Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat A multicolor electrochromic device having hybrid capacitor architecture with a porous carbon electrode Zhuang Liang, Kazuki Nakamura, Norihisa Kobayashi * Graduate School of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan ARTICLE INFO Keywords: Multicolor electrochromism Hybrid capacitor Porous carbon electrode Electrical double layer ABSTRACT With the progress and development of electrochromic (EC) technologies in various applications, the demand for multicolor EC devices (ECDs) has been increasing, and it has a huge connection with the expansion of the market. This paper reports on a novel multicolor ECD with a simple hybrid capacitor architecture (glass/ITO/ multi-EC gel/carbon/glass), containing both anodic and cathodic EC materials. By employing the carbon ca- pacitor electrode having porous morphology and large surface area as a counter electrode, even large charge amount consumed by multiple redox pairs’ cathodic materials was adequately compensated. The successful fabrication of the device, consequently, it provides four well-defined colors (white, green, yellow, and red) in one single device. 1. Introduction Electrochromic (EC) phenomenon is the ability to change color re- versibly by some materials when subjected to an electric charge which causing electrochemical redox reaction. With regard to the properties of EC materials, lots of significant improvements have been achieved, including smart windows [1–3], viologen-based antiglare rearview mirrors [4–6], and now it is being tried to be applied to some displays just like electronic paper since its reflective characteristic [7–9]. To continuous expansion in EC market requirement, the development of multicolor switch is attracting the eyes of many researchers from the world [10–13]. Until now, various principles and strategies aimed at multicolor switches have been intensively explored, such as multi- electrode configurations and patterned-based devices [14–21]. How- ever, since each of them needs quite complex structures, resolution and color variation of these EC devices (ECDs) are significantly limited. Regarding this, we have proposed an easy-to-make ECD towards mul- ticolor representation just by two TCO electrodes and sandwiched anodic and cathodic EC materials [22]. Different from typical ECDs using some complementary redox species as the counter-reaction ma- terial [23–31], ITO particle-modified electrode as the counter side was selected in this device. With this novel and simple device structure, since the amount of charge accumulated by the formation of the elec- trical double layer on the counter electrode equaled to the total amount of charge consumption in the two-electrode ECD, individual color switching has been successfully achieved, even though containing two kinds of both anodic and cathodic EC materials. This kind of strategy is named by hybrid capacitor architecture. Although some works have been investigated to achieve color change which is controlled by EC reaction with ion storage layer on the counter electrode [32,33], it should be noted that there is no another report on ECDs using electrical double layer capacitor as a counter electrode to achieve multicolor switching at least within our knowledge. In principle, this hybrid ca- pacitor ECD can provide many colors by using EC materials showing multicolor switching by the redox reaction. Usually, polymeric materials can show different colors change by different EC units, the work on triarylamine based electrochromic polymers, for example, has demonstrated multicolor switching within a single polymer [34,35]. Contrast with this, low weight small organic molecules can show superior good color purity, which enable the dis- tinct transition from colorless to colored states [36–41]. Furthermore, among of them, some molecules having multiple redox reactions without complicated synthesis procedures can exhibit multicolor switches within one single device [25,42,43]. We also have already reported the multicolor EC reaction of 4,4′-biphenyl dicarboxylic acid diethyl ester (PCE), which shows two kinds of color change (yellow and red) [44]. To achieve the color variation of hybrid capacitor ECD, we selected the PCE molecule as a multicolor EC material. However, as we observed in our previous paper, when driving voltage increased to the higher color density in the case of ITO particle-modified electrode as https://doi.org/10.1016/j.solmat.2019.109914 Received 27 February 2019; Received in revised form 25 April 2019; Accepted 30 April 2019 This manuscript was presented at the 13th International Meeting on Electrochromism, Aug.27–31, 2018. * Corresponding author. E-mail address: koban@faculty.chiba-u.jp (N. Kobayashi). Solar Energy Materials and Solar Cells 200 (2019) 109914 0927-0248/ © 2019 Elsevier B.V. All rights reserved. T