NOVELTY - Producing a light-absorbing layer for a solar cell by forming a group I-III-VIB compound thin film, involves depositing a single precursor including Group III and VI elements on a substrate by metal organic chemical vapor deposition (MOCVD) to form a Group III-VI or IIIB-VIC compound thin film; depositing a precursor including a Group I element by MOCVD to form a I-III-VI compound thin film; and heating the I-III-VI compound thin film or depositing a Group VI element-including precursor by MOCVD.    USE - For producing a light-absorbing layer for a solar cell by forming a group I-III-VIB compound thin film such as CuInSe2 thin film.    ADVANTAGE - The method is capable of producing a light-absorbing layer for a solar cell that is capable of producing a stoichiometric ratio of a high-quality I-II-VIB compound thin film without unnecessary waste of a Group III element. The method produces a high quality thin film at considerably low costs, as compared to conventional methods for forming CuInSe2 thin film (CIS) absorbing layers for solar cells.    DETAILED DESCRIPTION - Producing a light-absorbing layer for a solar cell by forming a group I-III-VIB compound thin film on a substrate involves: a) depositing a single precursor including Group III and VI elements on a substrate by metal organic chemical vapor deposition (MOCVD) to form a Group III-VI or IIIB-VIC compound thin film; b) depositing a precursor including a Group I element on the III-VI or III-VIC compound thin film by MOCVD to form a I-III-VI compound thin film composed of Group I, III and VI elements; and heating the I-III-VI compound thin film under a Group VI element-containing gas atmosphere or depositing a Group VI element-including precursor on the I-III-VI compound thin film by MOCVD to form an I-III-VIB compound thin film. The method further involves: d) depositing a single precursor including a Group III' or VI' element different from the Group III or VI element of the single precursor used in step (a) on the I-III-VIB compound thin film formed in step (c) by MOCVD to form a I-III1-xIII'x-VIB, I-III-(VI1-yVI'y)2, or I-III1-xIII'x-(VI1-yVI' y)2 compound thin film, where x and y are 0-1. The method further involves e) depositing a precursor including only a Group VI' element different from the Group VI element of the single precursor used in step (c) on the I-II-VIB compound thin film formed in step (c) by MOCVD to form a I-III-(VI1-yVI'y)2, where y is 0 to 1.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the process for producing CuInSe2 thin film.