Inkjet Printing PEDOT:PSS using Desktop InkjetPrinter

Chavis Srichan1* Thitirat Saikrajang2 Tanom Lomas1 Apichai Jomphoak1 Thitima Maturos1 Disayut Phokaratkul1

Teerakiat Kerdcharoen2 Adisom Tuantranont1*1 Nanoelectronics and MEMS Laboratory, National Electronics and Computer Technology Center (NECTEC) ,

NSTDA , Thailand2 Center of Intelligent Materials and Systems, Faculty of Science, Mahidol University, Rama VI Road.,

Bangkok, Thailand*Corresponding Author, email: chavis.srichan@nectec.or.th.adisorn.tuantranont@nectec.or.th

Abstract- PEDOT:PSS has been used recently into manyorganic-based devices in order to help charge transfer andimprove efficiency of the devices. PEDOT:PSS exhibit variousinteresting properties. It posses relatively good electrochemical,ambient, and thermal stability of its electrical properties ascompared with the other polythiophenes. One aim ofmanufacturing organic-based device is to lowering thefabrication cost. Due to PEDOT:PSS's stability, it is possible topattern PEDOT:PSS using inkjet printing. We found that usingthe CANON IP4500 desktop inkjet printer, the structure of 150micron could be patterned on PET substrate. By modifying thesurface properties of the substrate , the structure of 20 microncould be achieved. The conductivity of inkjet printedPEDOT:PSS could be further enhanced by annealing at 80 C.The conductivity could be 3 times improved. The morphology ofthe annealed PEDOT:PSS was further investigated using atomicforce microscopy(AFM) and the cause for conductivityenhancement could be explained via localization length extensionin variable range hopping theory.

I. INTRODUCTION

In the recent years, interest in organic electronics has beenlargely increasing due to their low cost material and hightheoretical efficiencies and potential manufacturability[I].One of the most prominent material is Poly(3,4­ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS).PEDOT:PSS exhibit various interesting properties. It possesrelatively good electrochemical, ambient, and thermal stabilityof its electrical properties as compared with the otherpolythiophenes. The processibility of PEDOT itself is verypoor since it is highly insoluble in water. This could berelieved by polymerizing it in a water soluble electrolytePSS[2]. The resultant PEDOT:PSS is p-doped material. It ismoderately transparent dark blue and has relatively highelectrical conductivity (1-10 S /cm). Due to its remarkableproperties , PEDOT:PSS could be patternable by printingtechnique. Spin coating has several disadvantages such as itcan not pattern the structure directly and the instrument used isquite expensive. There are many kinds of printing techniqueavailable such as inkjet, gravure, screen printing andimprinting. In this study we will focus only on the inkjet

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printing technique. Inkjet printing has several advantages overspin coating. It is low cost mask-less non contact processand consumes small amount of material as compared withspin coating technique. Nevertheless one of the limiting factorof inkjet printing technique is its resolution which is related tothe smallest dot being able to be inkjet printed.

For typical desktop inkjet printer, dots with diameter about50-150 micrometer are dispersed over the printed image area.A standard desktop inkjet printer are usually able to controlpicoliter amount of liquids in a precise pattern. This capabilitymakes the desktop printer one of the promising device formaterial patterning.

Surface morphology, shapes and thickness of the dropletsdepend on several factors such as surface energy of thesubstrate and ink and evaporation process. By controlling ormodulating these parameters, one can engineer the dots's sizeand their morphology.

There are several techniques based on microactuatorprinciple being used in inkjet printing technology. There aretwo major principles being commercialized in the marketnamely piezoelectric based and thermal based actuator. Oneof the most widely used desktop inkjet printer is thermalbased inkjet printer. Thermal print head consists of an inkchamber having a heater with a nozzle nearby [3]. With acurrent pulse running through the heater, heat is generated andtransferred from the heater to the ink. The ink becomes heatedto the critical temperature for bubble nucleation. When thenucleation occurs, a water vapor bubble instantaneouslyexpand to force the ink out of the nozzle. Heat from thermalinkjet printing might help dispersing polymer in the solution,however it might also introduce thermal load to the polymer.

Figure 1. Interdigitated Pattern used to determine resolution of the inkjetprinted pattern,

II. EXPERIMENTAL

An unmodified commercial available CANON IP4500desktop inkjet printer was used in this study. The inkjet print­ready PEDOT:PSS solution (Clevios P Jet N from HCStarck,USA) was loaded into the Grey Scale Cartridge. Theinterdigit pattern was designed in Adobe Photoshop 7.0 withthe resolution of 200 pixel per em. Therefore one pixel wouldresult in a dot of 50 micron diameter. Three sizes of 50 100and 150 micron interdigitated pattern were drawn as shown infigure 1 . The smallest feature size patternable by inkjetprinting technique could be found by checking if there is ashort circuit between the two electrodes. The image wasprinted on the flexible Polyethylene terephthalate (PET)substrate and let it dry in room temperature for 24 hours. Inorder to measure the conductivity of the PEDOT film. The3xl0 mm rectangular pattern was printed and the resistancewas measured. The thickness of the film was determined byAtomic Force Microscopy (AFM). From the geometry ,conductivity can be approximated using:

cr = liRA. (1)

17.7nm

Onm

Figure 2. AFM topographic image of the sample annealed at 80°C for 2hours

The thickness of the inkjet printed PEDOT:PSS is found to beapproximately 300nm. The edge of the structure is not sharpbut extends approximately 50 micrometer due to reflowing ofthe solvent before its evaporation.

The resistance of the annealed sample and sample withoutannealing are found to be 8 kn and 28 kn respectively. Withthe thickness of 300 nm determined from the AFM, thecorresponding conductivities are 1388.9 and 396.8 S/mrespectively. The influence of annealing temperature waschecked by annealing the samples at various temperatures for2 hours. The surface morphology of samples were investigatedusing AFM , as shown in figure 2 and 3.

Finally the PEDOT:PSS was also printed on the PDMSsubstrate. The morphology of the dots was examined usingScanning Electron Microscopy (SEM).

III. RESULTAND DISCUSSION

The interdigitated patterns with size of 50 and 100 micrometerare found to have short circuits except for one having size of150 micrometer. This indicates that for feature size smallerthan 100 micrometer the solvent might reflow before it isevaporated. Therefore the resolution of the PEDOT:PSS inkjetprinted on the unmodified PET surface was limited to 150micron. Since PEDOT:PSS has relatively good hydrophilicproperties, if the hydrophobic substrate is used then theresolution limit could be further reduced.

11.4nm

Onm

Figure 3. AFM topographic image of the inkjet printed sample withoutannealing

The conductivity also depends on the morphology of PEDOTand PSS. By annealing, the conductivity changes due to thechange of morphology. As can be seen from figure 4 andfigure 5 that after annealing, the PEDOT area is moreconnected and packed in comparison with the sample which isnot annealed. According to the manufacturer data, high ratioof PSS per PEDOT will yield better conductivity.Eventhough from the phase image of the annealed sample, thePSS-rich region seems to have smaller area than the PSS-rich

Figure 5. AFM phase image of the sample without annealing uponthresholding. Red area represents PEDOT-rich region.

According to Mott's VRH theory [4] , the probability oftheelectron for hopping between site i and j having energy stateEiand Ej respectively is

(4)

(3)

(6)

(5)

L = Y£ -1/3

Assuming that hopping distance is

{3 ;}-3/4

e= -kT­2 r

Putting this into eq.(3) we obtain

d(-2r )- ---e/kT =0de e1/3

;

By solving eq. (3) ,one can find

Thus by extending the localization length, the ratio ofhopping distance and localization length will decrease andaccording to eq. (6) this would result in higher hoppingprobability.

where B is energy difference for 3 dimensional system, theenergy at which hopping probability reach its maximum valuecould be calculated.The criteria for maximum hopping probability is

hopping through the PSS region must be able to take place.By enlarging the PEDOT-rich region , localization length isextended and consequently yielding higher hoppingprobability .This higher hopping probability would result inhigher conductivity. Nanoparticles were also found in theannealed samples. According to the AFM phase image, theparticles are expected to be PEDOT-rich material. Theseparticles are the result of PEDOT-phase condensation afterannealing.

The grain size of the PEDOT-rich region can be determinedby setting the threshold of the phase image. The thresholdvalue are chosen so that to homogenize nanoparticles of theannealed samples with respect to the AFM phase. Averagegrain size could be statistically computed by the imageprocessing software Gwyddion 2.9 .The average grain size forannealed samples and samples without annealing were foundto be 126 nm and 7.1 nm, respectively.

208.5 nm

99.8 nm

67.2 nm

148.5 nm

Figure 4. AFM phase image of the annealed sample upon thresholdingRed and yellow area represent PEDOT-rich region and PSS-rich regionrespectively.

region of the samples without annealing, they contains thesame ratio of PEDOT per PSS. Therefore the advantage ofannealing is to improve the conductivity by changing themorphology of the PEDOT:PSS. The larger grain of PEDOTwill result in higher conductivity. This effect could beexplained by variable range hopping(VRH) theory.

(L M .. )p. "-'exp _2----1)

I) ; kT(2)

From eq. (2) supposing that conductivity is linearlyproportional to the hopping probability and hopping distanceremains unchanged for the sake of simplicity , one can say that

where ~Eij = Ei- Ej and L , ~ are hopping distance andlocalization length respectively. PSS has poor conductivityand result in non conductive region. For conduction to occurbetween two PEDOT-rich regions separating by PSS, electron

.!!- = exp(- 2[ L - £]]a o ~ ~o

Solving for L / ~o ' we obtain

(7)

(8)

1 a-In-2 0'0

1- ;0;

Let's assume further that localization length scales linearly

according to grain size thus ;0/; = 7.1nm/126nm=0.056 .

Inserting this into eq.(7) and alao = 28 kQ/8 kQ =3.5 , we get

L /;0 = 0.663. Using this model, we could estimate the ratio

ofhopping distance and localization length.

Figure 6. SEM image of the PEDOT:PSS droplet on PDMS

Without plasma treating the PDMS, PEDOT:PSS film can notbe formed. Due to high surface tension of PEDOT:PSS andPDMS, the droplets of 20 micron are formed as illustrated inFig. 6. If we could print array of these dots and force them toconnect to each other, it might be possible to make amicrostructure with the minimum feature size of20 micron.Nevertheless in this study, only the maximum value of thegrey scale image was used. By modulating the grey scalevalue of the pattern, film thickness , solvent reflow lengthand the droplet's size could be changed. Change in solventreflow length would have an influence on minimum spacingrequired for making patterns and change in droplet size wouldcorrespond to the minimum feature size achievable by inkjetprinting.

IV. CONCLUSION

By inkjet printing PEDOT:PSS, resolution of 150 micron andthickness of 600 nm on PET substrate could be realized. Thefeature size could be reduced to 20 micrometer by printing onthe hydrophobic substrate such as PDMS. The conductivity ofthe inkjet printed PEDOT:PSS depends strongly on itsmorphology. Further annealing would result in differentevaporation rate and the reorientation of PEDOT and PSS. Byannealing at 80°C for 50 minutes the conductivity could be 3times enhanced. Not only does the conductivity ofPEDOT:PSS depend on the PEDOT to PSS ratio but also onthe morphology of the PEDOT:PSS as well. By annealing onthe PET substrate, PEDOT rich regions are enlarged.According to VRH theory, this would result in higher hoppingprobability and hence higher conductivity.

ACKNOWLEDGMENT

This work has been supported by printed electronics projectunder NECTEC. We would like to thank Mr. MatiHorprathum for his help and support in using instruments andall of our colleagues at Nanoelectronic and MEMS LaboratoryNECTEC for giving fruitful discussion and helps.

REFERENCES

[1] Erik Garnett and David Ginley, "Electrical and Morphologicalproperties of inkjet printed PEDOTIPSS films," US department ofenergy Journal ofUndergraduate Research, pp.24-29

[2] Yuka Yoshioka, Ghassan E. Jabbour, Desktop Inkjet printer as a tool toprint conductive polymers, Elsevier Synthetic Metal vol. 156 , pp 779­783,2006

[3] Hue P. Le, Progress and Trends in Ink-jet Printing Technology, Journalof Imaging Science and Technology, vol. 42. Number 1, pp.49-62,January /February 1998

[4] A. M. Nardes, On the conductivity of PEDOT:PSS thin film,Eindhoven: Technische Universiteit Eindhoven ,2007 Proefschrift ISBN978-90-386-1178-5