Nanocrystal Discovery Has Solar Cell Potential

Scientists at Los Alamos National Laboratory have discovered a process that increases the potential for using nanocrystals as solar cell materials for producing higher electrical outputs than current solar cells.

The discovery, announced last week and published in the journals Nature Physics and Applied Physics Letters, sheds light on the mechanism for carrier multiplication, a phenomenon in which semiconductor nanocrystals respond to photons by producing multiple electrons. Scientists now believe that carrier multiplication can be applied to more materials than previously thought. The findings herald the possibility of new photovoltaic technologies that make use of traditional solar cell materials such as cadmium telluride.

According to Los Alamos, carrier multiplication isn't unique to lead selenide nanocrystals, but also occurs in nanocrystals of other compositions like cadmium selenide.

"Our research of carrier multiplication in previous years was really focused on analyzing the response of lead selenide nanocrystals to very short laser pulses," Los Alamos scientist Richard Schaller said in a prepared statement last week. "We discovered that absorption of a single photon could produce two or even three excited electrons. We knew, somewhat instinctively, that carrier multiplication was probably not confined to lead selenide, but we needed to pursue the question."

Scientists believe carrier multiplication occurs through instantaneous photoexcitation of multiple electrons and it 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," lead project scientist Victor Klimov explained in a written statement. "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."

Carrier multiplication could be used in solar-fuel technologies and specifically, the production of hydrogen by photo-catalytic water splitting, according to the team of Los Alamos researchers. The production of hydrogen by photo-catalytic water splitting requires four electrons per water molecule and its efficiency can be dramatically enhanced if multiple electrons can be produced through a single-photon absorption event.