Scientists are focusing on nanometre-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. 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.
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 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.
Over the past five years, scientists have succeeded in greatly increasing the efficiency of nanocrystal solar cells, yet even in the best of these solar cells just 9 percent of the incident sunlight on the cell is converted into electrical energy.