NOVELTY - The method comprises: providing (110) a semiconductor film comprising indium (In) and selenium (Se) upon a substrate; heating (120) the substrate and the semiconductor film to a desired temperature; and performing (130) a mass transport through vapor transport of a copper chloride vapor and Se vapor to the semiconductor film within a reaction chamber. The step of heating the substrate with the semiconductor film comprises heating the substrate to a temperature of 475-600 degrees C before flowing the copper chloride vapor into the reaction chamber.    USE - The method is useful for fabricating a thin film device (claimed) such as a thin film photovoltaic cell or a thin film solar cell made of copper indium diselenide or copper indium gallium diselenide.    ADVANTAGE - The method simply and economically fabricates the thin film device with high energy conversion efficiency.    DETAILED DESCRIPTION - The method comprises: providing (110) a semiconductor film comprising indium (In) and selenium (Se) upon a substrate; heating (120) the substrate and the semiconductor film to a desired temperature; and performing (130) a mass transport through vapor transport of a copper chloride vapor and Se vapor to the semiconductor film within a reaction chamber. The step of heating the substrate with the semiconductor film comprises heating the substrate to a temperature of 475-600 degrees C before flowing the copper chloride vapor into the reaction chamber. The copper chloride vapor is produced by heating solid copper chloride at a temperature of 275 degrees C. The step of performing the mass transport comprises injecting the copper chloride vapor and Se vapor into the reaction chamber until the film includes a copper-rich region. The method further comprises: treating (140) the film in a flux of In by providing a source of In to the reaction chamber; treating the film in a flux of gallium (Ga) by providing a source of Ga to the reaction chamber; and maintaining the substrate temperature at the desired temperature while treating the film in a flux of In. The flux of In is produced by an In source. The flux of Ga is produced by an Ga source. The substrate comprises a back contact layer that includes a molybdenum film. An INDEPENDENT CLAIM is included for a method of forming a p-type film for a photovoltaic device.    DESCRIPTION OF DRAWING(S) - The diagram shows a flow chart of a method for fabricating a copper indium selenide or copper indium gallium selenide film.    Fabricating a thin film device (100)    Providing semiconductor film comprising indium and selenium upon substrate (110)    Heating substrate and semiconductor film to desired temperature (120)    Performing mass transport through vapor transport (130)    Treating film in flux of indium. (140)