The mechanism of enhancing the light harvesting efficiency of dye-sensitized TiO2 solar cells by coupling TiO2 inverse opals or disordered scattering layers to conventional nanocrystalline TiO2 films has been investigated. Monochromatic incident photon-to-current conversion efficiency (IPCE) at dye-sensitized TiO2 inverse opals of varying stop band wavelengths and at disordered titania films was compared to the IPCE at bilayers of these structures coupled to nanocrystalline TiO2 films and to the IPCE at nanocrystalline TiO2 electrodes. The results showed that the bilayer architecture, rather than enhanced light harvesting within the inverse opal structures, is responsible for the bulk of the gain in IPCE. Several mechanisms of light interaction in these structures, including localization of heavy photons near the edges of a photonic gap, Bragg diffraction in the periodic lattice, and multiple scattering events at disordered regions in the photonic crystal or at disordered films, lead ultimately to enhanced backscattering. This largely accounts for the enhanced light conversion efficiency in the red spectral range (600-750 nm), where the sensitizer is a poor absorber.