Cyanophenol and benzyl phosphonic acid grafted electrodes for poly(butyl viologen) thin film-based electrochromic device

Abstract

Viologens are considered among the most promising electrochromic materials for utilization in electrochromic devices (ECDs). In this regard, poly(butyl viologen) (PBV) has gained interest due to its stability, conductivity, and ease of synthesis, thus making it suitable for electrochromic applications. Regrettably, the poor adhesion of PBV thin film with substrates/electrodes deters the redox properties and long-term stability of PBV-based ECDs. To address this issue, herein, two types of surface-modified indium tin oxide (ITO) electrodes, grafted with 4-cyanophenol (P–CN-ITO) and benzyl phosphonic acid (BPO₃-ITO), are introduced for the deployment in PBV-based ECDs. The optical contrast (ΔT, %) for substrate-modified P–CN and BPO₃-based ECD was measured to be ∼57.0 and ∼59.5, respectively, which was remarkably higher (an improvement of ∼66 %) than that of ECD with bare ITO (∼34.3). Meanwhile, the coloration times (τ_c) of the P–CN, BPO_3, and bare ITO-based ECD were registered to be ∼1.7, ∼2.0, and ∼4.1 s, respectively, thus revealing the superiority of utilized functional groups over the unmodified substrate. The P–CN and BPO₃-based ECD retained ∼70.2 % and ∼55.5 % of initial ΔT, respectively, after continuous switching of 10,000 cycles, thus showing high endurance and reversibility. The XPS results indicated the strong covalent bond formation between the utilized functional groups (P–CN and BPO₃) and ITO, which delivered improved electrochemical, optical, and stability behavior when deployed in ECDs. Our study suggests that P–CN and BPO₃-based substrates can be promising for deployment as terminal electrodes in viologen-based ECDs for improved overall performance.