Low-temperature optimization of tungsten oxide and cerium oxide electrodes for electrochromic applications on flexible substrates

Abstract

In a global context marked by the climate emergency, improving the energy efficiency of buildings has become an absolute priority. The development of electrochromic smart windows represents an innovative solution for reducing building energy consumption while improving comfort. These devices are capable of modulating visible light to reduce glare from the sun and infrared radiation, which is responsible for heat transfer. This research work aims at the fabrication of electrochromic devices on flexible substrates (ITO-PET) via a liquid deposition method: ultrasonic spray pyrolysis (USP). The main objective was to validate a low-temperature (150 °C), environmentally friendly and scalable (roll-to-roll) manufacturing process, enabling the development of electrochromic devices on flexible plastic substrates. In terms of materials, this study optimized the deposition of thin films of amorphous tungsten oxide (WO3) and developed an innovative counter electrode based on cerium oxide doped with molybdenum (33% at.). The use of non-/low-polluting solvents (water/ethanol) and the absence of energy-intensive heat treatments confirm this study's commitment to sustainability and environmental friendliness. Electrochemical and optical characterizations of 25 cm² prototype devices demonstrate a significant transmittance modulation (61.6% transmittance for the transparent state, comparable to the current commercial devices, and 39.9% for the colored state) associated with a low charge density (Qin= 3.65 mC/cm² / Qout =3.44 mC/cm²), although the switching kinetics of the device (15 min) has still to be improved. In addition, the application of this technology to smart labels (PECLABEL project) has enabled the creation of functional electrochromic QR codes on flexible plastic with a switching time of 1 minute. This kinetic difference arises from the fact that the QR code pattern facilitates charge insertion, and that maximum coloration intensity is not required in the colored state. This thesis establishes the viability of low-temperature deposition for obtaining flexible electrochromic devices and paves the way for a breakthrough in the use of electrochromic technologies in buildings and for other applications requiring on-demand modulation of optical properties.