A method of fabricating an Ag—(Cu—)In—Ga—Se (A(C)IGS) based thin film using Se—Ag2Se core-shell nanoparticles, an A(C)IGS based thin film fabricated by the method, and a tandem solar cell having the A(C)IGS thin film are disclosed. More particularly, a method of fabricating a densified Ag—(Cu—)In—Ga—Se (A(C)IGS) based thin film by non-vacuum coating a substrate with a slurry containing Se—Ag2Se core-shell nanoparticles, an A(C)IGS based thin film fabricated by the method, and a tandem solar cell including the A(C)IGS based thin film are disclosed. According to the present invention, an A(C)IGS based thin film including Ag is manufactured by applying Se—Ag2Se core-shell nanoparticles in a process of manufacturing a (C)IGS thin film, thereby providing an A(C)IGS based thin film having a wide band gap.