Abstract: The invention relates to a method and a device for reacting metallic precursors with sulfur and/or selenium to chalcopyrite layers of CIGSS solar cells in a reaction chamber of an RTP furnace. The aim of the invention is to produce thin-layer solar modules. Said aim is achieved by introducing a substrate coated with the precursor as well as an amount of sulfur and/or selenium that is sufficient to carry out the reaction into a sealingly closable reaction box which is provided with at least one discharge valve that can be controlled from outside the reaction chamber. The reaction box is introduced into the reaction chamber of the RTP furnace, the reaction chamber is evacuated, the reaction box is heated to a predetermined temperature in the reaction chamber along with the substrate and is maintained at said temperature for a certain process time, the pressure in the reaction box being measured and being controlled via the at least one discharge valve during the process time.