Reflective and absorptive/transmissive polymer electrochromic devices (ECDs) composed of ..... 7 P. M. S. Monk, The Viologens, Wiley, Chichester, 1998.
Electrochromic devices based on soluble and processable dioxythiophene polymers{ Ali Cirpan, Avni A. Argun, Christophe R. G. Grenier, Benjamin D. Reeves and John R. Reynolds* Department of Chemistry, Center for Macromolecular Sciences and Engineering, University of Florida, Gainesville, FL 32611, USA Received 9th June 2003, Accepted 23rd July 2003 First published as an Advance Article on the web 15th August 2003 Reflective and absorptive/transmissive polymer electrochromic devices (ECDs) composed of spray-coated films of poly(3,3-bis(octadecyloxymethyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT(CH2OC18H37)2) and poly(3,3-bis(2-ethylhexyloxymethyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT(CH2OEtHx)2) layers as cathodically coloring polymers and poly(3,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-methyl-9Hcarbazole) (PBEDOT-NMeCz) as the anodically coloring polymer have been constructed and evaluated. These devices exhibit low switching voltages (¡1.2 V), high visible contrast values, sub-second switching times, and high switching stability in atmospheric conditions. The reflective ECD comprising PProDOT(CH2OEtHx)2 as the active layer demonstrates an unusual electrochromic behavior. By regulating the applied voltage, a high reflectance contrast of greater than 70% DT was achieved in the NIR region (2 mm) without any noticeable color change of the device.
Introduction Electrochromic devices (ECDs) are designed to modulate the reflectance or absorbance/transmittance of incident electromagnetic radiation by applying an external potential. ECDs have been developed for mirrors, optical displays, camouflage materials, spacecraft thermal control, and solar control glazings for ‘‘smart’’ building windows.1–9 Oxides of many transition metals have been shown to possess strong electrochromic (EC) properties.4,10,11 However, conducting polymers (CPs) have gained recent attention for use as ECDs in the last decade.12–19 Electroactive and conducting polymers are potential electrochromic materials, exhibiting a diverse variation of color and high contrast ratios, and as is especially illustrated by the work reported here, the desirable processability of thermoplastic polymers. As synthetic organic chemistry allows the derivatization of the monomer structure, controlling the electronic properties of the conjugated backbone allows fine-tuning of the EC properties. Recent focus has been placed on the application of solution processable electroactive polymers in ECDs due to their promising potential in the fabrication of large area devices.20,21 Our group has developed a new family of alkyl substituted poly(alkylenedioxythiophene) (PXDOT) derivatives due to their ease of synthesis, high chemical stability in the oxidized state, high optical contrast values, and fast switching between redox states.22–27 Early studies have shown that PEDOT is a useful material for electrochromic devices, switching from dark blue to transparent sky blue by applying a positive voltage.28,29 By increasing the size of the alkylenedioxy ring, and by increasing the number and size of the substituents on the ring, the electrochromic properties, including switching times and contrast ratios are enhanced.25,26 Dual polymer electrochromic devices have been fabricated and evaluated using a dimethyl propylenedioxythiophene derivative, PProDOT(Me)2, as the cathodically coloring {Electronic supplementary information (ESI) available: details of the synthesis of PProDOT(CH2OC18H37)2 and PProDOT(CH2OEtHx)2 and their polymerization. See http://www.rsc.org/suppdata/jm/b3/ b306365h/
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polymer.14,15,30 PProDOT(Me)2 is one of the most promising polymers for EC applications in the alkylenedioxythiophene based family, with a %DT of 78% and sub-second switching times.26 Transmissive/absorptive EC devices have been fabricated by sandwiching a gel electrolyte between two ITO coated glass slides, one in which PProDOT(Me)2 has been electrochemically deposited and another in which a complementary anodically coloring polymer has been deposited, such as poly(3,6-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9-methyl-9Hcarbazole) (PBEDOT-NMeCz) or an N-propane sulfonated poly(3,4-propylenedioxypyrrole) (PProDOP-NPrS).15 By applying a potential bias, the optical density of the device can be controlled from highly transmissive to deeply colored. As the PProDOT(Me)2 is oxidized, changing from a dark purple to a transmissive sky blue, the anodically coloring polymer becomes reduced, switching from a dark blue to a transmissive yellow. The devices have an overall colorimetrically measured luminance change of 60% and retain the optical response after tens of thousands of double potential steps. Dual polymer electrochromic devices have been fabricated which can modulate the reflectivity of electromagnetic radiation from a surface.14,30 PProDOT(Me)2 can be electrodeposited onto working electrodes consisting of slit gold14 or gold-coated porous membranes.30 The working front facing electrode is connected to a complementary polymer deposited onto a counter electrode hidden behind the EC active electrode. The reflectivity of the gold surface is observed as the cathodically coloring polymer switches from absorptive to transmissive with an applied potential. A device consisting of PProDOT(Me)2 deposited on top of the gold surface and PBEDOT-NMeCz deposited on the counter electrode gave a %DR of 55% at 600 nm.14 The device has a %DR of 80% in the NIR region (1.3–2.2 mm), displaying electrochromic reflectivity in the visible as well as in the infrared regions. Recent synthetic work has involved placing sufficiently long alkyl chains on the alkylenedioxy ring to induce solubility in order to chemically polymerize an organic soluble and processable polymer. Many methods for the chemical polymerization of thiophene derivatized monomers have been developed.31 Rieke32 and McCullough33 and their coworkers were the first to accomplish this task by synthesizing and characterizing
J. Mater. Chem., 2003, 13, 2422–2428 This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b306365h
regioregular poly(3-alkylthiophene)s followed by Iraqi,34 Guillerez,35 and many others. The latest method developed by McCullough and coworkers has utilized a Grignard metathesis polymerization,36 which does not require long reaction times, cryogenic temperatures, or the nontrivial preparation of reagents often seen with other methods and affords the polymer in the neutral state. Our group has extended Grignard metathesis coupling to the synthesis of soluble and processable derivative of poly(3,4-propylenedioxythiophene)s.37 In one example, dibutyl derivatization allows the synthesis of polymers with adequate molecular weights and solubility in THF and toluene to allow high quality films to be obtained. The polymer is electroactive, exhibiting an electrochromic switch from deep red–purple in the neutral state to highly transmissive sky blue in the oxidized state. Our group is now developing a new series of alkyl and alkoxy substituted PXDOTs that are highly soluble in common organic solvents, exhibit high optical contrast, and possess sub-second switching times.37,38 In this article, we report the construction and characterization of absorptive/transmissive and reflective ECDs based on two of these soluble conjugated polymers, PProDOT(CH2OC18H37)2 and PProDOT(CH2OEtHx)2.38
Experimental Tetrabutylammonium hexafluorophosphate (TBAPF6), lithium perchlorate (LiClO4), acetonitrile (ACN), and propylene carbonate (PC), were purchased from Aldrich. ACN and PC were distilled before use. PBEDOT-NMeCz, PProDOT(CH2OC18H37)2, and PProDOT-(CH2OEtHx)2, were prepared according to methodologies previously reported.38,39 The soluble polymer films were spray-coated onto ITO-coated glass slides (20 V sq21) and gold-coated polycarbonate membranes (5 V sq21) using an airbrush (Testors Corp.) from 0.6% w/w solution of polymer in toluene. Redox charges of polymer films were determined by stepping the potential from 0.6 V to 20.6 V (vs. Fc/Fc1), and monitoring the charge versus time in a three-electrode cell containing 0.1 M supporting electrolyte solution (TBAPF6–PC). Electrochemical polymerization of BEDOT-NMeCz39 was carried out in 0.1 M LiClO4–ACN using an EG&G Model PAR273A potentiostat/ galvanostat. A three-electrode cell, consisting of an ITO-coated glass slide as the working electrode, a platinum flag as the counter, and a silver wire as the pseudo-reference electrode, was used for electrodeposition of polymer films. The polymer gel electrolyte used in this study was prepared by mixing poly(methyl methacrylate) (PMMA), TBAPF6, acetonitrile, and propylene carbonate. The ratio of the composition of TBAPF6–PMMA–PC–ACN was 3 : 7 : 20 : 70 by weight. The film thickness was measured utilizing a Sloan Dektak 3030 profilometer. Colorimetry results were obtained by the use of a Minolta CS-100 Chroma Meter. The sample was illuminated from behind by a D50 (5000 K) light source in a light booth designed to exclude external light.40 A background measurement was taken using a blank device containing two ITO glasses held together by the gel electrolyte. Optical characterization of the ECDs was carried out using a Cary 500 UV-VisNIR spectrophotometer. Gold deposition (99.99% purity) on porous membranes (polycarbonate, 10 mm pore size, Osmonics Inc.) was carried out using a high vacuum chamber thermal evaporator (Denton DV-502A).30 The characterizations of the reflective ECDs were carried out using an integration sphere attached to the UV-Vis-NIR spectrophotometer.
Fig. 1 (a) Structures of the EC polymers used in this study. (b) Schematic representation of an absorptive/transmissive type PProDOT-R2/PBEDOT-NMeCz device.
in their neutral (colored) states onto substrates such as ITOcoated glass slides and gold-coated polycarbonate membranes using an airbrush. After removal of solvent by vacuum drying overnight, the resulting films of PProDOT(CH2OC18H37)2 and PProDOT(CH2OEtHx)2 were highly homogeneous and observed as blue and reddish purple, respectively. The average thickness of the films was around 150 nm with 20 nm surface roughness, as characterized by profilometry. Much like PProDOT(Bu)2,37 the diethylhexyl derivative, PProDOT(CH2OEtHx)2, exhibited a reddish purple color upon casting from solution. Once the film was fully oxidized and reduced back to the neutral colored state, the color changed to a blue–violet. This is due to doping induced order, which is a phenomenon brought about by the relaxation of the structure from a planar quinoidal oxidized form to an aromatic neutral form. Therefore, the band gap decreases in comparison to the randomly oriented cast film due to a longer effective conjugation length after redox switching. The polymer, PProDOT(CH2OC18H37)2, has a blue–violet color upon spray casting which does not change after redox switching. Upon spray coating from solution, the polymer films were in their neutral (colored) states. EC switching was achieved by inserting the polymer films into an electrolyte solution in a three-electrode cell configuration and by applying potential steps. Both polymers are cathodically coloring polymers, which appear blue–violet in the neutral (dedoped) state and transmissive blue in the oxidized (doped) state.38 Two types of ECDs were constructed with these soluble polymeric EC materials to demonstrate their light modulation characteristics in the visible and in the NIR region. The first type is an absorptive/transmissive ECD that operates as an electrochromic window and the second type is a reflective ECD that comprises a cathodically coloring polymer deposited on a gold substrate as the electrochromic layer. Absorptive/transmissive ECDs
Results and discussions Two organic solvent soluble conjugated polymers, PProDOT(CH2OC18H37)2 and PProDOT(CH2OEtHx)2 (structures shown in Fig. 1a), were spray coated from toluene (0.6% w/w)
ITO-coated glass (20 V sq21) was used as the substrate material. PProDOT-(CH2OC18H37)2 and PProDOT-(CH2OEtHx)2 were used as cathodically coloring polymers and PBEDOT-NMeCz as the anodically coloring polymer as illustrated by the structures shown in Fig. 1a. Redox charges of the two J. Mater. Chem., 2003, 13, 2422–2428
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Fig. 2 Voltage dependence of the relative luminance of a PProDOT(CH2OEtHx)2–PBEDOT-NMeCz device and photographs of the device in the bleached and dark states. In the colored and bleached states, the device exhibits a relative luminance of 16%, and 93%, respectively, resulting in a 77% luminance contrast.
complementary colored polymer films were matched using chronocoulometry to provide a balanced number of redox sites for switching by stepping the potential between 11.0 V and 21.0 V. Cathodically coloring polymer films (PProDOT(CH2OC18H37)2 and PProDOT-(CH2OEtHx)2) were fully oxidized to their transmissive blue state and the anodically coloring polymer (PBEDOT-NMeCz) was fully neutralized to its transmissive yellow state. The films were then soaked with polymer gel electrolyte until the entire polymer surface was uniformly covered. ECDs were assembled by placing two EC polymer films (one doped and the other neutral) facing one another separated by a polymer gel electrolyte layer as shown in Fig. 1b. Relative luminance studies were performed on the ECDs as a function of applied voltage using the CIE 1931 color space. The voltage dependence of the relative luminance offers a perspective on the transmissivity of a material as it relates to the perception of the human eye over the entire visible spectrum as a function of doping on a single curve. The light source used is calibrated taking into account the sensitivity of the human eye to different wavelengths. Fig. 2 shows the percent relative luminance change with respect to applied voltage of a PProDOT-(CH2OEtHx)2–PBEDOT-NMeCz device along with photographs of the extreme states. At negative voltages, the device is absorptive purple with a luminance value of 16%. At positive voltages, the luminance increases to 93%, yielding a 77% luminance contrast. This value is greater than that of the individual polymer films. In the device, EC polymers operate in a complementary manner where both of the polymers contribute to the absorption in the colored state. When the bias is switched, the absorption of the polymers in the visible region decreases and the device is observed to be transmissive yellow in this bleached state. It is important to note that the redox charging–discharging of the polymer layers was balanced with respect to each other for optimal performance. Therefore, it is possible to access the extreme doped/dedoped states. As for the PProDOT(CH2OC18H37)2–PBEDOT-NMeCz device, the relative luminance contrast was somewhat lower (45%), which is due to a slightly lower optical contrastof PProDOT-(CH2OC18H37)2. Another remarkable characteristic of the PProDOT-(CH2OEtHx)2 polymer is that the electrochromic changes (luminance, %T) occur abruptly in a very narrow potential range (100 mV) contrary to the gradual changes observed in other PProDOT derivatives.37 Spectroelectrochemistry plays a key role in examining the 2424
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Fig. 3 (a) Spectroelectrochemistry of a PProDOT-(CH2OEtHex)2– PBEDOT-NMeCz device as a function of applied voltage: (a) 10.8, (b) 10.5, (c) 10.4, (d) 10.3, (e) 10.2, (f) 10.1, (g) 0.0, (h) 20.1, (i) 20.2, (j) 20.5 V. At positive voltages PProDOT-(CH2OEtHx)2 is in the oxidized state and PBEDOT-NMeCz is in the neutral state. (b) Spectra from the two extreme states of the device.
optical changes that occur upon doping or dedoping of an ECD. Fig. 3a shows a series of UV-Vis-NIR absorbance spectra of the PProDOT-(CH2OEtHx)2–PBEDOT-NMeCz ECD under various applied voltages. When a negative voltage (e.g. 21.0 V) is applied to the PProDOT-(CH2OEtHx)2 layer, the polymer film has an absorptive violet color with the interband p–p* transition split into three distinct peaks at 520, 556 and 607 nm. This splitting was also observed for poly(3,4propylenedioxythiophene) (PProDOT) which has been attributed to several factors including vibronic coupling due to a high degree of regularity along the polymer backbone,25,41 Davydov (exciton) splitting,42 or discrete polymer segment absorptions.43 At this voltage, the PBEDOT-NMeCz layer is oxidized and exhibits an absorptive blue color. When the voltage is increased stepwise to 10.6 V, the PProDOT-(CH2OEtHx)2 layer starts to oxidize and a new peak at y980 nm, corresponding to polaron charge carriers of PProDOT-(CH2OEtHx)2, starts to increase in intensity at the expense of the p–p* transition.44,45 At the same time, another absorption is induced at longer wavelengths (w1100 nm), which is attributed to bipolaron charge carriers.46,47 Upon further oxidation (11.0 V), the PProDOT-(CH2OEtHx)2 layer is completely oxidized attenuating the absorption between 500–600 nm, changing the film’s color from deep violet to transmissive blue. At this voltage, the PBEDOT-NMeCz layer is reduced, changing from blue to a transmissive yellow. This results in the bleaching of the device, changing from violet to transmissive yellow–green. The absorption spectra of the device in two extreme states (colored and bleached), extracted from the spectroelectrochemical
series, are shown in Fig. 3b for purposes of clarity. It is important here to note that the optical density (OD) of the p–p* transition of the PProDOT-(CH2OEtHx)2 layer (lmax ~ 607 nm) is much higher than that of the PBEDOT-NMeCz layer (lmax ~ 418 nm). Therefore the PProDOT-(CH2OEtHx)2 layer has a greater optical contribution to the device. This is an expected result of the charge optimization process carried out during the device construction. Before the device assembly, we individually measured the redox switching charges required to fully switch the polymer layers for different polymerization charges. We found that less PBEDOT-NMeCz is required to match the amount of redox charge contained in PProDOT(CH2OEtHx)2 for a full switch; thus we are able to use a lower amount of PBEDOT-NMeCz in order to optimize the exchanged charge. As a result of this, the PBEDOT-NMeCz layer has lower OD values. Fig. 3b, curve a, also clearly explains the lightly absorbing green-toned color observed in the bleached state. The neutral PBEDOT-NMeCz (lmax at 418 nm) is slightly yellow and, combined with the residual absorption of PProDOT-(CH2OEtHx)2 around 600 nm, prevents the device switching to a completely colorless bleached state at 11.0 V. While not addressed in this paper for the soluble PProDOT based devices, using a high gap N-alkylated derivative of poly(3,4-propylenedioxypyrrole) overcomes much of this residual color in the bleached state.15 One of the most important criteria for selecting an electrochromic material is its coloration efficiency (g). The composite coloration efficiency is a useful term for measuring the power efficiency of a device since it determines the amount of optical density change (DOD) per injected/ejected electronic charge (Qd) during a large magnitude redox potential step and is given by the following equation,2,23 g ~ DOD/Qd ~ log [Tb/Tc]/Qd where Tb and Tc are the bleached and colored transmittance values, respectively. Fig. 4a shows both the optical density (solid line) and charge accumulation (dashed line) change as a function of time while the device is stepped between the absorbing, colored state and the transmissive, bleached state. Contrast ratios are calculated by the transmittance change between the colored and the bleached state. A nearly complete 95% of the full switch is reached faster (0.3 s) for the PProDOT(CH2OEtHx)2 device than that of PProDOT-(CH2OC18H37)2 device (4.6 s). Our group previously reported the g values of ECDs containing derivatives of PEDOT as the cathodically coloring polymer and PBEDOT-NMeCz as the anodically coloring polymer to be between 400 cm2 C21 and 1500 cm2 C21.48 Table 1 shows the coloration efficiencies of the two absorptive/ transmissive ECDs comprising soluble PProDOT derivatives as the cathodically coloring layer. As an example, when 95% of the total contrast is obtained, the g of the PProDOT(CH2OEtHx)2–PBEDOT-NMeCz device was 4804 cm2 C21 at 609 nm while that of the PProDOT-(CH2OC18H37)2–PBEDOTNMeCz device was 1294 cm2 C21 at 595 nm. Coloration efficiency (g) is directly related to the contrast ratios in organic polymers, as well as the reciprocal of the injected charge. The higher g of the PProDOT-(CH2OEtHx)2–PBEDOT-NMeCz device is due to the faster switching rates and more abrupt optical changes in a narrow potential window of the PProDOT-(CH2OEtHx)2 polymer. A high g can provide large optical modulation with small charge injection or extraction and is a crucial parameter for practical ECDs. We have measured the coloration efficiency of a PProDOT(CH2OEtHx)2–PBEDOT-NMeCz device by passing a low current through the device galvanostatically leading to bleaching of the device over a period of 20 seconds. Fig. 4b shows the change in the transmittance of the device, along with the variation of the coloration efficiency, as a function of the amount of charge passed during this slow bleaching. The
Fig. 4 (a) Chronoabsorptometry (solid line) and chronocoulometry (dashed line) for a PProDOT-(CH2OEtHx)2–PBEDOT-NMeCz electrochromic device monitored at l ~ 609 nm as the voltage is stepped between the colored state (21.0 V) and the bleached state (11.0 V). Device area ~ 5.5 cm2. (b) Variation of the coloration efficiency (g) and %T as a function of the charge passed as the device is bleached slowly at a constant current value of 0.03 mA cm22. Table 1 Optical and electrochemical data collected for coloration efficiency measurements % of full switch
Qd/ mC cm2
DODa
g/ cm2 C21b
PProDOT-(CH2OEtHx)2–PBEDOT-NMeCzc 100 50.9 2 0.34 95 48.4 0.3 0.28 90 45.8 0.19 0.26 85 43.3 0.17 0.25 80 40.7 0.15 0.24
1.37 1.35 1.33 1.31 1.28
4022 4804 5229 5251 5428
PProDOT-(CH2OC18H37)2–PBEDOT-NMeCzd 100 49.2 6.00 1.06 95 46.7 4.63 0.96 90 44.3 4.01 0.92 85 41.8 3.63 0.86 80 39.4 3.31 0.82
1.27 1.25 1.22 1.20 1.18
1197 1294 1332 1398 1435
D%T
tox/s
a DOD ~ log (Tox/Tred). bg ~ DOD/Qd. clmax ~ 609 nm, electrode area ~ 5.5 cm2, %Tred ~ 2.2%, %Tox ~ 53.1%, D%T ~ 50.9%. d lmax ~ 595 nm, electrode area ~ 5.0 cm2, %Tred ~ 2.8%, %Tox ~ 52%, D%T ~ 49.2%.
overall %T changes for the slow switching of this device are similar to those observed with the large potential step switching experiment in Fig. 4a. The g value has a maximum of y3850 cm2 C21 and follows a form similar to that observed previously for other PEDOT-derived ECDs.49 Although the charging process is linear with time (constant current), the %T curve tends to saturate and this results in lower g values at higher doping levels. When 95% of the full contrast change is reached J. Mater. Chem., 2003, 13, 2422–2428
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Fig. 6 Spectroelectrochemistry of a PProDOT-(CH2OEtHex)2 containing reflective device as a function of applied voltage: (a) 20.8, (b) 20.5, (c) 20.15, (d) 20.1, (e) 20.08, (f) 20.06, (g) 20.04, (h) 20.02, (i) 0, (j) 0.05, (k) 0.1, (l) 0.2, (m) 0.3, (n) 0.4, (o) 0.5, (p) 0.6, (q) 0.8, (r) 1.0, (s) 1.2 V. Fig. 5 (a) Schematic device structure of a reflective ECD. (b) Photographs exhibiting the neutral and the oxidized appearances of the active layer on a gold reflective surface.
using this slow experiment, the device is operating with a coloration efficiency of 1550 cm2 C21. This is lower than the 4804 cm2 C21 obtained at 95% of full contrast for the large magnitude potential step experiment. This is due to the nonFaradaic currents involved in the slow doping–dedoping of the polymers that causes a higher amount of total charge (0.6 mC cm22, Fig. 4b) to be needed for complete bleaching. Using a large magnitude potential step eliminates this current; hence the device requires less charge to switch (v0.4 mC cm22, Fig. 4a).
Reflective ECDs A reflective ECD constructed as shown schematically in Fig. 5a and utilizing an organic soluble PProDOT-(CH2OEtHx)2 as the surface active layer has been investigated. The conducting polymer switches between a transmitting (conducting) and absorbing (insulating) state upon a reversible redox switch causing the device to be gold reflective and magenta absorptive, respectively. The polymer was deposited onto a gold-coated porous polycarbonate membrane (active layer electrode) and onto a gold-coated Kapton2 (counter electrode) by spray coating. Both films were switched in a TBAPF6–PC electrolyte to match the redox charges before building the device. Following the redox conditioning of the polymer films, the counter electrode polymer layer was fully neutralized while the active polymer was fully oxidized to ensure a charge balance prior to device construction. The reflective devices were built in a sandwich configuration with the gold-coated membrane spray coated with PProDOT-(CH2OEtHx)2 shown in Fig. 5b facing outward to allow convenient reflective mode characterization.50,51 The outward facing electroactive polymer is responsible for the surface reflectivity modulation whereas the counter electrode polymer only contributes to the balancing of the electroactive sites and its optical properties are not observed and do not affect the device operation. The underlying counter electrode was constructed using the same polymer to attain electrochemical compatibility with the visible outward facing electrode. The reflective type electrochromic device was assembled using a high viscosity polymeric electrolyte composed of TBAPF6 dissolved in a PMMA matrix swollen by acetonitrile–propylene carbonate. 2426
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Fig. 6 shows a full set of reflectance spectra for a device as a function of applied voltage. When a negative voltage is applied to the active layer, the spectrum exhibits the lowest reflectance (%Rmin) at 600 nm, due to the absorption from the p–p* transition which gives rise to the color of the active layer (reddish purple) (Fig. 5b, neutral state) and a high reflectivity in the near infrared from 800 nm to 2000 nm. By applying a positive bias to the active layer, the polymer oxidizes and the p–p* transition attenuates with an increase of charge carrier transitions in the NIR region. At this voltage, the polymer active layer is transparent (Fig. 5b, oxidized state), so the gold layer dominates the reflectance. The device exhibits a reflectance contrast (D%R) value of greater than 55% in the visible region (l ~ 600 nm), and a D%R of 75% in the NIR (l ~ 2000 nm). An unusual electrochromic switching property was observed as the device was switched between –1 V and 0 V as illustrated by Fig. 7a. The reflectance contrast at 2000 nm was greater than 70%, while at 609 nm, D%R was less than 3%. The result is the creation of an ECD that is IR active while undergoing no visible color change. This unexpected behavior is easily observed by spectroelectrochemistry where the two UV-VisNIR spectra of this device were compared at 20.80 V and 20.02 V as shown in Fig. 7b. Despite a large contrast in the NIR region, the spectra are quite similar in the visible region. As shown in Fig. 7c, when a voltage is applied to the active layer between 11.2 V and 10.05 V, there is essentially no change in the NIR reflectance contrast at wavelengths longer than 1600 nm while there is substantial color change as evident by the response in the visible region. We speculate that this asymmetric switching of the ECD can be attributed to the full penetration of the NIR light through the neutral polymer layer, as opposed to the strong absorption of the visible light (Fig. 7b, 20.8 V). Therefore, when the polymer is partially oxidized (20.8 V to 20.02 V), the reflected NIR light is more sensitive to the optical changes that occur upon oxidative doping close to the electrode surface than the reflected visible light. As a result, we observe the absorption of the charge carriers in the NIR region (%R decreases) with minimal depletion of the p–p* absorption in the visible region. This does not occur for the spectroelectrochemical series of the same polymer in the transmissive mode, since only the transmitted light that goes completely through the polymer layer is detected. Similar findings have been addressed previously in the literature.19,52–54
built from these spray-coated films have the greatest coloration efficiency values reported to date (4804 cm2 C21 at 95% of the full contrast), and high luminance contrasts (77%) with less than 1 second switching times. The judicious selection of polymers along with ingenious engineering will further improve the switching rates of these devices to make them promising candidates for dynamic display applications. Considering that the accessible switching speeds with perceptible contrast values are now down to approximately a tenth of a second (44% change in 0.16 seconds, Fig. 4a), it is reasonable to compare this rapid switching rate to video rates [y15–30 frames per second (fps)]. Reflective ECDs built with surface-active polymer layers using porous substrates prove to be superior light modulators both in the visible and in the NIR region. The state of the art assembly of the ECD reveals the best switching properties of the solution processed EC polymer films. The ability to modulate NIR absorption/reflection by controlling the applied voltage without any noticeable color change in the visible region is an ongoing research interest and an unusual phenomenon in polymeric ECD systems. Further studies to fully understand this remarkable unsymmetrical EC switching are in progress.
Acknowledgements We would like to acknowledge the ARO/MURI (DAAD 1999-1-0316) and AFOSR (F49620-03-1-0091) for financial support. A. Cirpan would like to thank the Scientific and Technical Research Council of Turkey (TUBITAK NATOA2) for financial support.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Fig. 7 (a) Electrochromic switching at l ~ 2000 nm and l ~ 609 nm as the voltage of a PProDOT-(CH2OEtHex)2 containing reflective device is stepped between 21.0 V and 0.0 V. (b) Reflective spectra of the same device at 20.8 V and 20.02 V illustrating the NIR electrochromism with minimal color change. (c) Spectral response of the device between 11.2 V and 10.05 V.
Summary and perspective In conclusion, soluble and processable dialkyl and dialkyloxy substituted PProDOT derivatives are promising candidates for ECD applications due to their synthetic versatility, fast switching times, and high coloration efficiencies. Spray coating techniques allow homogenous deposition of polymer films over large and irregular surfaces on various substrates such as ITO and gold. This solution processing of polymer films introduces the possibility of constructing large area ECDs and patterned devices. Further processing using printing techniques, such as inkjet printing, is envisioned. Absorption/transmission ECDs
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D. R. Rosseinsky and R. J. Mortimer, Adv. Mater., 2001, 13, 783. K. Bange and T. Gambke, Adv. Mater., 1990, 2, 10. M. Green, Chem. Ind., 1996, 17, 641. P. M. S. Monk, R. J. Mortimer and D. R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. N. Leventis, M. Chen, A. I. Liapis, J. W. Johnson and A. Jain, J. Electrochem. Soc., 1998, 145, L55. J. Liu and J. P. Coleman, Mater. Sci. Eng., 2000, 286, 144. P. M. S. Monk, The Viologens, Wiley, Chichester, 1998. D. R. Rosseinsky and P. M. S. Monk, J. Electrochem. Soc., 2000, 147, 1595. R. Kingston, Chem. Br., 1999, 35(10), 24. C. G. Granqvist, Electrochim. Acta, 1999, 44, 2983. R. D. Rauh, Electrochim. Acta, 1999, 44, 3165. J. L. Boehme, D. S. K. Mudigonda and J. P. Ferraris, Chem. Mater., 2001, 13, 4469. J. P. Ferraris, C. Henderson, D. Tores and D. Meeker, Synth. Met., 1995, 72, 147. I. Schwendeman, J. Hwang, D. M. Welsh, D. B. Tanner and J. R. Reynolds, Adv. Mater., 2001, 13, 634. I. Schwendeman, R. Hickman, G. Sonmez, P. Schottland, K. Zong, D. M. Welsh and J. R. Reynolds, Chem. Mater., 2002, 14, 3118. W. A. Gazotti, G. Casalbore-Miceli, A. Geri, A. Berlin and M. A. DePaoli, Adv. Mater., 1998, 10, 1523. M. A. De Paoli, G. Casalbore-Miceli, E. M. Girotto and W. A. Gazotti, Electrochim. Acta, 1999, 44, 2983. H. Pages, P. Topart and D. Lemordant, Electrochim. Acta, 2001, 46, 2137. P. Chandrasekhar, B. J. Zay, G. C. Birur, S. Rawal, E. A. Pierson, L. Kauder and T. Swanson, Adv. Funct. Mater., 2002, 12, 2137. H. W. Heuer, R. Wehrmann and S. Kirchmeyer, Adv. Funct. Mater., 2002, 12, 89. M. A. De Paoli, A. F. Nogueira, D. A Machado and C. Longo, Electrochim. Acta, 2001, 46, 4243. B. C. Thompson, P. Schottland, G. Sonmez and J. R. Reynolds, Synth. Met., 2001, 119, 333. C. L. Gaupp, D. M. Welsh, R. D. Rauh and J. R. Reynolds, Chem. Mater., 2002, 14, 3964. C. L. Gaupp, D. M. Welsh and J. R. Reynolds, Macromol. Rapid Commun., 2002, 23, 885.
J. Mater. Chem., 2003, 13, 2422–2428
2427
25 A. Kumar, D. M. Welsh, M. C. Morvant, F. Piroux, K. A. Abboud and J. R. Reynolds, Chem. Mater., 1998, 10, 896. 26 D. M. Welsh, A. Kumar, E. W. Meijer and J. R. Reynolds, Adv. Mater., 1999, 11, 1379. 27 B. D. Reeves, B. C. Thompson, K. A. Abboud, B. E. Smart and J. R. Reynolds, Adv. Mater., 2002, 14, 717. 28 M. Dietrich, J. Heize, G. Heywang and F. Jonas, J. Electroanal. Chem., 1994, 369, 87. 29 L. B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik and J. R. Reynolds, Adv. Mater., 2000, 12, 481. 30 J. R. Reynolds, K. Zong, I. Schwendemann, G. Sonmez, P. Schottland, A. A. Argun and P.-H. Aubert, PCT Int. Appl. WO 0346106, 2003. 31 R. D. McCullough and P. C. Ewbannk, in Handbook of Conducting Polymers, 2nd edn., eds. T. A. Skothheim, R. L. Elsenbaumer and J. R. Reynolds, Marcel Dekker, New York, 1998, p. 225. 32 T.-A. Chen and R. D. Rieke, J. Am. Chem. Soc., 1992, 114, 10087; T.-A. Chen, X. Wu and R. D. Rieke, J. Am. Chem. Soc., 1995, 117, 233. 33 R. D. McCullough and R. D. Lowe, J. Chem. Soc., Chem. Commun., 1992, 70; R. D. McCullough, R. D. Lowe, M. Jayarama and D. L. Anderson, J. Org. Chem., 1993, 58, 904. 34 A. Iraqi and G. W. Barker, J. Mater Chem., 1998, 8, 25. 35 S. Guillerez and G. Bidan, Synth. Met., 1998, 13, 123. 36 R. S. Loewe, S. M. Khersonsky and R. D. McCullough, Adv. Mater., 1999, 11, 250; R. S. Loewe, P. C. Ewbank, J. Liu and R. D. McCullough, Macromolecules, 2001, 34, 4324. 37 D. M. Welsh, L. J. Kloeppner, L. Madrigal, M. R. Pinto, B. C. Thompson, K. S. Schanze, K. A. Abboud, D. Powell and J. R. Reynolds, Macromolecules, 2002, 35, 6517.
2428
J. Mater. Chem., 2003, 13, 2422–2428
38 See Electronic Supplementary Information. 39 G. A. Sotzing, J. L. Reddinger, A. R. Katritzky, J. Soloducho, R. Musgrave and J. R. Reynolds, Chem. Mater., 1997, 9, 1578. 40 B. C. Thompson, P. Schottland, K. Zong and J. R. Reynolds, Chem. Mater., 2000, 12, 1563; F. Billmeyer and M. Saltzman, in Principles of Color Technology, ed. R. S. Berns, John Wiley & Sons, 2000. 41 S. D. D. V. Rughooputh, A. J. Heeger and F. Wudl, J. Polym. Sci., 1987, 25, 1071; I. Schwendemann, Ph.D. Dissertation, 2002. 42 M. D. Curtis, Macromolecules, 2001, 34, 7905. 43 R. D. McCullough, S. Tristramnagle, S. P. Williams, R. D. Lowe and M. Jayaraman, J. Am. Chem. Soc., 1993, 115, 4910. 44 J. J. Aperloo, J. A. E. van Haare and R. A. J. Janssen, Synth. Met., 1999, 101, 417. 45 Y. Furukawa, J. Phys. Chem., 1996, 100, 15644. 46 B. Sankaran and J. R. Reynolds, Macromolecules, 1997, 30, 2582. 47 D. M. Welsh, A. Kumar, E. W. Meijer and J. R. Reynolds, Chem. Mater., 1999, 11, 1379. 48 S. A. Sapp, G. A. Sotzing and J. R. Reynolds, Chem. Mater., 1998, 10, 2101. 49 R. D. Rauh, F. Wang, J. R. Reynolds and D. L. Meeker, Electrochim. Acta, 2001, 46, 2023. 50 R. B. Bennet, W. E. Kokonasku, M. J. Hannan and L. G. Boxall, US Patent 5 446 577, 1995. 51 P. Chandrasekhar, US Patent 5 995 273, 1999. 52 P. Chandrasekhar and T. J. Dooley, Proc. SPIE, 1995, 169, 2528. 53 V. Fattori, G. C. Miceli, A. Geri, G. Zotti and M. C. Galazzi, Synth. Met., 1999, 101, 182. 54 P. Topart and P. Hourquebie, Thin Solid Films, 1999, 352, 243.
