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

A detailed investigation addressing the effects of functionalizing conjugated polymers with oligo(ethylene glycol) (EG(n)) 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 anEG(n) functionalized bithiophene (g2T) and the acceptor fragment is diketopyrrolopyrrole (DPP) functionalized with branched alkyl or EG(n) sidechains, is extensively probed. ECDs are next fabricated and it is found that EG(n) 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 EG(n) 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 EG(n) 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.