Elsevier

Energy Conversion and Management

Volume 106, December 2015, Pages 1192-1200
Energy Conversion and Management

Hierarchical Bi–Te based flexible thin-film solar thermoelectric generator with light sensing feature

https://doi.org/10.1016/j.enconman.2015.10.052Get rights and content

Highlights

  • Hierarchical Bi–Te thin films are prepared with enhanced electrical conductivity.

  • Flexible thin-film STEG is fabricated integrated with the hierarchical Bi–Te films.

  • The thermal design is optimized in the thin-film solar thermoelectric generator.

  • The power output is largely enhanced due to the low resistance and the large ΔT.

  • The thin-film thermoelectric generator exhibits light sensing feature.

Abstract

Thin-film solar thermoelectric generators show much promise in effective use of solar energy as a power supply for microscale devices. In this paper, we fabricated a flexible thin-film solar thermoelectric generator on the polyimide substrate using simple mask-assisted deposition process. The p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 thermoelectric films with special hierarchical nanostructure were prepared, resulting in substantial improvement in the electrical conductivity. The solar thermoelectric generator with the hierarchical Bi–Te thin films is able to generate 220 mV of the output voltage and 80 μW of the maximum power for a temperature difference of 100 K. We also analyzed the optimized design for the solar thermoelectric generator. By employing a package for the thermoelectric legs and a perforated structure, the heat flow was restricted along the thermoelectric legs and a larger temperature difference was achieved. A fast electrical response to the on–off simulated solar light irradiation was also observed, indicating light sensing feature of the fabricated thin-film solar thermoelectric generator. Our results revealed that a remarkably enhanced output power has been achieved in the flexible thin-film solar thermoelectric generator due to the introduction of the improved thermoelectric films and the optimized thermal design.

Graphical abstract

A flexible thin-film solar thermoelectric generator (STEG) was fabricated on the polyimide using a simple mask-assisted deposition process. The p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 films with hierarchical nanostructures were prepared. A good contact was observed between the columnar Cu/Ni electrode and the thermoelectric films. The power output of the thin-film STEG is largely enhanced due to the low internal resistance and the large ΔT. The output voltage of 220 mV and maximum power of 80 μW were achieved at a temperature difference of 100 K. A fast electrical response against on–off simulated solar light was observed, indicating a light sensing feature.

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Introduction

Thermoelectric devices based on the Seebeck effect have been employed for a wide range of applications, including the power generation [1], [2], [3], [4], temperature measurement [5], [6], and the detection of infrared radiation [7]. In the past decade, thermoelectric generators received scientific and technological interests, driven by the increasing need for renewable energy sources. Applications of thermoelectric generators range from the portable energy sources powered by the human body heat to electrical power generation from vehicle the exhaust heat [8], [9], [10], [11], [12]. Solar energy is also an alternative heat source via a photo-thermal conversion, and solar thermoelectric generator (STEG) exhibits much potential to convert solar energy to the electricity [13], [14], [15]. In particular, flexible thin-film STEG is expected to be used as power supply for the wireless sensor and the microscale devices because of its portability and high integration [16].

To date, the maximum efficiency of 5% has been achieved in a bulk STEG reported by Kraemer et al. [17] using nanostructured thermoelectric materials with ZT = 1.03 (where ZT is the dimensionless thermoelectric figure-of-merit). ZT is the parameter used to evaluate the suitability of thermoelectric materials for energy conversion applications and it is defined as:ZT=α2σλT,where α,σ,λ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity of the thermoelectric materials, and the absolute temperature, respectively. In order to improve the efficiency of thermoelectric generators, a key is to develop higher ZT materials. “Conventional” bulk thermoelectric materials appear to have ZT values limited to around 1. Recently, advances in nanotechnology have opened a door to improve the ZT values of thermoelectric materials through nanostructuring [18]. The desirable nanostructures can modify the transport properties of the electrons and phonons in thermoelectric materials and lead to significantly improvements in ZT [19]. It has been reported that ZT = 2.6 has been achieved in SnSe single crystals with layered nanostructure, due to exceptionally low lattice thermal conductivity of 0.23 W m−1 K−1 [20]. In addition, the introduction of the hierarchical interfaces can scatter phonons more effectively and reduce the thermal conductivity. In reports published by Biswas et al. [21], all-scale hierarchical architectures in PbTe–SrTe (4 mol%) doped with 2 mol% Na was constructed to scatter phonons with different wavelengths, and consequently a ZT value of 2.2 at 915 K is achieved. Additionally, the Sb2Te3 thin film with unique hierarchical nanostructures was also reported with ZT  0.88 at room temperature [22].

The recent demonstrations of STEGs are mostly based on bulk TE materials and devices [11]. A thin-film STEG model has been recently reported by Weinstein et al. [23]. In their work, it was predicted that a theoretical efficiency of 5% can be obtained in a thin-film STEG. This value is comparable to that of existing bulk STEGs. Mizoshiri et al. [16] fabricated thin-film TE modules for power generation using focused solar light. However, the thin-film STEGs fabricated to date still exhibits low power output due to the insufficient consideration of the thermal design for the thin-film STEG. Generally, the maximum power output for STEG is given by:Pmax=(SΔT)24Rinwhere S is the Seebeck coefficient of the device, ΔT the temperature difference established across the thermoelectric legs, and Rin the internal electrical resistance of STEG. According to Eq. (2), an important concern to improve the performance of STEG is to establish a large ΔT. In STEG, the temperature difference across the thermoelectric legs is produced by solar illumination, which is largely affected by the design of the thin-film STEG, including the component configuration and the thermal management. In general, a larger ΔT can be established by increasing the thermal resistance of the thermoelectric device and/or increasing the incoming thermal energy [24].

Low conversion efficiency and fabrication complexity have been identified as the main obstacles to the development of thin-film STEG. Previous work mainly focused on the theoretical calculation on the efficiency by the optimization of geometry. To prepare a high-performance thin-film STEG, the key challenge is to establish a large temperature difference and use high ZT thermoelectric thin films. Bi–Te based compounds are the best thermoelectric materials at room temperature [25]. A number of techniques, such as implantation [26], electrochemical deposition [27] or physical vapor deposition [10], [16], [28] and inkjet or screen printing [29], [30], [31], have been reported for preparation of thermoelectric thin films. In this paper, we reported a flexible thin-film STEG fabricated using simple mask-assisted deposition method. In order to improve its performance, we conducted a comprehensive optimization by considering both the materials and the design of the device. The high-performance p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 thin films with hierarchical nanostructures were prepared. The device structures with optimal design to obtain a large ΔT across the thermoelectric legs were investigated. The performances of power generation and the response to light irradiation of the fabricated thin-film STEG were measured and discussed. The results show that the fabricated thin-film STEG exhibits high power output and light sensing feature.

Section snippets

Film deposition

In this work, the Bi0.5Sb1.5Te3, Bi0.5Te2.7Se0.3, and Cu/Ni electrode films were deposited by magnetron sputtering system (JGP-450a, SKY Technology Development Co., Ltd. Chinese Academy of Sciences). The sputtering targets are hot-pressed discs of Bi0.5Sb1.5Te3, Bi0.5Te2.7Se0.3, Cu and Ni (60 mm in diameter, 99.99%), purchased from General Research Institute for Nonferrous Metals, China. The targets were powered by a direct current power supplier of 30 W. The deposition conditions have

Thermoelectric thin films

The phase structures of the Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3 films prepared at room temperature were investigated by XRD and the results are shown in Fig. 2. For the both films, all peaks are indexed as the rhombohedral phase (JCPDS 49-1713 and 50-0954), implying the formation of the polycrystalline structure. However, the intensity of the peaks is low, especially for n-type Bi2Te2.7Se0.3 film, indicating poor crystallinity. This was caused by a high sputtering rate combined with a low

Conclusion

A flexible thin-film STEG was fabricated using simple mask-assisted sputtering technology. The hierarchical thermoelectric thin films were developed and implemented into the device, which resulted in a reduction of the electrical resistance and consequently enhanced power output of the thin-film STEG. The output voltage and maximum power obtained from this STEG are 220 mV and 80 μW, respectively for a temperature difference of 100 K. Furthermore, efforts were made to increase the temperature

Acknowledgements

This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2012CB933200), the National Natural Science Foundation of China (Nos. 51172008 and 51002006), the National Natural Science Fund Innovation Group (No. 50921003), the Beijing Technology Topic Program (No. Z111100066511009), the Research Fund for Doctor Station Sponsored by the Ministry of Education of China (20111102110035), the Fundamental Research Funds for the Central Universities and the

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