A Black State from Reversible Copper Electrodeposition without Metal Additives

Metal electrodeposition and dissolution on a transparent electrode enable dynamic switching between the opaque and transparent states, respectively. To be used as dynamic windows, a fully black state must be achieved while maintaining reversibility. Cu is a top candidate that meets the latter criterion but fails the former, producing its characteristic orange tint. As a result, metal additives are often mixed with Cu ions but at the expense of some degree of reversibility. Here, a truly black state is achieved without metal additives by enhancing the dissipative interaction between light and Cu. A galvanic etching method is used to transform a flat ITO surface into an array of nanopillars, forming a gradually varying index across the ITO interface. This elongates the light absorption path length over all wavelengths once Cu is electrodeposited. The electrode is demonstrated in dynamically tunable devices including one that transitions between mirror-like and opaque states with a coloration efficiency of 20.3 cm² C^-1. These results highlight the potential of our strategy in light management devices, particularly for energy-conserving dynamic windows.