Highly Transparent Conjugated Polymer as the Counter Electrode in Electrochromic Smart Windows

A highly transmissive counter electrode material (CEM) at both neutral and doped states in the visible range is essential for the optical performance of electrochromic devices (ECDs), but is rarely reported due to the challenge in molecular design. Herein a novel conjugated polymer HTCP is reported, which exhibits a wide bandgap (3.02 eV) and a strong absorption at 300 nm in its neutral state. Upon oxidation, the absorption spectrum is substantially red-shifted, peaking at 900 nm. Thus HTCP can keep high transparency throughout the whole visible range at both states, making it particularly well-suited as the CEM in ECDs, which is demonstrated by the excellent optoelectronic performance of a smart window-type ECD in this work. Furthermore, the geometric and electronic configuration study of its building block reveals that HTCP evolves from a closed-shell butterfly-like folded structure into a twisted geometry featuring a quinoidal open-shell character after oxidation, explaining the nature of its unique optical properties. This work provides new insights and explicit instructions for the structural design of conjugated polymer-based CEMs. In addition, it offers a clear scenario for the application of open-shell materials in optoelectronics, not only limited to the chemical fundamental studies.

Automated summary

This paper reports a novel conjugated polymer HTCP as the counter electrode material in electrochromic devices, which exhibits high transparency and excellent optoelectronic performance. The study of its building block reveals the change in its optical properties during oxidation. This work provides new insights for the structural design of conjugated polymer-based counter electrode materials.