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.

an organic semiconductor, whereas PSS functions as a dopant and provides hole carriers to PEDOT by removing a portion of H atoms from the -SO 3 H groups. Because of the transparent and conductive nature of PEDOT:PSS, previous works have explored the use of PEDOT:PSS in ECDs [26][27][28][29] . In these studies, PEDOT:PSS typically serves as an electrode or an electrochromic layer, instead of a proton source [30][31][32][33] . For example, the shuffling of Li + ion between Prussian Blue and PEDOT:PSS has been shown to provide high CE in ECDs 34-38 (a comparison of previous studies is provided in Supplementary Table 1).
In this Article, we report the development of an all-solid-state tandem structure for ECDs that is composed of an electrochromic WO 3 layer, a PEDOT:PSS layer as the proton source and a solid polymeric electrolyte (SPE) layer on top of the PEDOT:PSS layer. The tandem structure has a number of functions: the SPE provides Na + to the PEDOT:PSS and pumps protons to WO 3 , creating a relay of insertion ions; the insulating SPE layer carries most of the applied voltage so that H 2 formation in PEDOT:PSS is avoided during high-voltage operation; and the PEDOT:PSS as an electrochromic material supplements the light absorption of WO 3 and enhances the contrast ratio of the overall device. Our design exhibits excellent performance in terms of colouration speed, contrast ratio and cycling stability, and we fabricate large-area and flexible devices, illustrating its potential for large-scale applications.

Design of tandem-structure ECD
To achieve an all-solid-state design and use protons as insertion ions, we employed a PEDOT:PSS film as the proton source. We fabricated an ECD composed of five layers: ITO/WO 3  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.
where the indium tin oxide (ITO) layer serves as a transparent conducting layer and the SiO 2 layer between the WO 3 and PEDOT:PSS layers serves as an ion-conductive, electron-insulating layer. Figure  1b (top) shows the transmittance changes in this device in response to voltage pulses with widths varying from 40 to 5/16 s. The wavelength of the illumination light is 670 nm. Under a voltage pulse of 40 s, the transmittance change (that is, the difference between the maximum and minimum transmittances (ΔT)) is 15%. Even when the pulse width is reduced to 5/8 s, the device is still able to respond by a ΔT of 13%. When the pulse width is further reduced to 5/16 s, ΔT quickly drops to 8%, suggesting that the switching time is between 5/8 and 5/16 s. This is an encouraging result as it shows that PEDOT:PSS can serve as a proton source in an ECD and it can yield comparable switching time to the liquid proton sources, which is typically in the sub-second level 16,17 .
Even though the switching speed by using protons as insertion ions is fast, the small ΔT of 15% is not sufficient for electrochromic applications. We attributed this small ΔT to the limited amount of removable protons in the PEDOT:PSS layer. To resolve this problem, we design a tandem structure by adding an extra SPE layer containing NaClO 4 and ferrocene on top of the PEDOT:PSS layer with an aim to use Na + from the SPE layer to pump more protons to the WO 3 layer. Here ferrocene was added into the electrolyte as a redox mediator in the ECD to compensate the charge lost due to Na + extraction 39 . This design is motivated by the fact that NaClO 4 -containing SPE is a widely used Na + source in WO 3 -based ECDs; meanwhile, PSS is a well-known ion-exchange material 40,41 .
Before fabricating a full device, we first examined the response time of the SPE/PEDOT:PSS junction to voltage pulses of different widths. We used a device structure of ITO/SPE/PEDOT:PSS/ ITO. Figure 1b (bottom) shows ΔT as a function of the voltage pulse width. It can be seen that this device without the WO 3 layer also exhibits electrochromic behaviour. At a pulse width of 40 s, ΔT is 30%. When the pulse width is reduced to 5/4 s, ΔT is still maintained at 28%. When the pulse width is further reduced to 5/8 s, ΔT drops to 18%, suggesting that the SPE/PEDOT:PSS junction also has a switching time at the sub-second level and is suitable for use in a tandem structure with the PEDOT:PSS/WO 3 junction. Figure 1c shows a schematic of the tandem structure with five layers, namely, ITO/SPE/PEDOT:PSS/WO 3 /ITO. The expected working mechanism is that the protons are the main insertion ions to the WO 3 layer and responsible for the colouration. The SPE layer only provides Na + to the PEDOT:PSS layer, but Na + does not diffuse into WO 3 in a significant amount. Instead, Na + mainly plays the role of pumping protons out of PEDOT:PSS through an ion-exchange process.
To justify this expected mechanism, we first carried out density functional theory calculations. We compared the cases with either Na + or a proton from the PSS model injected into WO 3 (Fig. 1c). We found that the insertion of Na + into WO 3 is 0.46 eV higher in energy than a proton. The high coordination of Na atoms by O atoms from the two side chains of PSS ( Fig. 1c and Supplementary Fig. 1) plays the key role of stabilizing Na + in PSS over a proton.
We further verified this process by carrying out X-ray photoelectron spectroscopy (XPS) experiments on three different    Supplementary Fig. 2, respectively. Figure 2c shows the photographs of the device under different applied voltages. Figure 2d shows the corresponding transmittance spectra. When the device is in the off state (applied voltage, 0 V), the transmittance in the visible (400−750 nm) and near-infrared (NIR) region (750−1,360 nm) can exceed 90% and 85%, respectively, in our best-performing devices. After applying a positive voltage of 1.2 V, the transmittance is reduced to ~30% and ~40% in the visible and NIR regions, respectively. When the voltage is further increased to 2.4 V, the transmittance reaches 2.7% and 8.7% in the visible and NIR regions, respectively, which shows a much better contrast ratio than the device without the PEDOT:PSS layer (above 20% in both visible and NIR regions; Supplementary Fig. 3). These results are consistent with the absorption spectra ( Supplementary Fig. 4), where the absorption peaks are located at 620 and 860 nm. Figure 2e shows the voltage dependence of the transmittance of devices with and without the PEDOT:PSS layer. Clearly, the device with the PEDOT:PSS layer exhibits two stages in both colouration and bleaching processes. Figure 2f,g shows a comparison of the cyclic voltammetry (CV) curves of the two devices, where two redox peaks were observed in the device with the PEDOT:PSS layer at all the scan rates. The two peaks indicate that two different electrochemical processes occur in such devices. In contrast, the device without the PEDOT:PSS layer only shows a single redox peak. We also carried out the x*y* colour space (CIE 1931) measurements. As shown in Supplementary Fig. 5, the device with the PEDOT:PSS layer exhibits a shallow and a deep colouration state. As determined from the density functional theory calculation and XPS measurement above, the colouration of WO 3 is mainly the result of proton insertion. We attribute the first stage of colouration to a relay process of proton insertion into the WO 3 layer, which is 'pumped' by Na + insertion into the PEDOT:PSS layer (equation (1)).

Characterization of tandem-structure ECD
where Fc and Fc + refer to Fe(ii)(C 5 H 5 ) 2 and Fe(iii)(C 5 H 5 ) 2 , respectively, H@PEDOT:PSS and Na@PEDOT:PSS refer to proton and Na + residing in the PEDOT:PSS layer, respectively, and x is the number of charge transfers during the redox. The valence change in the Fe centre of Fc molecules in the SPE layer ensures the continuity of electric current in the ECD.
Due to the fact that the PEDOT:PSS layer is electrochromic by itself, we attribute the second stage of colouration to Na + accumulation in the PEDOT:PSS layer, as elaborated below. In the second stage, the voltage is as high as 2.4 V. No pure proton sources can survive at this high voltage. One of the highest applied voltage on the liquid proton source as reported in the literature is only 1.5 V (ref. 9 ). Fig. 2h shows the measured voltage drops at individual layers, where the voltage drops on the SPE (U 1 ) and WO 3 (U 2 ) layers are directly measured (Fig. 2h, inset), and the voltage drop on the PEDOT:PSS (U 3 ) layer is obtained by taking the difference between the applied voltage and the sum of U 1 and U 2 . It can be seen that U 3 is always below 0.35 V, which should prevent the evolution of H 2 gas. In Fig. 2h, it is also observed that U 3 exhibits a peak at about 1.6 V, which is consistent with the voltage onset for the second colouration stage (Fig. 2e). To confirm this behaviour, we fabricated three such samples and a similar peak position was observed ( Supplementary  Fig. 6). We attribute the second stage of colouration to Na + extraction from the SPE layer and injection into the PEDOT:PSS layer at a high voltage (equation (2)). The extra Na ions result in the colouration of the PEDOT:PSS layer.

{
Fc ↔ Fc + + e − Na + + e − + PEDOT : PSS ↔ Na@PEDOT : PSS To understand the performance difference with and without the PEDOT:PSS layer, Fig. 2i compares the diffusion coefficients (D) of the two devices determined from the data in Fig. 2f,g using the Randles-Sevcik equation 42 : where D is in the unit of cm 2 s −1 , i p is the peak current (mA), A is the area (cm 2 ) of the working electrode, c is the concentration of active ions (mol cm −3 ), v is the scan rate (mV s −1 ) and n is the number of electrons (assumed to be 1). In the case without the PEDOT:PSS layer, the diffusion current is contributed by Na + insertion from SPE to WO 3 (equation (4)).

{
Fc ↔ Fc + + e − xNa + + xe − + WO 3 ↔ NaxWO 3 (4) In the case with the PEDOT:PSS layer, the diffusion current is mainly contributed by proton insertion from PEDOT:PSS to WO 3 , which should be accompanied by Na + insertion from SPE to PEDOT:PSS to fill the H vacancies (Fig. 1c). As expected, the kinetics of proton diffusion is indeed significantly faster than Na + diffusion, which explains the enhanced performance by using the PEDOT:PSS layer.

Performance of tandem-structure ECD
We next examined the cycle life and switching time of the all-solid-state tandem-structure ECD. Real-time transmittance spectra were taken at −0.5 and 2.5 V and the period for each cycle was set to 20 s ensuring full colouration and bleaching. The result is shown in Fig. 3a. It can be seen that after 3,000 cycles, the device still retains over 90% of its initial contrast ratio (that is, less than 10% degradation). Figure 3a, inset, shows the transmittance spectra of the 1,000th, 2,000th and 3,000th cycle, confirming the consistency over cycling. In contrast, the traditional ECD based on the simple SPE/WO 3 structure not only has a slow switching speed (~3.9 s for colouration and ~9.8 s for bleaching; Supplementary Fig. 7) but also exhibits poor cycling stability, suggesting that protons introduce less damage to the lattice than the larger Na + during cycling. Figure 3b shows the transmittance spectra for a typical cycle from which we determine the switching times for the colouration and bleaching processes. We define the colouration time (t c ) and bleaching time (t b ) as the time taken for the relative change in transmittance reaching 90%. Our results show that t c is about 0.7 s at an applied voltage of 2.5 V, whereas t b is 7.1 s at an applied voltage of −0.5 V. However, an inspection of the spectrum shows that the bleaching process exhibits two stages. The first stage with the relative change in transmittance reaching 65% only takes 0.9 s. It is the second stage that is significantly slower. This slow second stage is attributed to the strong affiliation of Na + in the PEDOT:PSS layer. From the CV measurement (the red-coloured arrow in Fig.  2g), it can be seen that the peak current is nearly unchanged for high scan rates, indicating that the second stage involves a slow ion diffusion process. From Fig. 2g, it can also be seen that even colouration involves two peaks. But these two peaks show similar scan-rate dependence, that is, having similar ion diffusion rates, which explains that the colouration in Fig. 3b does not exhibit two stages. As a comparison, the result for the traditional SPE/WO 3 structure without the PEDOT:PSS layer is also shown in Fig. 3b. The significant improvement by introducing the PEDOT:PSS layer can be clearly seen.
It is worth noting that the PEDOT:PSS layer has an optimized thickness. If the layer is too thick, the intrinsic light absorption will reduce the transmittance in the off state, as shown in Supplementary  Fig. 8. If the layer is too thin, it cannot provide sufficient protons to the WO 3 layer. We fabricated a number of devices with varying thicknesses of the PEDOT:PSS layer. The thickness was controlled by the times of spin coating. The thickness of the WO 3 layer was fixed at about 300 nm, whereas the thickness of the SPE layer is usually much thicker (about 20 μm) and therefore does not significantly affect the device performance. As shown in Fig. 3c, the thicker the PEDOT:PSS layer, the shorter is the colouration time (until reaching the intrinsic limit of proton diffusion), which is estimated from our results to be 0.7-0.9 s (specific process can be seen in Supplementary Fig. 9). In the best-performing device, the PEDOT:PSS layer is of the order of 50 nm (Fig. 2b), which matches with the 300 nm WO 3 layer.
As PEDOT:PSS is prone to hydration in air, we investigated the effect of humidity on the device performance. We fabricated four devices at 60%, 70%, 80% and 90% relative humidity ( Supplementary  Fig. 10). The dehydrated PEDOT:PSS layer was exposed to these humidity levels for about 2 min before depositing the SPE layer. The performance of the four devices, including the transmittance spectra and colouration/bleaching time (summarized in Supplementary  Table 2), is shown in Supplementary Fig. 10. No apparent degradation of device performance was observed, suggesting that the effect of humidity is marginal.
Another important performance indicator for an ECD is the CE, defined as the change in optical density (ΔOD) per unit of charge injection (ΔQ). A high CE value indicates a large contrast ratio with a small energy input. As shown in Fig. 3d, the CE for the tandem structure with PEDOT:PSS is 109 cm 2 C −1 at 670 nm, whereas the CE for the device without PEDOT:PSS is 93.5 cm 2 C −1 . As a comparison, a typical LiTaO 3 -based ECD shows a CE of 73.5 cm 2 C −1 (Supplementary Fig. 11). Figure 3e compares the tandem structure (with the PEDOT:PSS layer) and the traditional structure (without the PEDOT:PSS layer) with regard to their responses to voltage changes with different pulse widths. The transmittance was measured at 670 nm. The initial ΔT with a pulse width of 20 s is ~87% and ~70% for the tandem and traditional structure, respectively. As the pulse width is reduced to 10, 5, 5/2, 5/4, 5/8 and 5/16 s, the ΔT of the tandem structure decreases to 86%, 81%, 71%, 65%, 58% and 48%, respectively, and the ΔT of the traditional structure decreases to 65%, 60%, 45%, 35%, 25% and 15%, respectively ( Supplementary Fig. 12). This result further demonstrates the superiority of the tandem structure over the traditional structure from the aspect of contrast ratio. The 17% increase in the CE can be attributed to the second colouration stage. It also suggests that the main contributor to colouration in the tandem structure is the WO 3 layer, whereas the PEDOT:PSS mainly serves as an intermediary layer for the relay of protons and Na + . This is consistent with the results (Fig. 1) that without synergy between the three layers in the tandem structure, the contrast ratio is rather low.
A high-performance ECD is typically characterized by the switching time and contrast ratio (that is, the maximum ΔT between the on/off states). We summarize the contrast ratio and switching time for a number of representative WO 3 -based ECDs (Fig. 3f). The corresponding references are listed in Supplementary  Table 3. Proton-based devices (Fig. 3f, top right) have a fast switching speed and high contrast ratio. Recent advances in ECDs based on Al 3+ typically yield a high contrast ratio, but the switching speed is slow due to the diffusion of trivalent ions. ECDs based on Li + and Na + are usually not comparable with proton-based devices.

Scale up of the tandem-structure ECD
A major advantage of all-solid-state ECDs is the convenience of fabrication and encapsulation for large-scale applications. Traditional inorganic electrolytes like LiTaO 3 also suffer from low deposition rates that result in high cost of fabrication and low uniformity of devices. Here the polymer-based electrolytes in our tandem-structure design can be easily prepared and scaled up. To test the scalability, we fabricated 10 × 10 cm 2 windows based on the tandem structure. The photographs in Fig. 4a show the uniformity of colouration at different applied voltages as inspected by the naked eyes. The transmittance changes at 670 nm recorded at the centre and edge of the window are shown in Fig. 4b. The full spectra are shown in Supplementary Fig. 13. In both regions, the transmittance decreases from about 90% to about 20% in 2.1 and 6.3 s. Figure 4b also compares the large-area devices based on both tandem structure and traditional structure without the PEDOT:PSS layer. The solar irradiance converted from the transmittance spectra is shown in Fig. 4c,d, demonstrating higher contrast ratio for the full solar spectrum. Besides, we also fabricated a 30 × 40 cm 2 window. The large-area devices were fabricated using the equipment shown in Supplementary Fig. 14. The PEDOT:PSS layer was obtained by spraying the PEDOT:PSS dispersion through a spray nozzle. Then, the film was dehydrated at 80 °C for 12 min. As shown in Fig. 4e, uniform colouration can also be achieved (more detailed results are given in Supplementary Fig. 15).
The tandem structure discussed above could be easily adapted to make flexible ECDs. By replacing the brittle ITO layers with Ag nanowires (NWs), the whole device can be fabricated on a polyethylene terephthalate (PET) film. The Ag NWs were transferred onto the PET film by a blade-coating method. The WO 3 , PEDOT:PSS and SPE layers between the electrodes can be similarly coated (Methods shows the detailed processes). The flexible ECD of 10 × 10 cm 2 under the on and off conditions is shown in Fig. 4f. The device works well when being bent, suggesting great potential for flexible applications of the all-solid-state tandem-structure ECD.

Conclusions
We have shown that PEDOT:PSS can be used as a solid-state proton source for electrochromism in WO 3 , which enables the development of an all-solid-state design of proton-based ECDs. To effectively extract protons from PEDOT:PSS, which is critical for enhancing the contrast ratio of the ECDs, we designed a tandem structure of SPE/PEDOT:PSS/WO 3 . This creates a relay process of Na + and protons in the PEDOT:PSS layer: that is, Na ions from the SPE layer are used to pump more protons from PEDOT:PSS to WO 3 . Our ECDs can achieve ultrafast responses (colouration to 90% in 0.7 s and bleaching to 65% in 0.9 s and 90% in 7.1 s), high contrast ratios (about 90%), good CE (109 cm 2 C −1 at 670 nm) and excellent cycling stability (<10% degradation of contrast ratio after 3,000 cycles). We also demonstrated that this approach can be scaled up by fabricating large-area ECDs (up to 30 × 40 cm 2 ) with encouraging performance. A two-stage electrochromism was observed in the tandem structure, which is dominated by proton insertion to WO 3 and Na + insertion to PEDOT:PSS. This two-stage ion insertion helps in enhancing the contrast ratio of the ECD, and could also be of use in applications such as information displays and triple-state optical devices.
Material and device characterization. The ultraviolet-visible transmittance/ absorption spectra were measured using a Hitachi U-4100 spectrophotometer. The Fourier transform infrared spectra were measured using a Thermo Fisher Scientific Nicolet iS10 spectrophotometer. Cyclic voltammograms and electrochemical impedance spectra were measured using a Zahner IME6 electrochemical workstation. Wetting contact angles were measured using a JY-82A contact-angle meter. The ECDs were analysed by an SEM using a Hitachi SU8220 equipped with an energy-dispersive X-ray spectrometer from Oxford X-Max N .
Preparation of WO 3 . The amorphous WO 3 layer was deposited by reactive d.c. magnetron sputtering for 30 min at a power of 70 W, Ar/O 2 ratio of 94:6 and pressure of 2 Pa.
Preparation of SPE. NaClO 4 /PC (1 mol l -1 ), ferrocene and TTA21 in a ratio of 1:0.1:1 g were mixed with 2 ml propylene glycol methyl ether acetate and then stirred for 30 min. After that, 0.01 g iodonium salt (1,2-dimethyl-3-propylimidazolium iodide) was added into the mixture and stirred for another 5 min. The pristine electrolyte precursor was a gel-like mixture (2 ml), which was spin-coated (1,000 r.p.m. for 20 s) on an ITO/quartz electrode and then capped by another electrode. After curing for 30 s by a 300 W ultraviolet lamp, the electrolyte is solidified, which simultaneously binds the two ITO layers. Here NaClO 4 dissolved in PC was used as the Na + source. Ferrocene was used as a reducing (charge-compensating) agent in the electrochromic process. TTA21 and iodonium salt (Adamas) were used as the ultraviolet-curing monomer and initiator, respectively. Preparation of PEDOT:PSS. The PEDOT:PSS dispersion was spun onto the bottom ITO/WO 3 electrode at 1,000 r.p.m. for 60 s followed by 3,000 r.p.m. for 20 s. Then, the substrates were heated to 80 °C at a rate of 10 °C min -1 followed by annealing under a vacuum condition for 12 min.
First-principles calculation. The first-principles calculation was implemented in the Vienna ab initio simulation package 43 . Projector-augmented wave potentials were used to describe the core-valence interaction 44 . Plane waves with kinetic energy up to 408 eV were used as the basis set. The Perdew-Burke-Ernzerhof (PBE) functional 45 was used in the calculation. All the structures were relaxed until the force on each atom was less than 0.03 eV Å -1 and then the energy was calculated. Structural models are described in Supplementary Fig. 1.

Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.