Ultrasonic spray coated inorganic thin films for electrochromic applications

Türel, Onur
Recently, thin film transition metal oxides have been an important topic of optoelectronic device research due to high performance and low cost. Active layers in chromic devices and sensors, charge injection/transport layers of optoelectronic devices are only a few examples where transition metal oxide thin films are utilized. Tungsten oxide (WO3), molybdenum oxide (MoO3), titanium oxide (TiO2) and vanadium oxide (V2O5) are the most frequently investigated thin film chemistries. Deposition of these materials in thin film form using different methods enables researchers to explore their properties and improve their performance. Vacuum and solution based methods are two main deposition routes for thin films. In general, vacuum processes offer high homogeneity and purity, while solution based methods are less costly and practical. Comparison of device performances fabricated by these two branches yield critical data for the opportunity cost analysis. Electrochromic devices have been a focus of research lately and they extensively utilize thin film transition metal oxides and conducting polymers. Transition metal oxides offer lower performance but higher stability compared to conducting vi polymers. In addition, they adhere to the transparent conductive layer well and are highly compatible with common electrolytes. In this work, the effect of thin film deposition route on the morphology and performance of MoO3 based electrochromic (EC) devices was investigated. For deposition, ultrasonic spray pyrolysis (USP), thermal evaporation and sequential deposition (SD) of these two methods were used. MoO3 thin films were then characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface characteristics and chemical structure of the deposited thin films were found to depend extensively on the deposition route. EC performance and reversibility of the samples were evaluated using spectroelectrochemical methods and cyclic voltammetry. Sample prepared by USP method showed low coloration efficiency (CE) (16 cm2 C-1), but high stability. In contrast, sample prepared by thermal evaporation had high CE (30 cm2 C-1), but low stability. Sample prepared by SD method showed the highest CE (33 cm2 C-1) among other samples and maintained its stability with cycling. All in all, the SD route investigated in this work was found to be highly promising and can be simply extended to other metal oxide systems.