NOVELTY - The method comprises mixing (S10) a copper indium gallium sulfur selenide (CIGSSe) powder, a solvent and an additive to form a CIGSSe slurry, coating (S20) the CIGSSe slurry on a back electrode layer to form a CIGSSe slurry coating layer, drying process (S30) including pre-drying and removing the solvent in the CIGSSe slurry coating layer, and densifying process (S40) including a rolling process, a high-pressure liquid spray pressing process and a high-pressure gas spray pressing process, in which a pressure is applied onto CIGSSe slurry coating layer to densify CIGSSe slurry coating layer.    USE - The method is useful for forming a copper indium gallium sulfur selenide (CIGSSe) absorption layer and a cadmium sulfide (CdS) buffer layer under non-vacuum condition, where the layers are useful in a solar cell.    ADVANTAGE - The method is capable of economically forming the copper indium gallium sulfur selenide (CIGSSe) absorption layer and the cadmium sulfide (CdS) buffer layer with high quality and high compactness and improved absorption efficiency and photoelectric conversion efficiency.    DETAILED DESCRIPTION - The method comprises mixing (S10) a copper indium gallium sulfur selenide (CIGSSe) powder, a solvent and an additive to form a CIGSSe slurry, coating (S20) the CIGSSe slurry on a back electrode layer to form a CIGSSe slurry coating layer, drying process (S30) including pre-drying and removing the solvent in the CIGSSe slurry coating layer, densifying process (S40) including a rolling process, a high-pressure liquid spray pressing process and a high-pressure gas spray pressing process, in which a pressure is applied onto the CIGSSe slurry coating layer to densify the CIGSSe slurry coating layer, a primary selenization/sulfurization reaction process (S50), a thermal process including carrying out a crystallizing process of the primary CIGSSe absorption layer, an impurity cleaning process including removing impurity compounds in the primary CIGSSe absorption layer using an impurity scavenger and carrying out cleaning and drying, a rear-stage selenization/sulfurization reaction process, and a cadmium sulfide (CdS) buffer layer growth process including immersing the CIGSSe absorption layer in an aqueous solution to form the CdS buffer layer on the CIGSSe absorption layer by a chemical bath deposition method and scraping edges and a back of a substrate to remove redundant materials. The CIGSSe absorption layer and the CdS buffer layer are sequentially formed on the back electrode layer on the substrate under non-vacuum conditions. The coating layer forming process includes a spray coating process, a coating process, and an immersing process. The primary selenization/sulfurization reaction process comprises a primary sulfurization reaction and a primary selenization reaction by introducing hydrogen sulfide and hydrogen selenide, and making the CIGSSe slurry coating layer to produce sulfide and selenide by elevating temperature to form a primary CIGSSe absorption layer. The rear-stage selenization/sulfurization reaction process comprises a rear-stage sulfurization reaction and a rear-stage selenization reaction which allow the primary CIGSSe absorption layer to form a rear-stage CIGSSe absorption layer, through the rear-stage sulfurization reaction and the rear-stage selenization reaction. The additive comprises dispersant, adhesive, and leveling agent. The thermal process is a rapid thermal process including a rapid thermal crystallization process, a multi-stage crystallization process at a constant temperature, and a multi-stage cooling process.    DESCRIPTION OF DRAWING(S) - The diagram shows a flow chart of a method for forming a copper indium gallium sulfur selenide (CIGSSe) absorption layer and a cadmium sulfide (CdS) buffer layer under non-vacuum conditions.    Mixing process (S10)    Coating layer forming process (S20)    Drying process (S30)    Densification process (S40)    Primary selenization/sulfurization reaction process. (S50)