Optical and electrical properties of solution processable TiO x thin films for solar cell and sensor applications
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Optical and electrical properties of solution processable TiOx thin films for solar cell and sensor applications Jiguang Li1, Lin Pu2 and Mool C. Gupta1* Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, U.S A. 2 Department of Chemistry, University of Virginia, Charlottesville, Virginia, 22904, U.S.A. 1
ABSTRACT Recently, few tens of nanometer thin films of TiOx 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 TiOx thin films and it’s applications in solar cell and sensor devices. The TiOx material was made through hydrolysis of the precursor synthesized from titanium isopropoxide, 2methoxyethanol, and ethanolamine. The TiOx thin films of thickness between 20 nm to 120 nm were obtained by spin coating process. The refractive index of TiOx thin films were measured using an ellipsometric technique and an optical reflection method. At room temperature, the refractive index of TiOx 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 TiOx phase and density changes under thermal annealing. The electrical resistance was measured by depositing a thin film of TiOx between ITO and Al electrode. The electrical resistivity of TiOx thin film was found to be 1.7×107 .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 TiOx 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 TiOx thin films have been demonstrated as a low cost solution processable antireflection layer for Si solar cells. The results indicate that the TiOx layer can reduce the surface reflection of the silicon as low as commonly used vacuum deposited Si3N4 thin films. INTRODUCTION Titanium dioxide (TiO2) thin films have been extensively used in optical applications, such as antireflection coatings, waveguides and so on [1-3]. Various methods can be used for the fabrication of the TiO2 thin films such as PECVD [4], sputtering [5] etc. Sol-gel method is one of the most promising technique because it is a low cost and simple process [6, 7]. Heeger’s group reported that a novel so-gel TiOx thin film could increase the efficiency and life time of organic solar cells. The synthesis and properties of TiOx material is different from typical TiO2 prepared by sol–gel processes. The TiOx thin films composition contains Ti:O = 42.1 : 56.4 as determined by X-ray photoelec
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