ORIGINAL PAPER

Electrochemical deposition of poly[ethylene-dioxythiophene] (PEDOT)films on ITO electrodes for organic photovoltaic cells: controlof morphology, thickness, and electronic properties

José Alfredo Del-Oso1,2& Bernardo Antonio Frontana-Uribe2,3

& José-Luis Maldonado4& Margarita Rivera5 &

Melina Tapia-Tapia2 & Gabriela Roa-Morales2

Received: 11 April 2017 /Revised: 26 January 2018 /Accepted: 28 January 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractIn this article, controlled changes on morphology, thickness, and band gap of poly[ethylenedioxythiophene] (PEDOT) polymerfilms fabricated by electrochemical polymerization (potentiostatically) are analyzed. Electropolymerization of the monomerethylenedioxythiophene (EDOT) was carried out on indium tin oxide (ITO) electrodes, in different dry organic electrolyticmedia, such as acetonitrile, acetonitrile–dichloromethane, and toluene–acetonitrile mixtures. It was found thatelectropolymerization kinetics can be controlled by changing the polarity of the electrolytic media, and kinetics is slower forthose with low polarity. This fact combined with an accurate control of EDOTmonomer concentration and electropolymerizationat Epeak/2 potential, allows to control the morphology and thickness of the electropolymerized PEDOT films (E-PEDOT:ClO4);toluene/ACN (4:1, v/v) and [EDOT] = 0.3 mM gave the best films for application in organic photovoltaic (OPV) cells. Theperformance of the E-PEDOT:ClO4 films was tested on ITO electrodes as anode buffer layer in OPV cells with the configurationITO/E-PEDOT:ClO4/P3HT:PC61BM/Field’s metal, where Field’s metal (cathode) is a eutectic alloy that lets to fabricate OPVdevices easily and in a fast and economical way at free vacuum conditions. The performance of these devices was compared withan OPV device constructed with a buffer layer anode, prepared using the classical spin coating of PEDOT:PSS on ITO. Resultsshowed that OPV cells fabricated with E-PEDOT:ClO4 have a slightly increased PV performance.

Keywords Electrochemical deposition . PEDOT . OPV cells . Anodic buffer layer

Introduction

Organic photovoltaic (OPV) cells [1–5] may be an alternativeto their silicon-based inorganic counterpart, CdS, and other

families of inorganic photovoltaic (PV) devices due to theirlow cost, light weight, better processing conditions, semi-transparency, and flexibility [3–6]. For the bulk heterojunction(BHJ) approach and binary OPV cells, two compounds are

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s10008-018-3909-z) contains supplementarymaterial, which is available to authorized users.

* José Alfredo Del-Osoalfredo.deloso@uacm.edu.mx; adeloso@uacm.edu.mx

* Bernardo Antonio Frontana-Uribebafrontu@unam.mx

1 Universidad Autónoma de la Ciudad de México, Prolongación SanIsidro No. 151, Col. San Lorenzo Tezonco, Del. Ixtapalapa, C.P.09790 Mexico City, Mexico

2 Centro Conjunto de Investigación en Química SustentableUAEM-UNAM, Carretera Toluca-Ixtlahuaca Km 14.5,Toluca 50200, Estado de México, Mexico

3 Instituto de Química, Universidad Nacional Autónoma de México,Circuito Exterior S/N Ciudad Universitaria, 04510 MexicoCity, Mexico

4 Research Group of Optical Properties of Materials (GPOM), Centrode Investigaciones en Óptica A.P. 1-948, C.P. 37000 León,Guanajuato, Mexico

5 Instituto de Física, Universidad Nacional Autónoma de México,Ciudad Universitaria, 04510 Mexico City, Mexico

Journal of Solid State Electrochemistryhttps://doi.org/10.1007/s10008-018-3909-z

used in their active layer, namely, a conjugated electron-richcompound, which behaves as the electro-donor, and, usually, afullerene derivative, acting as the electro-acceptor [4, 5, 7]. Inspite of the hard work and progress in this field, attempts forcommercializing OPV devices been have faced with low per-formance and stability problems [8, 9]. To overcome thesepoints, exhaustive research has been focused to reduce chargerecombination and to improve charge transfer through theactive layer and the electrode interfaces since it would reducethe charge transfer resistance and therefore an improvement inthe Voc [10–12]. Particularly, the OPV anode is commonly atransparent conducting electrode, in most of the cases glasscovered by indium–tin oxide (ITO). The ITO-active layer in-terface has been modified with different buffer layers [13] toimprove hole extraction, giving devices with better perfor-mance [14]. Among the most used buffer layers are organicc o n d u c t i n g p o l y m e r s l i k e p o l y ( 3 , 4 -ethylenedioxythiophene):poly(styrenesulfonate) [15](PEDOT:PSS, Fig. 1), other polyalkylthiophenes [16, 17], aswell as inorganic (V2O5) thin deposits [18].

PEDOT:PSS consists of a cationic polymer and an anionicpolyelectrolyte mixture in an aqueous dispersion, which hasbeen intensively used due to its electron-blocking properties[19, 20], high conductivity (ca. 300 S cm−1), stability, andtransparency in its oxidized state [21, 22]. PEDOT:PSS layersgive smoother surfaces (RMS roughness of the order of 1 nm)with an excellent hole transfer capacity between the ITO andthe active layer, resulting in OPV cells with enhanced holecollection [14, 23] and higher open-circuit voltages (Voc)[12]. In spite of these advantages, the presence of thepolyanion PSS has serious drawbacks because it is very hy-groscopic and water content is a major problem during thedevice fabrication and operation. The presence of water dur-ing the OPV cell operation can generate OH radicals, whichare very reactive species that erode the cell components [24,25]. PEDOT:PSS typically is applied onto ITO electrode byspin coating from an aqueous dispersion, giving films withconsiderable structural and morphological non-uniformitythat require an additional step (annealing) as well as a limitedcharge transfer [26–28]. It has been found that thePEDOT:PPS film deposited by this method, after some time,suffers a phase separation leading to a PSS polyanion-richregions, which dramatically reduce electrical conductivitylimiting the anode performance [29–31]. All these problemsaffect stability of OPV cells constructed with PEDOT:PPS;therefore, new alternatives to produce hole PEDOT-basedbuffer layers are needed.

One of the most attractive techniques to prepare PEDOTfilms is by electrochemical deposition [32]. ElectrodepositedPEDOT (E-PEDOT) on metallic electrodes was firstly report-ed by Heinze et al. [33], who showed that anodic polymeriza-tion of EDOT monomer (Fig. 1) at low oxidation potentialwas possible because during the polymerization process the

mesomeric effect of the oxygen atoms stabilizes theelectrogenerated radicals–cations, decreasing EDOT oxida-tion potential to 1.25 V vs standard hydrogen electrode (incomparison to thiophene oxidation at 1.95 V) [34]. As a con-sequence, E-PEDOT films of excellent quality can be obtain-ed by electrochemical polymerization of the correspondingEDOT monomer on electrodes. Electropolymerization canbe carried out in non-aqueous solvents [35, 36] like acetoni-trile [37] by using as supporting electrolyte LiClO4 or atetralkylammonium perchlorate (TBAP) salts, thus, the poly-mer can be obtained directly in the charged (conducting) stateas a perchlorate doped film (E-PEDOT:ClO4) on the electrodeused as anode. This methodology circumvents aqueous solu-tions and the associated problems to the PSS polyanion andwater traces in the films described previously.

However, E-PEDOT growth processes and film morpholo-gy, as well as conducting properties, fluctuate dramatically withthe experimental parameters used in the electrodeposition [38].Besides, the electrochemical response of E-PEDOT differs sub-stantially from that observed with PEDOT:PSS deposited byspin-coating, resulting in a better electrical performance for thefirst [39]. All these issues indicate that the electrochemical de-position can be used to control the thickness, conductivity, andmorphology of E-PEDOT layers, useful as anode buffer layerin OPV cells. This idea was initially developed by Meerholzet al. [40], and other groups have followed this approach [41].They have demonstrated the E-PEDOT electrodeposition onITO electrodes for OPV cells construction by using NaPSSpolyanion in aqueous media as electrolyte. Also, E-PEDOT:ClO4 electrodeposition on ITO electrodes in non-aqueous solvents (acetonitrile, ACN) for OPV cells has beenachieved by cyclic voltammetry by Nasabuyin [37]; this ideawas recently revised by Vladimis et al. [42] Both studies

Fig. 1 Chemical structures of the compounds P3HT, EDOT,PEDOT:PSS, and the fullerene derivative PC61BM used for thefabrication of OPV cells

J Solid State Electrochem

demonstrated that E-PEDOT films can be used successfully tosubstitute PEDOT:PSS buffer layer with a moderate increaseson the OPV cell parameters (30% fill factor, 23% short circuitcurrent density, and 5% on open-circuit voltage). Nevertheless,nothing was mentioned in these articles about the polarity in-fluence of the solvent used, neither about the effect of theemployed technique. These two parameters are important be-cause previous studies have demonstrated that they have im-portant influence on the morphology and electrical propertiesof the obtained conducting polymers [43, 44], consequentlyaffecting the stability and performance of the fabricateddevices.

In this work, the effect of different anhydrous solventmixtures (toluene/ACN) on EDOT electropolymerizationis presented; the mixture with the slowest kinetics wasused to prepare E-PEDOT:ClO4 thin films. Properties suchas morphology, thickness, and conductivity of the obtaineddeposits were studied. An important contribution of thiswork is the potentiostatic EDOT polymerization directlyonto the ITO electrode, which is an electrochemical meth-od rarely used in the preparation of OPV cells’ anodicbuffer layers. These electrodeposited E-PEDOT:ClO4

films, as buffer layer, were used in OPV cells with thearchitecture ITO/E-PEDOT/P3HT:PC61BM/Field’s metal.The use of the eutectic alloy Field’s metal (FM: Bi, In,and Sn with 32.5, 51, and 16.5%, respectively), which isfree-vacuum deposited at room atmosphere, has beenrecently reported by our group as cathode in highlyefficient OPV cells [45–47]. Eutectic alloys are attractivebecause they provide an easy and fast way for fabricatingtop-metal electrodes since they melt at low temperatures(below 100 °C); FM just melts and drops on top of thedelimited area. Moreover, the material and deposition costare significantly low compared with the traditional metaldeposition by high vacuum evaporation procedures.Finally, the performance and stability of the OPV devicesfabricated with E-PEDOT were compared with cells wherePEDOT:PSS was deposited by the classical spin-coatingtechnique.

Experimental section

Materials

Figure 1 shows the chemical structure of the organic com-pounds used in this study: poly(3-hexylthiophene)regioregular (P3HT, Rieke®, Mw = 36 K via GPC), themonomer 3,4-ethylenedioxythiophene (EDOT) (Sigma-Aldrich), the fullerene derivative [6,6]-phenyl-C61 butyric ac-idmethyl ester (PC61BM, SESResearch, assay > 99.9%, PM=910.9 g mol−1), and the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Sigma-Aldrich, 1.3%weight dispersion in H2O, conductive grade). The anhydroussolvents acetonitrile (ACN), dichloromethane, and tolueneused in this work (Sigma-Aldrich) were used as received. Theused supporting electrolyte was tetrabutylammonium perchlo-rate (TBAP, FLUKA, assay ≥ 99% electrochemical grade).Transparent indium–tin oxide (ITO, 10 Ω sq−1) deposited onglass substrates (polished grade, 25 × 10.0 × 1.1 mm) were pur-chased from KINTEC and used as anode. Field’s metal (eutec-tic alloy—32.5% bismuth, 51% indium, 16.5% tin, meltingpoint 62 °C) was purchased from RotoMetals Inc.

Instruments

The cyclic voltammetry experiments and electrochemical po-lymerizations were carried out in a classical three-electrodeelectrolytic cell: the working electrode (WE) was an ITO elec-trode (area ≈ 0.2 mm2 homemade using a piece of ITO that fitsinto a NeoCoat-RD® holder and making electrical contactwith an aluminum foil and conductive epoxy glue), the refer-ence electrode (RE) was an Ag+/Ag° electrode (0.01 MAgNO3/0.1 M TBAP in ACN, E° = 0.47 V vs. NHE) andthe counter electrode (CE) was a Pt foil (3 cm2). Nitrogengas (99.999%; INFRA) inlet via a long needle was used tokeep the system free of oxygen and to deoxygenate the elec-trolytic solution. For the electrodeposit characterization andsolar cell construction, the electrochemical cell (Fig. 2) wasconstructed using as WE an ITO electrode (25 mm× 10 mm)

Fig. 2 Scheme and photograph ofthe three-electrode electrolyticcell used for the electrodeposits ofE-PEDOT:ClO4

J Solid State Electrochem

submerging 15mm into the electrolytic solution (15 mm2 areafor electropolymerization), and the Pt counter electrode wasfitted in front of it and parallel at 50mm (foil 25 × 15mm); thereference electrode was placed nearby the WE, outside of thecurrent lines for an accurate potential control.

The potentiostat/galvanostat used in this study was aPrinceton Applied Research model PAR 273A controlled bya personal computer with PowerSuite 2.56 software. In situelectrochemical conductance measurements were carried outusing a bipotentiostat system specially built for this applica-tion (HEKACIP 2). Asworking electrodes, interdigital micro-arrange electrodes (IDMAE) of 1 cm × 0.5 cm were used,featuring separate Pt bands (10 μm) deposited on it.Morphology and thickness studies were carried out with anatomic force microscope (JEOL JSPM 4210 Scanning ProbeMicroscope and ASYLUM RESEARCH MFP-3D Origin).UV–Vis spectra were recorded with a Genesys10S UV–Visspectrophotometer (Thermo Scientific). Current–voltage (J–V) curves were measured using a Keithley SourceMeter2400 unit and a solar simulator Sol 3A of Oriel® and calibrat-ed with an Oriel® reference cell at 100 mW cm−2. Tests wereperformed at atmospheric conditions and data were acquiredby LabVIEW® software specially designed for this purpose.

Cleaning and preparation of the ITO and IDMAEelectrodes

All new ITO electrodes were initially cleaned with a soft clothsoaked with an aqueous solution of Triton X-100 (1:100) andsubsequently successive ultrasonic washes (10 min durationeach) with solution of Triton X-100 (1:100), deionized water,ethanol, 1 mM EDTA (pH = 13), and finally deionized water.After this cleaning procedure, the humidity of ITO was elim-inated through a stream of N2, and each electrode was storedin vials and kept in a desiccator. For interdigital micro-arrangeelectrodes (IDMAE), the new electrodes were submerged in apiranha solution (sulfuric acid concentrated, with hydrogenperoxide 30% 4:1, v/v). After 20 min, they were rinsed thor-oughly with deionized water, followed by acetone wash, thenagain rinsed with deionized water and dried with a stream ofN2 and stored in vials and kept in a desiccator.

Electrochemical analyses of EDOT and electrochemicaldeposits of E-PEDOT:ClO4 films on ITO electrodes

Cyclic voltammetry (CV) analyses were carried out to EDOTsolutions (1 mM) prepared in anhydrous acetonitrile (ACN),toluene/ACN (2:1, v/v), toluene/ACN (4:1, v/v), ACN/dichloromethane (2:1, v/v), ACN/dichloromethane (4:1, v/v),and anhydrous dichloromethane. Tetrabutylammonium per-chlorate (TBAP, 0.1 M) was used as supporting electrolytein all cases. The solutions were prepared directly in the elec-trolytic cell, under N2 atmosphere, adding the weighted

quantity of TBAP and injecting the required solvent and theEDOT volume with a syringe and micropipette, respectively.Once the solution was prepared, it was N2 bubbled for 10 min.Experiments were carried out at 25 °C, using ITO electrode(area ≈ 0.2 mm2 homemade using a piece of ITO that fits intoa NeoCoat-RD® holder and making electrical contact with analuminum foil and conductive epoxy glue) as WE. One cyclewas recorded to obtain the electrochemical behavior of themonomer EDOT in the studied media, by using for the initialstep high and low potential values Ei = 0.0 V, Eh = 1.5 V, andEl = 0.0 V, respectively. For electrodeposits of PEDOT in theoxidized state doped with perchlorate anions (E-PEDOT:ClO4) on ITO electrode (15 mm2), the used techniquewas double pulse chronoamperometry (potentiostaticelectropolymerization). The initial pulse potential wasEi=− 1.5 V (0.01 s), and the second pulse was set at the desiredpolymerization potential during the time selected to have con-stant charge consumption; single pulse chronoamperometrycan be also used in programming an adequate equilibriumtime at the EI = 0 potential instead of the pulse at − 1.5 V.The half-peak potential (Epeak/2) was selected for the EDOToxidation determined from the cyclic voltammogram in thedifferent solvent mixtures to electropolymerize. Different po-lymerization times gave E-PEDOT:ClO4 films with differentthickness and properties. Finally, the ITO electrode coveredwith E-PEDOT in its conducting state was rinsed with ACN,dried with a stream of N2, and kept in sealed vials with inertatmosphere until its subsequent UV–Vis, AFM thickness anal-ysis, and/or use as anode in OPV cells.

Thickness measurement of the E-PEDOT:ClO4 filmson ITO

AFM equipment working in tapping mode with a scan area of5 × 5 μm was used to measure the film thickness by means ofa transverse cut made to the E-PEDOT:ClO4 deposits obtainedat different polymerization times on ITO. An average of fourdeterminations in different regions of the film is reported.Films were oxidized at 0.9 V (maximal transparency) priorthe thickness measurement in order to have the best conditionsfor OPV cell construction.

Conductance analysis of E-PEDOT:ClO4 filmson interdigital micro-arrange electrodes (IDMAE)

In situ electrochemical conductance measurements were car-ried out using a HEKA CIP 2 bipotentiostat and a three-electrode electrochemical cell fitted with a Pt foil counterelectrode separated 0.5 cm from the Pt IDMAE (WE), whereboth electrodes (same size) were placed in parallel positions;the reference used electrode was Ag+/Ag°. Firstly, the EDOTresponse in this electrode was determined by cyclic voltamm-etry and the half-peak potential was selected for

J Solid State Electrochem

electropolymerization by chronoamperometry technique fol-lowing the described protocol. Once E-PEDOT:ClO4 filmswere electrodeposited on Pt IDMAE, the electrode was re-moved from the electrolytic cell, and it was ACN rinsed andtransferred to another electrochemical cell containingmonomer-free ACN TBAP 0.1 M solution. A constant poten-tial difference of 10 mV was applied between the IDMAEbranches by one potentiostat to generate the current that cir-culates through the polymer during conductance determina-tions. The electrochemical conductance experiment was car-ried out by chronoamperometry using different potential pulseduring 30 s; this excitation was carried out by a secondpotentiostat and the current flowing through it was monitoredto generate the conductance graphic. More information aboutthe experimental setup can be found elsewhere [48, 49].

Fabrication and test of the OPV cells

Devices were fabricated under the bulk heterojunction (BHJ)architecture, using as active layer a mixture of P3HT:PC61BMin a weight ratio of 1:2. This mixture was prepared in anhy-drous chloroform and the solution was used to deposit thinfilms by spin coating (thickness between 80 and 100 nm,AFM determined) over glass covered with ITO, previouslycoated with PEDOT:PSS (ca. 40 nm) through spin coatingusing the following parameters [15, 23, 45]: ramp speed of400 rpm for 4 s, plateau speed of 2000 rpm for 60 s, andannealing of 80 °C for 20 min to remove moisture beforedepositing the active layer. In the case of ITO/E-PEDOT:ClO4 prepared by electropolymerization (vide supra),and prior to the active layer deposition, a potential pulse of0.9 Vwas applied during 30 s to assure a constant doping leveland maximal transparency. Later, the electrode was rinsedwith ACN, dried with a stream of N2, and kept in a sealed vialwith inert atmosphere until its use. To deposit the Field’s metalcathode [47], pellets of this eutectic mixture were placed on aPyrex glass beaker, set on a hot plate, and heated between 90and 100 °C. The melted material was deposited by carefuldropping over the organic active layer (heated at the sametemperature to avoid spontaneous solidification). Thephotoactive area tested was about 0.09 cm2, selecting it withscotch tape; the photovoltaic parameters were determinedusing the instruments previously described.

Results and discussion

Electropolymerization of EDOT at 1 and 0.3 mM

Figure 3 shows the electrochemical behavior of EDOT mono-mer in the different organic media studied. As can be seen, in thefive media the typical behavior of electropolymerization is ob-tained, characterized by a rapid increase in the anodic current

until reaching the current peak corresponding to monomer oxi-dation (Epeak) [50], followed by the crossing of the curve in thereverse sweep due to nucleation or chemical reactions of themonomer on the bare electrode [51]. The electroactivity windowof all used solutions, determined by cyclic voltammetry, goesfrom −2.75 to 1.75 V showing a well-behaved behavior withacceptable resistance (inset Fig. 3). On the other hand, from theslope of the oxidation curve, it is possible to deduce some infor-mation about the kinetics of electropolymerization process in thestudied media; the higher the slope, the higher the electrochem-ical reaction rate. Thus, EDOT electropolymerization kinetics isvery similar in the case of ACN andACN/dichloromethane (2:1,v/v) or (4:1, v/v), but dramatically decreases in the case oftoluene/ACN (4:1, v/v). In the case of toluene/ACN (2:1, v/v)and dichloromethane, their slopes are lower than in the first’scases, but higher than the last ones, showing intermediate kinet-ics behavior. Additionally, another important parameter ofEDOT electrochemical oxidation is the onset potential becausethe properties of the conducting polymer obtained depend on thepotential in which the film is subjected during the polymeriza-tion. Thus, in the case of ACN and ACN/dichloromethane, theEDOTonset oxidation potential was shown to be the less anodic(1.1–1.13 V); on the other hand, EDOT oxidation in dichloro-methane and toluene/ACN (2:1, v/v) had higher oxidation values(1.2–1.25 V). The behavior of EDOToxidation in toluene/ACN(4:1, v/v) is shown to have particular characteristics that can beexploited to make thin films of E-PEDOT:ClO4 useful in OPVcells, namely, low polymerization kinetics and a low onset oxi-dation potential (1.13 V). These properties may give rise tocontrol the E-PEDOT:ClO4 morphology and properties usingelectrochemical polymerization. The electrochemical behaviorof EDOToxidation in toluene/ACN (4:1, v/v) explains why thismedia was used in previous investigations carried out byNasybulin [37], who did not justify adequately the solvent

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Fig. 3 Cyclic voltammetry of EDOT in different organic media.[EDOT] = 1 mM, [TBAP] = 0.1 M, CE = Pt, WE = ITO, and v = 50 mV/s. Inset corresponds to the electrolyte response without EDOT in eachanalyzed media

J Solid State Electrochem

selection used in his study. Toluene has a low Rohrschneider’spolarity parameter, in comparison to ACN and dichloromethane(toluene = 2.4, dichloromethane = 3.1, and ACN = 5.8) [52];therefore, the TBAP electrolyte is poorly soluble in it and thecyclic voltammetry experiments in this media are not useful.Nevertheless, when it is mixed with 20% ACN, the electrolytesolubilizes satisfactorily generating good conductivity to carryout the electropolymerization process. Conductivity was deter-mined for solutions containing TBAP as supporting electrolyte:acetonitrile gave 6.520 mS/cm, toluene gave 0.128 μS/cm, andthe mixture toluene/ACN (4:1, v/v) gave as expected an interme-diate value of 1.528 mS/cm. These results suggest that the dis-sociation degree of the supporting electrolyte in this latter mediamay be controlling the charge transfer rate during the polymer-ization; therefore, a lower kinetics of the electropolymerizationprocess is observed when it is compared with electrolyte solu-tions in ACN or CH2Cl2. In order to confirm this proposal,potentiostatic polymerization at their respective anodic half-peak potential (Epeak/2 indicated in Fig. 4) at the different studiedmedia was carried out. The I versus t curves for these experi-ments are shown in Fig. 4.

The chronoamperometric curves are in concordance with thecyclic voltammetry, where ACN and ACN/dichloromethane(2:1) media showed the highest electrochemical reaction rate,generating the faradaic current plateau with higher current forthese media. The initial electropolymerization rate for dichloro-methane and toluene/ACN (2:1, v/v) is lower than in the previousmedia and a small induction time is required to start the poly-merization. It can be associated with the higher oxidation poten-tial needed to oxidize EDOT as was shown in the cyclic volt-ammetry. Again the toluene/ACN (4:1, v/v) mixture had an in-termediate behavior, where the current performance resemblesthe first mixtures, but the polymerization kinetics is slow. Thisstatementwas confirmed evaluating the charge consumed duringthe 100 s pulse potential by means of curve integration. It was

calculated only for representative mixtures of organic media(Fig. S1, average of three experiments), namely, ACN, ACN/dichloromethane (2:1, v/v), and toluene/ACN (4:1, v/v).Dichloromethane evaporates very quickly under bubbling ofN2 at Toluca-Mexico height (2640 m, above sea level) and doesnot allow to obtain reproducible experiments. From this analysis,it is confirmed that electropolymerization kinetics is three timesfaster in ACN versus toluene/ACN (4:1, v/v). Therefore, com-bining the experimental parameters like media polarity, mono-mer concentration, and potential value, it may be possible tocontrol the morphology and thickness of the E-PEDOT:ClO4

films deposited in ITO. Previous studies in our group demon-strated that pure ACN and using cyclic voltammetry generatefilms with rough morphology and important thickness due to afast electropolymerization reaction [53]. Therefore, for the nextpar t of th is s tudy, i t was only selec ted for theelectropolymerization studies the toluene/ACN (4:1, v/v) mix-ture using two EDOT concentrations, 1 and 0.3 mM, lookingfor the preparation of ITO/E-PEDOT:ClO4 electrodes with suit-able characteristics to be used in OPV cell construction.

The electrochemical response of EDOT 0.3 mM (figure notshown) in the mixture toluene/ACN (4:1, v/v) showed thesame behavior of the previous cyclic voltammetry obtainedat higher concentration (Fig. 3, pink line), and the correspond-ing decrease of current due to a difference in concentrationand a small diminishment in the slope according to the effectof concentration in the dimerization kinetics was observed.This phenomenon can be useful to control the morphologyand thickness of the E-PEDOT:ClO4 films.

Characterization of the morphology and thicknessof the E-PEDOT:ClO4 films obtained in toluene/ACN(4:1, v/v)

E-PEDOT:ClO4 films obtained in the mixture toluene/ACN(4:1, v/v) with EDOT concentration at 0.3 and 1 mM werespectroscopically studied (UV–Vis) at various oxidation states.They showed the typical optical behavior previously describedfor the transition between semiconducting–conducting PEDOTfilms and at higher potentials the oxidative degradation of thepolymer film [53–55]. E-PEDOT:ClO4 films with better opticalproperties were obtained with EDOT concentration of 0.3 mM(for a detailed explanation of the optical behavior, please seeFigs. S9 and S10). Figure 5 shows the UV–Vis spectrum of E-PEDOT:ClO4 films that were grown at different pulse times,using a potential of polymerization at Epeak/2 = 1.222 V (seeFig. 3) in the mixture toluene/ACN (4:1, v/v) with EDOT con-centration at 0.3mM. Later, and to have a valid comparison, thefilms were uniformly p-doped by applying a pulse potential of0.9 V for 30 s, in a monomer-free solution; these charged poly-mer films were spectroscopically analyzed. This oxidation po-tential (0.9 V) was selected because at this value in the previousanalysis the E-PEDOT:ClO4 films showed high transparency

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Fig. 4 Potentiostatic polymerization of EDOT in different organic mediaat the anodic half-peak potential (Epeak/2). [EDOT] = 1 mM, [TBAP] =0.1 M, CE = Pt, WE = ITO, RE =Ag°/AgNO3

J Solid State Electrochem

without the risk of overoxidation. The absorbance value at λ =700 nm that corresponds to the isosbestic point of the UV–Visbehavior (see Figs. S9 and S10) was determined, and it wascorrelated with the film thickness determined by AFM (Fig. 6).As can be seen in this study, as the deposit time increases, theroughness and size of polymer grain also showed larger values.

E-PEDOT:ClO4 film thickness also depends on the deposittime, and this fact allowed us to obtain a correlation betweenabsorbance and thickness (Fig. 7a) as well as thicknesses anddeposit time (Fig. 7b). The linear equations obtained were T =− 0.096 nm + 441.348 nm ×A700 nm with r2 = 0.996 for the firstgraph, and T = − 4.510 nm+ 0.250 nm/s × t with r2 = 0.985,for the last one. From these relationships, it is possible to de-termine easily without AFM measurement the thickness of E-PEDOT:ClO4 deposits obtained by CA. These relationshipsshow a clear linear trend, which indicates that polymer thick-ness follows the Faraday law at those deposit times and con-centration studied. Especially in Fig. 7b, it can be observed thataround 100–200 s of deposit time, a convenient thickness (20–40 nm) of E-PEDOT:ClO4 is obtained, which is compatible forits use as buffer layer of OPV cells; likewise, the maximumpeak size of the polymer film fits into the maximum recom-mended for these layers (ca. 40–70 nm) [5, 6]. The roughnesshistogram comparison between two E-PEDOT:ClO4 films pre-pared at different concentrations (Fig. 8) indicates that using aconcentration of 0.3 mM, a better control of the polymer grainmorphology was obtained, although the double of deposit timewas used. This result is consistent with obtaining a better con-trol of the thickness at a concentration of 0.3 mM, as wasshown through AFM analysis (Fig. 6).

The RMS value obtained from the AFM study of thePEDOT:ClO4 films using 0.3 mM was 13.7 nm for a deposittime of 100 s and 17 nm for 200 s, values that are in accordance

with those obtained previously (5–20 nm) [37, 42] for ITO/PEDOT layers used in OPV cells. AFM analysis showedwell-defined ellipsoidal domains in both height and phase andmodulated images; the formation of crystalline regions cannotbe discarded because the limit of grain boundaries is very welldefined as has been reported for other E-PEDOT films [56].The polymer films deposited with 1 mM EDOT are useless forOPV cells due to the large RMS and thickness values that theypresented; the same was obtained for larger deposit times with0.3 mM. Also, using this higher concentration, the polymerpeaks’ height and roughness show to be higher than the maxi-mum recommended [5, 6] for the PEDOT buffer layers (seeSupporting information file Figs. S2–S4). From these studies, itcan be concluded that thickness, morphology, and roughnesscan be tuned using an adequate deposit time and monomerconcentration in the mixture toluene/ACN (4:1, v/v), using thehalf-peak potential (Epeak/2) of the EDOT oxidation in this me-dia. For this reason, the following characterization experimentswere carried out using the EDOT concentration of 0.3 mM.

Characterization of electrochemical behaviorof the E-PEDOT:ClO4 films obtained in toluene/ACN(4:1, v/v)

Cyclic voltammetry (CV) analysis at different scan rates, from0.005 to 1.000 V/s for E-PEDOT:ClO4 film growth by CAusing [EDOT] = 0.3 mM in toluene/ACN (4:1, v/v), is shownin Fig. S5. Figure S6 shows the behavior of ipa and ipc versuspotential scan rate (v). The linear relationship of the anodicand cathodic current peaks indicates that the film is tightlybound on the surface of the electrode ITO and good electricalcontact at the interface ITO/E-PEDOT:ClO4. The registeredcurrent is due to the redox process of the conducting polymeroccurring at the interface ITO/E-PEDOT:ClO4 and not at theE-PEDOT:ClO4/dissolution interface, which generates a non-diffusional current, leading to their linear dependence on v;this result is important because this polymeric deposit is beenproposed as buffer layer for the extraction of holes in OPVcells. A similar experiment was carried out, using EDOT con-centration of 1 mM (Figs. S7–S8), which also shows verygood electrochemical behavior.

Figure 9 shows the reversibility and stability analysis of aE-PEDOT:ClO4 film, obtained using the conditions used inprevious analyses. This study was carried out using cyclicvoltammograms (CV) (from − 1.5 to 0.5 V) reaching up to30 cycles. The variation between the first cycle and the sub-sequent cycles is due to the memory effect of the polymer[43]; therefore, for this analysis, cycles 1 and 2 were not con-sidered. The current oxidation diminishment is due to theBmechanical stress^ caused by the transition of going fromreduced to charged (doped) state. For this reason, the studyof stability and reversibility of the polymeric films gives in-formation about the integrity and electrochemical reversibility

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Fig. 5 UV–Vis spectra of E-PEDOT:ClO4 deposits on ITO generated byCA in the mixture toluene/ACN (4:1, v/v), [EDOT] = 0.3 mM, and[TPAP] = 0.1 M at the half-peak potential (Epeak/2 = 1.222 V) withdifferent pulse times

J Solid State Electrochem

of the conducting polymer during redox cycles. The amountof cathodic and anodic charge loss was about 10% for bothstates and the ratioQc/Qa was higher than 0.95, demonstratinggood stability and reversibility, both characteristics agree withthe very good electrochemical behavior of the E-PEDOT:ClO4 film observed during the scan rate study.

Conductance of the E-PEDOT:ClO4 films obtainedin toluene/ACN (4:1, v/v)

The study of the conductance coupled to cyclic voltammetryfor E-PEDOT:ClO4 films obtained by potentiostaticelectropolymerization at different potentials in the mixturetoluene/ACN (4:1, v/v) (Fig. 10) evidences that when thehalf-peak potential (1.222 V) is used, the conductance of thefilms reaches the highest values in the experimental conditionsused. This result is important because a higher conductance ofE-PEDOT:ClO4 films would allow a greater transfer of holes

to the anode (ITO) in the OPV cells. The value of 1.083 V inFig. 10 corresponds to a polymerization at the foot of thevoltamperometric signal and the polymerization at 1.5 Vwas carried out on the polymer overoxidation region; thesetwo last conditions are not useful for generating for E-PEDOT:ClO4 films with high conductance. With each oneof the three potential values studied, it was necessary to usedifferent electropolymerization times (Fig. 10) to maintaincomparable charge consumption values (Q = 10 mC) and tohave a reliable comparison. The charge was calculated inte-grating the I versus t chronoamperometric curve obtainedfrom the electrodeposition process; an easier way to controlelectropolymerization charge is using chronopotentiometrymethod (not studied), where the electropolymerization currentis fixed and the charge calculation is obtained directly fromthe charge definition equation (Q = It). The interdigital elec-trode used for this experiment requires a thicker film forobtaining reproducible results; therefore, 1 mM experiments

Fig. 6 2D AFM images inoscillation mode for E-PEDOT:ClO4 films on ITO,obtained by CA (Epeak/2 =1.222 V, [EDOT] = 0.3 mM) for atime of deposit: a 100 s, b 200 s,and c 300 s. d Left: AFM view ofthe transversal analysis of a film(200 s) cut by AFM; right:morphology of the transversalanalysis

J Solid State Electrochem

are shown here to discuss the conductance properties of thefilms. It can be noticed in the obtained curves that after someelectropolymerization time the conductance reaches a plateau,which gives the maximum conductance of the film. An equiv-alent behavior is observed when the polymer is grown bycyclic voltammetry, where the conductance of the film

increases and after a certain number of cycles reaches a max-imum value [57]. The determined property in these experi-ments was conductance and not conductivity. For calculatingthe latter parameter, the exact thickness value of the polymerfilm on the interdigital electrode is required; nevertheless. thelinear relationship between conductance and conductivityallowed to obtain a first approximation to this property [58].

The conductance behavior of E-PEDOT:ClO4 film for dif-ferent doping potentials (− 1.40 V to 1.40 V) in a monomer-free solution is shown in Fig. 11; the insert shows the potentialvalues where the polymer was analyzed. This behavior wasobtained registering the conductance of the film after applyinga potential pulse as showed in the inset if Fig. 11 during 50 s.As can be observed, the maximum value of conductance isobtained when the applied potential is around 0.7–0.9 V,

Fig. 8 Histograms of roughnessfor E-PEDOT:ClO4 films growthon ITO, obtained in the mixturetoluene/ACN (4:1, v/v) for a 50 sdeposit time, with [EDOT] =1 mM and b 100 s deposit time,with [EDOT] = 0.3 mM

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)mn(

ssenkcihT

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Fig. 7 a Absorbance (at λ = 700 nm) vs thickness and b thicknessvs deposit time relationship for E-PEDOT:ClO4 films on ITO, depositedby CA (Epeak/2 = 1.222 V, [EDOT] = 0.3 mM) for pulse time 100, 200,300, and 400 s. Values correspond to the average of three independentsamples. Adjustment obtained by linear regression for all pulse times andthe confidence interval is shown

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Qa

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Fig. 9 Qc/Qa analysis of the film E-PEDOT:ClO4 film growth during100 s, by CA at Epeak/2 = 1.222 V in the mixture toluene/ACN (4:1, v/v)with [EDOT] = 0.3 mM and [TPAP] = 0.1 M, CE = Pt, and WE= ITO.The squares correspond to cycles 1, 2, 3, 10, 20, and 30. Insert: 30consecutive cycles of the analyzed film in monomer-free solution(ACN, [TPAP] = 0.1 M)

J Solid State Electrochem

which agrees with previous studies, where at this range ofpotential the film showed a maximum transparency (Figs.S9 and S10). At − 1.4 V, the film is in neutral state and con-ductance was not observed; on the other hand, when a poten-tial pulse of 1.4 V is applied, the conductance drops dramati-cally since the beginning, and after 20 s, the conductance isnot observed. In this condition, over-oxidation is provokedand polymer undergoes chemical degradation.

The AFM study of the E-PEDOT:ClO4 film deposited ontothe interdigital electrode (Fig. S11) using these experimentalconditions confirms the morphology and polymer densityequivalent and comparable to those deposits obtained on theITO electrode. Besides, the roughness histogram showed also

good polymer grain control with the maximal values close tothe desired and similar to the previously found with ITO elec-trode. The transversal cut analysis ratifies that the deposit hasthe height required (around 25 nm) for the solar cell devices.The polymer grew on the interdigital electrodes (Pt) hassmoother surface compared with the one that grew on ITOelectrode; this is probably due to the smoother surface of themetallic electrode. With this AFM analysis, the qualitativeconductance trend found for the E-PEDOT:ClO4 films onPt-interdigital electrodes can be validated for the films depos-ited on ITO electrode.

Application of the E-PEDOT:ClO4 filmsin the construction of organic photovoltaic cells

OPV cells were fabricated under the architecture ITO/PEDOT:PSS/ or ITO/E-PEDOT:ClO4/P3HT:PC61BM/Field’s metal, where the difference is the anode buffer layercomposition; one was prepared using spin coating and theother by means of potentiostatic (CA) electropolymerization.J–V curves (Fig. 12) show that OPV cells where the PEDOTcoating was prepared by electropolymerization (E-PEDOT:ClO4) have better values of Voc and Jsc than the cor-responding one obtained through spin coating (PEDOT:PSS).It must be pointed out that 40–50 nm thickness of PEDOTfilm is the usual value for using this conducting polymer asABL in OPV cells [15, 23, 45], and this value can be reachedfollowing the conditions described in the literature. For com-parison and using graph b of Fig. 7, E-PEDOT:ClO4, filmswith an equivalent thickness (about 50 nm) were used; this canbe done using a deposition time of 220–230 s.

Table 1 shows the comparison of the characteristic parame-ters of Voc, Jsc, FF, and η for the tested OPV cells prepared withthe different anodic buffer layer (ABL). As can be seen, cellswhere the PEDOT was obtained through the electrochemicaltechnique showed the best values, especially for the OPV

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E (V) vs Ag°/AgNO3

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ance

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Fig. 11 Conductance behavior of E-PEDOT:ClO4 film obtained bypotentiostatic electropolymerization (Epeak/2 = 1.222 V and a depositiontime of 150 s) in a monomer-free solution (ACN, [TPAP] = 0.1 M) fordifferent doping potentials. CE = Pt, WE = IDMAE and RE = Ag°/AgNO3. Insert: cyclic voltammetry of E-PEDOT:ClO4 film in amonomer-free solution (ACN, [TPAP] = 0.1 M); the color arrowsindicated the potential values where the E-PEDOT:ClO4 film wasanalyzed

Fig. 10 Conductance behavior of EDOT during electropolymerization atdifferent potential and time values in mixture toluene/ACN (4:1, v/v) with[EDOT] = 1 mM, CE = Pt, WE = IDMAE, RE = Ag°/AgNO3 andt = 150 s

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J(m

A c

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)

V (mV)

Fig. 12 J–V curves of three OPVs cells fabricated under the architectureITO/PEDOT:PSS/ or ITO/E-PEDOT/P3HT:PC61BM/Field’s metal, thefist prepared by spin coating and the second potentiostatically (deposittime is indicated in the line label)

J Solid State Electrochem

devices identified as E-PEDOT 230 s. Increments of Jsc,Voc, andFF are clearly observed, where first is 50% higher and the latter14% resulting in an efficiency increment of 140%. The latterresult is consistent with the optimum thickness (~ 40–50 nm)for buffer layer of PEDOT. Comparing the increase of photocur-rent efficiency ηE−PEDOT:PSS−ηPEDOT:PSS=ηPEDOT:PSS

�� 100� ��

obtainedby Vlamidis and colleagues [42] (23%) with that obtained in thiswork (140%), the potentiostatic (CA) deposition results are veryattractive for future OPV device fabrication. Direct comparisonof the photocurrent efficiency reported by Vlamidis and col-leagues is not possible because both cells do not have the samearchitecture, whereas they used evaporated aluminum as cath-ode; here, a eutectic metal mixture (Field’s metal) deposited bydrop casting was used. It is true that some eutectic alloy deteri-oration could take place in real applications because devicesmust tolerate temperatures up to 65 °C. Other eutectic alloyswith melting point about 140 °C, such as that composed by Biand Sn (58 and 42%, respectively), could be used to avoid in-conveniences due to sunlight heating under real conditions; testsof this alloy for the top electrode implementation on OPV de-vices are currently taking place in our group and will be reportedin due time.

From Fig. 12, it is possible to mention that cells presenthigh series resistance (Rs) and low shunt resistance (Rsh). Theseries and shunt resistance values play an important role on thePV performance. For efficient OPVs, the smallest Rs and thelargest Rsh values are required, which favor good charge ex-traction and small current leaks [45, 59]. As discussed in thiscontribution, several factors (film morphology, roughness, in-terfaces quality, etc.) are provoking low PV performance ofour simple fabricated devices, and therefore this is an indica-tion that they could have a considerable Rs value, which maybe determined (first approximation) from the dark and illumi-nated current–voltage J–V curves and by using the equivalentone-diode circuit model [45, 59]. Future work will be devotedto determine these values.

Conclusions

In this contribution, the electrochemical, spectroscopic, mor-phological, and conductance analyses of PEDOT films obtain-ed by potentiostatic electropolymerization in different dry

organic electrolytic media were carried out. These studiesshowed that the mixture toluene/ACN (4:1, v/v) controls theelectropolymerization kinetics of EDOTmonomer, generatinga well-ordered growth of the polymer layer. Furthermore, itwas also found that experimental parameters like concentra-tion of monomer, electropolymerization potential, and dopinglevel of the polymer film are critical to control the thickness,morphology, and electronic properties of the E-PEDOT:ClO4

films. With the best values determined in our experimentalconditions, anode buffer layers were prepared to constructsimple OPV cells with the architecture ITO/PEDOT:PSS/ orITO/E-PEDOT/P3HT:PC61BM/Field’s metal; it was foundthat OPV devices where the anode buffer layer was preparedby electropolymerization had the best values of Voc, Jsc, FF,and η.

Acknowledgments The technical support from Mario Monroy andCitlalit Martinez is recognized. This research was supported bySECITI-UACM PI2014-34, CONACYT 179356, FOMIX CONACYT-GDF 189282, Ce-MIE-Sol 207450/27 (Mexico) call 2013-02 andCONACyT-SENER grant 245754 (Mexico) call 2014-02, and FondoSectorial CONACYT-SENER-Sustentabilidad energética.

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Table 1 Comparison of the parameters Voc, Jsc, FF, and η for the OPV cells under the architecture ITO/PEDOT:PSS or E-PEDOT/P3HT:PC61BM(1:2,w/w)/Field’s metal (PV parameters correspond to an average of three independent experiments; the standard deviation is shown)

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