A novel technique to improve CIGSe solar cell performance was developed. Introduction of water vapor to a vacuum-deposition chamber reduces defects in CIGSe thin films. Open circuit voltage (V_oc) and short circuit current density (J_sc) of the films increased as a result. A solar cell with a wide-gap (1.3 eV) CIGSe film gave 18.1% efficiency, V_oc of 0.744V, J_sc of 32.4 mA/cm² and fill factor of 0.752. The novel technique would lead to development of practical large area and/or flexible CIGSe solar cells.