TiO2 thin films as protective material for transparent-conducting oxides used in Si thin film solar cells

Nb-doped TiO2 films have been fabricated by RF magnetron sputtering as protective material for transparent-conducting oxide (TCO) films used in Si thin film solar cells. It is found that TiO2 has higher resistance against hydrogen radical exposure, utilizing the hot-wire CVD (catalytic CVD) apparatus, compared with SnO2 and ZnO. Further, the minimum thickness of TiO2 film as protective material for TCO was experimentally investigated. Electrical conductivity of TiO2 in the as-deposited film is found to be ∼10−6 S/cm due to the Nb doping. Higher conductivity of ∼10−2 S/cm is achieved in thermally annealed films. Nitrogen treatments of Nb-doped TiO2 film have been also performed for improvements of optical and electric properties of the film. The electrical conductivity becomes 4.5×10−2 S/cm by N2 annealing of TiO2 films at 500 °C for 30 min. It is found that the refractive index n of Nb-doped TiO2 films can be controlled by nitrogen doping (from n=2.2 to 2.5 at λ = 550 nm) using N2 as a reactive gas. The controllability of n implies a better optical matching at the TCO/p-layer interface in Si thin film solar cells.