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From the Bottom Up – Flexible Solid State Electrochromic Devices Jacob Jensen and Frederik C. Krebs* Optoelectrical devices that alter their absorption spectrum under application of a suitable electric potential supplied by an external power source (battery, solar cell, etc.) are known as electrochromic devices (ECDs), a subgroup of electrochemical cells.[1] Fundamentally, in electrochemical cells the change in external potential across the cell leads to oxidation or reduction reactions in response to perturbing the equilibrium state of the cell. In electrochromic devices these redox reactions are perceived optically as color changes, by shifting the absorption from within to outside the visible region of the electromagnetic spectrum. This resulting on/off effect where the device is colored in one redox state while being transparent in the other is applicable to devices such as dimmable windows, displays, goggles or various forms of signage.[1–8] Examples of commercially available electrochromic (EC) technology includes Boeings Dreamliner where the manually operated window blinds has been substituted by EC technology allowing the passenger to block out sunlight by the push of a button.[9] Electrochromic window glass for use in housing are likewise commercially available, but the use of this is so far restricted to corporate office buildings.[4,10–12] Common to these applications are the use of WO3 as one of the electroactive species. WO3 is the most thoroughly studied inorganic EC species and due to its reliability it remains the most commercially successful EC technology to date.[13,14] Electrochromic devices based on conjugated polymers (CPs) are a maturing EC technology that is commercially interesting due to several favorable qualities. Electrochromic polymers (ECPs) are found in a wide variety of colors, which broadens the applicability in devices such as large area displays, signs, architectural elements and optical filters in addition to the established window applications.[15] Furthermore, ECP based devices have low power consumption during operation due to low potential requirements for oxidation/ reduction and an optical memory, whereby devices remain in a given redox state for an extended period of time when taken to open circuit.[1] From a manufacturing perspective the solubility of ECPs in common organic solvents, due to the incorporated alkyl side chains, is of prime importance. Through simple laboratory techniques like spin- and spray-coating, thin electrochromic films (50–300 nm) are realizable. However, in order to

Dr. J. Jensen, Prof. F. C. Krebs Department of Energy Conversion and Storage Technical University of Denmark Frederiksborgvej 399 DK-4000, Roskilde, Denmark E-mail: [email protected]

DOI: 10.1002/adma.201402771

Adv. Mater. 2014, DOI: 10.1002/adma.201402771

progress from research based experiments and take full advantage of ECP solubility, high throughput coating techniques such as screen printing and slot die coating is a requirement.[16,17] ECD structure and manufacturing is to a wide extent challenged by the electrolyte component. As it remains common practice to employ a semisolid adhesive gel electrolyte, fabrication of devices is limited to separately coating of the two electrodes before finalizing the device in a lamination step; a technical challenge in a simple R2R process and an impossibility in advanced R2R processes with 2D registration requirements. Without lamination, an adhesive electrolyte component is incompatible with device manufacturing by high speed R2R processing, as depicted in Figure 1A. Due to unwinding and rewinding of the flexible foil all layers are required to be solid, dry and non-sticky, which can be accomplished by in-situ curing of the electrolyte layer as shown in Figure 1B (step D). Other research groups have addressed the fabrication of solid state devices, and a promising strategy is based on polymerizing the ECPs inside a pre-cured electrolyte matrix, thereby creating a semi-interpenetrating network.[18–22] In this report we describe the structure and fabrication of flexible solid state ECDs, as an extension of our previous reports on ECD manufacture.[23–25] The key processing step in the novel devices is photo crosslinking of an acrylate based electrolyte solution immediately following slot die coating. Using a specially developed “curing chamber” mounted on a mini roll coater we show how solid state electrochromic devices can be manufactured continuously in one direction (i.e., from the bottom and up) using slot-die coating and flexographic printing; hereby considerably simplifying the manufacturing process. The work herein constitutes an important step in

Figure 1. Semisolid vs solid state electrolyte in ECD processing. A) Problems associated with roll to roll coating of a gel/semisolid electrolyte. The electrolyte solution is deposited through a slot die coating head, but due to adhesiveness it will stick to the drums and the foil during stretching and rewinding as marked by “!!!” (The electrolyte is shown as yellow for visualization) B) Successive deposition of layers continuously in one direction. The key step is photo-crosslinking of the electrolyte solution (step D).

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ionic liquid 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide was also slot-die coated (Figure 2D) and subsequently photo crosslinked by UV light. Due to atmospheric oxygen it was not possible to satisfactorily cure the electrolyte layer in an ambient atmosphere. This was solved by reducing the O2 concentration in the vicinity of the liquid electrolyte film, using the curing chamber depicted in Figure 3. The chamber consists of an aluminum chamber (15 × 12 × 9 cm3) wherein a UV light source is mounted. Two gas inlets supply inert gas, thereby reducing the oxygen concentration in the chamber and plastic “lips” mounted on the side of the chamber helps to minimize the gap between the drum and the chamber as shown in Figure 3. Figure 2. Structure and coating of a solid state electrochromic device. The individual layers Although a solid electrolyte is imperaare successively coated in the A to F direction. All the layers, except the top electrode (F), are tive during processing, a tradeoff between slot die coated as depicted in the upper left picture. The electrolyte layer (D) is slot-die coated followed by in-situ photo-curing as depicted in lower right picture. The top silver-grid electrode viscosity and response time is inevitable. (F) is deposited by flexographic printing as depicted in the upper right picture. The structure of Highly viscous electrolytes show reduced the primary electrochromic polymer is shown in the lower left side of the figure. ionic mobility and hence increased response times, as the ionic conductivity in polymeric electrolyte solution depends on the number of ionic species process compatibility for R2R coating of ECDs using fully scaland their mobility.[28,29] Polymeric electrolytes comprising able and additive methods that do not employ brittle materials such as indium-tin-oxide (ITO). ionic liquids are useful in ECDs owing to their high conducThe solid state devices were prepared by successively slottivity, low viscosity and a suitable electrochemical window in die coating of the individual layers (except the top and bottom which they are stable.[23,30–34] Furthermore ionic liquids can be grid electrodes that were prepared by flexographic printing) employed for their plasticizing properties, leading to higher as shown in Figure 2. As previously reported, homogenizing ion concentrations compared to electrolytes based on disthe electrical field is important to ensure an even and comsolved salts. Increasing the amount of IL resulted, as expected plete electrochromic switch in ECDs where thin silver grids in faster response times, up to a point (>50% w/w) where the are used as electrodes.[25,26] This was accomplished by coating mechanical properties (dryness, solidness, flexibility) were compromised. a conductive PEDOT:PSS layer on top of the silver grid foil The optical contrast and response time at 548 nm (Absmax) (Figure 2A). The primary electrochromic polymer Poly(3,3bis(((2-ethylhexyl)oxy)methyl)-3,4-dihydro-2Hthieno[3,4-][1,4] for a 1 cm2 solid state ECD switched between +1.0 V and –0.4 V dioxepine)-co(3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine-3,3-yl) with a pulse width of 40 seconds is shown in Figure 4A. bis(methylene)bis(2-methylacrylate) (methacrylate magenta), The devices switch between a neutral magenta state and of which a complete spectroelectrochemical analysis are found a transparent faint blue oxidized state that origins from the elsewhere, is coated on top of the conductive PEDOT:PSS reduced PEDOT:PSS film on the counter electrode. The layer (Figure 2B).[27] The methacrylate functionalities in this polymer are thought to account for an observed increase in mechanical stability of the photo-curable PEG-based electrolyte, since devices based on the homopolymer poly(3,3-bis(((2ethylhexyl)oxy)methyl)-6,8-dimethyl-3,4-dihydro-2H-thieno[3,4b][1,4]dioxepine-6,8-diyl) (ECP-magenta) lead to blisters in the electrolyte layer during processing of the following layers (see supplementary information Figure S1). Our assumption is that differences in surface energies between the alkoxy substituted homopolymer and the acrylate based electrolyte are causing mechanical stress leading to instability, and that introducing similar acrylate functionalities in the ECP relieve some of this stress. Following the primary EC layer is a layer of poly [(9,9-bis(3′-(dimethylethylammoniumbromido)propyl)Figure 3. In situ curing of gel electrolyte. A) UV-curing of the electrolyte. 2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene) (PFN – Figure 2C) The drum is mounted with foil and the UV-curing chamber is seen at the that enables sufficient wetting of the liquid electrolyte solution bottom of the picture. B) The UV-curing chamber seen from below. The (prior to crosslinking) onto the ECP film. A liquid electrolyte lamp is placed between two gas inlets (black plastic tubing), that supply solution comprised of polyethylenglycol diacrylate and the inert gas thereby reducing the oxygen concentration in the chamber.

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COMMUNICATION Figure 4. Optical contrast and response time for solid state electrochromic switched at +1 V/–0.4 V with a pulse width of 40 seconds A) unprotected devices B) devices protected in µbarrier foil. As the two samples are from the same batch, the lower optical contrast (31% vs 35%) observed for the protected samples are attributed to transmission losses in the barrier foil.

maximum optical contrast for the full device was found to be 35% (+/–1%), and the response time for 95% of a full switch was 25 seconds for reduction (coloring) as well as oxidation (bleaching). The applied potentials were chosen to be within the potential windows of the constituent polymers, as potentials outside of these have been shown to significantly reduce the optical contrast due to degradation.[35] However, it should be kept in mind that potential windows obtained by cyclic voltammetry can only be approximately applied to regular devices, as the former is obtained in a three electrode set-up relative to a reference electrode and the latter is a two electrode configuration. In this study it was likewise found that although increasing the potentials lead to faster response time, the operational stability of the devices was severely compromised. As described by several groups, inefficient charge consumption (i.e., charge that does not lead to electrochromic change) in an ECD may promote degradation of the polymer through processes such as overoxidation or nucleophilic attack involving the electrolyte.[35–38] To minimize inefficient charge consumption the pulse width was kept at 40 seconds as the absence of electrochromic response (i.e., the derivative of the transmittance vs. time is approaching zero) beyond 40 seconds of switching, might accelerate degradation. The average operational degradation rate for the novel solid state device was found to be 0.022% pr. double potential step (dps). In order to further increase the operational stability the devices were encapsulated in oxygenprotecting barrier foils similar to those previously employed for UV protection (Figure 4B).[39] This prolonged the lifetime by a magnitude compared to unprotected devices such that after 7500 dps. the devices showed an optical contrast of 16% (equivalent to 7 days of switching; operational degradation rate 0.002% pr. dps.) confirming that packaging the device using barrier material can significantly increase the operational stability. It seems reasonable to assume that oxygen (and/or moisture) reacting with the electrochemically charged polymers is the cause of the operational degradation since the minimum transmittance increases (i.e., the devices bleach), while the maximum transmittance remains largely unchanged. This can be explained by unprotected devices being more susceptible to irreversible oxidation (bleaching) by atmospheric oxygen during operation than protected devices.[23,39] Further supporting this line of thought is that after the initial decrease, the

Adv. Mater. 2014, DOI: 10.1002/adma.201402771

optical contrast somewhat stabilizes which could result from consumption of oxygen (without simultaneously replenishing) inside the protected devices, thereby prolonging the lifetime. In this report we have presented solid state electrochromic devices, manufactured by sequentially stacking layers in one direction using flexographic printing and slot-die coating methods. Building the devices from the bottom up was possible through in situ photo-crosslinking of the electrolyte layer in a curing chamber with oxygen concentrations reduced to a level suitable for radical polymerization of the electrolyte. The resulting devices showed an optical contrast (at Absmax) of 35%, and a response time of 25 seconds (95% of a full switch) at +1 V/–0.4 V. The devices showed an operational degradation rate of 0.02% pr. double potential step, which could be reduced to 0.002% pr. dps by packaging the devices in flexible barrier foil. It is to the best of our knowledge the first time that electrochromic devices have been manufactured continuously in one direction, eliminating the need for a lamination step and enabling fully additive roll-to-roll processes with 2D registration. The results presented here are important as a step towards efficient and cheap mass production of flexible ITO-free electrochromic devices using roll-to-roll techniques. We emphasize that the ITO- and vacuum free grid electrodes that we employ are fully 2D-printable still require further optimization to achieve the same optical transmission as the brittle ITO.

Experimental Section Instrumentation: Transmission spectra in the 900–300 nm range were determined using a Pharma Spec UV-1700 from Shimadzu. Transmission spectra were obtained relative to air, except for the barrier protected samples in which case the reference comprised two layers of barrier foil. A Keithley 2401 Sourcemeter was used for switching the devices and custom developed software for logging data. Profilometry measurements were performed on a Dektak 3030 Veeco Instruments Inc. Chemicals: Poly(3,3-bis(((2-ethylhexyl)oxy)methyl)-3,4-dihydro2Hthieno[3,4-][1,4]dioxepine)-co(3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine3,3-yl)bis(methylene)bis(2-methylacrylate) (methacrylate magenta) with an methacrylate content of 30% was synthesized as described elsewhere.[27] Polyethyleneglycol diacrylate (Mn 700) from Aldrich was run through a thin plug of basic Al2O3 prior to use, to remove the inhibitors 4-methoxy phenol (MEHQ) and butylhydroxytoluene (BHT).

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www.MaterialsViews.com 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide), acetonitrile (HPLC grade), isopropanol (HPLC grade) and 2,2-Dimethoxy2-phenylacetophenone were from Aldrich and used as received. Ebecryl 3000/30TP and Ebecryl 116 were from Cytec and used as received. Ag SI P1000X screen printing silver formulation was from Agfa. The conductive PEDOT:PSS formulation was a Clevios F010 from Heraeus Electrolyte Composition: The electrolyte composition was optimized by varying the concentration of the components and evaluating the properties in a setting similar to the one used for fabrication of the working devices (e.g., the maximum concentration of IL in an electrolyte retaining the desired mechanical properties was found by stepwise increasing the amount of ionic liquid and crosslinking the resulting electrolyte solution). These trials resulted in the following standard electrolyte solution used for all reported experiments: Polyethyleneglycol diacrylate (Mn 700) (6g); Ebecryl 3000/30TP (2g); 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (7g); Ebecryl 116 (0.2g); Acetonitrile (2ml); 2,2-Dimethoxy-2-phenylacetophenone (20 mg). Device Fabrication: Electrochromic devices (1 cm2) were fabricated on a mini roll coater described elsewhere using a slot-die coating head and a 13 mm meniscus guide made of stainless steel.[24] The substrate (bottom electrode) comprised barrier foil from Amcor (72 µm thickness) with a hexagonal silver grid that was photonically sintered.[26,40] The individual layers (as depicted in Figure 2) were coated using the following parameters: Layer A: PEDOT:PSS F010 (1:2 vol:vol isopropanol solution); coated at 1.2 m/min; deposition rate 0.4 ml/min; 60 °C. Layer B: Methacrylate magenta (20 mg/ml toluene solution); coated at 1.2 m/min; deposition rate 0.25 ml/min; 60 °C. Layer C: PFN (0.5 mg/ml methanol solution); coated at 0.7 m/min; deposition rate 0.15 ml/min; 60 °C, dry thickness app. 200 nm. Layer D: Electrolyte solution; coated at 0.3 m/min; deposition rate 0.8 ml/min; 60 °C; In situ curing was done using an Osram HTT 150-211 UV lamp following slot-die coating which left the gel solid and dry to the touch. The thickness of the solid electrolyte was determined by profilometry measurements to be 20 µM. Layer E: PEDOT:PSS F010 (1:1 vol:vol isopropanol solution); coated at 1.2 m/min; deposition rate 0.4 ml/min; 60 °C. Layer F: Flexographic printing of a hexagonal silver grid as the top electrode using; Ag SI P1000X; coated at 1.2 m/min; 60 °C. The devices were subsequently baked at 110 °C for 10 minutes. Amcor barrier PET foil carrying a layer of MP467 PTA adhesive (from 3M) was used to finally seal the device and study the effect of limited oxygen access during operation.

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[13] [14] [15] [16] [17] [18] [19] [20] [21]

[22] [23] [24] [25] [26] [27] [28] [29]

Supporting Information

[30]

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

[31]

Acknowledgements The authors would like to thank Torben Kjær for technical support during design and construction of the chambered UV-light source. Received: June 22, 2014 Revised: July 29, 2014 Published online:

[32] [33] [34] [35] [36]

[1] P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, Electrochromism and Electrochromic Devices, 1st ed., Cambridge University Press, 2007. [2] R. J. Mortimer, A. L. Dyer, J. R. Reynold, Disp. 2006, 27, 2–18. [3] E. S. Lee, S. E. Selkowich, R. D. Clear, D. L. DiBartolomeo, J. H. Klems, L. L. Fernandes, G. J. Ward, V. Inkarojrit, M. Yazdanian, 2006, CEC-500-2006-052. [4] C. M. Lampert, Proc Annu Tech Conf Soc Vac Coaters 2005, 675–667.

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Adv. Mater. 2014, DOI: 10.1002/adma.201402771

From the bottom up--flexible solid state electrochromic devices.

Solid-state flexible polymer-based electrochromic devices are fabricated continuously by stacking layers in one direction. This novel bottom-up approa...
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