Abstract: Sintered films of Cd.sub.1-x Zn.sub.x S (0.ltoreq.x<1) with high optical transmittance are provided. These films are produced by applying a paste composed of powdered CdS and CdCl.sub.2 or of a mixture of CdS, ZnS and CdCl.sub.2 onto an appropriate substrate such as borosilicate glass, under an inert gas atmosphere containing ZnCl.sub.2 vapor or thereafter resintering the paste on the substrate under a nitrogen atmosphere.The ZnCl.sub.2 vapor is produced by heating ZnCl.sub.2 at 400.degree. C. or higher but below the temperature of 500.degree.-700.degree. C. to which the specimen is heated.The resulting sintered films have optical transmittance of 80% at long wavelength ranges or higher and can be used as a window layer having a superior optical transmittance in solar cells.