Implementation and characterization of films for electrochromic devices, energetically self-sufficient, to develop the energy saving of buildings

Abstract

The need to reduce the energy consumption from the excessive use of non-renewable resources such as fossil fuels and the growing emissions of CO2 into the atmosphere, estimated at about 30-40 % [1] has helped to direct research towards the development of technologies dedicated to the delicate issue of energy saving. Within the construction sector, the largest contribution to energy loss, about 25-35 % comes from the use of inefficient windows [2]. The idea of improving the efficiency of buildings by developing the "Zero Energy Buildings" (ZEB) technology is based precisely on the use of a modern concept of window, now defined as dynamic and better known as "Smart Window" [3]. Industrial research is therefore projected toward the improvement of existing devices and the implementation of new solutions able to control the transmission values of the incident solar radiation, the energy recovery that follows, and the control of the flow of energy as a function of the cyclical variation of the seasons [4]. The introduction of electrochromic technology in the manufacture of smart windows has marked a significant improvement in the construction sector, as it is estimated that it is possible to reduce the energy consumption necessary for the heating or cooling of the rooms of a building, by up to 30% [5]. The theme of the research is the synthesis and characterization of electrochromic materials, to be used in the realization of electrochromic devices energetically self-sufficient for the promotion of energy-saving of buildings. First, the study of tungsten trioxide (WO3) and titanium dioxide (TiO2) thin films has been reported. WO3 and TiO2 thin films were synthesized with the Sol-gel method and then, they were annealed at different annealing temperatures. The structural properties regarding to the morphological characterization of the synthesized films (amorphous and crystalline) have been evaluated through Micro-Raman Spectroscopic investigations. The electrochemical properties of films have been characterized by Cyclic Voltammetry. The diffusion coefficients of lithium ions have been evaluated for each film electrode that has been immersed in liquid electrolyte and gel polymer electrolyte systems. Each film has been used as the electrode for assembling the electrochromic devices in the classic "sandwich" configuration, using a PMMA- gel polymer electrolyte polymeric in a solution of lithium perchlorate in propylene carbonate. The resulting electrochromic devices have been characterized by Cyclic Voltammetry. Subsequently, the main technological properties such as the variation of transmittance, the switching times, and the coloration efficiency, which are depending on the morphology of the individual electrodes and by the temperature of thermal treatment, were investigated by UV-Vis-NIR spectroscopy. Later, two classes of potential electrolytes valid for the construction of an electrochromic device have been investigated. The first consists of a liquid electrolyte based on a lithium salt, lithium perchlorate in propylene carbonate solution, and in a mixture of propylene carbonate and ethylene carbonate. The second class involves a polymer gel electrolyte based on Poly(methylmethacrylate). A detailed structural, electrochemical, and thermal characterization by ATR-FTIR Spectroscopy, Cyclic Voltammetry, Differential Scanning Calorimetry technique, and Thermal Gravimetric Analysis have been proposed to select the best electrolyte system to be used in electrochromic devices. Afterward, the realization of an electrochromic device "all-in-one" based on an electrochromic mixture of viologen-ferrocene of dimensions (10 cm ×10 cm) has been discussed. The realization of a homogeneous mask of spacers inside the device has been realized by using the photolithography technique. The electrochromic properties of the device have been investigated through Cyclic Voltammetry. The structural properties of the device in the OFF/ON states have been characterized by Micro-Raman Spectroscopy, while the optical performance of the device in terms of transmission modulation, color contrast ratio, and coloration efficiency were estimated by UV-Vis-Nir Spectroscopy. Finally, the industrial proposal concerning the design of systems to be used for the construction of large-area electrochromic devices is presented. Three specific industrial prototypes have been developed to perform the automated operations of device filling, thin film deposition on substrate, and device closure with a UV system. Moreover, the Etching process for the production of ITO tracks on glass sheets has been discussed. The treated glass sheets have been subsequently used for the construction of a prototype of an "all-in-one" electrochromic device based on the electrochromic mixture of a salt of viologen and ferrocene of dimensions (16 cm × 16 cm). The exclusivity of the developed prototype, concerns the matrix of pixels that compose it, where every single pixel can be electrically powered.