Semiconductors Amorphous-to-crystalline Sb 2 Se 3 and Sb 2 Se 2 S films Optical properties a b s t r a c t Nearly stoichiometric thin films of Sb 2 Se 3 and Sb 2 Se 2 S were deposited, by conventional thermal evaporation of the presynthesized materials, on glass substrates held at room temperature. X-ray diffraction studies on the annealed films, at T a ΒΌ 373, 423, and 473 K, revealed an amorphous-tocrystalline phase transition at T a ΒΌ 423 and 473 K for Sb 2 Se 3 and Sb 2 Se 2 S, respectively. The crystal structure of the polycrystalline films of Sb 2 Se 3 and Sb 2 Se 2 S is single-phase of an orthorhombic type structure, with space group Pbnm (62) and Z ΒΌ 4, as that of their source materials. The optical constants (n,k) and the thickness (t) of the investigated films were determined from optical transmittance data, in the spectral range 600-2500 nm using the Swanepoel method. The dispersion parameters of the indicated compositions were determined from the analysis of the refractive index. The analysis of the optical absorption spectra revealed that the polycrystalline films exhibited both direct and indirect energy gaps, on the contrary to the amorphous films which showed only a non-direct energy gap with a relatively higher Urbach's tail.