Synthesis of ITO Nanoparticles with Shape Control and their Assembly for Solution-Processed Transparent Electrodes
Indium tin oxide (ITO) is a well-known transparent conductive oxide. ITO thin films made by sputtering exhibits transparency higher than 80 percent in visible spectral range and the resistivity of 10-4 cm, and they are now widely used as a transparent electrode in flat panel displays, touch screens, and solar cells. In order to be used in these applications, ITO has to be patterned, but sputtering requires multiple steps of masking, applying photoresists, and etching to make patterned ITO. Solution-process is a promising alternative method for easy patterning. Using ITO nanoparticles (NPs) dispersed in solution as an ink, these NPs can be readily deposited on substrates, and their tendency to make a self-assembly upon the solvent drying facilitates building uniform assemblies.
Here, we present the ITO NPs synthesis and assembly and the optoelectronic properties of these assemblies. The decomposition of In(acac)3 and Sn(acac)2Cl2 in an organic solvent at 250C in the presence of oleylamine as surfactants leads to the formation of ITO NPs with two different shapes of ITO NPs: nanospheres and nanocubes. As-synthesized ITO NPs are stably dispersed in organic solvent such as hexane or chloroform. Prior to deposition, the surface of ITO NPs was chemically treated to facilitate surfactant removal. The uniform assemblies of both ITO nanospheres and nanocubes were made by spin-coating and dip-coating, respectively.
The thickness of these assemblies is readily tuned from 30 to 200 nm by the concentration of ITO NP solution. After annealing at 300C, these assemblies exhibited the transparency over 88% in visible spectral range and the resistivity of 2.6 x 10-3 cm order (sheet resistance down to 133 sq)-the best performance among NP-based ITO reported to date, and it is applicable to resistive touch screen. This high optoelectronic performance is attributed to monodispersity, stable dispersion, and uniform, densely-packed assembly of ITO NPs.