Nano Lett.http://dx.doi.org/10.1021/nl202915p (2011)

Colloidal semiconductor quantum dots can be synthesized and solution-processed in large quantities, and their optical properties have motivated applications ranging from photodetectors to solar cells. In particular, multiple exciton generation has been suggested as a route towards enhanced photovoltaic efficiencies, as photocurrents could increase if photons from the high-energy part of the solar spectrum could be harvested to produce multiple electron-hole pairs in low-energy-gap compounds. Although this process has been reported for some systems in solution, its efficiency remains under debate. Using time-resolved microwave conductivity measurements, Laurens Siebbeles and colleagues have now found evidence for the generation of multiple mobile charge carriers from single incident photons in PbSe quantum dot films. The films were fabricated in a layer-by-layer deposition and ligand-exchange process to increase the electronic coupling between the quantum dots. The researchers found quantum yields of up to 300%, which they attribute to inelastic scattering in strongly delocalized conduction states in the films, and they suggest that the effect could enhance solar-power conversion efficiencies by up to 24%.