Photocurrent generation in heterostructured ultrathin films fabricated by layer-by-layer deposition of polyelectrolytes bearing tris(2,2'-bipyridine)ruthenium(II) and ferrocene moieties

The photoelectrochemical properties of single-component and heterostructured layer-by-layer deposited films bearing tris(2,2'-bipyridine)ruthenium(II) (Ru) moieties were investigated by photocurrent measurements in solutions in the presence of sacrificial reagents. The photocurrent increased with an increase in the thickness of the films and then had a maximum at a thickness of 10 nm. This increase demonstrates a light-harvesting effect based on excitation energy migration among the Ru moieties to the film/electrolyte interface. A cathodic photocurrent was observed for a heterostructured film where bilayers bearing ferrocene (Fc) moieties and bilayers bearing Ru moieties were deposited on an indium tin oxide (ITO) substrate in the order (ITO/Fc/Ru). On the other hand, an anodic photocurrent was observed for the reverse order film (ITO/Ru/Fc). These results show that the direction of the photocurrent is determined by the gradient of the redox potentials formed in the heterostructured films. The internal quantum efficiency for the ITO/Ru/Fc film was twice that for the single-component film (ITO/Ru). This enhancement of the quantum efficiency is due to suppression of charge recombination by successive electron transfers in the heterostructured film.