Comparison of Electrodeposited and Sputtered Tungsten Trioxide Films for Inorganic Electrochromic Nanostructures

High Resolution Image Download MS PowerPoint Slide Electrochromic materials and their implementation with structural colors are currently being intensely researched because of their promising applications as non-emissive display devices utilizing ambient light. In particular, several fully inorganic devices that rely on electrochromic tungsten trioxide (WO 3 ) have been presented. For preparing nanoscale films of this material, sputtering is the most established technique, but electrodeposition has recently been shown to be capable of achieving exceptionally high electro-optical modulation contrast without the need for expensive equipment. In this work, we investigate the possibilities of electrodeposited WO 3 and present a systematic comparison with sputtered WO 3 with respect to performance in electrochromic devices. Importantly, we show that “ultralarge” electro-optical modulation (∼95% change in transmission) is possible for both types of films. However, it is only the sputtered films that enable such high contrast in a stable electrolyte such as LiClO 4 in propylene carbonate. The electrodeposited films are less uniform and difficult to make thicker than ∼500 nm. We find no evidence that the electrochromic properties of the electrodeposited WO 3 are intrinsically better than those of sputtered WO 3 . However, the electrodeposited films are much rougher and/or porous on the nanoscale, which increases the switching speed considerably. We conclude that electrodeposited WO 3 is mainly useful in applications in which high contrast is not essential while switching speed is. As an example, we present the first electrodeposited WO 3 integrated with structural colors by sandwiching the material between two metal films. By electrical control, the reflective colors can then be tuned at least one order of magnitude faster (a few seconds) than previously reported while having fair color quality and without any loss of brightness.