Effects of Sn doping on the structural, optical, photoluminescence, and electrical properties of transparent SeO₂ thin films

The pure and Sn-doped SeO₂ thin films on borosilicate glass substrates were synthesized using a spray pyrolysis method. X-ray diffraction analysis revealed that higher and appropriate Sn doping concentrations would induce the thin film to transform from amorphous to crystalline behavior as the 7 wt% Sn-doped SeO₂ thin film had a polycrystalline hexagonal crystal structure. The Fourier transform infrared spectroscopy spectra show that high Sn doping concentrations in thin films exhibit the O–Sn–O bending and Sn–O stretching modes of SnO₂, and the O = Se = O bond symmetric stretching vibration for all thin films. The high-resolution X-ray photoelectron spectroscopy measurement revealed the oxidation states of Se⁰, Se⁴⁺, and Se⁶⁺ for SeO₂ and Sn⁴⁺ for SnO₂. The photoluminescence spectra revealed the intense ultraviolet and blue emission peaks and the weak green emission peak. A maximum optical transmittance of ~ 90% in the visible region can be displayed for the pure SeO₂ thin film and decreased with increasing Sn doping concentration as a window layer in solar cells and other optoelectronic devices. The energy band gap value for the pure SeO₂ was evaluated as 3.08 eV and reduced to 2.97, 2.88, 2.86, and 2.79 eV for 1, 3, 5, and 7 wt% Sn: SeO₂, respectively. The lowest sheet resistance value was found for 5 wt% Sn: SeO₂, which is 286 GΩ/sq. Furthermore, the high figure of merit values of 9.46 × 10^–2 Ω^−1/12 and 8.86 × 10^–2 Ω^−1/12 were obtained for pure and 5 wt% Sn: SeO₂ thin films, respectively.