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

Abstract Nb-doped TiO 2 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 TiO 2 has higher resistance against hydrogen radical exposure, utilizing the hot-wire CVD (catalytic CVD) apparatus, compared with SnO 2 and ZnO. Further, the minimum thickness of TiO 2 film as protective material for TCO was experimentally investigated. Electrical conductivity of TiO 2 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 TiO 2 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 N 2 annealing of TiO 2 films at 500 °C for 30 min. It is found that the refractive index n of Nb-doped TiO 2 films can be controlled by nitrogen doping (from n =2.2 to 2.5 at λ = 550 nm) using N 2 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.