NOVELTY - The process comprises depositing a precursor structure (35), which has a first metallic sub-layer and a second sub-layer, on a base (30), and reacting the first sub-layer and the second sub-layer with each other by increasing the temperature of the precursor structure to a first temperature range (220-450 degrees C) for a first time period of 0.5-5 minutes and then to a second temperature range (500 degrees C) that is higher than the first temperature range for a second time period of 0.5-10 minutes to form a copper indium gallium selenide film on the base. The first sub-layer is formed on the base.    USE - The process of forming a copper indium gallium selenide film on a base is useful in a multilayer precursor film structure to form a group IBIIIAVIA absorber layer for solar cell fabrication (claimed).    ADVANTAGE - The process is capable of effectively and economically forming the copper indium gallium selenide film on the base with less effort thus ensuring economical fabrication of the solar cells with high quality and high-throughput.    DETAILED DESCRIPTION - The process comprises depositing a precursor structure (35), which has a first metallic sub-layer and a second sub-layer, on a base (30), and reacting the first sub-layer and the second sub-layer with each other by increasing the temperature of the precursor structure to a first temperature range (220-450 degrees C) for a first time period of 0.5-5 minutes and then to a second temperature range (500 degrees C) that is higher than the first temperature range for a second time period of 0.5-10 minutes to form a copper indium gallium selenide film on the base. The first metallic sub-layer is formed on the base, and the second sub-layer is formed on the first sub-layer. The second sub-layer comprises a selenium layer formed on the first sub-layer, a gallium layer formed on the selenium layer, and copper disposed over the selenium layer. The reaction step forms the copper indium gallium selenide film to include a surface region, a bulk region and a transition region between the surface region and the bulk region. A gallium/(gallium+indium) molar ratio in the transition region is lower than gallium/(gallium+indium) molar ratio in the surface region and gallium/(gallium+indium) molar ratio in the bulk region. The gallium/(gallium+indium) molar ratio in the first sub-layer is 0.2-0.5, and a copper/(indium+gallium) molar ratio of the precursor structure is 0.7-1.0. A thickness of the gallium layer in the second sub-layer is 10-50 nm. An INDEPENDENT CLAIM is included for a multilayer precursor film structure to form a group IBIIIAVIA absorber layer for solar cell fabrication.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a precursor structure.    Base (30)    Contact layer (31)    Precursor layer (32)    Top surface (33)    Precursor structure. (35)