Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat Effects of counter ions on electrochromic behaviors of asymmetrically substituted viologens Gaurav K. Pande a , Namhyeon Kim a , Jin Hui Choi a , Gopal Balamurugan a , Hong Chul Moon b,∗∗ , Jong S. Park a,∗ a Department of Organic Material Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea b Department of Chemical Engineering, University of Seoul, Seoul, 02504, Republic of Korea ARTICLE INFO Keywords: 1-Hexyl-1′-nonafluorohexyl asymmetric viologen Counter ion Electrochromic behaviors Anion exchange Ionic liquid ABSTRACT We present effects of counter anions on electrochromic behavior of asymmetrically substituted viologens, 1- hexyl-1′-nonafluorohexyl viologens (HERFVs). For this purpose, two anions, hexafluorophosphate ([PF 6 ] – ) and bis(trifluorosulfonyl)imide ([TFSI] – ), were employed for both HERFVs and 1-butyl-3-methylimidazolium ([BMIM] + )-based ionic liquids. The HERFV-containing electrochromic devices (ECDs) exhibited apparent de- pendence on the counter ions in terms of optical contrast, switching speed, long-term cyclic stability, coloration efficiency, and memory effect. Specifically, the ECDs based on [HERFV][PF 6 ] 2 showed a redox response at −1.0 V with a color change from colorless to blue. Meanwhile, the coloration of [HERFV][TFSI] 2 appeared at −3.0 V, and a significantly long optical memory effect was observed with unstable cyclic repeatability. We attributed this discrepancy to the nature of counter ions, such as size and electronegativity, which may influence the electron transfer process during redox reactions. These results demonstrated that counter anions can exert a significant impact on the EC behaviors of asymmetrically substituted viologens. The presented approach suggests a potential tool for controlling EC properties of viologen derivatives for applications in transmissive windows and eye wear. 1. Introduction Electrochromism is the phenomenon of a reversible spectral or op- tical change in a material due to the redox reaction upon applying potential biases. Various electrochromic materials have been in- vestigated, including viologens [1–11], inorganic metal oxides [12–14], and conducting polymers [15–19]. Electrochromic devices (ECDs) fea- ture user-controllable colors with acceptable switching time, low op- erating voltage, and high coloration efficiency. ECD-based technologies have received wide attention in a variety of applications, for example, energy-saving smart windows, antiglare rear view mirrors in vehicles, and EC displays. Among EC materials, viologens have been investigated in depth because of their superior properties: stable redox states, desirable electron-accepting capability, and the tunability of nitrogen sub- stituents. The dicationic species (V 2+ ) exist as air-stable salts, and their radical cations are among the most stable organic radicals. In the transition state, V 2+ undergoes two reversible reductions to form a radical cation (V +• ) and a neutral species (V 0 )[20]. The dicationic species is colorless and the radical cation forms a brilliant violet-blue, whereas the neutral species is colorless or yellow. The colors can be precisely adjusted according to the quaternized substituents. As counter anions, halides are mostly employed, but triflate and hexafluoropho- sphate have also been utilized as non-coordinating anions, which allow high solubility in organic solvents [21,22]. Recently, intensive efforts have been made to demonstrate colorless-to-neutral transitions in aryl- substituted viologens [23–29]. Among them, 1-alkyl-1-aryl asymmetric viologen exhibited colorless-to-neutral EC behavior in an ion gel [25] and a phosphorous functionalized viologen showed colorless-to-black/ grey EC transition [26]. In this study, we newly synthesized asymmetric viologens (HERFVs) substituted with hexyl and nonafluorohexyl side-chains as quaternized substituents. We expected that the end with a hexyl group would fa- cilitate the charge transfer for the bipyridine moiety, but the end with a nonafluorohexyl group would inactivate the aromatic ring due to fluorines with high electronegativity. The proposed asymmetric vio- logen is different from previous viologen compounds because a strong electron withdrawing nature was provided by a perfluorohexyl group, https://doi.org/10.1016/j.solmat.2019.04.004 Received 18 February 2019; Received in revised form 21 March 2019; Accepted 5 April 2019 ∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: hcmoon@uos.ac.kr (H.C. Moon), jongpark@pusan.ac.kr (J.S. Park). Solar Energy Materials and Solar Cells 197 (2019) 25–31 0927-0248/ © 2019 Elsevier B.V. All rights reserved. T