Reflective and Complementary Transmissive All‐Printed Electrochromic Displays Based on Prussian Blue

By combining the electrochromic (EC) properties of Prussian blue (PB) and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), complementary EC displays manufactured by slot‐die coating and screen printing on flexible plastic substrates are reported. Various display designs have been realized, resulting in displays operating in either transmissive or reflective mode. For the transmission mode displays, the color contrast is enhanced by the complementary switching of the two EC electrodes PB and PEDOT:PSS. Both electrodes are either exhibiting a concurrent colorless or blue appearance. For the displays operating in reflection mode, a white opaque electrolyte is used in conjunction with the EC properties of PB, resulting in a display device switching between a fully white state and a blue‐colored state. The developments of the different device architectures, that either operate in reflection or transmission mode, demonstrate a scalable manufacturing approach of all‐printed EC displays that may be used in a large variety of Internet of Things applications.


Introduction
Smart windows have grown in interest for their ability to reduce heating and cooling in buildings with large glass façades, thereby enhancing the energy efficiency and comfort as well as decreasing the environmental impact of a building. [1,2] Many different technologies are utilized in the development of smart windows, such as the electro-optical switching of liquid crystals, thermochromism, and electrochromism. [3] Electrochromic (EC) materials and devices present an attractive alternative due to their potential low-cost manufacturing and their low energy consumption. Due to their advantages, EC devices have also found use as smart labels for Internet of Things (IoT) and wearable electronics. [4][5][6][7] Extensive research has been performed in the field of electrochromics to enhance the color contrast or alter the color hue of the resultant EC device. Researchers have adjusted the architecture of the devices, chemically modified the individual EC materials, and introduced various performance-enhancing additives to achieve these goals. [8][9][10][11] Another research approach in the field of electrochromics is the manufacturing of displays via printing and coating processes. [5,[12][13][14][15] Utilizing such deposition techniques to fabricate EC displays may enable commercialization due to the allowance of a drastically lowered cost per device. Techniques such as inkjet printing, slot-die coating, and screen printing have been prevalent in the scientific literature to produce all-printed EC devices. While inkjet printing allows enhanced resolution and is a proven technology, slot-die coating and screen printing allow higher throughput and have the most potential when considering large-scale production. Slot-die coating allows for the continuous deposition of a material using a heavy metal die with a shim inside. The shim can be cut to allow large line patterns, but more complex patterns are difficult to achieve with slot-die coating. On the contrary, screen printing is a common printing technology that allows fast deposition of patterns at relatively high resolution (%50-100 μm). Additionally, screen printing allows for accurate alignment of subsequently deposited layers, which is a prerequisite in the manufacturing of all-printed multilayered functional devices. Both screen printing and slot-die coating can be based on either sheet-by-sheet or roll-to-roll processing, allowing for production runs ranging from simple prototyping to pilot production. These techniques may be used to deposit a variety of EC materials, whereby Prussian blue and poly (3,4- 6 ] 3 )) is a well-known inorganic material with a long history within the field of electrochromism. [16] PB and its analogues [17,18] represent a class of complex compounds with interesting EC properties. [19] PB forms an octahedral coordination framework, in which the Fe 2þ and Fe 3þ cations are bridged by cyanide (CN À ) ligands. In this cubic lattice structure cations, DOI: 10.1002/adem.202201299 By combining the electrochromic (EC) properties of Prussian blue (PB) and poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), complementary EC displays manufactured by slot-die coating and screen printing on flexible plastic substrates are reported. Various display designs have been realized, resulting in displays operating in either transmissive or reflective mode. For the transmission mode displays, the color contrast is enhanced by the complementary switching of the two EC electrodes PB and PEDOT:PSS. Both electrodes are either exhibiting a concurrent colorless or blue appearance. For the displays operating in reflection mode, a white opaque electrolyte is used in conjunction with the EC properties of PB, resulting in a display device switching between a fully white state and a blue-colored state. The developments of the different device architectures, that either operate in reflection or transmission mode, demonstrate a scalable manufacturing approach of all-printed EC displays that may be used in a large variety of Internet of Things applications.
such as Li þ , Na þ , or K þ , can be intercalated at interstitial positions for charge compensation. [20] PB as an anodically coloring EC material switches reversibly from a blue Fe 3þ /Fe 2þ mixed-valence state to a completely colorless, fully reduced Fe 2þ /Fe 2þ state (Prussian white, PW, or Everitt's salt) at negative voltages. Partial oxidation of PB results in Prussian green (PG) and complete oxidation forms Prussian yellow (PY), in which all iron centers are in the Fe 3þ state. The oxidation to PG and PY is not fully reversible, which results in a loss of cycling stability. Thus, in EC devices PB thin films are commonly switched between PB and PW. [21,22] PB thin films can be deposited via hydrothermal methods [23] or wet-chemical processes, such as electrodeposition, [24,25] spin coating, [26,27] inkjet printing, [28] or roll-to-roll slot-die coating. [14,15] Besides the application in EC devices, where PB is usually used as the anode material, it can be also considered as a cathode material for energy storage devices, e.g., sodium-ion batteries, [29,30] due to the similar working mechanisms to lithium-ion batteries. Furthermore, PB thin film electrodes have been investigated for cholesterol [28] and glucose sensing applications. [31] PEDOT is a conductive polymer which, when combined with PSS, forms a polymeric complex that possesses the ability to become easily dispersed in water. The latter property has led to its commercialization and implementation in many optoelectronic applications. Once dispersible, PEDOT:PSS can be developed into inks for utilization in various printing methods such as inkjet printing, slot-die coating, and screen printing. Therefore, PEDOT:PSS has become the most common conductive polymer within the fields of organic and printed electronics. [32] Numerous applications relying on printed PEDOT:PSS have been reported, including transistors, [33] sensors, [34] solar cells, [35] organic light emitting diodes, [36] supercapacitors, [37] and EC displays. [5,[38][39][40] The combination of PB and PEDOT:PSS allows for a complementary EC device. [41,42] As PB becomes colorless in its reduced state and blue colored in its oxidized state (anodically coloring), and PEDOT:PSS exhibits the opposite EC behavior (cathodically coloring), the display device exhibits high color contrast EC switching between an almost colorless state and a deep bluecolored state upon alternating the polarity of the applied voltage. This device is well suited for both smart window and smart label applications. Within this report, we use ITO-coated plastic substrates for roll-to-roll slot-die coating of PB, followed by sheet-based screen printing to complete the EC displays according to both transmissive and reflective designs. Hence, the combination of PB and PEDOT:PSS in flexible all-printed EC displays is demonstrated for the first time. The all-printed EC devices combining PB and PEDOT:PSS showed excellent properties with high color contrast values and good stability with respect to both color retention and cycling. The success of the EC devices described herein highlights the ability to produce excellent performing devices using printing technology, thereby demonstrating a potential large-scale manufacturing approach to obtain a cost-effective large-area display technology.

Results and Discussion
The manufacturing approach relies on the combination of slot-die coating and screen printing. The processing steps and the formation of all-printed EC devices are outlined in Section 4 (Electrochromic Display Manufacturing) and the slot-die coating and screen printing processes are illustrated in Figure 1.

Electrochromic Displays Operating in Reflection Mode
The manufacturing approach and materials composition of the reflective displays are similar to the conventional screen printed EC displays that have been developed by RISE. [43,44] The differences are that PEDOT:PSS is replaced by PB as the EC electrode and due to the poor electrical conductivity of PB, [45] an underlying ITO coating is also required to facilitate electron transport. The layers incorporated into the all-printed reflective EC display can be seen in Figure 2A. Electrochemical reduction of PB results in a fully colorless film, while the oxidized state changes the optical absorption characteristics and transforms the color of the thin film to light blue. EC displays based on PEDOT:PSS exhibit a residual faint blue color in the oxidized state; [46] hence, the fully colorless state of PB is of particular interest in optoelectronic applications. Spectroelectrochemical characterization data of the roll-to-roll fabricated PB thin films on ITO/PET are added to the Figure S1, Supporting Information. A white opaque electrolyte is used to hide the counter electrode, which implies that the display device exhibits a blue and white color when PB is switched to its oxidized and reduced state, respectively (see Figure 2B). The color contrast of the reflective displays was recorded in 1 cm 2 pixels manufactured according to the same Figure 1. Photographs showing the manufacturing processes. A) and B) Roll-to-roll slot-die coating of PB onto ITO/PET substrates (coating width: 500 mm), followed by C) screen printing to complete the all-printed EC displays. It should be noted that the screen printing process involves several layers (insulator, electrolyte, complementary EC electrode or counter electrode, and silver as an optional layer).
www.advancedsciencenews.com www.aem-journal.com device architecture, which resulted in ΔE* values exceeding 33. Conventional PEDOT:PSS-based reflective EC displays screen printed on PET substrates typically show ΔE* values lower than 30. [47] This is to some extent contradictory because PEDOT:PSS is switched to a deeper blue color in comparison with PB. However, the high color contrast obtained in the reflective PBbased EC devices reported here is explained by the colorless feature of PB switched to its reduced state, which visually is observed as a fully white-colored state due to the white and opaque electrolyte in the background. The ability to fully hide the segment content ("OK") in the OFF state ( Figure 2B) is an additional feature that is difficult to obtain in PEDOT:PSS-based EC devices. This is explained by that the pristine color of a PEDOT:PSS-based EC display originates from a semioxidized state, while EC switching from the reduced state typically results in a deeper oxidation state, and hence a brighter color, as compared to the color of the semioxidized PEDOT:PSS surrounding the EC display segments. Figure 2C,D show the current versus time characteristics when switching the PB in a rectangular 2.3 Â 7.9 mm 2 display segment to the oxidized (colored) and reduced (colorless) state, respectively. The voltage used to switch this display segment was þ/À2, 2.5, or 3 V. From a cycling stability perspective, it is advantageous to keep the applied voltage at a minimum level to avoid side reactions, however, there is also a trade-off with other performance parameters, e.g., switching time and color contrast. The carbon that serves as the counter electrode in the reflective displays exhibits lower charge capacity in comparison with counter electrodes based on conducting polymers, which explains the relatively high voltages that were applied in this test. This is also reflected in the values of the charge required for each color change; switching at þ/À2.5 and 3 V doubled and tripled the charge consumption, respectively, as compared to the charge consumed at þ/À2 V. Note that the EC displays reported herein are nonencapsulated, and the additional charge consumption is therefore caused by elevated parasitic reactions due to the water content of the electrolyte. Hence, the higher voltages will in this case not result in higher color contrast, only a slightly shorter switching time at the expense of operational lifetime.

Electrochromic Displays Operating in Transmission Mode
Various attempts on obtaining a transmissive display have been carried out in this study. The common theme for these attempts has been to develop an electrolyte formulation that is printable, curable, and transparent. A polyanionic electrolyte formulation, based on poly(sodium 4-styrenesulfonate) (PSS), resulted in The display segment is switched between its oxidized OFF state (left) and reduced ON state (right). ΔE* color contrast values higher than 33 were observed in the reflective display architecture. A movie of this display illustrating the color change can be viewed in the Supporting Information (Movie S1). C) Current versus time plots (logarithmic scale in the inset) when switching the PB in a rectangular 2.3 Â 7.9 mm 2 display segment to the oxidized state. The different curves were obtained by applying three different voltages (À2, À2.5, and À3 V), and the required charge for each EC transition is also indicated. D) The same display segment upon using the opposite voltage polarity, to enable EC switching to the reduced colorless state.
www.advancedsciencenews.com www.aem-journal.com all-printed EC displays. However, although possessing good printability, this electrolyte was exhibiting a hazy appearance deemed unsuitable for smart window applications. Despite being considered a failure, the switchability of this device type is shown in Figure S2 (see Supporting Information), wherein the oxidation state is alternated through the 7-segment display. Another attempt to obtain a transparent electrolyte was based on the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate (EMIM ES). While this electrolyte was acceptable with respect to transparency, unfortunately, its printability was poor and therefore unacceptable ( Figure S3 in the Supporting Information).
The next approach was instead relying on a transparent polycationic electrolyte formulation, which resulted in successful screen printing of transmissive EC displays operated in a complementary manner. Figure S4 in the Supporting Information, shows the cyclic voltammetry data of PB and PEDOT:PSS when using this electrolyte formulation, and Figure S5, Supporting Information, shows the impedance spectroscopy data; the latter also revealed an electrical conductivity of 7.4 mS m À1 for the transparent electrolyte ink formulation. Figure 3 shows photographs ( Figure 3A) and a schematic ( Figure 3B) of these displays, along with the values obtained from various color contrast measurements. Figure 3C shows the color contrast when sweeping the voltage applied to a transmissive display. The dark state is obtained by sweeping the voltage applied to the PEDOT:PSS electrode from 0 to À2 V (black curve), while the colorless state is attained by reversing the polarity of the voltage. The measurement is split into two separate sweeps, both starting at 0 V, and the deviation at this voltage is explained by the difficulty to reach exactly the same oxidation state in the subsequent sweep. A voltage window of 4.8 V (À2 to 2.8 V) was used in this measurement, which resulted in a maximum ΔE* of around 30. The graphs also reveal that the blue-colored and colorless states are reached at around À1.5 and 2 V, respectively. However, an even higher ΔE* of around 35 was obtained in a display with a 1 Â 1 cm 2 segment area upon increasing the voltage window to 6 V (þ/À3 V). Figure 3D shows the ability of the transmissive display to maintain its oxidation state as a function of time. The display was initially switched to either its colored state (À1.5 V applied to the PEDOT:PSS electrode) or its colorless state (2 V applied to the PEDOT:PSS electrode), and the ΔE* values were then recorded for the subsequent 10 min while keeping the display in open-circuit mode. The colored state of both curves fades by 10-15% during the first two minutes, but the color retention was then stabilized for the remainder of the measurement. The decay from the colorless state toward the blue-colored state was approximately 24% after 10 min, while the ΔE* color contrast of the initially blue-colored state only dropped 14% during the same time.  The conventional reflective PEDOT:PSS-based EC display from RISE typically shows a maximum ΔE* value of 30, [47] depending on various process parameters. Therefore, ΔE* around 35 for the transmissive displays based on PB and PEDOT:PSS reported herein is considered very promising. However, it should also be noted that it is difficult to compare these two display types because they are operating differently. The reflective display relies on the whiteness of the electrolyte used as the background, and its L*, a*, and b* color coordinates measured 94, À0.8, and 0.3, respectively. The standard copy paper used as the background in the color contrast measurements of the transmissive display showed similar L* and a* values, but the b* coordinate showed a value of almost À10, hence, the copy paper is slightly blue-colored. Having this in mind, the ΔE* values of the transmissive displays reported herein should be even higher by using a whiter background instead of the copy paper. Independent of the true ΔE* value, the transmissive displays clearly show that sufficiently high transparency has been achieved in the screen-printed electrolyte layer as well as in the complementary EC electrode materials based on PB and PEDOT:PSS. This is further verified by the spectroelectrochemical data of the PB ( Figure S1 in the Supporting Information) and PEDOT:PSS ( Figure S6, Supporting Information) EC electrodes, the spectroelectrochemical data of the all-printed transmissive EC display including the transparent electrolyte ink formulation sandwiched by the PB and PEDOT:PSS electrodes ( Figure S7, Supporting Information), and the spectroscopical data of the transparent electrolyte ink formulation ( Figure S8, Supporting Information).
Electrical measurements reveal several important parameters, e.g., manufacturing yield, switching time, and energy consumption. The measurements were performed by recording the current versus time data upon applying different voltages to the 2.3 Â 7.9 mm 2 segments of the bar graph display shown in Figure 3A, and the results are shown in Figure 4. The current versus time characteristics (Figure 4A,B) as well as the current versus voltage characteristics displayed in Figure 4D show almost identical switching behavior for all segments upon applying the respective voltage level. Such reproducibility indicates that this type of EC display can be reliably manufactured at a high yield. Figure 4A reveals information about the switching time versus the applied voltage upon switching the segments to the colorless state. The segments are fully decolored at the transitions occurring after approximately 1-2 s in all graphs. For the black curves (at 2 V), the transitions occur at approximately 1.5 s, while the switching time of the red curves (at 1.75 V) is shifted toward 2 s. The charge consumption at 2 V is approximately 20% higher as compared to the measurements at 1.75 V, which is an indication of minor parasitic reactions. Figure 4B shows the current versus time characteristics upon switching the segments to their blue-colored state at two different voltages. Here, the curve shape is almost identical for all segments, independent of the applied voltage, and this is further indicated by the fact that the average www.advancedsciencenews.com www.aem-journal.com charge consumption only differs by %10% when comparing the two voltages. Figure 4C gives an indication of the cycling stability of these display devices. One of the segments in this display was cycled for 200 min by applying þ/À1.5 V at a frequency of 0.4 Hz and 50% duty cycle, which corresponds to 4800 full switching cycles. No degradation was observed through visual inspection, and the electrical measurements support the same conclusion, both in terms of curve shapes and charge consumption. It should though be noted that there is a difference indeed when comparing the black and red decoloration curves at approximately 4 s. The reason for this deviation is yet to be understood. However, as this occurs toward the end of the measurements, at low-current levels, neither the switching time nor the charge consumption is affected. By comparing the switching characteristics of this transmission mode displays with conventional PEDOT: PSS-based displays operating in reflection mode, the displays containing PB electrodes show relatively higher current levels after reaching either the fully reduced or the fully oxidized state, which in turn implies increased energy consumption. Despite the increased energy consumption (%5 mJ cm À2 (at 2 V) versus <2 mJ cm À2 for conventional reflective PEDOT:PSS-based EC displays), the complementary EC displays developed herein are still of high interest because they enable all-printed EC displays in applications requiring transmission mode operation. The most plausible explanation for the increased energy consumption is that the ITO in the devices reported herein takes part in the electrochemical reaction. [48] The conventional PEDOT:PSS-based ECDs do not contain any ITO, they instead rely on the conductivity of the PEDOT:PSS and are therefore considered less sensitive to high voltages. But replacing PEDOT:PSS with PB also brings the requirement of ITO to support the electronic conductivity, and the voltages applied in these EC displays are most often beyond the onset of ITO reduction, which is an irreversible and undesired side reaction that eventually results in degradation and transformation of the ITO layer into a brown color. Such side reaction is even more probable in a humid environment, and as these EC displays are nonencapsulated, yet operated in an ambient environment, water will for sure be present within the devices.
To further emphasize on the transparency of the electrolyte, a white paper with a printed rainbow was used as the background. Complementary transmissive EC displays with an area of 1 Â 1 cm 2 were used to resemble smart windows, and the display segments were then switched between their colorless and blue-colored states at voltages of þ/À3 V (see Figure 5). The whiteness of the background, in addition to the vivid colors of the rainbow, observed when the display is switched to its colorless state underlines the high transparency of the electrolyte layer. The perceived color of the dark state is formed by a superposition of reduced PEDOT:PSS and oxidized PB, i.e., electroactive materials both exhibiting a blue tint.

Conclusion
The reported work demonstrates the possibility to manufacture all-printed EC displays operating in either reflection mode or by complementary switching in transmission mode using solutionprocessing methods. PB, which is one of the EC electrodes, is deposited via roll-to-roll slot-die coating onto ITO/PET, and the other materials (insulator, electrolyte, counter electrode, silver-based bus bars), required to complete the EC display architectures, are deposited via screen printing. The mode of operation is defined through the selection of the active materials and by the transparency of the electrolyte, thereby creating a wide range of possible EC display and smart window applications by using the same manufacturing approach.

Experimental Section
Materials: Indium tin oxide (ITO)-coated PET films were purchased from Eastman Chemical Company (Flexvue OC50, sheet resistance: The aqueous PB nanoparticle ink was synthesized according to a literature procedure. [49] The UV-curable insulating ink was purchased from Marabu (UVAR). The two electrolyte compositions (VV001 and TE001) used in the evaluated EC displays were provided by RISE, they are both based on polyelectrolytes with quaternized nitrogen atoms. VV001, which was used in the reflective EC displays, also contains TiO 2 as the opacifier. [50] Initially, two other electrolytes were also tested in the development of EC displays operating in transmission mode: poly(sodium 4-styrenesulfonate) (PSS, M W ¼ 70 000) and the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate (EMIM ES), both purchased from Merck Sigma-Aldrich. The PEDOT:PSS ink was purchased from Heraeus (Clevios S V4). The carbon ink (7102) and the silver ink (5000) were both purchased from DuPont.
Electrochromic Display Manufacturing: Flexible plastic films coated with a uniform layer of ITO were used as transparent and conductive substrates for the reported display devices. The PB nanoparticle ink was deposited onto ITO-coated PET using a roll-to-roll slot-die coating process [14,15] to form a uniform and blue-colored PB thin film. The width and the charge capacity of the coated PB thin film were 500 mm and approximately Figure 5. All-printed EC transmissive displays exhibiting A) a blue-colored (dark) state and B) a colorless (bright) state. A rainbow printed on a paper is used as the background. A movie of this display switching between the two oxidation states can be viewed in Movie S2 in the Supporting Information.
www.advancedsciencenews.com www.aem-journal.com 4.5 mC cm À2 , respectively. To ensure good adhesion on the ITO surface, the film was annealed at 100°C after coating. The other materials (insulator, electrolyte, counter electrode, silver-based bus bars) required to complete the EC display architectures were deposited on top of the PB layer by using a flatbed, sheet-fed, screen printing equipment (DEK Horizon 03iX). Photographs of the roll-to-roll slot-die coating line and screen printing equipment can be found in the Figure S9, Supporting Information. The screen printing tools were based on standard polyester mesh, and the pattern of the screen mesh design had the dimensions of %24 Â 40 cm. An insulating layer was screen printed onto the PB, followed by UV-curing. The insulating layer prevents short circuits between the EC electrodes and openings in the insulating layer define the active areas of the EC display segments. Two different electrolyte compositions were developed and provided by RISE. The electrolyte composition TE001, which is transparent, is utilized in the transmission mode displays. The electrolyte composition VV001, which is white and opaque and commonly used in the manufacturing of conventional screen printed PEDOT:PSS-based EC displays, [43,44,50] is utilized in the reflection mode displays to hide the counter electrode layer. The electrolyte layer, either based on the transparent or the opaque electrolyte formulation, was subsequently printed into the openings of the insulating layer and then UV-cured. The electrolyte provides the ions, quaternized polycations, and chloride anions, required to achieve the EC display functionality. [50] A counter electrode layer, i.e., PEDOT:PSS for the transmission mode displays or carbon for the reflection mode displays, was screen printed on top of the electrolyte layer, followed by thermal curing at 120°C for 3 min. Hence, each EC segment is created by two complementary EC electrodes (PB and PEDOT:PSS) or by one EC electrode (PB) and one counter electrode (carbon), sandwiching the patterned electrolyte layer. Optionally, silver-based conductors and contact pads were screen printed and thermally cured to lower the total resistance of the resulting display devices.

Characterization of Electrochromic Electrodes and Displays:
The EC displays were characterized in different ways. The color contrast of the display segments has been measured by using a spectrophotometer (Mercury from Datacolor). This gives the CIE L * a * b* color coordinates as well as the ΔE* color contrast value when comparing the different oxidation states of the EC displays; Equation (1) is used to calculate ΔE*, where the index c and b represent the colored and the bright state, respectively Using this color coordinate system is a standard method in the graphical industry, and it is therefore applied when reporting color contrast in reflective displays. [51] The same approach was used to determine the color contrast in the transmissive displays. A white paper was utilized as the background color in the transmissive displays, instead of the white opaque electrolyte used in the reflective displays.
Characterization of the current versus time switching behavior was performed by using a semiconductor parameter analyzer (HP/Agilent 4155B). The electrical characterization gives an indication of the display manufacturing yield, switching time, and energy consumption. All measurements were performed at a temperature of %20°C and at a relative humidity (RH) of %45-50%RH. Optical microscopy has been used to observe issues related to the manufacturing process, e.g., dewetting of printed inks in the multilayered structures.
The cyclic voltammetry scans of the PEDOT:PSS and PB electrodes ( Figure S4, Supporting Information) were obtained by using a potentiostat (OctoStat 5000 from Ivium), Ag/AgCl was used as the reference electrode and a platinum wire was used as the counter electrode. The polyelectrolyte ink formulation used in the all-printed EC displays was deposited on top of the working electrodes in these measurements.
Impedance spectroscopy ( Figure S5, Supporting Information) was conducted using a potentiostat (Biologic SP-200), which was connected to a lateral device comprising a layer of the printed transparent electrolyte bridging two printed carbon electrodes, as reported elsewhere. [6] The thickness of the printed electrolyte layer was assessed with an optical profilometer (Sensofar PLu neox 3D) and an evaluation unit for data acquisition (Heidenhain Evaluation Electronics ND287).
Spectroelectrochemical ( Figure S6 and S7, Supporting Information) and spectroscopical ( Figure S8, Supporting Information) experiments were conducted using a setup that combined a fiber optic absorption spectrometer (AvaSpec) and a potentiostat (Biologic SP-200). In Figure S6, Supporting Information, a platinum wire was used as the counter electrode in the spectroelectrochemical cell and Ag/AgCl was used as the reference electrode. Together with the PEDOT:PSS working electrode, all three electrodes were engulfed by a 0.01 M phosphate buffer in H 2 O. FTO-coated glass, purchased from Redox.me, was used as the substrate material for this spectroelectrochemical experiment.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.