NOVELTY - Method (M1) of producing coating solution for forming light-absorbing layer of chalcopyrite solar cell involves: dissolving copper or copper selenide and chalcogen in solvent having dimethyl sulfoxide and hydrazine to obtain solution; adding alcohol solvent to crystallize hydrazine-coordinated Cu chalcogenide complex; re-dissolving complex in dimethyl sulfoxide to obtain solution (I); dissolving hydrazine-coordinated indium chalcogenide complex in DMSO to obtain solution (II); dissolving hydrazine-coordinated Ga chalcogenide complex in DMSO to obtain solution (III); and mixing solutions.    USE - For producing a coating solution for forming a light-absorbing layer of a chalcopyrite solar cell (claimed).    ADVANTAGE - The thin-film solar cells prepared using the coating solution exhibit high photoelectric conversion efficiency. In the method, since Cu, In and Ga are dissolved in not hydrazine but dimethyl sulfoxide (DMSO), there is no need to use dangerous hydrazine in the application of the coating solution. As a result, safety of the process in the formation of a light-absorbing layer is assured. The coating solution for forming a light-absorbing layer has no precipitation of Cu2Se. Therefore, the coating solution exhibits excellent storage stability. The coating solution for forming a light-absorbing layer contains no amines as a dissolution promoter. As a result, the photovoltaic (PV) performance is improved as compared to conventional coating solutions. In the method, DMSO is used as a solvent. As a result the storage stability is improved as compared to conventional coating solution. The coating solution for forming a light-absorbing layer is not deteriorated even after 1 month, meaning that the coating solution exhibits excellent storage stability. Since the coating solution itself does not contain hydrazine, the chemical properties (explosiveness) of hydrazine in the formation of light-absorbing layer are not problematic, thereby improving the safety of the production process. Since the chalcopyrite solar cell produced by the method contains no hydrazine in the coating solution, the safety of the production process can be improved. Since the coating solution for forming a light-absorbing layer exhibits improved storage stability, limitation on the production process can be reduced.    DETAILED DESCRIPTION - A method (M1) of producing a coating solution for forming a light-absorbing layer of a chalcopyrite solar cell, involves: dissolving copper (Cu) or copper selenide (Cu2Se) and a chalcogen in a solvent comprising dimethyl sulfoxide and hydrazine to obtain a solution; adding an alcohol solvent to the resulting solution to crystallize a hydrazine-coordinated Cu chalcogenide complex; re-dissolving the crystallized hydrazine-coordinated Cu chalcogenide complex in dimethyl sulfoxide to obtain a solution (I); dissolving a hydrazine-coordinated indium (In) chalcogenide complex in dimethyl sulfoxide to obtain a solution (II); dissolving a hydrazine-coordinated Ga chalcogenide complex in dimethyl sulfoxide to obtain a solution (III); and mixing together the solution (I), the solution (II) and the solution (III). INDEPENDENT CLAIMS are included for the following:    (1) a method (M2) of producing a coating solution for forming a light-absorbing layer of a chalcopyrite solar cell, involving: dissolving Cu or Cu2Se and a chalcogen in a solvent comprising dimethyl sulfoxide and hydrazine to obtain a solution; adding an alcohol solvent to the resulting solution to crystallize a hydrazine-coordinated Cu chalcogenide complex; re-dissolving the crystallized hydrazine-coordinated Cu chalcogenide complex in dimethyl sulfoxide to obtain a solution (I'); dissolving a hydrazine-coordinated indium (In) chalcogenide complex and a hydrazine-coordinated Ga chalcogenide complex in dimethyl sulfoxide to obtain a solution (II'); and mixing together the solution (I') and the solution (II'); and    (2) a method of storing coating solution for forming a light-absorbing layer of a chalcopyrite solar cell, involving: producing the coating solution by the method (M1); and storing the coating solution for longer than two weeks without producing Cu2-Se precipitates; or producing the coating solution according to the method (M2); and storing the coating solution for longer than two weeks without producing Cu2-Se precipitates.