Morphological and Structural Engineering of Conjugated Polymers for Near-Infrared Electrochromic Devices

The recent device engineering and material optimization of electrochromic devices (ECDs) has enabled new frontiers beyond the original focus of smart-window applications and display devices, including: NIR-IR optical filters, energy storage, active camouflage, and broadband communication. For conjugated polymer ECDs, the highly tunable polymer structures provide numerous handles for modifying the optoelectronic and physicochemical properties facilitating efficient optical switching across the visible wavelength range (400-700 nm) and into the NIR (>750 nm). Here, we first disclose the optimization of a porous nanostructured conjugated polymer morphology enabling vertically stacked redox-active p-type and n-type polymer multilayer ECDs. Specifically, efficient oxidative electrochromic switching at lower potentials (+0.4 V versus +1.2 V with a dense top layer) or dynamic oxidative-reductive electrochromic switching is found with a multilayered ECD architecture. Next, we disclose the synergistic optimization of the CP sidechain (either branched alkyl or EG_n) and the electrolyte cation identity for a series of diketopyrrolopyrrole (DPP) and 3,4-ethylenedioxythiophene (EDOT) or glycolated bithiophene (g2T) copolymers. It was found that by fine-tuning the extent of EG_n sidechain incorporation and the identity of the electrolyte cation a 2x increase in optical contrast (from 12 to 24%) and >60x reduction in switching time (from 20 to 0.3 s) could be realized in NIR-ECDs. To elucidate the influence of the polymer morphology/microstructure on these performance metrics, atomic force microscopy (AFM) and grazing incidence wide-angle X-ray scattering (GIWAXS) were performed. These findings provide material design guidelines for the development of next-generation CPs and mixed ionic-electronic conductors.