Anomalous photovoltaic effect in nanocrystalline Si /SiO2 composites

We have observed an anomalous photovoltaic effect in films of Si nanocrystals embedded in SiO₂. Using conductive-probe atomic force microscopy and global transport measurements, we found, close to the percolation threshold of the Si crystallite phase, a large photovoltaic effect of up to about 7V. Following the dependence of this effect on the size of the Si nanocrystals and on the relative tip position with respect to the counterelectrode, we suggest a model based on charge separation of excited electron-hole pairs governed by the size-dependent quantum confinement and charging energies.