NOVELTY - Manufacturing (M1) a high density copper-indium-selenide (CIS) thin film involves: preparing CIS nanoparticles; dispersing the CIS nanoparticles and dissolving a binder in a solvent to form CIS slurry; dissolving a filling element-containing salt and a binder in a solvent to form a filling element-containing slurry; mixing the CIS slurry and the filling element-containing slurry to form a slurry mixture; and coating the slurry mixture on a substrate to form a CIS thin film, followed by heat treatment.    USE - For manufacturing a high density copper-indium-selenide (CIS) thin film used for a photo-absorption layer of a thin film solar cell (claimed), where solar cell is useful as energy source.    ADVANTAGE - The CIS thin film is less expensive, stable after long term use, and provides highly efficient solar cell.    DETAILED DESCRIPTION - Manufacturing (M1) a high density copper-indium-selenide (CIS) thin film, involves: either (a): preparing CIS nanoparticles; dispersing the CIS nanoparticles and dissolving a binder in a solvent to form CIS slurry; dissolving a filling element-containing salt and a binder in a solvent to form a filling element-containing slurry; mixing the CIS slurry and the filling element-containing slurry to form a slurry mixture; and coating the slurry mixture on a substrate to form a CIS thin film, followed by heat treatment; or (b) preparing CIS nanoparticles; dispersing the CIS nanoparticles and dissolving a binder in a solvent to form CIS slurry; coating the CIS slurry on a substrate to form a CIS thin film; dissolving a filling element-containing salt and a binder in a solvent to form a filling element-containing slurry; and coating the filling element-containing slurry on the CIS thin film, followed by heat treatment. INDEPENDENT CLAIMS are included for the following:    (1) a high density CIS thin film (F1) used for a photo-absorption layer of a solar cell, comprising: filling elements placed in cavities within the CIS thin film, where the filling elements are at least two elements selected from copper, indium, gallium, zinc, and tin; and    (2) a thin film solar cell comprising the high density CIS thin film (F1) as a photo-absorption layer.