Solar superabsorber to improve photovoltiac efficiency

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Researchers from North Carolina State University have developed a so-called superabsorbing design that may improve the light absorption efficiency of thin film solar cells and lower manufacturing costs.

It is claimed 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 100nm thick to capture the majority of the available solar energy,’ said Dr. 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 per cent of available solar energy using only a 10nm thick layer of amorphous silicon.

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

Cao noted 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 said in a statement, 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.

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.

Cao said: ‘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.

‘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.

‘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,’ has been published in Scientific Reports