NOVELTY - The glazing unit comprises a transparent glass substrate equipped with an antireflection coating. The coating comprises a porous material layer comprising silicon (30-35%), oxygen (58-65%), carbon (3-8%) and hydrogen in which the atomic proportion (P(C)) of carbon with respect to the sum of atomic contributions of silicon, oxygen and carbon locally varies, in a thickness direction of the layer, from a first surface to a second surface according to an increase between a first minimum value (P(Cmin1)) and a maximum value (P(Cmax)), where the ratio between the PCmax and PCmin1 is 1.2.    USE - The glazing unit is useful in a photovoltaic cells or solar collectors.    ADVANTAGE - The glazing unit exhibits high transmission energy, improved antireflective properties, and increased abrasion and chemical resistance.    DETAILED DESCRIPTION - The glazing unit comprises a transparent glass substrate equipped with an antireflection coating. The coating comprises a porous material layer comprising silicon (30-35%), oxygen (58-65%), carbon (3-8%) and hydrogen in which the atomic proportion (P(C)) of carbon with respect to the sum of atomic contributions of silicon, oxygen and carbon locally varies, in a thickness direction of the layer, from a first surface to a second surface according to an increase between a first minimum value (P(Cmin1)) and a maximum value (P(Cmax)), where the ratio between the PCmax and PCmin1 is 1.2 and according to a decrease the proportion of carbon between the PCmax and a second minimum value (P(Cmin2)), where the ratio of the PCmax and the PCmin2 is 1.2. The porous material layer has a chemical composition, according to the respective atomic proportions between the silicon, oxygen and carbon entering in composition of porous material that consists of 28-38% of silicon, 55-68% of oxygen and 2-10% of carbon. The antireflection coating constitutes the porous material layer in which the P(Cmin1) of the proportion of carbon in the porous layer is leveled with its air side surface and in which the P(Cmin2) of the proportion of carbon in the porous layer is leveled close to the surface of the substrate. The total amount of carbon in the material constituting the layer is less than 5 at.%. Ratios of P(Cmax)/P(Cmin1) and/or P(Cmax)/P(Cmin2) are greater than 2. The porous material layer has a thickness of 50-120 nm, and a refractive index of less than 1.35. INDEPENDENT CLAIMS are included for:    (1) a porous layer; and    (2) a process for fabricating a glazing unit.