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Slashing Costs In Thin Film Solar Cell Manufacturing

Scientists have designed onion-like structures to match light-trapping efficiency with the absorption efficiency of the semiconductor materials in thin film CdTe and CIGS solar cells




Researchers from North Carolina State University have developed a 'superabsorbing' design that may significantly improve the light absorption efficiency of thin film solar cells and drive down manufacturing costs.

The superabsorbing design could decrease the thickness of the semiconductor materials used in thin film solar cells by more than one order of magnitude without compromising the capability of solar light absorption.

“State-of-the-art thin film solar cells require an amorphous silicon layer that is about 100 nm thick to capture the majority of the available solar energy," says Linyou Cao, an assistant professor of materials science and engineering at NC State and senior author of a paper describing the work. “The structure we’re proposing can absorb 90 percent of available solar energy using only a 10 nm thick layer of amorphous silicon."

“The same is true for other materials. For example, you need a CdTe layer that is one micron thick to absorb solar energy, but our design can achieve the same results with a 50 nm thick layer of CdTe. Our design can also enable a 30 nm thick layer of copper indium gallium selenide to fully absorb solar light. That’s a huge advance."

Cao notes that the deposition of semiconductor materials stands as a major bottleneck for improving manufacturing productivity and lowering the cost of thin film solar cells. “A decrease in the thickness of semiconductor materials by one order of magnitude would mean a substantial improvement in manufacturing productivity and reduction in cost," Cao says, because the cells would use less material and the thin films could be deposited more quickly.

In cross-section, the new design looks like a rectangular onion. The light-absorbing semiconductor material coats a rectangular core. The semiconductor, in turn, is coated by three layers of anti-reflective coating that do not absorb light.

To develop the design, the researchers began by examining the maximum light absorption efficiency of semiconductor materials using light-trapping techniques. They found that maximising solar absorption requires a design in which the light-trapping efficiency for solar light is equal to the intrinsic absorption efficiency of the semiconductor materials.

In other words, in order to maximise solar absorption, you need to match the amount of solar light trapped inside the structure and the amount of solar light that could be absorbed. The researchers then designed the onion-like structures to match their light-trapping efficiency with the absorption efficiency of the semiconductor materials in thin film solar cells.

“We first theoretically predicted the maximum solar light absorption efficiency in given semiconductor materials, and then proposed a design that could be readily fabricated to achieve the predicted maximum. We developed a new model to do this work, because we felt that existing models were not able to find the upper limit for the solar absorption of real semiconductor materials," Cao says.

“And if this works the way we think it will, it would fundamentally solve light-absorption efficiency problems for thin film solar cells.The superabsorbing structure is designed for the convenience of fabrication, and we are looking for partners to produce and test this design," Cao adds. “The structure should be very easy to produce with standard thin film deposition and nanolithography techniques. We are happy to work with industry partners to implement this design in the production of next-generation solar cells.

The paper, “Semiconductor Solar Superabsorbers," by Yiling Yu et al was published in the journal Scientific Reports.

DOI: 10.1038/srep04107





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