Multiwall carbon nanotubes (MWCNTs) are grown via chemical vapour deposition method directly on a stainless steel substrate. Raman spectroscopy and transmission electron microscopy are the techniques chosen to characterize the structure of the synthesized carbon nanotubes: few structural defects are detected. After their removal from the stainless steel substrate, the as-grown MWCNTs are then airbrushed on a crystalline n-type silicon substrate to build up a simple photovoltaic device based on the MWCNT/Si Schottky heterojunction. Several devices have been made up by varying the thickness of the MWCNT film. External quantum efficiency (EQE) measurements performed in planar configuration show that Si substrate plays the major role in the generation of the electron-hole pairs upon illumination. For this reason, the EQE lineshape closely resembles the behaviour of a p-n junction solar cell. In addition, a sizeable variation of the EQE as a function of the MWCNT film thickness is observed. On-off cycles, performed on the device showing the larger value of EQE (18%), illustrate the genuine photovoltaic effect of our MWCNT/Si device.