Fluorinated benzyl viologens for enhanced electrochromism and remarkable stability in electrochromic devices: An in-situ mass exchange probing through EQCM

• Published in: Solar energy materials and solar cells Vol. 260; p. 112460
• Main Authors: Silori, Gaurav Kumar, Yu, Hsin-Fu, Huang, Yu-Jia, Ho, Kuo-Chuan
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.09.2023
• Subjects: Conjugated π-framework; Dimerization; Electrochromic device; EQCM; Fluorinated benzyl viologen; Conjugated π-framework; Electrochromic device; Dimerization; Fluorinated benzyl viologen; EQCM
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2023.112460

Benzyl viologens (BVs) have garnered massive attention due to their remarkable functional ability as a cathodically coloring material and glare reduction agent in electrochromic devices (ECDs). However, BV-based ECDs have some challenges, of which dimer formation due to viologen radicals is foremost, as it extensively damages the device performance. Herein, a new approach is proposed to address this issue in which five novel fluorine-substituted benzyl viologen (FSBV) are synthesized through a multi-step route based on the fluorine-atom count on the viologen moiety. Once synthesized, the FSBVs (0.01 M) were employed as an electrochromic material (ECM) along with tetrabutylammonium tetrafluoroborate/propylene carbonate (0.5 M TBABF4/PC) as a supporting electrolyte and ferrocene (0.05 M Fc) as a counter electrode material in an ECD assembly. We observed that the family of FSBV-based ECDs demonstrated suppressed dimerization tendency, higher optical contrast, fast response time, and enhanced cycling stability compared to the pristine benzyl viologen-based device (BV/Fc ECD). In particular, PFBV/Fc ECD (from the FSBV family), that utilizes 1,1ʹ-bis(2,3,4,5,6-pentafluorobenzyl)-4,4ʹ-bipyridine 1,1ʹ-diium tetrafluoroborate (PFBV) as an ECM, exhibited higher optical contrast (ΔT) of ∼63.6%, high coloration efficiency of ∼304 cm2/C, fast switching time of ∼1.2 s and an excellent ΔT retention of ∼97% after 10,000 cycles, all at 603 nm. The exciting feature of this study lies in the deployment of a highly sensitive and non-invasive technique, electrochemical quartz crystal microbalance (EQCM), to monitor the ultrasmall mass transmutations caused by synthesized chromophores at the electrode/electrolyte interface. The EQCM analysis revealed that the molecular structure of maneuvered viologen derivatives has a decisive role in determining the mass exchange behavior at the quartz crystal surface, which is reflected in a dissimilar degree of electrochromic performance. [Display omitted] •New schemes have been proposed to mitigate dimerization issues in viologen-based ECDs.•Five new chromophores are introduced based on the fluorine-atom count on the viologen moiety.•An electrode mass exchange probing of synthesized species is being carried out through the EQCM technique.•A systematic study has been instituted to realize the effect of the electron-withdrawing group on benzyl viologen moiety.