Tunning the optoelectronic properties of Sm³⁺-doped mesoporous TiO₂ thin films via PVP assisted sol-gel process for solar cells

Mesoporous TiO₂ thin films with adequate characteristics like high specific surface area and improved light harvesting properties are required for their use as photoanodes in semiconductor-sensitized solar cells. Hence, the optimization of the structure, morphology, chemical composition, and optical properties of Sm³⁺-doped mesoporous TiO₂ films via the PVP (0–0.5 g) assisted sol-gel method is investigated. The results indicate that low PVP contents in the synthesis process promote an improvement in the porosity of the film by causing the dispersion of the TiO₂:Sm³⁺ nanoparticles and, consequently, the formation of a mesoporous structure. However, the average particle size and porosity are reduced for PVP contents above 0.3 g. A similar behavior is found for crystallite size, varying between 16–18 nm. Furthermore, the film thickness is increased (92–233 nm) with the PVP content (0–0.5 g) due to the increased viscosity of the precursor solution used for depositing the films. Interestingly, PVP causes defects and oxygen vacancies that are related to the distortion of the TiO₂ crystal lattice, causing a decrease in the E_g (3.32–3.07 eV). Despite these defects, all the films exhibit the pure anatase phase, as demonstrated by XRD and Raman. Additionally, the films are highly luminescent under n-UV excitation, reaching a maximum intensity for 0.1 g of PVP. Results on the evaluation of Sb₂(S/Se)₃-sensitized solar cells indicate the best performance by using films processed with 0.3 g of PVP exhibiting a V_oc of 449 mV and J_sc of 5.026 mA/cm².