Enhanced Coloration Efficiency and Cycling Stability in Ultrathin NiO _<i>x</i> Films Utilizing Zeroth-Order Fabry–Perot Resonance

Electrochromic materials exhibit a reversible change of color on application of external voltage and have important applications in smart windows, low energy display technology, and active camouflage. Transition metal oxides have better long-term stability than organics but are limited by their narrow range of colors. For good optical contrast, the active electrochromic layer has a typical thickness of 0.1-1 μm, which determines the energy requirement for switching and the response time. Thicker layers have better optical contrast between states but can be brittle, or prone to delamination, limiting their long-term stability and use on flexible surfaces. In this work, we investigate the electrochromic properties of ultrathin (<100 nm) inorganic electrochromic films. We show that with the addition of a metal backing, we can use an optical resonance known as the zeroth-order Fabry-Perot mode to achieve a high optical contrast of 41.2%. Coloration efficiency values of 205 cm²/C are demonstrated for films as thin as 25 nm and a time response of 2.1 s and 2.5 s for coloring and bleaching, respectively. Cycling stability and optical memory (bistability) are key parameters for practical electrochromic applications. We demonstrate that a sub1 nm Ti adhesion layer is sufficient to stabilize the films for >250 cycles without degrading the optical contrast. The adhesion layer is shown to also significantly improve the optical memory (bistability) of the films. With an extremely low energy consumption of 1.76 mJ/cm² per switch and an optical contrast loss rate of 3% per hour, the films have a power requirement of 170 nW/cm² when refreshed every 3 h. Compared to the state of the art, we demonstrate a reduction of 10 times the energy required to switch using 20 times less material.