Science 328, 1543–1547 (2010)

Semiconductor photovoltaic cells are usually most efficient under light with the same energy as that of the cell's electronic bandgap. Less-energetic light is not absorbed, whereas more-energetic light creates electron–hole pairs that quickly shed their excess energy by emitting lattice vibrations, or phonons. If, instead, these charges could be extracted before they relax, while they are still 'hot', efficiencies could be greatly increased. Now, Xiaoyang Zhu, Eray Aydil, David Norris and colleagues from the University of Minnesota and the University of Texas at Austin have observed hot-electron transfer from a light-absorbing semiconductor to an electrode.

The team observed this transfer from one or two monolayers of PbSe nanocrystals resting on a substrate made of the commonly used electron acceptor, titania. The small nanocrystal size caused an energy-level spacing that was large relative to phonon energies, which reduced cooling rates. Furthermore, PbSe excited states are spatially large, increasing the chance of electron transfer into the substrate. When light from a red laser was pulsed onto the monolayers, they reflected a blue second-harmonic signal, which was very sensitive to the presence of any electric field generated by charge transfer.

By studying the second-harmonic time dynamics and temperature dependence, the team concluded that hot carriers were being transferred to titania. The effect is expected to be relevant to other semiconductor nanocrystals and conducting substrates and, if combined with a minimization of energy loss in the conducting substrate, may enable highly efficient hot-carrier solar cells to be made.