PhD thesis examines the potential of WO3 sol-gel material to be used as functional ink for inkjet printing and integration of WO3 printouts in printed optoelectronic systems. The research evaluates the correlation of the complex composition of functional WO3 sol-gel material to its physico-chemical and rheological properties and evaluates the printability of the material with inkjet printing technique. The main focus of the research was the modification of the sol-gel synthesis and the composition of WO3 sol, which enabled the development of functional inks for inkjet printing on unconventional and complex surfaces, such as glass and transparent conductive oxide (TCO). The challenge was very complex as the starting WO3 sol was tailored for dip-coating technique and has never been used before in inkjet printing. The replacement of highly volatile solvent - ethanol, with of 2-propanol and 2-propoxy ethanol was needed in synthesis of WO3 sol. Rheological properties of the modified WO3 sols were also suitable for analogue flexographic technology, as demonstrated with precise lines and homogeneous surfaces on glass substrate made with flexographic printing. In addition, incorporation of additives in the WO3 sol enabled production of test viscous WO3 paste, which allowed the deposition of thick layers with screen printing and doctor blading technique.
An important contribution to the understanding and usefulness of sol-gel materials for inkjet printing represents the study of time and thermal stability of sol-gel materials associated with the transition of sol to gel, which evaluates the applicability of the material to be used in printing industry and mass production. The results of rheological and micro-rheological properties of sol in connection with structural changes have described the effect of temperature and aging on the process of gelation, which is strongly affected also by storage conditions, sol concentration and the type of the solvent used, therefore the stability of sols varies from few days up to 10 months. Useful information about the drop formation process of WO3 sols were given with Reynolds, Weber, Ohnesorge and Z dimensionless numbers. Adhesion, optical properties, topology and the spectrophotometric characteristics of WO3 printouts were evaluated. The functionality of printed WO3 layers was tested in simple photochromic (PC) and hybrid electrochromic systems (EC) and organic solar cell prototype (OPV). Realized EC system enabled optical modulation in the visible part of the spectrum (Tvis) between 50 % and 6 %, while PC system between 85 % and 42 % under one-hour exposure to the sunlight (standard test conditions; 1000 W /m2, AM 1.5, T = 25 °C). We were faced with numerous problems in the development of OPV system, like surface interactions and interactions between the layers, which have a negative impact on the homogeneity and functionality of the individual layer, therefore the response of OPV system was insufficient.
In overall, we believe, that the study made a significant contribution to a better understanding of the sol-gel materials and their applicability in inkjet printing and moreover in the broad spectrum of graphic technologies. We answered to many complex questions in the field of physico-chemical properties of WO3 sol-gel materials, and demonstrated that the modification of sol-gel synthesis route enabled the preparation of printable WO3 material. We have shown that inkjet printing is a useful technique in the research of printing unconventional materials on complex substrates, due to its fast and easy adaptability to the user and contact-less and precise transfer, required by optoelectronic systems.
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