NOVELTY - The method comprises forming oxygen-free first, second and third precursor layers (106) on a flexible, aluminum foil substrate, where the precursor layers comprise discrete layers and group IB elements and group IIIA elements, forming a layer containing elemental chalcogen particles on the precursor layers, and heating the first precursor layer to a first temperature that is sufficient to melt the chalcogen particles to form molten chalcogen thus allowing the particles of the first layer to react with molten chalcogen having group IB elements and group IIIA elements in the precursor layer.    USE - The method is useful for forming a film of a group IB-IIIA-chalcogenide compound in the fabrication of solar cells, where: the group IB-IIIA-chalcogenide compound is copper-indium-gallium-sulfur-selenium (CuzIn(1-x)GaxS2(1-y)Se2y); z is 0.5-1.5; x is 0-1; and y is 0-10.    ADVANTAGE - The method is capable of effectively forming the film of the group IB-IIIA-chalcogenide compound with high-throughput thus reducing the fabrication cost and time of the solar cells and improving the power conversion efficiencies of the solar cells.    DETAILED DESCRIPTION - The method comprises forming oxygen-free first, second and third precursor layers (106) on a flexible, aluminum foil substrate, where the precursor layers comprise discrete layers and group IB elements and group IIIA elements, forming a layer containing elemental chalcogen particles on the precursor layers, heating the first precursor layer to a first temperature that is sufficient to melt the chalcogen particles to form molten chalcogen thus allowing the particles of the first layer to react with the molten chalcogen having the group IB elements and group IIIA elements in the precursor layer, prior to heating to a second temperature, to form a film of a group IB-IIIA-chalcogenide compound on the substrate, depositing a sodium containing layer on the film, depositing a gallium rich second precursor layer on the sodium containing layer, and reacting the second precursor layer with group VIA material. The first precursor layer is formed over the second layer, or the second layer is formed over the first layer. The forming step comprises wet depositing the first precursor layer containing group IB elements and different group IIIA elements as particles on the substrate, wet depositing a second layer (108) containing surplus chalcogen particles over the first precursor layer or below the first precursor layer, providing a source of excess chalcogen such as chalcogen-rich chalcogenide particles to the second layer, where neither the first precursor layer nor the second layer is heat treated until heated together, forming a dispersion including nanoparticles containing group IB elements and nanoparticles containing group IIIA elements, and spreading a film of the dispersion onto the substrate. The first precursor layer contains elemental chalcogen particles, and groups the IB elements in the form of a group IB-chalcogenide and the IIIA elements in the form of a group IIIA-chalcogenide. The third precursor layer contains elemental chalcogen particles. The steps of forming the precursor layer, disposing the layer containing elemental chalcogen particles over the precursor layer and/or heating the precursor layer and chalcogen particles are performed by roll-to-roll manufacturing method. The step of heating the precursor layer and chalcogen particles includes heating the substrate and precursor layer from an ambient temperature to a plateau temperature of 200-600 degrees C, maintaining a temperature of the substrate and precursor layer in the plateau temperature for a fraction of a second to 60 minutes, and reducing the temperature of the substrate and precursor layer. The film is further formed from the precursor layers of the particles and an ink containing a sodium compound with an organic counter-ion or a sodium compound with an inorganic counter-ion.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of a photovoltaic active layer.    First precursor layer (106)    Second layer containing chalcogen particles. (108)