NOVELTY - Producing photovoltaic modules having multiple solar cells made of silicon, comprises (a) producing multi-crystalline string-ribbon-wafer by silicon crystallization of a liquid silicon melt according to the string-ribbon-method (1), (b) post treatment of the solidified string-ribbon-wafers, where the string-ribbon-wafers during the post treatment are subjected to several post treatment processes, preferably e.g. an etching process (4) and/or a diffusion process (5), and (c) producing the photovoltaic modules (3) by assembling and electrically connecting the solar cells in a housing.    USE - The photovoltaic module is useful for converting sunlight into electrical energy.    ADVANTAGE - The method exhibits: improved mechanical properties without affecting the electrical properties of the solar cells; and low production costs.    DETAILED DESCRIPTION - Producing photovoltaic modules having multiple solar cells made of silicon, comprises (a) producing multi-crystalline string-ribbon-wafer by silicon crystallization of a liquid silicon melt according to the string-ribbon-method (1), (b) post treatment of the solidified string-ribbon-wafers for producing solar cells, where the string-ribbon-wafers during the post treatment are subjected to several post treatment processes, preferably an etching process (4), a diffusion process (5), a coating process and/or a metallization process, and (c) producing the photovoltaic modules (3) by assembling and electrically connecting the solar cells in a housing, where: the string-ribbon-wafers are subjected to a temperature annealing process at least once during the post treatment; the string-ribbon-wafers are heated to a predetermined annealing temperature during the temperature annealing process, within a heating period, then subsequently annealed during a retention time according to the annealing temperature, and then cooled during a cooling period; the annealing temperature is 750-1050 degrees C; and the sum of heating time and retention time is 30-240 minutes. INDEPENDENT CLAIMS are also included for:    (1) a string-ribbon-wafer for producing solar cells for photovoltaic modules comprising subjecting the string-ribbon-wafer at least once to a temperature annealing process, then subsequently annealed during a retention time according to the annealing temperature, and then cooled during a cooling period; and    (2) a photovoltaic module having multiple solar cells made of silicon, which are produced from the string-ribbon-wafers.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the process steps involved in producing the solar modules (Drawing includes non-English language text).    String-ribbon-method (1)    Post treatment (2)    Producing solar modules (3)    Etching process (4)    Diffusion process (5)