Optical properties of ITO/TiO 2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO2) single layer and double layer ITO/TiO2 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 SiO2 substrates either at room temperatures (RT) or heated to 200–400 8C. Under optimized conditions, transmission of ITO films in the visible (vis) rangewas above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 8C enhances the transmission of TiO2 films in the vis-NIR from about 70% to 92%. High transmission (90%) was observed for the double layer ITO/TiO2 with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (Eg), as well as the refractive indexes (n) for the films were estimated. n 2.03 and 2.04, respectively for ITO films and TiO2 film deposited at 400 8C in the visible region. Post-annealing of the TiO2 films for 3 h at 300 and 500 8C was performed to enhance n. The refractive index of the TiO2 films increases with the postannealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 8C, respectively. The TiO2 films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO2 films and when annealed at 400 8C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO2 films deposited at 200 8C. The shift decreases by half when the TiO2 film was deposited at 400 8C. Post-annealing was also performed on double layer ITO/TiO2.