Sol-Gel processing of organically modified ITO thin films and characterization of their optoelectronic and microstructural properties

Kesim, Mehmet Tümerkan
Indium tin oxide (ITO) thin films were formed on glass substrates by sol-gel method. Coating sols were prepared using indium chloride tetrahydrate (InCl3•4H2O) and tin-chloride pentahydrate (SnCl4•5H2O) stabilized in organic solvents (acetylacetone and ethanol). First attempt was to synthesize ITO thin films using standard/unmodified coating sols. The effect of calcination treatment in air (300 – 600 °C) and number of coating layer(s) (1, 4, 7 or 10) on optoelectronic properties (electrical conductivity and optical transparency), crystal structure and microstructure of ITO thin films were investigated. In addition, single-layer ITO thin films with optoelectronic properties comparable to multi-layered films were prepared by employing organically modified coating sols. Oxalic acid dihydrate (OAD) –a drying/microstructure control agent– addition to standard sol formulation was achieved. The rationale was to improve the optoelectronic properties of ITO films through enhancement in microstructure and chemical characteristics upon OAD addition. The effects of OAD content in the sol formulation and post-coating calcination treatment on electrical/optical properties of ITO films have been reported. Finally, the effects of post coating drying temperature (100 – 200 °C) and time (10 – 60 min) on optoelectronic and microstructural properties of OAD-modified ITO thin films were discussed. Thin films have been characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD),x-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-Vis) spectroscopy, fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM) and four-point probe measurement techniques. It was shown that film formation efficiency, surface coverage and homogeneity were all enhanced with OAD addition. OAD modification also leads to a significant improvement in electrical conductivity without affecting the film thickness (45±3 nm). Highly transparent (98 % transmittance in visible region) ITO thin films with a sheet resistance as low as 3.8±0.4 kΩ/sqr have been obtained by employing coating sols with optimized OAD amount (0.75 M). The optimum post-coating drying temperature (100 °C) and drying time (10 min) was also determined for 0.75 M OAD-modified ITO thin films.