Fabrication of flexible electrochromic film based on amorphous isopolytungstate by low-temperature inkjet-printed process with a solution crystallization kinetic-controlled strategy

Polyoxometalate (POM) is a promising functional material in the area of electrochromic device due to its good solution processable characteristic, but the low-quality film formation limits its application. Here, a crystallization kinetic-controlled strategy is suggested to form the amorphous isopolytungstate (IPT) film. By controlling the solution alkalinity, metatungstate clusters are dissociated into tiny tungstate monomers, inhibiting spontaneous crystallization nucleation and crystal growth. The amorphous IPT film is prepared using inkjet print technology with a series of low-temperature process (<70 °C), and it shows crack-free and smooth morphology, which results in a lower haze than crystalline IPT film. The electrochromic optical modulation ability of IPT film is enhanced from 14.8% to 46.2% after amorphous transformation. The coloring and bleaching response time of IPT film reaches 2.8 s and 10.4 s, respectively. The cyclic voltammetry test demonstrates that amorphous IPT film owns more excellent cyclic voltammetry stability (1800 cycles without capacity degradation) than crystalline IPT film (200 cycles with > 93% capacity degradation), which is attributed to its high-quality film structure. Additionally, the amorphous IPT film also shows more excellent flexibility than crystal IPT film due to eliminating the rigid lattice structure of salt crystal. The flexible IPT electrochromic device can maintain electrochromic modulation well after 1000 times bending with a radius of 10 mm, which is attributed to the excellent flexibility of amorphous IPT film.