Optical properties of ITO/TiO_2 single and double layer thin films deposited by RPLAD

摘要

Indium tin oxide (ITO) and titanium dioxide (TiO_2) single layer and double layer ITO/TiO_2 films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO_2 substrates either at room temperatures (RT) or heated to 200-400 C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 C enhances the transmission of TiO_2 films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO_2 with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated, n ≈ 2.03 and 2.04, respectively for ITO films and TiO_2 film deposited at 400 C in the visible region. Post-annealing of the TiO_2 films for 3 h at 300 and 500 C was performed to enhance n. The refractive index of the TiO_2 films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 C, respectively. The TiO_2 films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO_2 films and when annealed at 400 C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO_2 films deposited at 200 C. The shift decreases by half when the TiO_2 film was deposited at 400 C. Post-annealing was also performed on double layer ITO/TiO_2.