NOVELTY - A method (M1) of treating a copper-indium-gallium-selenide (CIGS) material (106), involves (a) contacting the CIGS material with an ionic solvent comprising sulfur or selenium-containing compounds; and (b) heating the CIGS material.    USE - For treating a CIGS material disposed on an electrically conducting substrate used to form a CIGS solar cell (Claimed).    ADVANTAGE - The method suitably dissolves impurity phases of binary oxides and selenides of copper, indium and gallium utilizing safe reagents which improves photovoltaic properties of a solar cell comprising the treated material. By removing impurity phases of binary oxides and selenides of copper, indium and gallium from the surface of the CIGS material, the sheet resistance of a CIGS thin film increases. The method improves methods for making CIGS solar cells by increasing sheet resistance of CIGS thin films. The treated material enables to make CIGS solar cells having enhanced solar cell conversion efficiencies by reducing the surface defect density of states and improving bulk material semiconductor properties. The CIGS film is treated with S or Se containing compounds dissolved in an ionic solvent (e.g., a eutectic solvent), which behaves like a low cost, nontoxic ionic solvent. Eutectic solvents used possess ionic liquid like properties allowing them to dissolve several metal oxides, and salts which are normally difficult to dissolve in ordinary solvents.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method (M2) of preparing a CIGS solar cell (100), involving:(a) providing a CIGS film deposited on an electrically conducting substrate (108); (b) contacting the CIGS material (106) with a solvent comprising Sulfur or Selenium-containing compounds; (c) heating the CIGS material; (d) preparing the CIGS material for inclusion in a solar cell; and (e) disposing a front contact (102) on the CIGS material to form the solar cell.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic illustration of a CIGS solar cell produced by the present method.    CIGS solar cell (100)    Front contact (102)    Buffer layer (104)    CIGS material (106)    Substrate (108)    Glass substrate (110)