Results of surface photovoltage spectroscopy on free-standing porous silicon films fabricated from boron-doped Si wafers of various resistivities are presented. We find that all the films have bandtails, which are about 0.3 eV wide, and their optical band gap is about 2 eV. The majority carriers in the strongly luminescent and poorly photoconducting films are holes, while in the weakly luminescent but photoconducting films they are electrons. This difference between the films appears to be due to different oxygen coverage of the silicon nanocrystallites. We conclude that the origin of the strong red-light luminescence is in the electron optical transitions from the conduction bandtail to the valence bandtail. {copyright} {ital 1997} {ital The American Physical Society}