Ultrasonic spray deposition of mesoporous, nanocrystalline tungsten oxide films for smart windows

Abstract

Electrochromic smart windows modulate their optical transparency in response to a small voltage, and are employed to tailor lighting conditions within a structure with the potential of significant energy savings. The two bottlenecks preventing wide spread implementation of electrochromic windows are cost and performance. In this thesis ultrasonic spray deposition (USD) is developed as a scalable, low cost manufacturing alternative to vacuum processes for the fabrication of mesoporous, nanocrystalline tungsten oxide thin films, the leading electrochromic. In the first approach, tungsten oxide nanoparticles (NP) were synthesized by hot-wire chemical vapor deposition (HWCVD), suspended in ethanol, and deposited on FTO substrates using USD. Nanoparticle morphology and size was controlled by tuning the HWCVD parameters such as filament temperature, substrate temperature, and oxygen partial pressure. Electrochromic performance like coloration efficiency and charge capacity improved by reducing the size of nanoparticles and consequently increasing the specific surface area. However, it was difficult to reduce NP size below ~15 nm, and scaling HWCVD to large volumes is challenging. In the second approach conventional template-assisted sol-gel chemistry was adapted to ultrasonic spray deposition. It was found that the performance is strongly related to annealing procedure, resulting in films with good optical modulation (75%) and fast switching (<10 s). The kinetics of the sol-gel process was quantified using UV-Vis spectroscopy, and performance was correlated to film composition (FTIR, Raman), morphology (SEM, BET) and crystallinity (TEM, XRD, SAD). Systematic manipulation of sol chemistry produced micron thick films with high specific surface area (>100 m2/g), mean pore sizes of ~5 nm, and narrow pore size distributions. These films display world record electrochromic performance, modulating >98% of incident solar radiation in the visible spectrum (450 - 900 nm). Elimination of haze enables full transmission in the bleached state, while the broadband coloration is attributed to the exceptionally high charge capacity (> 120 mC/cm2). The long term durability of these nanocrystalline films was assessed and it was found that the charge capacity decreased due to nanoparticle detachment during cycling. We explored the efficacy of protective coatings applied by evaporation and atomic layer deposition (ALD). It was found that the application of 1-3 cycles of ALD alumina enabled >2500 cycles with no appreciable change in electrochromic performance. Lastly, we examined the potential nanocomposite polymer electrolytes from both a theoretical and experimental perspective. In a collaborative effort a Poisson-Boltzmann model was developed to describe the space charge layer and its impact on conductivity for nanocomposite systems. This model provided guidelines to help understand how ionic conductivity varies as a function of nanoparticle size, volume fraction, configuration, and distribution within the bulk material. Experimentally, USD was used to synthesize nanocomposite films comprised of polymethyl methacrylate (PMMA) and fumed silica. It was shown that post-deposition annealing was critical to remove solvent and increase transmission. A surfactant, cetyltrimethylammonium bromide, was identified that successfully dispersed silica nanoparticles, enabling high transmission with loadings as high as 20 vol. %. The addition of fumed silica was found to enhance the electrochemical and mechanical properties of polymer films, with the latter property exceeding that of commercial PMMA.