NOVELTY - The method comprises depositing a precursor thin film containing group IB, group IIIA and bismuth compounds, and heating the precursor thin film in an atmosphere containing group VIA species. A mole ratio of the IB-IIIA-VIA to the bismuth is from 10:1-2000:1. The depositing step further comprises adding group IA and/or group VIA compounds into the film, and thermally treating the precursor thin film before the heating step. A depositing method of the depositing step is a vacuum film-coating process and a non-vacuum film-coating process, and comprises coating, sputtering and/or evaporation.    USE - The method is useful for manufacturing bismuth-doped group IB-IIIA-VIA compounds based light absorber layers that is useful in a solar cell (all claimed) such as copper indium gallium selenide solar cell.    ADVANTAGE - The method is capable of improving a grain size and a grain growth of the light absorber layers and improving electrical characteristics of the group IB-IIIA-VIA compounds so as to increase photovoltaic conversion efficiency of the solar cell.    DETAILED DESCRIPTION - The method comprises depositing a precursor thin film containing group IB, group IIIA and bismuth compounds, and heating the precursor thin film in an atmosphere containing group VIA species. A mole ratio of the group IB-IIIA-VIA to the bismuth is from 10:1-2000:1. The depositing step further comprises adding group IA and/or group VIA compounds into the precursor thin film and thermally treating the precursor thin film before the heating step. The precursor thin film is thermally treated at 50-650 degrees C. The depositing step is carried out by a vacuum film-coating process and a non-vacuum film-coating process, and comprises coating, sputtering and/or evaporation. The coating method comprises spin coating, slot coating, extrusion coating, curtain coating, slide coating, dipping and/or doctor blade coating. The atmosphere comprises vacuum or non-vacuum. An INDEPENDENT CLAIM is included for a solar cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic sectional view of a solar cell.    Solar cell (100)    Substrate (110)    Contact layer (120)    Light absorber layer (130)    Buffer layer. (140)