ACADEMIA

Researchers are developing solar cells that might be manufactured using special ink printed onto sheets of a supporting material, an approach that could lead to new low-cost solar cells economically competitive with other energy technologies.

The project, funded by the U.S. Department of Energy, strives to achieve critical requirements not met by other solar technologies: It should be able to be mass-produced at low cost and not limited by the availability of materials.

"To date, none of the photovoltaic technologies simultaneously meets all these constraints," said Rakesh Agrawal, Purdue University's Winthrop E. Stone Distinguished Professor in the School of Chemical Engineering.

The new solar cells hinge on developing an ink using tiny "nanocrystals" made of a material called copper zinc tin sulfide, or CZTS. Because the materials are abundantly available, the CZTS technology poses no resource hurdles, he said.

Agrawal's lab was the first to make CZTS nanocrystals, which enabled creation of a light-absorbing ink.

"The concept is that, once you have an ink you can print photovoltaic cells very fast, so they become very inexpensive to manufacture," Agrawal said.

The project is led by Agrawal; Mark Lundstrom, Purdue's Don and Carol Scifres Distinguished Professor of Electrical and Computer Engineering; Ashraf Alam, a Purdue professor of electrical and computer engineering; and Eric Stach, a researcher at Brookhaven National Laboratory.

To be competitive with other energy technologies, solar cells must be capable of generating terawatts, or trillions of watts, at a cost of 50 cents per peak watt of electricity.

"These goals can only be met with a truly transformational technology," Agrawal said.

The cells might be produced in a process that includes creating the nanocrystals, formulating the ink and printing it on a flexible supporting material. After the ink is applied, the cells are heated to about 500 degrees Celsius, a procedure called "sintering," to fuse the nanoparticles together.

The research involves advanced modeling, nanoelectronics and materials science. The researchers will create models showing how to precisely heat nanoparticles so that they are properly sintered, leading to optical and electrical properties.

"Manufacturing requires expertise in chemical engineering, but optimizing solar cell efficiency also requires electrical engineering expertise," Lundstrom said. "Putting these two viewpoints together is what makes this project so interesting."

The work is funded with a $750,000 grant as part of DOE's SunShot Initiative, which includes work to improve solar technologies.

Agrawal's lab has produced the second-highest efficiency CZTS-based solar cells, at 8.4 percent efficiency, and the goal of the research is to reach 15 percent efficiency or higher, he said.

Unlike some materials used for solar cells, the inks do not break down quickly in sunlight, offering promise as a long-lasting, economical photovoltaic technology.

CZTS solar cells were invented in the 1990s but have required the use of a procedure called chemical vapor deposition. That process, performed inside a chamber, is expensive and impractical for mass production. Using inks would make it possible to deposit the material at a lower cost. One challenge will be to produce CZTS nanocrystals that are free of impurities. Another aim is to increase efficiency by controlling its sintering and optoelectronic properties.

About three graduate students will be involved in the project. The Purdue Energy Center in Discovery Park also is supporting the research.

Agrawal recently received the National Medal of Technology and Innovation from President Barack Obama. The award is the highest honor for technological achievement bestowed by the president of the United States.