Tuning band gap, color switching, optical contrast, and redox stability in solution-processable BDT-based electrochromic materials

Abstract

Abstract 2,6-bis(trimethylstannane)-4,8-bis(2-octyldecyloxy)benzo[1,2-b:4,5-b′]dithiophene coupled with 2,5-didecyloxy-1,4-dibromobenzene and 4,7-dibromo-5,6-dioctyloxy-2,1,3-benzothiadiazole using Stille-coupling polymerization to afford the non D-A type polymer P1 and D-A type polymer P2, respectively. Two polymers displayed excellent solubility in normal organic solvents, so series of chemical instrumental analysis, such as NMR, GPC and elemental analysis, were carried out to confirm the inner structure and composition of the polymers. Different structure types exerted great influence on polymer's optical, electrochemical and electrochromic properties. Compared with P1 film, the introduction of the strong electron-withdrawing group benzothiadiazole improved conjugate effect along the polymer backbone of P2, which caused the redshift of ultraviolet–visible absorption spectrum and significant reduction in energy band gap. In addition, electrochromic performance test results showed that the P1 polymer film can switch between the orange-yellow and gray, while P2 film changed from blue in the neutral state to light yellow in the oxidation state. Moreover, the optical contrasts and response times of the polymer films also showed great differences due to the different molecular structure types. Higher transmittance changes and less sensitivity to the step interval on optical contrast indicated the fast switching speed and the superior cycle stability of P2 film.