PEDOT appears opaque in the oxidized state and transparent in the reduced state. Fig. 1 (a) presents the transmittance (T) spectra from 2.5 to 25 μm of 150 nm-thick PEDOT film deposited on grid-patterned gold/DSP silicon obtained at every 0.1 V between +0.8 V and −0.8 V versus the Ag/AgCl (KClsat) reference electrode. The resulting spectra clearly show changes in transmittance as a consequence of the reduction and oxidation of the PEDOT film. As the electrode potential shifts from positive to negative voltage, the transmittance of infrared from 2.5 μm to 25 μm increases resulting from the progressive reduction of PEDOT from its doped state to its neutral state.
From the cyclic voltammograms of the PEDOT films shown in Fig. 1 (b), the forward scan of the PEDOT films leads to the doped state, and the backward scan of the PEDOT films leads to the neutral state, which is transparent in the IR region. An increase in the anodic and cathodic current of thicker film either pumps more ions into the film or extracts more of them from the film. The increased reaction activity of the PEDOT films eventually enhances the transmittance contrast ratio ((Tneutral-Tdoped)/Tneutral) in thicker film. Fig. 1 (c) shows the transmittance of the neutral and the doped states of the PEDOT film deposited on grid-patterned gold/DSP silicon with variations in thickness at a wavelength of 10 μm. When the thickness of the PEDOT film increased, the transmittance contrast ratio increased. However, the transmittance contrast (Tneutral-Tdoped) does not show a consistent tendency as seen in the inset of Fig. 1 (c). As the thickness of the PEDOT film increases, the transmittance contrast increases rapidly, it is maintained at more than 0.16 between 70 nm and 150 nm, and decreases. Since both the transmittance contrast and the transmittance contrast ratio are important for transmissive electrochromic devices, we chose a 150 nm-thick PEDOT film for the optimized device.
Figure 1 (d) shows thermal images of i) AR-coated germanium and 150 nm-thick PEDOT, ii) doped and iii) neutral, over grid-patterned gold/AR germanium on a heated KAIST logo. The temperature of the doped state and the neutral state of PEDOT film was 32.2 °C and 31.6 °C, while the background was 30.6 °C, and the temperature of the substrate without PEDOT film was 34.2 °C. As shown in the FT-IR transmittance spectra result (Fig. 1 (a)), thermal images also reveal that the neutral state of PEDOT film is more transparent than its doped state.
To minimize transmittance loss while maintaining the electrical conductivity, we adopted four different geometries of gold grid whose linewidths were 5 μm and 20 μm and line spaces were 200 μm and 500 μm. As expected, Additional file 1: Figure S1 (a), shows the linear correlation between the planar coverage of gold and transparency. The small deviation could be due to contamination of carbon residues and patterning error.
Transmittance has a positive correlation with open area ratio; sheet resistance also has a positive correlation with open area ratio. To find out the effect of the sheet resistance, a double potential step chronoamperometric experiment was performed (E1 = +1.0 V, E2 = −1.0 V versus Ag/AgCl(KClsat) reference electrode; t1 = t2 = 30 s). The current versus time profile is shown in Additional file 1: Figure S1 (b). The sheet resistance of the gold grid was lower, and the electrochemical insertion and desertion of ions into PEDOT were saturated faster.
The grid-patterned gold is transparent and electrically conductive; however, it has low durability due to the metal decomposition during doping and undoping of PEDOT as shown in Fig. 2 (c). Therefore, we introduced grid-patterned PEDOT as an ion storage layer. The grid-patterned PEDOT was electrochemically polymerized on pre-patterned grid gold where electrochemical polymerization occurred. Fig. 2 (a) and Additional file 2: Figure S2 show AFM and SEM images of the grid-patterned gold before electrochemical polymerization and after electrochemical polymerization. Because the PEDOT was only polymerized on the grid-patterned gold part, the PEDOT on the grid-patterned area does not seem to have had much effect on the transmittance of the device as shown in Fig. 2 (b). After grid-patterned PEDOT deposition, the device showed stable cyclability, and the transmittance contrast rate was saturated at 80 %. All things considered, including open area ratio, which determines the thickness of the grid-patterned PEDOT, transmittance, conductivity, and response time, we adopted grid-patterned gold with line widths of 20 μm and line spaces of 500 μm as an electrode.
To maximize the transmittance contrast of the device, it is important to use a transparent electrolyte. Additional file 3: Figure S3 shows the transmittance of silicon/electrolyte/silicon the thickness of the electrolyte was varied, which was done using a spacer (2, 10, and 250 μm). When the thickness of the electrolyte was 250 μm, less than 5 % of IR light (λ = 10 μm) could penetrate; however, 60 % of IR light penetrated when the thickness was lower than 10 μm. The thickness of the electrolyte was fixed at 10 μm to maximize the transmittance while preventing electrical short circuits that can be caused when the thickness of the electrolyte is less than 10 μm.
We made two types of devices; one had an asymmetrical configuration, and the other had a symmetrical configuration. The asymmetrical device consisted of 150 nm-thick PEDOT film as a working electrode and grid-patterned PEDOT as a counter electrode; the symmetrical device consisted of two 150 nm-thick PEDOT films as a working electrode and a counter electrode. The spectra were measured using FT-IR at the voltages of +2.5, 0, and −2.5 V. Fig. 3 (a) shows the transmittance spectra of the neutral state (−2.5 V), the slightly doped state (0 V), and the doped state (+2.5 V) of the asymmetrical device, and Fig. 3 (b) shows the transmittance spectra of the opaque state (0 V) and the transparent state (+2.5 V and −2.5 V) of the symmetrical device. The transmittance contrast and transmittance contrast ratio of the symmetrical device were poorer than those of the asymmetrical device as shown in Additional file 4: Figure S4. The PEDOT film of the asymmetrical device can affect the device transmittance without any disturbance of the counter electrode. However, in the case of the symmetrical device, when one side is doped/neutral, the other side is neutral/doped, and the overall transmittance does not change.
Figures 3 (c) and (d) show thermal images of the neutral state (−2.5 V) and the doped state (+2.5 V), respectively, of the IR electrochromic device above a heated bulb. The temperature of the bulb is 40 °C. When the device was in the doped state, it blocked most of the IR light, so the heat source was measured, and its temperature was found to be 31.1 °C, which differs by only 0.9 °C compared to the temperature of the background. On the other hand, the bulb temperature was 34.0 °C when the device was in the neutral state, which makes it possible to clearly distinguish the heat source from the background using the thermal image. Supporting movie file (Additional file 5) also shows the transmittance change of the device. To measure the kinetics of the device, the transmittance changes of the device, shown in Fig. 4, were monitored using real-time FT-IR at the same time as a square wave voltage (−2.5 V to +2.5 V) was applied to the device. The doping and undoping times (to 90 % of equilibrium value) were 1.4 s and 23 s, respectively. The slower switching time of the undoping reaction compared to that of the doping reaction is consistent with those observed in other reports [28–31].