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Thermoelectric power enhancement of PEDOT:PSS in high-humidity conditions

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Applied Physics Express 7, 031601 (2014) http://dx.doi.org/10.7567/APEX.7.031601

Thermoelectric power enhancement of PEDOT:PSS in high-humidity conditions Qingshuo Wei*, Masakazu Mukaida*, Kazuhiro Kirihara, Yasuhisa Naitoh, and Takao Ishida Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8564, Japan E-mail: [email protected]; [email protected] Received January 9, 2014; accepted February 12, 2014; published online February 28, 2014 We report an increase in the thermoelectric power factor of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) from 23 + 5 to 225 + 130 µW/(m&K2) in high-humidity conditions. This enhancement was caused mainly by an increase in the apparent Seebeck coefficient, which could be related to morphological change after water absorption or electrochemical reaction of PEDOT in air. Our results demonstrate a positive effect of water in the PEDOT:PSS system and indicate the need for well-controlled measurement conditions, particularly humidity, in evaluating the performance of conducting organic materials. © 2014 The Japan Society of Applied Physics

hermoelectric devices, which convert heat directly to electricity, are promising candidates for harvesting waste heat and solar thermal energy.1) The Seebeck coefficient (S), electrical conductivity (·), and thermal conductivity (¬) are the most important parameters for evaluating thermoelectric materials. Pioneering studies on thermoelectric materials focused primarily on inorganic semiconductors, such as bismuth–telluride (Bi–Te) alloys, magnesium–silicon (Mg–Si) alloys, and metal oxides.2) Most of these materials operate at temperatures higher than 200 °C, although waste heat and solar-thermal energy typically occur at temperatures lower than 150 °C.3) The efficiency of thermal power conversion at low temperatures is low because of the small temperature difference between the heat source and ambient environment. Therefore, to harvest the large amount of thermal energy available at low temperatures, large-area thermoelectric devices are necessary. The most common lowtemperature thermoelectric material is Bi2Te3. However, Te is an expensive rare metal and is not environmentally friendly, making it unsuitable for large-area thermoelectric devices. Unlike their inorganic counterparts, organic semiconductors have not been thoroughly investigated, owing to their relatively low electrical conductivities and Seebeck coefficients.4–7) Recent advances in organic electronics for devices such as organic solar cells and transistors have improved the physical and chemical properties of organic semiconductors. Organic semiconductors can now be tuned over a fairly wide range, which may make them suitable for thermoelectric devices. Early studies on organic thermoelectrics focused mainly on polyaniline, polypyrrole, and polythiophene.8–14) The stable power factors (P = S2·) for these materials are less than 10 µW/(m0K2). Recently, several groups reported high power factors and excellent figures of merit (ZT = PT/¬, where T is temperature) for materials based on poly(3,4-ethylenedioxythiophene) (PEDOT).15–18) Katz and coworkers reported a promising power factor of 47 µW/ (m0K2) for commercial highly conductive PEDOT:PSS.15) Crispin and colleagues reported that de-doping highly conductive PEDOT:tosylate (tos) with tetrakis(dimethylamino)ethylene (TDAE) can yield a remarkable power factor greater than 300 µW/(m0K2) and ZT of 0.25, mainly because of a high Seebeck coefficient (>200 µV/K).16) Very recently, Pipe and colleagues achieved a power factor of 469 µW/(m0K2) and the highest reported ZT of 0.42 at room temperature by adding dimethyl sulfoxide (DMSO) to commercial PEDOT: PSS.17) Thus, the performance of organic thermoelectric materials can rival that of their inorganic counterparts, and

T

organic thermoelectric materials are very promising for harvesting energy at low temperatures.19–22) Studies on the relationship between water content and thermoelectric properties in the PEDOT:PSS system have been limited. PEDOT:PSS readily absorbs water vapor from air, and the weight percent of water in PEDOT:PSS is appreciable.23) Therefore, effects of water on the thermoelectric properties of PEDOT:PSS should be considered. In this paper, we report effects of atmospheric water vapor on the thermoelectric properties of commercial PEDOT:PSS. We used free-standing films because they are easy to handle and the amount of water in such films can be readily determined. PEDOT:PSS solution (PH1000) containing 3% ethylene glycol was used as ink because adding a second solvent such as ethylene glycol or DMSO in an aqueous dispersion of PEDOT:PSS can dramatically enhance its electrical conductivity.24–27) Initially, we attempted to float dropcast films using solvents. However, PEDOT:PSS films are not self-supporting on solvents; therefore, low-surface energy materials were used as substrates instead. Crosslinked poly(dimethylsiloxane) (PDMS) was used as the substrate because it displays both hydrophobic and oleophilic properties.28) The PEDOT:PSS solution was dropped onto the PDMS surface. After the solvent evaporated over several hours, the PEDOT:PSS film was easily detached by bending the PDMS substrates slightly. The free-standing films were then annealed at 150 °C for 30 min in air. The thickness of the prepared films was in the range of 20–40 µm. The density of the films as determined from their mass and volume was in the range of 1.5–1.6 g/cm3, which was close to the calculated value (1.45 g/cm3).29) This suggests that the as-prepared freestanding PEDOT:PSS films had a dense structure, which is important for studying their electrical conductivity and intrinsic thermal properties. The electrical conductivity of the PEDOT:PSS film was greater than 800 S/cm. Figure 1 shows the normalized change in weight of the as-prepared free-standing PEDOT:PSS films in air. The weight of the PEDOT:PSS films increased with time and reached saturation after approximately 15 min. If the films were heated again at 150 °C, their weight returned to its initial value. This is because PEDOT:PSS films readily absorb water from air. The amount of water absorbed from the air was strongly dependent on humidity (Fig. 1). At high humidity (84%), the increase in weight was greater than 35 wt %, and the conducting donor material, that is, PEDOT, constituted only 30 wt % of the PEDOT:PSS. The weight ratio of PEDOT to PSS was approximately 1 : 2.5;23)

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Appl. Phys. Express 7, 031601 (2014)

Q. Wei et al. (a) 27000 ppm (84%@25 °C)

1.3

1.2

17700 ppm (55%@25 °C)

1.1 5000 ppm (16%@25 °C)

1.0

0

5

10 Time (min)

15

20

5.0x10-4

(b) Electromotive force (V)

Normalized weight (a.u.)

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Fig. 1. Plot of the weight of free-standing PEDOT:PSS films versus time for exposure to air at different humidity levels.

therefore, the amount of water absorbed by the PEDOT:PSS was greater than the amount of PEDOT. Even at very low humidity (90% 35 µV/k ~ 65 µV/k

3.0x10-4 2.0x10-4 1.0x10-4

30% 17 µV/k 0.0 0

1

2 3 4 5 6 7 Temperature difference (K)

8

9

Fig. 2. (a) Schematic representation of the Seebeck measurement setup; (b) Seebeck coefficient measurements of PEDOT:PSS films at different humidity levels. Closed circles represent Seebeck coefficient measurements at a humidity value of 30% and open circles represent the Seebeck coefficient measurements at a humidity value larger than 90%.

interaction because of the solvent’s high dielectric constant. Second, PEDOT is an oxygen reduction catalyst, which suggests the possibility that PEDOT:PSS could undergo electrochemical reactions in air. At high humidity, the combination of PEDOT and water could function as a thermogalvanic cell, which would enhance the Seebeck coefficient.33–35) Other possible mechanisms cannot be ruled out at this stage. We also demonstrated that power output increased at high humidity. We fabricated a simple module containing 11 thermocouples printed on paper. A PEDOT:PSS layer 2.5 © 40 mm2 in size and approximately 20 µm thick was screenprinted on 300-µm-thick paper. Eleven PEDOT:PSS arrays were printed on one piece of paper, and silver paste was then screen-printed on the PEDOT:PSS arrays to form series connections. The paper was placed between a hot plate and cool plate, and the temperature difference between the two plates was maintained at approximately 35 °C [Fig. 3(a)]. Figure 3(b) shows the normalized power output versus time. When water was sprayed onto the surface, the power output increased 10- to 20-fold, and it decreased as the water evaporated. When we sprayed water onto the surface again, the enhancement in power output was repeated. Electrical conductivity increased slightly from 805 to 870 S/cm as humidity increased (Fig. 4). This could be due to enhanced carrier mobility in PEDOT because polar solvents can reduce the Columbic interaction between PSS and PEDOT. The power factor increased from 23 « 5 to 225 « 130 µW/(m0K2). Using the in-plane electrical conductivity, in-plane apparent Seebeck coefficient, and reported thermal conductivity [0.3–0.5 W/(m0K)],17) we calculated the apparent ZT at room temperature (303 K) to be as high as 0.3.

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(a)

Acknowledgments This work was supported by TherMAT, Future Pioneering Projects of the Ministry of Economy, Trade and Industry, Japan. We also acknowledge financial support from the Ministry of Education, Culture, Sports, Science and Technology through a Grant-in-Aid for Scientific Research (No. 20111015: Emergence in Chemistry of Nano-scale Molecular Systems).

(b)

Fig. 3. (a) Schematic representation of the thermoelectric module containing PEDOT:PSS; (b) normalized power output of the module plotted as a function of time.

Electrical Conductivity (S/cm)

1000 800 600 400 200 0

30

40

50 60 70 Humidity (%)

80

90

Fig. 4. Humidity-dependent electrical conductivity of PEDOT:PSS films.

In conclusion, we studied the effect of water content on the thermoelectric properties of the benchmark conducting polymer, PEDOT:PSS. The apparent Seebeck coefficient of PEDOT:PSS increased with the amount of water absorbed by PSS. This increase could be related to morphological changes in the films or the electrochemical reaction of PEDOT in air. Our results indicate a positive effect of water in the PEDOT:PSS system and highlight the need for wellcontrolled measurement conditions, particularly humidity, in evaluating the performance of conducting organic materials.

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