A series of results, based on charge transfer studies on complete functional devices, using as sensitizers organic and heteroleptic Ruthenium(II) polypiridyl dyes has led us to establish a direct relationship between the dye structure-the TiO₂/electrolyte⁺/ and device efficiency. The results are in good agreement with previous work on π-extended Ruthenium (II) complexes and the use of hydrophobic alkyl chains as inhibitors of back electron transfer reactions from the semiconductor to either the oxidized dye or the oxidized electrolyte. In an effort to understand the details of such differences we have also utilized high level DFT computational calculations to explore the role of the HOMO-LUMO energetics on the interfacial charge transfer kinetics.