Metal copper induced the phase transition of MoO3 to MoO2 thin films for the CdTe solar cells

Abstract Multilayered MoO3/Cu thin films were prepared by thermal evaporation in combination with the thermal treatments. As-deposited multilayers are amorphous structure. With the increase of temperature, Mo4O11 phase forms in the thin films without copper while MoO2 phase with the weak reduced molybdenum oxide appears in the copper-containing thin films. After annealing at the temperature higher than 450 °C predominant MoO2 appears in the multilayered MoO3/Cu thin films. Meanwhile, CuO and Cu2O are also present in the thin films. The formation mechanism of reduced molybdenum suboxide can be explained by the oxygen-vacancy generation reaction or the reduction of MoO3 to MoO3-x via the inter-diffusion at the MoO3/Cu interfaces. The band gaps and conductivity activation energies of multilayered MoO3/Cu thin films decrease as the annealing temperature increases, in which the smallest band gap of ~2.05 eV and activation energy of ~0.02 eV for the MoO2 thin films are found. Our results open a new road to broaden the application of reduced molybdenum suboxides as a back contact material for CdTe solar cells.