High temperature resistant coatings of transparent and conductive Nb doped Titania (TiO2:Nb) for solar cell applications

Niobium doped titania (TiO2:Nb) is a new n - type conducting transparent oxide (TCO). Beside the good electrical and optical properties, TiO2:Nb is characterized by a very good high temperature resistance. A 100 nm TiO2:Nb thin film shows after annealing in vacuum at 450°C a resistivity of 7.22 x 10-4 Omega*cm with a good transparency in the visible range and an extinction coefficient k of 0.0197 at 550 nm. This enables the application of such TCO coatings in the solar cell production, where high temperatures are required in the manufacturing process, e.g. for µc-Si or CdTe solar cells. The TiO2:Nb thin films were deposited by DC and pulsed DC sputtering from a ceramic titania target with a content of approximately 6 wt.-% Nb. We used a dynamic deposition process in a large area in - line sputtering plant up to a substrate width of 400 mm. We investigated the influence of several process parameters on the characteristics of the TiO2:Nb layers. The high temperature stability of the TiO2:Nb thin films was in focus of the investigations. After the deposition of TiO2:Nb thin films a thermal annealing in the range of 450 to 500°C in vacuum and hydrogen was performed. The optical properties of TiO2:Nb thin films were measured by VIS/ NIR spectrometry. Hall-measurements were used to determine the free carrier density, free carrier mobility and the resistivity. For an evaluation of the crystalline phases of the thin films, XRD measurements were carried out.