PEDOT:PSS Films with Metallic Conductivity through a Treatment with

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PEDOT:PSS Films with Metallic Conductivity through a Treatment with Common Organic Solutions of Organic Salts and Their Application as a Transparent Electrode of Polymer Solar Cells Zhimeng Yu,†,‡ Yijie Xia,† Donghe Du,† and Jianyong Ouyang*,†,‡ †

Department of Materials Science and Engineering and ‡Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, Singapore 117574, Singapore ABSTRACT: A transparent electrode is an indispensable component of optoelectronic devices, and there as been a search for substitutes of indium tin oxide (ITO) as the transparent electrode. Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a conducting polymer that is very promising as the next generation of materials for the transparent electrode if it can obtain conductivity as high as that of ITO. Here, we report the treatment of PEDOT:PSS with organic solutions to significantly enhance its conductivity. Common organic solvents like dimethylformamide and γ-butyrolactone and common organic salts like methylammonium iodide and methylammonium bromide are used for the organic solutions. The conductivity of pristine PEDOT:PSS films is only ∼0.2 S/cm, and it can be increased to higher than 2100 S/cm. The conductivity enhancement is much more significant than control treatments of PEDOT:PSS films with neat organic solvents or aqueous solutions of the organic salts. The mechanism for the conductivity enhancement is the synergetic effects of both the organic salts and organic solvents on the microstructure and composition of PEDOT:PSS. They induce the segregation of some PSSH chains from PEDOT:PSS. Highly conductive PEDOT:PSS films were studied as the transparent electrode of polymer solar cells. The photovoltaic efficiency is comparable to that with an ITO transparent electrode. KEYWORDS: PEDOT:PSS, conductivity enhancement, organic solution, organic salt, polymer solar cells

1. INTRODUCTION Optoelectronic devices include light emission diodes (LEDs), solar cells, liquid crystal displays (LCDs), detectors, lasers, and touch panel displays. They have attracted significant attention because they can obtain electricity from light or turn electricity into light. There should be at least one transparent electrode for light to pass through for optoelectronic devices. The traditional transparent electrode material is indium tin oxide (ITO). Nevertheless, ITO has several drawbacks. Indium is a scarce element on earth; its fabrication cost is high, and it is a brittle material.1,2 ITO is thus not a good material for flexible electronic devices that are considered as the next-generation of electronic devices. Hence, significant effort has been put toward the development of new transparent conductive materials as a substitute for ITO. Several types of materials, such as conducting polymers,3−16 carbon nanotubes,17,18 graphenes,19,20 and metal nanowires21−23 have been reported as ITO substitutes. One promising candidate is poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS; chemical structure shown in Scheme 1). PEDOT:PSS has unique merits in comparison with other transparent conductors and conducting polymers. An aqueous dispersion of PEDOT:PSS is commercially available, and it can be easily processed into thin films on rigid or flexible substrates by coating or printing. Additionally, PEDOT:PSS has high © 2016 American Chemical Society

Scheme 1. Chemical Structures of PEDOT:PSS, P3HT, and PC61BM

transparency in the visible range, and it is mechanically flexible.24 However, as-prepared PEDOT:PSS films from aqueous solution exhibit low conductivity of lower than 1 S/ cm. This conductivity is strikingly lower than that of ITO. The conductivity of ITO on plastic is approximately 2000 S/cm and on glass is 3000−6000 S/cm. For PEDOT:PSS to be used as the transparent electrode, its conductivity must be greatly increased. Since the first report by Kim et al. using polar organic solvents,13 a couple of approaches have been developed for conductivity enhancement of PEDOT:PSS. Those approaches are summarized in review articles.24−26 Jönsson et Received: January 11, 2016 Accepted: April 26, 2016 Published: April 26, 2016 11629

DOI: 10.1021/acsami.6b00317 ACS Appl. Mater. Interfaces 2016, 8, 11629−11638

Research Article

ACS Applied Materials & Interfaces al. proposed that the charge transport among the conducting grain is promoted by the solvent treatment.27 Ouyang et al. observed similar conductivity enhancement when the post PEDOT:PSS films were treated with polar organic solvents.28 Apart from high-boiling point orgainic solvents, other chemical agents like surfactants, ionic liquids, aqueous solutions of inorganic salts, zwitterions, organic and inorganic acids, and cosolvents of organic solvents and water were investigated for the conductivity enhancement of PEDOT:PSS.29−47 There are two main factors for conductivity enhancement. One is the segregation of some excess PSS chains from PEDOT:PSS, and the other is the change in the PEDOT chain conformation.8,27−29,48−50 Recently, a conductivity of higher than 3000 S/cm was observed by Xia et al. after PEDOT:PSS films were treated with sulfuric acid.29 Kim et al. found that the conductivity could be higher than 4000 S/cm when fuming sulfuric acid was used.30 Ouyang observed conductivities of up to 3300 S/cm when PEDOT:PSS was treated with organic acids like methanesulfonic acid.31 Other acids were also exploited to improve the conductivity.32−36 However, acids bring severe environmental and safety concerns because they have high chemical activity and many are highly corrosive for other materials. Using common chemicals with mild chemical properties to treat PEDOT:PSS can have practical meaning. In this work, we treated PEDOT:PSS films with common organic solutions, such as dimethylformamide or γ-butyrolactone (GBL) of methylammonium iodide (MAI), and observed the conductivity enhancement by up to 4 orders of magnitude. The conductivity can be enhanced to 2195 S/cm, much higher than that treated with neat organic solvents or aqueous solutions of these organic salts. Additionally, these highly conductive PEDOT:PSS films were studied as the transparent electrode of polymer solar cells (PSCs). The PSCs show power conversion efficiencies comparable to that with ITO as the transparent electrode.

Table 1. Conductivities of PEDOT:PSS Films Treated with 0.1 M MAI or MABr Solutions with Various Solvents at 140 °C solution

conductivity (S/cm)

solution

conductivity (S/cm)

MAI/ethanol MAI/methanol MAI/DMF MAI/acetone MABr/DMF

1020 1370 1660 740 1280

MAI/EG MAI/GBL MAI/DMSO MAI/water MABr/GBL

1570 1640 1210 970 1260

when treated with an acetone solution of 0.1 M MAI. The treatments with DMF or GBL solutions of 0.1 M MAI give rise to the highest conductivities with both higher than 1600 S/cm. The conductivity was enhanced by 4 orders of magnitude. Conductivity enhancement of up to 5 orders of magnitude was reported when PEDOT:PSS films prepared from Clevios P VP Al 4083 solution were treated.51 A control study was carried out to test the use of neat organic solvents for the treatment of PEDOT:PSS films (Table 2). The Table 2. Conductivities of PEDOT:PSS Films Treated with Neat Solvents at 140 °C along with the Chemical Structures and Physical Properties of the Solvents

2. EXPERIMENTAL SECTION MAI was obtained from Dysol. Dimethyl sulfoxide (DMSO), dimethylformamide (DMF), ethylene glycol (EG), acetone, ethanol, methanol, and GBL were supplied by Sigma-Aldrich. The suppliers for PEDOT:PSS aqueous solution (Clevios PH 1000), ITO, and glass substrates were the same as in our previous work.29 The procedures to treat PEDOT:PSS films with organic solutions and fabricated PSCs are similar to our previous work as well,27 as are the methods and instruments to used characterize the PEDOT:PSS films and PSCs.29

3. RESULTS AND DISCUSSION 3.1. Conductivity Enhancement of PEDOT:PSS Films. The PEDOT:PSS films were prepared on glass substrates by spin coating the aqueous PEDOT:PSS solution (Clevios PH1000). They were treated by organic solutions. Acetone, ethanol, methanol, DMF, ethylene glycol (EG), GBL, or DMSO was used as the solvent, and MAI or methylammoniun bromide (MABr) was used as the salt of the organic solution. After the solvent vaporized, the films were rinsed with deionized (DI) water or isopropyl alcohol (IPA). Table 1 presents the conductivities of the treated PEDOT:PSS films with organic solutions of 0.1 M MAI or MABr. The conductivity of PEDOT:PSS is significantly enhanced, and the conductivity enhancement is related to the solvent of the solutions. The conductivity is only ∼0.2 S/cm for pristine PEDOT:PSS films, and it is higher than 1000 S/cm except

chemical structures, dielectric constants, boiling points, and melting points are listed as well to understand the dependence of the conductivity enhancement on the structure and properties of the solvents. The conductivities of PEDOT:PSS films with the solutions are always much higher than with the neat solvents. The conductivities of PEDOT:PSS films treated with EG or DMSO are 960 and 890 S/cm, and they become 1570 and 1210 S/cm after being treated with EG or DMSO solution of 0.1 M MAI, respectively. The conductivities differ by more than 3 orders of magnitude for other organic solvents. The conductivities of PEDOT:PSS films treated with DMF and GBL are only 1.2 and 0.87 S/cm, respectively. They are higher than 1600 S/cm after being treated with the DMF or GBL solution of 0.1 M MAI. These results suggest that the 11630


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change or increases only slightly. The conductivity then dramatically increases with the increase in the MAI concentration. It does not change too much at concentrations higher than 0.1 M. The highest conductivities are 1713 and 1832 S/cm after being treated with 0.5 M MAI/DMF and 1 M MAI/GBL solutions, respectively. The conductivity enhancement of PEDOT:PSS also depends on the temperature during the solution treatment. Figure 2(b) displays the conductivities of PEDOT:PSS films treated with 0.1 M MAI/DMF or 0.1 M MAI/GBL at different temperatures. In the temperature range of 80−140 °C, the conductivity increases with increasing temperature. It then decreases when we further increased the temperature to 200 °C. The optimal treating temperature is 120−140 °C. Presumably, the treating temperature effect on the conductivity of the PEDOT:PSS films is relevant to the thermal properties of the PEDOT:PSS films. The polymer chains must experience a conformational change for the conductivity enhancement, and the thermal energy can facilitate the conformational change. Although the glass transition temperature of PEDOT:PSS cannot be observed by differential scanning calorimetry (DSC), it may be close to the optimal temperature of 120−140 °C. When the treating temperature is too high, polymer degradation can occur and can lead to decreased conductivity. The conductivity of the PEDOT:PSS films can be further enhanced by repeating the treatment with solutions multiple times. It is 2195 S/cm when treated twice with 0.1 M MAI/ GBL. 3.2. Characterization of PEDOT:PSS Films. The charge transport mechanisms of the solution-treated PEDOT:PSS films was investigated by measuring the resistances of PEDOT:PSS films in the temperature range of 110−350 K (Figure 3(a−c)). As shown in Figure 3(a), the pristine PEDOT:PSS film exhibited an increase in resistance with lower temperature. The temperature dependence of the resistance differs for the PEDOT:PSS films treated with DMF or GBL solutions of MAI. Although the resistances also increases with decreasing temperature in the range of 110−300 K, they increase with increasing temperature in the range of 320−350 K (Figure 3(b)), which indicates that the treated PEDOT:PSS films behave as a metal or semimetal in the latter temperature range.

conductivity enhancement of PEDOT:PSS arises from the synergetic effects of both MAI and the solvents. Figure 1 presents the variations of the conductivities of PEDOT:PSS films treated with neat solvents or 0.1 M MAI

Figure 1. Variations of the conductivities of PEDOT:PSS films treated with neat solvents or 0.1 M MAI solutions with the dielectric constant of the solvents.

solution with the dielectric constant of the solvents. The optimal dielectric constant of the solvents is in the range of 35− 40 in terms of the conductivities of the PEDOT:PSS films treated with solutions. The dependence for the treatment with solutions is different from that for the treatment with neat solvents. For the treatment with neat solvents, although the dielectric constant range of 35−50 may be the optimal range, there are remarkable exceptions, including DMF and GBL. The salt effect is further confirmed by using MABr to treat PEDOT:PSS films. The conductivities of PEDOT:PSS films treated with DMF and GBL solutions of 0.1 M MABr are 1280 and 1260 S/cm, respectively. These conductivities are lower than that with the solutions of 0.1 M MAI. More characterizations were performed for the conductivity enhancement of PEDOT:PSS films by DMF and GBL solutions of MAI because they can have the most significant conductivity enhancements. Figure 2(a) illustrates the dependences of the conductivities of the PEDOT:PSS films on the concentration of MAI in DMF or GBL. When the MAI concentration is lower than 10−4 M, the conductivity does not

Figure 2. (a) Variation of the conductivities of PEDOT:PSS films treated with DMF or GBL solutions with the MAI concentrations. The treatments were performed at 140 °C. (b) Dependences of the conductivities of PEDOT:PSS films treated with DMF or GBL solutions of 0.1 M MAI on the treating temperature. The concentration of MAI was 0.1 M in the solutions. 11631


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Figure 3. Temperature dependences of the normalized resistances of pristine and treated PEDOT:PSS films for (a) normalized resistances versus temperature, (b) resistances in the temperature range of 300−330 K, and (c) resistance dependence according to the 1D VRH model. The resistances are normalized to those of the corresponding PEDOT:PSS films at 110 K. The concentration of MAI is 0.1 M.

The one-dimensional variable range hopping (VRH) model was used to analyze the temperature dependences of the resistances of the PEDOT:PSS films,13,45 ⎡⎛ T ⎞1/2 ⎤ R(T ) = R 0exp⎢⎜ 0 ⎟ ⎥ ⎢⎣⎝ T ⎠ ⎥⎦

(1)

where T0 = 16/kBN(EF)L∥L⊥2 is the energy barrier between localized states, N(EF) is the density of the states at the Fermi level, and L∥ (L⊥) is the localization length in the parallel (perpendicular) direction. As shown in Figure 3(c), the temperature dependences of the resistances of the pristine PEDOT:PSS films are consistent with the one-dimensional VRH model. However, the resistance of the PEDOT:PSS films treated with DMF or GBL solution of MAI follow the VRH model only when the temperature is lower than 200 K. According to the analyses of the data with the VRH model, the energy barriers for all of the PEDOT:PSS films can be obtained. The energy barriers of pristine, 0.1 M MAI/DMF-, and 0.1 M MAI/GBL-treated PEDOT:PSS films are 1060, 81, and 71 K, respectively. The energy barrier due to the interchain charge hopping saliently decreases after a solution treatment. This suggests that the solution treatments of the PEDOT:PSS films can facilitate interchain charge hopping, presumably affected by two factors. One is the presence of a PSSH insulator, which inhibits the interchain charge hopping. Another is related to the conformation of the PEDOT chains. Charge hopping across linear PEDOT chains is much easier than across PEDOT chains with coil conformation. With the aim of understanding the effects of the solution treatments on the structure of PEDOT:PSS films, the pristine and treated PEDOT:PSS films were characterized by UV−vis spectroscopy and X-ray photoelectron spectroscopy (XPS). Figure 4 presents the UV absorption spectra of the untreated and treated PEDOT:PSS films. There are two absorption bands

Figure 4. UV−vis spectra of pristine and solvent- or solution-treated PEDOT:PSS films.

at wavelengths below 250 nm, which are due to the aromatic ring of PSS.28,37,38,40 Their intensities drop after each treatment, which suggests that some PSSH chains are rinsed away from the PEDOT:PSS films after the treatment. The removal of PSSH chains is confirmed by the XPS spectra of PEDOT:PSS films (Figure 5(a)). There are 4 S 2p XPS bands. The two in the range of 166−172 eV correspond to the sulfur atoms of PSS, and the other two in the range of 162−166 eV originate from the sulfur atoms of PEDOT.40,52 The relative S 2p intensity ratio of PEDOT to PSS hardly changed for the treatment with neat DMF or GBL, whereas it significantly increases for the treatment with DMF or GBL solution of MAI. The S 2p XPs bands indicate a sulfur atom ratio of PEDOT to PSS of 0.496, 0.570, 0.503, 1.287, and 1.043 for pristine, DMF-, GBL-, MAI/DMF-, and MAI/GBL-treated PEDOT:PSS films, respectively. The increase in the sulfur atom ratio after a 11632


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Figure 5. (a) S 2p XPS spectra of pristine and solvent- or solution-treated PEDOT:PSS films. (b) I 3d XPS spectra of pristine and solution-treated PEDOT:PSS films. The concentration of MAI was 0.1 M in the solutions used to treat the PEDOT:PSS films.

anions in the PEDOT:PSS film after treatment.53,54 Because MAI has good solubility in water, the remaining iodide in PEDOT:PSS can originate from the ion exchange between PEDOT:PSS and the MAI solution. Some iodide anions may replace PSS as the counteranions of PEDOT. The morphology of the PEDOT:PSS films can be changed by the removal of PSSH chains and conformational change of the PEDOT chains. Figure 7 presents AFM images of a pristine PEDOT:PSS film and PEDOT:PSS films treated with neat solvents or solutions. The surface morphologies of the PEDOT:PSS films treated with DMF or GBL are similar to that of the pristine PEDOT:PSS film, but the domain size increases from 50 to more than 100 nm after treatment with neat solvent. The surface morphology saliently changes after being treated with a solution. Nanosized fibers can be observed for the PEDOT:PSS films treated with DMF or GBL solution of MAI. The surface of the polymer films becomes rougher after a treatment. The roughnesses of the pristine, DMF-, GBL-, 0.1 M MAI/DMF-, and 0.1 M MAI/GBL-treated PEDOT:PSS films are 1.124, 1.078, 1.105, 1.388, and 1.402 nm, respectively. Palumbiny et al. proposed that the core/shell structure becomes smaller in the highly conductive PEDOT:PSS film after a treatment.48,49 This may happen during the treatment of PEDOT:PSS films with organic solutions of organic salts because the change in the core/shell size can also lead to a conformational change of the PEDOT chains. The results presented above suggest that the mechanism for the conductivity enhancement by MAI solutions is different from that with neat solvents. The conductivity enhancement by EG or DMSO solutions of MAI is much more significant than that by the corresponding neat solvent. Presumably, the phase segregation of PSSH from PEDOT:PSS can happen only when solvent or salt screens the Coulombic attraction between PEDOT having positive charges and PSS with negative charges. The Coulombic attraction even disappears if PSS changes to neutral PSSH. Polar solvent can screen the Coulombic attraction between PEDOT and PSS as the solvent molecules can solvate the polycations and polyanions, but the screening depends on not only the dielectric constant but also the hydrophilicity/hydrophobicity of the solvent used to treat PEDOT:PSS because PEDOT is hydrophobic and difficult to solvate with water. The screening effect of water is thus weaker than that of some polar organic solvents like EG and DMSO. As a result, a treatment of PEDOT:PSS with water only slightly enhances conductivity. As PEDOT has low polarity, there is an

solution treatment is the result of the removal of some PSSH from the PEDOT:PSS films. The PEDOT:PSS films were also characterized by cyclic voltammetry. Figure 6 shows the cyclic voltammograms (CV)

Figure 6. CVs of pristine and solution-treated PEDOT:PSS films in 0.1 M NaCl aqueous solution. The concentration of MAI was 0.1 M in the solutions used to treat the PEDOT:PSS films.

of pristine and solution-treated PEDOT:PSS films. The electrochemical activity of the PEDOT:PSS films increases after the treatment with a solution. The pristine PEDOT:PSS film exhibits electrochemical activity in the potential range from −0.2 to 0.8 V vs AgCl/Ag. Additional electrochemical activity was observed for the PEDOT:PSS films treated with DMF or GBL solution of MAI. The increase in the electrochemical activity can be attributed to the removal of PSSH from PEDOT:PSS and the change of the PEDOT chain conformation in PEDOT:PSS. There are core/shell structures for PEDOT and PSS chains. The conjugated PEDOT chains locate in the center, and they are surrounded by the PSS insulator chains. The insulator shell may block the charge transfer between the work electrode and PEDOT chains. The core/ shell structure disappears as a result of the removal of the excess PSSH chains. Iodide was detected in the solution-treated PEDOT:PSS films even after a careful rinse of the polymer films several times. As shown in Figure 5(b), the I 3d3/2 band appears in the range of 617−621 eV and the I 3d5/2 band in the range of 628−632 eV, whereas they are absent in the spectrum of pristine PEDOT:PSS. This suggests the presence of some I− 11633


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Figure 7. AFM images of PEDOT:PSS films that are (a) pristine and or treated with (b) DMF, (c) GBL, (d) 0.1 M MAI/DMF, and (e) 0.1 M MAI/ GBL. The AFM images are in μm.

positive charges on PEDOT chains. Therefore, the decrease in the Coulombic attraction between PEDOT and PSS by salts can be more significant than by neat solvent. Anion exchange between PEDOT:PSS and solution can even take place as evidenced by the presence of I− species in PEDOT:PSS films after a treatment with an MAI solution. The results and discussion presented above indicate a synergetic effect of both solvent and salt of solutions on the structure of PEDOT:PSS. The synergetic effect is schematically presented in Scheme 2. This module can provide a reasonable

optimal dielectric constant range for the solvent molecules to solvate the PEDOT cations. The presence of a salt like MAI is important for the conductivity enhancement, particularly for solvents like DMF and GBL. The salt effect can be attributed to interactions between cations and anions of the salt and the polycations and polyanions of PEDOT:PSS. These interactions are related to the soft parameters of the cations and anions.37,38,55 Methylammonium has a high soft parameter of 10.7 and can thus strongly bind to PSS, and iodide ions can compensate the 11634


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transparent conductors.56 The relationship between the transmittance (T550) at 550 nm and sheet resistance is shown in following equation57

Scheme 2. Schematic Diagram of the Synergetic Effect of Both Solvent and Salt of Solutions on the PEDOT:PSS Structure

T550 =

1

(

1+

2 Z0 σop 2R s σdc

)

(2)

where Z0 ≈ 377 Ω is the impedance of free space. In terms of this equation, the FOM of treated PEDOT:PSS is 27. The schematic device structure is shown in Figure 8(b). Poly(3-hexylthiophene) (P3HT; chemical structure shown in Scheme 1) and phenyl-C61-butyric acid methyl ester (PC61BM; chemical structure shown in Scheme 1) were used as the donor and acceptor. The current density (J)−voltage (V) curves of solar cells are presented in Figure 8(c). The J−V curve of a control device with an ITO transparent electrode is also included. The photovoltaic parameters, including open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE), are listed in Table 3. The series resistance (Rs) and shunt resistance (Rsh) were extracted from the inverse of the slopes of J−V curves of devices in the dark at 1 and 0 V, respectively.58−60 They are also provided in Table 3. The device with a PEDOT:PSS film treated with DMF solution of MAI exhibits a PCE of 3.51%, quite close to that of the control devices with ITO. The main difference in the photovoltaic performance for the PSCs with different transparent electrodes is Jsc. This can be attributed to the lower

explanation for the results observed in this study. When PEDOT:PSS is treated with neat DMF or GBL, the screening effect by these solvents is not strong, and it can be reversed after removal of the solvents. The more significant conductivity enhancement of PEDOT:PSS by MAI than MABr can be attributed to the different soft parameters of Br− (0.17) and I− (0.5).37,38,55 The higher the soft parameter, the stronger the interaction of the anion with PEDOT. 2.3. Application of Highly Conductive PEDOT:PSS Films in PSCs. The PEDOT:PSS films treated with solution exhibit high transparency. Figure 8(a) shows the optical transmittance spectra of ITO and PEDOT:PSS films. The ratio of direct circuit conductivity (σdc) to optical conductivity (σop) is usually used as the figure of merit (FOM) for

Figure 8. (a) Optical transmittance spectra for ITO and PEDOT:PSS films all on glass substrates. (b) Device architecture of PSCs. (c) J−V characteristics of PSCs with ITO or a PEDOT:PSS film treated with a DMF solution of MAI as the transparent electrode under AM 1.5 G illumination (100 mW/cm2). The concentration of the MAI/DMF solution was 0.1 M. 11635


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Table 3. Photovoltaic Performances of P3HT:PC61BM PSCs with ITO or a PEDOT:PSS Film Treated with 0.1 M MAI/DMF Solutiona

a

electrode

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

Rsh (kΩ cm2)

Rs (Ω cm2)

ITO treated PEDOT:PSS

0.58 ± 0.01 0.58 ± 0.01

9.55 ± 0.12 8.71 ± 0.11

0.66 ± 0.02 0.65 ± 0.02

3.66 ± 0.33 3.29 ± 0.21

7223 ± 306 1119 ± 75

2.48 ± 0.11 4.78 ± 0.60

The parameter values were obtained from 10 devices.

transmittance of the PEDOT:PSS film than that of ITO. The FF of the former is also slightly lower than the latter. This arises from the larger Rs value of the former.

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4. CONCLUSIONS The conductivity of PEDOT:PSS films can be significantly enhanced through a treatment with organic solutions of MAI. The conductivity of PEDOT:PSS films can be enhanced to 1800 after a single treatment and 2195 S/cm after being treated twice. The conductivity enhancements are much more significant than the treatments with neat organic solvents or an aqueous solution of MAI. It depends on the salt concentration and solvent of the solutions. The mechanism for the conductivity enhancement by organic solutions of organic salts is attributed to the synergetic effects of both the solvent and salt of the solutions. They induce the phase segregation of PSS chains from PEDOT:PSS and the conformational change of PEDOT chains induced by the organic salts and organic solvents. This method is mild and safe. DMF has a relatively low boiling point, and it can be readily removed. The highly conductive PEDOT:PSS films obtained by the treatment with organic solution can be used as the transparent electrode of PSCs. This method provides a new route for the development of high-performance conductive polymers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially supported by a grant from the Ministry of Education of Singapore (R-284-000-113-112). The Solar Energy Research Institute of Singapore (SERIS) is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB).

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