Abstract
Stable doping of indacenodithieno[3,2-b]thiophene (IDTT) structures enables easy color tuning and significant improvement in the charge storage capacity of electrochromic polymers, making use of their full potential as electrochromic supercapacitors and in other emerging hybrid applications. Here, the IDTT structure is copolymerized with four different donor–acceptor–donor (DAD) units, with subtle changes in their electron-donating and electron-withdrawing characters, so as to obtain four different donor–acceptor copolymers. The polymers attain important form factor requirements for electrochromic supercapacitors: desired switching between achromatic black and transparent states (L*a*b* 45.9, −3.1, −4.2/86.7, −2.2, and −2.7 for PIDTT–TBT), high optical contrast (72% for PIDTT–TBzT), and excellent electrochemical redox stability (Ired/Ioxca. 1.0 for PIDTT–EBE). Poly[indacenodithieno[3,2-b]thiophene-2,8-diyl-alt-4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole-7,7′-diyl] (PIDTT–EBzE) stands out as delivering simultaneously a high contrast (69%) and doping level (>100%) and specific capacitance (260 F g–1). This work introduces IDTT-based polymers as bifunctional electro-optical materials for potential use in color-tailored, color-indicating, and self-regulating smart energy systems.
Highlights
The concept of energy storage in conjugated polymers was introduced more than three decades ago by Heeger et al.,[14−16] and thereafter, plentiful research efforts have been focused on improving the charge storage capacity of the “pseudocapacitive” polymers for use as electrodes in solid-state batteries and supercapacitors.[17−19] An intriguing concept follows, as the electrochromic and energy storage properties of conjugated polymers can be merged into a single, bifunctional electro-optical material for use in electrochromic supercapacitors and other hybrid applications where the stored energy level can be visually observed or more precisely optically detected in real time.[20]
Chemical structures of the four copolymers PIDTT−TBT, PIDTT−EBE, PIDTT−TBzT, and PIDTT−EBzE are shown in Scheme 1, while the detailed synthetic procedures for the donor− acceptor−donor (DAD) segments and the copolymers are represented in Schemes S1 and S2 in the Supporting Information
Different combinations of the electron-rich EDOT donor monomer and either of the electron-deficient benzo[c][1,2,5]thiadiazole or benzo[d][1,2,3]triazole acceptor monomers are commonly used in well-functioning electrochromic polymers.[26,28,64,67]
Summary
Fast-growing interest in electrochromic materials has resulted in a steady supply of conjugated polymers capable of changing color between two or more states, with the common feature of delivering reversible color change, high contrast, and stable long-term operation.[1−5] This feeds the promised commercialization of smart windows/mirrors,[6−8] eyewear,[9] epaper, and other energy-efficient passive displays[10,11] and their scale up by solution-based methods.[12,13] On the other hand, conjugated polymers exhibit an intrinsic property of switching between conjugated and quinoidal forms upon electrochemical oxidation (p-type doping) and reduction (n-type doping), and this property has opened another interesting possibility to store energy through sufficient stabilization of the doped states. Article phene building blocks because of their low oxidation potential, fast color switching, and relatively simple chemical structures that allow easy color tuning, as spearheaded by the Reynolds group.[23−27] Other groups have developed similar structures to further tune the doping characteristics, optical contrast, and color purity of the polymers, for example, by means of chemical doping and electropolymerization.[28−31] polymers based on the EDOT and dioxythiophene structures are often identified by a blue transmission window in their neutral state or remaining absorption band in the low-energy visible spectral region, causing a characteristic blue tint to the oxidized state. Article applications of these and other (published and unpublished) IDTT-based polymers
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