A method for processing a thin-film absorber material with enhanced photovoltaic efficiency includes forming a barrier layer on a soda lime glass substrate followed by formation of a stack structure of precursor layers. The method further includes subjecting the soda-lime glass substrate with the stack structure to a thermal treatment process with at least H2Se gas species at a temperature above 400° C. to cause formation of an absorber material. By positioning the substrates close together, during the process sodium from an adjoining substrate in the furnace also is incorporated into the absorber layer.