ETH researchers have developed a comprehensive model to explain how electrons flow inside new types of solar cells made of tiny crystals. The model allows for a better understanding of such cells and may help to increase their efficiency.
Scientists are focusing on nanometer-sized crystals for the next generation of solar cells. These nanocrystals have excellent optical properties. Compared with silicon in today’s solar cells, nanocrystals can be designed to absorb a larger fraction of the solar light spectrum. However, the development of nanocrystal-based solar cells is challenging: “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, Professor of Materials and Device Engineering at ETH Zurich. Until now, the physics of electron transport in this complex material system was not understood so it was impossible to systematically engineer better nanocrystal-composites.
Wood and her colleagues conducted an extensive study of nanocrystal solar cells, which they fabricated and characterized in their laboratories at ETH Zurich. 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. 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.
The reason for the enthusiasm of many solar cell researchers for the tiny crystals is that at small dimensions effects of quantum physics come into play that are not observed in bulk semiconductors. One example is that the physical properties of the nanocrystals depend on their size. And because scientists can easily control nanocrystal size in the fabrication process, they are also able to influence the properties of nanocrystal semiconductors and optimize them for solar cells.
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, or – in other words – with an energy greater than the so-called band gap energy of the semiconductor. 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. Thus 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 of nanocrystal semiconductors is that they 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. Thus, 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.
This story is reprinted from material from ETH Zürich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.