NREL Solar Scientists Epitomize Teamwork
December 22, 2010
A Colorado carpenter's son, an African American from Indiana, a post-doctoral researcher from Senegal, and a young woman from China are working together to solve one of the most important problems in solar-cell efficiency.
When they're not laughing with each other, or meeting with a group of 20 to share strategies, the foursome of scientists at the U.S. Department of Energy's National Renewable Energy Laboratory is trying to control semiconductors' band gaps to make solar cells less expensive and more efficient. And along the way they're attempting to solve fundamental scientific questions about the nature of new opto-electronic materials.
They're the new face of science — collaborative and diverse, living proof that the age of the solo scientist shouting "Eureka!" has been replaced in the 21st century by multi-disciplinary teams with complementary skills.
David Ginley, the son of a carpenter, who grew up in suburban Denver near NREL's Golden, Colo., campus, leads the team.
He is joined by Joe Berry, a senior scientist, who hails from Indiana; Yi Ke, a graduate student from China who is doing her doctoral dissertation work at Colorado School of Mines and is experimenting with materials in NREL's Pulse Laser Deposition (PLD) lab; and Paul Ndione, the post-doc who oversees the PLD.
NREL Team Searching for Better Top Layer
Despite tremendous gains made in processing and developing solar cells, most arrays on individual rooftops or in grid-connected solar fields still operate well below the nearly 30 percent theoretical conversion efficiency possible for a single absorber device. There are many opportunities to significantly improve the existing efficiency, sometimes while reducing the cost.
That means any breakthrough to add a percentage point or two to that efficiency is huge, and a big step toward making clean solar energy competitive with fossil fuels.
Most solar photovoltaics are composed of an active semiconductor absorber that absorbs the light, a junction to turn photons into charge carriers, and contacts to efficiently remove the carriers without blocking light. To accomplish this, the top layer, the one facing the sun, needs to be both transparent and able to conduct electrons with very little loss.
It's that top layer that is the subject of the NREL team's work. Lately, solar manufacturers have been using indium tin oxide for that transparent conducting oxide layer. However, indium is difficult to extract and is very expensive. So, scientists are searching for alternatives.
Zinc oxide is a promising candidate because it is both highly transparent and conductive, as well as being much more abundant, Ke said. It is also about 1 percent of the cost of indium tin oxide.
Ginley and his team want to add magnesium to the zinc oxide to improve its transparency and then to dope the ZnMgO with another material to boost its conductivity, all in the name of developing more efficient and more cost-effective solar cells.
Searching for an Elusive Element
They're rarely without their deluxe-model periodic tables called up on their iPads or iPhones, searching for that elusive element that can best pair with zinc oxide and magnesium to boost the number of electrons that can conduct electricity. This impurity only need be present at less than 1 percent and should not significantly change the structure of the ZnMgO, but it adds the electronic carriers (doping) that are so critical to getting the photogenerated charges out of PV devices without significant loss.
In the transparent conductive semiconductors, most electrons (carriers) are in the conduction band, which means they're free to move and carry an electric current. The valance band in the material is a lower energy state in which carriers are not as mobile
Between the conduction band and the valence band is the energy gap, or band gap. It's this gap that is so intriguing to scientists, who think they can change the size of the bandgap and simultaneously but independently control the electronic conductivity by doping with an appropriate impurity.
Ginley, Berry and Ke are looking for the best doping agents to push electrons from the atoms in the ZnMgO material to the conduction band of the semiconductor, where they would be in a free electronic state and can help improve the efficiency and lower the cost of PV devices.
When the NREL team finds a promising material, as with the addition of magnesium, the resulting semiconductor layer has a larger band gap and would be more transparent. However, the materials that dope the ZnO do not add electronic carriers to the Mg substituted materials as well. So, this drives the search to look for new dopants.
"As you crank that gap open, you basically make something that is increasingly transparent," Berry said. "That means you can look through it, and for solar that's what you need.
"The fact that you can change the sensitivity to color at which this thing responds means you can make a detector or window that's selective for a particular wave length," Berry said. "Being able to tune that gap is useful in terms of optoelectronics."
The trick is to get the electrons moving without changing the fundamental nature of the semiconductor material.
"What's cool in this system, you can crank substantial amounts of magnesium into ZnO and it basically stays zinc oxide," Ginley said. "You change the electronic properties, but nothing else changes. It gets much more transparent and its electronic properties are better."
To explore these systems the group uses pulsed laser deposition. Inside the PLD chamber, they aim lasers at ceramic targets containing the chosen material, inducing tremendous energy in the atoms on the surface. What erupts is a plasma plume of partially ionized gas that knocks out some atoms and moves some of the electrons from those atoms to a higher energy state.
Imagine a Water Pistol and Some Mud
A bright light forms in the plasma plume, as those excited electrons release energy while relaxing back into a lower energy state.
Meantime, the fast-moving ions and atoms in the plasma stop abruptly when they run into the plate (or substrate) for the materials being deposited. They solidify into a thin film suitable for incorporation into a next generation of solar cells.
A down-to-earth analogy? "Imagine using a powerful water pistol to shoot at a mound of mud," Ke said. The resulting slurry "sprays on your beautiful clothes."
The water pistol is the laser, the clothes the substrate. The slurry of mud and water is akin to the plasma of atoms and ions, "except the plasma is much more gorgeous and interesting," Ke said.
"For a dopant we're looking at yttrium, scandium, and titanium as possible replacements for the conventional aluminum used to dope zinc oxide," Berry said. "In the magnesium-substituted materials, the question is can you restore the critical ability to dope by going to new dopants."
"If you can control the band gap, while controlling the doping, you can have a huge impact on organic photovoltaic, organic LEDs, silicon, copper indium gallium, PV as a whole," Ginley said. "It would have an immense applicability." A number of other technologies such as flat panel displays and transparent electronics also depend on these same materials, Ginley said.
Forming a Scientific Team
Ginley says one if his major duties is hiring good people.
"You should hire people that scare you, they're so good," Ginley said, "people who are more likely to replace you than anything else. You shouldn't be timid when you hire. With grad students, we look for people with outstanding potential who have good communications skills and some indication of being able to be team players.
"The era of the lone scientist is over," Ginley said. "The kinds of problems we deal with, you just don't have the horsepower to do it by yourself. That's an increasing realization nationwide. Look at the Energy Frontier Research Centers. These new centers are a reflection of that. People are realizing that big problems take critical-mass teams."
"We don't know enough on our own," Ginley said. "It's that shared knowledge base and experience base that makes things go faster."
Joe Berry, the African American son of a professor, whom Ginley plucked from the National Institute of Standards and Technology up the road in Boulder, concurred. The breadth of the solar cell project, together with the collegiality of the team, gave him a new enthusiasm for his work. "When I was at NIST, I was doing something by myself at a bench," he said. "But the number of people who cared about what I was doing, or who would be impacted by what I was doing, was equally as large."
From China with Aspirations
Ke got her undergraduate degree in China, majoring in electrical engineering. "I started feeling enthusiastic about solar cells" during her college years, she said. "They're things that can really help humans, can give utilities the power to solve a lot of problems. I figured out I could have a great career trying to move that along."
Ke applied to graduate school in the United States because "it has the best higher education in the world," she said. "I feel so fortunate to be here and working with NREL. They have the best scientists, the best mentors ever. Graduate work here is more challenging that in China."
The chance to work at NREL was the main reason she applied to the Colorado School of Mines, Ke said. "I got accepted at Stanford, but my advisor at Mines talked about the possibility of joining this group and working at NREL. He mentioned Dave, I looked him up and I came here for an interview. I was very lucky to get it."
Ke would be very content working in photovoltaics and renewable energy the rest of her career.
"I really want to be a person who understands the science and the R&D," she said. "But also someone who can apply the technology to industry so I can make some difference. To let people use this and become less dependent on fossil fuels."
She sees herself living part time in the United States, part time in China. "A greener future in both America and China can lead to cooperation between the two countries."
Senegal to France to Quebec to NREL
Ndione, the post-doc who hails from Senegal, earned his undergraduate degree in Bordeaux, France, and got his doctorate in Quebec, Canada.
Just a thin slice of Senegal's population is college-educated, and to specialize in certain specific areas, a Senegalese has to leave the country. "Now, there are more opportunities for scientists in Senegal," Ndione said. "But to specialize in a field that includes semiconductors and lasers is difficult. We have to go abroad to do it.
"This is what I love," Ndione said, while setting up for another experiment. "The aim in the future is to adapt this technology to our realities in Africa and also, to promote intensive collaboration between the USA and Africa in the field of renewable energy."
Finding a Life of Science
Berry got his undergraduate degree at Goshen College in Goshen, Ind., where his father was a political science professor.
Science and math always suited Berry. It might have suited his father as well, but the elder Berry grew up in the segregated south, the first in his family to get a college degree. "Back then, it was one thing for an African American to learn how to read, quite another to learn how to do trigonometry or calculus," Berry said. "I don't think he ever had the opportunities to do that. He might have been inclined."
The son took the next step, cultivating his love for the sciences. Berry's dad helped him with his homework until high school chemistry, after which he was on his own.
Berry went on to get a doctorate at Pennsylvania State University in condensed material physics, specializing in photon detection.
"These kinds of band-engineering things are meat and potatoes to a semiconductor device physicist," Berry said. "But they're much more challenging than the ones we considered challenges in graduate school.
"NREL being NREL, there's this balance between what we do on the fundamental level and the need to find a way to produce these things at low cost and at scale," Berry said. "NREL is one of those places you always think that it would be nice to work here," Berry said. "I didn't know whether I had the appropriate skill set. But there was an opening and I applied. Dave in his infinite wisdom decided I'd be a reasonable fit for his group. It's been four years. I've been happy as a clam ever since."
Learn more about NREL's photovoltaic research.
— Bill Scanlon