Hunting black color via absorption engineering: EDOT and thiophene-benzothiadiazole based black-to-transmissive copolymer and its electrochromic device application

Neutral state black electrochromics are prominent materials for smart applications such as displays, car rear views, e-papers etc. This work has aimed to obtain a neutral state black electrochromic polymer, by tuning the optical absorbance to whole visible region. Two monomers, 4,7-bis(5-(thiophen-2-yl)thiophen-2-yl)benzo[c] [1,2,5]thiadiazole (TTBTT) and 3,4-ethylenedioxythiophene (EDOT), were selected according to their complementary absorbance behaviors, and they were combined in the same polymer backbone via electrochemical copolymerization technique. Copolymerization was performed using three different monomer feed ratios (2:1, 1:1 and 1:2 (EDOT: TTBTT)). Even though all the copolymers exhibited dark to transmissive colors from neutral to oxidized states, changing the monomer feed resulted in different optical contrast, switching response and color hues for each copolymer. The higher optical contrast (from 17% to 27%) and faster switching response (from 3.6 s to 1.8 s) were recorded as EDOT unit increase in the copolymer chain. A dual type electrochromic device was also constructed using the copolymer and the resulting device was successfully changed its color from black to transmissive.

Automated summary

This study aimed to obtain a neutral state black electrochromic polymer by adjusting the optical absorbance across the entire visible region. Two monomers, TTBTT and EDOT, were combined in the same polymer backbone through electrochemical copolymerization. Varying the monomer feed ratios resulted in different optical contrast, switching response, and color hues for each copolymer. The copolymer with more EDOT units showed higher optical contrast and faster switching response. A dual-type electrochromic device was also successfully constructed using the copolymer.