TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Process: In the process (M1), the CIS thin film is formed by non-vacuum coating selected from spraying, ultrasonic spraying, spin coating, doctor blade coating, screen printing, and inkjet printing. The process (a) further involves: drying the CIS thin film after forming the CIS thin film and before the heat treatment. The process (b) further involves: drying the CIS thin film after forming the CIS thin film; and drying the CIS thin film after coating the filling element-containing slurry on the CIS thin film. The heat treatment is selenization heat treatment performed at 400-550 degrees C while supplying Se vapor.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The CIS nanoparticles comprise CIS nanoparticles, Cu-In-Ga-Se (CIGS) nanoparticles, or Cu2-Zn-Sn-S4 (CZTS) nanoparticles. The filling element-containing salt is a mixed salt of at least two selected from copper salt, indium salt, gallium salt, zinc salt, and tin salt. The filling element-containing slurry further comprises a selenium compound. The selenium compound is selected from SeO2, SeCl4, and selenourea. The copper salt is selected from CuCl, Cu-acetate, CuI, and CuSO4. The indium salt is selected from In(NO3)3, In2(SO4)3, and In-acetate. The gallium salt is selected from GaCl3, GaI3, Ga(NO3)3, and Ga-acetate; the zinc salt is selected from ZnCl2, Zn(NO3)2 and ZnI2; and the tin salt is SnCl4.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The solvent is an alcohol-based solvent including methanol, ethanol, pentanol, propanol, and butanol. The binder is selected from ethylene glycol, propylene glycol, ethyl cellulose, and polyvinylpyrrolidone. EXAMPLE - CuI (0.343 g) and InI3 (0.991 g) were mixed with distilled pyridine solvent (30 ml), followed by stirring for 10 minutes. After stirring, the opaque solution became transparent. The mixture of Cu/In was mixed with a solution of Na2Se (0.5 g) in distilled methanol (20 ml). The resultant mixture had atomic ratio of Cu:In:Se of 0.9:1:2. The mixture was reacted for 1 minute while stirring at 0 degrees C, to obtain CIS nanoparticles. The prepared CIS colloid was subjected to centrifugation for 30 minutes and ultrasonic treatment for 5 minutes, followed by washing with distilled methanol. This process was repeated to obtain highly pure CIS nanoparticles. The CIS nanoparticles (0.3 g) and propylene glycol (0.3 g) were dissolved in methanol (1.2 g), followed by ultrasonic treatment for 60 minutes to obtain a CIS slurry. Cu(NO3)2 (0.15 g), InCl3 (0.184 g), and propylene glycol (2 ml) were mixed in ethanol (8 ml), and stirred for 60 minutes, to obtain a filling element-containing slurry. The CIS slurry was mixed with the filling element-containing slurry, followed by ultrasonic treatment for 60 minutes, to obtain a slurry mixture. The slurry mixture was coated on a glass substrate, followed by drying the substrate at 60 degrees C for 5 minutes, and at 180 degrees C for 2 minutes to remove the alcohol solvent and the binder. The coating and drying were repeated 3 times to form a CIS thin film on the substrate. The film was subjected to heat treatment while supplying Se vapor at 400 degrees C to obtain a high density CIS thin film.