Transparent Conducting Coatings Made from Redispersable Crystalline Nanoscaled Powders

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C. Goebbert*, H. Bisht*, R. Nonninger**, M. A. Aegerter*, H. Schmidt** *Department of Coating Technology "**Departmentof Chemistry and Technology of Non-metallic Inorganic Materials INM-Institut ffir Neue Materialien, Im Stadtwald, Gebdude 43, D-66123 Saarbrticken, GERMANY ABSTRACT

A new wet chemical concept is presented for the preparation of electrically conducting SnO 2:Sb (ATO) and In20 3:Sn (ITO) coatings. It is based on the deposition by spin, dip or spray techniques of a solution containing crystalline nanoscaled particles fully redispersable in a solvent. The particle synthesis is carried out in a solution at temperatures < 200 'C by a controlled growth reaction using SnCl4 and InCl3 as precursors and SbC13 and SnCl4 as dopant, respectively. The aggregation of the particles is avoided by in-situ surface modification with bifunctional organic molecules. After drying the nanocrystalline particles can be fully redispersed in ethanol at pH < 6 (for ITO) or water at pH > 8 (for ATO) with solid contents up to 8.8 vol. %. Single layers with thickness up to 200 nm (ATO) and 400 nm (ITO) have been fabricated. Their sheet resistance decrease with the sintering temperature. Typical lowest values are 430 Q0 (550 'C) for ATO and 90 QE (900 'C) for reduced ITO. The resistivity of as fired ATO and ITO coatings is stable but it slightly increases with time for ITO in the reduced state. All coatings have a transmission in the visible range of about 90 %. Anti-glare conducting coatings on glass with 70 GU as well as antistatic coatings (RD 1 MQO) on polycarbonate substrates have been obtained with chemically modified ITO suspensions. INTRODUCTION

Transparent conducting oxides deposited on glass are important materials in the field of optoelectronic devices such as electrochromic windows, solar cells, electroluminescence and liquid crystal displays [1]. N-type semiconductors such as indium tin oxide (ITO), fluorine or antimony doped tin dioxide (FTO, ATO) or aluminium or gallium doped zinc oxide (AZO, GZO) are widely used as transparent electrodes in these applications. Practically all known coating processes have been used for their preparation [1, 2]. The coatings obtained by the sol-gel route present a resistivity, p, higher than those obtained by other processes. Typical values reported are 3.10.3 Qcm [3, 4] for SnO 2:Sb coatings and 8.10' Qcm for In 20 3:Sn coatings [5]. The reason for the low conductivity of such coatings is due to their high porosity (low density) even after sintering at high temperature [6]. Structures with large and densely packed crystallites obtained by spray pyrolysis, sputtering and CVD processes exhibit higher electron mobility and carrier density and consequently lower resistivity [I]. The paper discusses a new concept for the preparation of ATO and ITO coatings on glass and plastic substrates based on the use of solutions containing already crystalline fully redispersed SnO 2:Sb or In20 3:Sn nanoparticles. The influence of the densification temperature and sintering time on the resistivity a