NOVELTY - Producing (I) chalcopyrite compound thin film, comprises (a) mixing first metal precursor, first organic binder, and first water-soluble solvent to obtain first paste, (b) mixing second metal precursor, second organic binder, and second water-soluble solvent to obtain second paste, (c) coating first paste on conductive substrate to form first paste layer, (d) coating second paste on first paste layer to form second paste layer, (e) thermally treating coated conductive substrate in air or oxygen atmosphere to obtain mixed oxide thin film, and (f) thermally treating mixed oxide thin film.    USE - The method is useful for producing chalcopyrite compound thin film (claimed) which is used as light-absorbing layers for thin-film solar cells.    ADVANTAGE - The method provides high-quality chalcopyrite compound thin-film solar cells at low cost.    DETAILED DESCRIPTION - Producing (I) a chalcopyrite compound thin film, comprises (a) mixing a first metal precursor, a first organic binder, and a first water-soluble solvent to obtain a first paste, (b) mixing a second metal precursor, a second organic binder, and a second water-soluble solvent to obtain a second paste, (c) coating the first paste on a conductive substrate to form a first paste layer, (d) coating the second paste on the first paste layer to form a second paste layer, (e) thermally treating the coated conductive substrate in air or an oxygen atmosphere to obtain a mixed oxide thin film, and (f) thermally treating the mixed oxide thin film in a sulfur gas, a selenium gas or a sulfur/selenium mixed gas atmosphere to form a sulfide or selenide thin film. The first metal precursor and the second metal precursor are identical to or different from each other and are each independently a precursor of at least one of group IB metals and/or group IIIA metals. The precursor of group IB metals and the precursor of group IIIA metals are each independently included in either the first metal precursor or the second metal precursor or both of them. The first and the second water-soluble solvent are identical to or different from each other and are each independently comprising water, alcohol and/or acetone. The first and the second organic binder are identical to or different from each other and are each independently comprising ethyl cellulose, polyvinyl acetate, palmitic acid, polyethylene glycol, polypropylene glycol, polypropylene carbonate and/or propylenediol. The conductive or non-conductive substrate is made of indium tin oxide, fluorine-doped indium tin oxide, glass, molybdenum-coated glass, a metal foil, a metal plate and/or a conductive polymer material. INDEPENDENT CLAIMS are also included for:    (1) producing (II) chalcopyrite compound thin film, comprising (a)-(d) as above per se, thermally treating the coated conductive substrate in air or an oxygen atmosphere to obtain a copper indium gallium mixed oxide thin film, and thermally treating the copper indium gallium mixed oxide thin film in a sulfur gas, a selenium gas or a sulfur/selenium mixed gas atmosphere to form a copper indium gallium selenide thin film, where the first metal precursors and the second metal precursors are identical to or different from each other and are each independently comprising copper, indium and gallium precursors and the copper, indium and gallium precursors are each independently included in either the first metal precursors and/or the second metal precursors;    (2) a chalcopyrite compound thin film comprising either: as light-absorbing layers, at least two coating layers formed on the conductive substrate where the light-absorbing layer positioned relatively close to the conductive substrate has a higher average density than the light-absorbing layer relatively remote from the conductive substrate; or as light-absorbing layers, at least three coating layers formed on the conductive substrate, where the ratios of the concentration of the gallium element to that of the copper element in the light absorbing layer positioned closest to the conductive substrate and the light-absorbing layer remotest from the conductive substrate are greater by 0.1-0.9 than the ratio of the concentration of the gallium element to that of the copper element in the light-absorbing layer positioned between the two other light absorbing layers; and    (3) a chalcopyrite compound thin film produced by solution processing where the concentrations of gallium in the upper and lower portions are higher than the concentration of gallium in the central portion.