Flexible and transparent organic-inorganic hybrid thermoelectric

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Flexible and transparent organic-inorganic hybrid thermoelectric modules Xinyun Dong, Sixing Xiong, Bangwu Luo, Ru Ge, Zaifang Li, Jing Li, and Yinhua Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08696 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Applied Materials & Interfaces

Flexible and transparent organic-inorganic hybrid thermoelectric modules

Xinyun Dong, Sixing Xiong, Bangwu Luo, Ru Ge, Zaifang Li, Jing Li and Yinhua Zhou*

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China.

∗Corresponding author: [email protected]

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Abstract: Light-weight, mechanically flexible, transparent thermoelectric modules are promising as portable, and easy-to-integrate energy sources. Here, we demonstrate flexible, transparent thermoelectric

modules

by

using

a

conducting

polymer

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the p-type leg and indium tin oxide (ITO)-PEDOT:PSS as the n-type leg. Main observations include: (1) The bilayer combination of ITO-PEDOT:PSS (PEDOT:PSS coated on top of the ITO) displays a negative Seebeck coefficient (S) and the value is similar to that of the ITO single layer; (2) The S value of the ITO-PEDOT:PSS is almost not dependent on the area ratio of the stacked PEDOT:PSS and ITO; (3) The conducting polymer PEDOT:PSS deposition on top of ITO helps the ITO not to generate cracks during bending, which enhances the mechanical flexibility of the ITO. Based on these observations, thermoelectric modules with 8 pairs of junctions are fabricated and the thermoelectric modules’ ∆V/∆T (modules’ generated thermovoltage per temperature difference) is nearly the addition of S values of all legs. Thermoelectric modules show good mechanical flexibility and air stability. Applications of thermoelectric modules have also been demonstrated to produce thermovoltage via the temperature difference produced by a human hand or warm water.

Keywords: Thermoelectric, hybrid thermoelectric module, conducting polymer, charge carrier mobility, flexible and transparent thermoelectric device

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1. Introduction Organic-based thermoelectric materials, with merits of excellent mechanical flexibility, easy processing and light weight, are able to directly convert heat to electricity (Seebeck effect) and have been attracting a lot of attentions of the community.1-7 They are considered as a promising class of thermoelectric materials for wearable energy generators3, 8 and self-powered electronics.9 To achieve sufficient thermovoltage and thermopower to drive electronics, it is necessary to integrate n- and p-type of thermoelectric units in series into thermoelectric modules.10-14 Recently, p- and n-type thermoelectric materials have been investigated for high-performance and flexible organic thermoelectric modules.9, 15

For

p-type

materials,

commercial

organic

polymer

poly(3,4-ethylenedioxythiophene):

poly(styrenesulfonate) (PEDOT:PSS) has been studied, and it exhibits a high thermoelectric figure of merit (ZT) value through removing the non-ionized PSS.16 Tuning the oxidation level,17-18 synthesizing composites,5, 19-21 and designing multilayer structures22-23 have been used to enhance the performance of PEDOT:PSS. For n-type materials, novel n-type molecules24-26 and dopants (such as metals27-28 and organic dopants29-31) have been developed for n-type thermoelectrics. However, most n-type organic materials suffer from poor air stability and low electrical conductivity. These generally yield low ZT values of n-type thermoelectric devices and modules.7 Among the well-developed inorganic thermoelectric materials,32-34 metal oxides have good air stability and high conductivity with proper doping. In the literature,35-38 indium tin oxide (ITO) was reported as an air-stable thermoelectric material with a negative Seebeck coefficient. ITO also has high optical transparency. As the n-type thermoelectric leg, it allows us to fabricate transparent thermoelectric modules combing with the p-type PEDOT:PSS thermoelectric leg. 3



In our work, we first report a flexible and transparent thermoelectric module consisting of PEDOT:PSS as the p-type leg and ITO-PEDOT:PSS as the n-type leg. We find that the stacked layer of ITO-PEDOT:PSS (PEDOT:PSS coated on top of ITO) exhibits a negative Seebeck coefficient (S) and the S value is close to that of single layer of ITO. The S values of ITO-PEDOT:PSS almost remain constant with area ratio (AITO/APEDOT:PSS) changing from 5 to 100%. The nearly unchanged S value of the ITO-PEDOT:PSS alleviates the requirement of precise patterning of p-type and n-type legs that could simplify the fabrication of thermoelectric modules. Furthermore, the coated PEDOT:PSS reduces the stress on ITO and therefore the stacked ITO-PEDOT:PSS exhibits a better mechanical flexibility compared with the single layer of ITO on the plastic substrate. Then, we fabricated flexible and transparent thermoelectric modules consisting of PEDOT:PSS as the p-type legs and ITO-PEDOT:PSS as the n-type legs.

2. Experimental Section

Materials PEDOT:PSS aqueous solution (Clevios PH1000) was purchased from Heraeus. Ethylene glycol (EG) purchased from Sigma-Aldrich. Indium tin oxide (ITO) on polyethylene terephthalate (PET) or glass were purchased from Zhuhai Kaivo Optoelectronic Technology Co. Ltd.

Fabrication of thermoelectric modules ITO strips (0.5 cm × 5 cm) on PET or glass were patterned by laser photoetching. The blank area between two of ITO films was also 0.5 cm × 5 cm. PEDOT:PSS mixed with 5 wt.% ethylene glycol (EG) was spin-coated on the substrate. The entire substrate including both the ITO and the blank area 4


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was covered by PEDOT:PSS film. The substrate were annealed at 150 °C on a hot plate for 10 min in air. After the spin coating, the adjacent PEDOT:PSS legs and ITO-PEDOT:PSS legs were isolated to form p-type and n-type legs by the knife blade or the laser beam. The π-shaped thermoelectric module was obtained by using silver paste to connect the adjacent p- and n-type legs.

Characterization and measurement Thermoelectric Seebeck coefficients of single legs (PEDOT:PSS or ITO-PEDOT:PSS) and modules were measured by a homemade system. Temperature of the hot side and cold side of devices were measured by thermocouples coupled with thermal conductive silicone grease. The thermoelectric voltage between two Ag electrodes was recorded by using a nanovoltmeter (NI PXIe-1073, National Instruments Inc.). Multiple (5-10 times) measurements were performed to obtain average values with standard deviations. Electrical conductivity, bulk carrier concentration and carrier mobility of ITO and PEDOT:PSS films were determined by the Hall coefficient analyzer. Sheet resistances of samples were also measured using Four Probe Method. Film thicknesses were measured by a surface profiler (Dektak X-T, BRUKER).

3. Results and Discussion Figure 1a illustrates the device structure of flexible and transparent organic-inorganic hybrid thermoelectric modules. PEDOT:PSS is the p-type leg whereas ITO-PEDOT:PSS is the n-type leg on the flexible polyethylene terephthalate (PET) substrate. The thermoelectric module comprises alternating PEDOT:PSS and ITO-PEDOT:PSS legs. The ITO-PEDOT:PSS denotes the ITO layer coated by a PEDOT:PSS film. Both the p-type leg and the n-type leg contain the PEDOT:PSS which 5



could alleviate the request of precise patterning of PEDOT:PSS films. The PEDOT:PSS is typically processed by spin coating from the aqueous solution which is not suitable for precise patterning. Figure 1b shows the systematic ∆V/∆T of thermoelectric modules with 1, 2, 4 and 8 pairs on PET substrates. It can be seen that systematic ∆V/∆T values of modules linearly depend on p-n pairs. The ∆V/∆T values are 37.8 ± 2.9, 73.5 ± 7.4, 155.4 ± 15.5 and 293.5 ± 7.9 µV/K, respectively. It indicates the uniformity and reproducibility of the pair of p-n legs. The resistance of module is also linearly dependent on the numbers of pairs. Resistance values are 1.7 ± 0.3，3.3 ± 0.6，6.8 ± 1.1 and 13.8 ± 2.1 kΩ for 1, 2, 4 and 8 p-n pairs, separately. The resistance is mainly from PEDOT:PSS since it has lower conductivity of about 750 S/cm compared with the ITO (7280 S/cm, Table 1). Besides PET substrates, we also fabricated the thermoelectric modules on glass substrate. As shown in Figure S1, the systematic ∆V/∆T of the thermoelectric modules on glass with 1, 2, 4 and 8 pairs are 28.9 ± 3.1, 58.6 ± 5.4, 123.4 ± 6.6, 257.1 ± 5.6 µV/K. The ∆V/∆T values on glass substrates are lower than those on PET substrates. The lower S values of ITO on glass (-15.2 ± 0.8 µV/K, Table 2) than those on PET substrates (-22.0 ± 1.1 µV/K) is associated to the different fabrication conditions of ITO on PET and on glass, which is consistent with the earlier reports.36, 39-40

An important feature we would like to emphasize is that ITO-PEDOT:PSS legs display similar S values to that of single ITO films. The PEDOT:PSS coating on ITO doesn’t change thermoelectric properties of ITO. This feature makes the demonstration of thermoelectric modules easier without the need of precise patterning. To understand this feature, we fabricated different samples and tested their thermoelectric properties on glass substrates. Figure 2a and 2b show the thermoelectric voltage as a function of temperature difference (ΔT ). The S values of PEDOT:PSS on glass and ITO on glass were calculated to 17.62 and -14.27 µV/K, respectively. The PEDOT:PSS displays a p-type property 6


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and the ITO displays a n-type property. The S value of PEDOT:PSS is comparable to the value reported in the literature.16-17, 41 The vertically stacked ITO-PEDOT:PSS with the same area displays a negative S of -14.01 µV/K that is close to the single ITO film. We also measured the S value of the sample of the ITO film and the PEDOT:PSS film in parallel connection as shown in Figure 2a. The result is also a negative S value of -13.93 µV/K that is still close to that the single ITO film. Sheet resistances of ITO-PEDOT:PSS films (30.9 ± 1.8 Ω/sq on PET, 9.7 ± 0.2 Ω/sq on glass) are slightly lower than that of ITO films (31.4 ± 1.4 Ω/sq on PET, 10.6 ± 0.3 Ω/sq on glass, Table 2). The PEDOT:PSS coating slightly improves the charge transport of the ITO.

We further fabricated ITO-PEDOT:PSS samples that contain the stacked ITO and PEDOT:PSS layers on glass substrates, but with different area ratio of the ITO film and the PEDOT:PSS film (as shown in the inset of Figure 2b). The area ratio (AITO/APEDOT:PSS) varies from 5 to 100%. Figure 2b shows S values of these ITO-PEDOT:PSS samples. It can be seen that S values of the ITO-PEDOT:PSS samples almost remain constant around -15 µV/K. It indicates that in the ITO-PEDOT:PSS samples, ITO is the mainly dominant factor and it determines the direction and value of the final output voltage. As long as ITO is contained in the ITO-PEDOT:PSS, it always displays the n-type property and the S values are close to that of ITO. Therefore, when we fabricate the thermoelectric module, PEDOT:PSS and ITO-PEDOT:PSS are separately used as the p-type and n-type leg. Precise patterning of p- and n-type legs can be alleviated. The oversized PEDOT:PSS films on the ITO will not affect the final thermoelectric output of ITO-PEDOT:PSS samples. These allow us to easily demonstrate the thermoelectric module with multiple p-n legs.

To understand why ITO mainly dominates the direction and value of the final thermoelectric property of ITO-PEDOT:PSS samples, we measured the conductivity, bulk carrier concentration and 7



carrier mobility of ITO and PEDOT:PSS films. Values of these parameters are shown in Table 1. The electrical conductivity of ITO film is 7280 S/cm, which is nearly 10 times as high as that of the PEDOT:PSS film (750 S/cm). ITO shows a carrier mobility of 3.64 × 101 cm2/V·s whereas the PEDOT:PSS shows a carrier mobility of 6.50 × 10-2 cm2/V·s. Thicknesses of the PEDOT:PSS and ITO films are 130 and 170 nm, respectively. Since we have tested the ITO-PEDOT:PSS sample with an area ratio (AITO/APEDOT:PSS) as low as 5% (1/20), the result rules out the possibility that S is determined by the resistance of the ITO and PEDOT:PSS layers. We believe that the S of the ITO-PEDOT:PSS sample is determined by higher carrier mobility and electrical conductivity of ITO.42

To demonstrate the mechanical flexibility of the hybrid thermoelectric module, bending test of the 8-pair thermoelectric module was performed. Changes of the systematic ∆V/∆T value and the resistance after different bending cycles with a radius of 12.5 mm were recorded as shown in Figure 3a. After 5000 consecutive bending cycles, the change of the S value is less than 3%, which shows negligible degradation during the bending process with such bending radius. As for the resistance, the resistance gradually increases with bending cycles. After 5000 bending cycles, the final R value reaches about 1.1 times of the initial value. Such good mechanical flexibility is surprising because it is known that ITO is easy to crack during bending.43-44 So, we performed the bending test on ITO and ITO-PEDOT:PSS samples on PET substrates. As shown in Figure 3b, the sheet resistance of the ITO rapidly increases. It increases to about 50 times larger than the initial value after 5000 cycles bending with a radius of 12.5 mm. The dramatic increase of ITO sheet resistance is attributed to the formation of cracks (the optical microscopy is shown in Figure 3d). In contrast, the sheet resistance of the ITO-PEDOT:PSS almost doesn’t change after the bending. In the optical microscopy image (Figure 3c), no cracks are observed in the ITO-PEDOT:PSS films after 5000 cycles bending. The PEDOT:PSS 8


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deposited on the top of ITO reduces the stress on ITO during the bending and improves the mechanical flexibility.

To test the air stability, the 8-pair thermoelectric module is kept in air without encapsulation at room temperature and a relative humidity of 40%. Figure 3e shows changes of the systematic ∆V/∆T and resistance as a function of storage time. After kept in air for 30 days, the module remains a ∆V/∆T of approximately 80% of the initial value. The resistance of the module increases by 40% compared with the initial value. The thickness of PEDOT:PSS increases to about 1.2 times of the initial value after exposed in air for 30 days at the relative humidity of 40% because of the water adsorption (Figure S2).45-46 The adsorbed water increases the film thickness and separates the interconnection of PEDOT chains, thus increasing resistances induced by lower electronic coupling and conductivity.47-48 Considering the main reason of the decreased systematic ∆V/∆T from the water adsorption, proper encapsulation against water will improve the module lifetime.

Figure 4 shows the output power and voltage of the flexible thermoelectric module with 8 n-p pairs under different loaded resistance with a ∆T of 20 K and 30 K, respectively. Testing circuit is shown in Figure 4a. As shown in Figure 4b and 4c, when the loaded external resistance increases, the output voltage and power increase for both the ∆T of 20 K and 30 K. When the loaded resistance is about 11.5 KΩ, approximately equal to the resistance of the 8-pair thermoelectric module, the output power reaches a maximum value for both the ∆T cases of 20 K and 30 K. Maximum output thermoelectric powers for the module (8 pairs) are 0.86 nW and 1.75 nW for the ∆T of 20 K and 30 K, respectively.

The demonstrations of the module for practical applications are also performed. The

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thermoelectric module with 8 p-n pairs was used for the demonstration. Figure 5a and 5b shows the demonstration of voltage generation via a ∆T between a human hand and air. Before the hand holds the module, the voltage is nearly zero. When the hand holds one side of the module, the temperature of this side rises up. The ∆T produces a thermoelectric voltage of 3.15 mV. Figure 5c is an image taken by an infrared camera that the ∆T is about 11.6 K. The thermovoltage of 3.2 mV via a ∆T of 11.6 K tells the modules’ ∆V/∆T value of about 275 µV/K that is similar to what we obtained in the above-mentioned value for the modules with 8 p-n pairs. The good flexibility of thermoelectric modules enables them to be used on curved surfaces. Figure 5d and 5e show the demonstration of the module attached to the curved surface of a beaker to produce thermovoltage with a ∆T produced by a warm water. A thermovoltage of about 6.8 mV is generated with a ∆T of 21.3 K shown from the infrared camera (Figure 5f). A video in the supporting information shows that the generation of the thermovoltage when the hot water is added into the beaker (Movie. S1). These results show the flexible thermoelectric module has a good potential for converting heat into electricity.

4. Conclusions In this work, we have demonstrated the flexible and transparent thermoelectric module with PEDOT:PSS as the p-type leg and ITO-PEDOT:PSS as the n-type leg. It is important that the PEDOT:PSS coated ITO sample (ITO-PEDOT:PSS) shows a negative Seebeck coefficient and the value is similar to that of the single ITO film. The ITO dominates the thermoelectric property in ITO-PEDOT:PSS. Even when the area ratio (AITO/APEDOT:PSS) varies from 5 to 100%, S values of ITO-PEDOT:PSS samples on glass almost remain constant around -15 µV/K. The reason of the ITO

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dominating the thermoelectric property is probably due to the much higher carrier mobility and electrical conductivity of the ITO than those of the conducting polymer PEDOT:PSS. These findings enable us to easily fabricate the flexible and transparent thermoelectric module with the PEDOT:PSS and ITO-PEDOT:PSS legs without the need of stringent precise patterning. The thermoelectric module shows a linear dependence of S values on pair numbers of p-n legs. The module with 8 p-n pairs displays a systematic ∆V/∆T that is nearly the addition of all the single legs. Applications of thermoelectric modules have also been demonstrated to produce thermovoltage via the ∆T produced by human hand or warm water. ITO- and PEDOT:PSS-based flexible and transparent thermoelectric modules have the advantage of easy fabrication and integration. The PEDOT:PSS coating on the top of ITO is helpful to promote the mechanical stability during the consecutive bending. Good mechanical flexibility and air stability that show promise as an energy source for harvesting heat. This is also a good example that the understanding of the thermoelectric performance would simplify the processing and fabrication of devices.

Associated Content Supporting Information. Systematic ∆V/∆T and resistance of modules on glass; Thickness change of PEDOT:PSS films with humidity; a movie demonstrating thermovoltage generation of a curved transparent and flexible thermoelectric modules with adding hot water into a beaker.

Acknowledgements The work is supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 21474035, 51773072), the HUST Innovation Research Fund 11



(Grant No. 2016JCTD111, 2017KFKJXX012). We thank Dr. Hengda Sun and Prof. Xavier Crispin for helping us with the fruitful suggestions and discussions.

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Polymeric Mixed Conductors. Adv. Funct. Mater. 2016, 26 (34), 6288-6296.

Table 1 The conductivity, carrier concentration and carrier mobility of ITO and PEDOT:PSS films on PET substrates. Sample

Conductivity (S/cm)

Carrier concentration (cm-3)

Carrier mobility (cm2/V·s)

ITO film

7.28 × 103

1.23 × 1021

3.64 × 101

PEDOT:PSS film

7.74 × 102

7.44 × 1022

6.50 × 10-2

Table 2 The Seebeck coefficient and sheet resistance of ITO, PEDOT:PSS and ITO-PEDOT:PSS films on PET and glass substrates. Sample

Seebeck (µV /K)

Sheet Resistance (Ω/sq)

PET/ITO

-21.9 ±1.1

31.4 ± 1.4

PET/PEDOT:PSS

18.6 ± 1.1

96.7 ± 2.1

PET/ITO-PEDOT:PSS

-22.2 ± 1.1

30.9 ± 1.8

Glass/ITO

-15.2 ± 0.8

10.6 ± 0.3

Glass/PEDOT:PSS

19.5 ± 1.8

99.2 ± 4.6

Glass/ITO-PEDOT:PSS

-15.8 ± 1.2

9.7 ± 0.2

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Figure 1 (a) Schematic illustration of the flexible and transparent organic-inorganic hybrid thermoelectric module on the PET substrate with PEDOT:PSS as the p-type legs and the ITO-PEDOT:PSS as the n-type legs. The ITO-PEDOT:PSS displays n-type thermoelectric properties regardless the area ratio of the PEDOT:PSS and ITO. (b) Thermoelectric modules’ systematic ∆V/∆T (µV/K) and Resistance (kΩ) as a function of the numbers of thermoelectric pairs on PET.

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Figure 2 (a) Thermoelectric output voltage at a function of ∆T for ITO film, PEDOT:PSS film, ITO-PEDOT:PSS bilayer, ITO and PEDOT:PSS films in parallel connection, respectively, on glass substrates. (b) The thermoelectric Seebeck coefficient of ITO-PEDOT:PSS with different area ratios of the ITO coated by PEDOT:PSS film on glass substrates.

18


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Figure 3 (a) Normalized thermoelectric modules’ systematic ∆V/∆T and resistance of flexible and transparent thermoelectric 16-leg (8 p-n pairs) modules as a function of bending cycles with the bending radius of 12.5 mm. (b) Normalized sheet resistance of ITO and ITO-PEDOT:PSS on flexible PET substrates during repeated bending with the bending radius of 12.5 mm. Optical microscopy images of (c) ITO-PEDOT:PSS and (d) ITO film after 5000 cycles of consecutive bending. (e) Normalized thermoelectric modules’ systematic ∆V/∆T and resistance of the thermoelectric modules as a function of exposure time in air without encapsulation.

19



Figure 4 (a) Diagram of the electrical circuit for evaluating the output power of the thermoelectric modules. Output voltage and power with different external load with different ∆T: (b) 20 K; (c) 30 K.

(a)

V Load PEDOT:PSS

A

ITO-PEDOT:PSS Sliver paste Substrate

2.5

Output voltage Output power

5 4

2.0 1.5

3

1.0

2

7

0.5

1 0

2

4

6

0.0

Output voltage Output power

5

3.0 2.5

4

2.0

3 2

1.5

1

1.0

0

8 10 12 14 16 18 20 Load (kΩ)

3.5

∆T = 30 K

6

0

2

20


4

6

0.5

8 10 12 14 16 18 20 Load (kΩ)

Output power (nW)

∆T = 20 K

4.0

8 Output voltage (mV)

6

0

(c)

3.0

7

Output power (nW)

(b) Output voltage (mV)

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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Figure 5 Demonstrations of the flexible thermoelectric modules for applications. (a-c): Temperature difference from the hand and air produces an output voltage of 3.1 mV. (d-f) Temperature difference from the hot water and air produces an output voltage of 6.8 mV. (c) and (f) are infrared thermal images to show the temperature differences between the hand and air, the hot side and cold side of the beaker were 11.6 K and 21.3 K, respectively.

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Table of Content

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Flexible and transparent organic-inorganic hybrid thermoelectric

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