Photo and Electrochemical Characteristics Dependent on the Phase Ratio of Nanocolumnar Structured TiO₂ Films by RF Magnetron Sputtering Technique

Abstract

Nanostructured titanium dioxide (TiO 2 ) prepared by RF magnetron sputtering was grown at substrate temperatures ( T sub ) ranging from RT (25 °C) to 450 °C. At lower temperatures, the TiO 2 film deposited at a high sputtering pressure (12 mTorr) was amorphous. This was due to adatoms being unable to migrate effectively on the substrate. However, an increase in T sub provides additional thermal energy and promotes the formation of the rutile at high temperature. The postannealing process provides additional driving force for nucleating the amorphous phase to anatase or rutile, followed by the growth of the rutile grain size. A 2.2 μm thick TiO 2 film sputtered at 350 °C had a columnar structure and showed the best performance as a photoanode layer for a dye-sensitized solar cell (DSSC), with a V oc of 0.65 V, a J sc of 7.93 mA/cm 2, a fill factor of 0.533, and efficiency of 2.74%. In addition, the durable electrochromic performance was also confirmed using 400 nm thick TiO 2 films sputtered at 350 °C. The improved properties were attributed to the formation of a mixed crystalline phase with anatase and rutile, a porous structure, and the narrowing of the optical band gap because of the formation of rutile, trap sites, and surface states. Furthermore, the calculated porosity of the TiO 2 film sputtered at 350 °C also showed a high value (∼60%), which contributes to the increase in surface area and is in close contact to the electrolyte.