Toward soluble high-performance electrochromic polymer: Optimizing cross-link density

Cross-linked conjugated polymers attract great attentions in optoelectronic field due to their unique properties resulting from the existence of intramolecular pore and network structure. However, the poor solubility restricts the application of cross-linked polymers in photoelectric device. Here, the strategy of changing cross-link density is employed to optimize solubility and properties of cross-linked conjugated polymer. Five polythiophene derivatives with different cross-link densities were synthesized through direct arylation reaction. The cross-linked conjugated polymers achieve good solubility in organic solvent through decreasing cross-link density, and possess similar optical absorption regions compared to conjugated polymer without cross-linked structure. Through optimizing cross-link density, the electrochromic properties of conjugated polymers could be enhanced, including optical contrast (55 %), response time (0.6 s), coloration efficiency (CEox = 510 cm2 C-1; CEred = 3567 cm2 C-1), and stability, which is more excellent than that of reported cross-linked polymers. And, the flexible electrochromic device based on PET-(1,9) was constructed by simple solution processing and could achieve fast color change between wine red and light blue in low voltage driving force, which indicating the applicability of soluble cross-linked conjugated polymer. Besides, the mechanism about ionic transport and stability improvement of cross-linked conjugated polymers is discussed. These results demonstrate that changing cross-link density is an effective strategy to optimize the properties of cross-linked conjugated polymers.