Dissertation Defense – Ning Xia
Prof. Rosario Gerhardt, Advisor, MSE
Prof. Vladimir Tsukruk, MSE
Prof. Dong Qin, MSE
Prof. Nazanin Bassiri-Gharb, ME/MSE
Prof. Manos Tentzeris, ECE
Dr. David Gottfried, IEN
"PROCESSING, CHARACTERIZATION AND MODELING OF SOLUTION-PROCESSED INDIUM TIN OXIDE FILMS"
Indium tin oxide (ITO) is the most widely used transparent conducting material because of its excellent combination of high optical transparency and low electrical resistivity. ITO films have been used in numerous optoelectronic devices, such as photovoltaic cells, displays and smart glasses. Vacuum-based deposition is the commercially available method to fabricate ITO films currently, in which strict vacuum conditions and high-cost equipment are required. In contrast, solution-processed methods are promising low-cost alternatives to deposit ITO films and patterns under atmospheric conditions.
The objectives for this research are to investigate the optimal conditions for obtaining high-quality ITO films with solution-processed methods, to understand the microstructural evolution of multi-layer ITO films via a non-destructive characterization method and to study the impedance behavior of ITO films through experiments and simulations. The structure of this dissertation is based on achieving these objectives. 1) Spin-coating method and ink-jet printing method were optimized to fabricate multi-layer ITO films. The surface morphology, electrical and optical properties were compared. 2) Neutron reflectometry was first used to investigate the buried microstructure of solution-processed multi-layer ITO films. The porosity in ITO films were quantitatively calculated and related to the electrical properties. 3) The electrical properties of multi-layer solution-processed ITO films were experimentally characterized by impedance spectroscopy. The conduction mechanism in ITO films and the influence of open circuit capacitance from the measurement configuration and equipment were discussed. 4) The electrical properties of ITO films were further studied by finite element analysis in 2D and 3D models. The effects of sample geometry, film conductivity and electrodes geometry were simulated. It was proved that correct measurement of impedance spectroscopy requires a well-controlled instrument setup and well-defined sample-electrode geometry to interpret the intrinsic impedance and capacitance of thin film samples.
The best ITO films made in this work had a combination of very low surface roughness (< 2 nm), high optical transmittance (> 90%) over the visible light range and controllable low sheet resistivity (10-2 to 10-3 Ω∙cm). These low-cost solution-processed patternable ITO films, with sub-micrometer thickness, have great potential for most optoelectronic applications.