A method for fabricating thin-film optoelectronic devices (100), the method comprising: providing a alkali-nondiffusing substrate (110), forming a back-contact layer (120); forming at least one absorber layer (130) made of an ABC chalcogenide material, adding least one and advantageously at least two different alkali metals, and forming at least one front-contact layer (150) wherein one of said alkali metals comprise Rb and/or Cs and where, following forming said front-contact layer, in the interval of layers (470) from back-contact layer (120), exclusive, to front-contact layer (150), inclusive, the comprised amounts resulting from adding alkali metals are, for Rb and/or Cs, in the range of 500 to 10000 ppm and, for the other alkali metals, typically Na or K, in the range of 5 to 2000 ppm and at most 1/2 and at least 1/2000 of the comprised amount of Rb and/or Cs. The method (200) is advantageous for more environmentally-friendly production of photovoltaic devices on flexible substrates with high photovoltaic conversion efficiency and faster production rate.