Lambertian light trapping is a well-known method for enhancing the light harvesting efficiency of solar cells. Since it is based on ray optics, it is conventionally considered as inapplicable for subwavelength ultrathin films. Here we show a way around this limitation by separating the light confinement and absorption sites within the stack of materials comprising the entire cell. We demonstrate this approach for ultrathin film hematite (α-Fe2O3) photoanodes designed for renewable hydrogen production via solar water splitting. Attaching a Lambertian back reflector (that is, a white scattering sheet) to the backside of the cell results in a photocurrent enhancement of 25% to 30%, depending on the hematite thickness, in comparison to the same cell with a specular back reflector (i.e., a mirror). Theoretical analysis suggests that even higher enhancement may be possible, exceeding 40% in some cases, if light escape through the cell edges could be prevented. The proposed approach is not material-specific and can be readily implemented in other materials and other types of solar cells. Another advantage of this approach is that the light management is achieved using simple commercial products, making the proposed approach cost-effective and easy to implement in a variety of solar cells and photodetectors.