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

Here, we demonstrate for the first time the ability of a porous π-conjugated semiconducting polymer film to enable facile electrolyte penetration through vertically stacked redox-active polymer layers, thereby enabling electrochromic switching between p-type and/or n-type polymers. The polymers <b>P1</b> and <b>P2</b>, with structures diketopyrrolopyrrole (DPP)-π_bridge-3,4,-ethylenedioxythiophene (EDOT)-π_bridge [π_bridge = 2,5-thienyl for <b>P1</b> and π_bridge = 2,5-thiazolyl for <b>P2</b>] are selected as the p-type polymers and <b>N2200</b> (a known naphthalenediimide-dithiophene semiconductor) as the n-type polymer. Single-layer porous and dense (control) polymer films are fabricated and extensively characterized using optical microscopy, atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. The semiconducting films are then incorporated into single and multilayer electrochromic devices (ECDs). It is found that when a p-type (<b>P2</b>) porous top layer is used in a multilayer ECD, it enables electrolyte penetration to the bottom layer, enabling oxidative electrochromic switching of the <b>P1</b> bottom layer at low potentials (+0.4 V versus +1.2 V with dense <b>P2</b>). Importantly, when using a porous <b>P1</b> as the top layer with an n-type <b>N2200</b> bottom layer, dynamic oxidative-reductive electrochromic switching is also realized. These results offer a proof of concept for development of new types of multilayer electrochromic devices where precise control of the semiconductor film morphology and polymer electronic structure is essential.