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フレキシブル有機熱電変換素子の実用化に向けて

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九州大学学術情報リポジトリ

Kyushu University Institutional Repository

フレキシブル有機熱電変換素子の実用化に向けて

黄, 善彬

https://doi.org/10.15017/1806988

出版情報:Kyushu University, 2016, 博士(工学), 課程博士 バージョン:

権利関係:Fulltext available.

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(様式2)

氏 名 : 黄 善彬(ファン ソンビン)

論 文 名 : Toward the Practical-Use of Flexible Organic Thermoelectric Generators (フレキシブル有機熱電変換素子の実用化に向けて)

区 分 : 甲

論 文 内 容 の 要 旨

Thermoelectric energy conversion is one attractive possible solution as it offers an extremely reliable and silent power source with no moving parts. Furthermore, electricity can be generated from the waste heat produced by things such as power plants and machinery even at low temperatures because only a small temperature difference is required across a thermoelectric generator (TEG). However, if TEGs are too bulky, heavy, or rigid, the number of existing industrial and natural environments to which they can be easily ada pted will be limited. Additionally, although the thermoelectric effect driving the devices has been known for nearly two centuries, growth of the inorganic TEG industry has been limited because of very low efficien cies and high production costs.

Organic thermoelectric generators (OTEGs) replace the traditional inorganic active materials in TEGs with organic compounds and have additional advantages such as low thermal conductivity, light weight and flexibility. Such properties give OTEGs the potential to be used to easily cover large heat-exchanger surfaces of any shape and size. Organic materials also offer the prospect of low-temperature solution processing, which could enable the roll-to-roll mass printing of large-area, integrated modules and reduce costs. Therefore OTEGs could become the most versatile distributed power generation technology in the near future with applications from wearable electronics to power supplies for mobile devices a nd distributed sensor networks.

The state-of-art and challenges for OTEGs are described in Chapter 1. The research in recent years on organic thermoelectric materials has predominantly aimed at improving their thermoelectric efficiency.

The main goal has been to find materials capable of highly efficient thermoelectric energy conversion.

According to this trend, this dissertation undertakes the study of various factors affecting the characteristics of organic thermoelectric devices to advance the development and future commercialization of high performance thermoelectric devices.

A solution process for the fabrication of the uni-leg type flexible modules consisting of only a p-type semiconductor is developed using a photo-etching technique for patterning the micrometer-thick active layers in Chapter 2. This solution-processing method may be suitable for roll-to-roll mass production and the reduction of production costs compared to conventional processes while working around the problem of a scarcity of good candidates for n-type materials.

In the following chapter, the scarcity of good candidates for organic n-type materials, which limits the use of π-leg module structures because of unbalanced transport coefficients between p- and n-type materials leading to power losses, is addressed. In particular, the extremely low electrical conductivity of n-type materials compared with those of p-type materials is a serious challenge. Thus, Chapter 3 explores soluble organic n-type conducting polymers with controllable carrier densities and thermoelectric

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performance similar to or exceeding that of p-type materials.

Although improvements in n-type thermoelectric properties are obtained, electrical conductivity degradation in air is still an issue for all n-type organic materials. Because the work function of doped n- type organic materials become shallower with increasing dopant concentration, electrons can easily to transfer from doped n-type materials to the components of air (e.g., oxygen and water). Chapter 4 focuses on poly(metal 1,1,2,2-ethenetetrathiolate)s, which are polymers with metal-ligand conjugation forming metal-organic-frameworks (MOFs) and are attractive for n-type OTEGs because of their excellent TE properties and relatively good stability in air.

However, these MOF-based material systems suffer from their insolubility, which makes them unlikely as candidates for realizing all-solution-processed OTEGs. This insolubility, which interferes with the fabrication of smooth thin films, leads to difficulties when analyzing physical and optical properties and electronic states, inhibiting further development. In Chapter 4, a sol -gel technique with oxidation is demonstrated to be very effective for the fabrication of smooth thin films of MOFs.

Furthermore, the thermoelectric properties and the atmospheric stability of electrical conductivity of the MOF thin films are discussed in terms of the electronic structures to analyze the factors affecting the stability of n-type OTEGs in air.

Finally, the summary of this thesis and future strategy are introduced in Chapter 5.The efficiency of thermoelectric energy converters is limited by the fundamental properties of thermoelectric material, and further improvement of efficiency cannot be expected unless innovative new classes of materials are explored. This chapter introduce the band structure engineering to enhance the local density of states in conduction or valence bands which can be explained based on the Mahan-Sofo theory. Use of this new physical principle could further enhance thermoelectric performance for organic thermoelectric materials and enable more widespread use of organic thermoelectric generators.

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