NOVELTY - Fabrication of thin-film optoelectronic device (100) involves providing substrate (110), forming back-contact layer (120), forming at least one absorber layer (130), adding alkali metal and forming at least one cavity at the surface of the absorber layer. The cavity is formed by dissolving at least one crystal aggregate comprising at least one alkali crystal comprising at least one alkali metal way from surface of the absorber layer. The absorber layer is made of group 11-group 13-group 16 element-containing chalcogenide material (I).    USE - Fabrication of thin-film optoelectronic device e.g. solar cell.    ADVANTAGE - The method enables fabrication of thin-film optoelectronic device with high photovoltaic conversion efficiency, by simple, economical and environmentally-friendly process with reduced consumption of chemical product.    DETAILED DESCRIPTION - Fabrication of thin-film optoelectronic device involves providing substrate, forming back-contact layer, forming at least one absorber layer, adding alkali metal and forming at least one cavity at the surface of the absorber layer. The cavity is formed by dissolving at least one crystal aggregate comprising at least one alkali crystal comprising at least one alkali metal way from surface of the absorber layer. The absorber layer is made of group 11-group 13-group 16 element-containing chalcogenide material of formula: AB'C' (I), where A is elements of group 11 of the periodic table of chemical element as defined by the International Union of Pure and Applied Chemistry including copper and silver, B' is elements in group 13 of periodic table including indium, gallium and aluminum, and C' is elements in group 16 of periodic table including sulfur, selenium and tellurium. The chalcogenide material includes ternary, quaternary, pentanary, or multinary variations.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional of thin-film optoelectronic device.    Thin-film optoelectronic device (100)    Substrate (110)    Back-contact layer (120)    Absorber layer (130)    Buffer layer (140)