Effect of reannealing temperature on characteristics of nanocrystalline Sn-doped In_2O_3 thin films for organic photovoltaic cell applications

摘要

In this study, nanocrystalline Sn-doped In_2O_3 (ITO) films were deposited by electron beam evaporation technique and were annealed in air atmosphere from 300℃ to 500℃ for 30 min. Then, the annealed ITO films in air at 450℃ were reannealed in vacuum for 1 h at different temperatures from 300℃ to 500℃. The effects of reannealing temperature on structural, electrical, and optical properties of the ITO films were investigated. Increasing reannealing temperature from 300℃ to 500℃ reduced sheet resistance of ITO thin films from 38 to 12(Ω/sq). The highest transparency over the visible wavelength region of spectrum (95%) was obtained for reannealed films at 450℃. The optimum reannealing temperature for these films is 450℃. Refractive index at 550 nm and porosity for ITO films reannealed at 450℃ were 1.92% and 21.2%, respectively. The allowed direct bandgap at different reannealing temperature was evaluated to be in the range of 4.1-4.28 eV. X-ray diffraction results showed that the reannealed films were polycrystalline and a rise in grain size was observed in them. The average grain size in the films reannealed in vacuum at 450℃ is about 48.6 nm. Atomic force microscope images indicated that the grain size and root-mean-square roughness films depend on the reannealing temperature. It has been found that reannealing temperature is a key factor in controlling the structural, electrical, and optical properties of ITO films. The power conversion efficiency of the device with ITO films reannealed at 450℃ is 1.22% and it is about 58% higher than that of the device without it. This indicates that this film is a promising transparent electrode for organic photovoltaic cells.