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Shedding new light on solar cells

Slide Show

Erik Helgren, assistant professor of physics, demonstrates how the thermal evaporating chamber transforms a pellet of copper or aluminum into thin metal films used in creating polymer solar cells.



One physics faculty member’s efforts to engineer a more efficient solar energy cell are also creating well-prepared future scientists among students at Cal State East Bay. 

 Assistant Professor Erik Helgren joined CSUEB after working at University of California, Berkeley, where he conducted research on silicon solar cells. Armed with a research background, Helgren arrived at Cal State East Bay in fall 2008 ready to continue his work in the lab, with the addition of student assistants. 

 “The type of research I wanted to pursue, in particular the renewable energy materials research, is ideally suited for incorporating undergraduates as part of the research plan,” Helgren says.

In January 2009, Helgren acquired lab space in the corner of the North Science Building on the Hayward campus. On an overcast day, sunlight struggles to brighten the otherwise dimly-lit lab. It is here Helgren and his students search for ways to illuminate alternative solar energy cells. 

The lab contains all the traditional makings of a scientific laboratory; but among the beakers, microscope slides, and charcoal-colored countertops, are two tools that look like remnants from a sci-fi film from decades past. Nestled beside the door is the “glove box.” Long rubber gloves inserted into the front glass piece allow researchers to manipulate objects within the box without coming into contact and contaminating the material inside. At the center of the room stands a thermal evaporating chamber, a silver contraption resembling a simpler R2D2. The chamber and the glove box are critical pieces of equipment in Helgren’s research on alternative solar energy cells. 

Instead of continuing research on silicon solar cells — which make up 95 percent of solar cells used commercially — Helgren is looking at organic polymers as an alternative to the widely-produced silicon cells. While silicon cells, the same type used in computer chips, have been most efficient at converting energy from the sun, the cells are expensive to produce and are not flexible, causing breaks in the cells. Organic polymers, including the P3HT Helgren works with, are cheaper to construct and are more malleable.

“The polymer can be put on a plastic sheet, sprayed on, and can be flexible,” Helgren says. 

But there is a hurdle to making polymer the solar cell material of choice: its reduced efficiency. While silicon solar cells can capture and convert sunlight at an approximate 15 percent efficiency rate, the polymer efficiency rate ranges from 3 percent to 5 percent, says Helgren.

This is where Helgren hopes his research will pay off. He and his student assistants are looking at the material properties of polymer cells and investigating how to increase their efficiency.

Inside the lab, Helgren and his students construct, manipulate, and experiment on the polymer solar cells. Seven students have worked in Helgren’s renewable energy resource lab, including an intern from Contra Costa Community College and CSUEB students whose majors range from physics to ethnic studies, including one who hopes to become a midwife. 

Attracted by the opportunity to work in a campus laboratory, physics major Trevor Billings transferred to Cal State East Bay from De Anza Community College during fall quarter. 

“I like to be in the lab, whether it’s for a grade or not,” says Billings, during a break from his studies. “(It’s) passionately fun for me. It’s something that I’m interested in just doing to do it.” 

Billings did not assume he’d get to work in a lab right away when he arrived at the University, so he was excited when Helgren invited him to work alongside him. “(I) crunch some numbers. Come up with ideas. Carry heavy things,” Billings says. Throughout fall quarter, Billing’s lab work has focused on construction of the vacuum chamber, including taking measurements, testing the power supply, and drilling and placing posts in the chamber. 

The vacuum chamber of the thermal evaporator was completed in December. In the vacuum environment, thin metal films are created to produce the polymer solar cells. The film layers must be transparent enough to allow sunlight to hit the polymer cell sandwiched between the film strips. Tiny pellets of metal, typically copper or aluminum, are loaded into the machine, where they are vaporized and collected on glass slides. In January, Helgren’s students were trained on how to safely run the machine and create the films.

Applying polymer to the films also requires a painstaking procedure. The polymer layer is applied inside the glove box to keep oxygen from contacting the polymer. If oxygen touches the polymer, it will cause the organic material to oxidize, making the polymer useless, Helgren explains. Oxygen is pumped out of the air inside the glove box and replaced with an alternative gas, such as nitrogen.

Helgren and his team have been trying to improve the cell’s efficiency by adding carbon-based derivatives to the cell, in this case carbon nanotubes. Published research papers report that adding nanotubes increases the efficiency, but the reason is still being studied. “Do the nanotubes help the polymer blend absorb more sunlight or does the colloid of nanotubes and polymer conduct current better? That is one of the research questions I hope to answer,” Helgren says. 

After adding the nanotubes and finishing construction of the solar cell, Helgren’s team members take the polymer solar cell outside into sunlight and test its efficiency using voltage measuring equipment. They then analyze and interpret the outcomes. 

“Ideally, we (will) find some really great breakthrough on how to increase the efficiency of polymer cells,” Helgren says.

“I’d like to see results from the research to have the feeling of accomplishment that we did something,” Billings adds. “Even if the accomplishment is just making sure the vacuum chamber isn’t leaking.”

Helgren hopes to publish his team’s findings and attend and present at scientific conferences. Billings, for instance, may get to present at an upcoming conference at the Naval Postgraduate School in Monterey. Faculty in the physics and chemistry departments recently received a grant from the National Science Foundation to purchase new research instruments. Helgren and his students also will benefit from the grant by receiving commercial-quality equipment, which is expected to be in place by summer. 

“Students would get (improved) hands-on experience using thin film deposition equipment and other techniques involved in solar cell manufacturing similar to ones being used in industry,” Helgren explains.

For Billings, experience working in the lab offers the promise of bringing him closer to his dream job of working for a think tank. And working on a timely project offers additional rewards.

“It’s pop science,” he says. “Making spreadable photovoltaic cells is really in vogue in the scientific papers … It’s something that can open up a lot of scientific horizons.” 

Helgren, on the other hand, maintains a long-range vision about the relevancy of his research. It addresses an important and pressing issue, he says: the future of energy. 

“We are an economy and a country so dependent on fossil fuels, but as David Goldstein in his book Out of Gas puts it, the cheap petroleum we are so dependent on will soon run out,” he says. 

Helgren will share his vision for combating fossil fuel dependence with alternative materials, such as polymer solar cells, at research seminars and eventually, he hopes, by developing a materials science engineering program at Cal State East Bay. But for now, he hopes his research will provide breakthroughs in the green energy field, diminishing the energy problem and contributing to a brighter future.

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