Organic Thermoelectrics E-Book
142,99 €
Mehr erfahren.
- Herausgeber: Wiley-VCH GmbH
- Kategorie: Fachliteratur
- Sprache: Englisch
Presents the fundamental mechanism of thermoelectric conversion, design strategy and advances in different materials, devices fabrication and characterization of thermoelectric parameters.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 705
Veröffentlichungsjahr: 2022
Ähnliche
Table of Contents
Cover
Title Page
Copyright
Preface
1 Introduction of Organic Thermoelectrics
1.1 Brief Introduction and Historical Overview
1.2 Thermoelectric Effect
1.3 Thermoelectric Parameters
1.4 Challenges and Perspectives
References
2 Theoretical Model and Progress of Organic Thermoelectric Materials
2.1 Introduction
2.2 Charge Transport
2.3 Thermal Transport
2.4 Theoretical Progress in OTE Materials
2.5 Conclusion
References
3 P‐Type Organic Thermoelectric Materials
3.1 Introduction
3.2 Charge Transfer Complexes
3.3 Conventional Conducting Polymers
3.4 Doped High Mobility Semiconductors
3.5 Perspective
References
4 N‐Type Organic Thermoelectric Materials
4.1 Introduction
4.2 Materials and Properties
4.3 Strategies for the Performance Optimization
4.4 Summary and Perspective
References
5 Hybrid/Composite Organic Thermoelectric Materials
5.1 Introduction
5.2 Fundamental Effect and Theory
5.3 Materials and Properties
5.4 Strategies of Hybrid/Composite OTE Materials Fabrication and Optimization
5.5 Conclusion and Perspective
References
6 Organic Ionic Thermoelectric Materials and Devices
6.1 Introduction
6.2 Fundamentals of Ionic Thermoelectrics
6.3 Organic i‐TE Materials Based on Electrolytes
6.4 Organic i‐TE Materials Based on Mixed Conductors
6.5 Organic i‐TE Devices and Applications
6.6 Differences Between Ionic and Electronic Thermoelectrics
6.7 Perspectives and Challenges
References
7 Engineered Doping of Organic Thermoelectric Materials
7.1 Introduction
7.2 Chemical Doping
7.3 Electrochemical Doping
7.4 Electric‐Field Induced Interfacial Doping
7.5 Photodoping
7.6 Doping Strategies for OTE Materials
7.7 Conclusions and Perspectives
References
8 Organic Thermoelectric Devices
8.1 Introduction
8.2 Power Generator
8.3 Peltier Cooler
8.4 Multifunctional Applications
8.5 Conclusion
References
9 Single‐Molecule Thermoelectric Devices
9.1 Introduction
9.2 Fundamental Background and Experimental Techniques
9.3 Advances in Single‐Molecule Thermoelectric Devices
9.4 Perspectives
References
10 Measurement Techniques of Thermoelectric‐related Performance
10.1 Introduction
10.2 Measurement of Electrical Conductivity
10.3 Measurement of Seebeck Coefficient
10.4 Measurement of Thermal Conductivity
10.5 Simultaneous Measurement of Key Parameters
10.6 Determination of Carrier Concentration
10.7 Determination of Electronic Structure
10.8 Summary
References
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Summarized performance of typical p‐doped OTE materials.
Chapter 4
Table 4.1 Thermoelectric properties of some typical n‐doped semiconducting ...
Table 4.2 Thermoelectric properties of some typical n‐doped semiconducting ...
Chapter 5
Table 5.1 Summary of the optimized thermoelectric performance of composites...
Chapter 6
Table 6.1 Exemplary liquid electrolytes and their i‐TE properties.
Table 6.2 Exemplary solid and semisolid i‐TE materials.
Table 6.3 Exemplary organic
i
‐TE materials based on OMIECs.
Chapter 8
Table 8.1 Summary of output power of OTE‐based generators in different devi...
List of Illustrations
Chapter 1
Figure 1.1 Schematic diagrams of thermoelectric (a) power generator and (b) ...
Figure 1.2 Temperature‐dependent thermopower curve of β‐(BEDT‐TTF)
2
BrI
2
.
Figure 1.3 Basic principle of (a) Seebeck effect and (b) Peltier effect.
Figure 1.4 Schematic illustration of Hall effect and the other three effects...
Figure 1.5 Thermoelectric power conversion efficiency as a function of ZT at...
Figure 1.6 Carrier concentration dependence of electrical conductivity, Seeb...
Chapter 2
Figure 2.1 The schematic diagram of charge transport and heat transport in o...
Figure 2.2 The schematic diagram of energy dispersion of...
Figure 2.3 Potential energy of the charge transfer reaction [20]
Figure 2.4 The diagram of electron motion path following the thermal transpo...
Figure 2.5 Simply schematic illustration of carrier transport in hopping mod...
Figure 2.6 The diagram of
S
and
σ
. (a)
S
–
σ
modeling and (b) Charge...
Figure 2.7 Fitting curve of
S
–
σ
from experimental results [31].
Figure 2.8 Dispersion curves of phonon vibrations in a diatomic lattice.
Figure 2.9 (a) Vectorial representation of a Normal phonon scattering proces...
Figure 2.10 Heat transport process with phonon scattering in (a) Crystalline...
Figure 2.11 (a) Crystal structure lattices of C8‐BTBT in the ac plane and ab...
Figure 2.12
S
–
σ
relationship comparison between theory (line, filled do...
Figure 2.13 (a) Schematic illustration of thermal and electrical transport i...
Figure 2.14 (a) Representative snapshots of coarse‐grained MD simulations of...
Figure 2.15 Radar charms for predicting n‐doping ability of TAMs (a) and the...
Chapter 3
Figure 3.1 Molecular structures of representative p‐type OTE materials.
Figure 3.2 Effect of de‐doping time on (a)
σ
,
S
, and (b) PF of PANI‐CSA...
Figure 3.3 Diagrams of various polymerization methods of PEDOT: (a) in situ ...
Figure 3.4 Schematic diagram of film formation process based on GIWAXS evalu...
Figure 3.5 (a) Diagram of PEDOT:PSS structural rearrangement via a post‐trea...
Figure 3.6 Chemical structure and absorbance spectra of PEDOT:PSS treated wi...
Figure 3.7 The open‐circuit voltage of PEDOT:Tos, PEDOT:PSS, and PSSNa versu...
Figure 3.8 (a) Schematic diagram of molecular rearrangement of PEDOT:PSS tre...
Figure 3.9 (a) Schematic diagram of set‐up for TE characterization of PEDOT:...
Figure 3.10 (a) Reaction mechanism of PEDOT by TDAE. The relationship among ...
Figure 3.11 Electronic structure and density of PEDOT in polaron and bipolar...
Figure 3.12 Effect of current density on (a)
S
, σ, (b)...
Figure 3.13 Chemical structures and simulation snapshots of (a) ra‐P3HT and ...
Figure 3.14 (a) Conjugated polymer structures and acronyms, (b)
S
,
σ
, P...
Figure 3.15 (a) Molecular structures of PDPPS‐12 and PDPPSe‐12 and the effec...
Figure 3.16 (a) The structure illustration and (b) TE performance of multila...
Figure 3.17 Illustration of molecular design in OSCs toward TE applications ...
Figure 3.18 Critical properties for designing conjugated backbones for OTE m...
Figure 3.19 Critical factors for side‐chain strategies of OTE materials: mis...
Figure 3.20 The molecular design concepts for OTE materials [166].
Chapter 4
Scheme 4.1 Factors that affect the performance of n‐type OTEs. The elements ...
Figure 4.1 (a) Exhibition of flexibility, the structure of ett is shown in t...
Figure 4.2 (a) Representations of the structures, (b) SEM image, (c) thermoe...
Figure 4.3 Molecular structures of some typical (a) n‐type conjugated polyme...
Figure 4.4 (a) Electrical conductivities, (b) Seebeck coefficients, (c) Powe...
Figure 4.5 (a) Chemical structures and calculated HOMO/LUMO levels of PzDPP,...
Figure 4.6 (a) Combining the advantages of both imide and cyano functionalit...
Figure 4.7 (a) Schematics of the film and chemical structures of BBL and PEI...
Figure 4.8 Molecular structures of some n‐type small molecules mentioned in ...
Figure 4.9 (a) Electrical conductivity, (b) Seebeck coefficient, (c) PF of T...
Figure 4.10 (a) Electrical conductivity, (b) Seebeck coefficient, (c) PF of ...
Figure 4.11 Temperature dependence of thermoelectric parameters of N‐DMBI do...
Figure 4.12 (a) Molecular structures of 1,2‐DBNA‐2 and 1,2‐DBNA‐5, (b) elect...
Figure 4.13 Molecular structures of (a) P(NDI2OD‐T2) and (b) P(NDI2OD‐Tz2), ...
Figure 4.14 Chemical structure and power factor of N–N, A–N, A–A as a functi...
Figure 4.15 (a) Calculated HOMO‐LUMO levels, (b) electrical conductivity, (c...
Figure 4.16 Chemical structures of (a) P(NDI2OD‐T2), TEG‐N2200, and PNDI2C
8
T...
Figure 4.17 (a) Electrical conductivity, (b) Seebeck coefficient, and (c) po...
Figure 4.18 Schematics of the proposed molecular packing models in (a) the i...
Figure 4.19 (a) Chemical structures of PNDICITVT and NDI‐TBAF; (b) Electrica...
Figure 4.20 (a) Schematic representation of the transition metal facilitated...
Figure 4.21 (a) the chemical structure of P(PymPh) and NaNap, and the OTE pr...
Figure 4.22 (a) Molecular structures and electron affinity of the conjugated...
Chapter 5
Figure 5.1 Percolation theory and nonlinear changes in the transport propert...
Figure 5.2 Schematic illustration of the energy filter effect in hybrid OTE ...
Figure 5.3 (a) Schematic image of PEDOT hybrid with carbon quantum dots (CQD...
Figure 5.4 Chemical structures of typical OTE used for hybrid materials.
Figure 5.5 (a) Electrical conductivity of PEDOT:PSS/Te nanowire film as a fu...
Figure 5.6 (a) Stress–strain curves of PP/T were posted‐treated with EG (E‐P...
Figure 5.7 (a, e) Seebeck coefficient, (b, f) electric conductivity and (c, ...
Figure 5.8 (a) Schematic illustration of in situ Bi
2
Te
3
‐PEDOT:PSS hybrid nan...
Figure 5.9 (a) SEM image of PVDF/Ni nanowires hybrid film with 80-wt% Ni. (b...
Figure 5.10 (a–c) SEM images of cross sections along out‐of‐plane direction ...
Figure 5.11 (a)
σ
, (b)
S
, and (c) PF of the CuPc/SWCNTs and CuPcI/SWCNT...
Figure 5.12 (a) Schematic diagram of preparation process of PANI/GP/PANI/DWC...
Figure 5.13 (a) Seebeck coefficient and (b) electrical conductivity of rGO
x
/...
Figure 5.14 SEM images of (a) MWCNT bundles and (b) graphene platelets. TEM ...
Figure 5.15 (a) Seebeck coefficient and (b) electrical conductivity of PEDOT...
Figure 5.16 Schematic illustration of the DOS for polymer composites. Upper ...
Figure 5.17 Illustrative sketch of the continuous assembly line for a p–n ju...
Figure 5.18 (a) Redox reaction mechanism of Te
IV
with EDOT. (b) Schematic il...
Figure 5.19 (a) Schematic diagram of structure and preparation process and t...
Figure 5.20 (a) The
σ
and
S
of Te‐PEDOT:PSS nanowires are plotted as a ...
Figure 5.21 (a) Schematic illustration of π‐conjugative...
Figure 5.22 Schematic diagram for the preparation of PEDOT:PSS/Bi
2
Te
3
hybrid...
Chapter 6
Figure 6.1 Schematic diagram of thermodiffusion. (a) particles without charg...
Figure 6.2 Illustration of the working principle of a full HCCD cycle for IT...
Figure 6.3 Schematic of Ionic charge transport mechanisms. (a) Grotthuss mec...
Figure 6.4 The illustration of the ionic Seebeck coefficient measurement set...
Figure 6.5 Chemical structures of typical molecules used as organic i‐TE mat...
Figure 6.6 Chemical structures of typical cations and anions that are common...
Figure 6.7 (a) Chemical structures of WPU and EMIM:DCA. (b) Schematic prepar...
Figure 6.8 Material classes of OMIECs containing CPs. (a) Heterogeneous blen...
Figure 6.9 (a) Digital image of the oxidized, aligned cellulosic membrane. (...
Figure 6.10 (a) Schematic illustration of cutting‐healing process for PANI:P...
Figure 6.11 (a) Schematic diagram of the ionic thermoelectric‐gated inverter...
Figure 6.12 (a, b) Pressure sensing. (a) The resistance of the sensor has a ...
Figure 6.13 (a) Schematic of molecule structure of COF fabricated with Tp an...
Chapter 7
Figure 7.1 The typical doping methods of OSCs for TE application: chemical d...
Figure 7.2 The schematic illustration of chemical doping engineering in OSCs...
Figure 7.3 Schematic of doping mechanisms of OSCs including charge transfer ...
Figure 7.4 Molecular structures and corresponding LUMO or HOMO levels of typ...
Figure 7.5 Molecular structures of typical p‐type dopants.
Figure 7.6 Molecular structures of typical n‐type dopants.
Figure 7.7 Schematic diagram of typical doping methods [2].
Figure 7.8 Schematic illustration of (a) anion‐exchange doping. (b) Molecula...
Figure 7.9 (a) Mechanism of Bi interfacial doping of TDPPQ; (b) The
σ
o...
Figure 7.10 (a) Schematic view of the OECT‐based thermoelectric device with ...
Figure 7.11 (a) Device structure of OFET‐based organic thermoelectric device...
Figure 7.12 Mobilities of carriers introduced by field‐effect (filled square...
Figure 7.13 (a)
S
(filled symbols) and PF (open symbols) as a function of
σ
...
Figure 7.14 (a) The structure of MEH‐PPV photo‐thermoelectric device; (b) Sc...
Figure 7.15 (a) Schematic illustration of the NDI(2OD)(4
t
BuPh)‐DTYM2‐based o...
Figure 7.16 Three patterns of dopants distribution within the microstructure...
Figure 7.17 Schematic illustration of regulating DOS distribution in composi...
Figure 7.18 The strategies to enhance the stability of doped OTE materials, ...
Figure 7.19 The concept of homogeneous doping of OSCs: (a) The XPS depth‐pro...
Figure 7.20 The typical doping methods and key aspects of TE‐oriented doping...
Chapter 8
Figure 8.1 Module structures of (a) OTE‐based generator and (b) Peltier cool...
Figure 8.2 Schematic illustration of OTE devices with (a) vertical, (b) hori...
Figure 8.3 (a) Device geometry of poly(Ni‐ett) based OTE prototype consistin...
Figure 8.4 Characteristics of output power in a logic circle. (a) Electric l...
Figure 8.5 (a) Schematic illustration of inkjet printing technique, and (b) ...
Figure 8.6 Schematic illustration of R2R printing of a flexible OTE device [...
Figure 8.7 Schematic illustration of continued deposition of OTE prototype v...
Figure 8.8 Schematic diagram showing various wearable sensors and correspond...
Figure 8.9 (a) Illustration of device fabrication on flexible polyethylene n...
Figure 8.10 Schematic illustration of fabric OTE generators, which are desig...
Figure 8.11 (a) Neat (left) and PEDOT:PSS dyed silk yarns submerged for one-...
Figure 8.12 (a) Schematic image of a 3D fabric TEG and (b) Photograph of cor...
Figure 8.13 Strategies summarized for stretchable OTE materials and devices....
Figure 8.14 (a) Molecular structure and solution process used for OTE compos...
Figure 8.15 (a) Stress–strain curve of a free‐standing PEDOT:PSS film withou...
Figure 8.16 The development of TE materials for Peltier cooling.
Figure 8.17 Schematic diagram of the Peltier effect. (a) Structure diagram a...
Figure 8.18 (a) Photograph of flexible poly(Ni‐ett) films on suspended paryl...
Figure 8.19 (a) Photograph of Te‐nanowire/P3HT composite‐based self‐powered ...
Figure 8.20 Basic mechanism of OTE‐based photo response.
E
F
is the Fermi lev...
Figure 8.21 (a) Schematic illustration of a PEDOS‐C6‐based NIR detection via...
Figure 8.22 (a) Schematic diagram of PTE devices under NIR (808-nm) laser ir...
Figure 8.23 (a) Device structure of CNT‐based THz sensor. (b) THz imaging of...
Figure 8.24 (a–d) Schematic illustration of the MFSOTE sensing devices [10]....
Figure 8.25 (a) Schematic illustration of ionic thermoelectric gated P3HT in...
Figure 8.26 (a) Schematic illustration of electronic and ionic transfer in h...
Chapter 9
Figure 9.1 Schematic illustration of research topics in single‐molecule ther...
Figure 9.2 (a) Schematic of Seebeck effect in molecular junctions. The tempe...
Figure 9.3 (a) Top: schematic of the CSW‐479 molecule with a side group whic...
Figure 9.4 Schematics of experimental techniques for measuring single‐molecu...
Figure 9.5 (a) Schematic of STM‐BJ based setup. The STM tip is kept at ambie...
Figure 9.6 (a) Schematic of MCBJ setup. Three‐point support configuration al...
Figure 9.7 (a) Schematic of the experimental arrangement: molecules are self...
Figure 9.8 (a) Schematic of a calorimetric scanning thermal microscopy (C‐ST...
Figure 9.9 (a) Schematic of experimental device. Molecule monolayer was firs...
Figure 9.10 (a) A series of photographs of the formation of a conical tip of...
Figure 9.11 (a) Magnified SEM image of a nanogap junction formed by an as‐fa...
Figure 9.12 (a, c) Applied tip displacement as function of time. When the ti...
Figure 9.13 (a) Schematic sketch of STSM circuits for measuring the graphene...
Figure 9.14 (a)
d
I
/d
U
spin mapping of the Fe/Ir (111) square Skyrmion lattic...
Figure 9.15 The progress on the experimental explorations of single‐molecule...
Figure 9.16 Schematic diagrams of (a) the modified MCBJ configuration and (c...
Figure 9.17 (a) The measured thermovoltage as a function of temperature diff...
Figure 9.18 (a) The measured thermovoltages from S1 to S4 are plotted as a f...
Figure 9.19 (a, b) Experimental thermopower values at contact for a single C
Figure 9.20 (a) Schematic diagram of the Au–TDO...
Figure 9.21 (a) Schematic diagrams of the modified STM‐BJ setup for conducta...
Figure 9.22 (a) Measured thiol‐ and isocyanide‐terminated molecules. (b) The...
Figure 9.23 (a) Relation between the measured Seebeck coefficients of substi...
Figure 9.24 (a) Schematic illustration of molecular junctions with Au electr...
Figure 9.25 (a, b) Conductance and thermopower simultaneously acquired while...
Figure 9.26 (a) The thermovoltage of Au–(
para
‐OPE3)–Au junction. (b) The the...
Figure 9.27 (a) Schematic illustration of destructive quantum interference. ...
Figure 9.28 Seebeck coefficient of the Au‐BPDT‐Au junction (a) and Au–C
60
–Au...
Figure 9.29 (a) Schematic diagrams of experimental configuration. Maps of (b...
Figure 9.30 (a) Experimental set‐up for quantifying heat transport in single...
Figure 9.31 (a) Schematic diagram of the experimental platform. (b) The phys...
Chapter 10
Figure 10.1 Illustration of measurement of electrical conductivity for infin...
Figure 10.2 Commonly used parallel electrode method for measuring the resist...
Figure 10.3 Impact of electrode and channel width to length ratio on the rel...
Figure 10.4 Some sample shapes for van der Pauw method measurement: (a) irre...
Figure 10.5 (a) Schematic diagram of Seebeck coefficient testing.
In situ
re...
Figure 10.6 Application of infrared thermometry in Seebeck coefficient testi...
Figure 10.7 Single point method (a) and quasi‐static method (b) to determine...
Figure 10.8 Effect of contact [13] (a, b) and electrode size [2] (c, d) on S...
Figure 10.9 Schematic of the (a) absolute and (b) comparative techniques for...
Figure 10.10 The time‐dependent temperature difference...
Figure 10.11 (a) The bifilar spiral structure of a hot‐disk sensor. (b) Cros...
Figure 10.12 The 3‐omega method. (a) Device structure [30] and (b) experimen...
Figure 10.13 Schematic illustration of the cross‐section for the geometry fo...
Figure 10.14 (a) Cross‐section of the sample, (b) In‐phase and out‐of‐phase ...
Figure 10.15 (a) Schematic of the out‐of‐plane and the in‐plane thermal cond...
Figure 10.16 (a) Schematic of a typical transient thermoreflectance setup. T...
Figure 10.17 Schematic diagram of (a) cross‐plane (b) in‐plane thermal diffu...
Figure 10.18 (a) Device structure for TFA measurement. (b) Chip with a shado...
Figure 10.19 (a) Temperature distribution of the two heated membranes with t...
Figure 10.20 Schematic illustration of (a) device structure and (b) transfer...
Figure 10.21 Principle of Hall effect.
Figure 10.22 Device structure of Hall bar method.
Figure 10.23 Summary of the commonly used electronic structure characterizat...
Figure 10.24 Schematic illustration of some of the important parameters deri...
Figure 10.25 (a) Seebeck coefficient versus conductivity curves and (b) corr...
Figure 10.26 Electronic structures of occupied and unoccupied states of C60‐...
Guide
Cover Page
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
Wiley End User License Agreement
Pages
iv
xiii
xiv
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
Organic Thermoelectrics
From Materials to Devices
Edited by Daoben Zhu
Editor
Prof. Daoben Zhu
CAS Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Zhongguancun North First Street 2
Beijing, 100190
China
Cover Image: © Daniel Grill/Getty
Images
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication Data
A catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d‐nb.de>.
© 2023 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐34985‐2
ePDF ISBN: 978‐3‐527‐83548‐5
ePub ISBN: 978‐3‐527‐83550‐8
oBook ISBN: 978‐3‐527‐83549‐2
Preface
With an increasing demand for electricity, a burning desire in scientific fields is to harvest electricity from renewable energy in a green route. Thermoelectric (TE) materials can satisfy this requirement not only because they can allow direct energy harvesting from waste heat and natural heat resources but also owing to the existence of huge energy in the form of waste heat, solar heat, and body heat in our daily life. The development of TE materials was initialed from the discovery of Seebeck effect in 1821 by Thomas J. Seebeck. Thereafter, a reverse energy conversion, namely, the Peltier effect, has been observed in 1834, which can enable both refrigerator and heating applications. After 200 years' evolution, an important trend in TE materials is to meet the increasing demand for distributed energy, which relies on the development of flexible and low‐cost devices. Organic materials possess excellent flexibility than conventional thermoelectric materials, which constitute key advantages in flexible applications. Moreover, benefitting from striking developments in organic electronics, organic materials have been widely characterized with unique features of fine‐tuned electrical properties via molecular design, solution processability, and lightweight. More importantly, their low intrinsic thermal conductivity offers the potential of possessing high thermoelectric performance, especially at a low temperature lower than 400 K. In combination with the attractive feature, scientists are focusing on developing state‐of‐the‐art organic thermoelectric (OTE) materials to open up new opportunities for thermoelectrics.
In the past 50 years, we have been engaged in the research of conductive molecular systems and have experienced the breeding and rapid development of OTE materials. As early as the 1980s, my group and collaborators began to use the Seebeck effect to study the basic scientific problem of charge transport in organic conductors. Since 2005, we carried out more comprehensive research on the thermoelectric properties and device functionalization of both conventional conducting polymer and novel high‐mobility organic semiconductors. Based on these research backgrounds, this book attempts to comprehensively summarize the latest research results in the field of OTE materials and expounds on the strategies and methods to improve TE performance. We hope this book can help researchers in relevant fields and promote the rapid development of OTE materials and devices.
The book is divided into 10 chapters. The first chapter is the introduction, which briefly describes the fundamentals of TE conversion and overviews the development of the OTE field. The second chapter gives a brief introduction to the mechanism and basic physical process in carrier transport and thermal transport for TE conversion. Chapters 3–5 summarize the recent development and key strategies to develop p‐type, n‐type, and composite/hybrid OTE materials. Chapter 6 introduces the fundamentals and the development of organic ionic TE materials and devices. Chapter 7 is an overview of the doping engineering toward high‐performance OTE materials, which introduces the basic mechanisms, fundamental requirements, and recent advances of doping for OTE applications. Chapter 8 summarizes the geometries and construction methods of OTE devices and introduces their applications in flexible generators, Peltier cooling elements, and multifunctional sensors. Chapter 9 simply introduces the theoretical and experimental understanding of single molecular TE devices, together with the recent development in related detection methods. Chapter 10 summarizes the principles and methods for measuring TE parameters of OTE materials and devices, providing the readers with the basic knowledge of current analysis techniques of OTE.
This book is mainly from the worldwide innovative research results in the past 10 years and has been strongly supported by many collaborators! We sincerely thank our authors for their contributions to the chapters! Thanks to the permission of many academic journals sponsored by the American Chemical Society, Springer Nature Publishing Group, the Royal Chemical Society, John Wiley & Sons, Elsevier, AIP Publishing, American Physical Society, American Association for the Advancement of Science, Chinese Chemistry Society, Multidisciplinary Digital Publishing Institute and other institutions. In the process of publishing this book, we have received strong encouragement from Wiley Press. I would like to express my heartfelt thanks for their enormous patience and support.
1 August 2022Beijing
Daoben Zhu
1Introduction of Organic Thermoelectrics
Yingqiao Ma, Ye Zou, Chong‐an Di, and Daoben Zhu
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Haidian District, Beijing, 100190, China
1.1 Brief Introduction and Historical Overview
In the past two decades, energy and environmental issues have become major challenges for human society. The development of a new category of green energy is then becoming an important direction to deal with these challenges. From the viewpoint of energy conversion, the efficiency is less than 40% in average, whereas the remaining energy is mainly lost in heat. Taking into consideration of energy resources, solar is an abundant energy resource, which can be converted into electricity by photovoltaic technology. Although the state‐of‐the‐art efficiency of photovoltaic cells has exceeded 30% [1], of the rest more than 60% cannot be converted into electricity in a simple way. From the perspective of new applications, the scientists began to pay attention to the distributed energy supply mode featured by portability to satisfy the growing requirements of wearable electronics. The utilization of environmental and even body heat is considered as a promising solution to satisfy such requirements. These joint demands indicate that the highly efficient application of ubiquitous heat energy is extremely important in the next 10–30 years, which may breed a new energy industry.
Thermoelectric (TE) materials can directly convert the heat into electricity or vice versa and provide a straightforward way to exploit the waste and natural heat energy. The so‐called thermoelectric conversion is resulting from the interference of electric current and heat flow in various materials. Basically, there are two kinds of TE devices (Figure 1.1), including thermoelectric power generator and thermoelectric cooler, which have no moving parts and require no maintenance. The power generator possesses a great potential for natural and waste heat recovery from solar, industrial facilities and various engines, thus providing reliable power in distributed and even remote areas, such as in deep space and mountaintop telecommunication sites. In addition, TE cooler offers a reverse route to enable refrigeration and temperature control in electronic packages and medical instruments.
Figure 1.1 Schematic diagrams of thermoelectric (a) power generator and (b) cooling device.
Source: Ye Zou.
To have a clear map of thermoelectric behavior, we start with a brief history of TE effects. In 1821, Thomas Johann Seebeck found that the temperature difference between two different metals led to the deflection of the magnetic needle [2]. Subsequent studies showed that the phenomenon was due to the potential difference caused by the temperature difference, which led to the change in current and magnetic field. The basic connotation of this experimental phenomenon constitutes the Seebeck effect. Thirteen years later, in 1834, Jean Charles Athanase Peltier discovered a reverse process – that the passage of electric current through a thermocouple produces heating or cooling effect depending on its direction [3]. This led to another TE effect, the Peltier effect, which can realize electric heating and cooling in a direct way. In 1854, William Thomson (Lord Kelvin) discovered that if a temperature difference exists in a current‐carrying conductor, heat is either liberated or absorbed depending on the direction of current and materials [4], which is supplementary to the Peltier heating. This is called the Thomson effect. Moreover, he analyzed the relationship between Seebeck effect and Peltier effect from a theoretical point of view. These three kinds of TE effects and the Joule heat effect constitute the physical basis of the TE conversion process.
In the first 100 years after the discovery of Seebeck effect, TE materials suffer from a slow development. The material category was limited to metals and are mainly used for temperature measurement thermocouples. In the mid‐twentieth century, with the emergence and rapid development of quantum mechanics and semiconductor theory, narrow‐band gap semiconductors represented by Bi2Te3 and PbTe have been developed. The TE performance began to improve rapidly, and the figure‐of‐merit ZT was close to 1.0 [5]. Although TE materials‐based power generator and solid‐state refrigerator begin to be applied, the theoretical model is not perfect, and the performance needs to be further optimized. Since the 1990s, the theoretical model of “phonon glass and electronic crystal” [6], the design strategy of low‐dimensional thermoelectric materials, and the cross‐scale control method of material structure have been put forward one after another, which promote the rapid development of the fundamental theories and material categories. The ZT value has broken 1.0 and enabled a variety of functional applications.
In the past decade, with the prospects of flexible devices in artificial intelligence, health monitoring, and Internet of things (IoTs), TE materials begin to show a variety of new development trends. The first trend is to continuously develop new generation of model materials and drive new breakthroughs in ZT; the second is to develop high‐performance low‐temperature flexible TE materials to expand the TE application in micro‐temperature difference power generator; the third is to give full play to the advantages of reversible energy conversion of TE materials and focus on solid‐state refrigeration with TE materials.
Organic material is assembled by weak intermolecular interactions, which endows them with unique optoelectronic functionalities. As far as TE conversion is concerned, organic thermoelectric (OTE) materials have many advantages, such as good flexibility, low intrinsic thermal conductivity, and excellent performance at room temperature [7–9]. They are expected to complement traditional inorganic TE materials and become a key element for the next‐generation flexible devices. Although OTE materials have attracted great attention in the past decade, it is not a new research direction. Several decades ago, scientists began to use Seebeck effect to investigate the charge transport property in molecular crystals. For one example, Prof. D. Zhu from Institute of Chemistry, Chinese Academy of Sciences, and Prof. D. Schweitzer in Germany studied the electrical properties of two‐dimensional organic conductor β‐(BEDT‐TTF)2BrI2 in 1986 [10]. They accurately measured the Seebeck coefficient at different temperatures and discovered the sign change behavior of Seebeck coefficient around 110 K, which revealed its excellent bipolar charge transport behavior (Figure 1.2). It is worth noting that the p‐type and n‐type power factors of this material at different temperatures reach 2.2 and 0.05 μW m−1 k−2, respectively. These early research progress laid the foundation for the rapid development of OTE materials in the past decade.
Figure 1.2 Temperature‐dependent thermopower curve of β‐(BEDT‐TTF)2BrI2.
Source: [10]. Reproduced with permission of Elsevier.
The recent rapid developments of OTE materials are inspired by the breakthrough in PEDOT and poly[Ni‐ett] [11–14]. For p‐type materials, PEDOT is the most widely studied conductive polymer, which shows a broad application potential in organic solar cells, transparent electrodes, and so on. Based on the previous research on conductive molecular system, Joo et al. in Korea University studied the influence of solvents on the electrical conductivity and Seebeck coefficient of PEDOT:PSS thin films, which was the early work to investigate the TE properties of the system [15]. Thereafter, Xu et al. investigated the TE properties of PEDOT:PSS films doped with DMSO and glycol, and reported a ZT value of 0.00175 [16]. In 2011, Crispin et al. studied the influence of doping levels to the TE properties of PEDOT:Tos. By precisely adjusting the oxidation level, the figure‐of‐merit of the film reached 0.25 [11]. Compared with p‐type materials, the development of n‐type OTE materials is more challenging. After several years of systematic research, Zhu et al. reported n‐type metal–organic complex OTE materials, namely, poly(Ni‐ett), in 2012. By using ion coordination to control the performance and polarity of the materials, the ZT value over 0.2 was obtained [12]. After that, polycrystalline thin films were prepared by electrochemical method, and the ZT value was further increased to 0.3 [14]. These breakthroughs in high‐performance p‐type and n‐type OTE materials have attracted increasing attention and directly promoted the rapid development of OTE materials and devices. More recently, increasing attention has been paid to design novel OTE molecules and combine conventional TE theory with organic materials, leading to tremendous progress in composite/hybrid OTE materials, chemical doping of OTE semiconductors, and multi‐functional OTE devices. These remarkable progresses constitute the current status of OTE field.
1.2 Thermoelectric Effect
The TE conversion is based on the diffusion transport of carriers. It includes three basic physical phenomena, namely, the Seebeck effect, the Peltier effect, and the Thomson effect. The three effects are called by a joint name the TE effect.
1.2.1 Seebeck Effect
The Seebeck effect, which is also known as the first TE effect, was firstly discovered in 1821 by the German scientist Thomas Johann Seebeck. As shown in Figure 1.3a, when two different wires “a” and “b” are connected at both ends to form a circuit loop, if a temperature difference is created between the two junctions W and X by heating one of the junction W, the carriers in both wires will move from the high‐temperature junction W to low‐temperature junction X, resulting in a potential difference ΔV between the two junctions and an electric current in the loop. Through the Seebeck effect, the temperature difference in environment can be directly converted into electricity, thus achieving the power generation by temperature difference. It is found that the magnitude of the thermoelectric potential difference is proportional to the temperature difference at the junctions. Assuming that the temperature at the hot junction W is Th and at the cold junction X is Tc, the potential difference ΔV between the two junctions can be expressed as:
where Sab is the differential Seebeck coefficient of the two materials. The direction of the potential difference ΔV is dependent on the properties of the two materials forming the loop and the direction of temperature difference. For instance, suppose the temperature at junction W is higher than junction X (Figure 1.3a), if the current generated by the Seebeck effect is in the clockwise direction from “a” to “b,” then the loop a‐b has a positive differential Seebeck coefficient. The Seebeck coefficient is usually measured in μV K−1. It should be noted that the Seebeck coefficient has also been called the thermoelectric power or the thermal electromotive force (EMF) coefficient.
Figure 1.3 Basic principle of (a) Seebeck effect and (b) Peltier effect.
Source: Ye Zou.
The Seebeck effect introduced above is based on two different wires forming a current loop. For a homogeneous material, the absolute Seebeck coefficient at temperature T is defined as:
The relationship between the differential Seebeck coefficient Sab in the loop and the absolute Seebeck coefficients (Sa and Sb) of the two materials can be illustrated as: Sab = Sa − Sb.
The absolute Seebeck coefficient is independent of the direction of temperature difference but is only determined by the property of the thermoelectric material itself. For any TE materials, the carriers (both the holes and electrons) move from hot end to cold end under the temperature difference. However, due to the different sign of carriers, the direction of thermal EMF of p‐type materials (holes are majority carriers) is from the hot end to the cold end, resulting in a positive absolute Seebeck coefficient. On the contrary, the direction of thermal EMF of n‐type materials (electrons are majority carriers) is from the cold end to the hot end, resulting in a negative absolute Seebeck coefficient. The absolute Seebeck coefficient is more commonly used than the differential Seebeck coefficient in assessing TE material property. In the following part of this book, the Seebeck coefficient is referred to the absolute Seebeck coefficient unless otherwise specified.
1.2.2 Peltier Effect
The Peltier effect, also known as the second thermoelectric effect, is physically an opposite process of the Seebeck effect and was first discovered by French scientist Jean Charles Athanase Peltier in 1834. The Peltier effect can basically convert electricity directly into temperature difference, which therefore has great application in thermoelectric cooling or heating (heat pumping). As shown in Figure 1.3b, when an electrical current passes through a loop composed of two different wires “a” and “b,” a small change in heat arises at both the two junctions W and X in addition to the Joule heating by resistance. Thereby, the two junctions appear to absorb or liberate heat creating a cooling or heating effect, respectively, related to the direction of current flow. The Joule heating is irreversible, while Peltier heating or cooling is reversible between heat and electricity without a loss of energy. The magnitude of the heat absorbed or liberated is proportional to the current flow. Assuming that the current I clockwise flows from “a” to “b,” the heat absorbed or liberated per unit time at junction X is:
where Πab is the differential Peltier coefficient and the sign of Πab is negative if the clockwise current I leads to a heat liberation at junction W and a heat absorption at junction X. Therefore, the way in which heat is exchanged, either heat absorption or liberation, at the junctions is related to the properties of the two materials and the current direction. The Peltier coefficient is usually measured in volts (V).
The Peltier effect stems from the different potential of electrical charge carriers in different materials. In detail, when current flows across a junction between two different wires, energy exchange occurs due to different potential energy of carriers, leading to cooling or heating effect at the junction. For instance, if electrons move from the high‐energy level material to low‐energy level material, they will release energy and the junction will exhibit the heat liberation effect on the macroscopic level. On the contrary, if electrons move from the low‐energy level material to high‐energy level material, they will absorb energy and the junction will show the heat absorption effect.
Similar with the case of the Seebeck coefficient, the relationship between the differential Peltier coefficient Πab at the junction and the absolute Peltier coefficients (Πa and Πb) of the two materials forming the loop follows: Πab = Πa − Πb. The absolute Peltier coefficient is independent of the current direction and is only determined by the properties of the material itself. The reported Peltier coefficient of organic material is in the order of tens of mV [17].
1.2.3 Thomson Effect
In 1855, the British scientist William Thomson (later Lord Kelvin) established the relationship between the Seebeck coefficient and Peltier coefficient by analyzing the thermoelectric effect in a homogeneous material with thermodynamic theory. His research also indicated that a third thermoelectric effect existed in the homogeneous material, which is later known as the Thomson effect. In detail, when current I flows through a homogeneous conductor with a temperature gradient (ΔT), in addition to produce Joule heat, the conductor needs to absorb or release heat in order to maintain the original temperature gradient ΔT. This effect was later successfully and experimentally verified by other scientists in 1867. The rate of Thomson heat absorption or release (dQ/dt) across the homogeneous conductor is:
where β is the Thomson coefficient with a unit in V K−1. If the direction of current coincides with the temperature gradient and the conductor absorbs heat, then the Thomson coefficient is positive.
Both the Seebeck effect and the Peltier effect are discovered involving the formation of current loops in which two wires are joined together. They originated from the energy difference of carriers in different materials. Nevertheless, the Thomson effect is a phenomenon that occurs in homogeneous materials, i.e. the carriers in a homogeneous material have different energies at different temperatures, resulting in heat exchange when the carriers transport through a temperature gradient. In the design of thermoelectric devices, the Thomson effect is usually ignored because of its relatively small contribution compared with the Peltier effect to thermoelectric energy conversion.
The interrelationship between the three thermoelectric parameters, namely, the Seebeck coefficient (S), the Peltier coefficient (Π), and the Thomson coefficient (β), thermodynamically derived by Thomson is expressed as:
These two equations later came to be known as the Kelvin relationship. Their rigorous derivation requires the use of non‐reversible thermodynamic theory. The Kelvin relationship is important in understanding the basic phenomena, which indicates that the values of β and Π can be calculated directly from S. These three parameters are important for characterizing the thermoelectric properties of TE materials, where S and Π are widely used to evaluate the ability of thermoelectric power generation and thermoelectric cooling, respectively.
1.2.4 Other Related Effects
When a charge carrier is moving in the direction perpendicular to a magnetic field, the moving charge will be deflected by the Lorentz force of the magnetic field. Similar to the effect of magnetic field on charge transport property, the thermoelectric effects also become changed under perpendicular magnetic field, resulting in some new phenomena, which are called as the thermogalvanomagnetic effects or the transverse magnetothermoelectric effects. Generally speaking, the impact of magnetic field on the Seebeck effect and the Peltier effect can be obvious only when the applied magnetic field is strong and the carrier mobility of the material is high.
Figure 1.4 summarizes the Hall effect and the other three effects of magnetic field on thermoelectric properties, which correspond to the phenomena of generating a new electric field or a new temperature gradient under a current flow or a temperature gradient in a perpendicular magnetic field. In detail, in the spatial three‐dimensional coordinate, when an isotropic sample is subjected to a uniform magnetic field Bz with direction along the longitudinal axis (z), by applying a current Ix or a temperature gradient dT/dx to the sample along the horizontal axis (x), a new electric field Ey or a new temperature gradient dT/dy in the vertical axis (y) direction will be generated under the transverse force of magnetic field Bz.
Figure 1.4 Schematic illustration of Hall effect and the other three effects of transverse magnetic field on thermoelectric properties: (a) Hall effect, (b) Nernst effect, (c) Ettingshausen effect, and (d) Righi–Leduc effect.
Source: Ye Zou.
The charge deflection direction in a perpendicular magnetic field can be determined by Fleming's left‐hand rule. For the well‐known Hall effect, as illustrated in Figure 1.4a, if the longitude current density is Ix, the perpendicular magnetic field is Bz, and the transverse generated electric field strength is Ey = dV/dy, then the Hall coefficient RH can be expressed as: . The Hall effect is not directly related to the energy conversion, but it is an extremely important and effective method for investigating the charge carrier transport behavior. The Hall effect will be further introduced in Section 10.6.2.
Among different transverse magnetothermoelectric effects, the Nernst effect and the Ettingshausen effect are more directly relevant to the TE energy conversion. The Nernst effect is somewhat similar to the Hall effect in that when a perpendicular magnetic field is applied in the direction of the sample temperature gradient or heat flow, an electric field then generally appears in a direction perpendicular to both the temperature gradient (dT/dx) and the magnetic field (Bz) (Figure 1.4b). The Nernst coefficient N is determined by the relationship: , where dV/dy is the generated transverse electric field. The sign of Nernst effect is illustrated in Figure 1.4b. The sign of the Hall effect is related to the polarity (sign) of the carriers, while the sign of the Nernst effect does not depend on the carrier positivity or negativity but is only related to the temperature gradient and the magnetic field direction.
The Ettingshausen effect refers to the phenomena that a transverse temperature gradient appears in the direction perpendicular to the orthogonal current flow and magnetic field (Figure 1.4c). The relationship between the Ettingshausen effect and the Nernst effect is similar to that between the Peltier effect and the Seebeck effect. They differ in that the temperature difference in the Ettingshausen effect or the Nernst effect is perpendicular to the electric field, while the temperature difference in the Peltier effect or the Seebeck effect is parallel to the electric field. The Ettingshausen coefficient P is defined by: , where dT/dy is the resulting vertical transverse temperature gradient. The relationship between the Nernst coefficient and Ettingshausen coefficient is Pκ = NT, where κ is the thermal conductivity.
In addition, there is another transverse magnetothermoelectric effect, namely the Righi–Leduc effect (Figure 1.4d). The Righi–Leduc effect is the phenomenon that a transverse temperature gradient (dT/dy) arises in the vertical direction when heat (dT/dx) flows across a vertical magnetic field (Bz). The Righi–Leduc coefficient M is given by: .
The magnetothermoelectric effect can be applied for a new type of energy conversion. Although the transverse magnetothermoelectric effect has not yet been widely applied in practice, the Ettingshausen effect has potential advantages compared with the Peltier effect for thermoelectric cooling, and the Nernst effect also has some unique advantages compared with the Seebeck effect for thermal radiation detection. For example, in cooling application, the heat source and heat sink of the Ettingshausen device are in direct contact with the side face of magnetothermoelectric material, while the heat source and heat sink of the Peltier device are in electrical contact.
1.3 Thermoelectric Parameters
1.3.1 Basic Parameters
The basic parameters associated with thermoelectric power generation (Seebeck effect) are the Seebeck coefficient, electrical conductivity, and thermal conductivity. As described in Section 1.2.1, the Seebeck coefficient is used to characterize the magnitude of Seebeck effect and is expressed as S = ΔV/ΔT, where ΔT is the temperature difference and ΔV is the corresponding potential difference under the temperature difference. The Seebeck coefficient is measured in V K−1 or μV K−1.
The electrical conductivity (σ), which is the reciprocal of resistivity (ρ), is used to describe the ability of electrical transport. For isotropic solid materials, the resistivity is determined by the equation ρ = (U/I)(A/L), where A and L are the cross‐sectional area and length of the sample, respectively; I is the current flowing through the cross‐sectional area, and U is the potential difference of the sample. The electrical conductivity is then determined by σ = 1/ρ = (I/U)(L/A) and is usually measured in S m−1 or S cm−1.
The thermal conductivity (κ) reflects a fundamental ability to transfer heat through a material by conduction. It is defined as the heat transfer per unit area of a specimen under per unit temperature difference and per unit time, and thereby is given by the formula: κ = (Q/t)(L/(AΔT)), where ΔT is the temperature difference, t is the time, and Q is the heat transferred. The thermal conductivity is mostly composed by the carrier contribution (carrier thermal conductivity, κc) and the lattice contribution (lattice thermal conductivity, κL), which can be described as: κ = κc + κL. The thermal conductivity is measured in W m−1 K−1.
The basic parameters associated with thermoelectric cooling (Peltier effect) are the Peltier coefficient, electrical conductivity, and thermal conductivity. As described in Section 1.2.2, the Peltier coefficient is used to characterize the magnitude of the Peltier effect and is expressed as , where dQ/dt is the heat absorbed or liberated per unit time and I is the current. The Peltier coefficient is measured in V or mV.
1.3.2 Power Conversion Efficiency and TE Figure‐of‐Merit
There are two common types of thermoelectric devices based on their operational modes, namely, the power generation devices and cooling devices. They have similar structure but opposite energy conversion process. Figure 1.1 shows their operating principles. As illustrated previously, thermoelectric devices usually consist of p‐type and n‐type thermoelectric elements, which are connected in series to form the basic unit of a thermoelectric circuit loop. In practical application, multiple p‐type and n‐type units are connected alternately to form a thermoelectric module which makes heat flow in parallel and current flow in series, thereby effectively increasing the open‐circuit voltage of power generator or cooling capacity of refrigerator.
1.3.2.1 Power Conversion Efficiency
In Figure 1.1a, when the p‐type and n‐type thermoelectric legs are connected to form a unit thermocouple for a power generator and a temperature difference ΔT is created between the hot and cold ends, then a thermopower will be generated between the p‐type and n‐type legs at the two cold ends and a current will appear in the circuit loop. In this way, the thermoelectric materials can realize the TE energy conversion. Power conversion efficiency is the most important index to evaluate the performance of thermoelectric devices. For thermoelectric power generation devices, the power conversion efficiency η or the thermal efficiency is the ratio of the power output over the heat absorbed at the hot end. For power generation device with homogeneous legs shown in Figure 1.1a, assuming that the thermocouple legs transport coefficients σ, S, and κ are temperature independent, the thermoelectric power conversion efficiency can be expressed as:
where P is the power output to the load RL and Qh is the absorbed heat at the source. If the current in the loop is I, then the power output in Figure 1.1a can be defined by the external load resistance as:
Since the Seebeck voltage V in the loop is V = SΔT = S(Th − Tc), the electrical current flow through the load, and the power output can then be described as:
In the power generation device, assuming there is no heat transport at the heat sink other than through the two thermocouple legs and neglecting the Thomson effect and the lateral heat loss, then the total heat transfer from heat source to sink, which is composed of absorbed heat at the hot end Qh and half the overall Joule heating travels to each of the ends, is balanced to the thermal conduction along the thermocouple legs and the Peltier cooling associated with the current flow. Therefore, the heat absorbed at the hot end with temperature Th can be expressed as:
where S is the total Seebeck coefficient (absolute value) of the p‐type and n‐type thermoelectric elements, Th and Tc are the temperatures at the ends of heat source and heat sink, respectively, R is the internal total electrical resistance, and K is the internal total thermal conductance of the p‐type and n‐type thermocouple legs of the device, and ΔT = Th − Tc is the temperature difference between the hot and cold ends. R and K can be expressed as:
and
Here, ρ and κ are the resistivity and thermal conductivity of the thermocouple elemental material, and A and L are the cross‐sectional area and length of the thermocouple legs. The subscripts p and n represent the p‐type or n‐type thermocouple legs. The first, second, and third terms on the right side of Eq. (1.11) are the Peltier cooling associated with the current flow, half the overall Joule heating travels to each of the ends, and thermal conduction along the thermocouple legs, respectively. Therefore, the power conversion efficiency η can be expressed as:
Defining the value of Z as Z = S2/KR, the above Eq. (1.14) can be expressed as:
Formula 1.15 indicates that the power conversion efficiency η varies with the ratio of RL/R in addition to the properties of the TE materials themselves and the temperature difference. The maximum power conversion efficiency ηmax can be obtained by differentiating the power conversion efficiency η in Eq. (1.15) with respect to the ratio of the load resistance to the internal resistance and setting it to zero. The result yields a relationship of . Then, the maximum power conversion efficiency ηmax is:
