Indium-free, acid-resistant anatase Nb-doped TiO₂ electrodes activated by rapid-thermal annealing for cost-effective organic photovoltaics

Indium-free and acid-resistant anatase Nb-doped TiO₂ (NTO) electrodes are promising as economical substitutes for high-cost Sn-doped In₂O₃ (ITO) films used in organic photovoltaics. By rapid-thermal annealing under an ambient vacuum, an insulating amorphous NTO film of low transparency was changed dramatically into a transparent and conductive anatase NTO electrode. Metallic conductivity of the annealed NTO electrode could be attributed to formation of the anatase phase and activation of the Nb dopant. Based on synchrotron X-ray scattering and high-resolution transmission electron microscopy, the electrical properties of the NTO electrode could be correlated with the microstructure of the NTO film. The acid-stability of NTO film also supports its use as a substitute for ITO electrode. Unlike Ga:ZnO and Al:ZnO films, which were easily etched by acidic PEDOT:PSS solution, the NTO film was stable against this reagent. Importantly, the annealing temperature influenced the performance of the organic solar cell fabricated with the NTO electrode. This indicates that activation of Nb dopants and formation of the anatase phase play an important role in the extraction of carrier from the organic layer to the anode electrode.