NOVELTY - The method comprises: forming a metal precursor film, and heating the film in the presence of a chalcogen at a temperature of 100-700 degrees C. The step of forming the metal precursor film comprises: forming a first layer above a surface of a substrate (302), where the first layer comprises copper (Cu) and gallium (Ga) and is formed using a sputtering process from a first sputtering target comprising 25-66 atm.% of Ga; and forming a second layer above the first layer, where the second layer comprises Cu and indium (In) and is formed using a sputtering process from a second sputtering target.    USE - The method is useful for forming a semiconductor material on a substrate (claimed) in the fabrication of a thin film photovoltaic device such as copper indium gallium sulfide or copper indium gallium selenide solar cells.    ADVANTAGE - The method is capable of economically forming copper indium gallium sulfide absorber layers having a graded bandgap thus improving efficiency of the CIGS solar cells.    DETAILED DESCRIPTION - The method comprises: forming a metal precursor film, and heating the metal precursor film in the presence of a chalcogen at a temperature of 100-700 degrees C. The step of forming the metal precursor film comprises: forming a first layer above a surface of a substrate (302), where the first layer comprises copper (Cu) and gallium (Ga)and is formed using a sputtering process from a first sputtering target comprising 25-66 atm.% of Ga; and forming a second layer above the first layer, where the second layer comprises Cu and indium (In) and is formed using a sputtering process from a second sputtering target comprising more than 50% of In. The heating step comprises: heating the substrate and the metal precursor film to a temperature of 300-400 degrees C in an atmosphere comprising 1 molar% of the chalcogen; heating the substrate and the metal precursor film to a temperature of 400-550 degrees C in an atmosphere comprising 1 molar% of the chalcogen; and heating the substrate and the metal precursor film to a temperature of 550-650 degrees C in an atmosphere comprising 1 molar % of the chalcogen. The method further comprises forming a third layer above the second layer before the heating step. The first and second layers are formed by a process comprising batch or in-line (co)evaporation, batch or in-line physical vapor deposition (PVD) (sputtering), atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma enhanced CVD, plasma enhanced ALD, atmospheric pressure CVD, ultra-fast atmospheric ALD, plating, printing or spraying of inks, screen printing, inkjet printing, slot die coating, gravure printing, or wet chemical depositions. The heating step is performed in a batch system and in an in-line system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic sectional view of a simple substrate thin film photovoltaic stack.    Substrate (302)    Back contact layer (304)    Absorber layer (306)    Buffer layer (308)    Front contact interface layer. (310)