Absorption Coefficient and Optical Contrast Modulation through Side Chain Engineering of Electrochromic Polymers

Conjugated polymers can be electrochemically bleached to achieve colored-to-transmissive electrochromic switching. This work presents a side chain engineering approach to enhance colored state absorptivity of an electrochromic polymer, while maintaining high transmissivity in the bleached state. A series of dioxythiophene copolymers with systematic variations in their branched alkyl side chains have been used to understand side chain influence on solubility, aggregation, film morphology and optical constants. By increasing the chromophore density through an appropriate choice of side chains, the absorption coefficient of the colored polymer can be improved by almost 50%, with minimal influence on bleached-state absorption coefficients over the visible region. As such, solution-processed films of high absorbance can be prepared using thin polymer layers to achieve fast electrochromic switching and highly transmissive bleached states. Under optimized conditions, optical contrasts nearing 80% can be reached at the wavelength of maximum absorbance, among the highest reported for solution-processed, cathodically coloring electrochromic polymers. The improved absorption coefficients are especially useful in enhancing the electrochromic performance in the low transmittance regime (colored state transmittance <20%), providing useful insights for designing high contrast electrochromic materials.