Chemical bath deposition (CBD) has proved top be the most favorable method for application of a buffer layer to semiconductor substrates, for example, chalcopyrite thin-film solar cells, whereby previously cadmium sulphide (CdS) was deposited and as cadmium is a highly toxic heavy metal, alternatives have been required. According to the invention, the semiconductor substrate is dipped in a solution for approximately 10 minutes, produced by the dissolution of zinc sulphate (0.05-0.5 mol/l) and thiourea (0.2 to 1.5 mol/l) in distilled water at a temperature being held essentially constant throughout said period. For the first time, the ZnS layer permits comparable or higher efficiencies than conventionally only achieved with toxic cadmium compounds. The method is hence much more environmentally-friendly with the same result.