Higher PEDOT Molecular Weight Giving Rise to Higher Thermoelectric

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Higher PEDOT Molecular Weight Giving Rise to Higher Thermoelectric Property of PEDOT:PSS: A Comparative Study of CleviosTM P and CleviosTM PH1000 Zeng Fan, Donghe Du, Hongyan Yao, and Jianyong Ouyang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15158 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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ACS Applied Materials & Interfaces

Higher PEDOT Molecular Weight Giving Rise to Higher Thermoelectric Property of PEDOT:PSS: A Comparative Study of CleviosTM P and CleviosTM PH1000 Zeng Fan, Donghe Du, Hongyan Yao, and Jianyong Ouyang*

Department of Materials Science and Engineering, National University of Singapore, Singapore 117574, Singapore

1 ACS Paragon Plus Environment


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Abstract:

Poly(3,4-ethylenedioxuthiphene):poly(styrenesulfonate)

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(PEDOT:PSS)

is

a

promising candidate as the next-generation thermoelectric (TE) material. Its TE properties are strongly dependent on its chemical and electronic structures. In this paper, we investigated the effect of PEDOT molecular weight on the TE properties of PEDOT:PSS films by a comparative study on two commercial grades of PEDOT:PSS, CleviosTM P and CleviosTM PH1000. Dynamic light scattering (DLS) and Raman spectra imply that the PEDOT of CleviosTM PH1000 possesses longer conjugated chains than that of CleviosTM P. The TE properties of both the CleviosTM P and CleviosTM PH1000 films can be significantly enhanced through various post treatments, including solvent treatment, germinal diol treatment, organic solution treatment and acid treatment. After these treatments, the treated CleviosTM PH1000 films constantly show both superior Seebeck coefficients and electrical conductivities over the treated CleviosTM P films. It is attributed to the higher molecular weight of PEDOT for the former than the latter. For the treated CleviosTM PH1000, longer PEDOT chains result in large PEDOT domains, facilitating the charge conduction a semi-metallic behaviour. Tuning the oxidation level of PEDOT:PSS is a facile way to enhance their TE property. A base treatment with sodium hydroxide was subsequently performed on both the treated CleviosTM P and CleviosTM PH1000 films. The power factors of both grades of PEDOT:PSS films were remarkably increased by a factor of 1.2-3.6. Still, both the conductivity and the Seebeck coefficient of a based-treated CleviosTM PH1000 film are superior over those of a control CleviosTM P film. The highest power factor the former is 334 μW/(m·K2) for the former while only 11.4 μW/(m·K2) for the latter. They are different by a factor of about 30 times.

KEYWORDS: PEDOT:PSS, molecular weight, post treatment, Seebeck coefficient, electrical conductivity, thermoelectric property

1. INTRODUCTION 2 ACS Paragon Plus Environment

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Organic thermoelectric (TE) materials are regarded as the next-generation green energy materials owing to their various advantages over inorganic TE materials, such as low cost, good mechanical flexibility, abundant elements and low toxicity 1, 2. The main factor to evaluate the energy conversion of a TE material is the dimensionless figure-of-merit, ZT=S2σT/k, where S stands for the Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity and T is the absolute temperature thermoelectric

application.

3-6

. Conducting polymers have been investigated for

Among

them,

poly(3,4-ethylenedioxuthiphene):

poly(styrenesulfonate) (PEDOT:PSS) has attracted the most intense attention due to its solution processability, high transparency in the visible range and good thermal stability 7-9. However, as-prepared PEDOT:PSS films from its aqueous solution usually have an electrical conductivity of less than 1 S/cm, severely restricting their practical applications. Various posttreatment methods have been developed to enhance the electrical conductivity of PEDOT:PSS 8, 10-12

. The post treatments can enhance the electrical conductivity of PEDOT:PSS films by a

factor of four orders of magnitude. Hence, an appropriate post-treatment on PEDOT:PSS can effectively improve its TE properties because the ZT value is proportional to conductivity 1318

. There are several grades of PEDOT:PSS products commercially available in market, such

as CleviosTM P and CleviosTM PH1000. These two grades of aqueous solutions are prepared through a similar approach and have the same PEDOT:PSS concentration (1.3 wt.%) and PEDOT:PSS ratio (1:2.5 by weight)

19, 20

. Although the conductivities of the as-prepared

PEDOT:PSS films are almost the same for the two grades, their conductivities after a treatment can be different by a factor of more than 10 7, 20 The different conductivities are attributed to the different PEDOT molecular weights of the two PEDOT:PSS grades. The TE properties of polymers depend on both the conductivity and the Seebeck coefficient 21, 22. The conditions for the optimal power factor are usually different from that of the conductivity because the Seebeck



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coefficient and conductivity have different dependences on the doping level

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23-25

. When the

doping level is lowered, the conductivity decreases whereas the Seebeck coefficient increases 26-33

. It is important to understand how the PEDOT molecular weight affects the TE properties

of PEDOT:PSS after some treatments. In this work, a comparative study was performed on the TE properties of CleviosTM P and CleviosTM PH1000 films. Their conductivities are significantly enhanced through a post treatment with a solvent, germinal diol, organic solution or an acid. The treated PEDOT:PSS films are then treated with base, which further improves the power factor (power factor = S2σ). Apart from the conductivity, the Seebeck coefficient of a CleviosTM PH1000 film is significantly higher than that of a control CleviosTM P film after base treatment. The highest power factor of the former is almost 30 times of the latter.

2. EXPERIMENTS 2.1 Materials and treatments of PEDOT:PSS films. Materials and the procedures for the post treatments of CleviosTM P and CleviosTM PH1000 can refer to our previous works

7, 8

.

After treatment, the films were taken down from the hot plate, cooled down to room temperature in air, rinsed with sufficient de-ionized (DI) water for 15 seconds three times and dried again on the hot plate at 120 °C. 2.2 Characterization. Characterizations of the electrical conductivities, thicknesses, Seebeck coefficients, dynamic light scattering (DLS), UV-vis absorption spectra, X-ray photoelectron spectroscopy (XPS), atomic force microscopic (AFM) images, Raman spectra and temperature-resistivity dependences of the PEDOT:PSS films can refer to our previous works 7, 8, 34.

3. RESULTS AND DISCUSSION


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3.1 TE property of the two grades of PEDOT:PSS films. Both the as-prepared CleviosTM P and CleviosTM PH1000 films have electrical conductivities of not exceeding 1 S/cm, and their Seebeck coefficients cannot be measured accurately

27

. They are thus not suitable for TE

application. The TE properties of both the CleviosTM P and CleviosTM PH1000 films can be significantly enhanced through a post treatment with ethylene glycol (EG), hexafluoroactone (HFA⋅3H2O), 0.1 M methylammonium iodide/ dimethylformamide (MAI/DMF) solution or 1 M sulphuric acid (H2SO4) solution (Table 1). The Seebeck coefficients of the PEDOT:PSS films depend on the treatment method. For the CleviosTM PH1000 films treated with HFA·3H2O, EG or MAI/DMF solution, the Seebeck coefficients are 22.7-24.8 μV/K. They are higher than that (14.5-14.7 μV/K) of the CleviosTM P films treated by the same method. For the H2SO4 treatment, the Seebeck coefficients of the treated CleviosTM PH1000 films are also higher than those of the treated CleviosTM P films. They are ~17 μV/K for the former and ~11 μV/K for latter. In comparison with other post treatment methods, the Seebeck coefficients by the H2SO4 treatment are low for both the two PEDOT:PSS grades. Presumably, PEDOT:PSS is further doped during the H2SO4 treatment 11, 35. Higher doping level leads to lower Seebeck coefficient.

Table 1 A comparison of TE property of the PEDOT:PSS films (CleviosTM P and CleviosTM PH1000) treated with various post treatments.

Untreated

Seebeck coefficient (μV/K) CleviosTM CleviosTM P PH1000 -

Electrical conductivity (S/cm) CleviosTM CleviosTM P PH1000 0.3 0.3

Power factor (μW/(m·K2)) CleviosTM CleviosTM P PH1000 -

HFA·3H2Otreated

14.5±0.2

24.8±1.0

226±23

1206±20

4.7

74

EG-treated

14.6±0.3

22.6±0.8

211±12

1222±82

4.5

69.2

MAI/DMFtreated

14.7±0.7

22.7±1.4

236±26

2100±112

5.1

108.2

H2SO4-treateda

12±0.8

16.5±0.7

281±18

2156±92

4.0

58.7

H2SO4-treatedb

10.8±1.1

17.3±1.0

420±36

3088±123

4.9

92.4



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a

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Treated by H2SO4 once. b Treated by H2SO4 three times.

These post treatments also significantly enhance the conductivity of the PEDOT:PSS films. But the conductivity enhancement is much more salient for CleviosTM PH1000 over CleviosTM P. The electrical conductivity is 211-281 S/cm for CleviosTM P, while it is 1206-2156 S/cm for CleviosTM PH1000 after the post treatments. After treated with H2SO4 for three times, the electrical conductivities of the CleviosTM P and CleviosTM PH1000 films are enhanced to 420 and 3088 S/cm, respectively. Accordingly, the power factors of the CleviosTM PH1000 films after treatment are remarkably higher than that of the treat CleviosTM P films. The highest power factors were observed for the PEDOT:PSS films treated with MAI/DMF solution. It is 108.2 µW/(m⋅K2) for the MAI/DMF-treated CleviosTM PH1000 films, while it is only 5.1 µW/(m⋅K2) for the MAI/DMF-treated CleviosTM P films. They are different by a factor of about 20. As reported in literature

13, 36, 37

, these post treatments can facilitate the charge transport

across the PEDOT chains. Their effects on the Seebeck coefficient and the conductivity of PEDOT:PSS films can be ascribed to the enhancement of the charge mobility by these treatments. The treated PEDOT:PSS films are subjected to a further treatment with NaOH. The TE properties of the CleviosTM P and CleviosTM PH1000 films after the NaOH treatment are listed in Table 2. The base treatment can slightly lower the conductivity but remarkably increase the Seebeck coefficient of the PEDOT:PSS films. After the base treatment the Seebeck coefficients are always lower for CleviosTM P than for CleviosTM PH1000. They are 16.5-19.5 μV/K for the former and 28.5-39.2 μV/K for the latter. The latter Seebeck coefficients are almost double of the former by the same treatments. It is interesting to find that the post treatment with H2SO4 only gives rise to the Seebeck coefficients lower than that with other chemicals for both grades of PEDOT:PSS. But the treatments with H2SO4 then base lead to the


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highest Seebeck coefficients. These results are consistent with the effect of the H2SO4 treatment on the doping level of PEDOT:PSS. PEDOT:PSS is further doped by protons that attach to the thiophene ring during the H2SO4 treatment 33, 38. The protonic acid doping lowers the Seebeck coefficient. Because the base treatment eliminates the protonic acid doping of PEDOT:PSS, it saliently increases the Seebeck coefficient of PEDOT:PSS.

Table 2 A comparison of TE property of the PEDOT:PSS films (CleviosTM P and CleviosTM PH1000) treated with various post treatments and further base treatment. Seebeck coefficient (μV/K) CleviosTM CleviosTM P PH1000

a

Electrical conductivity (S/cm) CleviosTM CleviosTM P PH1000

Power factor (μW/(m·K2)) CleviosTM CleviosTM P PH1000

HFA·3H2Otreated

16.5±1.7

28.5±2.9

203±27

995±99

5.5

80.8

EG-treated

17.1±1.5

34.5±2.4

174±4

966±43

5.1

115.0

MAI/DMFtreated

18.8±1.9

28.5±2.2

176±18

1550±155

6.2

125.9

H2SO4-treateda

19.2±1.9

38.1±0.6

229±32

1178±118

8.4

171.0

H2SO4-treatedb

19.5±2.0

37.1±2.9

299±18

2170±201

11.4

334 .0

b

Treated by H2SO4 once. Treated by H2SO4 three times.

Although the Seebeck coefficients increase saliently, the electrical conductivities of the NaOH-treated CleviosTM P and CleviosTM PH1000 films only slightly decrease to 176-299 S/cm and 966-2170 S/cm, respectively. As a result, the power factors of the both grades of PEDOT:PSS are enhanced. The enhancement in the power factor for CleviosTM PH1000 is more significant than for CleviosTM P. The highest power factors were observed for the PEDOT:PSS films treated with H2SO4 triply and then with NaOH. It is only 11.4 µW/(m⋅K2) for CleviosTM P, whereas it becomes 334.0 µW/(m⋅K2) for CleviosTM PH1000. They are different by a factor of about 30.



3.2 Mechanism for the different TE properties of the two grades of PEDOT:PSS films. By different post treatments, the TE property enhancements of both CleviosTM P and CleviosTM PH1000 can be attributed to the PSS removal/segregation and the PEDOT conformation change 7-9, 36. They are caused by the weakening of the Coulumbic attraction between PEDOT and PSS during a treatment. Presumably, the different TE properties of CleviosTM P and CleviosTM PH1000 films before and after the treatments arise from the different PEDOT molecular weights of them 34. It is impossible to measure the PEDOT molecular weight by gel permeation chromatography (GPC) or viscosity study because the molecular weight of PEDOT is much less than PSS of PEDOT:PSS and PEDOT is attached to PSS through the Columbic attraction. The PEDOT molecular weights in CleviosTM P and CleviosTM PH1000 were characterized and compared by DLS, using their diluted aqueous solutions (0.013 wt.%). As shown in Figure 1, the gel particles of CleviosTM P have a size distribution of 90-712 nm peaked at 255 nm. In contrast, two size distribution peaks with the majority (>90%) of 255-712 nm and the minority ( 220K the MAI/DMF-treated CleviosTM PH1000 films behaviour as a metal or semimetal 39, 40. This suggests charge hopping of disorder system at low temperature but charge transport of a system with energy band structure at T > 220 K. The thermal energy is large enough to overcome the energy barrier for charge hopping. The different charge transport mechanisms of MAI/DMF-treated CleviosTM P and CleviosTM PH1000 films arise from the different PEDOT molecular weight. We estimated the carrier mobility of the untreated and treated PEDOT:PSS films by using the formula: σ = neμ, where n, e and μ are the carrier concentration, electronic charge and charge carrier mobility, respectively. The carrier mobilities are ~(4-5)×10-3 cm2V-1s-1 for both the untreated CleviosTM P and CleviosTM PH1000 films, whereas they increase to 1.5-2.4 and 8-18 cm2V-1s-1, respectively, after a post treatment. After the further treatment of MAI/DMF-treated CleviosTM P and CleviosTM PH1000 films with NaOH, their resistances monotonically decreases with the elevating temperature in the whole temperature range. The T0 values are 759 K and 153 K for the MAI/DMF-then-NaOH treated CleviosTM P and CleviosTM PH1000 films, respectively. These results indicate the obvious relationship between the T0 value and conductivity. Higher T0 value, lower the conductivity

17

. Nevertheless, the T0 value is not directly related to the Seebeck coefficient.


Page 11 of 22

Although the conductivities of both MAI/DMF-treated CleviosTM P and CleviosTM PH1000 films decrease after the further treatment with NaOH, their Seebeck coefficients increase. The main factor for the Seebeck coefficient is the doping level of the polymer. (a)

(b) 1

1.0 Untreated Clevios P Untreated PH1000

0.8

Normalized resistance


Untreated Clevios P Untreated PH1000

0.6

0.4

0.2

0.36788

0.13534 100

150

200

250

300

0.05

0.06

0.07

0.08

0.09

0.10

T-1/2(K-1/2)

Temperature (K)

(d)

(c) 1.0

1

0.9 0.8



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


0.7 0.6 0.5 0.4 0.3

MAI/DMF treated Clevios P MAI/DMF-NaOH treated Clevios P MAI/DMF treated PH1000 MAI/DMF-NaOH treated PH1000

0.2 0.1 0.0

100

150

200

250

0.05

300

MAI/DMF treated Clevios P MAI/DMF-NaOH treated Clevios P MAI/DMF treated PH1000 MAI/DMF-NaOH treated PH1000

0.36788

0.06

0.07

0.08

0.09

0.10

T-1/2(K-1/2)

Temperature (K)

Figure 2. The normalized resistances of untreated, MAI-treated and MAI-NaOH treated CleviosTM P and CleviosTM PH1000 films as a function of temperature. (a, c) Normalized resistances versus temperature. (b, d) Analysis of resistance-temperature relationship with the VRH model. To understand the chemical structure, the CleviosTM P and CleviosTM PH1000 films before and after treatments were studied by Raman spectroscopy (Figure 3). The CleviosTM P and CleviosTM PH1000 films exhibit quite similar Raman bands within the range of 1300-1600 cm1

. For the untreated CleviosTM P, the Raman bands assigned to the Cα=Cβ stretching are located

at 1443.6 cm-1, whereas those of the untreated CleviosTM PH1000 appear at 1441.3 cm-1. This implies a higher conjugation length for CleviosTM PH1000, because this C=C Raman band shifts to red at high conjugation length

41, 42

. This is consistent with the DLS results. After 11

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treated with MAI/DMF solution, the Cα=Cβ stretching Raman bands of both CleviosTM P and CleviosTM PH1000 films show a significant red-shift, revealing the transition of PEDOT chains from benzoid structure to quinoid structure after treatment

36, 37

. The subsequent NaOH

treatment does not remarkably affect the position of the Raman bands but make them narrower. This can be attributed to the decrease in the doping level of PEDOT:PSS during the NaOH treatment 43. The Cα=Cβ Raman bands are located at 1437.0 and 1435.1 cm-1 for the MAI/DMFNaOH treated CleviosTM P and CleviosTM PH1000 films, respectively. Thus, the MAI/DMFthen-base treated CleviosTM PH1000 may have longer linear PEDOT segments than the MAI/DMF-then-base treated CleviosTM P. This agrees well with their R-T dependences as discussed above. Crispin et al. also reported that the crystalline structure can lead to a higher Seebeck coefficient than the amorphous structure 39, 40. Therefore, the Raman results confirm the effect of the PEDOT molecular weight on the Seebeck coefficient of PEDOT:PSS.

Intensity (a. u.)

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(a)

Clevios P Untreated MAI/DMF-treated MAI/DMF-NaOH-treated

(b)

PH1000 Untreated MAI/DMF-treated MAI/DMF-NaOH-treated

1300

1350

1400

1450

1500

1550

1600

-1

Raman shift (cm )

Figure 3. Raman spectra of the (a) CleviosTM P and (b) CleviosTM PH1000 films before and after the treatments with MAI/DMF then NaOH solutions. The different conformation of CleviosTM P and CleviosTM PH1000 lead to distinct morphologies in their untreated and treated films. Figure 4 displays the AFM images of the 12 ACS Paragon Plus Environment

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pristine, MAI/DMF-treated, and MAI/DMF-NaOH treated PEDOT:PSS films. In the comparison of Figures 4(a) and (b), the surface of the untreated CleviosTM P film is quite smooth whereas the untreated CleviosTM PH1000 film has small grains of less than 50 nm. After a treatment with MAI/DMF solution, the MAI/DMF-treated CleviosTM P shows a circular shape with growing domain sizes reaching up to ~80 nm, while a nanosized fibrous structure obviously appears for the CleviosTM PH1000 films after the treatment. Such fibrous structure of CleviosTM PH1000 films arises from the long and rigid polymer chains

34

. A subsequent

NaOH treatment shows no significant effect on altering the surface morphologies of both the two grades of PEDOT:PSS films, for which respectively, the elliptical-shape domains and the fibrous structure are still preserved. However, the grain walls for CleviosTM P and the fibrous structure for CleviosTM PH1000 appear to be thicker than those of their films without the NaOH treatment. As revealed by Raman spectroscopy, a treatment can affect the conformation of the PEDOT chains of PEDOT:PSS. This is further investigated by the roughnesses of the films. The untreated CleviosTM P and CleviosTM PH1000 films are quite smooth, and their RMS roughnesses are 0.77 and 1.28 nm, respectively. After the MAI/DMF treatment, both films become significantly rougher, and their roughnesses increase to 1.39 and 1.60 nm, respectively. Domains with circular shape appear for the MAI/DMF-treated CleviosTM P film, whereas fibrous structures can be observed for the MAI/DMF-treated CleviosTM PH1000 film. Longer PEDOT chains make the structures more anisotropic. After the subsequent NaOH treatment, there is no significant change in the RMS roughness of the polymer films. They are 1.31 and 1.49 nm, respectively. Thus, the NaOH treatment does not significantly affect the polymer conformation. This is consistent with the Raman spectra of the polymer films. This further evidences that both the doping level and the morphology can affect the Seebeck coefficient.



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(a)

(b)

(c)

(d)

(e)

(f)

Page 14 of 22

Figure 4. AFM images of PEDOT:PSS films, (a) untreated CleviosTM P, (b) MAI/DMF-treated CleviosTM P, (c) MAI/DMF-NaOH treated CleviosTM P, (d) untreated CleviosTM PH1000, (e) MAI/DMF-treated CleviosTM PH1000 and (f) MAI/DMF-NaOH treated CleviosTM PH1000. All the image areas are 2 × 2 μm2. The electronic structures of the CleviosTM P and CleviosTM PH1000 films before and after treatments were further compared by cyclic voltammetry (CV) because the electrochemical activity of PEDOT:PSS is sensitive to its conductivity and polymer chain conformation

36

.

Figure S3 compares the CVs of the untreated and treated CleviosTM P and CleviosTM PH1000 within the potential range of -0.8 to 0.8 V vs. Ag/AgCl. The results suggest that higher PEDOT molecular weight always shifts the redox activity to lower potential. The redox potentials become lower when an organic compound or polymer has longer conjugation length 34, 44. This is similar to that of poly(3-hexythiophene) 45, 46. Poly(3-hexythiophene) with higher molecular weight can have broader electrochemical behaviour. After the NaOH treatment, the cathodic and anodic peaks of both CleviosTM P and CleviosTM PH1000 slightly shift to higher potentials. This is probably due to the conductivity decrease by the base treatment. But the redox peaks of the MAI/DMF-then-base treated CleviosTM PH1000 appear at lower potentials than those of


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the MAI/DMF-then-base treated CleviosTM P. This also confirms that the NaOH treatment may not significantly affect the chain conformation of PEDOT:PSS.

4. CONCLUSIONS CleviosTM P and CleviosTM PH1000 films having different PEDOT molecular weights exhibit significantly different Seebeck coefficients and electrical conductivities, therefore giving rise to different power factors. CleviosTM PH1000 with high PEDOT molecular weight always has higher conductivity and Seebeck coefficient than CleviosTM P with low PEDOT molecular weight after treatment(s). The power factor of the former can be about 30 times of the latter. This research work provides deep understanding on the effect of molecular weight on the TE property of the PEDOT:PSS films. It indicates that to increase the PEDOT molecular weight can be an effective way to further increase the TE properties of PEDOT:PSS. In principle, this should be applicable for other conducting polymers.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting information. UV absorption, 2p XPS spectra and cyclic voltammograms of the untreated, MAI/DMF-treated and MAI/DMF-NaOH treated CleviosTM P and CleviosTM PH1000.



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ACKNOWLEDGEMENTS This work was financially supported by a research grant from the Ministry of Education, Singapore (R-284-000-156-112).

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TOC graph Treated CleviosTM PH1000 Power factor = 126 μW/(m·K2)

Treated CleviosTM P Power factor = 6.2 μW/(m·K2)


Higher PEDOT Molecular Weight Giving Rise to Higher Thermoelectric

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