Transparent and conducting ITO thin films by spin coating of an aqueous precursor solution

Abstract

An environmentally friendly aqueous sol-gel process has been developed to fabricate thin films of indium tin oxide (ITO). A stable sol was prepared from indium nitrate and tin acetate precursors together with acetic acid and ethylene glycol. The sol transformed into an amorphous gel during heating, which decomposed and crystallized further to nano-crystalline ITO at $300 C. The nanocrystalline ITO powders prepared from the precursor gel were homogeneous and single phase with particle sizes around 15 nm. The aqueous sol was applied for spin coating of ITO films on glass substrates. The deposited thin films were homogeneous and continuous with no cracks or pin-holes and exhibited very good and reproducible optical transparency and electrical conductivity, with a specific resistance of 4.59 Â 10 À3 U cm, thereby demonstrating the potential of this Pechini related sol-gel process. Experiments to determine the robustness of the process with respect to the concentration of the precursors and substitution of the organic components with other alcohols or acids were also performed, and some important aspects of the chemistry of the sol-gel process are addressed.

Modified Pechini Synthesis of Oxide Powders and Thin Films

“…By this polymer-complex solution method, less amount of organics is used and for the synthesis of CaAl2O4 a phase pure material was prepared with a higher amount of cations than the number of functional groups in the polymer (Gülgün et al 1999) For some specific cations it has more recently been observed that complexing agents with -OH groups like ethylene glycol and polyvinyl alcohol (PVA) are better compounds to promote homogeneity of the final oxide than carboxylic acids. This has empirically been found to be of great advantage for amphoteric cations like In 3+ , Sn 4+ and Bi 3+ in the preparation of indium tin oxide (Kundu and Biswas 2008;Sunde et al 2012) and BiFeO3 (Selbach et al 2007;Liu et al 2010). An amino carboxylic acid, e.g.…”

“…This is an interesting aspect with this synthesis approach. Upon further heating the metastable phase will be transformed to the thermodynamically stable phase (Schumm et al 2011;Sunde et al 2012). During preparation of oxides with very basic cations, the formation of intermediate carbonates might occur due to the availability of CO2 from the decomposition of the organics.…”

“…The crystallization temperature is dependent on the system but can be as low as 400 °C as for example in ITO (Sunde et al 2012). The decomposition is exothermic, sometimes causing the decomposition and crystallization to occur simultaneously, thereby making it challenging to identify the real crystallization temperature (Sunde et al 2012). …”

“…The spongy-like precursor material is first turned into an amorphous material that is further crystallized into a single phase target material (Tai and Lessing 1992a;Sletnes et al 2016). The crystallization temperature is dependent on the system but can be as low as 400 °C as for example in ITO (Sunde et al 2012). The decomposition is exothermic, sometimes causing the decomposition and crystallization to occur simultaneously, thereby making it challenging to identify the real crystallization temperature (Sunde et al 2012).…”

“…In most of these studies, nano-crystalline In2O3 and ITO were prepared by co-precipitation (Kim et al 2002;Kim et al 2006), although a few reports with modified Pechini-methods exist (Sunde et al 2012). The modified Pechini methods avoid the simultaneous precipitation of both indium and tin hydroxides, which ruins the homogeneity on the atomic scale using coprecipitation.…”