Organic Thermoelectric Materials

Zhiqun Lin, Ming He, Zhiqun Lin, Ming He

• Publisher: Royal Society of Chemistry
• Year: 2019
• Print ISBN: 9781788014700
• eBook ISBN: 9781788019163
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Energy

Thermoelectric materials have received a great deal of attention in energy-harvesting and cooling applications, primarily due to their intrinsic low cost, energy efficient and eco-friendly nature. The past decade has witnessed heretofore-unseen advances in organic-based thermoelectric materials and devices. This title summarises the significant progress that has been made in the molecular design, physical characterization, and performance optimization of organic thermoelectric materials, focusing on effective routes to minimize thermal conductivity and maximize power factor. Featuring a series of state-of-the-art strategies for enhancing the thermoelectric figure of merit (ZT) of organic thermoelectricity, and highlighting cutting-edge concepts to promote the performance of organic thermoelectricity, chapters will strengthen the exploration of new high-ZT thermoelectric materials and their potential applications. With contributions from leading worldwide authors, Organic Thermoelectric Materials will appeal to graduate students as well as academic and industrial researchers across chemistry, materials science, physics and engineering interested in the materials and their applications.

CHAPTER 1

Introduction

Jaeyoo Choi ^a , Madeleine P. Gordon ^{a , b} , Pengyu Yuan ^{a, c , d} , Hyungmook Kang ^{a , c} , Edmond W. Zaia ^{a , e} and Jeffrey J. Urban^*a

^a The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, USA,

^b Applied Science and Technology Graduate Group, University of California, Berkeley, California, USA

^c Department of Mechanical Engineering, University of California, Berkeley, California, USA

^d School of Engineering, University of California, Merced, California, USA

^e Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California, USA
^*E-mail: [email protected]

Organic thermoelectric materials and generators are a promising field of research due to numerous attractive characteristics that suggest the ability to overcome the shortcomings of existing inorganic TE systems. This book aims to reprise some of the key directions in this field and provide some context on the key developments and opportunities that remain to be explored. Therefore, to facilitate comprehension of the book, this introduction chapter will present a brief overview of essential topics spanning the fields of chemistry, materials science, engineering, and physics, including a brief historical overview of organic TE materials. TE measurement techniques, including carrier-energy and thermal transport in organic TE systems, will also be briefly discussed, as they are relevant to understanding what challenges remain in understanding and optimizing the performance of these organic TE materials.

The field of thermoelectrics (TEs) has existed for over 150 years since the Peltier and Seebeck effects were first observed in the mid 19th century. Broadly speaking, TEs play a dual role in the broad energy landscape – they have the ability to interconvert thermal and electrical gradients – a connection that no other type of technology can offer. Traditionally, these TE effects have been realized predominantly in inorganic materials, and this has been fruitful for the development of solid state devices of use in mostly niche applications in refrigeration and power generation. However, there has been recent momentum toward exploring so-called “soft TEs” ^1,2 in the past decade that this book aims to capture. This was primarily driven by new developments in organic and hybrid materials, and the observation that new TE transport rules, flexible form factors, and low-cost manufacturing processes are all available in these materials.

This book aims to reprise some of the key directions in this field and provide some context on the key developments and opportunities that remain to be explored. In order to quickly develop functional knowledge of this field, a wide-ranging bundle of skills including chemistry, materials science, characterization methods, and transport physics is needed. The following chapters in this book will cover the intriguing electrical and thermal properties of some featured organic materials and their unique transport characteristics will also be discussed. In addition, various interfacial engineering approaches including hierarchal architectures, inorganic/organic hybrid interfaces, and organic intercalation will be introduced.

1.1 Motivation

The long-term consequences of global reliance on fossil fuels have led to a significant need for alternative energy and innovative energy-harvesting technologies. Among them, TE energy conversion may be a key player in the suite of next-generation energy technologies. Because they can directly convert thermal energy, such as waste heat, into useful electrical energy via the Seebeck effect or provide both active solid-state cooling and heating from an applied current via the Peltier effect. This is because at least 60% of primary energy produced in the world is wasted in the form of heat. ^3,4 Home heating, automotive exhaust, and industrial processes all generate an enormous amount of unused waste heat. Energy-intensive industries, such as industrial manufacturing, oil and gas operations, and transportation, operate around the clock and produce enough waste heat to generate over 15 GW of electricity per year. ⁵ Over the last decade, significant research investments have actually been made for the purpose of waste heat recovery to increase energy efficiency. Several car manufacturers have investigated the impact of converting waste heat to electricity in the automobile engine and have applied TE generators in real automobile production. ⁶ Some studies targeted the development of TE generators that can be attached onto round-shape hot pipes in plant and to power networking sensors, lights, and portable electronic devices, etc. ^7,8 However, despite significant potential, TE generators have not yet experienced broad commercial deployment due to a number of challenging issues. Commercially available TEs are typically fabricated with rare or toxic inorganic materials (tellurium, selenium, lead, etc.), and possess other common aspects which limit practical utilization: (1) mass production of TE materials due to scarce resources; (2) shape compatibility to uneven or curved surfaces of heat sources due to rigid form factors; and (3) large-area fabrication for mass-energy conversion owing to energy-intensive fabrication processes. To solve these problems, novel TE material design is required to circumvent the aforementioned limitations. From these reasons, soft TE materials, which are typically based on organic materials, are emerging as a promising candidate for the future TE energy conversion technologies.

Organic materials are an especially appealing area of research for TE applications because they typically possess high levels of mechanical flexibility, are composed of earth-abundant materials, are cost-effective, and have shown the ability for scalable manufacturing. The scalable and low-cost manufacturing of organic materials (polymers, small molecules, etc.) has been demonstrated throughout history for a vast array of different commercial applications. Organic materials also offer the opportunity for leveraging pre-established device fabrication and processing techniques (roll-to-roll, inkjet, etc.) which would enable the production of lightweight and flexible next-generation TE devices with a many different geometries and designs. ^2,9 Thus, fully realizing the potential of organic TEs would enable the fabrication of robust, lightweight, and flexible devices appropriate for a wide range of new applications.

Over the last decade there has been a global proliferation of low-power electronics such as personal, wearable and healthcare devices. TE generators are ideal for this type of device and are expected to be widely applicable as a local power supply that doesn't require the charging or replacement of batteries for wearable electronic devices, sensors, and personal medical and health-care devices. ¹⁰ For example, some implantable medical devices such as pacemakers require only 10 microwatts of power for semi-permanent operation without battery replacement surgery. ¹¹ These applications are quite interesting because human body heat is one of the most abundant and reliable wasted energy sources, which can be conveniently harvested by utilizing TE energy conversion. Given that the average person emits between 100 and 120 W of heat energy into the air, ¹² TE offer promising value when compared to the operational power needs of a smartphone (5 W) or laptop (45 W). ¹³ Although there is clearly opportunity for the implementation of TE devices, significant progress in the TE generator for wearable applications has lacked thus far. One significant challenge faced by most traditional inorganic TEs is the inherent curvature of the human body and the need for high flexibility and durability in order to ensure tight and stable attachment of the device to the skin. Additionally, traditional inorganic TEs are incompatible with the wide-area manufacturing techniques that would be essential for maximizing energy conversion. Therefore, especially for wearable applications, organic TE materials have many advantages such as flexibility, light-weight, and processability beyond the intrinsic properties of inorganic TE materials despite the low TE performance.

In summary, organic TE materials and generators are a promising field of research due to their many attractive characteristics that suggest the ability to overcome the shortcomings of existing inorganic TE systems. In the following sections, we will present a brief overview of essential topics spanning the fields of chemistry, materials science, engineering, and physics, including a brief historical overview of organic TE materials. TE measurement techniques, including carrier-energy and thermal transport in organic TE systems, will also be briefly discussed, as they are relevant to understanding what challenges remain in understanding and optimizing the performance of these organic TE materials.

1.2 History of TE Materials: Past to Present and Future

Historically, all commercially available TE generators have been based on doped narrow-bandgap semiconductors. A prototypical example of a high-performance TE material is bismuth telluride (Bi₂Te₃), discovered by H. J. Goldsmid and coworkers in the UK in 1954. ¹⁴ The majority of basic theory and common design standards established for TE systems are based on these types of inorganic systems. In the early 1990s, M. S. Dresselhaus and L. D. Hicks proposed the innovative theory that low-dimensional materials are favorable for enhancing material efficiency. ^15,16 This theory predicts that certain materials such as bismuth, which is a poor bulk TE material, can realize high TE performance in 2D quantum-well or 1D quantum-wire structures. ^17,18 After the 2000s, owing to improvement of atomic-scale synthesis and fabrication techniques, new categories of materials with high TE performance such as skutterudites, clathrates, half-heuslers and quantum dot superlattices have been reported. ^19,20 As a leading TE material, inorganic semiconductors such as the bismuth–tellurium–antimony–selenium (Bi–Te–Sb–Se) alloy family have been widely investigated so far. However, despite their promising TE performances, mechanical brittleness, energy-intensive processing methods, and material scarcity remain critical obstacles for further commercial deployment. ²

Organic TE materials on the other hand, have attracted increasing amounts of attention throughout the years in an effort to overcome the challenges faced by inorganic TE materials. These “soft” TE materials are exciting for a number of reasons; they demonstrate novel and unique transport mechanisms and also enable the development of conformal devices that leverage low manufacturing costs. Organic materials including conjugated polymers, polythiophene derivatives, nanoscale carbon-based materials, graphene derivatives, and assorted organic–inorgan...

Table of contents

• Cover
• Half Title
• Series Editor
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Introduction
• Chapter 2 Thermoelectric Transport Theory in Organic Semiconductors
• Chapter 3 Synthesis of Organic Thermoelectric Materials
• Chapter 4 PEDOT-based Thermoelectrics
• Chapter 5 Carbon Based Thermoelectric Materials
• Chapter 6 Organic Hierarchical Thermoelectric Materials
• Chapter 7 Conducting Polymer-based Organic–Inorganic Thermoelectric Nanocomposites
• Chapter 8 Thermoelectric Materials by Organic Intercalation
• Chapter 9 Flexible Organic-based Thermoelectric Devices
• Subject Index