NOVELTY - A photovoltaic device comprises multi-layer anti-reflective coating formed over transparent substrate. The coating comprises nanostructured layers, each layer has tuned porosity and some layers have different porosities to create a different index of refraction for those layers.    USE - A photovoltaic device.    ADVANTAGE - The anti-reflective coating is easily processed with minimal environmental impact upon high production volume manufacturing; and is mechanically stable and abrasion resistant, uniformly formed even on large surface areas, and simply processed without the need for silica nanoparticles. The device applied with the coating has more ordered surface structure than the acid-etched roughness of the prior art, higher uniformity than the aqueous sol coating of other prior art, and simpler manufacturing process than the sol-gel embedded, multi-modal nanoparticle strategy employed in other prior art.    DETAILED DESCRIPTION - The multi-layer anti-reflective coating has graded index of refraction. Each nanostructured layer has different index of refraction, and has porosity that is different from that in any other layer to alter the index of refraction for that layer. The layers define a three-dimensional porous network that provides an optical path which captures most of the visible light which enters the network. The porous network increases light transmission through the substrate to an underlying photovoltaic absorber layer. The pores are filled with transparent pore-filling material to define nanowires in the nanostructured porous layers. The transparent pore-filling material is titania, organic material, dyes, pigments or conjugated polymers. The device further comprises surface protecting layer, UV absorber, moisture resistance, scratch resistance layer, fluorine resin as surface protecting layer, encapsulant, inorganic material for protecting against exposure to water or air, and photovoltaic absorber layer. The light collection is greater than or equal to 95 (preferably greater than or equal to 85)% of incoming light in wavelengths of 300-1300 nm, or greater than or equal to 95 (preferably greater than or equal to 85)% of incoming light in wavelengths of 400-1600 nm. The pores have diameter of 2-500 nm, preferably 10-30 nm. An INDEPENDENT CLAIM is included for a method for forming an anti-reflective film, comprising forming nanostructured porous layers by sol-gel process, surfactant templation and/or forming nanoporous coatings from polymeric precursors and using decomposition of organic portion by heat, plasma or ozone. The anti-reflective film is formed on individual solar cells, on transparent front layer of solar panel, or directly on solar cell with no glass at all on the cell.