Roll type conducting polymer legs for rigid-flexible thermoelectric generator

Thermoelectric generators (TEGs) are composed of a number of p-n type legs which are electrically connected in series and sandwiched between the electrical insulating layers. In this structure, a current is generated according to the thermoelectric properties of the n- and p-type materials under temperature difference (ΔT) between the heat sources against the cold side. Under the condition that the thermoelectric properties of materials are limited, a large ΔT is necessary to increase the thermoelectric output voltage, which is given as¹ 

where m is the number of thermocouples, S is the mean Seebeck coefficient, and K_TEG and K_other are the thermal resistance of the thermocouples and the total thermal resistance of the other parts (such as the contact thermal resistances), respectively. However, it is difficult to reach a large ΔT because of the limited cooling efficiency, particularly in micro-thermoelectric generators that require a high integration density of thermocouples. Furthermore, it is also difficult to prepare high density thermocouple structures through the semiconductor fabrication processes because of the difficulty in preparing thermoelectric semiconductors as thick films. Thus, several attempts have been made to reach a high integration density, such as high aspect ratio of the thermocouples.^2–4 However, this requires the use of micro-fabrication methods for the thermocouples with a small diameter. Furthermore, an increase in the aspect ratio of the thermocouples is limited because K_TEG is also affected by the aspect ratio.³ 

Since conducting polymer (CP) thin films have unique properties as energy harvesters, such as intrinsic flexibility and color tunability,⁵ CPs have been explored for use as a bendable thermoelectric (TE) harvester, which has been difficult to realize with thermocouples composed of the rigid inorganic semiconductors and ceramic heat conductors.^3,6 Thus, bulk-type CPs as a p-type thermoelectric leg have been prepared by multiple coating of CPs, multiple solution casting polymerizations, vapor phase polymerization, or a pelletizing process. As the thickness of CPs increases, the inflexibility of the legs is increased. Moreover, the heat dissipation into air was not efficient to apply them for wearable TEGs that use air cooling in most cases.

As CPs are intrinsically flexible, the geometry of CP films could be designed to maximize ΔT while minimizing the electrical resistance for the connection of multiple thermocouples. Among the various geometric shapes, we propose a roll-type thin poly(3,4-ethylenedioxythiophene) (PEDOT) film doped with tosylate as a p-type leg, which insures keeping the low electrical resistance and flexibility of the film in the module.^7,8 Furthermore, the ΔT from the roll-type TE legs should be larger than the conventional rod-type legs because of the large void space in the roll. Such a flexible p-type leg could be integrated with the rigid n-type leg to afford a rigid-flexible thermoelectric generator (RF-TEG). This combination of a rigid part with a flexible part means that it can function like skeletal fingers consisting of several rigid phalange bones connected through flexible joints.

Here we report a new method for the preparation of a flexible p-type thermoelectric leg using a CP film, which affords low electrical resistance in the multiple connections of legs and a large ΔT. In addition, the thermocouples made of p-type CP films and n-type Bi₂Te₃ were fabricated to provide a high thermoelectric output under a ΔT of 7.9 K.

Among the CPs, the previously reported PEDOT doped with tosylate (PP-PEDOT)⁵ was selected as the p-type material because of its high thermoelectric properties. The PP-PEDOT films are light, flexible and could be prepared in a large area film so that they can be cut into small legs. Therefore, the PP-PEDOT film (8 × 8 cm²) was prepared on a thin polyethylene terephthalate (PET) film (40 μm) via solution casting polymerization according to our previous method, to afford a 600 nm thick PP-PEDOT film (Figures 1(c) and 1(d)). The film was cut into a small area film (1.8 × 0.2 cm²) and rolled up in an elliptical shape with a 4:1 ratio in the horizontal diameter (8 mm) against the vertical diameter (2 mm), as shown in Figure 1. The roll-type PP-PEDOT leg enabled easy contact to both the top and bottom electrodes due to its flexibility.

Simulation results on the ΔT of vertically roosted rod- and roll-type PP-PEDOT leg were obtained using the COMSOL Multiphysics software platform. Time-dependent heat transfer and convective cooling were reflected in the simulation. Figure 2(a) shows the temperature gradient for a TE leg of the rod-type against the roll-type after the contact to a hot plate at 60 °C for 30 s. We have simulated the effect of PP-PEDOT on ΔT, considering three different cases as follows: (1) a 600 nm thick PP-PEDOT on a 6 μm PET, (2) a 6 μm-thick PP-PEDOT on a 6 μm thick PET, and (3) a 6 μm PP-PEDOT on a 60 μm PET layer. Figure S1 of the supplementary material illustrates that the roll-type TE leg always shows 7 K higher heat dissipation than the conventional rod-type legs after 30 s heating. This could be attributed to the void in the roll that allows effective air cooling.

The temperature of the bottom part in contact with the hot plate was almost the same for both TE legs, but that of the top part was 32 °C and 25 °C, resulting in ΔT of 26 °C and 33 °C for rod- and roll-type modules, respectively. Thus, roll-type TE legs are expected to have a much higher ΔT than the rod-type PP-PEDOT film, and so the roll-type PP-PEDOT leg provided not only a better connection during the fabrication of the TE module but also a higher ΔT than the rod-type PP-PEDOT leg.

The above p-type PP-PEDOT legs were combined with Bi₂Te₃ because of the high electrical conductivity and power factor of the latter.⁹ A Bi₂Te₃ block was prepared under high temperature and pressure (6 Mpa, 80 °C) and then cut into cubic shapes (2 × 2 × 2 mm³), as illustrated in Figure 1. The patterned gold electrodes were coated on a polyimide (PI) film (50 μm). To achieve the flexibility of a TEG, a dangling type rigid-flexible thermoelectric generator (RF-TEG) was explored using a flexible wire electrode covered with polytetrafluoroethylene according to the previous work.¹⁰ The connection between the wire and the bottom electrode was further optimized by using a thin flexible PI substrate, which allowed firm adhesion of the wire electrode. After the wiring and soldering process for each p- and n-type leg, the thermocouples were combined to produce a 36-couple RF-TEG. The dangling type TEG structure maintained connections between the electrodes when the device was bent. Since the dangling type of RF-TEG was directly open to air, without any packaging that decreased the output voltage, the ΔT was kept large by the air cooling effect.

The internal resistance of the 36-couple RF-TEG was determined as 3.1 kΩ, which is 0.6 kΩ higher than the estimated resistance (∼2.5 kΩ) calculated from the average resistance for one of the roll-type PP-PEDOT leg (60 Ω) and n-type Bi₂Te₃ block (1 Ω). Such a difference in resistance could be attributed to the connection resistance of legs and thermocouples.

The maximum power output (P_max) from a TEG is dependent on the open circuit voltage (U) and internal resistance (R),¹¹ 

The 36-couple RF-TEG showed a ΔT of 7.9 K between the top (open to air) and bottom of the TEG upon contact with a hot plate at 70 °C. Under this condition, the maximum output current and voltage from the 36-couple RF-TEG were determined as 12 μA and 36.7 mV, respectively. Figure 2(b) shows the power output of the 36-couple RF-TEG under ΔT = 7.9 K with various load resistances, leading to P_max of 115 nW under a resistance of 3.0 kΩ (Figure 2(c)). P_max linearly increased as ΔT (Figure 3(a)) and reached 160 nW under a ΔT of 10 K. Although there are several types of flexible TEGs known, such as single wall carbon nanotubes (SWNTs) and film type Bi₂Te₃, the integration of thermocouples is low (m = 4 or 6) in SWNT based TEGs^12,13 and is very high (ΔT > 50 K) to reach high output in Bi₂Te₃.¹⁴ As compared in Table I with the TE modules consisting of CPs or carbon nanotubes (CNTs),^15–18 the 36-couple RF-TEG module in this work shows high output in a very low ΔT and unit output in pW/K/thermocouple (Table I).

As described above, the RF-TEG is bendable and the internal resistance change by the bending radius of the 36-couple RF-TEG was negligible up to a 4 cm radius of the module’s curvature (Figure 3(b)), indicating that it can be easily worn on the human wrist. Furthermore, the internal resistance of the 36-couple RF-TEG module was stably longer than 120 days under the exposure to air (Figure 3(c)). Thus, an arm warmer was prepared and combined with the 36-couple RF-TEG. Parts of the arm warmer were cut out and connected to the 36-couple RF-TEG to make the temperature gradient larger by exposing the top of 36-couple RF-TEG to air (Figure 4). The output voltage of the 36-couple RF-TEG was determined as 10.6 mV, which was generated constantly and steadily from human body heat (Figure 4: inset). Although the generated output voltage is small, it is large enough to be used for a small low-power wearable electronics such as AllSee device (gesture recognition)¹⁹ or an embedded microchip in a wristband.²⁰ In addition, the low output voltage could be accumulated and converted to a high voltage through the circuitry for a static energy harvester.^21,22 In summary, this is the first example of a rigid-flexible TEG based on the organic and inorganic hybrid TE legs with high output performance. It can generate more than 160 nW under a ΔT of 10 K and is flexible enough to apply it on the human wrist, taking advantage of the flexibility of PP-PEDOT thin film. The internal resistance of the RF-TEG was stable even after exposing the device to air for longer than 100 days. Finally, this approach could be applicable to a wearable device because the generated output energy could be accumulated in daily life.

See supplementary material for the COMSOL simulation and the experimental details.

This research was supported by Global Research Laboratory (GRL) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2016K1A1A2912753).

The authors declare no conflict of interest.