NOVELTY - The method comprises preparing a mixture solution by mixing precursor solutions comprising copper chloride, indium chloride, selenourea and gallium trichloride under a nitrogen atmosphere at room temperature, spraying the mixture solution on a substrate (210) and drying the mixture solution thus forming a thin film, and selenizing the thin film under a selenium atmosphere. The step of preparing the mixture solution comprises adjusting a solution composition of the precursor solutions. The step of spraying is performed in a non-vacuum state.    USE - The method is useful for manufacturing a thin-film light-absorbing layer, which is useful for a solar cell (claimed).    ADVANTAGE - The method is capable of easily and economically manufacturing a thin-film light-absorbing layer without any environmental contamination thus increasing the light conversion efficiency and long-term stability of a large-area solar cell.    DETAILED DESCRIPTION - The method comprises preparing a mixture solution by mixing precursor solutions comprising copper chloride, indium chloride, selenourea and gallium trichloride under a nitrogen atmosphere at room temperature, spraying the mixture solution on a substrate (210) and drying the mixture solution thus forming a thin film, and selenizing the thin film under a selenium atmosphere. The step of preparing the mixture solution comprises adjusting a solution composition of the precursor solutions. The mixture solution is expressed as CuxInySez, where x is 0.5-1.4, y is 0.5-1.4 and z is 1-6. The step of spraying is performed in a non-vacuum state. The substrate is heated at a temperature of 300-500 degrees C for 30-60 minutes using a heater provided in an apparatus for manufacturing a thin-film light-absorbing layer (230). The apparatus for manufacturing a thin-film light-absorbing layer comprises an inlet port into which the mixture solution is introduced, a spray nozzle for spraying the mixture solution, a spray gun provided between the inlet port and the spray nozzle so that the mixture solution is transferred by a carrier gas including nitrogen, where the carrier gas is sprayed at 3-50 ml/h, and a rotary shaker provided under the heater. The rotary shaker is rotated at 10-50 rpm. The step of drying comprises cooling the mixture solution to room temperature at 8-12 degrees C/minute. The step of selenizing is performed at a temperature of 50-610 degrees C for 30-120 minutes. INDEPENDENT CLAIMS are included for:    (1) a thin-film light-absorbing layer;    (2) a method of manufacturing a copper indium diselenide-based thin-film solar cell; and    (3) a thin-film solar cell.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a thin-film solar cell.    Substrate (210)    Back contact layer (220)    Light-absorbing layer (230)    Buffer layer (240)    Window layer. (250)