NOVELTY - Making thin film solar cell on substrate (10) involves providing thin film (30) coated on back contact (20) on first substrate; depositing selenium layer onto second substrate; arranging the substrates so that the thin film is parallel to, and facing selenium layer, where thin film and selenium layer are spaced apart; and heating the substrates. The thin film is copper indium diselenide (CIS) or copper indium gallium diselenide (CIGS). The thin film is CIGS film that is deficient in copper atoms or is CIGS film that contains sodium ions. The back contact contains molybdenum.    USE - For making thin film solar cell on a substrate (claimed); for making solar cell devices and other products used to convert sunlight into electricity; and for making photovoltaic absorber layer for an optoelectronic application such as a solar cell.    ADVANTAGE - The method can be used to control stoichiometry of metal atoms in making a solar cell for targeting a particular concentration and providing a gradient of metal atom concentration. The polymeric precursor compounds are stable in inert atmosphere and advantageously allow control of the stoichiometry, structure, and ratios of the atoms in a semiconductor material or layer, in particular, metal atoms and atoms of Group 13. Using polymeric precursor compounds in any particular semiconductor manufacturing process, the stoichiometry of monovalent metal atoms and Group 13 atoms can be determined and controlled. For processes operating at relatively low temperatures, such as certain printing, spraying, and deposition methods, the polymeric precursor compounds can maintain the desired stoichiometry. As compared to processes involving multiple sources for semiconductor preparation, the polymeric precursors can provide enhanced control of the uniformity, stoichiometry and properties of a semiconductor material. These advantageous features allow enhanced control over the structure of a semiconductor material made with the polymeric precursor compounds. The polymeric precursors are superior building blocks for semiconductor materials because they may provide atomic-level control of semiconductor structure. The polymeric precursor compounds allow direct and precise control of the stoichiometric ratios of metal atoms. The polymeric precursor can be used alone, without other compounds, to readily prepare a layer from which CIS, CIGS materials of any arbitrary stoichiometry can be made. Using the method chemically and physically uniform semiconductor layers can be prepared with polymeric precursor compounds; and solar cells and other products can advantageously be made in processes operating at relatively low temperatures using the polymeric precursor compounds and compositions. The polymeric precursor compounds can provide enhanced processability for solar cell production; provide the ability to be processed at relatively low temperatures, as well as the ability to use a variety of substrates including flexible polymers in solar cells.    DETAILED DESCRIPTION - Making a thin film solar cell on a substrate (10) involves: providing a thin film (30) coated on a back contact (20) on a first substrate; depositing a selenium layer onto a second substrate; arranging the first and second substrates so that the thin film is parallel to, and facing the selenium layer, where the thin film and the selenium layer are spaced apart; and heating the first and second substrates. The thin film is copper indium diselenide (CIS) or copper indium gallium diselenide (CIGS). The thin film is CIGS film that is deficient in copper atoms. The thin film is CIGS film that contains sodium ions. The thin film is spaced apart from the selenium layer by a distance of 10 to 3000 (preferably 10-500, especially 50-150) micrometers. The selenium layer is deposited by vapor deposition; or is deposited by spraying, printing or coating a selenium-containing ink. The heating step anneals the thin film. The thin film contains 0.01 to 2.0 (preferably 0.1-1) atom% alkali ions. The space between the thin film and the selenium layer is enclosed. The thin film is crystalline. The back contact contains molybdenum. The back contact on a first substrate is a conducting substrate. An INDEPENDENT CLAIM is included for solar cell made by the process.    DESCRIPTION OF DRAWING(S) - The figure shows schematic representation of a solar cell.    Substrate (10)    Electrode layer/molybdenum-containing layer (20)    Absorber layer/copper indium diselenide or copper indium gallium diselenide layer (30)    Buffer layer/cadmium sulfide layer (40)    Transparent conductive layer/indium tin oxide layer or doped zinc oxide layer (50)