Fine-Tuning the Color Hue of π-Conjugated Black-to-Clear Electrochromic Random Copolymers

A family of dioxythiophene (DOT)-based conjugated random copolymers, capable of reversibly switching between a neutral black and an oxidized transmissive state, are reported for electrochromic (EC) applications. By replacing 3,4-ethylenedioxythiophene (EDOT) in the previous generation of a core donor-acceptor-donor (D-A-D) terheterocycle with a methyl-alkylated 3,4-propylenedioxythiophene (ProDOT), we enhance the solubility of precursors and monomers in this new generation of broadly absorbing electrochromic polymers (ECPs). By adjusting the newly designed terheterocycle monomer feed ratio, we obtain three polymers with varying hues of color neutrality (a* and b* < +/- 10) throughout the entire switching voltage range and with integrated contrasts 45-49% throughout the visible region (380-700 nm). The a* values of the polymers ranged between -8 (green hue) and +5 (red hue), with their b* values ranging between -4 and -7 (blue hue) in their neutral states; additionally, the a* and b* values are all below +/- 8 in their oxidized states. In electrochromic devices (ECDs), we exploit this variation in color hue to achieve color balance with the counter electrode material-a minimal color-changing polymer (MCCP) that is colorless in its fully neutral and oxidized states but light red at intermediate potentials. This red hue was balanced by the green hue of ECP-Random40, leading to an ECD that was color-neutral (a* ranging from -0.8 to -11 and b* ranging from -0.5 to 1.7) throughout the switching voltage range. This family of high-contrast and color-neutral ECPs are scalable with high yielding syntheses for all monomers due to the enhancement of monomer solubility and the usage of commonly available DOT moieties, which will aid in advancing the development of large-scale polymer-based EC window and display applications. This method provides materials that switch from black to transmissive through only gray intermediates as needed for switchable color tinting applications.