Light absorption in the near-infrared and photocurrent synergism observed with vacuum-sublimed films of zinctetraphenylporphyrin

Abstract

Photocurrent peaks are found in the near-infrared (NIR) as well as in the visible for zinc(II) tetraphenylporphyrin (ZnTPP) films contacting redox solutions, in contrast to no appreciable photocurrent in the NIR for a sandwich-type cell of Al/ZnTPP/Au. The spectral responses of photocurrents in the NIR for the wet cells resemble that of photocurrent synergism reported earlier for the sandwich-type cell, although the ZnTPP solid is almost transparent in this wavelength region. The observed electronic transition in the NIR is ascribed not to a singlet-to-triplet excitation which is commonly observed with phthalocyanine solids, but to the absorption of the NIR light due to a trace of a ZnTPP photoproduct, most probably an isoporphyrin. The photoproduct, which diminishes a generation efficiency of photocurrents by acting as a recombination center, is responsible for the increase in photocurrents due to Soret band excitation by simultaneous illumination with the NIR light. The difference in the NIR photoresponse between the wet cells and the dry cell is explained on the basis of an energy-level consideration.