Effect of B2O3 addition on optical and structural properties of TiO2 as a new blocking layer for multiple dye sensitive solar cell application (DSSC)

Abstract

TiO₂–B₂O₃ sol–gel films were prepared using titanium(IV) isopropoxide as a Ti source and boric acid (H₃BO₃) as B precursor. B₂O₃ doping was found to improve the characteristics of TiO₂ films making them suitable to use as blocking TiO₂ layers in DSSC B₂O₃ acted as a flux and glass forming oxide leading to amorphous vitreous layers having an average thickness of 50 nm. Films were transparent, adherent and perfectly continuous without any leakage current. X-ray diffraction measurements prove that crystallinity decreases with boron content meaning that amorphous phase was favoured. Surface morphology was investigated by atomic force microscopy. It showed that film surface became more and more smooth. The E_g Raman-active phonon mode at 145 cm⁻¹ reveals the same arrangement of TiO₂ octahedra observed in the anatase phase. The Lorentzian multipeak fitting showed the emergence of a new mode at 152 cm⁻¹ whose mode intensity increased with boron content; we attributed it to the presence of boron. The influence of B₂O₃ dopant on the optical properties was examined by UV-visible spectroscopy and spectroscopic ellipsometry. Refractive index, extinction coefficient and optical band gap have been extracted by fitting ellipsometric spectra with the double new amorphous model. Difference between the optical gap values obtained from UV-visible spectra and those calculated by ellipsometry did not exceed 0.3 eV. The optical band gap increased from 3.4 to 3.9 eV by increasing boron content from 0 to 20%. The increase of E_g is expected to induce an enhanced output ddp into DSSC.