Facile method to prepare high-performance WO3·nH2O films by potassium sulfate-assisted chemical bath deposition

Chemical bath deposition (CBD) is regarded as an ideal approach for tungsten oxide (WO 3 ) electrochromic films because of its low cost and high efficiency. However, the currently reported WO 3 films prepared by CBD suffer from poor stability, delayed switching speed, and inadequate optical modulation (ΔT) due to undesirable morphology, oxygen vacancies, and crystallinity, which limit their potential application. This study presents WO 3 ·nH 2 O films by CBD assisted with different sulfates (Na 2 SO 4 , (NH 4 ) 2 SO 4 , and K 2 SO 4 ) as capping agents. The K 2 SO 4 -assisted CBD-prepared WO 3 ·nH 2 O films (W-K) possess optimal performance with large optical modulation (ΔT = 87 % at 630 nm), fast switching speed (t c /t b= 7.4/4.2 s), and outstanding cycling stability (5000 cycles). The effect of capping agent on growth mechanism and structural morphology of WO 3 ·nH 2 O is investigated in detail. Benefit from the optimal capping effect of K 2 SO 4 , W-K has the larger specific surface area, rich oxygen vacancies, and amorphous/crystalline structure, which endow W-K with ample redox reaction sites, easier for ions and electrons to migrate, and leading to the release of reaction stress. Besides, the electrochromic devices assembled by W-K demonstrate good electrochromic performance overall. This research offers a fresh perspective on the low-cost and high-efficiency electrochromic WO 3 films and devices, and provides a feasible guide for their implementation of commercial applications.