Mixed Viologens-Based Electrochromic Curtain: Recipe to Achieve Multiwavelength Switching and Superior Power Efficiency

Abstract

In pursuit of sustainable architecture, aesthetically pleasing smart windows with additional functionalities have emerged as a viable way to improve energy efficiency in modern buildings, where smart electronic curtains are an integral part. Developing smart windows with low power consumption and multifunctionality, along with high flexibility, is the primary focus in recent years. Here, a recipe to achieve a four-digit coloration efficiency (CE) with a notably high color contrast (CC) has been achieved in an all-organic solid-state electrochromic device (ECD). Two different viologens (Vio) with Cl– and PF6– counterions, namely, Vio2+(Cl–) and Vio2+(PF6–), have been synthesized and mixed in various concentration ratios (1:3 (ECD1-3), 1:1 (ECD1-1), and 3:1 (ECD3-1)) for fabricating ECDs, using polythiophene (PT) as another auxiliary EC material. The PT-M2-ECD device, which was created by mixing viologens in equal ratio (1:1), exhibits the highest CC value of 96% at 515 nm (visible region) and blocks the near-infrared (NIR) wavelength by 26% (at 800 nm). The significantly high four-digit coloration efficiency of 1413 ± 16, 1680 ± 69, and 1251 ± 69 cm2/C was estimated for ECD1-3, ECD1-1, and ECD3-1, respectively. The devices also show fast response with a switching time of as low as 1 s. A flexible device using the same material retains a similar performance as the rigid one, suggesting a potential application as a wearable electronics gadget. The high performance of ECDs in terms of CC and CE in rigid and flexible forms makes them a viable and appropriate choice for smart window applications, providing a sustainable way to balance energy efficiency with occupant comfort and design flexibility in smart building constructions. © 2026 American Chemical Society