Enhanced Optical Contrast and Switching in Near‐Infrared Electrochromic Devices by Optimizing Conjugated Polymer Oligo(Ethylene Glycol) Sidechain Content and Gel Electrolyte Composition

Abstract

A detailed investigation addressing the effects of functionalizing conjugated polymers with oligo(ethylene glycol) (EGn) sidechains on the performance and polymer‐electrolyte compatibility of electrochromic devices (ECDs) is reported. The electrochemistry for a series of donor‐acceptor copolymers having near‐infrared (NIR)‐optical absorption, where the donor fragment is 3,4‐ethylenedioxythiophene (EDOT) or an EGn functionalized bithiophene (g2T) and the acceptor fragment is diketopyrrolopyrrole (DPP) functionalized with branched alkyl or EGn sidechains, is extensively probed. ECDs are next fabricated and it is found that EGn sidechain incorporation must be finely balanced to promote polymer‐electrolyte compatibility and provide efficient ion exchange. Proper electrolyte‐cation pairing and polymer structural tuning affords a 2x increase in optical contrast (from 12% to 24%) and >60x reduction in switching time (from 20 to 0.3 s). Atomic force microscopy (AFM)/grazing incidence wide‐angle X‐ray scattering (GIWAXS) characterization of the polymer film morphology/microstructure reveals that an over‐abundance of EGn sidechains generates large polymer crystallites, which can suppress ion exchange. Lastly, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) indicates sidechain/electrolyte identity does not influence the electrolyte penetration depth into the films, and EGn sidechain inclusion increases electrolyte cation uptake. The material structural design insight and guidelines regarding the polymer‐electrolyte ion insertion/expulsion dynamics reported here should be of significant utility for developing next‐generation mixed ionic‐electronic conducting materials.

Conjugated polymers with near-infrared (NIR) optical absorption: structural design considerations and applications in organic electronics

“…86–89 Additionally, the incorporation of EG n sidechains can provide improved electrolyte penetration and exchange, which is essential for ECDs and OECTs. 90–94…”

“…Specifically, for ECDs and OECTs, where device operation is tethered to the ability of the electrolyte to penetrate and exchange within the polymer film, having a CP with large crystalline domains can lead to electrolyte ion trapping which is detrimental to device performance. 92,100,101 Conversely, in OFETs having a large crystalline domain size can be beneficial to ensuring efficient charge transport and high hole and electron mobilities. 98,102,103 Larger crystalline domain sizes are also important in OPV, but in this setting the domain sizes must be carefully controlled to minimize extensive aggregation within the active layer blend to provide efficient photocurrent generation and charge extraction.…”

“…Briefly, starting with a neutral CP a positive or negative bias is applied to electrochemically oxidize or reduce the CP layer, which in turn generates a p/n-polaron, respectively. 2,92,114 This leads to bleaching of the ground state optical absorption band and emergence of a polaron/bipolaron absorption band. Note, since this is an electrolyte coupled process, efficient and reversible electrochemical oxidation or reduction and electrolyte exchange within the CP layer are of critical importance.…”

A black electrochromic device based on a CuI film for energy-efficient applications

“…Electrochromic materials can be divided into inorganic, 10–12 organic, 13,14 and metal–organic materials. Owing to the diversity of material energy levels in metal ions and ligands, electrochromic devices based on metal–organic materials can display different colors in a single device, 15 which has attracted considerable research attention. The mechanism responsible for electrochromism in metal–organic electrochromic materials is the metal–ligand charge transfer (MLCT), 16–19 and their optical modulation ability can be adjusted through the exchange of functional groups.…”

Influence of Cross-Link Density on Electrochromic Properties of Conjugated Polymer

“…Electrochromism indicates the optical absorption properties of materials could vary reversibly between different potentials, leading to the apparent color varying reversibly under low energy consumption. , The features endow electrochromic materials with wide applications in smart glass of buildings and automobiles, camouflage materials, multifunctional electronics, etc. − The reported electrochromic materials include organic materials (polymer and small organic molecule), − inorganic materials (WO 3 , TiO 2 , NiO, etc. ), − and organic/inorganic composites. , Conjugated polymer as an organic electrochromic material exhibits many unique advantages relative to others, such as good flexibility, lightweight, rich color, fast respond time, etc. − A series of high-performance conjugated polymers as electrochromic materials were reported, such as black-transparent polymers, , multicolored polymers, , near-infrared polymers, , etc. It must be pointed out that many reported conjugated polymers often exhibit good performance only at one electrochromic parameter, which is adverse to their commercial application.…”