Citation: Lin, C.-L.; Cheng, T.-L.; Wu, N.-J. Micropatterned Poly(3,4-ethylenedioxythiophene) Thin Films with Improved Color-Switching Rates and Coloration Efficiency. Polymers 2022, 14, 2951. https://doi.org/10.3390/ polym14142951 Academic Editor: Paola Scarfato Received: 9 June 2022 Accepted: 19 July 2022 Published: 21 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Micropatterned Poly(3,4-ethylenedioxythiophene) Thin Films with Improved Color-Switching Rates and Coloration Efficiency Cheng-Lan Lin 1,2, * , Tzung-Lin Cheng 1 and Nian-Jheng Wu 3 1 Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan; 135800@mail.tku.edu.tw 2 Water Treatment Science and Technology Research Center, Tamkang University, New Taipei City 25137, Taiwan 3 CNRS, Institut des Sciences Moléculaires d’Orsay, Université Paris-Saclay, 91405 Orsay, France; nianjheng.wu@u-psud.fr * Correspondence: cllin@mail.tku.edu.tw or cllintw@gmail.com; Tel.: +886-2-26215656 (ext. 2723) Abstract: Electrochromic materials carry out redox reactions and change their colors upon external bias. These materials are the primary component in constructing smart windows for energy saving in buildings or vehicles. Enhancing the electrochromic performances of the materials is crucial for their practical applications. Micropatterned poly(3,4-ethylenedioxythiophene) (mPEDOT) thin films are electrodeposited on indium tin oxide conducting glass in this study. Their electrochromic properties, including transmittance modulation ability, color-switching rates, and coloration effi- ciency, are investigated and compared with nonpatterned PEDOT thin films. The mPEDOT thin films exhibited faster coloring and bleaching speeds and higher coloration efficiency than the PEDOT thin films while keeping similar transmittance modulation ability. The results suggest that micropat- terning an electrochromic material thin film might enhance its electrochromic performances. This research demonstrates the possibility of promoting the color-switching rate of a PEDOT thin film by micropatterning it. Keywords: poly(3,4-ethylenedioxythiophene); micropattern; electrochromic material; electrode 1. Introduction Electrochromic (EC) materials are species that can switch their absorptivity toward incident light upon external bias switches [1]. EC materials can be prepared as thin films on transparent conducting substrates, and the resulting electrodes can be used to construct electrochromic devices (ECDs). The operating mechanisms of ECDs are used for constructing smart window systems [2]. The systems manipulate light and heat interchange between a building or a vehicle and its surroundings. For example, reducing the heat transfer into a building through windows during summertime could decrease the loading of the air-conditioning system, which could achieve energy-saving purposes [3]. One of the major challenges in developing ECDs for practical applications is to improve the color-switching rate. Decreasing the coloring and bleaching response times of EC material thin films plays a pivotal role in promoting ECD performances. Poly(3,4-ethylenedioxythiophene) (PEDOT) is a well-known conducting polymer [4]. PEDOT derivatives electrodeposited from various functionalized 3,4-ethylenedioxythiophenes exhibit the possibility for multiple and a wide variety of color changes [5–7]. Their applications in many areas, such as energy conversion and storage devices [8], sensors [9], and displays [10], have been extensively explored. PEDOT is a polymeric EC material that can be switched between light-blue and dark-blue states. The coloring-switching mechanism corresponds to its electrochemical redox reactions [11]. The redox reactions of the PEDOT thin film can be expressed as Equation (1). PEDOT n+ : nClO - 4 +ne - ⇄ PEDOT + nClO - 4 (1) Polymers 2022, 14, 2951. https://doi.org/10.3390/polym14142951 https://www.mdpi.com/journal/polymers