MICRO STRUCTURED ELECTROCHROMIC DEVICE BASED ONPOLY(3,4-ETHYLENEDIOXYTHIOPHENE) (PEDOT)

T.Deutschmann, S.Roth, E.Oesterschulze

Physics and Technology of Nanostructures, University of Kaiserslautern, Kaiserslautern D-67663, GermanyNano Structuring Center, Department of Physics, University of Kaiserslautern, Kaiserslautern D-67663, Germany

Abstract — Because of their low powerconsumption and cheap fabrication, electrochromicdevices represent an interesting alternative basisfor many optical devices, including solar cells,paper-like displays, tuneable windows and mirrors,OLEDs, and optical filters. The broad commercialavailability of electrochromic devices remainshindered by technological obstacles, however. Forexample, many devices rely on the spatial control ofthe optical transmittance established by individualsegmented electrodes carrying the electrochromicmaterial, but structuring the segmented electrodeswithout degrading the optical properties of thepolymer is still a challenge. Here we present a viabletechnical solution combining UV lithography andboth wet and dry etching to define independentsegments with controllable transmittance.Another obstacle to commercial devices is sealant.Fast response times require solvent-based liquidelectrolytes, which are very volatile and aggressiveto most sealing materials; also the electrochromicpolymers must be protected from oxygen andmoisture entering the electrolyte solution. Herewe also introduce a three-step sealing techniqueto define the necessary parallel spacing betweenthe segmented working electrode and the counter-electrode with airtight sealing of the device. Wedemonstrate the operation of electrochromic devicesfabricated by combining our two techniques, andinvestigate the achieved optical contrast.

Keywords: Electrochromism, PEDOT, ITO,Conductive Polymer

I – Introduction

Electrochemical materials (ECM) are capable of re-versibly changing their absorption spectra and thus theirvisible color in response to an external voltage. Whenan electric potential is applied, the material is oxidized(or reduced) and small ions (Li+, H+, ...) diffuse fromthe electrolyte into the ECM to balance the charge. Bothprocesses, the oxidation and the incorporation of ions,change the chemical structure of the ECM reversibly.This structural change leads to a change in the bandstructure of the ECM and thus to a change in its color.In special cases, when one of the ECM absorbtions isin the IR- or UV-range, thin layers of the material canbe transparent in the visual range. These electrochromiclayers with electrically switchable transmittance are ofspecial interest as active layers in optical devices.

Deb introduced these types of material in 1969 [1]; thefirst ones were inorganic oxides like tungsten trioxide(WO3) and iridium dioxide (IrO2), and were expensive,complex to synthesize, and had rather long switchingtimes of several minutes. In recent years, however, thedevelopment of polymer electrochromic materials hasintroduced many improvements and opened the doorfor several practical applications. Polymer ECMs arecheap, easily processable, flexible, rapidly switchableand can be designed to achieve almost any color [2, 3].Electrochromic devices (ECD) make use of this colorswitching capability. They have appealing advantagesover established technologies like LCD (liquid crystaldisplay), including low power consumption, roll-to-rollfabrication, and high mechanical flexibility [4]. ECDsare already used for commercial products like smartwindows [5] or tunable car mirrors. Other applicationslike optical filters [6], paper-like displays or e-booksare interesting emerging fields. In order to turn suchcomplex devices into commercial products, however,some obstacles have to be overcome.

This work deals with two of these issues. Func-tionalization of the device requires structuring of thepolymer layer and the underlying transparent electrodeto a micrometer scale, without diminishing the opticalproperties. We present a combination of UV lithographyand dry and wet etching processes to achieve discreteelectrode segments with a separation width of 100 µm.This method can potentially realize any desirable struc-ture with a size of less then 10 µm utilizing conven-tional UV lithography.

The second obstacle to electrochromic devices thatwe address is the problem of device sealing. For sta-bility and safety, currently available ECDs have solidelectrolytes. However, liquid electrolytes would offer afaster response time for the electrochromic switching,due to higher mobility of the ions in the electrolyte. Touse liquid electrolytes the sealant has to fulfil certainrequirements. The sealing materials must be chemicallystable to withstand the solvent. Furthermore, they mustalso seal the device airtight, to prevent the solvent fromevaporation and to protect the polymer from moistureand oxygen. Since most solvents are also toxic, reason-able resistance against breaking and leaking of the sol-vent has additionally to be ensured. We present a triplesealing method, using an o-ring-seal as an inner barrierand spacer, an epoxy glue to mechanically stabilize thedevice, and an epoxy resin to seal the device airtight.

II – Experimental Details

A. Materials and reagents

All chemicals mentioned were purchased fromSigma-Aldrich and used as received if not otherwisestated. The ITO covered glass slides were also boughtfrom Sigma-Aldrich and are 25 x 25 x 1.2 mm in size.The ITO layer has a thickness of 100-150 nm, aresistivity of 8-12 Ω/sq and a light transmittance ofabout 86% in the visible region. The electrolyte solutionwas prepared from 0.1 M lithium salt (LiCF3SO3; 96%)in acetonitrile (ACN, HPLC grade 99.99% anhydrous)without further purification. For the electrochemicaldeposition EDOT monomer (97%) was added to theelectrolyte solution.

B. Design and concept

Electrochromic devices are electrochemical cells,which are generally realized as a layer structure wherean electrolyte is sandwiched between two electrodes.The schematic in figure 1 illustrates the design ofour layer system. The transparent working electrodeis an ITO layer on a glass substrate. ITO is a semi-conductive metal oxide with good conductivity andtransparency and is commonly used for transparentelectrodes. An electrochromic polymer PEDOT layer iselectrochemically deposited on top of the ITO. PEDOTis a well-researched [7] and commercially availableelectrochromic polymer. It is light blue and almosttransparent in the oxidized state, and dark blue in thereduced state. The ITO and PEDOT layer representthe working electrode of the electrochemical cell. It isseparated from the counter-electrode made of a secondITO covered glass by a liquid electrolyte, that is elec-trically insulating, but ionically conducting. The elec-trolyte also stores enough ions due to its thickness andconcentration rendering a special layer for that functionunnecessary. Eliminating the latter should presumablyraise the overall transparency.

+electrolyte

PEDOT

ITO

glass superstrate

glass substrate

ITO

Figure 1: Layer structure used for the ECD.

III – Results and Discussion

A. Electrochromic Film Deposition

The PEDOT polymer film was deposited froma solution containing 0.01 M EDOT monomer and

0.1 M LiCF3SO3 in acetonitrile. The deposition wasperformed in a three electrode cell, with ITO asworking electrode, a silver wire as reference electrode,and a platinum wire as counter-electrode. To controlthe process we used a ”Reference 600” (Gamry)potentiometer in constant potential mode. Whenthe potential of the working electrode rises above1.2 V against the reference electrode, the monomerin the solution is oxidized and begins forming thepolymer film on the working electrode [8]. We useda voltage of 1.5 V for our deposition to ensure fastand homogeneous formation of the polymer layer. Thepotentiometer was also used to measure the depositedcharge which is proportional to the layer thickness. Wedeposited about 20 mC/cm2 to obtain a layer thicknessof 250-300 nm. The thickness was measured using asurface profiler (Dektak 8, Veeco).

B. Structuring of the Working Electrode

After deposition the electrode was structured using aphotoresist as an etching mask. The resist (AZ 5214E,MicroChemicals) was spread out on the polymer with aSuss Delta 80BM Gyrset spin-coater at 4000 rpm andheated to 110C for one minute. The resist was thenexposed to UV light with a dosis of 100 mJ/cm2 througha mask and developed for 45 seconds in AZ 400K de-veloper (MicroChemicals). The remaining photoresistwas then utilized as a mask for the etching processes.In a first step the PEDOT was etched by an oxygenplasma with 100 W at 2 mbar pressure for 10 minutes inan plasma-etcher (PICO-UHP, Diener electronic). Sincethere is no noticeable selectivity between the resist andthe PEDOT layer, etching time and power had to becarefully controlled. After removal of PEDOT, the ITOwas etched by a HCl solution (40 %(vol)) for 4 minutes.Finally, the remaining photoresist was removed in N-Ethyl-2-pyrrolidon (NEP) for 20 minutes at 100 C,followed by rinsing the substrate with acetonitrile anddrying with nitrogen. We realized structures with a sizedown to 100 µm. The resolution was limited by thechoice of the mask, and could be further reduced to lessthan 10 µm using high resolution fused silica masks.

A typical example of a structured electrode with threeannular segments is shown in figure 2. It is operated inan electrochemical cell as working electrode togetherwith a silver reference wire and a platinum counter-electrode. The figure show that all three segments canbe switched individually and that no electrical crosstalkoccurs.

C. Device Assembly

Before assembling the device, an inlet port for theliquid electrolyte was prepared. For that purpose a0,5 mm hole was drilled into the counter-electrode witha diamond-coated drill. After cleaning the electrodesin acetone and isopropanol, a stack composed of theworking electrode, a spacer and the counter electrode

Figure 2: Images of the structured working electrode in theelectrochemical cell. A silver wire is shown behind the work-ing electrode to demonstrate the transparency contrast. Thethree segments can be switched individually from transparentto deep blue, without any crosstalk.

was formed. The spacer constituted the inner barrierfor the electrolyte solution, and needed to ensure aplanar well defined distance between the electrodes.We used an o-ring seal made of viton (chemicallystable) with a thickness of 1 mm. The stack wasfixed using an epoxy glue (Endfest 300, UHU) at atemperature of 100C with a pressure of about 10 N forone hour. After cooling, the electrolyte solution wasfilled through the inlet port using a syringe (Solaronixinc.). Subsequently, the inlet port and the glue werecovered with an epoxy resin (TorrSeal). After 24 hoursof drying the resin sealed the device airtight. In a laststep the ITO contacts were protected from physicaldamage by a conductive adhesive copper tape (3M).The finished device is shown in figure 3.

D. Characterization of the Device

One of the most interesting properties of elec-trochromic devices is their contrast. It is defined as

∆%T = T1(λ )−T2(λ ) (1)

where T1 and T2 are the transmittance at a certainwavelength λ for the transparent and the coloredstate, respectively. Our spectra were measured witha spectrometer (plasmacalc-2000; Mikropak) and alight microscope (A1m; Zeiss) for illumination. Tocompare the measure data in this section, we divided allintensities by the intensity measured with a referencedevice, which was of identical design but had nopolymer film deposited. The spectra for the opaque andtransparent state as well as the contrast of the deviceare shown in figure 4. The contrast reaches a maximumof over 50% at a wavelength of about 600 nm.

Figure 3: The optical image shows the completely sealedand fully functional device. The outer annular segment wasswitched into the opaque state, while the two inner segmentsare transparent. The three-step sealing consists of the blacko-ring in the center, the almost transparent epoxy glue, andthe white epoxy resin around the device and on top of the inletport.

Further measurements were made to investigate thememory effect of the device which describes theability of the ECM to hold a redox state in an opencircuit. To measure this effect a potential of 2 V or-2 V was applied for 10 seconds to switch the device

0.2

0.4

0.6

0.8

1

450 500 550 600 6500

20

40

60

80

100

rel.

inte

nsity

cont

rast

[%]

wavelength [nm]

transparentopaquecontrast

Figure 4: Spectral measurements in both opaque and trans-parent mode of the device. Rel. Transmission 1 is a referencedevice with no ECM. The resulting contrast is the differenceof both measurements and reaches over 50% at a wavelengthof 600 nm.

into the transparent or opaque state, respectively. Afterthis initial voltage pulse the intensity of transmittedlight with a wavelength of 633 nm was recorded over10 minutes. The results were normalized to the intensityof the reference device at the same wavelength and areshown in figure 5. While the transparent state staysstable, the opaque state slowly degrades. After 10minutes the overall contrast at this specific wavelengthdropped by only 5%. Depending on the application,refreshing the ECM state may only be needed a fewtimes per hour, which underlines the very low powerconsumption expected from electrochromic devices.

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500 6000.2

0.3

0.4

0.5

0.6

0.7

rel.

tran

smis

sion

time [s]

transparent

opaque

Figure 5: The memory effect was investigated in the followingway. After applying a 10 second voltage pulse the electrodeswere kept at floating potential. The transmitted intensity nor-malized to that of the reference device was observed at awavelength of 633 nm, revealing a contrast drop of only 5%after 10 minutes.

IV – Conclusion

An approach for the design and fabrication ofa micro-structured organic electrochromic device ispresented. Processes were developed for the photo-lithographical structuring and etching of stack of an ITOand PEDOT layer to accomplish segmented electrodes.It enables to realize optically active segmented elec-trodes of any desired design, suitable for a variety ofpractical applications.

To demonstration the feasibility of this approach, wefabricated an electrochromic device with three coaxialannular segments. They could be switched individuallywithout any crosstalk occurring. The device reached amaximum contrast of 50% (@ 600 nm). Without con-nection to the power supply, the contrast dropped byonly 5% after 10 minutes duration. This memory effectis the main reason for the very low power consumptionof ECDs.

In the future the presented EC device will be thebasis to establish an tuneable micro-structured irisaperture for the use in active micro-optics.

V – Acknowledgements

We thank the German Research Foundation (DFG)for their financial support through Priority Program1337 ‘Active Micro-optics’ . We also wish to thankthe Nano Structuring Center (NSC) for their technicalsupport.

References

[1] S. K. Deb. A novel electrophotographic system.Applied Optics, 8:192–195, 1969.

[2] Aubrey L. Dyer, Emily J. Thompson, and John R.Reynolds. Completing the color palette with spray-processable polymer electrochromics. ACS Appl.Mater. Interfaces, 3:1787–1795, 2011.

[3] Chao Ma, Minoru Taya, and Chunye Xu. Flexi-ble electrochromic device based on poly (3,4-(2,2-dimethylpropylenedioxy)thiophene). Electrochim-ica Acta, 54:598–605, 2008.

[4] Tunga Tsai-Shih and Ho Kuo-Chuan. Cyclingand at-rest stabilities of a complementary elec-trochromic device containing poly(3,4- ethylene-dioxythiophene) and prussian blue. Solar EnergyMaterials & Solar Cells 90 (2006) 521537, 90:521–537, 2006.

[5] Alexander Kraft and Matthias Rottmann. Proper-ties, performance and current status of the lami-nated electrochromic glass of gesimat. Solar En-ergy Materials & Solar Cells, 93:2088–2092, 2009.

[6] S. Roth, M. Ignatowitz, P. Mueller, W. Moench, andE. Oesterschulze. Non-mechanical variable aper-tures based on poly(3,4-ethylenedioxythiophene)(pedot). Microelectronic Engineering, 88:2349–2351, 2011.

[7] Manrico Fabretto, Kamil Zuber, Carlos Jariego-Moncunill, and Peter Murphy. Measurement proto-cols for reporting pedot thin film conductivity andoptical transmission: A critical survey. Macromol.Chem. Phys., 212:2173–2180, 2011.

[8] Serge Cosnier and Arkady Karyakin, editors. Elec-tropolymerization; Concepts, Materials and Appli-cations. Wiley-VCH, 2010.