Effect of Silver thickness and Annealing on Optical and Electrical Properties of Nb 2 O 5 /Ag/Nb 2 O 5 Multilayers as Tr
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Effect of Silver thickness and Annealing on Optical and Electrical Properties of Nb2O5/Ag/Nb2O5 Multilayers as Transparent Composite Electrode on Flexible Substrate Aritra Dhar1 and T. L. Alford1,2 1
Department of Chemistry and Biochemistry,
2
School for Engineering of Matter, Transport, and Energy,
Arizona State University, Tempe, Arizona 85287
ABSTRACT Multilayer structures of Nb2O5/Ag/Nb2O5 have been deposited onto flexible substrates by sputtering at room temperature to develop indium free composite transparent conductive electrodes. The optical and electrical properties of the multilayers are measured by UV–Visible spectroscopy, Hall measurement and four point probe and the effect of Ag thickness has been studied. The critical thickness of Ag to form a continuous conducting layer is found to be 9.5 nm and the multilayer stack has been optimized to obtain a sheet resistance of 7.2 Ω/sq and an average optical transmittance of 86 % at 550 nm. The Haacke figure of merit (FOM) has been calculated for the films, and the multilayer with 9.5 nm thick Ag layer has the highest FOM with 31.5 x 10-3 Ω/sq, which is one of the best FOM reported till date for room temperature deposition on flexible substrates. The multilayered samples are annealed in vacuum, forming gas, air and O2 environments and the optical and electrical properties are compared against the as-deposited samples. INTRODUCTION Transparent conductive oxides (TCO) are useful materials that exhibit unique combination of high transparency in the visible region along with high electrical conductivity. TCOs play an important role as transparent electrodes for flexible optoelectronic devices such as solar cell panels, liquid crystal displays and organic light emitting devices (OLED)[1-3]. Other common applications for TCOs include heat mirrors, optical filters, low emittance films for advanced glazing, and protective or decorative coatings. The material most commonly used for transparent electrodes in optoelectronic applications is indium tin oxide[4-5] (ITO) due to its low resistivity (~ 10−4 Ω-cm), high transmittance (~ 80 %), and fabrication maturity. However, the limited supply of indium and the growing demand for ITO make the resulting fabrication costs prohibitive for future industry. Thus, cost factors have promoted the search for inexpensive materials with good electric-optical properties. Recently, electrical resistivity has been improved by inserting a thin metal or metal alloy film between two TCO layers[4,6-9]. These composites have significant lower resistivity than a single-layer TCO film. Amongst metals, Ag is a good candidate for such multilayer films because of its low resistivity[10].
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Niobium oxide films has high refractive index and wide band gap (3.4-3.8 eV) and thus the dielectric film has a high transparency in the visible region[7]. The Nb2O5 coatings show excellent thermal and chemical stability Thus the high transparency of Nb2O5 in visible region and low resistivity of thin Ag layer lead us to the detailed study of optical and
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