NOVELTY - Producing a film of compound semiconductor involves: providing a substrate (210); providing a compound bulk material having a first chemical composition that includes at least one first chemical element and second chemical element; depositing a film on substrate using compound bulk material as a single source of material, where deposited film has a composition same as the first chemical composition; and inducing residual chemical reaction in deposited film using a source containing second chemical element to increase the content of second chemical element in the deposited film.    USE - For producing compound semiconductor film useful for producing a thin-film solar cell (claimed) such as a thin-film photovoltaic device.    ADVANTAGE - The three-stage thin film formation process provides several advantages over the existing film forming processes. The use of a pre-reacted bulk material as a single-source deposition precursor facilitates the engineering of various material compositions and properties, e.g. an optical bandgap, and provides superior compositional uniformity in the deposited films. Subsequent residual chemical reactions in the deposited film enable more flexible control of the film's morphology. Specifically, they promote polycrystalline grain growth and grain fusing. The volumetric expansion of the film associated with these chemical reactions removes pinholes and improves intergrain electrical transport. The interlayer adhesion can be compromised by the film's exposure to the high temperatures typically required by chemical reactions in the existing deposition processes. The three-stage process improves the adhesion between the absorber layer and the first contact layer by reducing the amount of chemical reactions required in the film after deposition.    DETAILED DESCRIPTION - Producing a film of compound semiconductor involves: providing a substrate (210); providing a compound bulk material having a first chemical composition that includes at least one first chemical element and a second chemical element; depositing a film on the substrate using the compound bulk material as a single source of material, where the deposited film has a composition same as the first chemical composition; and inducing a residual chemical reaction in the deposited film using a source containing the second chemical element to increase the content of the second chemical element in the deposited film so that the deposited film has a second chemical composition. At least one first chemical element comprises at least one element from group I and group III chemical elements. The second chemical element comprises an element from group VI. INDEPENDENT CLAIMS are included for the following:    (1) producing (M1) a thin-film solar cell involving: depositing a first contact layer on a substrate; producing an absorber layer over the first contact layer; depositing a window layer and a second contact layer over the absorber layer, where the absorber layer is produced by a process that includes: providing a compound bulk material having a first chemical composition that includes at least one first chemical element and a second chemical element; depositing a film over the first contact layer using the compound bulk material as a single source of material, where the deposited film has a composition same as the first chemical composition; and inducing a residual chemical reaction in the deposited film using a source containing the second chemical element to increase the content of the second chemical element in the deposited film so that the deposited film has a second chemical composition; and    (2) producing a compound semiconductor film having a second chemical composition that includes a first element involving: forming a compound bulk material having a first chemical composition that is deficient in the first chemical element with respect to the second chemical composition; forming a film on a substrate from the compound bulk material; and increasing an amount of the first chemical element in the film so that it has a composition equal to the second chemical composition.    DESCRIPTION OF DRAWING(S) - The figure shows a view of a substrate configuration of a thin-film solar cell.    Thin-film photovoltaic device (200)    Substrate (210)    First contact layer (220)    Thin absorber layer (230)    Window layer (240)    Second contact layer (250)