Sustainable Energy

A Super-Absorbent Solar Material

A new material, patterned at the nanoscale, absorbs a broad spectrum of light and could make thin-film solar cells more efficient.

A new nanostructured material that absorbs a broad spectrum of light from any angle could lead to the most efficient thin-film solar cells ever.

Light catcher: This scanning electron microscope image shows the super absorbent nanostructures, which measure 400 nanometers at their base.

Researchers are applying the design to semiconductor materials to make solar cells that they hope will save money on materials costs while still offering high power-conversion efficiency. Initial tests with silicon suggest that this kind of patterning can lead to a fivefold enhancement in absorbance.

Conventional solar cells are typically a hundred micrometers or more thick. Researchers are working on ways to make thinner solar cells, on the order of hundreds of nanometers thick rather than micrometers, with the same performance, to lower manufacturing costs. However, a thinner solar cell normally absorbs less light, meaning it cannot generate as much electricity.

Some researchers are turning to exotic optical effects that emerge at the nanoscale to solve this conundrum. Harry Atwater, a professor of applied physics and materials science at Caltech and a pioneer of the field, has now come up with a way of patterning materials at the nanoscale that turns them into solar super-absorbers.

Atwater worked with Koray Aydin, now an assistant professor of electrical engineering and computer science at Northwestern University, to develop the super-absorber design, which takes advantage of a phenomenon called optical resonance. Just as a radio antenna will resonate with and absorb certain radio waves, nanostructured optical antennas can resonate with and absorb visible and infrared light. The length of a structure determines what wavelength of light it will resonate with. So Atwater and Aydin designed structures that effectively have many different lengths: wedge shapes with pointy tips and wide bases. The thin, nanoscale wedges strongly absorb blue light at the tip and red light at the base.

Atwater and Aydin demonstrated this broadband effect in a 260-nanometer-thick film made of a layer of silver topped with a thin layer of silicon dioxide and finished with another thin layer of silver carved with arrays of wedges that are 40 nanometers at their tips. Atwater says they chose these materials because they are particularly challenging: in their unpatterned state, they’re both highly reflective; but the patterned films can absorb an average of 70 percent of the light across the entire visible spectrum. This work is described in the online journal Nature Communications.

Kylie Catchpole, a research fellow at the Australian National University in Canberra, says the design is promising because it works over a broad band of the spectrum. These effects, Catchpole says, “are usually very sensitive to wavelength.” However, she notes, the designs will have to be applied to other materials to work in solar cells.

Aydin and Atwater are now doing just that. The researchers have made a 220-nanometer-thick silicon film that absorbs the same amount of light as an unpatterned film 25 times thicker.

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