Generation of Long-Lived Redox Equivalents in Self-Assembled Bilayer Structures on Metal Oxide Electrodes

We report on the synthesis and photophysical properties of a photocathode consisting of a molecular bilayer structure self-assembled on p-type NiO nanostructured films. The resulting photocathode and its nanostructured indium-tin oxide analog absorb visible light and convert it into injected holes with injection yields of similar to 30%, measured at the first observation time by nanosecond transient absorption spectroscopy, and long-lived reducing equivalents that last for several milliseconds without applied bias. An initial quantum yield of 15% was achieved for photogeneration of the reduced dye on the p-NiO electrode. Nanosecond transient absorption experiments and detailed analyses of the underlying electron transfer steps demonstrate that the overall efficiency of the cell is limited by hole injection and charge recombination processes. Compared with the highly doped indium tin oxide photocathode, the NiO photocathode shows superior photoconversion efficiencies for generating reducing equivalents and longer lifetimes of surface-bound redox-separated states due to an inhibition toward charge recombination with the external assembly.