All-solid-state proton-based tandem structures for fast-switching electrochromic devices

All-solid-state electrochromic devices can be used to create smart windows that regulate the transmittance of solar radiation by applying a voltage. However, the devices suffer from a limited ion diffusion speed, which leads to slow colouration and bleaching processes. Here we report fast-switching electrochromic devices that are based on an all-solid-state tandem structure and use protons as the diffusing species. We use tungsten trioxide (WO 3 ) as the electrochromic material, and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as the solid-state proton source. This structure exhibits a low contrast ratio (that is, the difference between on and off transmittance); however, we add a solid polymeric electrolyte layer on top of PEDOT:PSS, which provides sodium ions to PEDOT:PSS and pumps protons to the WO 3 layer through ion exchange. The resulting electrochromic devices exhibit high contrast ratios (more than 90% at 650 nm), fast responses (colouration to 90% in 0.7 s and bleaching to 65% in 0.9 s and 90% in 7.1 s), good colouration efficiency (109 cm 2 C −1 at 670 nm) and excellent cycling stability (less than 10% degradation of contrast ratio after 3,000 cycles). We also fabricate large-area (30 × 40 cm 2 ) and flexible devices, illustrating the scaling potential of the approach.