A method (200) and deposition zone apparatus (300) for fabricating thin-film optoelectronic devices (100), the method comprising: providing a potassium-nondiffusing substrate (110), forming a back-contact layer (120); forming at least one absorber layer (130) made of an ABC chalcogenide material, adding at least two different alkali metals, and forming at least one front-contact layer (150) 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 (470) from back-contact layer (120), exclusive, to front-contact layer (150), inclusive, the comprised amounts resulting from adding at least two different alkali metals are, for potassium, in the range of 500 to 10000 ppm and, for the other of said at least two different alkali metals, in the range of 5 to 2000 ppm and at most ½ and at least 1/2000 of the comprised amount of potassium. The method (200) and apparatus (300) are advantageous for more environmentally-friendly production of photovoltaic devices (100) on flexible substrates with high photovoltaic conversion efficiency and faster production rate.