Nature Commun. 5, 4148 (2014)

As a first step to generate free charge-carriers in a solar cell, photons impinging on the device must promote electrons from the valence band to the conduction band of the absorbing material. This means that the energy of the photons must be equal or higher than a threshold value corresponding to the material energy bandgap. The energy in excess of this threshold is usually lost in rapid thermal relaxation of the excited charges; yet it may be used to promote additional electrons via the process of carrier multiplication. Victor Klimov and colleagues now show that PbSe nanocrystals surrounded by a thick CdSe shell are ideal candidates to recycle the excess photon energy into additional free charges. Indeed, a photon having energy slightly higher than twice the bandgap of the PbSe core has non-zero probability to excite two charge carriers. The researchers suggest that this enhanced carrier multiplication is due to reduced electronic coupling between the core and the shell states, which slows down the thermal relaxation of shell-localized charges.