Novel photocathodic characteristics of organic bilayer composed of a phthalocyanine and a perylene derivative in a water phase containing a redox molecule

Abstract

An organic bilayer composed of metal-free phthalocyanine (H 2Pc, p-type semiconductor) and perylene derivative (PTCBI, n-type semiconductor) was found to involve novel characteristics of photoelectrode working in the water phase. When the photoelectrode characteristics were investigated when the PTCBI has contact with water containing the redox molecule ( Fe III ( CN ) 6 3 - (electron acceptor)), it was shown by voltammetry that the photocathodic current due to the Fe III ( CN ) 6 3 - reduction occurs at the present electrode, which is different from the ordinary characteristics at the n-type semiconductor/water interface of the Schttoky junction. Separate voltammetric studies showed that the photocathodic characteristics of the H 2Pc/PTCBI bilayer are consistent with those of the H 2Pc single layer, indicating that there are pin-holes in the PTCBI layer of the bilayer; that is, the H 2Pc has direct contact with Fe III ( CN ) 6 3 - dissolved in water. Thus, it is inferred that the photocathodic current occurs at the H 2Pc surface. However, the action spectrum for the photocathodic current indicated that a broad visible light absorption (400–750 nm) by only PTCBI can also induce photocurrent generation especially at wavelengths shorter than 500 nm where absorption of the H 2Pc is absent or relatively weak. This is supported by the previous knowledge that the PTCBI exciton alone can contribute to carrier generation through charge separation at the H 2Pc/PTCBI interface. This study showed the novel photocathodic characteristics at the organic solid/water interface coupled with electron conduction through the n-type semiconductor, in addition to the ordinary characteristics at the p-type semiconductor/water interface.