Disclosed herein is a method of fabricating a CIS or CIGS thin film, comprising: forming, on a substrate, a seed particle layer comprising copper-indium-compound seed particles comprising copper (Cu); indium (In); and at least one selected from the group consisting of gallium (Ga), sulfur (S) and selenium (Se), applying, on the seed particle layer, a water-soluble precursor solution comprising: a water-soluble copper (Cu) precursor; a water-soluble indium (In) precursor; and at least one selected from the group consisting of a water-soluble gallium (Ga) precursor, a water-soluble sulfur (S) precursor and a water-soluble selenium (Se) precursor, and forming a thin film at high temperature.