Are tiny crystals the next big thing in solar cells?

A solar-cell chip based on nanocrystals fabricated by researchers at ETH Zurich. (Photo: Deniz Bozyigit / ETH Zurich)

Tomorrow’s solar cells will likely be made of nanocrystals.

Compared with silicon in today’s solar cells, these tiny crystals can absorb a larger fraction of the solar light spectrum. But, until now, the physics of electron transport in this complex material was not understood, making it impossible to systematically engineer better nanocrystal-composites.

“These solar cells contain layers of many individual nano-sized crystals, bound together by a molecular glue. Within this nanocrystal composite, the electrons do not flow as well as needed for commercial applications,” explains Vanessa Wood, a professor of materials and device engineering at ETH Zurich.

Wood and her colleagues conducted an extensive study of nanocrystal solar cells, which they fabricated and characterized in their laboratories. They were able to describe the electron transport in these types of cells via a generally applicable physical model for the first time.

“Our model is able to explain the impact of changing nanocrystal size, nanocrystal material, or binder molecules on electron transport,” says Wood.

Optimized for solar cells

The model will give scientists in the research field a better understanding of the physical processes inside nanocrystal solar cells and enable them to improve solar cell efficiency.

One reason scientists are excited about nanocrystals is that their physical properties vary at different sizes. And because scientists can easily control nanocrystal size in the fabrication process, they are able to optimize them for solar cells.

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One such property that can be influenced by changing nanocrystal size is the amount of sun’s spectrum that can be absorbed by the nanocrystals and converted to electricity by the solar cell.

Semiconductors do not absorb the entire sunlight spectrum, but rather only radiation below a certain wavelength.

In most semiconductors, this threshold can only be changed by changing the material. However, for nanocrystal composites, the threshold can be changed simply by changing the size of the individual crystals. That means scientists can select the size of nanocrystals in such a way that they absorb the maximum amount of light from a broad range of the sunlight spectrum.

An additional advantage is that nanocrystal semiconductors absorb much more sunlight than traditional semiconductors. For example, the absorption coefficient of lead sulfide nanocrystals, used by the ETH researchers in their experimental work, is several orders of magnitude greater than that of silicon semiconductors, used traditionally as solar cells.

A relatively small amount of material is sufficient for the production of nanocrystal solar cells, making it possible to make very thin, flexible solar cells.

The work is described in Nature Communications.

Source: ETH Zurich