Oxidative-Reductive Near-Infrared Electrochromic Switching Enabled by Porous Vertically Stacked Multilayer Devices

Here, we demonstrate for the first time the ability ofa porous pi-conjugated semiconducting polymer film to enable facile electrolytepenetration through vertically stacked redox-active polymer layers,thereby enabling electrochromic switching between p-type and/or n-typepolymers. The polymers P1 and P2, with structuresdiketopyrrolopyrrole (DPP)-pi(bridge)-3,4,-ethylenedioxythiophene(EDOT)-pi(bridge) [pi(bridge) = 2,5-thienylfor P1 and pi(bridge) = 2,5-thiazolyl for P2] are selected as the p-type polymers and N2200 (a known naphthalenediimide-dithiophene semiconductor) asthe n-type polymer. Single-layer porous and dense (control) polymerfilms are fabricated and extensively characterized using optical microscopy,atomic force microscopy, scanning electron microscopy, and grazingincidence wide-angle X-ray scattering. The semiconducting films arethen incorporated into single and multilayer electrochromic devices(ECDs). It is found that when a p-type (P2) porous toplayer is used in a multilayer ECD, it enables electrolyte penetrationto the bottom layer, enabling oxidative electrochromic switching ofthe P1 bottom layer at low potentials (+0.4 V versus+1.2 V with dense P2). Importantly, when using a porous P1 as the top layer with an n-type N2200 bottomlayer, dynamic oxidative-reductive electrochromic switchingis also realized. These results offer a proof of concept for developmentof new types of multilayer electrochromic devices where precise controlof the semiconductor film morphology and polymer electronic structureis essential.