New all-plastic electrochromic devices have been manufactured using commercial PEDOT foils and classical polymer electrolyte electrodeposition techniques. Several devices with different areas have been electrically characterized by using an impedance spectroscopy technique. After a brief description of the manufacturing process, we discuss electrical properties with the size of the device. Real and imaginary parts of the impedance have been plotted as a function of frequency. Some interesting conclusions have been derived from those plots, and some improvements in manufacturing process of this kind of devices are also proposed.Publicad

Barrios Puerto, David

Sánchez Pena, José Manuel

Vergaz Benito, Ricardo

Vázquez García, María Carmen

English

Universidad Carlos III de Madrid e-Archivo

ctdPOoorOleerealaeHpcav±3ItocdsssiihmrvlsdihtodOE LETTERSElectrical analysis of newall-plastic electrochro-mic devicesRicardo Vergaz, J. M. S. Pena, MEMBER SPIE,David Barrios, and Carmen VázquezUniversidad Carlos III, Departamento de TecnologíaElectrónica E.P.S., Grupo de Displays & AplicacionesFotónicas, C/Butarque 15, E-28911, Leganés,Madrid, SpainE-mail: jmpena@ing.uc3m.esAbstract. New all-plastic electrochromic devices have beenmanufactured using commercial PEDOT foils and classicalpolymer electrolyte electrodeposition techniques. Severaldevices with different areas have been electrically character-ized by using an impedance spectroscopy technique. After abrief description of the manufacturing process, we discusselectrical properties with the size of the device. Real andimaginary parts of the impedance have been plotted as afunction of frequency. Some interesting conclusions havebeen derived from those plots, and some improvements inmanufacturing process of this kind of devices are alsoproposed. © 2006 Society of Photo-Optical Instrumentation Engineers.DOI: 10.1117/1.2388278Subject terms: electrochromic devices; PEDOT; electricalcharacterization; impedance analysis.Paper 060511LR received Jun. 27, 2006; revised manuscriptreceived Aug. 30, 2006; accepted for publication Sep. 6, 2006;published online Nov. 14, 2006.1 IntroductionThe classical research on electrochromic materials is fo-cused mainly on solid devices in which glass is the trans-parent surface used as substrate. Recent investigations arefocused on plastic substrates due to its flexibility, whichwill impel their applications as flexible displays and swit-chable optical filters. However, no industrially all-plasticelectrochromic devices exists to date, due to a number ofdrawbacks such as low quality of the transparent conductorlayer onto plastic substrates,1 among others. As a conse-quence of this trend, we have manufactured all-polymerflexible devices as described elsewhere.2,3In this work, we will focus on the electrical character-ization of the devices, depending on their size. We willdescribe the impedance measurements carried out in them.The impedance measurement permits us to obtain impor-tant information for the modeling of the device. Electricalmodels should show the difference between the usual elec-trochromic devices and these ones, because the reducednumber of layers should result in simpler electrical equiva-lent circuits than traditional EC ones.2 Experimental ProcedureAs it has been described,2 electrochromic devices with anew configuration have been constructed Fig. 1. This con-figuration avoids the use of a conductor layer to make acontact electrode. Instead, PEDOT acts as both the electro-0091-3286/2006/$22.00 © 2006 SPIEOptical Engineering 110501-1hromic layer and the electrode. PEDOT is colored inransparent-blue when oxidized and in dark-blue when re-uced. We used polyethylene terephtalate foils covered byEDOT, commercialized by AGFA under the trademark ofRGACON. A tetrahydrofurane solution of a polyethylenexide-lithium triflate polymer electrolyte is deposited ontone of them by casting. Afterwards, the solvent is evapo-ated, at room temperature during 3 hours, and the secondRGACON EL is glued on top of the polymer electrolyteayer.In this configuration, one of the PEDOT layers acts aslectrochromic material and the other layer acts as counter-lectrode. The construction is totally symmetric; thus, theole of the PEDOT layers could be interchangeable. Theasy construction of the devices, with only commerciallyvailable materials, simplifies from five to three the activeayers number and may result in many technologicaldvantages.Devices of different sizes were manufactured, and theirlectrical behavior has been tested. An impedance analyzerewlett-Packard© Model 4194A was used to derive the im-edance characteristics of the devices. The whole system isontrolled via HP-IB bus from a personal computercquiring 401 points in each spectrum. Different DC biasoltage can be applied to the sample with an accuracy of0.12% ±12 mV.Results and Discussionmpedance spectroscopy measurements were carried out forhe different devices tested. Fig. 2a for the increasing partf a 3 V ramp with 0.5 V steps, and Fig. 2b for the de-reasing part of the cycle, both show the results for two ECevices with active areas of 92 mm2 and 1538 mm2, re-pectively. The steps are applied at a 60-s rate to ensure thewitching conditions acquired in the transmittance mea-urements.From the analysis of both figures, it can be seen thatmpedance is higher for the smaller device. Studying thempedance modulus, we can observe a mainly resistive be-avior in low and medium frequencies. It stands for theainly resistive role that the charge movement has in theeactions involved during the natural operation of the de-ice, always at frequencies lower than 100 Hz. The value atow frequencies is changing with voltage, which corre-ponds to the changing electrochemical situation inside theevice. In both devices, the impedance module is decreas-ng as the voltage increases. It shows that the active speciesave better conducting properties than the original ones andhat the main current efficiency takes place at the beginningf the switching. Because of that, the consumption of theevice should be reduced in normal operation, with the cellFig. 1 Configuration of the new electrochromic devices.November 2006/Vol. 4511OE LETTERSFig. 2 Impedance modulus and phase plots for two devices with 92-mm2 and 1538-mm2 active areas,during the application of potential steps of 0.5 V: from 0 to 3 V a, and from 3 to 0 V b.Fig. 3 Nyquist plots for a device with 92 mm2 of active area, while switching from 0 to 3 V a and from3 V to 0 V b.Optical Engineering November 2006/Vol. 4511110501-2tovRitsnoeidtcvaidttAWoSPRToCRCRCRWOE LETTERSin the colored state, and with the application of short cur-rent pulses to maintain coloration because this current willcross a less resistive material. For the decreasing part ofthe ramp figures labeled as b, a change in the trend can bedetected from 1.5 V to 0 V. This is also shown in chrono-amperometric experiments. The 0 V signal should beapplied during higher times to reach stochiometricequilibrium.At high frequencies, the behavior is not resistive, but itsmodulus is the same for all the voltages. According also tothe phase plots, these features seem to be similar to those ofa Randles cell, as suggested by Bell et al. for this kind ofdevices.4Nyquist plots of the smallest device are shown in Fig. 3.As it can be seen, at high frequencies low ReZ values-the linear trend is added to the resistance effect, constantfor all the voltages, thus standing for the contacts of thedevice, and with a value of around 10 k. The differentvalues of the low-frequency parts of the plots high ReZvalues show the different resistances of the cell at chang-ing voltages, from around 800 k 0 V to 200 k 3 V.The asymmetrical response in the two ramps is also de-tected in these plots comparing Fig. 3a and 3b. Theseplots confirm a behavior that can be reasonably describedby an equivalent circuit made by a Randles circuit withWarburg impedance simulating the ionic diffusion at lowfrequencies Fig. 4. Table 1 shows the different elementsthat integrate the proposed electrical equivalent circuit. Ad-ditionally, this equivalent circuit may be useful to showsome problems related to the homogeneity of the adhesivecontact layers and the EC materials, not seen in the normaloperation. This can be revealed by simulating the equiva-lent circuit through a resistance and capacitance in parallelconfiguration, which can serve as a feedback method todesign appropriate materials with ionic and electronic prop-erties to make devices durable over many cycles.5 Tradi-tionally, organic ECs tended to suffer from problems withsecondary reactions during switching affecting the stabilityand durability of these systems.6 The electrical analysispresented in this work may also contribute to develop morestable organic systems for switchable glazing applications,truck mirrors, among others.4 ConclusionsNew all-plastic electrochromic devices, constructed byusual techniques and with commercial materials, and with aconfiguration of five layers instead of the typical seven lay-ers PEDOT is contact and active layer, have been electri-cally characterized.Electrical characterization has been made in order to testtheir dynamical properties in operation. Charge injectionshows that bleaching is slower and that a small current isdriven to the cell with 3 V applied to maintain coloration.Impedance analysis and Nyquist plots can be used to deriveFig. 4 Proposed equivalent circuit for the EC devices.Optical Engineering 110501-3he electrical parameters that may be useful to simulate theperation of the cell in a real polarization circuit. The de-ice is supposed to act as an equivalent circuit made by aandles circuit with Warburg impedance simulating theonic diffusion at low frequencies. The simple behaviorcompared with usual ones for electrochromic devices de-ected here must be related to the simple symmetrical con-truction of the device, especially because of the reducedumber of layers used. This work will enable better controlf organic electrochromic devices, leading to improvedlectrooptical performance as well as a better understand-ng of the electrochemical charge injection process in theseevices.The devices show promising results for electrically con-rolled window applications because switching time is not aritical parameter. Additionally, organic electrochromic de-ices can be used in truck mirrors, sunglasses, optical vari-ble attenuators, displays, etc. In any case, polymerizedonic-liquids could improve the dynamic response of theseevices. Further research on polymer electrolyte composi-ions and in the sticking or sealing processes will be doneo improve the dynamic behavior of the EC devices.cknowledgmentse thank Agfa, Daiso, and 3M for supplying free samplesf materials. This work was partially supported by thepanish Ministry of Science and Technology grant no.TR95-0940.01.OP.eferences1. D. S. Ginley and C. Bright, “Transparent conducting oxides,”MRS Bull. 25, 15–18 2000.2. D. Mecerreyes, R. Marcilla, E. Ochoteco, H. Grande, J. A. Pomposo,R. Vergaz, and J. M. Sánchez-Pena, “A simplified all-polymer flex-ible electrochromic device,” Electrochim. Acta 49, 35559 2004.3. R. Vergaz, J. M. S. Pena, A. B. Gonzalo, J. M. Ollero, C. Vázquez,J. A. Pomposo, H. Grande, and D. Mecerreyes, “Characterization ofnovel all-plastic electrochromic devices: electro-optic and voltam-metric response,” Opt. Eng. 4312, 2975 2004.4. J. M. Bell, J. P. Matthews, and L. L. Skryabin, “Modelling switchingof electrochromic devices—a route to successful large area devicedesign,” Solid State Ionics 152–153, 853–860 2002.5. C. M. Lampert, “Smart switchable glazing for solar energy anddaylight control,” Sol. Energy Mater. Sol. Cells 52, 207–221 1998.6. C. M. Lampert and C. G. Granqvist, Large-Area Chromogenics:Materials and Devices for Transmittance Control, Optical Engineer-ing Press-SPIE, Bellingham, WA 1990.able 1 Components of the proposed electrical equivalent circuit forrganic EC devices.omponent Source Meanings Contacts Resistance of the contactsp Sealing problems,bubblesCapacitorp Resistancedl Active layers Double-layer capacitancect Charge transfer resistances Ions Ionic diffusionWarburg impedanceNovember 2006/Vol. 4511

A simplified all-polymer flexible electrochromic device,” Electrochim.

and 3M for supplying free samples f materials. This work was partially supported by the panish

Characterization of novel all-plastic electrochromic devices: electro-optic and voltammetric response,”

Large-Area Chromogenics: Materials and Devices for Transmittance Control, Optical Engineering Press-SPIE,

Modelling switching of electrochromic devices—a route to successful large area device design,” Solid State Ionics 152–153,

Electrical analysis of new all-plastic electrochromic devices

Abstract

Similar works

Full text

Available Versions

Universidad Carlos III de Madrid e-Archivo

Name not available