Organic dyes were adsorbed on a compact TiO2 thin film and nanoporous TiO2 film for use as a photosensitizer and their photoelectrochemical (PEC) water-splitting responses were compared under visible light illumination. The compact TiO2 thin film exhibited a much better PEC response than the nanoporous TiO2 film, even though the latter adsorbed a much higher amount of dye, due to its higher surface area. This can be attributed to the depletion layer formed in the compact film morphology, because its electric field suppresses the back transfer (or recombination rate) of the electrons photogenerated by the dye molecules.