We report on a significant photocurrent generation from a planar device obtained by coating a bare n doped silicon substrate with a random network of multiwall carbon nanotubes (MWCNTs). This MWCNT/n-Si hybrid device exhibits an incident photon to current efficiency reaching up to 34% at 670 nm. We also show that MWCNTs covering a quartz substrate still exhibit photocurrent, though well below than that of the MWCNTs coating the silicon substrate. These results suggest that MWCNTs are able to generate photocurrent and that the silicon substrate plays a fundamental role in our planar device. The former effect is particularly interesting because MWCNTs are generally known to mimic the electronic properties of graphite, which does not present any photocurrent generation. On the basis of theoretical calculations revealing a weak metallic character for MWCNTs, we suggest that both metallic and semiconducting nanotubes are able to generate e-h pairs upon illumination. This can be ascribed to the presence of van Hove singularities in the density of states of each single wall carbon nanotube constituting the MWCNT and to the low density of electrons at the Fermi level. Finally, we suggest that though both MWCNTs and Si substrate are involved in the photocurrent generation process, MWCNT film mainly acts as a semitransparent electrode in our silicon-based device.

Photocurrent generation measured with an electrochemical cell on carbon nanotubes has been obtained mainly from single-walled nanostructures. In fact, multiwall carbon nanotubes (MWCNTs) were not expected to show analogous low-dimensional effects, due to their close similarity to metallic graphite. Recently we reported on the ability of MWCNTs to generate photocurrent in the visible and ultraviolet spectral range. In this paper we show a significant enhancement in the photocurrent generation when the walls of the MWCNTs were decorated with dispersed Cu nanoparticles. This result is of particular relevance for photovoltaic nanodevices and solar energy conversion applications.