The photovoltaic characteristics of optimized iodine‐doped thin‐film sandwich cells of the structure Al/Al2O3/magnesium tetraphenylporphyrin/Au, fabricated and tested under high vacuum, are reported. The action spectra of the short‐circuit photocurrent show that only light absorbed near the Al‐Al2O3electrode, to a depth of ∼70 nm in the porphyrin layer, is effective in producing charge carriers, consistent with the presence of a depletion region at this contact. Comparison of observed and modeled action spectra indicate an underlying enhancement of carrier generation which increases with increasing excitation energy above ∼2 eV. The photovoltaic efficiency increases with decreasing porphyrin thickness between 200 and 100 nm. Maximum values of &eegr;wh=1×10−3 %, &eegr;’&egr;=7×10−2 % and &fgr;&egr;=4.3 % are obtained for a 100‐nm thick film. For the thinner cells, the photocurrent varies linearly with light intensity up to 106 mW/cm2, indicating that little recombination of charge carriers occurs during transport to the electrodes. The porphyrin films degrade rapidly in the presence of oxygen/water vapor; however, the stability of the films and devices is good under vacuum. Possible mechanisms of charge‐carrier generation are discussed.