S54

Thermoelectric Properties of Printed Bismuth Telluride Films
Kunihisa Kato, Hisashi Iwata, and Koji Miyazaki
Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu, miyazaki@mech.kyutech.ac.jp
ABSTRACT
We prepared a mixture of thermoelectric bismuth telluride particles, a conductive polymer (PEDOT:PSS),
poly(acrylic acid) (PAA), and several organic additives to fabricate thermoelectric films using printing or coating
techniques. In the mixture, the organic components (PEDOT:PSS, PAA, and an additive) act as a binder to connectbismuth telluride particles mechanically and electrically. Among the organic additives used, glycerol significantly
enhanced the electrical conductivity and bismuth telluride particle dispersibility in the mixture. Bi0.4Te3.0Sb1.6 films
fabricated by spin-coating the mixture showed a thermoelectric figure of merit (ZT) of 0.2 at 300 K when theBi0.4Te3Sb1.6 particle diameter was 2.8 μm and its concentration in the elastic films was 95 wt%.

Key words: Bi0.4Te3.0Sb1.6,Thermoelectric, Printing, Conductive polymer

1. INTRODUCTION
Thermoelectric generators can convert low-grade waste heat into electricity, making it a key technology,
contributing to sustainability through scavenging of waste heat or heat sources.1 The performance of thermoelectric
device is determined by the thermoelectric figure of merit, ZT, which is defined as ZT = S2σT/κ, where S is theSeebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the temperature. For thepractical uses of thermoelectric devices to spread, improvements in device performance and reductions in
manufacturing costs are required. It has recently been reported that using nanostructured thermoelectric materials
efficiently increases ZT as a result of reduced thermal conductivity by phonon scattering.2-6 Generally, thermoelectric
devices are fabricated by vacuum processing, particularly for thin film applications, but these processes are expensive.
Printing or coating processes such as screen- and inkjet-printing are attractive methods for reducing the manufacturingcosts of thermoelectric devices. In recent reports, a number of methods for obtaining thermoelectric materials byprinting using polymer materials have been described.7-9 The electrical conductivities and Seebeck coefficients ofprinted thermoelectric materials are lower than those of bulk materials. To improve the electrical conductivity andSeebeck coefficient of a thermoelectric compound, it is necessary to anneal the thermoelectric compound or fill thegrain boundaries with conductive materials. Polymer materials are very attractive as they have low thermal
conductivity, good flexibility, and good printability. We fabricated flexible printed thermoelectric thin films
containing thermoelectric particles, a conductive polymer, and various additives, and here we report the thermoelectric
properties of the composite thermoelectric thin films.

2. EXPERIMENTS
The thin films of the thermoelectric compounds we produced consisted of thermoelectric particles, a conductive
polymer, poly(acrylic acid) (PAA), and various organic additives. The films were prepared using two processes. As
the starting material, we used p-type Bi0.4Te3.0Sb1.6 particles (Kojundo Chemical Laboratory) with an average particle
size of 180 μm. The particles were milled using a jet-milling apparatus for 30 min at 0.15, 0.30, and 0.95 MPa under anitrogen atmosphere. The size distribution of each particle was measured using a laser diffraction particle sizeanalyzer. The resulting average particle sizes after jet milling were approximately 0.66, 2.8, and 5.5 μm, respectively.
We prepared a polymer compound from a conductive polymer (PEDOT:PSS), PAA, and various organic additives,
namely glycerol, N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO).
The milled Bi0.4Te3.0Sb1.6 particles were added to the polymer compound to make printable slurries. The
thermoelectric thin films were prepared by spin-coating on a glass or alumina substrate and heated at 150 °C for 10
min under an argon atmosphere. The thicknesses of the thin films were determined using a stylus profilometer, andwere approximately 0.1–10 μm. The surface and cross-section morphologies of the thermoelectric thin films wereinvestigated using scanning electron microscopy (SEM). The in-plane electrical conductivities of the films weremeasured at room temperature using a four-point probe method, and the in-plane Seebeck coefficients S weremeasured at room temperature. One end of the thin film was connected to a heat sink and the other end to a heater.
The Seebeck coefficient was determined as the ratio of the potential difference V along the film to the temperature
difference T. The cross-plane thermal conductivity was determined at room temperature using a differential 3ω
method with an accuracy of 10%.10 The figure of merit ZT for each film was estimated from the results of theelectrical conductivity, Seebeck coefficient, and the thermal conductivity.

3. RESULTS AND DISCUSSIONThe thermoelectric properties of the hybrid thermoelectric thin films were studied at room temperature.

Theelectrical conductivities, Seebeck coefficients, and power factors of the thin films were investigated, as shown in Fig.
1. The electrical conductivities of the films were reduced when the particle diameter increased from 0.66 to 2.8 μm.
The electrical conductivities of the thermoelectric particles were lower than that of the conductive polymer compound
film itself, so the particles acted as electrical resistors. As the thermoelectric particle concentration increased, theSeebeck coefficients were significantly enhanced. The power factors of the thin films consisting of particles withdiameters of 0.66 and 2.8 μm particles reached 74.1 and 237.2 μW/(m·K2), respectively. The power factors of the thinfilms consisting of particles with diameters of 2.8 μm particles larger than previous report. 7 The thin films of thethermoelectric material by filling the particle boundaries by the addition of conductive polymer and organic additives.
The cross-plane thermal conductivities of the p-Bi0.4Te3Sb1.6 composite films with diameters of 0.66 and 2.8 μm
particles were 0.24, 0.36W/(m·K), respectively. The measured thermal conductivities were close to the conductivepolymer(0.32W/(m·K))and

significantly

less

than

thoseofstandard

p-typebulk

bismuthtelluridematerials(1.5W/(m·K)). The reason for the low thermal conductivity is scattering of phonons at the grain boundaries
and filling of the boundaries with materials of low thermal conductivity. The hybrid thermoelectric films wereprepared by spin-coating on a PET substrate and heating at 150 °C for 10 min under an argon atmosphere. Figure 2shows the visible appearance of the hybrid thermoelectric thin films on the polymer substrate. There were no cracks ordefects in the material when the film was bent several times. The ZT values of the 0.66- and 2.8-μm p-Bi0.4Te3Sb1.6
particle films were estimated to be 0.09 and 0.20, respectively. The reduction in the thermal conductivity effectively
improved the ZT value. In order to further improve the thermoelectric performance, it is necessary to enhance theelectrical properties.

Fig.1 Electrical properties of hybrid thermoelectric films

Fig.2 Fabricated printed flexible thermoelectric thin films

4. CONCLUSION

In summary, we have proposed a method for fabricating flexible printing-type thermoelectric thin films containing ptype Bi0.4Te3.0Sb1.6 particles, a conductive polymer, and various additives. The dispersibilities of the thermoelectric
particles in ink were improved by milling the particles. We used sub-micron thermoelectric particles, and the crossplane thermal conductivities of p-type Bi0.4Te3.0Sb1.6 thin films were significantly reduced by phonon scattering at thegrain boundaries. Flexibility could be imparted to the thin films of the thermoelectric material by filling the particle
boundaries by the addition of small amounts of conductive polymer and organic additives. When the particle
concentration was 95 wt%, we achieved the highest ZT value, estimated to be 0.2.

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