In this study, we propose low power consumption, all-in-one type electrochromic devices (ECDs) for effective heat shutters. Considering diffusion-controlled device operation, polymeric viologens (poly-viologens) are synthesized to lower the diffusivity of EC chromophores and to minimize self-bleaching. In comparison with devices based on mono-viologens corresponding to the monomer of poly-viologens, poly-viologen-containing ECDs exhibit advantages of lower coloration voltage (ca, −0.55 V) and higher coloration/bleaching cyclic stability (>1500 cycles). In particular, poly-viologen ECDs show remarkably reduced self-bleaching as designed, resulting in extremely low power consumption (∼8.3 μW/cm2) to maintain the colored state. Moreover, we successfully demonstrate solar heat shutters that suppress the increment of indoor temperature by taking the advantage of low-power operation and near-IR absorption of the colored poly-viologen-based ECDs. Overall, these results imply that the control of the diffusivity of EC chromophores is an effective methodology for achieving single-layered, low-power electrochemical heat shutters that can save indoor cooling energy when applied as smart windows for buildings or vehicles.
Numerous conjugated polymer-based electrochromic devices (ECDs) have been developed, but the correlation between electronic properties of electrochromic polymers (ECPs) and their EC performance, especially response speeds, has been rarely explored. In this work, we propose two new dithienopyran-based ECPs containing different heteroatoms with distinct electronic characteristics to elucidate the effect of the electronic structure of ECPs on EC performance. When electron-donating alkoxy groups are fused into ECPs (P(DT-P)), their highest occupied molecular orbital energy level becomes higher than the alkylthio (electron withdrawing group)-containing ECPs (P(DT-TP)) and the oxidation of P(DT-P) is easier. In addition, the P(DT-P) in the oxidized state is relatively stable due to the stabilization through the resonance of the lone-pair electron of the oxygen atom. These features contribute to the rapid bleaching of the P(DT-P) (∼0.5 s). In contrast, the alkylthio group of P(DT-TP) pulls electrons and destabilizes the radical cation in the oxidized state. Thus, when the P(DT-TP) is oxidized (bleached), the recovery to the neutral state (colored state) is preferred, leading to fast coloration (∼0.4 s) and unprecedentedly high coloration efficiency (∼1323 cm2/C). Both systems can retain more than 90% of their initial optical contrast after 5000 cyclic switchings, indicating their high feasibility for commercialization.
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