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.

These new results also shed light on the mechanism for carrier multiplication, which likely occurs via the instantaneous photoexcitation of multiple electrons. Such a process has never been observed in macroscopic materials and it explicitly relies on the unique physics of the nanoscale size regime.

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.

—Lead project scientist Victor Klimov

The Los Alamos findings point toward practical photovoltaic technologies that may utilize such traditional solar cell materials as cadmium telluride, which is very similar to cadmium selenide.

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 dramatically enhanced if these multiple electrons can be produced via a single-photon absorption event.

Research on carrier multiplication at Los Alamos is funded by the DOE’s Office of Basic Energy Sciences and by Los Alamos’ Laboratory-Directed Research and Development (LDRD) program.

Resources:

• "High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states; Richard D. Schaller, Vladimir M. Agranovich and Victor I. Klimov; Nature Physics 1, 189-194 (2005) doi:10.1038/nphys151

• "Effect of electronic structure on carrier multiplication efficiency: Comparative study of PbSe and CdSe nanocrystals"; Richard D. Schaller, Melissa A. Petruska, and Victor I. Klimov; Appl. Phys. Lett. 87, 253102 (2005)

• LANL Quantum Dot Research website