Viologen-Immobilized 2D Polymer Film Enabling Highly Efficient Electrochromic Device for Solar-Powered Smart Window

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) are an emerging class of organic layered materials with well-defined permanent porosity. Thanks to their tailor-made structures and properties, these porous crystalline polymers have exhibited the potential for broad functions in optoelectronics, membrane, catalysis, and energy storage and conversion. Recent advances have demonstrated that 2D COFs integrated with color-switching, redox-active units such as triphenylamine in the backbones could present reversible color switching and are, therefore, excellent candidates for electrochromic devices (ECDs). Nevertheless, these 2D COF-based ECDs have not unleashed their full potential to display high coloration efficiency (CE) (while few examples could reach over 800 cm² C⁻¹) due to the random stacking of crystallites and the presence of amorphous defects that lead to restricted ion diffusion. In this regard, achieving related materials with highly ordered and oriented columnar-like pores could provide a large interface area with the electrolyte, high utilization of built-in color-switching components, and fast ion diffusion, which are crucial for realizing a quick and efficient EC switching.

The researchers from Prof. Xinliang Feng’s group recently demonstrated the high-performance ECDs with a fully crystalline viologen-immobilized 2D polymer (V2DP) thin film as the color-switching layer. The high density of vertically oriented pore channels (pore size ≈ 4.5 nm; pore density ≈ 5.8 × 10¹⁶ m^-2) in the synthetic V2DP film enables high utilization of redox-active viologen moieties and benefits for Li⁺ ion diffusion/transport. As a result, the as-fabricated ECDs achieve a rapid switching speed (coloration, 2.8 s; bleaching, 1.2 s), and a high CE (989 cm² C^-1), and low energy consumption (21.1 µW cm^-2). Moreover, it is managed to fabricate transmission-tunable, self-sustainable EC window prototypes by vertically integrating the V2DP ECDs with transparent solar cells. This work sheds light on designing electroactive 2D polymers with molecular precision for optoelectronics and paves a practical route toward developing self-powered EC windows to offset the electricity consumption of buildings.

Reference: Zhiyong Wang, Xiangkun Jia, Panpan Zhang, Yannan Liu, Haoyuan Qi, Peng Zhang, Ute Kaiser, Sebastian Reineke, Renhao Dong, Xinliang Feng, Viologen-Immobilized 2D Polymer Film Enabling Highly Efficient Electrochromic Device for Solar-Powered Smart Window, Adv. Mater. 2021, 2106073. https://doi.org/10.1002/adma.202106073

Acknowledgments: This work was financially supported by EU Graphene Flagship (GrapheneCore3, No. 881603), ERC starting grant (FC2DMOF, No. 852909), ERC Consolidator Grant (T2DCP), Coordination Networks: Building Blocks for Functional Systems (SPP 1928, COORNET), H2020-MSCA-ITN (ULTIMATE, No. 813036), H2020-FETOPEN (PROGENY, 899205), CRC 1415 (Chemistry of Synthetic Two-Dimensional Materials, No. 417590517), as well as the German Science Council and Center of Advancing Electronics Dresden (cfaed). R.D. thanks Taishan Scholars Program of Shandong Province (tsqn201909047). Z.W. and X.J. gratefully acknowledge funding from China Scholarship Council. The authors acknowledge the Centre for High Performance Computing (ZIH) in Dresden, Germany, and the support of computational calculation from Chenghao Liu in Mcgill University, Canada.