Visible and near-infrared electrochromic thiophene–diketopyrrolopyrrole polymers

Abstract

A series of low bandgap polymers containing thiophene and diketopyrrolopyrrole (DPP) units were synthesized and their electrochromic (EC) properties were investigated. It was found that the ratio (R_da) of thiophene to DPP units correlates to the EC properties, in particular optical contrast (ΔT) and coloration efficiency (CE) in the visible and near-infrared spectral regions (e.g., 1310 nm). At the wavelength of 1310 nm, the CE of polymer P2 (R_da = 3 : 1) and ΔT of polymer P4 (R_da = 4 : 1) reached the high values of 1383 cm² C⁻¹ and 81.5%, respectively. Polymer P3 (R_da = 3.5 : 1) had a relatively short switching time of 1.24 s from the doped state to neutral state. Colors of polymer films changed reversibly between green or blue (neutral state) and grey (doped state) upon the redox cycling.