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/H(2)tpp (5, 10, 15, 20-tetraphenylporphyrin) Schottky-barrier cell did not show a clear rectification property because of the large elec tric resistance of the H(2)tpp solid, the rectification property was remark ably improved when o-chloranil was added into the H(2)tpp solid. The short- circuit dark-current was observed for the Al/dye cells with the mixed solid in contrast to that with the pure H(2)tpp solid, and it increased with inc reasing molar ratio (R) of o-chloranil to H(2)tpp. Furthermore, a much larg er photocurrent was observed for the mixed-solid cells than for the pure H( 2)tpp cell, but in the former cells in contrast to the latter cell, the ope n-circuit photovoltage was approximately the same as the open-circuit dark- voltage. These results indicate that the photocurrent of the mixed-solid ce lls was from the photocorrosion of the Al electrode. The short-circuit phot ocurrent action spectra obtained by irradiating from the Al side followed t he 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 t he spectra. This difference arises because a small amount of metal-free por phyrin dications, which is hardly detected by W-visible spectra, was produc ed 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 o f the hydrogen bonds (between H(2)tpp and o-chloranil) and the electric fie ld (in a Schottky barrier built during the spin coating). Thus the metal-fr ee porphyrin dications efficiently underwent a photoinduced charge-separati on by the potential gradient in the Schottky barrier. (C) 1999 The Electroc hemical Society. S0013-4651(98)09-033-8. All rights reserved.