In this paper we investigate the distribution of the electrically available states near the band-edge in pentacene thin films of different thicknesses, aiming to the identification of the active thickness of pentacene layers in fully operational devices such as organic thin film transistors (OTFTs). The film structure has been studied by X-ray diffraction technique, while their relative electronic density of states distribution (DOS) around the band-edge has been investigated by photocurrent (PC) spectroscopy analyses. The effects of ion implantation on OTFTs have been investigated by PC analyses of OTFTs implanted with N+ ions of different energy and doses. We show how PC spectroscopy has the remarkable ability to detect modifications of the DOS distribution in a non invasive way, thus allowing the direct study of the active semiconductor film in fully operational OTFTs.