Heterostructured p- and n-type Two-Dimensional Polymer Electrochromic Films for Ambipolar Multistate Optical...

Redox-active two-dimensional polymers (2DPs) are promising as electrochromic layers due to their structural order and permanent porosity. However, most 2DP electrochromic layers can only be used in binary switching, which prevents multi-state optical profiles from being accessed in a single device. This challenge is in part due to the difficulty of fabricating multi-layer 2DP devices in which both redox-active layers can participate in electrochemically driven color switching. In this report, we show how interfacial polymerization methods can provide multilayer, n- and p-type 2DP layers that exhibit rapid reversible optical modulation, decent coloration efficiencies (up to 101 cm² C^-1), and high stability over 100 cycles, with both layers actively participating in electrochromic switching. In stacked devices, the 2DP layer directly contacting the gel electrolyte exhibits electrochromic responses (rates, efficiencies, etc.) comparable to those of the corresponding single-layer device, while the bottom-buried layer shows a slight reduction in electrochromic performance. We attribute this slightly diminished performance to limitations in ion transport in the buried electrochromic layer, which are more pronounced in conventional non-porous, multi-layer electrochromic materials. These observations broadly suggest that multilayer semiconducting 2DPs may be useful in a range of mixed ionic-electronic conductor applications, where multi-state or ambipolar performance is desirable.