Efficient optical extraction of hot-carrier energy.

Light-induced generation of free charge carriers in semiconductors constitutes the physical basis of photodetection and photovoltaics. To maximize its efficiency, the energy of the photons must be entirely used for this purpose. This is highly challenging owing to the ultrafast thermalization of 'hot' carriers, which are created by absorption of high-energy photons. Thermalization leads to heat generation, and hence efficiency loss. To circumvent this, dedicated schemes such as photovoltaic hot-carrier cells are being explored. Here we consider optical extraction of the excess energy of hot carriers by emission of infrared photons, using erbium ions in combination with silicon nanocrystals. We determine the external quantum yield of the infrared photon generation by the erbium ions, and demonstrate that cooling of the hot carriers induces a steep, step-like, increase in erbium-related external quantum yield by up to a factor of 15 towards higher excitation energies. Finally, we comment on the potential of our findings for future photovoltaics in the form of an optical ultraviolet-to-infrared spectral converter.