Optical and electrical properties of solution processable TiO_x thin films for solar cell and sensor applications

摘要

Recently, few tens of nanometer thin films of TiO_x have been intensively studied in applications for organic solar cells as optical spacers, environmental protection and hole blocking. In this paper we provide initial measurements of optical and electrical properties of TiO_x thin films and it's applications in solar cell and sensor devices. The TiO_x material was made through hydrolysis of the precursor synthesized from titanium isopropoxide, 2-methoxyethanol, and ethanolamine. The TiO_x thin films of thickness between 20 nm to 120 nm were obtained by spin coating process. The refractive index of TiO_x thin films were measured using an ellipsometric technique and an optical reflection method. At room temperature, the refractive index of TiO_x thin film was found to be 1.77 at a wavelength of 600 nm. The variation of refractive index under various thermal annealing conditions was also studied. The increase in refractive index with high temperature thermal annealing process was observed, allowing the opportunity to obtain refractive index values between 1.77 and 2.57 at a wavelength 600 nm. The refractive index variation is due to the TiO_x phase and density changes under thermal annealing. The electrical resistance was measured by depositing a thin film of TiO_x between 1TO and Al electrode. The electrical resistivity of TiO_N thin film was found to be 1.7×10~7 Ω.cm as measured by vertical transmission line method. We have also studied the variation of electrical resistivity with temperature. The temperature coefficient of electrical resistance for 60 nm TiO_x thin film was demonstrated as - 6× 10~(-3)/°C. A linear temperature dependence of resistivity between the temperature values of 20 - 100 °C was observed. The TiO_x thin films have been demonstrated as a low cost solution processable antireflection layer for Si solar cells. The results indicate that the TiO_x layer can reduce the surface reflection of the silicon as low as commonly used vacuum deposited Si_3N_4 thin films.