A Black State\nfrom Reversible Copper Electrodeposition\nwithout Metal Additives

Metal electrodeposition and dissolution on a transparent\nelectrode\nenable dynamic switching between the opaque and transparent states,\nrespectively. To be used as dynamic windows, a fully black state must\nbe achieved while maintaining reversibility. Cu is a top candidate\nthat meets the latter criterion but fails the former, producing its\ncharacteristic orange tint. As a result, metal additives are often\nmixed with Cu ions but at the expense of some degree of reversibility.\nHere, a truly black state is achieved without metal additives by enhancing\nthe dissipative interaction between light and Cu. A galvanic etching\nmethod is used to transform a flat ITO surface into an array of nanopillars,\nforming a gradually varying index across the ITO interface. This elongates\nthe light absorption path length over all wavelengths once Cu is electrodeposited.\nThe electrode is demonstrated in dynamically tunable devices including\none that transitions between mirror-like and opaque states with a\ncoloration efficiency of 20.3 cm<sup>2</sup> C<sup>–1</sup>. These results highlight the potential of our strategy in light\nmanagement devices, particularly for energy-conserving dynamic windows.