Atomistic Control of Metal–Molecule Junctions for Efficient Photo-Induced Uphill Charge Transfer

Abstract

Metalloporphyrins were immobilized at single crystalline gold surfaces of (111), (100), and (110) orientations via imidazole-terminated alkanethiols. This method enabled us to simultaneously control the surface density of porphyrins, the distance between the electrode surface and porphyrins, and the atomistic structure of Au–S junctions. The Au–S binding sites were determined to be twofold bridge configuration at Au(100) and Au(110) and threefold hollow configuration at Au(111). Photoinduced uphill charge transfer under potentiostatic control was measured for these well-defined molecular assemblies. The quantum efficiency of photocurrent generation was higher at Au(100) and Au(110) than at Au(111), showing that the Au–S junction is more conductive at the bridge binding site than at the hollow binding site. This difference is well explained by the electronic coupling strength of Au–S junctions. For efficient photo-energy conversion in organic photovoltaic devices, the metal–organic contact barrier can be reduced by controlling the atomistic structures of the interface.