Highly transparent to truly black electrochromic devices based on an ambipolar system of polyamides and viologen

Abstract

A novel electrochromic device (ECD) based on an electroactive ambipolar system was constructed and designed through an absorption-complementary approach. The system consisted of electroactive polyamides (PAs) with N,N,N′,N′-tetraphenyl-p-phenylenediamine (TPPA) and tetraphenylbenzidine (TPB) units in the backbone and heptyl viologen (HV) in the supporting electrolyte. Each of the electrochromic materials (ECMs), including TPPA-PA, TPB-PA and HV, provided one of the three primary colors that merged into a black color. Because of the suitable counter electrode materials used in this study, the overall operating voltage was effectively reduced; thus, the electrochemical stability and lifetime of the ECD were greatly enhanced. Furthermore, the whole system was completely transparent in its neutral or bleaching state, and the transmittance was reduced to only 6% in the colored state in both the visible and near-infrared (NIR) regions. The ECD demonstrated a high L* change (ΔL*) of 81 and a significant transmittance change (ΔT) of 60% in the visible region. Thus, through the excellent combination of the electrochromic and ambipolar characteristics of the system, a genuine ‘highly transparent to truly black’ ECD was successfully fabricated, implying the great potential of this device as a shutter in transparent displays and related devices. Stray light that makes it hard to spot objects in see-through displays can be reduced by 95% with a new colour-changing polymer. The approach, from Guey-Sheng Liou of the National Taiwan University and co-workers, involves solution casting a blend of electrochromic materials to achieve a ‘transparent-to-black’ transformation for contrast-improving shutters. The primary component is an anodic electrochromic film made from two triarylamine-based polymers with reversible electrochemical properties. When a gel electrolyte containing cathodic viologen molecules is sandwiched between two ITO-coated glass electrodes with the polymer film, the device could merge the colors of red, green or blue and turned from transparent to black with just mild voltage stimulation, thanks to built-in charge trapping effects. These colours spanned most of the visible-light spectrum and could be subtracted to generate the final target colour of black. High-performance electrochromic device based on ambipolar system of polyamides and viologen exhibits extremely high absorption over the visible and near-infrared (NIR) regions, indicating the device could be changed from highly transparent to truly black. This electrochromic device implies high potential of application as shutter for transparent displays and energy-saving devices.