fulton Schools at ASU.

Initial support for this research came from the Science Foundation Arizona in 2007, and following funding from the National Science Foundation, the Army Research Office, the Air Force Research Laboratory and Air Force Office of Scientific Research have paved the way to enabled many of these new ideas developed in the past 10 years, according to Zhang.

 

Top photo: A novel approach to a materials science challenge has birthed a record-breaking monocrystalline cadmium telluride solar cell, which boasts the highest voltage ever for its type of cell. Photo by Cheng-Ying Tsai/ASU

 

 

Pete Zrioka

Communications specialist , Ira A. Fulton Schools of Engineering

480-727-5618

 
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ASU engineers' edible supercapacitor can wipe out E. coli or power a camera.
Interdisciplinary invention a recipe that cracks a number of problems.
May 17, 2016

ASU engineers create edible supercapacitors with range of health-application possibilities

Health food just took on a whole new meaning.

Engineers have created an edible supercapacitor that can wipe out E. coli or power a camera from inside the body.

Foods like activated charcoal, gold leaf, Gatorade, seaweed, egg white, cheese, gelatin and barbecue sauce can store and conduct electricity. Sandwich them together, and you have a supercapacitor — a high-capacity electrical component that can store electrical energy temporarily.

“We know it’s possible to make devices from food,” said Hanqing Jiang, an associate professor of mechanical engineering in the School for Engineering of Matter, Transport and Energy in Arizona State University’s Ira A. Fulton Schools of Engineering.

Researchers proved in the lab that the devices (seen above) can kill E. coli. “We’re trying to kill other bacteria as well,” Jiang said.

They also proved the devices can power a camera while in the stomach.

“The main application is to pass through a (gastrointestinal) tract, doing whatever a GI doctor needs,” Jiang said.

The supercapacitor could replace endoscopies with real-time monitoring of the gastrointestinal tract.

You wouldn’t want to pass them around at a party. Asked what the combination tastes like, Jiang replied, “It is cheese.”

The paper (not the recipe) was published Monday in Advanced Materials Technologies (not Bon Appetit). Recipes usually don’t read like this:

“The slurry was coated on the current collector by doctor’s blading followed by overnight drying in ambient environment and 6 hours drying in room temperature, low pressure (10 Pa) chamber to avoid thermal stress as well as remove the water in the electrode.”

Ingestible electronics do exist, but they need to be passed from the body. There are other concerns as well, Jiang said.

“The concern is that it’s not digestible,” he said of the previous ingestible electronics. “If it breaks, there is a possibility of contamination.”

The invention cracks a number of problems. Implantable electronics require surgery. Biodegradable electronics exist, but they have low energy density and battery size is limited. Edible materials proposed in the past have toxic components that can cause stomach pain and nausea.

Jiang and his team went interdisciplinary, weaving together the food industry, material sciences, device fabrication and biomedical engineering.

Carbon is already used in supercapacitors. Jiang chose activated charcoal and gold leaf because they both have high electrical conductivity and chemical stability. Gold is used extensively in Indian cuisine, and the European Union classifies gold as a drug. It acted as a current collector in the research.

Edible supercapacitors.

Hanqing Jiang (left) and his students, chemical engineering student Wenwen Xu and mechanical engineering student Xu Wang, with the ingredients for the supercapacitor "recipe" in Jiang’s lab on May 10 on the Tempe campus. Photos and video by Ben Moffat/ASU Now

 

The devices were made by hand. In future they’ll be made by 3-D printers and will be much smaller than the “sandwiches” made by Jiang and his students, which are a little bit bigger than a soy sauce packet.

Jiang and three students have been working on the project since August. Currently he is discussing the next steps in application with Mayo Clinic officials.

Meanwhile, business operations managers thought Jiang was catering a party on the university dime when he filed his expense report.

“The funny thing is when we got all the materials in, I had a hard time getting reimbursed,” he said. “It was all food.”

Professor Shelley E. Haydel, Center for Infectious Diseases and Vaccinology, the Biodesign Institute, and professor Lenore Dai, director of the School for Engineering of Matter, Transport and Energy, collaborated with Jiang on the research. Other co-authors were 2016 ASU grads Prithwish Chatterjee, currently working at Intel Corporation; 2016 ASU graduate Zeming Song; mechanical engineering PhD student Cheng Lv; and PhD student in Biodesign John Popovich.

Scott Seckel

Reporter , ASU Now

480-727-4502