NOVELTY - Forming (M1) a thin film of copper indium gallium diselenide on a substrate, involves forming an ionic liquid composition comprises an ionic solvent and ionic species of copper, indium, gallium, and selenium; immersing a surface of the conductive substrate into the ionic liquid, immersing a counter electrode into ionic liquid, electrically connecting the conductive substrate and counter electrode to a power supply; and electrochemically depositing in a one-pot process a film comprised of copper, indium, gallium, and selenium having a stoichiometry copper indium gallium diselenide.    USE - For forming thin film of copper indium gallium diselenide on a conductive substrate useful for forming solar cell and optoelectronic device (claimed).    ADVANTAGE - The method is an economical, reproducible electrodeposition method for producing stoichiometric copper indium gallium diselenide films which is scalable and is use in industries for the production of large surface area solar cells without the requirement of any post-deposition sintering or selenization; provides low-cost and large-area solar cells, without heating above 60-100 degrees C or post-deposition thermal sintering in electrodeposition processes; and provides high efficiency for solar energy conversion at very low production cost.    DETAILED DESCRIPTION - Forming (M1) a thin film of copper indium gallium diselenide of formula CuInxGa1-xSe2 on a conductive substrate, involves forming an ionic liquid composition comprises an ionic solvent and ionic species of copper, indium, gallium, and selenium; immersing a surface of the conductive substrate into the ionic liquid, immersing a counter electrode into the ionic liquid, electrically connecting the conductive substrate and the counter electrode to a power supply; and electrochemically depositing in a one-pot process a film comprised of copper, indium, gallium, and selenium having a stoichiometry CuInxGa1-xSe2.    x=0.6-0.8.    An INDEPENDENT CLAIM is included for forming (M2) a multilayer structure involving depositing a layer of CuInxGa1-xSe2 on a conductive substrate, depositing at least one n-type semiconductor layer on the CuInxGa1-xSe2, and depositing a top electrode on the n-type semiconductor layer to form the multilayer structure; or depositing at least one n-type semiconductor layer on a conductive substrate, depositing a layer of CuInxGa1-xSe2 on the at least one n-type semiconductor layer, and depositing a top electrode on the CuInxGa1-xSe2 layer to form the multilayer structure.