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 (WO3) 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 WO3 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 cm2 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 cm2) and flexible devices, illustrating the scaling potential of the approach. Solid-state tandem structures that use protons as the diffusing species can be used to create electrochromic devices that exhibit high contrast ratios, fast responses, good colouration efficiency and excellent cycling stability.