Photoelectrode characteristics of an organic bilayer in water phase containing a redox molecule

Abstract

We have recently reported that the organic bilayer of 3,4,9,10-perylenetetracarboxyl-bisbenzimidazole (PTCBI, n-type semiconductor) and 29H,31H-phthalocyanine (H₂Pc, p-type semiconductor), which is a part of a photovoltaic cell, acts as a photoanode in the water phase (Abe et al., ChemPhysChem 5:716, [2004]); in that case, the generation of the photocurrent involving an irreversible thiol oxidation at the H₂Pc/water interface took place to be coupled with hole conduction through the H₂Pc layer, based on the photophysical character of the bilayer. In the present work, the photoelectrode characteristics of the bilayer were investigated in the water phase containing a redox molecule ${\left( {{\text{Fe}}\,^{{{{\text{III}}} \mathord{\left/ {\vphantom {{{\text{III}}} {{\text{II}}}}} \right. \kern-\nulldelimiterspace} {{\text{II}}}}} {\left( {{\text{CN}}} \right)}^{{{3 - } \mathord{\left/ {\vphantom {{3 - } {4 - }}} \right. \kern-\nulldelimiterspace} {4 - }}}_{6} } \right)}$, where the photo-induced oxidation and reduction for the ${\text{Fe}}\,^{{{{\text{III}}} \mathord{\left/ {\vphantom {{{\text{III}}} {{\text{II}}}}} \right. \kern-\nulldelimiterspace} {{\text{II}}}}} {\left( {{\text{CN}}} \right)}^{{{3 - } \mathord{\left/ {\vphantom {{3 - } {4 - }}} \right. \kern-\nulldelimiterspace} {4 - }}}_{6}$ couple were found to take place at the bilayer. The photoanodic current involving the ${\text{Fe}}\,^{{{\text{II}}}} {\left( {{\text{CN}}} \right)}^{{4 - }}_{6}$ oxidation efficiently occurred at the interface of H₂Pc/water, similar to the previous example. In the view of the voltammograms obtained, it was noted that there are pin-holes in the H₂Pc layer of the bilayer, leading to a cathodic reaction with ${\text{Fe}}\,^{{{\text{III}}}} {\left( {{\text{CN}}} \right)}^{{3 - }}_{6}$ at the PTCBI surface especially in the dark; that is, the band bending at the PTCBI/water interface can essentially be reduced by applying a negative potential [e.g., < ∼ 0 V (vs Ag/AgCl)] to the PTCBI, when the cathodic reaction may take place through the conduction band of the PTCBI. Moreover, under that applied potential condition of irradiation, the photogenerated electron carrier part can move to the PTCBI surface, thus enhancing the reduction of ${\text{Fe}}\,^{{{\text{III}}}} {\left( {{\text{CN}}} \right)}^{{3 - }}_{6}$.