Design Rules for High Contrast Mid-Infrared Electrochromism in Conjugated Polymers

The ability to modulate absorption in the mid-infrared on-demand has broad applications ranging from camouflage to thermal management. While the ability of conjugated polymers to modulate mid-IR is known through tuning of the oxidation state, the specific structure-property relationships that govern the modulation amplitude remain largely unexplored. Our study introduces a series of dioxythiophene-based copolymers, where we adjust backbone planarity, electron-richness, and side chain polarity to investigate their effects on spectral traits. We find a clear correlation between higher conductivity and larger optical modulations attained through a broadening of the charge carrier bands and an intensification of specific vibrational modes. As thin films, our top-performing copolymer attains an 85% IR-contrast between its doped and undoped states. To probe their performance under electrochemical control, we fabricated and tested electrochromic cells with the best cells reaching a mid-IR contrast of nearly 60%. These findings enhance our understanding of design rules impacting mid-IR spectral features for conjugated polymers.