We calculate the maximal absorption enhancement obtainable by guided mode excitation in a weakly absorbing dielectric slab over wide wavelength ranges. The slab mimics thin film silicon solar cells in the low absorption regime. We consider simultaneously wavelength-scale periodicity of the texture, small thickness of the film, modal properties of the guided waves and their confinement to the film. Also we investigate the effect of the incident angle on the absorption enhancement. Our calculations provide tighter bounds for the absorption enhancement but still significant improvement is possible. Our explanation of the absorption enhancement can help better exploitation of the guided modes in thin film devices.