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

Abstract

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 ITO and Al electrode. The electrical resistivity of TiO x thin film was found to be 1.7×10 7 .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 3 N 4 thin films.

Solution Processable Semiconducting TiOx Thin Films for Photovoltaic Applications

“…The anatase or rutile crystalline structure was observed after annealing at temperature higher than 500 °C . The surface morphology, which will affect the contact resistance with electrode also changed after annealing [54] .…”