PHOTOVOLTAIC PROPERTIES OF IODINE-DOPED MAGNESIUM TETRAPHENYLPORPHYRIN SANDWICH CELLS .2. PROPERTIES OF ILLUMINATED CELLS

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-Al2O3 electrode, to a depth of approximately 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 approximately 2 eV. The photovoltaic efficiency increases with decreasing porphyrin thickness between 200 and 100 nm. Maximum values of eta-(wh) = 1 x 10-3 %, eta'-epsilon = 7 x 10-2 % and phi-epsilon = 4.3 % are obtained for a 100-nm thick film. For the thinner cells, the photocurrent varies linearly with light intensity up to 106 m W/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.