Contribution of Electric‐Field‐Induced Metal‐Free Porphyrin Dication to Photocurrent in Mixed Solid of Metal‐Free Porphyrin and o‐Chloranil/Al Schottky‐Barrier Cell

Although a Al/H2tpp (5, 10, 15, 20-tetraphenylporphyrin) Schottky-barrier cell did not show a clear rectification property because of the large electric resistance of the H2tpp solid, the rectification property was remarkably improved when o-chloranil was added into the H2tpp solid. The short-circuit dark-current was observed for the Al/dye cells with the mixed solid in contrast to that with the pure H2tpp solid, and it increased with increasing molar ratio (R) of o-chloranil to H2tpp. Furthermore, a much larger photocurrent was observed for the mixed-solid cells than for the pure H2tpp cell, but in the former cells in contrast to the latter cell, the open-circuit photovoltage was approximately the same as the open-circuit dark-voltage. These results indicate that the photocurrent of the mixed-solid cells was from the photocorrosion of the Al electrode. The short-circuit photocurrent action spectra obtained by irradiating from the Al side followed the absorption spectra of the dye solid films on the Al substrate below the R value of about 1, but above R = 1.5, a clear difference was observed in the spectra. This difference arises because a small amount of metal-free porphyrin dications, which is hardly detected by UV-visible spectra, was produced in the immediate neighborhood of the Al electrode when spin-coated with a larger R value because of dark-corrosion of aluminum by both assistance of the hydrogen bonds (between H2tpp and o-chloranil) and the electric field (in a Schottky barrier built during the spin coating). Thus the metal-free porphyrin dications efficiently underwent a photoinduced charge-separation by the potential gradient in the Schottky barrier