Optimized ion diffusion depth for maximizing optical contrast of environmentally friendly PEDOT:PSS electrochromic devices

Because the electrochromic (EC) performance significantly affects the diffusion of ions into the EC layer, EC layer thicknesses vary, ranging from approximately 100 nm to 400 nm. Their optimized and maximized EC performance in both optical contrast and color-switching behaviors are investigated as a function of EC layer thicknesses and applied bias voltages. Commercially available poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonate) (PEDOT:PSS) is selected as the EC layer due to its environmentally friendly condition and expected reproducible uniformity. Among the four different thicknesses of the PEDOT:PSS layer fabricated, the ~200 nm thick PEDOT:PSS layer exhibits the highest optical contrast ratio at an applied bias voltage of + 3.0 V for the bleached state and - 3.0 V for the colored state. Moreover, through increasing the applied bias voltage slightly to + 3.5 V/- 3.5 V, the color-switching speed is also significantly improved from 10 s to 5 s for switching from the bleached state to the colored state.