A method of speeding up fabricating thin-film photovoltaic cells devices comprising at least two different alkali metals, with a roll-to-roll manufacturing apparatus at high speed, the method comprising the steps: (i) providing a substrate (210, 110) delivered between a delivery roll and a take-up roll of a roll-to-roll manufacturing apparatus; (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 an alkali-nondiffusing substrate; and that selectable quantities of at least one and preferably at least two different alkali metals are added between steps (iii) and (iv) by vapor deposition in elemental form or precursor thereof, the selectable quantities of alkali metal(s) being added from a vapor deposition source external to the alkali-nondiffusing substrate (210,110); wherein said at least one alkali metal(s) comprise Rb and/or Cs, and wherein 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 the at least one alkali metal(s) are, for Rb and/or Cs, in the range of 200 to 10000 atoms per million atoms (ppm) and, where at least another alkali metal is present in the range of 5 to 10000 ppm and at most 1/2 and at least 1/2000 of the comprised amount of Rb and/or Cs. (i) providing a substrate (210, 110) delivered between a delivery roll and a take-up roll of a roll-to-roll manufacturing apparatus; (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 an alkali-nondiffusing substrate; and that selectable quantities of at least one and preferably at least two different alkali metals are added between steps (iii) and (iv) by vapor deposition in elemental form or precursor thereof, the selectable quantities of alkali metal(s) being added from a vapor deposition source external to the alkali-nondiffusing substrate (210,110);