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Abstract
Titanium dioxide thin films were deposited by Metallorganic Chemical Vapor Deposition (MOCVD) at substrate temperatures ranging from 250oC to 450oC over soda lime glass and indium tin oxide (ITO) coated glass substrates. X-ray diffraction (XRD) studies show that the films have a crystalline anatase structure at all the deposition temperatures. Particle size decreases and texture changes with the increase in substrate temperature. Scanning electron microscope (SEM) measurements confirmed the XRD results. X-ray photoelectron spectroscopy (XPS) confirmed the appearance of a new well resolved state in the core level of Ti 2p spectrum shifted by 1.16 eV to lower binding energy due to the reduction of Ti+4 ions to Ti+3 upon litheation. Chronoamperometry, cyclic voltammetry and in situ UV-Vis spectrophotometry studies were carried out on the prepared samples to correlate the effects resulting from the increase in deposition temperature and the electrochromic properties. The particle size and crystallinity control the electrochromic performance. The 350oC film shows the highest ion storage capacity and the highest optical modulation and substantial band gap broadening.
Another set of films was deposited over substrates with higher sheet resistances at the same conditions as those mentioned above. They were characterized by XRD and their electrochromic performance was compared with the previous set.
The effect of increasing the precursor concentration was studied. Films were deposited at 250oC, 350oC and 450oC temperatures over ITO coated and sodalime glass substrates. Crystalline structure was investigated by XRD showing a pure anatase phase at all deposition temperatures. The texture changes with increasing deposition temperature. Morphology and thickness were studied by scanning electron microscopy (SEM) showing columnar growth and slight grain size decrease with increasing deposition temperature. Chronoamperometry, optical modulation, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) measurements were carried out for the 350oC and 450oC deposited samples indicating that the 450oC has a better electrochromic performance than the 350oC sample. This might be due to the relatively smaller particle size, the orientation and/or the thickness of the 450oC film.
Tungsten oxide thin films were deposited over ITO substrates by pulsed laser ablation (PLD) at various substrate temperatures and partial pressures of Ar and O2 gases using a WO3 ablation target. To study the effect of deposition temperature on the electrochromic properties chronoamperometry, optical modulation, and cyclic voltammetry experiments were conducted. Among the films studied, the film deposited at 500oC and 50% Ar/50% oxygen was the best from structural and electrochromical point of view.
Electrochemical and optical modulation studies were carried out for the two types of ITO films used in this study to see what role the substrates play in the electrochromic process. The sheet resistance of one of the substrates ranges from 5-15 Ω/□ and that of the other one ranges from 70-100 Ω/□. The color of the lithieated films was brown while the color of the TiO2 films was gray and that of WO3 was blue. So, it is obvious that they do not participate in coloring since after bleaching TiO2 and WO3 films go back to the transparent state. The electrochemical reproducibility of the high sheet resistance film was poor.