A photovoltaic assembly includes an optical element comprising a body having opposed planar major surfaces with a plurality of open-mouthed inclusions (grooves) formed on the light output surface. Each groove has a closed apex with an included apex angle. The body is formed from a material having an index of refraction of at least 1.3 and an induced absorbance rate ΔAbs/Dose less than or equal to about 0.4. The optical element is attached to a photovoltaic cell using an adhesive having an index of refraction in the range from about 1.34 to 1.50. The apex angle is selected such that light incident upon the body is conveyed the light output surface by the mechanism of total internal reflection from the boundary walls of the grooves and/or without retro-reflection toward the incident surface. The thickness of the adhesive layer and the dimension of the mouth of the grooves are selected such that light incident on the surface of the photovoltaic cell is not obscured by conductor lines on the cell's surface.