On-substrate polymerization of solution-processed, transparent PEDOT:DDQ thin film electrodes with a hydrophobic polymer matrix

Organic Electronics 12 (2011) 1518–1526

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On-substrate polymerization of solution-processed, transparent PEDOT:DDQ thin film electrodes with a hydrophobic polymer matrix Michael L. Machala a,b,⇑, Lars Mueller-Meskamp a,b,⇑, Sylvia Gang b, Selina Olthof a,b, Karl Leo a,b,c a

Dresdner Innovationszentrum Energieeffizienz Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, 01069 Dresden, Germany c Fraunhofer Institut für Photonische Mikrosysteme, Maria-Reiche Strasse 2, 01109 Dresden, Germany b

a r t i c l e

i n f o

Article history: Received 11 March 2011 Received in revised form 17 May 2011 Accepted 21 May 2011

Keywords: Conductive polymer PEDOT:DDQ Hydrophobic Electrode In situ PEDOT:PSS

a b s t r a c t Thin conductive, hydrophobic films of poly(3,4-ethylenedioxythiophene) or PEDOT were synthesized on-substrate in the presence of the organic electron acceptor and dehydrogenating agent 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) using a versatile processing procedure. Significant polymerization in the processing solution was delayed by using common aprotic, ethereal solvents with low dielectric constants to prevent solvating the EDOT:DDQ charge transfer complex into radicals. Polymerization was initiated by an increase in concentration upon solvent evaporation during spin coating. A hydrophobic polymer matrix additive of polyvinyl acetate was used to aid in film formation, and a post-treatment rinse with acetonitrile was necessary to obtain a conductive film. Conductivities ranged from 17 to 59 S/cm, where the higher values were achieved at the cost of transparency. A work function of 4.62 eV was determined by UV photoelectron spectroscopy for one film recipe. When comparing conductive AFM results of PEDOT:DDQ to highly conductive, ethylene glycol-treated PEDOT: poly(styrenesulfonate) or PSS, surface currents were orders of magnitude higher for PEDOT:DDQ than for PEDOT:PSS. If optimized further, less acidic and hydrophobic PEDOT:DDQ films have the potential to replace PEDOT:PSS for use as a transparent electrode or charge transport layer in organic solar cells, organic light emitting diodes, touch screens, and other optoelectronic devices. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Poly(3,4-ethylenedioxythiophene) or PEDOT has gained much attention for application in organic electronics due to its stability and high transparency in an oxidized, conductive state [1–4]. The introduction of alkyldioxy subsituents at the 3 and 4 positions of thiophene lowered its oxidation potential and simultaneously blocked undesirable b-linkage during polymerization, eventually leading to the synthesis of PEDOT [2]. Unfortunately, PEDOT is ⇑ Corresponding authors at: Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, 01069 Dresden, Germany. Tel.: +49 351 463 33504; fax: +49 351 463 37065. E-mail addresses: [email protected] (M.L. Machala), [email protected] (L. Mueller-Meskamp). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.05.017

insoluble and intractable in common solvents due to its rigid, stabilizing p-conjugated backbone [1–3]. However, it has been shown that this obstacle can be overcome by introducing a polyelectrolytic template (e.g. polystyrene sulfonic acid or PSS) during polymerization which acts as a dopant counterion for colloidal dispersion in an aqueous solution [3]. PEDOT:PSS is commercially available, and in its pristine form has a conductivity which can range from 10 5 to 1 S/ cm or more when water is used as the solvent [4]. Typical application of this product is in antistatic coatings for photographic film and as a hole transport layer (HTL) in organic solar cells and organic light emitting diodes (OLED) [1,2,4]. Further, if small amounts of high boiling point organic solvents or surfactants are added to solution, a significant increase in conductivity can be observed for thin

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films, reaching over 1400 S/cm at 89.5% transmission with solvent post treatment [5–8]. While PEDOT:PSS is most commonly used in commercial application, some alternative forms of PEDOT synthesis have been used in electrochemical polymerization (primarily for analytical studies) or in vapor phase polymerization (VPP) where a common polymerization catalyst is the Lewis acid Fe(III) p-toluene sulfonate or Fe(tos)3 [9,10]. Fe(tos)3 is also the most prominent oxidant used for in situ polymerization of PEDOT, where conductivities reach over 1000 S/cm when processed with the inhibitor pyridine and water for improved film quality and to extend pot life [11,12]. Ha et al. used relatively high concentrations of imidazole in the processing solution to reduce the high reactivity of Fe(tos)3 prior to processing and to ‘‘[preserve] the EDOT in monomeric form after spin-casting’’, where polymerization is initiated by heating the substrate [13]. They report 750 S/cm with 81% transmission at 270 X/sq. With the aforementioned values, PEDOT is becoming more attractive as a flexible alternative electrode to brittle transparent conductive oxides, namely preeminent and expensive indium tin oxide. For all its applications, PEDOT:PSS has some drawbacks. The inherent low pH (less than 3) and hygroscopic nature of PSS lead to degradation of surrounding interfaces [14,15]. To overcome corrosion issues, researchers at TDA Research developed a lauryl terminated bis-poly (ethyleneglycol) PEDOT doped with perchlorate, which is processed in nitromethane and reaches conductivities of 10–60 S/cm [16]. Additionally, Kim et al. produced an organic-processable, non-acidic solution of PEDOT using a hydrophobic, imidizolium-based polyionic liquid as a counterion for use as an HTL in OLEDs [15]. They reported conductivities of 10 2 S/cm. To further address some of the problems associated with PEDOT:PSS, we have attempted the synthesis of a hydrophobic, matrix-assisted PEDOT thin film using the organic electron acceptor and dehydrogentating agent 2,3dichloro-5,6-dicyanobenzoquinone (DDQ) as both the mediator for synthesis of PEDOT and for subsequent doping during spin coating. We also attempted synthesis of PEDOT with tetrachloro-1,4-benzoquinone, which has been used successfully in the synthesis of conductive polypyrrole along with other halogenated benzoquinones and in other oxidative coupling reactions requiring dehydrogenation [17–20]. However, within the chosen environment

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these reagents of lower oxidative potential relative to DDQ did not initiate an observable polymerization reaction with EDOT. In our research, we developed a versatile procedure to process PEDOT:DDQ from solution using solvents with much lower dielectric constants than MeCN, such that significant polymerization is inhibited until the solvent evaporates and the reactants reach a critical concentration which drives polymerization. It has been shown that the formation of charge transfer (CT) complexes decreases with an increase in the dielectric constant of the solvent [21]. Therefore, it was believed that in solvents with high dielectric constants, the EDOT:DDQ CT complex dissociates into a radical cation and anion which drives polymerization in solution. Conversely, the CT state is more prevalent in less polar solvents (e.g. 1,4-dioxane, THF) such that the rate of dissociation and polymerization is significantly slowed and is largely governed by concentration. When only solutions of EDOT and DDQ were spin coated in THF, a granular, highly resistive film was produced. However, upon the addition of PVAc or a related polymer to function as a templating matrix, smooth, conductive thin films were produced. Successful films turned from a pale brown–green to a bright blue after spin coating (Fig 1), which signals the formation of reduced PEDOT chains [22]. A subsequent rinse with MeCN after drying was necessary to further polymerize and oxidize the PEDOT chains as well as to remove excess PVAc, (un)reacted DDQ, and monomer. As stated previously, conductive polypyrrole has been synthesized in the presence of halobenzoquinones. Bruno et al. reports a two order of magnitude increase in conductivity from pressed pellets of poly(EDOT-co-pyrrole) over PEDOT as a control [23]. Thin continuous films of the copolymer may give favorable results which this procedure could offer. Thus far, we have achieved 17 S/cm with almost 87% transmission at 550 nm, where higher conductivities were achieved at a cost of decreased transmission. When comparing in situ processing procedures employing Fe(tos)3/ amine inhibitor or DDQ, PEDOT:DDQ conductivity and associated values are currently lower than those achieved by PEDOT:tos. Nevertheless, the processing procedure and properties of PEDOT:DDQ are still being investigated, and potential advantages include processing in non-polar solvents with organic-processable polymers. Further, the

Fig. 1. THF solution of EDOT and DDQ (a) prior to spinning (b) after 2 s of spinning.

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pot life of the Fe(tos)3/amine inhibitor system varies. For pyridine solutions, the pot life can last up to a week whereas certain imidazole solutions are preserved indefinitely; however, in the latter, crystal formation in solution can occur and high temperatures are necessary for initiating polymerization [10]. The pyridine procedure presented by Clevios suggests processing the complete solution immediately to prevent gelling [24]. For PEDOT:DDQ, the processing solution has a relatively long pot life, and the resulting smooth, conductive film could take on a range of mechanical and optoelectronic properties dependent on the polymer additive. 2. Materials and methods 2.1. Spectroscopy Solutions of approximately 73 lM DDQ (Aldrich 98%, stored under nitrogen) in a range of aprotic solvents (dry 1,4 dioxane, n-butyl acetate, dry THF, MeCN) were prepared and their spectra recorded. One molar equivalent of 3,4-ethylenedioxythiophene (EDOT, Aldrich) was added to the solutions, and the change in spectra was recorded over time. 2.2. Film formation Multiple series of EDOT:DDQ solutions were prepared under ambient conditions in the following way: to a designated amount of PVAc (Aldrich, avg. Mw of 100,000) and DDQ was added reagent grade, dry 1,4-dioxane. After stirring for approximately 1 h to dissolve all contents, EDOT was added, and the solution was stirred for 1 h. For example, to 40.2 mg PVAc and 86.9 mg (0.38 mmol) DDQ was added 2 mL 1,4-dioxane. After 1 h of stirring, 29.9 lL EDOT (0.28 mmol) was added and stirred for another hour to give an overall concentration of 0.14 M EDOT with EDOT:DDQ in a 1:4/3 mol ratio and EDOT:PVAc in a 1:1 wt ratio. The solutions were spin coated on glass cleaned by sonication sequentially in baths of acetone and isopropanol and then oxygen plasma-treated prior to use. All films were spun at 1200 rpm for 5 s and immediately placed under a petri dish lid (9 cm diameter) for 2 h unless otherwise specified. This film recipe will be referred to as a standard film. After 2 h, films were carefully rinsed 2–3 times with MeCN and allowed to dry. To a solution of PEDOT:PSS provided by H.C. Starck (Baytron PH 1000) was added 5% vol ethylene glycol (EG). The solution was filtered with a 0.45 lm Wattman membrane filter, spin coated at 2500 RPM for 30 s, and annealed at 120 °C for 1 h to serve as reference material – also referred to as standard. 2.3. Instrumentation Absorption and transmission spectra were obtained by UV–vis spectroscopy (Shimadzu MPC-3100). Sheet resistance values were calculated and reported as an average of 10 values per film by 4-point probe measurements (Lucas Labs., tungsten carbide tips). X-ray photoelectron

spectroscopy (XPS) and UV photoelectron spectroscopy (UPS) measurements were performed with a Poibos 100 setup (Specs) as described elsewhere [25]. Conductive AFM was performed on an AIST-NT Combiscope 1000 executed in contact mode with platinum or gold coated silicon tips at different bias voltages (PEDOT:PSS at 2.1 V, PEDOT:DDQ at 10 mV, tip radius 50 nm). 3. Theory In these experiments, we believe that the polymerization for conductive PEDOT with DDQ is based on the Diaz mechanism for polypyrrole [26–28]. One potential pathway to polymerization is illustrated in Scheme 1. As benzoquinones are known to undergo one-electron reduction in aprotic solvents [29], one possible mechanism involves the initial formation of a CT complex following the collision of DDQ and EDOT. Dissociation of the complex – accelerated by solvent evaporation – and collision of the resulting radical cations could yield a cationic dimer. Once two EDOT radical cations couple, surrounding DDQ species can abstract a-hydrogen atoms. Scheme 1 represents one simplified pathway to polymer formation which shows the lifecycle of DDQ to DDQH2. In this pathway a monoradical, cationic dimer is formed after hydrogen abstraction. Polymer chains grow by the addition of cationic species of EDOT. Many pathways are possible to the formation of EDOT dimers with different oxidation states, radical number, and bond order. While we do not have experimental evi dence for support, we believe that the DDQH radical found in these processes could perform subsequent oxidation of neutral PEDOT segments or that the DDQH anion can become the counterion of an already-oxidized sulfur, leading to conductive polymer chains. 4. Results and discussion 4.1. Spectroscopy Solutions of monomer, oxidant, and different solvents exhibited characteristic colors and color changes during processing. Common ethereal solvents were selected based on their solvating ability in search of a balance between charge-screening ability and vapor pressure relative to MeCN (e = 36.64, vp25 = 11.9 kPa) and include: THF (e = 7.52, vp25 = 21.6 kPa), 1,4-dioxane (e = 2.22, vp25 = 4.95 kPa) and n-butyl acetate (e = 5.07, vp25 = 1.66 kPa) [30]. A slower drying process was thought to lead to longer reaction times for increased polymerization and doping, thus, improving film quality. N-butyl acetate did not produce conductive films after drying and rinsing with MeCN, nor was a significant blue color change observed. Even after heating to remove more solvent, the films remained highly resistive, which was attributed to steric interference from the alkyl chain during the reaction. Films employing THF turned from a pale brown-green to a bright blue within seconds of spin coating (Fig. 1) while the rate of color change for 1,4-dioxane was slower. When spin coating films with 1,4-dioxane, polymerization on the pipette tip

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Scheme 1. One potential pathway for the formation of conductive PEDOT with DDQ as the oxidizing and dehydrogenating agent.

and on the substrate was slowed relative to THF due to the lower vapor pressure of the double ether. Further, THF produced films with holes after rinsing with MeCN while 1,4dioxane did not. Thus, 1,4-dioxane was the solvent of choice in this study. When adding EDOT to the bright yellow 1,4-dioxane DDQ solution at standard film concentrations, the solution turned dark initially and then faded to a clearer browngreen. At first, extensive polymerization was not observed in solution with the naked eye. However, in about a week, small crystals – believed to be DDQ – began to form on the bottom of the vial on which polymer grew. The addition of small amounts of toluene appeared to suppress this crystal growth which could extend the refrigerated shelf life to weeks or more. With the same concentrations of reactants but using MeCN as the solvent, the solution turned dark immediately upon addition of EDOT and formed a significant amount of precipitate. A dark film began to coat the inside of the glass vial within a day. Thus, it is believed that such rapid polymerization was a result of the solvation of the CT-complex which initiated significant polymerization in solution. To observe the solvent effect on DDQ and on DDQ in the presence of EDOT, UV–Vis absorption spectroscopy was performed in a polar (MeCN) and a non polar (1,4-dioxane) aprotic solvent (Fig. 2). Solutions containing only the monomer, 73 lM EDOT, show an absorption peak at around 260 nm and no other features in the observed wavelength range. Thus, the formation of a stable solution and there being little interaction of monomer and solvent were concluded. Solutions containing only 73 lM DDQ

show quite different behavior depending on the solvent. For DDQ in MeCN a characteristic double peak was observed at 270 and 280 nm and for the radical anion (DDQ
-) at 346, 432, 457, 547, and 588 nm which are in agreement with previous reports [31,32]. This shows a strong interaction of DDQ with MeCN and supports the formation of radicals in solution. Upon addition of EDOT to the DDQ solution, the DDQ peaks reduced over time as the radical peaks increased, which indicates that MeCN supports the formation of radicals which could lead to polymerization. However, a peak around 265 nm appeared which is at the position of the sum of the original, separated EDOT and DDQ peaks. Switching to a solvent with a much lower dielectric constant, 1,4-dioxane, the spectrum changed significantly. The combined EDOT and DDQ peak splits into separated peaks (around 264 nm and 290 nm, respectively), showing a further changing interaction with the solvent relative to MeCN. For higher wavelengths multiple peaks are visible with broad absorption bands centered around 500 and 682 nm which disappear upon the addition of EDOT. Sambhi reported the formation of charge transfer complexes of DDQ with the cyclic ethers 1,4-dioxane and THF which may account for such intense absorption peaks for this double ether [33]. For both solvents, the main peaks of EDOT and DDQ for short wavelengths were reasonably stable at this low concentration. The spectral characteristics differ between the solvents with regard to the interaction of DDQ and solvent as well as small temporal evolutions of the DDQ peaks upon the addition of EDOT. Definitive evidence for CT

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Fig. 2. UV–Vis absorption spectra, showing the temporal evolution of the solutions. Starting from 73 lM DDQ solutions (red) in aprotic solvents (a) MeCN and (b) 1,4-dioxane, the spectra begin to change upon the addition of 1 mol eq EDOT. The spectra were monitored for up to 2 h (black, several changing curves), and at 18 h post addition of EDOT (blue, last measurement). For comparison, the spectrum of 1 mol eq EDOT (dash) is given.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

formation in either case could not be produced from these experiments, but it is inferred that much more stable CT formation occurs in 1,4-dioxane.

4.2. Film transmission After spin coating a standard film, it was immediately transported for transmission measurements (T0 = 30 s after spin coating). The evolution of transmission during drying can be found in Fig. 3. The intense blue color observed immediately after spin coating was assumed to be reduced PEDOT which absorbs around 600 nm [22]. This dip in transmission reduced over time as absorption increased in the IR which suggests the formation of oxidized PEDOT and likely, longer chains. For shorter wavelengths, the curve shows a higher transmission initially and then evolves toward the shape of the absorption spectra of EDOT:DDQ in 1,4-dioxane around 350 and 450 nm found in Fig. 2. This dip in transmission is thus attributed to DDQ, indicating that unreacted DDQ and its other forms remain in the film. Once the film is rinsed

Fig. 3. Transmission measurements from the time evolution of drying a standard 0.14 M EDOT:DDQ film.

with MeCN, the sharp absorption of DDQ around 300 nm is significantly reduced (Fig. 4). Fig. 4 depicts the change in transmission of films containing varied concentrations of EDOT while the ratio of EDOT:DDQ was held at 1:4/3 and the wt ratio of EDOT:PVAc at 1:1. Films that followed this formulation had a greenish tinge after drying and were very resistive. A dramatic shift in transmission was observed after gently rinsing the films with MeCN, where acetone produced similar results but DMSO ruined the films. The shift is attributed to further polymerization and oxidation of PEDOT monomer and oligomers, accelerated by MeCN, while rinsing away excess (un)reacted DDQ, monomer and PVAc. The difference in magnitude of transmission is attributed to different film thicknesses, formed by the varying amounts of EDOT available. The transmission curves suggest that by these concentration changes the mechanism is not altered or limited somehow, so the chemical composition and the formation mechanism is nearly uniform for all ratios.

Fig. 4. Transmission spectra of EDOT:DDQ films at a ratio of 1:4/3 with an EDOT:PVAc wt ratio of 1:1. Before (dashed) and after rinsing (solid) with MeCN: 0.07 M EDOT (blue), 0.14 M EDOT (black), 0.21 M EDOT (red), 0.28 M EDOT (green) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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This uncontrolled secondary polymerization and oxidation during rinsing is a very important step in obtaining a conductive film. For the remainder of this paper, rinsing with MeCN was considered complete when the green tinge was visibly rinsed away (2–3 rinses). It should be noted that further improvement of this step has great potential and should be investigated further. A smooth PVAc:PEDOT layer, present after rinsing, suggests that as the PEDOT forms it migrates toward the bottom of the film due to its insolubility in common solvents. Similarly, as the film dries, it appears that DDQ and DDQH2 may rise to the top with the evaporating solvent. For films that were rinsed too hard with MeCN, the film peeled off of the glass surface but did not dissolve, maintaining film structure integrity. When water was added to the film surface, droplets condensed and ran off; contact angle measurements were not taken. It appears that the film material takes on the hydrophobic nature of PVAc while simultaneously taking on the insoluble nature of PEDOT. Such properties could be attractive when engineering mechanically robust, conductive plastics like those achieved by Hansen et al. by dissolving PEDOT:tos into a PMMA substrate [12].

trary to the trend up to that point. An EDOT:DDQ ratio of 1:4/3 was chosen as the standard film for reproducibility. Increasing the overall concentration of a standard film produced an increase in film thickness and a decrease in transmission (Table 2). The highest conductivity in this series was achieved with the standard film recipe. It appears that the act of rinsing with MeCN both polymerizes and oxidizes PEDOT while the rigid PEDOT structure continues to lock in PVAc as observed by the increase in film thickness in Table 2. There are slight differences in the values for the two standard films (separate solutions) in Tables 1 and 2. However, considering the non-standardized rinsing of the films, these values are remarkably similar. Varying the amount of PVAc in a standard film did not produce a trend in film thickness, where the most transparent and conductive film was at a wt ratio of 1:5/4 (Table 3). This supports the idea that the PEDOT chains migrate to the bottom of the ‘‘wet’’ film as they form. The supporting polymer matrix species will likely impact many aspects of the film quality for its role may include aiding in both templating chain growth as well as in providing structural stability for the film itself. 4.4. Conductive AFM

4.3. Concentration effects The effects of varying the EDOT:DDQ mol ratio, EDOT concentration, and the EDOT:PVAc wt ratio on film formation and on electronic properties were investigated. As can be expected, the films were much more resistive when one molar equivalent or less of DDQ was used, where excess oxidant is needed to dope the PEDOT (Table 1). Increasing the DDQ concentration to and above stoichiometric ratios produced an increase in film thickness and eventually a wavy surface morphology. Comparing ratios of 1:5/3 and 1:2, a decrease in sheet resistance concomitant with slightly higher transparency was observed which is con-

Fig. 5 compares conductive AFM images of a standard PEDOT:DDQ film and a PEDOT:PSS film with 5% vol EG deposited on glass. Both layers are not treated further before measurement, so the surface depicted is as it was after several hours of exposure to atmosphere. For both samples, the current appears to be centered on the grains. For PEDOT:PSS, the grain size can be adjusted by changing solvent concentration, where the size depicted in Fig. 5 is representative of PEDOT:PSS at approximately 750 S/cm [8,34]. Measuring local conductivity, both samples behaved quite differently. While the PEDOT:DDQ sample showed high currents at low bias

Table 1 Effects on film properties by varying the EDOT:DDQ molar ratio. EDOT (M)

0.14 0.14 0.14 0.14 0.14

EDOT:DDQ

EDOT:PVAc

Transmission (%)

Sheet resistance

Thickness

r

(mol ratio)

(wt ratio)

500 nm

700 nm

(kX/sq)

(nm)

(S/cm)

1:2/3 1:1 1:4/3 1:5/3 1:2

1:1 1:1 1:1 1:1 1:1

86.8 81.6 79.1 69.5 71.3

81.8 73.6 70.0 59.2 59.8

1580 76.5 3.03 2.68 2.11

26 56 73 125 wa 110 wb

0.24 0.23 45 30 43

w = wavy surface, avg. thickness a = pk–pk 12 nm b = pk–pk 14 nm.

Table 2 Effects on film properties by varying the overall concentration of the film solutions. EDOT (M)

0.07 0.14 0.21 0.28

EDOT:DDQ

EDOT:PVAc

Transmission (%)

Sheet resistance

Thickness

r

(mol ratio)

(wt ratio)

500 nm

700 nm

(kX/sq)

(nm)

(S/cm)

1:4/3 1:4/3 1:4/3 1:4/3

1:1. 1:1. 1:1. 1:1.

86.6 76.3 61.2 54.0.

83.3 65.1 50.3 34.6

16.4 2.88 3.68 6.10.

33 79 142 wc 217 wd

17 44 19 4.5

w = wavy surface, avg. thickness c = pk-pk 30 nm d = pk-pk 90 nm.

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Table 3 Effect of film properties by varying the EDOT:PVAc weight ratio. EDOT (M)

0.14 0.14 0.14 0.14

EDOT:DDQ

EDOT:PVAc

Transmission (%)

Sheet resistance

Thickness

r

(mol ratio)

(wt ratio)

500 nm

700 nm

(kX/sq)

(nm)

(S/cm)

1:4/3 1:4/3 1:4/3 1:4/3

1:1/2 1:3/4 1:5/4 1:3/2

76.1 75.5 78.3 77.4

67.4 65.3 69.5 68.2

3.90 4.54 2.12 2.72

80 r 92 80 85 s

32 24 59 43

r = rough, avg. s = tall spikes.

Fig. 5. Conductive AFM for standard (a–c) PEDOT:PSS and (d–f) PEDOT:DDQ. Tip bias was (f) are overlays of the subsequent images at certain current thresholds.

voltages, measuring currents on PEDOT:PSS required a relatively high bias voltage that was orders of magnitude higher than that of PEDOT:DDQ and which resulted in surface currents orders of magnitude lower. Thus, we believe there is a less conductive, PSS-rich phase present on the PEDOT:PSS surface which is in agreement with literature [35,36]. In combination with exposure to atmosphere, this results in an insulating top layer, causing poor contact properties. Such a surface layer is apparently not present on the PEDOT:DDQ film initially, resulting in significantly better contact properties. This difference in contact resistance was observed macroscopically when liquid metal was not needed to assist in sheet resistance measurements with a 4-point probe for PEDOT:DDQ while it was needed for macroscopically more conductive PEDOT:PSS. 4.5. UPS To determine the work function (WF) and to further characterize the material, XPS and UPS measurements

2.1 V for PEDOT:PSS and 10 mV for PEDOT:DDQ. Images (c) and

were performed. UP spectra of a standard PEDOT:DDQ film gave a work function of 4.62 eV. This rather low value compared to earlier measurements of 5.05 eV for PEDOT:PSS shows a significant shift of the work function due to a change of counteranion and suggests that DDQ is a weaker dopant [10,37]. However, both layers show similar intensities of occupied states near the Fermi level. Further, when comparing reported values for VPP PEDOT:tos of 4.3 ± 1 eV by UPS [38], the WF value is greater for PEDOT:DDQ. As the counterion of PSS and tosylate are identical, the surrounding chemical environment of a conductive polymer can also influence the WF which can be easily altered for PEDOT:DDQ (i.e. polymer matrix). XPS was performed to compare PEDOT:DDQ to PEDOT:PSS. Unfortunately, our PEDOT:DDQ layers contain a rather large amount of PVAc with similar elemental and chemical structures, thus obscuring the signal. Nevertheless, some conclusions could be drawn (see Appendix A). Doping of PEDOT by DDQ was concluded, and a shift in surface chemistry toward a less PEDOT-rich top layer was observed over time.

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5. Conclusion Thin conductive films of PEDOT:DDQ were synthesized by delaying polymerization in ethereal solvents with low dielectric constants. The resulting smooth films produced by spin coating from 1,4-dioxane with PVAc as a stable, templating polymer and by rinsing with MeCN showed good structural integrity with decent conductivity and transparency. The feasibility of this approach has been proven, and a basic process has been established. Following our recipe, a film with a conductivity of 17 S/cm at 33 nm thick with 86.6% transmission at 550 nm was achieved. According to our observations, the processing solutions were sufficiently stable depending on solvent polarity, where significant polymerization is triggered by solvent evaporation in the spin-coated film. The as-coated layers still contained significant amounts of DDQ and partially reacted species, which can be reacted further by rinsing with MeCN which simultaneously removes byproducts and enhances the degree of polymerization. We are confident that the thin film properties can be improved further by optimizing many parameters such as the polymer matrix/template, solvents (and ratios), and the rinsing procedure or by changing the monomer. There is more potential for further exploration of this procedure due to its inherent versatility, where this system could rival that of the reference PEDOT:PSS. The attractive properties of this film formulation include: lower acidity relative to PSS, hydrophobic properties and structural support of the 180

PEDOT:DDQ

intensity (counts/s)

120 60 0 180

PEDOT:PSS 120 60

PEDOT PSS

0 172

170

168

166

164

162

binding energy (eV) Fig. A1. Comparison of XPS spectra of sulfur S(2p) signal for standard PEDOT:DDQ and EG-treated PEDOT:PSS thin films.

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polymer additive, and a lower contact resistance relative to PEDOT:PSS. We believe PEDOT:DDQ has potential for many applications such as a hole transport layer or as a transparent electrode (e.g. in organic solar cells or other optoelectronic devices) and should be explored further. Acknowledgments The authors thank Olaf Hild and John Hofferberth for useful discussions and suggestions, H.C. Starck for supplying samples of Baytron PH 1000, Yong Hyun Kim and Christoph Sachse for support, and Christian May for supporting the DIZEeff. M. Machala thanks the German Federal Foreign Office for providing this experience through a Transatlantic Renewable Energy Fellowship and Dana C. Olson for suggesting this placement. This work was funded by the European Union (EFRE), the Free State of Saxony and the Fraunhofer Gesellschaft as part of the Dresdner Innovationszentrum für Energieeffizienz. Appendix A XPS spectra for the S(2p) signal of PEDOT:DDQ and PEDOT:PSS can be found in Fig. A1. In both samples, the peak originating from PEDOT is at the same position within experimental error at 163.77 eV for PEDOT:DDQ and at 163.79 eV for PEDOT:PSS. For PEDOT:DDQ, a small subpeak – likely a positively charged sulfur atom – is visible. Whether this peak is present for PEDOT:PSS is unclear, since it might be hidden in the signal originating from the PSS peak. The positions of the S(2p) peak confirms the successful polymerization of EDOT and subsequent oxidation. For a freshly prepared sample and after 2 days in ultrahigh vacuum, the S(2s) signal, attributed to PEDOT and the Cl(2p) signal, attributed to DDQ, were recorded by XPS. The signal intensity for the S(2s) peak decreased by a factor of 2.4 while that of the Cl(2p) peak increases by a factor of 1.5 (Fig. A2). From the relative peak intensities, the ratios between the different polymers/molecules in the upper few nanometers of the film can be estimated. The PEDOT:DDQ ratio was estimated by comparing Cl(2p) and S(2p) peak intensities. The values for PEDOT:PVAc and PVAc:DDQ are more error prone since the oxygen peak that is used for the calculation is present in PVAc as well as in DDQ and PEDOT. For PEDOT:PVAc, the S(2s) and O(1s) signals were

Fig. A2. XPS signals for a) S(2s) and b) Cl(2p) peaks in PEDOT:DDQ for a fresh sample and the same sample stored in ultra-high vacuum for 2 days.

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Table A1 An estimation of ratios of various chemical species in PEDOT:DDQ films by XPS.

Fresh sample 2-Day-old sample

PEDOT:DDQ

PEDOT:PVAc

PVAc:DDQ

1.0 0.3

0.3 0.1

0.3 0.3

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