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Synthetic Metals 194 (2014) 38–46

Contents lists available at ScienceDirect

Synthetic Metals

jo ur nal homep age: www.elsev ier .com/ locate /synmet

EDOT-PSS based 2-in-1 step-by-step films: A refined study

hristine de Saint-Aubina,∗, Mohammad El Hajj Hassana,1, Philippe Kunemanna,ilia Patoisb,2, Boris Lakardb, Roxane Fabrec, Joseph Hemmerléc, Pierre Schaafc,ichel Nardina, Marie-France Vallata

Institut de Science des Matériaux de Mulhouse, UMR 7361 CNRS – UHA, 15 rue Jean Starcky, BP 2488, 68057 Mulhouse cedex, FranceInstitut UTINAM, UMR 6213 CNRS – Université de Franche-Comté, 16 Route de Gray, 25030 Besanc on cedex, FranceInstitut National de la Santé et de la Recherche Médicale, U1121, 11 rue Humann, 67085 Strasbourg cedex, France

r t i c l e i n f o

rticle history:eceived 16 November 2013eceived in revised form 20 February 2014ccepted 8 April 2014vailable online 16 May 2014

eywords:EDOT-PSS-In-1 step-by-step film buildup

a b s t r a c t

The fine influence of several key parameters onto the recently reported 2-in-1 step-by-step constructionof PEDOT-PSS nanofilms by spin-coating is investigated by laser ellipsometry, UV–vis–NIR spectrometry,tapping-mode AFM and 4-point probe conductimetry following Van der Pauw geometry. First, the thick-ness of the film increases when deposited under good ventilation. Then, the linearity of film thicknesswith respect to the PEDOT-PSS deposition step number is maintained by thermal treatment at 423 K dur-ing 30 min, showing that the 2-in-1 deposition method is compatible with the thermal annealing stepsused in electronic devices containing PEDOT-PSS. Moreover, the concentration of the PEDOT-PSS suspen-sion used for the deposition exerts major influence on the film buildup rate, with a minimum one needed

hermal annealinganometric thickness controllectronic conductivitynelastic cut-offs of scaling

for the method to process at reasonable pace. Finally, analogously to what is known for films obtained bya sole deposition step, the conductivity of 2-in-1 PEDOT-PSS nanofilms is shown to behave differently atambient to high temperature (373 K) than at the lower temperatures where the conductivity studies areusually made. All these results will be precious for the construction of devices containing a PEDOT-PSSfilm with a thickness needing to be controlled reproducibly at the nanoscale.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

PEDOT-PSS is one of the key substances in the spreading fieldf organic electronics. Thanks to its good conductivity, relativeransparency and air stability, its films obtained from aqueoususpensions are used in many devices of organic electronics likerganic light emitting diodes (OLEDs) [1], organic solar cells (OSCs)2] or organic field effect transistors (OFETs) [3].

In these devices, very thin submicrometric films of PEDOT-PSS

re needed. In OSCs, the need for transparency imposes a first upperimit to the thickness of the PEDOT-PSS buffer layer, which must notxceed 100 nm [4]. Moreover, in a recently proposed OSC [2], the

∗ Corresponding author. Tel.: +33 3 89 60 87 00.E-mail addresses: christine.de-saint-aubin@uha.fr, chrisdsa@free.fr

C. de Saint-Aubin).1 Present address: Université de Liège, Département de chimie appliquée/Facultées Sciences Appliquées, Institut de Chimie-Bâtiment B6, Sart Tilman, 4000 Liège,elgium.2 Present address: Institute of Condensed Matter and Nanosciences/Bio and Softatter, Université Catholique de Louvain, Croix du Sud, 1, L7.04.02,

348 Louvain-la-Neuve, Belgium.

ttp://dx.doi.org/10.1016/j.synthmet.2014.04.003379-6779/© 2014 Elsevier B.V. All rights reserved.

PEDOT-PSS layer thickness has not only to be submicrometric butvery precisely tailored to embed only partially gold nanoparticles(Au Nps) deposited on the indium-tin oxide (ITO) electrode of theOSC. Otherwise, these Au Nps could not simultaneously assumetheir two roles i.e. near-field enhancer of the exciton generationand dissociation in the active layer as well as hole carrier extractionenhancer at the ITO electrode.

Consequently, the methods classically used for constructingPEDOT-PSS films like simple casting [1,5–8], spin-coating [2,9–14],spraying [15] or ink-jet printing [16] have to be turned into con-trollable and reproducible ones at the nanometric scale. Theirmain parameters need also to be identified and elucidated. How-ever, apart from general considerations like that the thicknessof a PEDOT-PSS film can be easily varied using different spin-coating speeds [15], there is still a need for fine studies aboutthe deposition parameters of PEDOT-PSS during nanometric filmbuildup.

Moreover, properties like conductivity of such films have to

be known in conditions close to the ones applied during deviceutilization, approximately between 300 and 400 K. Yet, most ofthe PEDOT-PSS film conductivity studies were done under 300 K[7–9,17] to bring theoretical understanding onto the electronic

C. de Saint-Aubin et al. / Synthetic

(a) S+

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NH

xN

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Fig. 1. Chemical formulae of the polyelectrolytes used in the present work: PEDOT(

tt

PoiittotbNlsolatc

asmbbetr

2

2

(pa(fo

a), PSS (b), bPEI (c).

ransport mechanism and they were seldom done over ambientemperature [11].

In a previous study, we proposed to cast new light on PEDOT-SS films by considering them in terms of view of layer-by-layerbtained polyelectrolyte multilayers [12]. PEDOT-PSS films havendeed to be considered like polyelectrolyte films where PEDOTs the positively charged oligomeric electrolyte and PSS the nega-ively charged polyelectrolyte (see Fig. 1 for the formulae of thesewo compounds), both being deposited from a unique suspensionf the PEDOT-PSS complex during step-by-step film buildup. Ashis 2-in-1 buildup fulfills the need for nanometric PEDOT-PSS filmuildup with controlled thickness, we decided to refine its study.ote that the strength of the 2-in-1 method applied to PEDOT-PSS

ies in the absence of any alternate deposition of the PEDOT-PSSuspension with another suspension, as this would degrade somef the properties of the films like conductivity [12]. Moreover, aayer-by-layer construction of the film with alternate deposition of

PSS polyanion-solution and a PEDOT polycation-solution can nei-her be considered, as no common solvent, especially not water, isapable of dissolving PEDOT [18].

Firstly, we wanted to extend the knowledge of this buildupbout the key roles of the strongly related drying conditions andpin-coating speed as well as to study the possible effect of a ther-al annealing. As in our previous work, the PEDOT-PSS films were

uilt via spin-coating but the results obtained have implicationseyond this archetypal technique, for example for the now clearlymerging ink-jet printing. Secondly, we characterized the evolu-ion of the conductivity of the films with temperature higher thanoom temperature.

. Experimental

.1. Materials

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)PEDOT-PSS) 1.3 wt% dispersion in H2O (conductive grade),oly(sodium 4-styrene sulfonate) (PSS) (Mw ∼ 70,000 g mol−1)

nd branched poly(ethylene imine) (bPEI) solutionMw ∼ 750,000 g mol−1; 50% (w/v) in H2O) were purchasedrom Aldrich. All the products were used as received. The formulaef the different products are shown in Fig. 1. Water purified

Metals 194 (2014) 38–46 39

via reverse osmosis (Millipore Elix system, min. 5 M� cm, max.15 M� cm) was the solvent used for all experiments. Phosphorusn-doped [1 0 0] oriented silicon wafers were bought from MatTechnology (Morangis, France). Soda-lime microscope glass slideswere purchased from Carl Roth (Lauterbourg, France). Quartzslides were bought from Hellma (Paris, France).

2.2. Preparation of substrates

Three kinds of substrates were used, silicon wafers, glass andquartz slides. Silicon-oxide-on-silicon substrates were preparedby cutting silicon wafers to approximately 5 mm × 10 mm pieces,rinsed with cyclohexane and dried under a nitrogen stream. Thiscleaning was sufficient because of the buffering role of the precur-sor layers (see below) directly deposited on the substrate. Glasssubstrates were obtained by cutting microscope glass slides to13 mm × 27 mm pieces, rinsed with cyclohexane and dried under anitrogen stream. Quartz slides (12.5 mm × 45 mm) were cleaned inthe same manner than glass slides.

2.3. Preparation of spin-coated films

A TP6000 spin-coater with CT60 controller (Karl Suss, SaintJeoire, France) was used for the preparation of the spin-coated films.

2.3.1. (bPEI)1 or (bPEI-al-PSS)2 Precursor layersDepending on the chosen substrate, underlying precursor lay-

ers built by alternate deposition steps of bPEI and PSS aqueoussolutions are sometimes needed to allow the deposition of thePEDOT-PSS film onto the substrate. As explained in our previousstudy [12], at least one bPEI precursor layer is necessary for the filmbuildup on the silicon wafer. A system combining alternate deposi-tion of bPEI and PSS can be used instead of the sole bPEI layer withno modification on the further PEDOT-PSS deposition. For the glasssubstrate, as we found out finally, no precursor layer is needed, butof course they can be used.

To construct these precursor layers, a 0.6 mg/mL ultrasoni-cated aqueous solution of the positively charged bPEI (pKa = 8.2–8.3[19]) was manually spread over the whole surface of the nega-tively charged substrate and immediately spin-coated during 30 sat 5000 rpm, with an initial acceleration of 5000 rpm/s. This proce-dure constituted the (bPEI)1 precursor system. For certain samples,a 0.7 mg/mL solution of the negatively charged PSS (pKa = 1 [20])was then deposited in the same manner, without any intermedi-ate rinsing or further drying. The whole procedure was repeatedto obtain precursor layers designated as (bPEI-al-PSS)2, “al” mean-ing that the two substances, bPEI and PSS, were alternativelydeposited [12]. In other words, (bPEI-al-PSS)2 represents a sys-tem where bPEI, PSS, bPEI and finally PSS were successivelydeposited.

2.3.2. (Precursor layers) (PEDOT-cx-PSS)n filmsA 0.25 wt% PEDOT-PSS dispersion, obtained by diluting the com-

mercial one, was spread over the whole surface of a precursor layercovered substrate and spin-coated in the same manner as for theprecursor layers. At low spin-coating rotational speed (1000 rpm),the sample was additionally hand-dried in a nitrogen-stream. Theprocess was repeated n times to reach a system designated as(PEDOT-cx-PSS)n: “cx” meaning that the PEDOT-PSS complex wasdeposited n times [12]. Because both positive and negative specieswere simultaneously present in the unique mixture used for the

film buildup, we named this step-by-step method the 2-in-1 method[12]. Following this nomenclature, the entire deposited system,including the precursor one if needed, was designated as (precursorlayers) (PEDOT-cx-PSS)n, with (precursor layers) referring to either

4 nthetic Metals 194 (2014) 38–46

bbnofisa

2

Ut

2

t7mbtnstlnasNk

2

L(sq

2

S(stpstb3R(ma

2

pifswtotm

Fig. 2. Film thickness, measured by ellipsometry, of spin-coated (PEDOT-cx-PSS)n

films built on Si/SiO2/(bPEI-al-PSS)2 wafers, as a function of the deposition step

0 C. de Saint-Aubin et al. / Sy

PEI or (bPEI-al-PSS)2 and n referring to the deposition step num-er. We intentionally avoid calling n the “layer number”, because

does not necessarily at all describe the deposition of a continu-us layer of matter. When the effect of a thermal annealing on thelm was investigated, the final (precursor layers) (PEDOT-cx-PSS)n

ample was heated at 423 K for 30 min on a hot plate in ambientir.

.4. Characterization

The PEDOT-PSS films were characterized by laser ellipsometry,V–vis spectroscopy, atomic force microscopy (AFM) and conduc-

ivity measurements.

.4.1. Laser ellipsometryNull-ellipsometric measurements were performed using a Mul-

iskop (Optrel GBR, Berlin, Germany), operating at 532 nm at a0◦ angle of incidence, with a beam spot of 0.6 mm. Measure-ents were made at six different spots on each sample, the data

eing acquired by the Optrel software “Multi” and treated withhe Optrel software “Elli”. The model used to calculate the thick-ess and the real refractive index N of the studied polyelectrolyteystem was composed of four layers: a silicon layer with a refrac-ive index of N = 4.1501 (real) and k = 0 (imaginary), a silicon oxideayer with N = 1.4607 and k = 0 and a thickness determined for eachew sample, the studied polyelectrolyte system layer with k = 0nd N and its thickness both determined for each new depositionequence (or else, but only if numerical solving failed, imposing

= 1.5000 and k = 0) and finally an air layer with N = 1.0000 and = 0.

.4.2. UV–vis–NIR spectroscopyUV–vis–NIR absorption of the samples was investigated using

ambda 750 (PerkinElmer, Shelton, USA) and Evolution 220Thermo Fisher Scientific, Courtaboeuf, France) spectrometers. Theamples were deposited on microscope glass substrates or onuartz slides; these were taken as references.

.4.3. Atomic force microscopy (AFM)Tapping mode AFM images in air were acquired on Multimode

canning Probe Microscopes Dimension 3000 and Nanoscope IVDigital Instruments Veeco Technology group, Plainview, USA) totudy the morphology, rms (root mean square) roughness Rq andhickness of the constructed films. Nanoworld Arrow-NC siliconrobes (radius of curvature <10 nm) were used for imaging theamples. Imaging was done at a fixed scan rate of 1 Hz with a resolu-ion of 512 × 512 pixels. The obtained images were processed withoth Nanoscope 6.13r1 (Digital Instruments, Veeco) and WSxM.0 (Nanotec Electronica S.L.) softwares [21]. The rms roughnessq was calculated by the Nanoscope software from height images1 �m × 1 �m or 10 �m × 10 �m) after prior automatic planefit and

anual third order flattening treatment to remove any tilt and bowrtifacts.

.4.4. Conductivity measurementsConductivity measurements were done with a home-made four

oint probe apparatus following the Van der Pauw geometry,nterfaced with Labview [22]. The measurements were conductedollowing the NIST recommendations for 4-points probe mea-urements [23]. In particular, the quality of the ohmic contactsas checked for each measure. This quality was quantified by

he values of the linear regression coefficients of the Ohm-plotf the 8 resistances used to determine the Van der Pauw resis-ances Ra and Rb [23]: the measure is only accepted if the

ean of all coefficients had a minimum value of 0.999. More

number n with variable ventilation conditions: circles: good ventilation; triangles:poor ventilation. The error-bars represent the expanded uncertainties calculated ata confidence level of 99%.

about the four point probe apparatus can be found elsewhere[22].

3. Results and discussion

3.1. More about the drying key-steps

First of all, we confirmed the key-role of the drying step follow-ing each deposition step [12] by varying the ventilation conditionsin the region of the spin-coater around the PEDOT-PSS film underconstruction, through variable opening of the apparatus towardambient air. Fig. 2 shows the evolution of the thickness, deter-mined by ellipsometry, of two (PEDOT-cx-PSS)n films built onSi/SiO2/(bPEI-al-PSS)2 as a function of the number of depositionsteps n. The first film (circles) was obtained under good ventila-tion, the second film (triangles) was obtained in a more confinedenvironment.

It can be clearly seen that enough ventilation is necessary for the2-in-1 film buildup to succeed: at a given deposition step number(e.g. 50), the thickness of the film obtained under good ventilation(e.g. 72 nm) is always higher than the one obtained with poor venti-lation (e.g. 32 nm). The consequence is that in these last conditions,the film hardly constructs. We can thus make one step further inour way to explain the key-role of the drying-step in the 2-in-1deposition method [12]. Under good ventilation, the solvent dry-ing step can take place, which follows the centrifugal expulsionof the big majority of the suspension deposited onto the substrate.Moreover, the adsorption of water on the PSS-shell of the depositedPEDOT-PSS particles becomes then smaller than that existing in amore confined and consequently richer moisture atmosphere. Thishas been shown previously by Okuzaki and co-workers [5] in theirstudy of water sorption on a PEDOT-PSS film, by means of sorptionisotherm analysis. In this study, water adsorption was shown toproceed in two steps under increasing relative water vapour pres-sure. First, a water monolayer constructs on the highly hydrophilicsulfonates of the external PSS-shell of the deposited PEDOT-PSS.Then, after completion of this first monolayer, water condenses onless active sites or on already adsorbed water molecules, finallyreaching the sorption of water on pure water. Hence, we proposethat good drying conditions during one spin-coating step of a 2-in-1 PEDOT-PSS film buildup promotes desorption of water from

the deposited PEDOT-PSS film, allowing the charged sites on thesurface to attract the charged sites of the next PEDOT-PSS particlesthat will be deposited in the next deposition step.

C. de Saint-Aubin et al. / Synthetic Metals 194 (2014) 38–46 41

Fig. 3. Film thickness, measured by ellipsometry, of spin-coated (PEDOT-cx-PSS)n

films built on Si/SiO2/(bPEI-al-PSS)2 wafers, as a function of the deposition step num-ber n: before thermal annealing (black full curve); just after 423 K 30 min thermalannealing (dashed curve); after 423 K 30 min thermal annealing and 4 days in ambi-ee9

3fi

lo3Potpttfi

ehmPmbsttwdishslm

(eelsdo

fi

Fig. 4. Tapping-mode AFM 3D-height image of a 1 �m × 1 �m surface of: (a) aspin-coated (PEDOT-cx-PSS)25 film built on a (bPEI-al-PSS)2 glass slide, thermallyannealed at 423 K for 30 min and left in ambient conditions for 1 week (b) a spin-

nt conditions (gray full curve). The curves serve only as a guide for the eye. Therror-bars represent the expanded uncertainties calculated at a confidence level of9%.

.2. Non-perturbation of the linearity of the 2-in-1 growth by anal thermal annealing

To reach better overall performance, devices using PEDOT-PSSike photovoltaic cells or organic light emitting diodes (OLEDs) areften thermally annealed at temperatures between approximately48 and 423 K [13,14]. During these thermal treatments, PEDOT-SS is known to dry and to change its morphology, reaching a betterverall conductivity [10,24,25]. It was thus important to verify thathe 2-in-1 PEDOT-PSS film nanoconstruction we proposed was noterturbed by a subsequent thermal treatment. In other words, arehe linearity of the buildup over the deposition step number and thehicknesses of the 2-in-1 PEDOT-PSS films maintained even with anal thermal treatment?.

To answer this question, PEDOT-PSS 2-in-1 films with differ-nt deposition step numbers were prepared as usual and finallyeated at 423 K during 30 min in ambient air, a thermal treatmentimicking the ones usually applied to devices containing PEDOT-

SS films [4,26]. The thickness of these films was then immediatelyeasured by ellipsometry (Fig. 3). Compared to the reference films

efore thermal treatment, the films after thermal treatment seemlightly thinner (1–2 nm so near 5% less thick: this lies withinhe measurement uncertainty of 1–3 nm but the systematics ofhe phenomenon could still indicate a real tendency to thinning)hereas the linearity of the evolution of the film thickness witheposition step number n is maintained. After leaving these films

n ambient conditions for four days, their thickness was again mea-ured: compared to their state before thermal treatment, the filmsave slightly gained in thickness (2–4 nm so near 9% thicker, lyinglightly over experimental uncertainty) and the linearity of the evo-ution of the film thickness with deposition step number n is still

aintained.The slight loss (respectively gain) in thickness after heating

resp. after leaving the samples in ambient air) can be simplyxplained by a drying (resp. swelling) of the films due to waterlimination (resp. uptake). The important result here is that theinearity of the film thickness with respect to the PEDOT-PSS depo-ition step number is maintained by the thermal treatment: the

rying step due to the heating does not affect this valuable propertyf the 2-in-1 film nanoconstruction.

Furthermore, the surface morphologies and the roughness of thelms are also unaffected by the thermal treatment as shown by AFM

coated (PEDOT-cx-PSS)50 film built on a precoated Si/SiO2/(bPEI-al-PSS)2 wafer andwithout any further treatment.

imaging (Fig. 4), contrary to results obtained by others [11]. Thegranular structure characteristic of PEDOT-PSS, reminiscent of thegel-like particles found in the aqueous suspension used to build thefilms [12], can be observed as well on the surface of the thermallyannealed film (a) as on the surface of the non-annealed film (b). Thetwo films are very smooth with an rms roughness of 1 nm for thethermally annealed film and 0.8 nm for the non-annealed film, ascalculated on a 1 �m × 1 �m surface.

3.3. Fine influence of the rotational speed of the spin-coater

In a further step, we wanted to get a finer understanding of theinfluence of the rotational speed of the spin-coater on the 2-in-1PEDOT-PSS film construction.

For films constructed by spin-coating in a non-step-by-stepmanner, it can logically be predicted that the faster the rotation,the thinner the PEDOT-PSS film obtained because of the centrifu-gal force exerted on the film increasing with increasing rotationalspeed and of the drying potential increasing because of the airvelocity increase above the sample. Nevertheless, as far as we know,the influence of the rotational speed on the film growth rate aswell as on the film surface morphology has never been examinedfor PEDOT-PSS films constructed with the step-by-step method.Therefore, 2-in-1 PEDOT-PSS films were built by spin-coating thedeposited PEDOT-PSS suspension at four different rotational speedsr. The evolutions of the ellipsometric thicknesses of these fourPEDOT-PSS films as a function of the deposition step number aredepicted in Fig. 5.

The overall thickness of the 2-in-1 film does indeed decreasewith increasing speed: for example, the thickness obtained after40 deposition steps decreases from 159 nm at 1000 rpm to 105 nmat 2000 rpm and 3000 rpm, reaching a minimum value of 93 nmat 5000 rpm. For nearly all deposition step numbers, the thicknesscan satisfactorily be modeled by a linear function of the recipro-cal speed of rotation (see “Supplementary data”), allowing rapidchoice of the deposition conditions (number of deposition steps androtational speed) to reach a chosen thickness value. Note that the1000 rpm-curve is more uncertain than the others. This is attributedto the supplementary hand-drying step which perturbs somewhatthe film buildup.

The results of tapping-mode AFM study of the effect of the rota-tional speed during spin-coating on the surface morphology of the2-in-1 PEDOT-PSS films is given in Fig. 6.

The surface morphologies of the four films are identical. For a

1 �m × 1 �m studied film surface, the roughness around 3 nm ofthe films constructed at 1000, 2000, 3000 and 5000 rpm is inde-pendent of the rotational speed of the spin-coater. Thus, the 2-in-1

42 C. de Saint-Aubin et al. / Synthetic Metals 194 (2014) 38–46

Fig. 5. Film thickness, measured by ellipsometry, of spin-coated (PEDOT-cx-PSS)n

films built on Si/SiO2/(bPEI)1 wafers, as a function of the deposition step numbern, with variable rotational speed: wide-dashed curve: 1000 rpm; narrow-dashedccu

Pt

at

3

mdti

otgote

fildpc3

Fig. 7. Film thickness, measured by ellipsometry, of spin-coated (PEDOT-cx-PSS)50

films built on Si/SiO2/(bPEI)1 wafer, as a function of the deposition step number n,with variable fd (factor of dilution) of the commercial PEDOT-PSS suspension: blackfull curve: fd = 1; gray full curve: fd = 5; gray dashed cruve: fd = 30; black dashed

Fo

urve: 2000 rpm; gray full curve: 3000 rpm and black full curve: 5000 rpm. Theurves serve only as a guide for the eye. The error-bars represent the expandedncertainties calculated at a confidence level of 99%.

EDOT-PSS film surface morphologies are not affected by the rota-ional speed of the spin-coater.

Finally, films obtained via spin-coating at high rotational speedre easier to obtain, because additional hand-drying steps (see “Sec-ion 2.3”) are unnecessary.

.4. Major influence of the PEDOT-PSS suspension concentration

As in polyelectrolyte films obtained by layer-by-layer depositionethod, the concentrations of the polyelectrolyte solutions to be

eposited can play a role in the film buildup [27]. It is necessaryo know if the concentration of PEDOT-PSS in the suspension couldnfluence the PEDOT-PSS 2-in-1 film buildup.

First, the thickness growth of five PEDOT-PSS suspensionsbtained by increasing dilution of the commercial one was moni-ored by ellipsometry; the results obtained are given in Fig. 7. Therowth of the film obtained with the commercial suspension couldnly be measured until deposition step number 3, because thenhe film began to be too thick for the ellipsometric method to bemployed (see above).

As the concentration of the PEDOT-PSS suspension used forlm construction decreases, the curves in Fig. 7 are shifted toward

ower thicknesses. For a given deposition step number, the more

iluted the suspension, the thinner the obtained film. For exam-le, films of deposition step number 10 obtained with dilutedommercial PEDOT-PSS suspension are respectively 42 nm, 6 nm,

nm and 2 nm thick if the factor of dilution is 5, 30, 50 and 500,

ig. 6. Tapping-mode AFM study of spin-coated (PEDOT-cx-PSS)20 films built on precoatef the film. The expanded uncertainties (level of confidence of 99%) are obtained from at

curve: fd = 50; small dotted curve: fd = 500. The curves serve only as a guide for theeye. The error-bars represent the expanded uncertainties calculated at a confidencelevel of 99%.

respectively. When the factor of dilution varies between 1 and30, the effect of the concentration is the biggest and the filmgrowth is non-negligible. Over this value of 30, the film hardlyconstructs. For the non-diluted films, the 5 and 30 times dilutedfilms, the film growth rates are respectively of about 16, of 2 and of0.2 nm/deposition step number respectively, so with about a factor10 between each. Thus, to construct a 2-in-1 PEDOT-PSS film, theconcentration of the PEDOT-PSS suspension has to be maintainedbetween 1.3 wt% and 0.04 wt% (factor of dilution between 1 and30 from the commercial solution used in this study). Within thisinterval, a compromise about the value of the factor of dilutionhas to be made between the resolution of the film thickness andthe growth rate of the film construction: a higher thickness can bereached with smaller deposition step numbers with a concentratedsuspension, but with less accurate control of the thickness.

Complementarily, the evolution with the deposition step num-ber of the absorbance at 910 nm of different 2-in-1 PEDOT-PSS filmsobtained by varying the factor of dilution of the commercial suspen-sion is given in Fig. 8: the more diluted the suspension, the smallerthe absorption at a given deposition step number. This can be quan-

tified by the linear regression slopes of 0.025, 0.0029 and 0.0003absorbance units per deposition step number obtained for theseconstructions, here again with a factor of about 10 between each.As PEDOT absorbs at 910 nm due to its polaronic and bipolaronic

d Si/SiO2/(bPEI)1 wafers: height image; rms roughness Rq of a 1 �m × 1 �m surfaceleast two measurements at different locations of the same film.

C. de Saint-Aubin et al. / Synthetic Metals 194 (2014) 38–46 43

Fig. 8. UV–vis–NIR absorbance at 910 nm of spin-coated (PEDOT-cx-PSS)20 filmsbuilt on (bPEI-al-PSS)2 precoated glass substrate, as a function of the deposition stepnumber n, with variable factor of dilution (fd) of the commercial PEDOT-PSS suspen-sion: black full curve: fd = 1; gray full curve: fd = 5; dotted curve: fd = 50. The dottedlines represent the curve obtained by linear regression. The error-bars represent theexpanded uncertainties calculated at a confidence level of 95%.

acMlt

op(dtdipcstaaPto

Fpfi(

Fig. 10. Film thickness, measured by ellipsometry, of 8 spin-coated (PEDOT-cx-PSS)n

bsorptions [28], this indicates that less matter can be deposited toonstruct a 2-in-1 PEDOT-PSS film if a diluted suspension is used.oreover, it confirms that the film buildup is indeed possible and

inear for a non-diluted PEDOT-PSS suspension like the one used inhis study, a fact that could not be proven by ellipsometry.

An AFM study confirms as well the impact of the concentrationf the PEDOT-PSS suspension onto the 2-in-1 film surface mor-hology, as shown in Fig. 9. In Fig 9(a), a complete and smoothRq = 0.8 nm on a 1 �m × 1 �m surface) film is obtained with 50eposition steps of a PEDOT-PSS suspension diluted 5 times fromhe commercial suspension. By contrast, in Fig. 9(b), only some ran-om spots of PEDOT-PSS emerge above the surface of the substrate

f 50 deposition steps of a more diluted (dilution factor 50) sus-ension of PEDOT-PSS is used; no PEDOT-PSS film forms under thisondition. So by using too dilute commercial PEDOT-PSS suspen-ions, the surface of the substrates are not completely covered byhe film. The competition between dissolution in the water solventnd PEDOT-PSS deposition [12] is won by the dissolution process,llowing only the deposition of some random spots of PEDOT-SS onto the substrate. This explains the stagnating ellipsometrichickness and absorbance curves for the most dilute suspensions,bserved in Figs. 7 and 8.

ig. 9. Tapping-mode AFM study of spin-coated (PEDOT-cx-PSS)50 films built onrecoated Si/SiO2/(bPEI)1 wafer, 3D-height images of a 1 �m × 1 �m surface of thelm with variable factor of dilution (fd) of the PEDOT-PSS commercial suspension:a) fd = 5; (b) fd = 50 (no film: only random spots of PEDOT-PSS onto the substrate).

films built on Si/SiO2/(bPEI-al-PSS)2 wafers, as a function of the deposition step num-ber n. The error-bars represent the expanded uncertainties calculated at a confidencelevel of 99%.

3.5. Stability and reproducibility of 2-in-1 PEDOT-PSS films

After one year storage of the PEDOT-PSS films under ambi-ent atmosphere, their thicknesses remain constant, indicatinggood stability under usual conditions. Besides, like PEDOT-PSSfilms obtained by a non layer-by-layer manner [28,29], our 2-in-1PEDOT-PSS dissolve after immersion in Millipore water.

The reproducibility of PEDOT-PSS film buildup is shown inFig. 10 for eight films constructed by two different operators from a0.25% PEDOT-PSS suspension, at 5000 rpm, under good ventilationand over a time period of four months. The relative uncertaintiesof the film thickness, calculated from the expanded uncertaintiesat a confidence level of 99% indicated in Fig. 10, lie between 9 and18% throughout the explored deposition number range, which isremarkable at this nanometric level.

Hence, the 2-in-1 method is indeed a reproducible method forthe buildup of PEDOT-PSS nanometric films.

3.6. Conductivity of 2-in-1 PEDOT-PSS films

To define the lower limit of the thickness needed for a PEDOT-PSS 2-in-1 film to conduct properly, films of variable deposition stepnumbers constructed with the commercial PEDOT-PSS suspensiondiluted 5 times were tested by a four point probe method with Vander Pauw geometry; examples of the results obtained are given inFig. 11.

In the example of Fig. 11(a), a conductivity of approximately0.40 S/cm is measured between 297 and 373 K for a sampleobtained with 5 deposition steps of the PEDOT-PSS suspension(diluted five times from the commercial one). But as can be seenon the curves, the conductivity behaves erratic with temperature.This erratic behavior is due to a poor ohmic contact taking placeduring the measurements between the film and the four probes, asindicated by the poor value of the ohmic correlation factor (smallerthan 0.94 and mostly very far from this limit). The origin of this poorohmic contact is attributed to the non-continuity of the PEDOT-PSS film onto the non-conducting substrate. The behavior of a filmobtained with 50 deposition steps of the same dilute PEDOT-PSSsuspension strongly contrasts with the first example, as can beseen in the example of Fig. 11(b). Here, the conductivity of the filmregularly increases with temperature, witnessing a semiconduct-

ing behavior for PEDOT-PSS in this temperature range. During thisexperiment, the ohmic correlation factor lies near 1 (between 0.998and 0.999). All this indicates that this new film is complete enoughto ensure good ohmic contact with its surroundings. Moreover, the

44 C. de Saint-Aubin et al. / Synthetic Metals 194 (2014) 38–46

Fig. 11. Four point probe conductivity of spin-coated (PEDOT-cx-PSS)n films built on precoated Si/SiO2/(bPEI-al-PSS)2 wafer as a function of temperature with variabledeposition step number n: (a) n = 5; (b) n = 50. Filled circles: first heating from 297 to 323 K; empty circles: first cooling from 323 to 297 K; filled triangles: second heatingf b): rea

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f1tthditAPotc

obcabi

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ing to the sample of Fig. 11(b). By testing different samples, ˛was found to lie between 1 and 1.5 with a correlation coefficientalways exceeding 0.999. Results close to these were found by Girtan

rom 297 to 373 K and empty triangles: second cooling from 373 to 297 K. Inset in (s a guide for the eye.

alues between 0.4 and 0.9 S/cm found for the conductivity agreeith the one of 1 S/cm announced by the furnisher (in unknown

onditions [31]), the difference between these values most proba-ly stemming from different measurement conditions.

By repeating the conductivity measurements with films of dif-erent PEDOT-PSS deposition step numbers (of 5, 6, 7, 8, 9, 10,5, 20, 25 and 50), the limit between bad and good ohmic con-act was found for deposition step number 15: this sample washe first to maintain good ohmic contact over the two cycles ofeating and cooling. Consequently, from the correlation betweeneposition step number and thickness, it can be inferred that a min-

mum PEDOT-PSS thickness of 40–45 nm is needed for the sampleo ensure alone proper electronic contact with metallic probes.lthough in an electronic device, the contact between a 2-in-1EDOT-PSS film and another layer might be optimized, this limitf 40–45 nm for the PEDOT-PSS layer has to be taken as a warninghreshold: under this limit, ohmic contact may become a matter ofoncern.

For the samples showing good ohmic contact, the evolutionf the conductivity � with temperature during the first heatingetween 297 K and 373 K (where PEDOT and PSS are known to behemically stable) was first modeled with different laws that hadlready been observed for PEDOT-PSS by several authors [7–9,17]ut at lower temperatures (from approximately 10 to 300 K). An

llustration for one such film is given in Table 1.As could be predicted from the high temperature range scanned

uring our experiments, neither model describes satisfactorily

able 1orrelation factor obtained for the model y = ax + b tested to describe the evolutionith temperature of the conductivity � of a spin-coated (PEDOT-cx-PSS)50 film built

n precoated Si/SiO2/(bPEI-al-PSS)2 during first heating between 297 and 373 K.he references correspond to the studies where PEDOT-PSS was shown to followhe model. VRH: variable range hopping; CELT: Charge Energy Limited Tunneling.

Model y x Correlation factor

VRH 1D; VRH withCoulomb gap; CELT;polaronic islands. [7,8]

ln � T−1/2 0.9908

VRH 2D [17] ln � T−1/3 0.9901VRH 3D [9] ln � T−1/4 0.9898

duced energy of activation W as a function of temperature T. The curves serve only

the behavior of our films, as shown by the poor values obtainedfor the correlation coefficients (hardly bigger than 0.99). Actu-ally, VRH describes rather low-temperature electronic transportbecause only at these temperatures does long range hopping com-pete with simple thermal activation [32]. Not surprisingly, thecorrelation coefficient obtained by modeling � as power law didnot give a better result (correlation coefficient 0.9887), as thiskind of law applies for disordered systems lying near their metal-insulator transition [33]: this condition was not fulfilled by ourfilm, as shown by its clearly decreasing reduced activation energyW(T) = d ln �/d ln T in the temperature range under study (see insetin (b)) [33]. Furthermore, the Arrhenius law did not apply as well(correlation coefficient 0.9928) [34]. By contrast, modeling ourdata with Inelastic Cut-offs of Scaling (ICS) using perturbationtheory [35] with � = �0 + CT−˛ (with �0 > 0 and C < 0) gave betterresults, as can be seen in the example given in Fig. 12, correspond-

Fig. 12. Spin-coated (PEDOT-cx-PSS)50 film built on precoated Si/SiO2/(bPEI-al-PSS)2: correlation coefficient obtained with ICS-modeling � = �0 + CT−˛ during firstheating between 297 and 373 K. The curve serves only as a guide for the eye.

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t al. [11] for PEDOT-PSS film obtained in conditions close to ours,he differences lying in the spin-coater rotation speed, in a sup-lementary thermal annealing at 383 K during 2 min and more

mportantly in the fact that the film was obtained via a sole depo-ition step.

Note the well-reported [10,24] hysteresis in the conductivityvolution vs. temperature shown by our samples: if temperaturesxceed 323 K, the conductivities are higher during the coolingequence up to 323 K. This hysteresis, mainly explained by the dry-ng process [10], has an impact on the fitting curve of ICS modelsi.e. the curve plotting the correlation coefficient vs. ˛) which ishifted toward different values in a non-reproducible manner. Thishift can be seen as the signature of the disruption occurring in thelm, which is cooling and hydrating in the same time. Note thaturing cooling, fitting with the other non ICS-models is even worsehan during heating.

Finally, in the thickness interval between 45 and 125 nm, ouresults show that the conductivity at 298 K of the PEDOT-PSS filmss independent of the thickness of the film, and reaches 25 ± 5 S/m.his result is important because it allows precise thickness controlithout perturbing the conduction properties of the nanometriclm.

. Conclusion

With this comprehensive study of the 2-in-1 PEDOT-PSS depo-ition, the combined influence of drying and speed of spin-coating,he non-perturbation of a thermal annealing and the effect ofhe concentration of the suspension to be deposited are demon-trated and explained. This identification of the main parametersllows controllable and reproducible PEDOT-PSS film construc-ion at the nanometric scale by spin-coating. But this study goeseyond this sole technique. For example, the necessity of good dry-

ng conditions between the deposition steps is of great importancef other deposition techniques, like spraying or ink-jet printing,re foreseen. If inkjet-printing seem to be ‘2-in-1 compatible’ forEDOT-PSS because drying steps can easily be imagined betweenhe deposition steps, the use of automated spraying techniqueseems less straightforward because of the necessity to introducerying steps in a layer-by-layer technique where constant rins-

ng by the escaping liquid in excess takes place at the surface ofhe film under construction. Moreover, PEDOT-PSS films obtainedy the 2-in-1 method behave, as far as their electric conductiv-

ty is concerned, like analog ones obtained with a sole depositiontep: the nanometric thickness control obtained from the 2-in-1ethod is not gained at the expense of this important property of

he conducting material.

cknowledgments

CdSA was supported by the French “Ministère de l’Éducationationale” and by the French “Agence Nationale de la Recherche”ANR-10-BLAN-0818 “Biostretch”).

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.synthmet.014.04.003.

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