NOVELTY - The method comprises forming a precursor film on a substrate, where the precursor film comprises the group of IB-IIB-IVA-VIA amorphous nanoparticles, and conducting a thermal process to the precursor film to form a light absorption layer on the substrate. The Group IB-IIB-IVA-VIA amorphous nanoparticles are formed by a chemical bath reaction synthesis method. The precursor film is formed by a spin coating, brush coating, knife coating, spraying, or printing method. The step of forming the precursor film on the substrate comprises coating a slurry on the substrate.    USE - The fabrication method is useful for a light absorption layer of a solar cell (claimed) such as compound thin film solar cell including copper indium gallium selenide solar cell and copper zinc tin sulfide thin film solar cell.    ADVANTAGE - The method economically and rapidly fabricates the light absorption layer of the solar cell with high conversion efficiency and stability and large area fabrication ability.    DETAILED DESCRIPTION - The method comprises forming a precursor film on a substrate, where the precursor film comprises the group of IB-IIB-IVA-VIA amorphous nanoparticles, and conducting a thermal process to the precursor film to form a light absorption layer on the substrate. The Group IB-IIB-IVA-VIA amorphous nanoparticles are formed by a chemical bath reaction synthesis method. The precursor film is formed by a spin coating, brush coating, knife coating, spraying, or printing method. The step of forming the precursor film on the substrate comprises coating a slurry on the substrate, where the slurry comprises the amorphous nanoparticles, and drying the substrate to form the precursor film on the substrate. The amorphous nanoparticles are formed by one, two or three kinds of nanoparticles. The Group IB element to the sum of the Group IIB element and Group IVA element has a ratio of 0.6-0.99. The Group IB element to the Group IVA element has a ratio of about 1.8-2.2. The Group IIB element to the Group IVA element has a ratio of about 0.9-1.5. The Group VIA element to the Group IB-IIB-IVA-VIA amorphous nanoparticles is 0.5-0.6. The thermal process is conducted at 200-5000 degrees C for 20 minutes to 4 hours and in an atmosphere.