Thin film silicon solar cells are an attractive option for the production of sustainable energy but their low response at long wavelengths requires additional measures for absorption enhancement. The most successful concepts are based on light scattering interface textures whose understanding is greatly facilitated by considering a superposition of periodic textures that diffract the light into oblique angles, ideally beyond the critical angle of total internal reflection. Because the thickness of the active layers is on the same scale as the wavelength, interference of diffracted waves gives rise to resonance phenomena. We discuss the absorption enhancement in terms of a perturbation approach using the modal structure of a corresponding device with flat interfaces.