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 Transparent Electrode of Polymer Solar Cells Zhimeng Yu, Yijie Xia, Donghe Du, and Jianyong Ouyang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00317 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 27, 2016

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ACS Applied Materials & Interfaces

PEDOT:PSS Films with Metallic Conductivity through a Treatment with Common Organic solutions of Organic Salts and Their Application as Transparent Electrode of Polymer Solar Cells Zhimeng Yu1,2, Yijie Xia1, Donghe Du1 and Jianyong Ouyang1,2* 1 Department of Materials Science and Engineering, National University of Singapore, Singapore 117574 2 Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, Singapore 117574 Keywords: PEDOT:PSS, conductivity enhancement, organic solution, organic salt, polymer solar cells

Abstract: Transparent electrode is an indispensable component of optoelectronic devices, and substitutes of indium tin oxide (ITO) have been searched as the transparent electrode. Poly(3,4ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) that is a conducting polymer is very promising to be the next generation materials for transparent electrode if it can has high conductivity as ITO. Here, we report the treatment of PEDOT:PSS with organic solutions to

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significantly enhance its conductivity. Common organic solvents like dimethylformamide (DMF) and γ-butyrolactone (GBL) and common organic salts like methylammonium iodide (MAI) and methylammonium bromide (MABr) 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 ITO transparent electrode.

1. Introduction Optoelectronic devices include light emission diodes (LEDs), solar cells, liquid crystal displays (LCDs), detectors, lasers and touch panel displays. They have attracted great attention since they can get electricity from light or turn electricity to 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 electronic devices. Hence, a lot of effort has been made on developing new transparent conductive materials as the substitute of ITO. Several types of materials such as conducting polymers,3-16 carbon nanotubes,17,18 graphenes19,20 and metal nanowires21–23 were reported as the ITO

substitutes.

One

of

the

promising

candidates

is

poly(3,4-


2

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ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, chemical structure shown in Scheme 1). PEDOT:PSS has unique merits in comparison with other transparent conductors and conducting polymers. 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 transparency in the visible range, and it is mechanically flexible.24 However, as-prepared PEDOT:PSS films from its 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 about 2000 S/cm, and that is 3000-6000 S/cm for ITO on glass. In order to use PEDOT:PSS 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 were developed for the conductivity enhancement of PEDOT:PSS. Those approaches were summarized in the review articles.24,25 Jönssonet al. proposed that the charge transport among the conducting grain is promoted by the solvent treatment.27 Ouyang et al. observed simliar 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 mainly two factors for the 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

ofhigher than 3000 S/cm was observed by Xia et al. after PEDOT:PSS films were treated with sulfuric acid.29 Kim et al. found 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


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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 of them are highly corrosive for other materials.

P3HT

PC61BM

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

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 solution or γ-butyrolactone (GBL) solution of methylammonium iodide (MAI) and observed the conductivity enhancement by up to 4 orders in 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 transparent electrode.

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.


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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 fabricate PSCs are similar to our previous work as well.27 The methods and instruments to characterize the PEDOT:PSS films and PSCs are also the same as in our previous work.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 de-ionized (DI) water or iso-propyl 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 that treated with acetone solution of 0.1 M MAI. The treatment with DMF or GBL solution of 0.1 M MAI gives rise to the highest conductivity. Both conductivities are higher than 1600 S/cm. The conductivity was enhanced by 4 orders in magnitude. Conductivity enhancement of up to 5 orders in magnitude was reported when PEDOT:PSS films prepared from Clevios P VP Al 4083 solution were treated.51 Control study was carried out to use treat neat organic solvents for the treatment of PEDOT:PSS films (Table 2). The chemical structures, the dielectric constants, the boiling points


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and the meting 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, they become 1570 and 1210 S/cm after treated with EG or DMSO solution of 0.1 M MAI. The conductivities are different by more than 3 orders 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 treated with DMF or GBL solution of 0.1 M MAI. These results suggest that the conductivity enhancement of PEDOT:PSS arises from the synergetic effects of both MAI and the solvents.

Table 1. Conductivities of PEDOT:PSS films treated with 0.1 M MAI or MABr solutions with various solvents at 140 ℃. Solution

Conductivity

Solution

(S/cm)

Conductivity (S/cm)

MAI/ethanol

1020

MAI/EG

1570

MAI/methanol

1370

MAI/GBL

1640

MAI/DMF

1660

MAI/DMSO

1210

MAI/acetone

740

MAI/water

970

MABr/DMF

1280

MABr/GBL

1260


6

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Figure 1 presents the variations of the conductivities of PEDOT:PSS films treated with neat solvents or 0.1 M MAI solution with the dielectric constant of the solvents. The optimal dielectric constant of the solvents is in the range of 35 to 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.

Table 2. Conductivities of PEDOT:PSS films treated with neat solvents at 140 ℃. The chemical structure and physical properties of the solvents are also provided.

Chemical

Dielectric

Melting point

Boiling point

Conductivity

structure

constant

(oC)

(oC)

(S/cm)

24.5

-114

78.4

0.35

32.7

-97.6

64.7

370

DMF

36.7

-60.5

153

1.2

EG

37

-12.9

197.3

960

solvent

ethanol methanol

CH3OH


7


GBL

39.1

-43.53

204

0.87

DMSO

46.7

19

189

890

acetone

20.7

-95

56

0.24

water

80.1

0

100

3.0

1800

solution solvent

1500

Conductivity (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1200 900 600 300 0 20

30

40

50

60

70

80

Dielectric constant

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.

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


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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 change or increases only slightly. The conductivity then dramatically increases with the increase in the MAI concentration. It does not change too much at the concentrations of higher than 0.1 M. The highest conductivities are 1713 and 1832 S/cm after 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 0.1M MAI/DMF or 0.1 M MAI/GBL at different temperatures. In the temperature range from 80 to 140 °C, the conductivity increases with the lifting temperature. It then decreases when we further increased the temperature to 200 °C. The optimal treating temperature is 120-140 ℃. 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 oC. When the treating temperature is too high, polymer degradation can occur and leads to the conductivity decrease.


9


1800 3

10

(b)

(a) MAI/DMF treated MAI/GBL treated

Conductivity (S/cm)

Conductivity (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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102

101

1500

1200

MAI/DMF treated MAI/GBL treated

900

100

10-1 -8 10

600 10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

80

100

Concentration (M)

120

140

160

180

200

Temperature (K)

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

The conductivity of the PEDOT:PSS films can be further enhanced by repeating the treatment with solutions for multiple times. It can be 2195 S/cm when treated twice with 0.1 M MAI/GBL.

3.2 Characterizations of PEDOT:PSS films. The charge transport mechanisms of the solutiontreated PEDOT:PSS films was investigated by measuring the resistances of PEDOT:PSS films in the temperature range of 110 to 350 K (Figure 3(a), (b), (c)). As shown in Figure 3(a), the pristine PEDOT:PSS film exhibited resistence increase with the lowering temperature. The temperature dependence of the resistance becomes different for the PEDOT:PSS films treated with DMF or GBL solution of MAI. Although the resistances also increase with the temperature decrease in the range of 110 to 300 K, they increase with the lifting temperature in the range


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from 320 to 350 K (Figure 3(b)). These indicate that the treated PEDOT:PSS films behavior as a metal or semimetal in the latter temperature range.

0.8

Pristine MAI/DMF treated MAI/GBL treated

0.6

0.4

0.2 100

150

200

250

0.74 MAI/DMF MAI/GBL

0.73

0.72

0.71 300

300

Temperature (K)

310

320

330

340

350

Temperature (K)

(c)

1


0.36788

0.05

(b) Normalized resistance

Normalized resistance

0.75

(a)

1.0

Normalized resistance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06

0.07 -1/2

T

0.08

0.09

0.10

-1/2

(K )

Figure 3. Temperature dependences of the normalized resistances of pristine and treated PEDOT:PSS films. (a) Normalized resistances versus temperature. (b) Temperature dependences of the resistances in the temperature range from 300 to 330 K. (c) The resistance dependence on temperature 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


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 T  1 2  R(T ) = R0 exp  0   (1)  T   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 is 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 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 the interchain charge hopping. Presumably, two factors can affect the interchain charge hopping. One is the presence of 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. Aiming at understanding the effect of the solutions 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 at the wavelength below 250 nm. They are due to the aromatic ring of PSS.28,37,38,40 Their intensities drop after each


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treatment, and it suggests that some PSSH chains are rinsed away from the PEDOT:PSS films after the treatment. 1.2

Pristine DMF treated GBL treated MAI/DMF treated MAI/GBL treated

1.0

Absorption (A.U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0.0

200

220

240

260

280

300

Wavelength (nm)

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

The removal of PSSH chains is confirmed by the XPS spectra of PEDOT:PSS films (Figure 5(a)). There are 4 S2p 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 S2p intensity ratio of PEDOT to PSS hardly changed for the treatment with neat DMF or GBL, while it significantly increases for the treatment with DMF or GBL solution of MAI. The S2p XPs bands indicate sulfur atom ratio of PEDOT to PSS are 0.496, 0.570, 0.503, 1.287, 1.043 for pristine, DMF treated, GBL treated, MAI/DMF treated and MAI/GBL treated PEDOT:PSS films, respectively. The increase in the sulfur atom ratio after a solution treatment is a result of the removal of some PSSH from PEDOT:PSS films.


13


1.2

Pristine DMF treated GBL treated MAI/DMF treated MAI/GBL treated

(a) 1.0

Pristine MAI/DMF treated MAI/DMF treated

0.8

(b)

Intensity (a.u.)

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2 0.0 170

168

166

164

162

615

620

Binding energy (eV)

625

630

635

Binding energy (eV)

Figure 5. (a) S2p XPS spectra of pristine and solvent- or solution-treated PEDOT:PSS films. (b) I3d XPS spectra of pristine and solution-treated PEDOT:PSS films. The concentration of MAI was 0.1 M in the solutions used to treat PEDOT:PSS films.

The PEDOT:PSS films were also characterized by cyclic voltammetry. Figure 6 shows the cyclic voltammograms (CV) 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 structure for PEDOT and PSS chains. The conjugated PEDOT chains locate in the center, and they are surrounded with 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 careful rinse of


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the polymer films for several times. As shown in Figure 5(b), the I3d2/3 band appears in the range of 617-621 eV and the I3d2/5 band in the range of 628-632 eV, while they are absent in the spectrum of pristine PEDOT:PSS. This suggests that the presence of some I- anions in the PEDOT:PSS film after treatment.53,54 Because MAI has good solubility in water, the remaining of iodide in PEDOT:PSS can originate from the ion exchange between PEDOT:PSS and MAI solution. Some iodide anions may replace PSS as the counter anions of PEDOT.


40

20

Current (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-20

-40

-0.8

-0.4

0.0

0.4

0.8

Potential (V)

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 PEDOT:PSS films.

The morphology of the PEDOT:PSS films can be changed by the removal of PSSH chains and the conformational change of the PEDOT chains. Figure 7 presents the AFM images of a pristine PEDOT:PSS film and PEDOT:PSS films treated with neat solvents or solutions. The surface morphology of the PEDOT:PSS films treated with DMF or GBL is similar to that of the


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pristine PEDOT:PSS film. But the domain size increases from 50 to more than 100 nm after a treatment with neat solvent. The surface morphology saliently changes after 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-treated, GBL-treated, 0.1M MAI/DMF-treated 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 It 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 the conformational change of PEDOT chains.

(a)

(b)


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(c)

(d)

(e)

Figure 7. AFM images of PEDOT:PSS films (a) pristine and treated with (b) DMF, (c) GBL, (d) 0.1 M MAI/DMF, and (e) 0.1 M MAI/GBL. The unit for the AFM images is µm.

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 solution 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 have


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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 is difficult to be solvated with water. The screening effect of water is thus weaker than some polar organic solvents like EG and DMSO. As a result, a treatment of PEDOT:PSS with water only slightly enhance the conductivity. As PEDOT has low polarity, there is an 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 the 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 it can thus strongly bind to PSS, while iodide ions can compensate the 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 a MAI solution. The results and discuss presented above indicate the 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 reasonable 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 reversible after the removal of the solvents. The more significant


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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 Higher the soft parameter, stronger the interaction of the anion with PEDOT.

Scheme 2. Schematic diagram of synergetic effect of both solvent and salt of solutions on the structure of PEDOT:PSS

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 transparent conductors.56 The relationship between the transmittance (T550) at 550 nm and sheet resistanceis shown in the following equation,57 𝑇𝑇550 =

1

𝑍𝑍 𝜎𝜎𝑜𝑜𝑜𝑜 2 (1+ 0 ) 2𝑅𝑅𝑠𝑠 𝜎𝜎𝑑𝑑𝑑𝑑

(2)

with Z0 ≈ 377 Ω as the impedance of free space. In terms of this equation, the FOM of treated PEDOT:PSS is 27.


19


100 90 80

Transmittance (%)

70 ITO MAI/DMF treated

60 50 40 30

For MAI/DMF treated PEDOT:PSS Rs=108.77 Ohm/sq T550=88.34%

20 10 0

(a)

350 400 450 500 550 600 650 700 750 800

Wavelenght (nm)

0

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

-2

ITO MAI/DMF treated

-4

-6

-8

(c) -10 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Voltage (V)

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

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 ITO transparent electrode is also included. The photovoltaic parameters, including


20

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

Table. 3 Photovoltaic Performances of P3HT:PC61BM PSCs with ITO or a PEDOT:PSS film treated with 0.1M MAI/DMF solution. Voc

Jsc

Electrode

Rsh

Rs

(%)

(kΩcm2)

(Ωcm2)

FF (V)

ITO

PCE

(mA/cm2)

0.58±0.01 9.55±0.12 0.66±0.02 3.66±0.33 7223±306 2.48±0.11

Treated 0.58±0.01 8.71±0.11 0.65±0.02 3.29±0.21

1119±75

4.78±0.60

PEDOT:PSS The parameter values were obtained from 10 devices.


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4. Conclusion 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 treated twice. The conductivity enhancements are much more significant than the treatments with neat organic solvents or 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 paves a new avenue for the development of high-performance conductive polymers.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research work was financially supported by a grant from the Ministry of Education of Singapore (R-284-000-113-112). 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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Table of Contents Graphic

1800

solution solvent

1500

Conductivity (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1200 900 600 300 0 20

30

40

50

60

70

80

Dielectric constant


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PEDOT:PSS Films with Metallic Conductivity through a Treatment with

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