A method of fabricating thin-film optoelectronic devices comprising at least two different alkali metals, the method comprising the steps: (i) providing a substrate (210, 110); (ii) forming a back-contact layer (220, 120); (iii) forming at least one absorber layer (230, 130), which absorber layer is made of an ABC chalcogenide material, including ABC chalcogenide material ternary, quaternary, pentanary, or multinary variations, wherein A represents elements of group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu and Ag, B represents elements in group 13 of the periodic table including In, Ga, and Al, and C represents elements in group 16 of the periodic table including S, Se, and Te; and (iv) forming at least one front-contact layer (250, 150) characterized in that the substrate (210,110) is a potassium-nondiffusing substrate; and that selectable quantities of at least two different alkali metals are added during step (iii) and/or between steps (iii) and (iv) by vapour deposition, the selectable quantities of alkali metals being added from vapour deposition sources external to the potassium-nondiffusing substrate (220,120); wherein one of said at least two different alkali metals is potassium and where, following forming said front-contact layer, in the interval of layers from back-contact layer, exclusive, to front-contact layer, inclusive, the comprised amounts resulting from adding the selectable quantities of at least two different alkali metals are, for potassium, in the range of 500 to 10000 atoms per million atoms (ppm) and, for the other of said at least two different alkali metals, in the range of 5 to 2000 ppm and at most 1/2 and at least 1/2000 of the comprised amount of potassium. (i) providing a substrate (210, 110); (ii) forming a back-contact layer (220, 120); (iii) forming at least one absorber layer (230, 130), which absorber layer is made of an ABC chalcogenide material, including ABC chalcogenide material ternary, quaternary, pentanary, or multinary variations, wherein A represents elements of group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu and Ag, B represents elements in group 13 of the periodic table including In, Ga, and Al, and C represents elements in group 16 of the periodic table including S, Se, and Te; and (iv) forming at least one front-contact layer (250, 150) the substrate (210,110) is a potassium-nondiffusing substrate; and that selectable quantities of at least two different alkali metals are added during step (iii) and/or between steps (iii) and (iv) by vapour deposition, the selectable quantities of alkali metals being added from vapour deposition sources external to the potassium-nondiffusing substrate (220,120); wherein one of said at least two different alkali metals is potassium and where, following forming said front-contact layer, in the interval of layers from back-contact layer, exclusive, to front-contact layer, inclusive, the comprised amounts resulting from adding the selectable quantities of at least two different alkali metals are, for potassium, in the range of 500 to 10000 atoms per million atoms (ppm) and, for the other of said at least two different alkali metals, in the range of 5 to 2000 ppm and at most 1/2 and at least 1/2000 of the comprised amount of potassium.