NOVELTY - The method involves supplying a zinc source and solution remained in a first bath (10) after supplying of an alkaline solution to a substrate (11) placed inside a second bath (20). The zinc source is diffused to inside of a light absorbing layer through a surface of the light absorbing layer. The substrate is moved to a third bath (30). A sulfur source and a mixed solution comprising a solution remained in the second bath after supplying and diffusing of the zinc source are supplied to the substrate placed inside the third bath. A buffer layer is formed on the light absorbing layer.    USE - Method for manufacturing a cascade copper-indium-gallium-selenide (CIGS)-based thin film solar cell.    ADVANTAGE - The method enables reusing the zinc source remaining after the use in a previous act in forming the buffer layer on the substrate with the light absorbing layer, thus sufficiently supplying the zinc source to a vacancy region near the surface of the light absorbing layer, which enables substituting and etching of the zinc source, and hence enabling the diffusion and distribution of a sufficient amount of the zinc source on the surface and the inside of the surface of the light absorbing layer. The method enables reusing alkaline solution to effectively remove copper-selenium material and oxides present on the surface of the light absorbing layer so as to sufficiently substitute and diffuse the newly supplied zinc source into the inside of the light absorbing layer, thus promoting growth of the buffer layer. The method allows the buffer layer to be continuously formed on the substrate on which the light absorbing layer is formed when the substrate is immersed into the mixed solution of the alkaline solution, the zinc source and the sulfur source within a concentration range and heat-treated in a temperature range, thus enabling reduction in manufacturing cost and resolving an environmental issue in handling waste liquid. The method enables decreasing reflection rate of incident light when the surface of transparent electrode layers is rough due to texturing, thus increasing an amount of trapped light. The method enables manufacturing the cascade thin film solar cell with improved productivity.    DETAILED DESCRIPTION - The alkaline solution comprises an ammonia compound i.e. ammonia water. The zinc source comprises zinc salt and zinc compound. The sulfur source comprises a thiourea-based compound. The buffer layer is made of zinc sulphide. INDEPENDENT CLAIMS are also included for the following:    (1) a device for manufacturing a thin film solar cell    (2) a thin film solar cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a manufacturing device for forming a buffer layer of a thin film solar cell.    Baths (10, 20, 30)    Substrate (11)    Supply devices (12, 22, 32)    Pumps (14, 24)    Blocking devices (16, 26)