NOVELTY - Forming (M1) a porous coating (210) on a substrate (200) involves coating a substrate with a sol-gel composition comprising at least one porosity forming agent, where the porosity forming agent is selected from at least one of dendrimers and organic nanocrystals; and annealing the coated substrate to remove at least one porosity forming agent to form the porous coating.    USE - For forming a porous coating on a substrate (claimed) useful for many applications including semiconductor device manufacturing, solar cell manufacturing, glass manufacturing, and energy cell manufacturing.    ADVANTAGE - The method allows forming porous low refractive index coatings on substrates; and the coating exhibits increased durability and controllable pore size. The porosity forming agent decomposes or combusts to form voids of desired size and shape upon heating. The porosity forming agent leads to the formation of stable pores with larger volume and an increased reduction in the refractive index of the coating. The coating can reduce reflections of the incident light and permits more light to reach the thin film semiconductor film of photovoltaic device, thus permitting the device to act more efficiently.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a composition (C1) for forming a sol-gel system, comprising a film forming precursor, an acid or base containing catalyst, an alcohol containing solvent, at least one porosity forming agent selected from at least one of dendrimers and organic nanocrystals, and water; and    (2) making (M2) a sol-gel system involving mixing a film forming precursor, an acid or base containing catalyst, water, and an alcohol containing solvent to form a reaction mixture by at least one of a hydrolysis and polycondensation reaction, and adding at least one porosity forming agent selected from at least one of dendrimers and organic nanocrystals to the reaction mixture.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of a photovoltaic cell comprising the porous coating.    Glass substrate (200)    Porous coating (210)    Photovoltaic device (300)    Photovoltaic semiconductor (320)    Reflector (340)