Effect of chromium doping on the band gap tuning of titanium dioxide thin films for solar cell applications

A simple and inexpensive spray pyrolysis deposition (SPD) approach was used to produce TiO₂ and Cr (2-8) at.%-doped TiO₂ thin films. To explore the morphological features of the films, FE-SEM micrographs were used and found that 6 and 8 at.% TiO₂:Cr films had fibrous patterns with diameters of 0.45 and 0.78 μm, respectively, while the remainder of the films were agglomerated particles. From X-ray diffraction investigation, it was found that the TiO₂ thin films had an anatase crystal phase (tetragonal) up to 6 at.% Cr doping, while an anatase-rutile mixed crystalline phase was identified for 8 at.% Cr doping. The crystallite size of the pristine TiO₂ film was 35 nm, while for TiO₂:Cr films, it ranges from 35 to 46 nm. The Fizeau fringes technique was employed to measure the thickness of the TiO₂ film and 165 nm was found for pristine TiO₂ and 164-180 nm for TiO₂:Cr films. UV-visible spectroscopy was used to study optical properties such as absorbance, refractive index, optical band gap, dielectric constant, and optical conductivity. As the Cr concentration increases, the optical band gap decreases from 3.40 eV to 2.70 eV. Using the four-point probe method, it was found that the resistivity changes with temperature and is also affected by the Cr content.