Controlling the Optical Properties of Dynamic Windows Based on Reversible Metal Electrodeposition

We describe physicochemical strategies that allow for control of the transmission and color of dynamic windows based on reversible metal electrodeposition. The electrolyte composition and voltage profile used for Bi and Cu electrodeposition significantly affect the color of the devices as quantified by La*b* space colorimetry (L = lightness, a* = red/green coordinate, b* = yellow/blue coordinate). First, in terms of electrolyte composition, we find that although increasing the Cu ion concentration improves the maximum attainable opacity of the devices, it decreases the color neutrality of the windows. Second, we demonstrate that altering the electrodeposition voltage profile affects the average size of the metal electrodeposits, which in turn dictates device color during tinting. Through a combination of SEM-EDX imaging and Mie theory calculations, we develop a model that satisfactorily predicts the relationship between electrodeposit morphology and color neutrality. By using pulsed electrodeposition to further tune electrodeposit morphology, we design dynamic windows with enhanced color neutrality without sacrificing switching speed or reversibility. These strategies enable us to construct devices that possess clear-to-gray-to-black transitions with La*b* color neutrality values less than 10 throughout switching. These measurements demonstrate that the windows are more color neutral than traditional electrochromic devices. Despite the intrinsic red color of bulk Cu, proper control of electrode and electrolyte chemistry can nonetheless yield highly color neutral devices that maintain the advantageous electrochemical properties of Cu.