Los Alamos National Laboratory scientists have discovered that a phenomenon called carrier multiplication, in which semiconductor nanocrystals respond to photons by producing multiple electrons, is applicable to a broader array of materials than previously thought.
The discovery means that nanoscrystals, used as solar cell materials, could produce higher electrical outputs than current solar cells.
In papers published recently in the journals Nature Physics and Applied Physics Letters, the scientists demonstrate that carrier multiplication is not unique to lead selenide nanocrystals, but also occurs with very high efficiency in nanocrystals of other compositions, such as cadmium selenide. In addition, these new results shed light on the mechanism for carrier multiplication, which is likely to occur via the instantaneous photoexcitation of multiple electrons. Such a process has never been observed in macroscopic materials.
According to Richard Schaller, a
Lead project scientist Victor Klimov explains, “Carrier multiplication actually relies upon very strong interactions between electrons squeezed within the tiny volume of a nanoscale semiconductor particle. That is why it is the particle size, not its composition that mostly determines the efficiency of the effect. In nanosize crystals, strong electron-electron interactions make a high-energy electron unstable. This electron only exists in its so-called ‘virtual state’ for an instant before rapidly transforming into a more stable state comprising two or more electrons.”
Other interesting opportunities may also be associated with the use of carrier multiplication in solar-fuel technologies and specifically, the production of hydrogen by photo-catalytic water splitting. The latter process requires four electrons per water molecule and its efficiency can be enhanced if these multiple electrons can be produced via a single-photon absorption event.
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