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Investigation of Polymer Based Materials in Thermoelectric Applications
Von der Fakultät für Elektrotechnik und Informationstechnik
der Technischen Universität Chemnitz
genehmigte
DISSERTATION
zur Erlangung des akademischen Grades
Doktoringenieur
(Dr.–Ing.)
vorgelegt
von : M. Sc. Jinji Luo
geboren am : 20. Sep 1985
in : Fujian, V. R. China
eingereicht am : 10. November 2014
Gutachter : Prof. Dr. Thomas Geßner
Prof. Dr. Carsten Deibel
Tag der Verteidigung 19. May 2015
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Bibliographic Description
Investigation of Polymer Based Materials in Thermoelectric Applications
Luo, Jinji - 117 pages, 72 Figures, 11 Tables, 148 References
Chemnitz University of Technology, Faculty of Electrical Engineering and Information Technology
Dissertation, 2014
Abstract
With the advancements in the field of wireless sensor networks (WSNs), more and more applica-
tions require the sensor nodes to have long lifetime. Energy harvesting sources, e.g. thermoelectric genera-
tors (TEGs), can be used to increase the lifetime and capability of the WSNs. Integration of energy harvest-
ers into sensor nodes of WSNs can realize self powered systems, providing the possibility for maintenance
free WSNs. TEGs can convert the existing temperature differences into electricity. The efficiency of TEGs is
directly related to the dimensionless figure of merit (ZT) of materials, which is given as 𝑍𝑇 = 𝜎𝑆2𝑇/𝑘,
where 𝜎 is the electrical conductivity, 𝑆 is the Seebeck coefficient, 𝑘 is the thermal conductivity, T is the
temperature and 𝜎𝑆2 is the power factor. Traditional thermoelectric (TE) materials are based on inorganic
materials, of which the thermal conductivity is high. Over the past decade, the use of nanostructuring tech-
nology, e.g. superlattice, could decrease the thermal conductivity in order to enhance the efficiency of TE
materials. However, the high cost and the rigidity of inorganic TE materials are limiting factors. As alterna-
tives, polymer based materials have become the research focus due to their intrinsic low thermal conductivi-
ty, high flexibility and high electrical conductivity. Moreover, polymer based materials could be fabricated in
solution form, giving the possibility for employing printing techniques hence a decrease in the production
cost.
Unlike the typical approach, in which secondary dopants are added into PEDOT:PSS solutions to
modify the power factor of polymer films, this thesis is focused on a more efficient method to improve TE
properties. This thesis demonstrates for the first time that post treatment of PEDOT:PSS films with the sec-
ondary dopant DMSO as the medium results in a much larger power factor than the traditional addition
method. The post treatment method also avoids the usually required mixing step involved in the addition
method. Different solvents were selected to discuss the impact factors in the modification of the power factor
by this post treatment approach. The post treatment of PEDOT:PSS films was then extended to utilize a
green solvent EMIMBF4 (an ionic liquid) as the medium. EMIMBF4 is found to exchange ions with
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PEDOT:PSS films. As a result, the EMIM+ cations remain in the films and reduce the oxidation level of
PEDOT chains, which affects the Seebeck coefficient and the electrical conductivity.
Furthermore, TE materials based on hybrid composites with polymer as the matrix and Te
nanostructures as the nanoinclusions were investigated. This thesis successfully developed a green synthesis
method to obtain Te nanostructures, in which a non toxic reductant and a non toxic Te sources were used.
Well controlled Te nanostructures including nanorods, nanowires and nanotubes were synthesized by wet
chemical and hydrothermal synthesis. Those as synthesized Te nanowires were then integrated into
PEDOT:PSS solution for composite films fabrication. A high Seebeck coefficient up to 200 𝜇𝑉/𝐾 was ob-
served in the composite film.
Keywords
thermoelectric, power factor, PEDOT:PSS, secondary dopant, post treatment, organic solvent, ionic liquid,
tellurium, nanostructure, composite
v
Referat
Mit den Weiterentwicklungen der Drahtlosen Sensornetzwerke (engl. WSN, wireless sensor net-
works) stellen immer mehr Anwendungen die Forderung einer langen Lebensdauer der Sensorknoten. Ener-
giegewinnungssysteme (engl. Energy Harvesters) wie z.B. thermoelektrische Generatoren (TEGs) können
genutzt werden, um die Lebensdauer und Leistungsfähigkeit der WSN zu steigern. Mit der Integration von
Energy Harvesters können WSN ohne äußere Stromversorgung realisiert und somit die Möglichkeit zur War-
tungsfreiheit geschaffen werden. TEGs liefern Energie durch die Umwandlung einer Temperaturdifferenz in
Elektrizität. Die Effektivität der TEG ist direkt verbunden mit der Material-Kennzahl ZT und ist gegeben
durch 𝑍𝑇 = 𝜎𝑆2𝑇/𝑘, wobei 𝜎 die elektrische Leitfähigkeit ist, 𝑆 der Seebeck Koeffizient, 𝑘 die thermische
Leifähigkeit, T die Temperatur und 𝜎𝑆2 der Leistungsfaktor. Herkömmliche thermoelektrische (TE) Mate-
rialien basieren auf anorganischen Materialien, von denen die thermische Leitfähigkeit hoch ist. Im Laufe
des letzten Jahrzehnts konnte durch den Einsatz der Nanostrukturierung die thermische Leitfähigkeit verrin-
gern werden um damit die Effizienz von TE-Materialien zu steigern. Die Steifigkeit dieser Materialien ist ein
anderer Aspekt. Als Alternative für anorganische TE Materialien sind Polymer basierte TE Materialien zum
Fokus der Forschung geworden aufgrund einer intrinsisch niedrigen thermischen Leitfähigkeit, hohen Flexi-
bilität und hohen elektrischen Leitfähigkeit. Des Weiteren können diese Polymere in gelöster Form verarbei-
tet werden, was die Möglichkeit für den Einsatz von Drucktechnologien und damit geringeren Produktions-
kosten gibt.
Anders als der herkömmliche Ansatz den Leistungsfaktor der Polymerfilme durch die Ergänzung
von sekundären Dotanten in PEDOT:PSS Lösungen zu verändern, wurde in dieser Arbeit eine effizientere
Methode zur Verbesserung der TE Eigenschaften gesucht. In dieser Arbeit wird zum ersten Mal gezeigt, dass
die Nachbehandlung von PEDOT:PSS Schichten mit sekundären Dotanten Dimethylsulfoxid (DMSO) als
Medium der Nachbehandlung zu einem viel höheren Leistungsfaktor führt als bei der Zugabemethode und
außerdem die sonst erforderliche Mischprocedur vermeidet. Es wurden verschiedene Lösungsmittel ausge-
wählt um die Einflussfaktoren bei der Modifikation des Leistungsfaktors durch die Nachbehandlung von
Polymerschichten zu diskutieren. Die Nachbehandlung von PEDOT:PSS Schichten wurde nachfolgend er-
weitert um das umweltfreundliche Lösungsmittel EMIMBF4 (eine ionische Flüssigkeit) als das Medium ein-
zusetzen. EMIMBF4 ist bekannt für den Austausch von Ionen mit PEDOT:PSS Schichten, so dass EMIM
Kationen in der Schicht verbleiben, die Oxidationsstufe der PEDOT-Ketten senken und damit den Seebeck-
Koeffizient und die elektrische Leitfähigkeit beeinflussen.
Des Weiteren konzentriert sich diese Arbeit auf TE Materialien basierend auf Kompositen aus
Polymeren mit Nanoeinlagerungen. Erfolgreiche Syntheseansätze wurden für Tellur-Nanostrukturen entwi-
ckelt, bei denen keine giftigen Reduktionsmittel und keine giftigen Tellur-Quellen zur Verwendung kamen.
Es erfolgte die Erzeugung von kontrollierten Tellur-Nanostrukturen, einschließlich Nanostäben, Nanodrähten
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und Nanoröhren, mit nass-chemischer und hydrothermaler Synthese. Die so hergestellten Nanodrähte wur-
den dann in PEDOT:PSS Lösungen integriert für die Herstellung von Komposite-Schichten. Dabei konnte
ein hoher Seebeck-Koeffizienten, bis zu 200 𝜇𝑉/𝐾, festgestellt werden.
Stichworte
thermoelektrisch, Leitungsfaktor, PEDOT:PSS, sekundäre Dotanten, Nachbehandelung, organische Lö-
sungsmittel, ionische Flüssigkeit, tellurium, Nanostruktur, Komposite
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Content Abbreviations ........................................................................................................................... 9
Symbols ................................................................................................................................... 11
1 Introduction .................................................................................................................. 13
2 Fundamentals of Thermoelectrics ............................................................................... 17
2.1 Thermoelectric Generation and Refrigeration ................................................................................................. 18
2.2 Conflicting Thermoelectric Material Properties .............................................................................................. 22
2.3 Overview of Thermoelectric Materials ............................................................................................................ 25
2.3.1 Traditional Inorganic Thermoelectric Materials .................................................................................. 25
2.3.2 Polymer Based Thermoelectric Materials ........................................................................................... 28
3 Characterization Techniques ....................................................................................... 35
3.1 Electrical Conductivity .................................................................................................................................... 35
3.2 Seebeck Coefficient ......................................................................................................................................... 36
3.3 Thermal Conductivity ...................................................................................................................................... 38
3.4 Atomic Force Microscopy ............................................................................................................................... 40
3.5 UV-Vis Spectroscopy ...................................................................................................................................... 41
3.6 Raman Spectroscopy ....................................................................................................................................... 42
3.7 X-ray Diffraction ............................................................................................................................................. 42
3.8 X-ray Photoelectron Spectroscopy .................................................................................................................. 43
3.9 Transmission Electron Microscopy ................................................................................................................. 44
4 Thermoelectric Properties of PEDOT:PSS by the Addition and Post Treatment with Organic Solvents ........................................................................................................... 45
4.1 The Role of Organic Solvents .......................................................................................................................... 45
4.2 Influence of DMSO Addition and DMSO Post Treatment .............................................................................. 46
4.2.1 Sample Preparation and Materials Characterization ............................................................................ 46
4.2.2 Effect of DMSO Addition and Post Treatment ................................................................................... 47
4.3 Influence of Post Treatment with Different Organic Solvents ......................................................................... 55
4.3.1 Sample Preparation and Materials Characterizations .......................................................................... 55
4.3.2 The Correlation between the Post Treatment Medium and the Thermoelectric Properties ................. 56
4.4 Conclusions ..................................................................................................................................................... 62
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5 Thermoelectric Properties of PEDOT:PSS Films by the Application of Ionic Liquid 63
5.1 Introduction to Ionic Liquids (ILs) .................................................................................................................. 63
5.2 Sample Preparation and Materials Characterization ........................................................................................ 64
5.3 Influence of EMIMBF4/DMSO Mixtures on the Thermoelectric Properties ................................................... 66
5.4 Physical and Chemical Properties of Films Modified with EMIMBF4/DMSO Mixtures ................................ 67
5.5 Conclusions ..................................................................................................................................................... 73
6 Hybrid Composites of Polymer with Nanoinclusions ................................................ 75
6.1 Synthesis and Morphology of Te Nanostructures ............................................................................................ 77
6.1.1 Wet Chemical Synthesis of Tellurium Nanostructures ........................................................................ 77
6.1.2 Hydrothermal Synthesis of Tellurium Nanostructures ........................................................................ 77
6.1.3 Characterization of Tellurium Nanostructures .................................................................................... 78
6.1.4 Morphology of Tellurium Nanostructures ........................................................................................... 79
6.2 PEDOT:PSS/Te nanostructures Hybrid Composites ....................................................................................... 86
6.2.1 Preparation of Hybrid Composites ...................................................................................................... 86
6.2.2 Thermoelectric Properties of Hybrid Composites ............................................................................... 87
6.2.3 Conclusions ......................................................................................................................................... 90
7 Summary and Outlook ................................................................................................. 93
7.1 Summary ......................................................................................................................................................... 93
7.2 Outlook ............................................................................................................................................................ 95
Appendix ................................................................................................................................. 96
A.1 Reduction Ability of LAC/MEA Mixture ............................................................................................................. 96
A.2 JCPDS Patern with Cu-Kα and Co-Kα as Radiation Sources ............................................................................... 98
A.2.1 JCPDS 00-036-1452 Card with Cu-Kα as Radiation Sources ............................................................... 98
A.2.2 JCPDS 00-036-1452 Card with Co-Kα as Radiation Sources ............................................................... 99
Bibliography ......................................................................................................................... 101
List of Figures ....................................................................................................................... 105
List of Tables ........................................................................................................................ 108
Versicherung......................................................................................................................... 109
Theses .................................................................................................................................... 110
Curriculum Vitae ................................................................................................................. 113
Publications........................................................................................................................... 115
Acknowledgements............................................................................................................... 116
Abbreviations
AFM Atomic Force Microscopy
CNT Carbon Nanotube
COP Coefficient of Performance
CP Coordination Polymer
CSA Camphorsulfonic acid
DEG Diethylene Glycol
DOS Density of State
DMSO Dimethyl Sulfoxide
EDOT 3,4-Ethylene Dioxythiophene
EG Ethylene Glycol
𝐸𝑀𝐼𝑀𝐵𝐹4 1-Ethyl-3-Methylimidazolium Tetrafluoroborate
EMIMTCB 1-Ethyl-3-Methylimidazolium Tetracyanoborate
Ett 1,1,2,2,-ethenetetrathiolate
GO Graphite Oxide
HOMO Highest Occupied Molecular Orbital
HRTEM High Resolution Transition Electron Microscope
IL Ionic liquid
ITO Indium tin oxide
LAC L-ascorbic acid
LUMO Lowest Unoccupied Molecular Orbital
MBE Molecular beam epitaxy
MEA Ethanolamine
NIR Near Infrared
PAC Polyacetylene
PANI Polyaniline
PEDOT:PSS Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)
PGEC Phonon Galss Electron Crystal
PHT Polythiophene
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PPY Polypyrrole
PSS Poly(styrene sulfonate)
PVDF Polyvinylfluoride
PVP Poly(vinylpyrrolidone)
QDSL Quantum Dot Superlattice
SAED Selected Area Electron Diffraction
SEM Scanning Electron Microscopy
SPS Spark Plasma Sintering
TE Thermoelectric
TEG Thermoelectric Generator
TEM Transmission Electron Microscopy
Tos Tosylate
UV Ultraviolet
Vis Visible
VRH Variable Range Hopping
XPS X-ray Photoelectron Spectroscopy
XRD X-ray Diffraction
WSN Wireless Sensor Network
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Symbols
𝛽 Thomson coefficient
∆𝑇 Temperature difference
∆𝑉 Voltage difference
𝜂 Efficiency of a TEG
𝛾 Hopping exponent for electrical conductivity
𝜅 Thermal conductivity
𝜅𝑒 Electron thermal conductivity
𝜅𝑙 Lattice thermal conductivity
𝜆 Wavelength
𝜇 Charge carrier mobility
𝜔 Angular frequency
𝜙 Coefficient of performance
Π𝑃𝑒𝑙𝑡𝑖𝑒𝑟 Peltier coefficient
𝜌 Electrical resistivity
𝜎0 Conductivity prefactor
𝜎𝐸 Energy dependent electrical conductivity
𝜑 Phase shift
𝐶𝑠𝑝𝑟𝑖𝑛𝑔 Spring constant
𝑑𝑓 Penetration depth
𝐸𝐹 Fermi energy level
𝑘𝐵 Boltzmann constant
𝐿0 Lorenz factor
𝑁(𝐸) Density of state
𝑛𝑙𝑒𝑔 Number of thermal couples
𝑞𝑐𝑜𝑛 Conduction heat
𝑞𝑐𝑜𝑜𝑙𝑖𝑛𝑔 Peltier cooling
𝑞𝑖𝑛 Power input to the hot junction
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𝑞𝐽 Joule heating
𝑞𝑃𝑒𝑙𝑡𝑖𝑒𝑟 Cooling power of a Peltier cooler
𝑇0 Characteristic temperature
𝑡𝑓 Film thickness
𝑤𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 Rate of consumed energy
Z𝑑𝑒𝑣𝑖𝑐𝑒 Figure of merit of a thermoelectric device
A Area
d Distance between atomic layers
𝑒 Charge of charge carriers
F Force
I Electric current
J Electric current density
K Thermal conductance
L Length
n Charge carrier concentration
P Power output of a TEG
PF Power factor
Q Heat
𝑞 Heat conduction per unit volume
R Electrical resistance
S Seebeck coefficient
s Distance between probes
𝑇 Temperature
V Voltage
𝑤 Half width of heater
𝑥 Cantilever deflection
Z Figure of merit
ZT Dimensionless figure of merit
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1 Introduction
Technological advances have promoted rapid decrease in the size of computational functionality,
growth in the number of network devices, reduction in the sensor sizes and a continual decline in
the cost. [1] Consequently, inexpensive wireless sensor networks (WSNs), which are composed of
sensor nodes with different functionalities, could be realized. WSNs are being explored in fields
such as home applications, environmental applications, health applications and other commercial
and military applications. [2] A WSN (shown in Figure 1.1a) is based on the collaborative effort of a
large number of sensor nodes and it is able to monitor conditions such as temperature, humidity,
pollutant concentration, noise level and etc. [2] Each of the sensor nodes (see Figure 1.1b) requires
energy and has its own functionality. Both power storages and power harvesting devices are inte-
grated to supply the sensor nodes with energy to maintain their performance.
One of the main issues of WSNs is the network lifetime. As soon as the stored energy is consumed,
the network lifetime is terminated. This could be critical especially when the replacement of the
power storage is challenging. To extend the lifetime of WSNs, improving the energy density of
storage systems and scavenging their own power from the environment have been researched in
literatures. [3] Currently the most prevalent used power sources are non-rechargeable batteries.
There have been progresses to increase the storage density of batteries. However, the lifetime of
batteries cannot be extended indefinitely. Replacing batteries is unavoidable. In applications, such
as inhospitable environment monitoring, replacing batteries in devices is cost prohibitive. To
(a) A WSN with sensor nodes[6] (b) A wireles sensor node[6]
Figure 1.1 An example of a simple wireless sensor network.
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overcome this problem, power management systems with high power efficiency [4,5] and embedded
energy harvesters to harvest energy from environment [7] are under study. The application of energy
harvesting devices is a promising solution towards maintenance-free WSNs. To demonstrate the
application of self-powered WSNs, a smart city concept with integrated WSNs is proposed by
Tanaka et al. (shown in Figure 1.2). In this study, one example is to assemble the maintenance free
self powered WSNs on energy plants. Through these WSNs, it is able to promote energy saving and
monitor the status for disaster prevention.
For sensor nodes, in which energy reservoirs, e.g. batteries, are used to supply the power, their life-
time depends on the fixed amount of energy that is stored in the device. Therefore, the power gen-
eration is as a function of the battery lifetime. On the contrary, power harvesting devices are able to
produce energy as long as they are in operation. When a WSN is designed for a short lifetime, the
usage of batteries is a reasonable solution as they can provide high power density. However when
an autonomous and maintenance free WSN with long lifetime is required, integration of energy
harvesters is required. Among diverse power harvesting devices, solar energy harvesters offer ex-
cellent power density in direct sun light. Piezoelectric energy harvesters also provide sufficient en-
ergy density when there are vibration sources. However, in areas when there is no sufficient light
exposure or no vibration source, these energy harvesting methods become unpractical. Thermal
gradient energy harvesters, which are also called thermoelectric generators (TEGs), provide with a
possibility to recover the environmental heat into electricity. In addition, home heating, automotive
Figure 1.2 A smart city concept with wireless sensor networks. [8]
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exhaust and industry processes all generate huge amount of unused waste heat. Recovering the
waste heat into energy by TEGs could greatly increase the energy efficiency. Moreover, TEGs do
not have moving parts and do not emit CO2. Therefore, they offer a green and solid state energy
technology with respect to the world's demand for sustainable energy.
TEGs are excellent options to recover the nature abundance of heat in our environment. The high
fabrication cost of traditional inorganic thermoelectric (TE) materials, however, limits their wide
spread application. In contrast, polymers based materials could be potential candidates to decrease
the material cost. In addition, polymer based TEGs could be fabricated by printing techniques,
which gives the opportunity of inexpensive mass production. Furthermore, the flexibility of poly-
mer based TEGs offers the advantage over rigid TEGs made from inorganic TE materials as they
could easily be adjusted to different application environments. There have been plenty of investiga-
tions on inorganic TE materials. Polymers as alternative TE materials, however, are comparatively a
new research focus.
The primary objectives of this work are
o To investigate the thermoelectric properties of PEDOT:PSS films by the modification with
solvents (secondary dopants, amines and ionic liquid). Direct secondary dopant addition into
the polymer solution and post treatment of polymer films with secondary dopant are em-
ployed to identify a more efficient approach in optimizing the thermoelectric properties. The
used solvents for the post treatment are varied to find the correlation between the solvent
properties and thermoelectric properties of the polymer films.
o To develop green synthesis method to synthesize tellurium nanostructures. The method is
designed to be carried out with non toxic tellurium source and non toxic reductant. The syn-
thesized tellurium nanostructures should be with controlled morphology. Synthesized tellu-
rium nanostructures are then integrated into polymers to understand their influence on the
thermoelectric properties of hybrid composites.
The outline of the thesis is as follows:
Chapter 2: Fundamentals of Thermoelectrics
A general introduction to thermoelectrics and devices based on thermoelectric materials is present-
ed. The challenge for designing thermoelectric materials is pointed out. Progresses about both tradi-
tional inorganic TE materials and polymer based TE materials are reviewed.
16
Chapter 3: Characterization Techniques
Techniques related to thermoelectric properties characterization are briefly introduced. Auxiliary
tools (Atomic Force Microscopy, UV-Vis spectroscopy, Raman spectroscopy, X-ray Diffraction, X-
ray Photoelectron Spectroscopy and Transmission Electron Microscopy) are described.
Chapter 4: Thermoelectric Properties of PEDOT:PSS by the Addition and Post Treatment
with Organic Solvents
Direct addition of DMSO into PEDOT:PSS solution and post treatment of PEDOT:PSS films with
DMSO both were employed to modify the power factor of polymer thin films in order to find a
more efficient method for high power factor. Afterwards the role of the post treatment medium is
discussed by the variation of used solvents during the post treatment.
Chapter 5 Thermoelectric Properties of PEDOT:PSS Films by the Application of Ionic Liquid
From a green chemistry point of view, ionic liquid is employed as an alternative to improve the
power factor of PEDOT:PSS thin films by means of post treatment. The influence of ionic liquid on
the physical and chemical properties of PEDOT:PSS films is carefully analyzed.
Chapter 6 Hybrid Composites of Polymer with Nanoinclusions
Green synthesis methods based on wet chemical and hydrothermal synthesis for tellurium
nanostructures are applied. With non toxic reductant and non toxic tellurium source, tellurium
nanostructures with different morphologies are synthesized. The as synthesized tellurium nanostruc-
tures are employed to fabricate polymer/tellurium nanostructure composites. The hybrid composites
are designed in order to obtain the high Seebeck coefficient of tellurium nanostructures and the in-
trinsic low thermal conductivity of the polymer matrix.
The thesis is closed with a summary and an outlook in Chapter 7.
In Appendix A.1 the reduction ability of LAC/MEA mixture is demonstrated with comparison to
LAC and MEA through the usage of graphite oxide dispersion. JCPDS patterns are shown in Ap-
pendix A.2 to show that same information about the crystal structure is obtained even with different
radiation sources.
17
2 Fundamentals of Thermoelectrics In a thermoelectric material, there are either free electrons or holes. Both types of charge carriers
also carry heat. When the material is imposed with a temperature difference, charge carriers at the
hot side (𝑇𝐻) prefer to diffuse to the cold side (𝑇𝐶) as it is shown in Figure 2.1. Consequently there
is a buildup of electric potential to resist the diffusion of charge carriers. The generation of an elec-
tric potential by placing a material in a temperature gradient is called the Seebeck effect. And the
proportionality constant of the voltage (∆𝑉) to the temperature difference (Δ𝑇) is defined as the
Seebeck coefficient: [9]
𝑆 = −∆𝑉 ∆𝑇� = −(𝑉𝐻 − 𝑉𝐶)/(𝑇𝐻 − 𝑇𝐶) (2.1)
A P-type material is dominant with holes and exhibits a positive Seebeck coefficient. On the other
hand, an N-type material is dominant with electrons which results in a negative Seebeck coefficient.
Peltier effect is the opposite of the Seebeck effect. It is observed as the induced temperature differ-
ence when an electric current is driven through a material. Depending on the direction of current,
this effect could be either heating or cooling. For example, in Peltier cooling, when an external elec-
tric current is applied to drive the charge carriers, the charge carriers will carry away heat. The heat
is then forced to flow from the cold side to the hot side. The ratio of the rate of cooling (𝑞𝑐𝑜𝑜𝑙𝑖𝑛𝑔) to
the electric current 𝐼 defines the Peltier coefficient (Π𝑃𝑒𝑙𝑡𝑖𝑒𝑟) by [9]
Π𝑃𝑒𝑙𝑡𝑖𝑒𝑟 = 𝑞𝑐𝑜𝑜𝑙𝑖𝑛𝑔𝐼� (2.2)
When an electric current is driven through a conductor, of which there is a temperature gradient,
Figure 2.1 Schematic description of Seebeck effect. [10]
18
there is either an emission of heat or absorption of heat. This phenomenon is called the Thomson
effect. If an electric current with density 𝐽 passes through a homogeneous material, heat conduction
per unit volume 𝑞 is [11]
𝑞 = 𝜌𝐽2 − 𝛽𝐽𝑑𝑇𝑑𝑥
(2.3)
where 𝜌 is the resistivity of the material, 𝑑𝑇/𝑑𝑥 is the temperature gradient along the material and
𝛽 is the Thomson coefficient. Here, 𝜌𝐽2 is the Joule heating per unit volume, which is not reversi-
ble. And 𝛽𝐽 𝑑𝑇𝑑𝑥
is the Thomson heat, which depends on the direction of applied current.
The above mentioned three thermoelectric effects are related by the Kelvin relation [9]
𝑆 = Π𝑃𝑒𝑙𝑡𝑖𝑒𝑟/𝑇 (2.4)
𝛽 = 𝑇𝑑𝑆𝑑𝑇
(2.5)
To evaluate the thermoelectric properties of a material, a figure-of-merit (Z) is employed and de-
fined as
𝑍 = 𝑠2𝜎𝜅
(2.6)
in which 𝜎 is the electrical conductivity, 𝜅 is the thermal conductivity. The product of 𝑆2 𝜎 is wide-
ly used in literatures as the power factor (𝑃𝐹) to evaluate the thermoelectric properties of materials.
2.1 Thermoelectric Generation and Refrigeration
Based on the Seebeck effect and Peltier cooling effect, devices for power generation and refrigera-
tion can be fabricated. The working principle of power generation and refrigeration devices is
demonstrated in Figure 2.2 by simplifying each device to only one thermal couple. The thermal
(c)
Figure 2.2 Thermoelectric refrigeration (a), power generation (b) and a practical TEG (c).
19
couple is consisted of a P-type and an N-type thermoelectric leg. Practical devices are consisted of
multiple thermal couples that are connected electrically in series and thermally in parallel (see Fig-
ure 2.2c). In a power generation device (or TEG), the thermal couple is connected to an external
load 𝑅𝐿. Upon a temperature difference, the charge carriers in both types of thermal legs will flow
from the hot side to the cold side to produce a voltage. The induced voltage drives a current flow
through the load, generating electrical power. The efficiency of the TEG depends on both the value
of 𝑅𝐿 and on the thermoelectric properties of the thermal couple. In a refrigeration device (or Pelti-
er cooler), an electric current is supplied as it is depicted in Figure 2.2b. The charge carriers keep
flow away from the cold side to the hot side. As the charge carriers also carry away the heat, the
cold side remains cold.
When a TEG is subjected to a temperature gradient Δ𝑇 = 𝑇𝐻 − 𝑇𝐶, the actual temperature drop of
TEG (Δ𝑇𝐺) is smaller than Δ𝑇 due to the electrical and thermal resistance between the thermal leg
and the metal connection. Under the assumption that the heat passes only through the thermal legs,
the temperature drop of TEG is Δ𝑇𝐺 ≈ Δ𝑇 = 𝑇𝐻 − 𝑇𝐶. The thermal energy of the hot side comes
from the following three contributions (shown in Figure 2.3) [12]: (I) the rate heat passing through
the thermal legs (𝐾𝑃 + 𝐾𝑁)(𝑇𝐻 − 𝑇𝐶) when the electric current is zero. K is the thermal conduct-
ance of thermal legs; (II) the Joule heating 𝐼2(𝑅𝑃 + 𝑅𝑁)/2 to the hot side. The heating is originated
from the current 𝐼 induced by thermoelectric voltage. R is the resistance of thermal legs; (III) the
rate of Peltier heat absorption (𝑆𝑃 − 𝑆𝑁)𝑇𝐻𝐼 from the hot side due to the current flowing to the cold
side. The power input to the hot junction is defined as: [12]
𝑞 𝑖𝑛 = (𝑆𝑃 − 𝑆𝑁) 𝑇𝐻 𝐼 − 12
(𝑅𝑃 + 𝑅𝑁) 𝐼2 + (𝐾𝑃 + 𝐾𝑁)( 𝑇𝐻 − 𝑇𝐶 ) (2.7)
The electrical power output is:
Figure 2.3 Demonstration of the energy flows through a thermal leg upon a temperature gradient. [12]
20
𝑃 = 𝑅𝐿 𝐼2 (2.8)
in which 𝑅𝐿 is the resistance of external load.
The current I induced by the thermoelectric voltage is defined as
𝐼 =( 𝑆𝑃 − 𝑆𝑁)∆𝑇
𝑅 + 𝑅𝐿=
( 𝑆𝑃 − 𝑆𝑁)∆𝑇 𝑅𝑃 + 𝑅𝑁 + 𝑅𝐿
(2.9)
The open circuit voltage is:
∆𝑉 = 𝑛𝑙𝑒𝑔 � ( 𝑆𝑃 − 𝑆𝑁)𝑑𝑇𝑇𝐻
𝑇𝐶
(2.10)
in which 𝑛𝑙𝑒𝑔 represents the number of thermal couples.
The efficiency of a thermoelectric generator is the ratio of the output power to the power input, giv-
en by [9]
𝜂 =𝑅𝐿𝐼2
𝑞𝑖𝑛=
𝑅𝐿𝐼2
( 𝑆𝑃 − 𝑆𝑁) 𝑇𝐻 𝐼 − 12 (𝑅𝑃 + 𝑅𝑁) 𝐼2 + (𝐾𝑃 + 𝐾𝑁)( 𝑇𝐻 − 𝑇𝐶 )
(2.11)
A maximum efficiency is achieved when the external load resistance is equivalent to the internal
resistance of the TEG. Under an assumption that parameters (S, 𝑘 and 𝜎) are constant within the
temperature range of interest, the efficiency can be interpreted as: [13]
𝜂𝑚𝑎𝑥 =𝑇𝐻 − 𝑇𝐶
𝑇𝐻�1 + 𝑍𝑑𝑒𝑣𝑖𝑐𝑒𝑇 − 1
�1 + 𝑍𝑑𝑒𝑣𝑖𝑐𝑒𝑇 + 𝑇𝐶𝑇𝐻�
(2.12)
with here 𝑍𝑑𝑒𝑣𝑖𝑐𝑒 as the thermoelectric figure of merit for the TEG and 𝑇 = (𝑇𝐻 + 𝑇𝐶)/2.
When a current 𝐼 passes through a Peltier cooler, the charge carriers in both thermal legs will move
away from the cold junction and carry away the heat. With a continuous supply of electric current,
the charge carriers keep carrying away to cool the cold side. For a Peltier cooler, the coefficient of
performance (𝐶𝑂𝑃) is introduced, which is defined as the ratio of heat extracted from the source to
the consumed electrical energy. [14] In an ideal case, in which the device is free from heat losses
associated with the heat conduction and electrical resistance, the Peltier cooling at the cold side is
given as [14]
𝑞𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = ( 𝑆𝑃 − 𝑆𝑁)𝐼𝑇𝐶 (2.13)
The heat conduction to the cold side which is opposite to the cooling effect is [14]
21
𝑞𝑐𝑜𝑛 = ( 𝑇𝐻 − 𝑇𝐶 )( 𝐾𝑃 + 𝐾𝑁) (2.14)
Considering the Joule heating 𝑞𝐽 = 12
( 𝑅𝑃 + 𝑅𝑁 ) 𝐼2 at the cold side, the cooling power is given
by[14]
𝑞𝑃𝑒𝑙𝑡𝑖𝑒𝑟 = 𝑞𝑐𝑜𝑜𝑙𝑖𝑛𝑔 − 𝑞𝑐𝑜𝑛 − 𝑞𝐽
= ( 𝑆𝑃 − 𝑆𝑁)𝐼𝑇𝐶 − ( 𝑇𝐻 − 𝑇𝐶 )( 𝐾𝑃 + 𝐾𝑁) − ( 𝑅𝑃 + 𝑅𝑁 )𝐼2/2
(2.15)
And the rate of consumed electrical energy by [14]
𝑤𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 = ( 𝑆𝑃 − 𝑆𝑁)𝐼( 𝑇𝐻 − 𝑇𝐶) + 𝐼2( 𝑅𝑃 + 𝑅𝑁 ) (2.16)
The COP (𝜙) is then given by [14]
𝜙 =( 𝑆𝑃 − 𝑆𝑁)𝐼𝑇𝐶 − ( 𝑇𝐻 − 𝑇𝐶 )( 𝐾𝑃 + 𝐾𝑁) − ( 𝑅𝑃 + 𝑅𝑁 )𝐼2/2
( 𝑆𝑃 − 𝑆𝑁)𝐼( 𝑇𝐻 − 𝑇𝐶) + 𝐼2( 𝑅𝑃 + 𝑅𝑁 )
(2.17)
In order to yield a maximum cooling power, the supplied current should be derived from Equation
2.15 to give
𝐼𝑞 = ( 𝑆𝑃 − 𝑆𝑁)𝑇𝐶( 𝑅𝑃 + 𝑅𝑁)� (2.18)
By replacing Equation 2.17 with Equation 2.18, the corresponding maximum cooling power is giv-
en [14]
𝜙𝑚𝑎𝑥 =𝑍𝑇𝐶2/2 − ( 𝑇𝐻 − 𝑇𝐶 )
𝑍𝑇𝐻𝑇𝐶
(2.19)
in which Z is equal to (𝑆𝑃 − 𝑆𝑁)2/(( 𝐾𝑃 + 𝐾𝑁)( 𝑅𝑃 + 𝑅𝑁)).
Equation 2.19 shows that the optimum COP of a Peltier cooler depends on the Z of the thermal
couple and the temperature difference between the heat source and the heat sink.
For both TEG and Peltier cooler, the performance of devices depends on Z of the thermal couples
(see Figure 2.4). To have high performance devices, the Z of the thermal couple should be high
enough. Because Z has a dimension of inverse temperature (see Equation 2.6), the dimensionless
figure of merit, which is defined as 𝑍𝑇 = (𝑠2𝜎𝑇)/𝜅 at a given temperature, is widely used. The
evaluation of ZT for a thermal couple depends on the ZT of both P-type and N-type thermal legs
and also the geometry of device configuration, resulting in the complexity in the ZT calculation. For
simplicity, ZT of a single thermoelectric material is employed to evaluate the material property.
22
(a) Conversion efficiency of TEG regarding ∆𝑇 and Z of the
thermal couples.[9]
(b) The coefficient of performance of a Peltier cooler regarding
∆𝑇 and Z of the thermal couples.[9]
Figure 2.4 The dependence of device efficiency on the Z of the thermal couples.
2.2 Conflicting Thermoelectric Material Properties
To maximize the ZT of a material, a large power factor, defined as 𝑆2𝜎 is required. At the same
time, a low thermal conductivity (𝑘) is demanded to maintain the temperature difference for effi-
cient energy conversion. However, the transport characteristics depend on interrelated material
properties. Therefore, an optimized ZT could be only achieved by tuning the material properties. [15]
To ensure a large Seebeck coefficient, the presence of a single type of charge carriers inside the
material would be beneficial. A mixed N-type and P-type charge carriers will cancel out the induced
thermal voltage, leading to lower Seebeck coefficient. The great challenge of TE materials lies in
increasing the Seebeck coefficient without diminishing the electrical conductivity. Mott equation
from Boltzmann transport theory provides a general understanding of the Seebeck coefficient. [16]
𝑆 = �𝜋2
3𝑘𝐵2𝑇𝑒
𝑑𝑙𝑛[𝜎(𝐸)]𝑑𝐸
�𝐸=𝐸𝐹
(2.20)
Here, 𝑘𝐵 is the Boltzmann constant, 𝐸𝐹 is the Fermi energy and 𝜎(𝐸) is a measure of the contribu-
tion of electrons with energy E to the total conductivity and 𝑒 is the charge of carriers. [17] With the
assumption that the electronic scattering is independent of energy, 𝜎(𝐸) is proportional to the densi-
ty of states (DOS) at energy E. Seebeck coefficient is in fact a measure of the variation in 𝜎(𝐸)
above and below the Fermi level. The Seebeck coefficient could be indicated from the DOS dia-
gram as it is shown in Figure 2.5. The one with fast changing of DOS near 𝐸𝐹 (Figure 2.5a) is ex-
pected to have larger Seebeck coefficient according to Equation 2.20. [16]
23
All the electrical conductivity, Seebeck coefficient and thermal conductivity strongly depend on the
temperature and the charge carrier concentration. The intercoupling relationship among the electri-
cal conductivity, the Seebeck coefficient and the thermal conductivity regarding the charge carrier
concentration is shown in Figure 2.6a. The electrical conductivity is improved when the charge car-
rier concentration is increased. However, there are simultaneous increase in the thermal conductivi-
ty and decrease in the Seebeck coefficient. Due to these conflicting thermoelectric material proper-
ties, only optimized power factor and ZT could be obtained through tuning the charge carrier con-
centration.
An efficient TE material requires a low thermal conductivity (𝑘). Thermal conductivity comes from
the heat contributed from charge carrier transport (𝑘𝑒, electron thermal conductivity) and also the
heat coming from the phonon travelling through the lattice (𝑘𝑙, lattice thermal conductivity). The
electron thermal conductivity is directly related to the electrical conductivity given by the
Wiedemann-Franz Law: [15]
𝑘 = 𝑘𝑒 + 𝑘𝑙 (2.21)
Figure 2.5 Hypothetical DOS with a) a large slope 𝑑𝑙𝑛[𝜎(𝐸)]/𝑑𝐸 and b) a small slope near 𝐸𝐹 . [16]
(a) Dependence of materials parameters on carrier con-
centration.[15]
(b) Influence of lattice thermal conductivity on ZT.[15]
Figure 2.6 Optimization of ZT through tuning the charge carrier concentration.
24
and
𝑘𝑒 = 𝐿0𝜎𝑇 = 𝑛𝑒𝜇𝐿0𝑇 (2.22)
in which 𝐿0 is the Lorenz factor, 𝑛 is the charge carrier concentration and 𝜇 is the charge carrier
mobility. The Lorenz factor is 2.4 × 10−8 𝐽2𝐾−2𝐶−2 for free electrons.
Wiedemann-Franz law addresses the challenge to tune the ZT through the charge carrier concentra-
tion. By replacing Wiedemann-Franz law into the definition of ZT, ZT can be converted into [15]
𝑍𝑇 =𝑆2/𝐿0
1 + 𝑘𝑙/𝑘𝑒
(2.23)
Equation 2.23 indicates that when a material is with high electrical conductivity or very low 𝑘𝑙
(where 𝑘𝑙/𝑘𝑒 ≪ 1), the critical parameter determining the ZT is the Seebeck coefficient.
To reduce the total thermal conductivity, both 𝑘𝑒 and 𝑘𝑙 should be minimized. However, lowering
the electron thermal conductivity through reducing the charge carrier concentration concomitantly
decreases the electrical conductivity. Modification of the lattice thermal conductivity, on the other
hand, is an effective way to manipulate the total thermal conductivity as it is not determined by the
electronic structure. To reduce the lattice thermal conductivity, introducing point defects to induce
strong phonon scattering is proven successfully. Figure 2.6b demonstrates that the value of ZT is
doubled by reducing the lattice thermal conductivity from 0.8 to 0.2.
Based on the intercoupling behavior among the thermoelectric material properties, materials with
“phonon glass electron crystal (PGEC)” structure are expected to have high ZT. [15] The demand for
electron crystal originates from the high power factor of crystalline semiconductors, which meets
the compromise required from the electronic properties. At the same time, the phonon glass is re-
quired in order to design materials with low lattice thermal conductivity.
Organic materials based TE materials, such as polymers, are also facing the same challenge in the
optimization of power factor. For lightly and moderately doped polymers, the electrical conduction
is dominated by the thermal activation or by the variable range hopping (VRH). [17] In this case, the
electrical conductivity of polymers can be written as [17]
𝜎 = 𝜎0𝑒𝑥𝑝 [−(𝑇0/𝑇)𝛾] (2.24)
25
in which 𝜎0 is the conductivity prefactor. It is weakly temperature dependent but depends on the
scattering. 𝑇0 is the characteristic temperature and it is related to the localization length. 𝛾 is the
hopping exponent (0.25-0.5). [18]
The Seebeck coefficient of polymers is dependent on the temperature in a relation of [19]
𝑆 = �𝑘𝐵22𝑒�𝑇0𝑇�12 (𝑑𝑙𝑛𝑁(𝐸)
𝑑𝐸)�𝐸=𝐸𝑓
(2.25)
with 𝑁(𝐸) as the density of state.
Upon doping, the electrical conductivity of polymers is increased. In addition, they exhibit a transi-
tion from semiconducting into metallic characteristic. Polyaniline doped with camphor sulfonic acid
(CSA) and PEDOT:PSS doped with dimethyl sulfoxide (DMSO) and ethylene glycol (EG) are both
observed with metallic behaviors. [18,20] For highly doped polymers with metallic behavior, the See-
beck coefficient could be explained by Mott Equation as it is expressed in Equation 2.20.
2.3 Overview of Thermoelectric Materials
The current TE materials can be categorized into traditional inorganic TE materials and polymer
based TE materials. Recent advancements in the ZT of inorganic TE materials are based on the
nanostructuring method. High ZT values have been achieved by this method to preferentially reduce
the lattice thermal conductivity. [21] However, the complexity involved in the nanostructuring meth-
od and the high-cost fabrication processes push the development for alternatives. Polymers have
come to the fore front as a promising alternative.[22] In the following, the progresses in inorganic
and polymer based TE materials are reviewed.
2.3.1 Traditional Inorganic Thermoelectric Materials
In inorganic TE materials, the current research trend is to look for new materials that either have
high ZT or are able to work within a broad temperature regime. [23] Up to now, alloys based on bis-
muth telluride (Bi2Te3) are successful in practical applications near room temperature. For applica-
tions in the mid temperature range (500 K to 900 K), materials based on group IV tellurides are
commonly used. The state of the art commercial bulk TE materials regarding the application tem-
perature are shown in Figure 2.7. Those traditional TE materials have been extensively investigated.
Until 1990s, the maximum ZT of most materials remained around 1, limiting them only to niche
applications. In 1993, Hicks and Dresselhaus proposed that the confinement of materials into low
26
dimensions (2D, 1D and 0D) could increase the ZT, which inspired intensive researches on the
thermoelectric properties of quantum wells and superlattices. [24] For conventional 3D systems, the
quantities (S, 𝜎 and 𝑘) are interrelated to each other. Hence, it is difficult to separately tune one of
the parameters for better ZT. [25] However, when the dimension is decreased, the change in the den-
sity of electronic states (DOS) (see Figure 2.8) allows new opportunities to modify S, 𝜎 and 𝑘. [26]
To design low dimensional materials with high thermoelectric properties, common method for ma-
terial nanostructuring is either introducing nanoscale constituents for the quantum confinement ef-
fect or the design of internal interfaces. With the use of quantum confinement effect, it is able to
obtain materials with enhanced DOS near the Fermi energy. As a result, a high power factor could
be obtained. [16] Through the utilization of the internal interfaces, phonons are more effectively scat-
tered than electrons, especially those which strongly contribute to the thermal conductivity. Corre-
spondingly, the lattice thermal conductivity could be greatly reduced. These nanostructuring strate-
gies have been used to fabricate nanostructured thin films and bulk nanostructured materials.
Thin Film Nanostructured Materials
Harman et al. utilized both the quantum confinement effect and the superlattice structure to develop
quantum dot superlattices (QDSLs) as shown in Figure 2.9. [27] To obtain quantum dots with good
quality, uniform size and well controlled separation distance, molecular beam epitaxy (MBE) is
used to deposit the QDSLs. It is confirmed experimentally that the thin films of QDSLs are with
Figure 2.7 Figure of Merit, ZT, of the state of the art inorganic TE materials regarding the temperature. [15]
Figure 2.8 Electronic densities of states of materials with different dimensions. [26]
27
improved Seebeck coefficient and higher ZT than bulk samples. The better thermoelectric perfor-
mance is associated with quantum dot structure and the designed interfaces. [27] A later developed
cooling device made from these QDSLs thin films demonstrates a much better cooling performance
than conventional materials under same test condition. [27]
Bulk Nanostructured Materials
Superlattice thin films and nanowires could be only produced in thin film and small quantity due to
the very low deposition rate of the MBE technique (e.g. 360 nm/hour for silicon). For practical ap-
plications that are compatible with commercially available devices, materials with high perfor-
mance are required to be synthesized in a large quantity. This promotes the development of bulk
process rather than the nanofabrication process to fabricate nanostructured materials. [28] Bulk
nanostructured nanocomposite materials are subdivided into two main groups: one is that nanoparti-
cles are embedded in a host material and the other one is that hetero-structure geometry with differ-
ent nanoparticles adjacent to each other is present in the bulk material. [28] The challenge behind
these types of materials is to create a nanoscale structure with high ZT through a bulk process. And
these bulk synthesized nanostructure materials must be thermodynamically stable to maintain their
performance.
A typical strategy for bulk nanostructured TE materials is to use either a chemical or a physical
route to synthesize nanopowders. Afterwards, the nanopowders are rapidly compacted to avoid
grain growth. [29] The physical route to prepare nanopowders is realized by a top down process: me-
chanical ball milling. The chemical route is a bottom up approach in which nanopowders are syn-
thesized in the solution phase. The followed powder compacting process is critical as the
nanograins should be maintained for high TE performance. In a conventional hot processing, grain
growth is unavoidable, which leads to deteriorated TE properties. Therefore, the key challenge lies
Figure 2.9 Schematic cross section and SEM image (surface) of QDSL. [25]
28
in how to maintain the bulk nanostructured materials thermodynamically stable to constrain the
grain growth. The recent developed spark plasma sintering (SPS) and direct current induced hot
press are able to compact the loose nanopowders into a dense bulk within several minutes. Thereby,
the grain growth is suppressed and the original nanostructures are kept within the bulk material. [29]
2.3.2 Polymer Based Thermoelectric Materials
Since the discovery of high electrical conductivity in polyacetylene (PAC) upon doping with
iodione or arsenic pentafluoride (AsF5), great interest is attracted to intrinsic conducting polymers.
The Nobel Prize in Chemistry in 2000 to Heeger, MacDiarmid and Shirakawa has highlighted the
importance of this type of polymers. Intrinsic electrical conducting polymers are conjugated poly-
mers with an alternation between single and double bonds. The alternation of single and double
bonds greatly lowers the band gap between the HOMO (Highest Occupied Molecular Orbital) and
LUMO (Lowest Unoccupied Molecular Orbital) levels, allowing the charge carriers to transport
through the 𝜋 orbital.
PACs are the first type of studied conjugated polymers. In their neutral state, polymers resemble
insulators. When the oxidation level (reduced state ↔ oxidized state) is increased, the electrical
conductivity could be increased to a value as high as 105 S/m. This high electrical conductivity
prompts great research interest of their applications in the electronics industry. However, the oxi-
dized PACs with high electrical conductivity degrade readily in air. Moreover, these polymers are
insoluble in solvents, making it difficult to process. Other conjugated conducting polymers, such as
polypyrrole (PPY), polyaniline (PANI) and polythiophene (PHT) are developed at the same time.
However, it is still a great challenge to dissolve these polymers for solution based processing.
A breakthrough in the electrical conducting polymers was made by Bayer AG. A water dispersible
poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) dispersion was produced.
PEDOT:PSS is a stable aqueous dispersion which is produced by the polymerization of (3,4-
ethylene dioxythiophene) (EDOT) with poly(styrene sulfonate) (PSS) as the surfactant. Uniform
PEDOT:PSS films can be fabricated from a commercial PEDOT:PSS dispersion by conventional
coating techniques, such as spin coating, inkjet printing, spray coating, screen printing and aerosol
jet printing. These polymer thin films are highly transparent, which is ideal for the application as
transparent electrodes for solar cells and as anti-static coating. Besides, high electrical conductivity
up to 3000 S/cm of PEDOT:PSS film, which is comparable to typical indium tin oxide (ITO) elec-
trode, is reported by a simple sulfuric acid treatment by Ouyang et al. [30]
29
With their high electrical conductivity, polymers are considered as alternative TE materials. The
other advantageous aspects of utilizing polymeric materials as TE materials are:
I) Thermal properties
The intrinsic thermal conductivity of polymers is reported to be lower than 1 W/(mK), which is 2-3
magnitudes lower than that of traditional inorganic TE materials. [21] Thus, the nanostructuring
method for low thermal conductivity could be avoided.
II) Mechanical properties
TEGs made from traditional inorganic TE materials are usually rigid. However, in applications
where wearable and bendable devices are required, the rigidity is an issue. [31] The flexibility of pol-
ymer based TE materials is an optimum for this type of application. In addition, the density of pol-
ymers is much lower, which allows the realization of light weight TEGs.
III) Fabrication process
Most polymers can be dissolved in organic solvents, supporting the application of conventional
printing techniques for the deposition. In turn, the production cost could be greatly lowered while
the production capability could be increased. Moreover, the printing of polymer based TE materials
could take place at room temperature. Compared with the high energy input of typical process for
inorganic TE materials, this allows low energy fabrication.
Conductive Polymer
Pristine conductive polymers have too low charge carrier concentration for an effectively electron
transport. This in turn results in a poor electrical conductivity (below 10−8 𝑆/𝑐𝑚 ). [21] The reason
is that the charge transport is dominated by hopping between polymer chains. The electrical con-
ductivity could be increased by doping. As the power factor is proportional to the electrical conduc-
tivity, doping is widely used for a better thermoelectric performance. Doping of polymers is usually
realized by electrochemical or chemical processes. [22] Thereby, more charge carriers are introduced
to increase the electrical conductivity and the charge carriers are transported along intra- or inter-
chains. [32] However, a side effect of this doping process is the concomitant decrease of the Seebeck
coefficient. Up to now, even though the electrical conductivity of polymers is high, the Seebeck
coefficient is still in the range of ~ 20 𝜇𝑉/𝐾. [30] Dedoping (the reverse of doping) is applied to
increase the Seebeck coefficient. Figure 2.10 demonstrates the result of a chemically dedoped P-
30
type PEDOT:Tosylate (PEDOT:Tos) film, of which the Seebeck coefficient is brought up to hun-
dreds of 𝜇𝑉/𝐾 at low oxidation level. [33] An optimum power factor is achieved through controlling
the oxidation level by dedoping. The trade-off between the electrical conductivity and the Seebeck
coefficient is attributed to the change in the Fermi energy: the Fermi energy is pushed inside the
conduction band and the DOS around the Fermi level becomes symmetrical during doping. [20–22,33]
As the Seebeck coefficient is a measure of the variation in 𝜎(𝐸) above and below the Fermi level,
this symmetry results in the decrease of the Seebeck coefficient. Therefore, the doping level has to
be well controlled to balance the electrical conductivity and Seebeck coefficient for an optimum
power factor.
Figure 2.10 The correlation of thermoelectric properties regarding the oxidation level of PEDOT chains. [33]
For PANIs, the typical used are camphor sulfonic acid (CSA), hydrochloric acid (HCl) and ammo-
nium hydroxide (NaOH). A high power factor value of ~ 1 𝜇𝑊/(𝑚𝐾2) is obtained by sequential
doping of PANIs with CSA and NaOH. [34] For PACs, iodione and ion chloride (FeCl3) are the typi-
cal doping agents. [22] PEDOT:PSS, one type of PHTs, is typically dedoped with hydrazine to tune
the oxidation level of PEDOT chains. [35,36]
Besides the doping of polymers, the other widely used approach is to use secondary dopants (espe-
cially for PEDOT:PSS), which could increase the electrical conductivity by several orders of mag-
nitudes. [37] The role of secondary dopants is not the same as the doping agents. They do not change
the electronic structure. Instead, they affect only the film morphology. Upon the removal of second-
ary dopants by heating the polymer films, this effect on the electrical conductivity remains. For ex-
ample, DMSO and EG (secondary dopants for PEDOT:PSS) have been proven to sufficiently in-
31
crease the electrical conductivity up to 1418 S/cm. [38] Taking advantage of the extreme high elec-
trical conductivity of PEDOT:PSS brought by secondary dopants, they are applied to explore their
roles in the thermoelectric properties of PEDOT:PSS. [39–43] Kim et al. reported that the
PEDOT:PSS film has achieved a ZT=0.42 with DMSO at room temperature. [44]
Coordination polymers (CPs), which are constructed from metal ions and ligands where metal ions
are used as connectors and ligands as linkers, are found to be promising TE materials. [45] Zhu 's
group reported high TE characteristic (best organic TE materials) of a series of N- and P-type CPs
which contain a 1,1,2,2,-ethenetetrathiolate (ett) linking bridge: polymer[Ax(M-ett)] (A =
tetradecyltrimethyl ammonium, tetrabutyl ammonium, Na+, K+, Ni2+, Cu2+, M = Ni, Cu). [46] An all
organic TEG device was fabricated based on 35 N-P thermal couples of synthesized CPs (N-type
poly[Nax(Ni-ett)] and P-type poly[Cux(Cu-ett)]) with the thermal leg size of 5 𝑚𝑚 × 2 𝑚𝑚 ×
0.9 𝑚𝑚. Under a temperature difference of ∆𝑇 = 80 𝐾, the TEG with an optimized load resistance
is able to deliver a voltage of 0.26 V, a current of 10.1 mA and a power of 2.8 𝜇𝑊/𝑐𝑚2.
Hybrid Composites Based on Polymers and Nanoinclusions
Hybrid composites are developed to use the advantageous characteristics of both the polymer ma-
trix and the nanoinclusions. The formulated composites feature the low thermal conductivity, flexi-
bility and solution processability based on the polymer matrix. In addition, composites could yield
the high electrical conductivity and the high Seebeck coefficient as the nanoinclusions. Carbon
nanotubes (CNTs) and inorganic particles are typically investigated as the nanoinclusions. The no-
tations for hybrid composites are polymer/CNTs hybrid composites and polymer/inorganic nanopar-
ticles hybrid composites.
Polymer/CNTs Hybrid Composites
CNTs have remarkable electronic, physical and mechanical properties, but their high thermal con-
ductivity is a big challenge when directly used as TE materials. As most polymers still have low
electrical conductivity, especially the non-conjugated ones, the combination of CNTs with polymers
could enhance the power factor by the increase of the electrical conductivity. In addition, the ther-
mal conductivity of the hybrid composite could remain low due to the intrinsic low thermal conduc-
tivity of the polymer matrix.
Yu's research group has developed polymer hybrid nanocomposites with a segregated network
structure. In this type of composites, an insulating polymer dispersion was employed as the matrix
32
and CNTs which are dispersed by PEDOT:PSS are the nanoinclusions. The CNTs are pushed into
the interstitial spaces between the insulting polymers to construct the segregated network. Their
study has shown that a CNTs network with junctions which is thermally disconnected but electrical-
ly connected is formed (see Figure 2.11).[47,48] A flexible hybrid composite with a power factor of
~160 𝜇𝑊/(𝑚𝐾2), an electrical conductivity of ~105 S/m and a thermal conductivity in the range
of 0.2 - 0.4 𝑊/(𝑚𝐾) at room temperature is reported recently. [48]
Mechanical blending of CNTs into the polymer matrix solution is also a commonly used technique
to fabricate polymer/CNTs composites. [47–51] Hewitt et al. demonstrated the possibility of embed-
ding CNTs into a polyvinylfluoride (PVDF) matrix by simple mechanical mixing in the solution
phase. [50,51] A flexible TEG was built-up by multi-layering the thermal legs of PVDF/CNTs hybrid
composites as it is shown in Figure 2.12.
In order to have better polymer structures, in-situ polymerized polymers with CNTs as template are
extensively investigated. [52–54] This approach could effectively tune the interface between polymers
and CNTs. Polymers are located within or in the immediate vicinity of the CNTs. Especially for
polymers, to which CNTs have strong 𝜋 − 𝜋 interactions, ordered polymer chains could grow along
Figure 2.11 Nanotubes are coated by PEDOT:PSS particles, making nanotube-PEDOT:PSS-nanotube junctions in the composites. [48]
Figure 2.12 Demonstration of a flexible TEG based on PVDF/CNTs hybrid composites. [51]
33
the surface of CNTs, leading to enhanced carrier mobility for better electrical conductivity. Meng et
al. demonstrated that the thermoelectric properties of PANI/CNTs hybrid composite outperform that
of pure CNTs and the PANI matrix. [52] Bounioux et al. developed a composite of PHT and CNTs
of which the achieved power factor (maximum 95 𝜇𝑊/(𝑚𝐾2)) significantly exceeds the values of
both PHT and CNTs. [54]
Besides CNTs, other carbon based materials such as graphite, graphite oxide, expanded graphite,
graphene and graphite nanofibers are proven to be able to increase the power factors of polymer
based hybrid composites.[55–58]
Polymer/Inorganic Nanoparticles Hybrid Composites
There is growing interest in developing composites based on polymers and inorganic nanoparticles
as a synergistic effect in both the flexibility, low thermal conductivity of polymers and high ther-
moelectric performance of inorganic nanoparticles could be generated. [21] Tellurium (Te), Bi2Te3
and Sb2Te3 are the most often used nanoparticles to fabricate hybrid composites. Evan's group has
developed dispenser printed hybrid composites based on ball milled Bi2Te3 and Sb2Te3 nanoparti-
cles and an epoxy matrix.[59–62] A printed P-type single leg TEG (legs made from silver paste are
deposited to complete the TEG) can produce a power output of 20.5 𝜇𝑊 at 0.15 mA and 130 mV
for a ∆𝑇 of 20 K (see Figure 2.13). [62] However, the epoxy matrix is insulating, which leads to the
low electrical conductivity of the hybrid composite. Replacing the epoxy matrix with conducting
polymers could be a solution for better power output.
PHTs, among conducting polymers, are extensively investigated as the matrix. [63–68] Flexible and
all solution processed P-type and N-type TE materials based on ball milled inorganic particles and a
commercial PEDOT:PSS solution were demonstrated by Katz's group (see Figure 2.14). [69] The
inorganic nanoparticles were drop casted on a substrate to form a thin layer of nanoparticles.
Figure 2.13 Illustration of the dispenser printed flexible polymer/inorganic nanoinclusion composites. [62]
34
Subsequently, a PEDOT:PSS solution was then drop casted on top of the nanoparticle layer in order
to form the hybrid composites. This work shows the possibility to process TE devices by an all so-
lution method. Later on He et al. developed a hybrid composite of PHT and Bi2Te3 nanowires, of
which the power factor is 4 times higher than that of pure PHT. [63] The improved Seebeck coeffi-
cient and power factor are attributed to the rational engineering at the organic-inorganic semicon-
ductor. In Lawrence Berkeley Laboratories, the researchers demonstrate that homogeneous Te
nanorods could be synthesized with PEDOT:PSS solution as the morphology directing agent. The
synthesized Te nanorods are coated with a thin layer of PEDOT:PSS (see Figure 2.15).[18,70,71] In
this type of hybrid composite, a ZT of about 0.1 is achieved.
A hybrid composite based on Te nanowires and PHT was further developed by Wang's group. [64]
The composite was deposited on a flexible Kapton substrate, which was then attached to the human
body to employ the body temperature for energy harvesting. Polymer/Te nanostructures hybrid
composites can also be realized through the vapor deposition of both the Te nanostructures and pol-
ymers as reported by Sinha et al. [68]
Figure 2.14 SEM picture of a PEDOT:PSS solution casted over a layer of ball milled Bi2Te3 particles. (a) Cross sec-tion; (b) Top view. [69]
Figure 2.15 Demonstration of a composite casted from PEDOT:PSS directed synthesized tellurium nanorods. [71]
35
3 Characterization Techniques This chapter discusses characterization techniques for the determination of thermoelectric proper-
ties: the electrical conductivity, the Seebeck coefficient and the thermal conductivity. In addition,
Atomic Force Microscopy (AFM), Raman spectroscopy, UV-Vis spectroscopy, X-ray Photoelec-
tron Spectroscopy (XPS), X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM)
are used for detailed analysis of material properties.
3.1 Electrical Conductivity
The electrical resistivity (𝜌) is a measure of the materials' inherent resistance to the applied current
flow. It is inversely proportional to the electrical conductivity, which is defined as 𝜎 = 1/𝜌. The
typical methods to determine the electrical resistivity are 2-Point Probe method, 4-Point Probe
method and the van der Pauw method.
The arrangement of a typical 2-Point Probe resistance measurement is shown in Figure 3.1. In this
method, the obtained resistances include the contact resistance (𝑅𝑐), the spreading resistance (𝑅𝑠𝑝)
and the resistance of the probes (𝑅𝑝). The major problem of this method is the contact resistance, in
particular, when the samples are high electrical conducting. Moreover, it is not applicable for sam-
ples with random shapes. In principle, 2-Point Probe resistance measurement is suitable for the
samples with high resistance, in which the contact resistance could be ignored.
To overcome the above mentioned problems, the 4-Point Probe and the van der Pauw method are
introduced. Both have become standard methods to measure the surface resistance of thin films. In
Figure 3.1 Schematic diagram of a 2-Point Probe method.
36
this thesis work, the focus is on the 4-Point Probe method, which is arranged as the sketch in Figure
3.2. In this method, a current I is injected through the two outer probes, and a voltage is recorded
between the inner two probes. Same as in the 2-Probe method, there is a probe resistance, a contact
resistance and a spreading resistance for each probe. However, the usage of a voltmeter with high
input impedance results in a very low current flowing through the voltage path. Therefore, 𝑅𝑐, 𝑅𝑠𝑝
and 𝑅𝑝 could be neglected. [72]
Figure 3.2 Schematic diagram of a 4-Point Probe method.
When the probes with a uniform spacing (𝑠1 = 𝑠2 = 𝑠3 = 𝑠) are placed on an infinite slab material
(𝑡𝑓 is the film thickness), the resistivity can be calculated as:
𝜌 = 2𝜋𝑠𝑉𝐼� ,𝑓𝑜𝑟 𝑡𝑓 ≥ 𝑠 (3.1)
and
𝜌 = (𝜋𝑡𝑓/𝑙𝑛2)𝑉𝐼� ,𝑓𝑜𝑟 𝑠 ≥ 𝑡𝑓 (3.2)
For thin films (𝑠 ≥ 𝑡𝑓), the surface resistance Rs is given by
𝑅𝑠 = 𝜌/𝑡𝑓 =𝜋𝑙𝑛2
𝑉𝐼
= 4.53𝑉/𝐼 (3.3)
3.2 Seebeck Coefficient
There are two popular approaches to measure the Seebeck coefficient: the integral method and the
differential method. In the integral method, samples are required to be fabricated in a long wire like
geometry with lead (Pb) in L geometry as the reference material. In some cases, it is difficult to
prepare test sample with long wire like geometry, especially for spin coated polymer films. The
differential method, on the other hand, is generally used to measure films and bulk samples. The
schematic diagram of a differential method is shown in Figure 3.3: the sample is imposed with a
37
temperature difference (∆𝑇) of 𝑇2 − 𝑇1; one end of reference wires is connected to the sample and
the other end is connected with copper (Cu) wires to the measurement devices; the temperature of
each site is depicted in the figure. In this method, two important conditions need to be fulfilled: 1st
The connection Cu wires and the samples should be homogeneous that 𝑆𝐶𝑢 and 𝑆𝑠𝑎𝑚𝑝𝑙𝑒 are only as
a function of the temperature, not of the position along the Cu wire or the sample; 2nd During the
measurement, the net current across the sample is zero that the electrical field E from the current
contribution is zero. The potential difference ∆𝑉 = −𝑆∆𝑇 is only due to the Seebeck coefficient of
test sample.
Under these conditions, the ∆𝑉 is given as
∆𝑉 = −(� 𝑆𝐶𝑢𝑑𝑇𝑇0
𝑇𝑎+ � 𝑆𝑟𝑒𝑓1 𝑑𝑇
𝑇1
𝑇0+ � 𝑆𝑠𝑎𝑚𝑝𝑙𝑒𝑑𝑇
𝑇2
𝑇1+ � 𝑆𝑟𝑒𝑓2 𝑑𝑇
𝑇0
𝑇2+ � 𝑆𝐶𝑢𝑑𝑇
𝑇𝑎
𝑇0)
(3.4)
with 𝑆𝐶𝑢 as the Seebeck coefficient of Cu wires, 𝑆𝑟𝑒𝑓1 the Seebeck coefficient of the Ref1 wire and
𝑆𝑟𝑒𝑓2 the Seebeck coefficient of the Ref2 wire.
When homogeneous references wires are used ( 𝑆𝑟𝑒𝑓1 = 𝑆𝑟𝑒𝑓2 = 𝑆𝑟𝑒𝑓) and the temperature difference
is much smaller than the average temperature (𝑇𝑎𝑣) of sample (∆𝑇 ≪ 𝑇𝑎𝑣 ≡ (𝑇1 + 𝑇2)/2), then
Equation 3.4 can be transformed into
∆𝑉 = −� �𝑆𝑠𝑎𝑚𝑝𝑙𝑒 − 𝑆𝑟𝑒𝑓�𝑑𝑇 ≈ �𝑆𝑠𝑎𝑚𝑝𝑙𝑒 − 𝑆𝑟𝑒𝑓�∆𝑇𝑇2
𝑇1
(3.5)
Figure 3.3 Sketch of a Seebeck coefficient measurement based on the differential method. [9]
38
And the Seebeck coefficient of the sample follows:
𝑆𝑠𝑎𝑚𝑝𝑙𝑒(𝑇𝑎𝑣) = −∆𝑉∆𝑇
+ 𝑆𝑟𝑒𝑓(𝑇𝑎𝑣) (3.6)
The Equation 3.6 is only valid when the Seebeck coefficient has no appreciable change in the tem-
perature change of ∆𝑇.
SRX, which is designed by Fraunhofer IPM, is commercial Seebeck coefficient characterization
equipment based on the differential method and it is able to characterize thin films. [73] Figure 3.4a
displays the sample holder in SRX. The test sample, e.g. polymer thin films on a glass substrate,
was fixed on top of the sample holder with the test sample film facing down. The thermal couples
for temperature sensing were then connected to the film by a pressure contact through the plate
spring (shown in Figure 3.4b). This avoids gluing the sample to the mounting plate or welding
thermocouples to the sample surface. The micro heaters embedded in both sides of the mounting
base are used to regulate the temperature gradient.
(a)
(b)
Figure 3.4 Sample holder for Seebeck coefficient measurement of the platform in SRX. [74]
3.3 Thermal Conductivity
When a heat Q passes through an object with a cross section area of A (shown in Figure 3.5a), a
temperature difference over a distance of ∆𝐿 will be created. The heat flux (𝑄/𝐴) is proportional to
the temperature gradient (∆𝑇/∆𝐿) in a relation of
𝑄𝐴∝∆𝑇∆𝐿
(3.7)
Under the condition that the heat flux only flows through the sample that the heat loss in the radial
direction is negligible and the heat flux is at a steady state, Equation 3.7 can be rewritten to give the
thermal conductivity 𝑘
39
𝑘 =𝑄/𝐴∆𝑇/∆𝐿
(3.8)
This formulation is valid only when the heat radiation from the sample surface is small compared to
the heat flux through the sample, e.g. the test temperature is below 50 K. However, when the test
temperature is above room temperature and the sample is a poor conductor of heat, most of the heat
is radiated away instead of fluxing through the sample. Under this circumstance, Equation 3.8 is no
more valid. To overcome this problem, the 3 omega method is introduced. In the 3 omega method,
an infinitely narrow metal line with four contact pads is deposited on the top of the sample surface
(see Figure 3.5b). An alternating current (𝐼 = 𝐼0cos (𝑤𝑡)) is applied through the metal heater line,
leading to the sample heating at a frequency of 2𝜔. Correspondingly, the temperature of the metal
line oscillates as 𝑇 = 𝑇0 + 𝑇2𝑤cos (2𝜔𝑡 + 𝜑), with 𝜑 as the phase shift. The temperature oscilla-
tion at the same time leads to the resistance oscillation of the metal heater line at a frequency of 2𝜔.
Combined with the source current at the frequency 𝜔, an oscillating voltage signal across the metal
line occurs at 3𝜔 and it is defined as[75]
𝑉3𝜔 =12𝑉0𝑅0𝑑𝑅𝑑𝑇
∆𝑇2𝜔 (3.9)
The 𝑉3𝜔 can be detected by a lock-in amplifier. Rearranging Equation 3.9, it is obtained
∆𝑇2𝜔 = 2𝑅0𝑉0𝑑𝑇𝑑𝑅
𝑉3𝜔 (3.10)
(a) Bulk sample (b) Thin film with deposited electrode for 3 omega method[76]
Figure 3.5 Schematic views of thermal conductivity measurements.
In a typical experimental setup, the temperature fluctuation is small enough that 𝑉0 = 𝑉𝜔. From the
temperature oscillation of the metal heater line which is as a function of heater frequency, the ther-
mal conductivity can be derived. [77] In the case of thin films on substrates, both the temperature
response of a film on the substrate and the temperature response for a substrate only are required.
To apply the 3 omega method for thermal conductivity measurement, the following conditions must
be fulfilled:
40
a) The film thickness (𝑡𝑓) must be far less than the thermal penetration depth in the film (𝑑𝑓).
b) The film must be much thinner than the heater half-width 𝑤.
c) The film should be deposited on the top of an electrical conducting substrate.
All the above discussions are applicable for insulating samples. If the interesting sample is electri-
cally conducting, e.g. the conducting polymers, a thin insulating layer between the metal heating
line and the test sample should be deposited.
3.4 Atomic Force Microscopy
Atomic Force Microscopy (AFM) consists of a cantilever spring (see Figure 3.6), to which a very
sharp tip is mounted at the end. In this technique, the tip scans the sample surface. The interaction
force between the tip and sample atoms is then recorded to provide a 3D profile of the surface on
nanoscale. This force depends on the spring constant of the cantilever and also the distance between
the probe and the sample surface. According to Hooke’s Law, the force, F, is given by [78]
𝐹 = −𝐶𝑠𝑝𝑟𝑖𝑛𝑔 ∙ 𝑥 (3.11)
in which 𝐶𝑠𝑝𝑟𝑖𝑛𝑔 is the spring constant and 𝑥 is the cantilever deflection.
The force regarding the tip distance from the sample is depicted in Figure 3.7. To obtain the surface
profile of samples, several modes can be applied. These modes are determined by the force: in the
contact mode, the tip is in contact with the sample surface all the time and the deflection of cantile-
ver is kept constant; in the non-contact mode, the tip does not touch the specimen at all with a con-
stant amplitude of the oscillation; in the intermittent mode or tapping mode, the tip intermittently
Figure 3.6 Basic principles of AFM. [79]
Figure 3.7 Atomic interaction force regardingthe tip distance from
sample. [78]
41
contacts the surface and oscillates with sufficient amplitude to prevent the tip from being trapped by
the soft sample layer. [80]
For soft materials, like polymers, contact mode AFM has specific limit as the lateral forces can
damage samples, causing artifacts and reducing the image resolution. Tapping mode AFM over-
comes these limitations and it is widely used for the morphology study of soft materials. In the tap-
ping mode, the cantilever is oscillated with the resonant frequency ranging from 50 kHz to 400 kHz.
When the cantilever is brought closer to the sample surface and touches it, the vibration amplitude
is reduced. The amplitude is measured and kept constant through a feedback mechanism. Hence,
topographical images are produced as the amplitude is dependent on the mean tip height. [80] In ad-
dition, the phase shifts of vibration are simultaneously detected when the oscillating cantilever in-
teracts with the sample surface. The phase shifts often correspond in a complex way with the sam-
ple surface, such as variations in the composition, adhesion and viscoelasticity. [80] Phase imaging is
the mapping of shifts. In particular for heterogeneous surfaces, it provides an enhanced phase con-
trast. This is important for mapping different components in the sample, for example, regions of
high and low surface hardness in the sample can be differentiated.
3.5 UV-Vis Spectroscopy
Visible light lies in the wavelength range 400-700 nm. When light is absorbed by matter, it pro-
motes the valence electrons of matter from their normal (ground) states to higher energy (excited)
states. This phenomenon can also occur in the ultraviolet region (down to 190 nm). The absorption
of light depends on the bonding system within the test sample. For example, 𝜋-electrons require
lower energy to the excited states than 𝜎 electrons. Therefore, increasing the number of 𝜋 bonds in
the test sample can shift the absorption to longer wavelength.
Figure 3.8 Monitoring the reduction of a polymer film on a glass substrate by UV-Vis spectroscopy. [81]
42
In a UV-Vis spectrometer, the sample is radiated with a range of wavelengths. A detector records
the absorbed wavelengths and the extent of absorption. With its sensitivity to the chemical bonds,
UV-Vis spectrometer can be utilized to monitor the condition in which a redox process occurs as
shown in Figure 3.8: The sample is reduced in a consequence of 𝑎 → 𝑏 → 𝑐. As a consequence, the
aborsption at shorter wavelength is observed.
3.6 Raman Spectroscopy
When light interacts with molecules, there are both elastic and inelastic scattering. In elastic scatter-
ing, the scattered light has the same frequency as the incident light. In inelastic scattering, the light
interacts with molecules in a way that its energy is either gained or lost. As a result, the scattered
light is shifted in frequency. The shifting in the frequency is called the Raman Effect. The principle
of spectroscopic transitions including Raman scatterings is depicted in Figure 3.9.
Raman Effect is based on the molecular deformation in an electric field determined by the molecu-
lar polarizability. If the polarizability is changed during the vibration, the test sample is Raman-
active. Therefore, Raman spectroscopy provides information about molecular vibrations that can be
used for sample identification. In Raman spectroscopy, a sample is illuminated with a monochro-
matic light source (e.g. laser) in the ultraviolet (UV), visible (Vis) and near infrared (NIR) range.
The scattered light is then collected by optics and filtered to obtain Raman spectra of the sample.
3.7 X-ray Diffraction
X-ray diffraction (XRD) is an analytical method used for the phase identification of a crystalline
Figure 3.9 Spectroscopic transitions underlying several types of vibrational spectroscopy. 𝜈0 indicates laser frequency, while 𝜈 is the vibrational quantum number. The virtual state is short-lived distortion of the electron distribution by the
electric field of the incident light. [82]
43
material and it provides information about unit cell dimensions. For example, the crystalline struc-
ture (lattice spacing) of tellurium can be determined by XRD. XRD is based on the wave interfer-
ence of a monochromatic X-ray and a crystalline sample. When a monochromatic X-ray hits the
sample (see Figure 3.10), the interaction of the incident X-ray with the sample could produce a con-
structive interference when Bragg’s law is satisfied.
Bragg’s law is defined as:
𝑛𝜆 = 2𝑑 𝑠𝑖𝑛𝜃 (3.12)
with 𝑑 as the distance between atomic layers in the crystal and 𝜆 is the wavelength of the incident
X-ray beam.
In XRD, a high regular structure is needed for the diffraction to occur. Therefore, only crystalline
solids will diffract and amorphous materials will not show up in the diffraction pattern.
A powder X-ray diffractometer (see Figure 3.11) is consisted of a source of X-ray tube, a sample
holder and an X-ray detector. In the cathode ray tube, the filament is heated to produce electrons.
By applying a voltage, the produced electrons are then accelerated towards a target and bombard the
target material. Consequently, a characteristic X-ray spectrum is generated. Target materials are
usually copper (Cu), cobalt (Co), iron (Fe), chromium (Cr) or molybdenum (Mo). These X-rays are
then directed onto the sample. When the sample and detector are rotated, the intensity of reflected
X-rays is recorded. According to Equation 3.11, a peak due to the constructive interference in inten-
sity will occur. From the XRD pattern, it is able to identify the crystal structure of known samples
by using the JCPDS cards from database.
Figure 3.10 Bragg’s diagram. [83]
Figure 3.11 Instrumentation of a X-ray diffractometer. [83]
3.8 X-ray Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (XPS) is an analytical technique to obtain the chemical infor-
mation (the composition and the chemicals state) about the surface of solid materials. It is realized
by using soft X-rays to examine core levels as shown in Figure 3.12: the atom in a molecule or a
44
solid absorbs photons of the X-ray, leading to the ionization and the emission of a core electron.
The energy of X-ray is the summation of the kinetic energy of emitted photoelectrons and the bind-
ing energy of the core electron. By measuring the kinetic energy of emitted photoelectrons, the
binding energy of electron could be obtained. Through plotting the intensity of photoelectrons ver-
sus the binding energy, XPS spectrum is generated. The XPS spectrum could be used for the ele-
mental identification and for the chemical state of element. To carry out an XPS measurement, an
ultrahigh vacuum system is needed to detect electrons and to avoid the surface reaction or contami-
nations.
Figure 3.12 Schematic diagram of XPS working principle. [84]
3.9 Transmission Electron Microscopy
A Transmission Electron Microscopy (TEM) consists of an electron source, an electromagnetic lens
system, a sample holder and an imaging system. The sample of interest is illuminated with electrons
under a high vacuum. The transmitted electrons through the sample are then collected. The trans-
mission of electron beam is highly dependent on the sample properties, such as the density and the
composition. Hence, the structural information of samples can be obtained by TEM.
For samples with the presence of crystal structures, the scattering from crystal planes introduces a
diffraction contrast. This contrast is determined by the orientation of the crystalline area inside the
sample with respect to the electron beam. [85] Based on this, TEM can be used to image individual
crystals. Moreover, the atomic arrangements within the crystalline structures can be clearly imaged
by the high resolution TEM (HRTEM).
45
4 Thermoelectric Properties of PEDOT:PSS by
the Addition and Post Treatment with Organic
Solvents
(Part of this work is published in Journal of Applied Physics, DOI: 10.1063/1.4864749)
4.1 The Role of Organic Solvents
It is well known that the addition of organic polar solvents, such as ethylene glycol (EG), [38,41,86–88]
diethylene glycol (DEG), [89] sorbitol [90] and dimethyl sulfoxide (DMSO) [41,43,87,91,92] into the
PEDOT:PSS solution could significantly increase the electrical conductivity of PEDOT:PSS films
by up to 2 or 3 orders of magnitude. The treatment of PEDOT:PSS solid thin films with salts, [93]
zwitterions, [94] acids, [30] co-solvents [95] and fluoro compounds [96,97] could also effectively improve
the electrical conductivity.
As the power factor of a TE material is proportional to its electrical conductivity, the addition of
polar solvents for higher electrical conductivity is confirmed to improve the power factor of
PEDOT:PSS films. [41,43] However, there is less study on the thermoelectric properties of
PEDOT:PSS films by the post treatment with organic solvents. Hence, this chapter focused on the
thermoelectric properties, in particular, the electrical conductivity of PEDOT:PSS films. 3 different
methods were applied. 1st DMSO addition method: DMSO was directly added into the PEDOT:PSS
solution. Magnetic stirring was used to make homogeneous mixtures.; 2nd Post treatment of the pris-
tine film method: sufficient amount of DMSO was dropped on top of the pristine film for a defined
time period with followed thermal annealing to dry up the solvents.; 3rd Combination of DMSO
addition and DMSO post treatment method: DMSO was added into the PEDOT:PSS solution to
prepare solid thin films, to which DMSO post treatment was further applied.
46
4.2 Influence of DMSO Addition and DMSO Post Treatment
4.2.1 Sample Preparation and Materials Characterization DMSO Addition
The designed mixture solutions were prepared by adding DMSO (Sigma Aldrich) directly into the
PEDOT:PSS solution (Clevios, PH 1000). Magnetic stirring at 800 rpm for 20 hours was used to
homogenize the mixture. Films with different volume concentrations of DMSO were then spin
coated on glass and silicon substrates (20 mm × 20 mm). These substrates were successively pre-
cleaned with deionized water, acetone and iso-propanol. The spin condition was fixed at 3000 rpm
for 20 s. Afterwards samples were dried on a hot plate at 130°C for 10 min. All steps were done
under air atmosphere. The labeling of sample is detailed in Table 4.1, with xvol% representing the
volume concentration of DMSO. The polymer film without DMSO addition is defined as the pris-
tine film or P_0vol%_w/o post.
Sample With DMSO Addition With DMSO Post Treatment
P_xvol%_w/o post YES NO
P_xvol%_w post YES YES
Table 4.1 Sample nomenclature regarding DMSO addition and DMSO post treatment.
DMSO Addition and DMSO Post Treatment
PEDOT:PSS films with different DMSO concentrations were prepared with the above mentioned
procedure. Then 150 𝜇𝐿 DMSO was dropped on top of these films to cover the whole surface. The-
se samples were left inside the fume hood in the ambient atmosphere for 30 min. Hot plate baking
at 130°C for 30 min was followed to dry up any residual solvents.
Materials Characterization
The surface resistance of samples (𝑅𝑠) on glass substrates was measured with the 4-point probe
method by using Omnimap RS-35C at room temperature. The corresponding film thickness (𝑡𝑓)
was characterized using an Alpha Step 500 surface profilometer. The electrical conductivity is then
calculated according to Equation 3.3. The Seebeck coefficient of sample, which was deposited on a
glass substrate, was characterized by SRX. The AFM images were recorded by using a 5500 AFM
(Agilent) in the tapping mode. AFM cantilever with a resonant frequency of 330 kHz is selected for
the soft polymer samples. The tip on the cantilever is with a radius less than 7 nm. To analyze the
AFM images, Gwyddion software was used. Surface roughness (Rq) is the root mean square aver-
47
age of height deviation of taken area provided by Gwyddion software analysis. Raman spectra were
conducted by using a 514 nm laser line as the excitation source on a LabRam HR800 (HORIBA
Scientific, Villeneuve d’Ascq, France). The samples for Raman spectroscopy were deposited on
silicon substrates. The surface composition was investigated using XPS on a PREVAC XPS setup
equipped with a VG Scienta MX-650 AlK𝛼 X-ray source and an XM-780 monochromator as well
as a VG Scienta R3000 hemispherical analyzer. The XPS investigations were carried out under an
ultra-high vacuum of 1⋯ 5 × 10−10 mbar.
4.2.2 Effect of DMSO Addition and Post Treatment During the characterization of Seebeck coefficient for the low conducting pristine thin film (high
resistance between thermocouples), huge noise (100%) was detected (shown in Figure 4.1), hinder-
ing the accuracy of results. Therefore, the Seebeck coefficient of a drop casted pristine thick film
was characterized in order to estimate the Seebeck coefficient of the thin pristine film. As it is
shown in Figure 4.2, the Seebeck coefficient of a thick pristine film is 16.55 ± 5% 𝜇𝑉/𝐾 at room
temperature. Its electrical conductivity is similar to that of a spin coated pristine thin film (shown in
Figure 4.2). As there are no chemicals were used during the preparation of both pristine thick and
thin films, it is reasonable to assume that the Seebeck coefficient of the pristine thin film is similar
to that of the drop casted thick film. Zhang et al. observed that a maximum power factor was found
for drop casted PEDOT:PSS thick film at a certain amount of DMSO addition into the
PEDOT :PSS solution (PH 1000). [69] Hence, in this chapter, the focus is to look for the optimum
power factor of PEDOT:PSS thin films by the addition of DMSO.
Figure 4.1 Seebeck coefficient of a pristine film by spin
coating.
Figure 4.2 Seebeck coefficient of a drop casted pristine
thick film.
48
It is shown in Figure 4.3, the addition of DMSO into PEDOT:PSS solution improves the electrical
conductivity and the electrical conductivity levels off at 5vol% DMSO. Similar to literature, the
Seebeck coefficient in Figure 4.4 is slightly reduced with increased DMSO volume fraction. Among
the studied PEDOT:PSS thin films, an optimized power factor of 18.2 µW/(mK2) is obtained for the
film with 5vol% DMSO addition (shown in Figure 4.5). On the other hand, a single DMSO post
treatment increases the electrical conductivity (see Figure 4.3) to a value, which is comparable to
the highest value achieved by the DMSO addition method. And the Seebeck coefficient falls in the
same range as for those films with only DMSO addition. Correspondingly, the power factor of film
with only DMSO post treatment (P_0vol%_w post) exhibits much higher power factor than films
Figure 4.3 Electrical conductivity of PEDOT:PSS thin
films by DMSO addition and DMSO post treatment.
Figure 4.4 Seebeck coefficient of PEDOT:PSS thin films by DMSO addition and DMSO post treatment.
Figure 4.5 Power factor of PEDOT:PSS thin films by DMSO addition and DMSO post treatment. with DMSO addition. The additional DMSO post treatment on DMSO added polymer films further
promotes the electrical conductivity. The higher DMSO volume fraction that is previously added
into the PEDOT:PSS solution, the smaller the improvement in the electrical conductivity is ob-
49
served. On the other hand, the Seebeck coefficient demonstrates a rather complex dependence on
the previously added DMSO concentration. At low DMSO addition (< 4vol%), the additional
DMSO post treatment is detrimental for the Seebeck coefficient that an intercoupling behavior be-
tween the electrical conductivity and the Seebeck coefficient is observed. In contrast, at high
DMSO concentration (>4 vol%), the additional post treatment results in a simultaneous increase in
the Seebeck coefficient and the electrical conductivity. Films with the combination of DMSO post
treatment and DMSO addition demonstrate higher power factor than those with only DMSO addi-
tion as shown in Figure 4.5. Among the studied samples, the film with only DMSO post treatment
is evidenced with a high power factor. In comparison to the DMSO addition method, DMSO post
treatment does not require long time mixing. The high power factor brought by DMSO post treat-
ment makes it much more effective than either DMSO addition or the combination of both methods
to tune the thermoelectric properties of PEDOT:PSS films.
To elucidate the effect of DMSO addition, film morphologies of selected samples were recorded by
AFM in the tapping mode shown in Figure 4.6. Films with low DMSO concentration were selected
as there is apparent difference in the electrical conductivity between films with and without DMSO
post treatment. The topology of the pristine film (P_0vol%_w/o post) shows no clear structure. The
topology pictures change significantly, with appearance of some elongated large domains, with the
addition of DMSO. It is well known that PEDOT:PSS film is composed of hard PEDOT-rich grains
and soft PSS-rich grains. As it is discussed in Chapter 2, phase images of AFM could be used to
differentiate the hardness of samples. In the case of phase image of PEDOT:PSS film, the bright
and dark phase shifts correspond to hard PEDOT-rich grains and soft PSS-rich grains, respectively. [89] In the phase image of the pristine film, there is low degree phase separation. In comparison, the
PEDOT:PSS films with DMSO addition (e.g. at 1 vol% and 3 vol%) all display distinguishable
elongated grain structures as observed in phase images. The well-defined grain structure in modi-
fied PEDOT:PSS films indicates that DMSO addition triggers phase separation, generating a con-
nected network of elongated PEDOT-rich grains. PEDOT-rich grains have a much higher intrinsic
electrical conductivity than the PSS-rich grains. The PSS-rich grains are essentially insulating be-
cause PSS is a weak ionic conductor. [90] Correspondingly, the electrical conductivity of films added
with DMSO is increased. Nevertheless, it is difficult to tell any differences from the morphologies
between the 1 vol% DMSO added film and the 3 vol% DMSO added film even though much higher
electrical conductivity was obtained for the film with 3 vol% DMSO addition.
50
Figure 4.6 AFM images of films with only DMSO addition with scan size 1 𝜇𝑚 × 1 𝜇𝑚. Upper row: topology; down row: phase image.
The topology and phase image of samples with post treatment are shown in Figure 4.7. The same
effect on the morphology is also observed for DMSO post treated films (P_0vol%_w post): an in-
terconnected network of elongated PEDOT-rich grains is generated. Even though the additional
DMSO post treatment on film with 1 vol% DMSO addition simultaneously increases the electrical
conductivity (4 times higher) and Seebeck coefficient, no observable morphological difference be-
tween P_1vol%_w/o post and P_1vol%_w post is found. This is also the case for the between sam-
ple P_3vol%_w/o post and P_3vol%_w post. Nevertheless, the surface roughness is altered (shown
in Table 4.2). The pristine film has a 𝑅𝑞 of 1.36 nm. By increasing the DMSO concentration in the
PEDOT:PSS solution, the roughness of thin films is changed to 1.44 nm and 1.82 nm with 1 vol%
and 3 vol% DMSO addition, respectively. This surface roughening confirms the phase separation
triggered by DMSO addition. The film with only DMSO post treatment has a roughness of 2.00 nm.
In addition, it is observed that the 2nd post treatment of previously DMSO added films further dete-
riorates the film surface. Kim et al. [38] and Alemu et al. [98] both pointed out that depletion of insu-
lating PSS chains from the film surface during the post treatment with organic compounds contrib-
51
utes to the film surface roughening. The diminishing of film thickness (shown in Figure 4.8) further
confirms that the DMSO post treatment depletes the insulating PSS chains.
Figure 4.7 AFM images of films with DMSO addition and DMSO post treatment. Scan size 1 𝜇𝑚 × 1 𝜇𝑚. Upper row:
topology; down row: phase image.
Sample Rq [nm] Sample Rq [nm]
P_0ovl%_w/o post 1.36 P_0ovl%_w post 2.00
P_1ovl%_w/o post 1.44 P_1ovl%_w post 1.85
P_3ovl%_w/o post 1.82 P_3ovl%_w post 2.20
Table 4.2 Surface roughness of PEDOT:PSS thin films subjected to DMSO addition and DMSO post treatment.
52
Figure 4.8 Evolution of film thickness regarding DMSO addition and DMSO post treatment.
The surface composition was recorded by XPS with the focus on the sulphur atom as shown in Fig-
ure 4.9. In the XPS sulphur spectra of the pristine film, there are two dominant peaks. One locates at
~164 eV, which is assigned to the sulphur atom in PEDOT chains. The other peak appears at ~168
eV, which is originated from the sulphur atom in PSS chains. [99] A small amount addition (1vol%)
of DMSO does not bring in noticeable change in the sulphur signal. However, the DMSO post
treated films, which are proven to be high conducting, exhibit much higher intensity of the sulphur
atom of PEDOT. The surface sulphur composition derived from XPS is listed in Table 4.3. Com-
pared to the pristine film, the PEDOT to PSS ratio of the film with 1 vol% DMSO inclusion is
Figure 4.9 Surface composition of the sulphur atom for PEDOT:PSS thin films by XPS.
53
Sample PEDOT/PSS ratio at surface
P_0vol%_w/o post 0.366
P_0vol%_w post 0.712
P_1vol%_w/o post 0.381
P_1vol%_w post 0.612
Table 4.3 The PEDOT to PSS ratio on the surface of PEDOT:PSS thin films.
increased from 0.366 to 0.381, indicating a phase separation for higher content of PEDOT chains.
For the film with only DMSO post treatment, the PEDOT/PSS ratio increases up to 0.712, suggest-
ing a high content of PEDOT chains are resulted in after the post treatment. The additional DMSO
post treatment for 1 vol% DMSO added film shows a PEDOT to PSS ratio of 0.612. The 2 times
increase in the PEDOT/PSS ratio after post treatment confirms the removal of PSS chains, which is
in accordance with the roughening of film surface and the thinning of film thickness. Furthermore,
the high extent of PEDOT chains on surface enhances the PEDOT feature in the XPS spectra: the
spin-split components with 1.5 eV energy difference for sulphur atom in PEDOT appear.
In the next, the Raman spectroscopy was used to analyze the possible change in the chemical struc-
ture and the chain conformation. These samples were selected as there is large difference in the
electrical conductivity for samples with or without post treatment. The chosen excitation laser at
514 nm is in resonance with the neutral form of PEDOT. [100] The weak spectra signal shown in
Figure 4.10 indicates that all the investigated structures are in the oxidative form. The principal
peak at ~1450 𝑐𝑚−1 in PEDOT:PSS film is related to the symmetric 𝐶 = 𝐶 stretching vibration.
Ouyang et al. reported that secondary dopants change the thiophene ring of PEDOT chains from a
benzoid to a quinoid structure. [101] Since the interaction among linear PEDOT chains in quinoid
structure is stronger than that in coiled chains in benzoid structure, the electrical conductivity is
increased. [101] According to his work, this change in PEDOT chain structure is evidenced in the
Raman spectra as a red shift and narrowing of the band between 1400 and 1500 𝑐𝑚−1. [101] Howev-
er, the Raman spectra of film with 1 vol% DMSO addition does not show a red shift for the band at
~1450 𝑐𝑚−1 with comparison to the pristine film (P_0vol%_w/o post). Even for the high conduct-
ing DMSO post treated films, of which the conductivities are enhanced more than 100 orders of
magnitude higher, the Raman spectra resemble that of the pristine film, indicating that the thiophene
ring structure is not changed.
Combining all the experimental observation, both DMSO addition and DMSO post treatment result
in the phase separation that an elongated, well connected network of conducting PEDOT-rich grains
54
is formed, leading to high electrical conductivity. In addition, DMSO post treatment removes the
insulating PSS chains from the film surface, further improving the electrical conductivity. The us-
age of DMSO, either by direct addition or post treatment, does not change the chemical structure of
PEDOT chains.
From above mentioned analysis, it is concluded that the secondary dopant does not change the
chemical structure. Instead, it modifies the morphology and depletes the insulating PSS chains from
the film surface. A remaining question is what could be the underlying reason behind the observed
thermoelectric properties. For a deeper understanding, the charge carrier mobility, charge carrier
concentration and also the crystallinity need to be considered regarding the effect of secondary do-
pants. For PEDOT:PSS films, the charge carrier mobility and concentration depend on the number
of non-ionized dopants PSS chains. [44] Removing the hydrophilic PSS chains by using secondary
dopants, could increase the charge carrier concentration, which in turn affects the electrical conduc-
tivity and Seebeck coefficient. [44] Furthermore, a better PEDOT crystallinity leads also to a higher
Seebeck coefficient. [102] It is well studied that the addition of secondary dopants into PEDOT:PSS
solution increases the charge carrier mobility and concentration, which leads to the high electrical
conductivity and relative stable Seebeck coefficient. [41] The subsequently performed DMSO post
treatment is also reported to result in a better crystallinity [103], which might attribute to the slightly
higher Seebeck coefficient. The combined effect on the charge carrier concentration, mobility and
film crystallinity might be assigned to the observed thermoelectric properties regarding the DMSO
addition and post treatment.
Figure 4.10 Raman spectra of PEDOT:PSS thin films under different fabrication conditions.
55
4.3 Influence of Post Treatment with Different Organic Solvents
In the previous section, post treatment is demonstrated as a simpler and more effective method to
enhance the electrical conductivity than direct addition. In particular, for chemicals with low boiling
points, the added chemicals evaporate before they could interact with polymer chains during the
film fabrication process, e.g. spin coating. [98] As a result, no high electrical conductivity is ob-
served. Instead, solvents based post treatment method allows solvents with sufficient time to inter-
act with polymer chains, which effectively increases the electrical conductivity. There have been
plenty of investigations on the effect of solvents addition, mainly secondary dopants, on the electri-
cal properties of PEDOT:PSS films. However, for a post treatment, the solvent variation remains an
open question on the TE properties of PEDOT:PSS films. Therefore, post treatment is employed in
this section with various chemicals, including secondary dopants (DMSO and EG) and amines (eth-
anolamine and ammonia solution) as the post treatment medium to tune the TE properties of
PEDOT:PSS thin films.
4.3.1 Sample Preparation and Materials Characterizations
Sample Preparation
Pristine thin films were prepared by spin coating (3000 rpm, 20 s) the as received PEDOT:PSS so-
lution (Clevios, PH 1000) on precleaned glass and silicon substrates (20 𝑚𝑚 × 20 𝑚𝑚). Hot plate
baking at 130°C for 10 min was used to make solid films. Pristine films were then subjected to a
post treatment with DMSO, ethylene glycol (EG, Sigma Aldrich) and ethanolamine (MEA, Sigma
Aldrich), respectively. The post treatment procedure is the same as in previous section 4.2.1. (Note:
Since the water content in the ammonia solution dissolves the pristine film, ammonia solution was
not used as the post treatment medium of the pristine film.) In addition, a two-step post treatment
method was performed, of which the DMSO pre-treated films were post treated further with an
ammonia solution and MEA, respectively. In this two-step post treatment method, the 2nd step of
post treatment was done by dropping MEA or ammonia solutions (25%, Merck) on top of films for
30 min. Followed thermal annealing on a hot plate at 130°C for 15 min was used to dry up films.
The detailed nomenclature for discussions is listed in Table 4.4.
Materials Characterizations
The electrical conductivity, Seebeck coefficient, morphological information and Raman spectra
were characterized as in Section 4.2.1. The UV-Vis absorption spectra from 300 nm to 1400 nm
56
Sample 1st post treatment with solvents 2nd post treatment with solvents
P0 NO NO
P1 YES, with DMSO NO
P2 YES, with EG NO
P3 YES, with MEA NO
P11 YES, with DMSO YES, with MEA
P12 YES, with DMSO YES, with ammonia solution
Table 4.4 Sample notation of PEDOT:PSS thin films treated with vary organic solvents.
wavelength were taken with a Shimadzu UV-3101PC UV-Vis-NIR scanning spectrometer. In the
beginning, the UV-Vis spectra of samples on glass substrates were recorded. Then the UV-Vis
spectra of the glass substrate were characterized. The UV-Vis spectra of samples were obtained by
subtracting the UV-Vis spectra of samples on glass substrates with the UV-Vis spectra of the glass
substrate.
4.3.2 The Correlation between the Post Treatment Medium and the Thermoelec-
tric Properties
As it is shown in Figure 4.11, the electrical conductivity of films post treated with either DMSO or
EG is ~ 900 times higher than that of the pristine film P0. Post treatment with MEA alone also in-
creases the electrical conductivity (from 1 S/cm to 85 S/cm). However, the extent is much smaller
than that by the post treatment with DMSO or EG. The 2nd step of MEA post treatment greatly di-
minishes the electrical conductivity (from 930 S/cm of P1 to 231 S/cm of P11). The same phenom-
enon is observed by using the ammonia solution for the 2nd step post treatment of P1, but with a
lower degree of degradation in the electrical conductivity (P12: 484 S/cm). The Seebeck coeffi-
cients for DMSO or EG post treatment modified films do not vary too much with respects to that of
pristine thick film. This confirms further secondary dopants, either by means of direct addition or
post treatment, do not change the chemical structure of PEDOT chains. MEA post treatment results
in the film P3 with much higher Seebeck coefficient than P0, P1 and P2. The additional MEA or
ammonia solution post treatment after DMSO post treatment both increases the Seebeck coefficient,
in which 54% and 16% increase is observed for MEA and ammonia solution post treatment, respec-
tively. The post treatment with either MEA or ammonia solution leads to the conflicting phenome-
non between the Seebeck coefficient and the electrical conductivity. The high electrical conducting
films post treated with either DMSO or EG, despite of their low Seebeck coefficients, exhibit larger
power factors as shown in Figure 4.12.
57
Figure 4.11 Seebeck coefficient and electrical conductivity of PEDOT:PSS films regarding the post treatment medium.
Figure 4.12 Power factor of PEDOT:PSS thin films regarding the post treatment medium.
Chemicals Dielectric Constant Boiling Point/°C[104]
DMSO 47 (25°C)[105] 189
EG 37.7 (20°C)[106] 195
MEA 31.94 (20°C)[107] 170.8
Ammonia solution - 37.7[108]
Table 4.5 Physical properties of the chemicals.
It is well studied that polar solvents of high boiling point, such as DMSO and EG, are able to trigger
the phase separation, leading to the formation of elongated, well connected networks of PEDOT-
rich grains. Hence, high electrical conductivity can be yielded. Alemu et al. stated that the
hydrophilicity and the dielectric constant are crucial factors, which determines the electrical con-
ductivity when chemicals are used to modify solid thin films. [98] To evaluate used chemicals, their
physical properties are listed in Table 4.5. DMSO, EG and MEA all have high dielectric constant
and high boiling point. Despite of the similarity in the dielectric constant and the boiling point,
MEA is observed with different effect on the electrical conductivity with comparison to DMSO and
EG. The additional MEA post treatment in the two-step post treatment method even degrades the
electrical conductivity of DMSO pretreated films. In previous section, it is shown that the electrical
conductivity of a PEDOT:PSS film is related to the morphology. Therefore, morphological infor-
mation was collected by AFM as presented in Figure 4.13 the topology and in Figure 4.14 the phase
images.
Pristine film P0 with low electrical conductivity shows a typical topology of PEDOT:PSS films, in
which no clear feature could be observed due to the presence of PSS chains on surface. Hydrophilic
solvents such as DMSO and EG (which are with high boiling point and applied with sufficient
58
amount during the post treatment) are able to deplete the hydrophilic and insulating PSS chains
from the surface. Consequently, an interconnected network of elongated PEDOT-rich grains for
high electrical conductivity is generated. The corresponding phase images in Figure 4.14 reveal the
significant change of these grains. At the same time, the film roughness is affected (see Table 4.6).
The DMSO and EG post treated films have surface roughness of 2.05 nm and 2.03 nm, respectively.
The roughening of film surface suggests the phase separation and the depletion of PSS chains after
the DMSO and EG post treatment. The post treatment of the P0 with MEA, which also has high
dielectric constant and high boiling point, results in a similar surface topology. However, it slightly
increases the electrical conductivity but results in much higher Seebeck coefficient. For the two-
steps-post-treated film P11, despite of the decrease in the electrical conductivity and the enhance-
ment in the Seebeck coefficient, the 2nd step post treatment with MEA does not modify the film
morphology as it is observed in AFM images. In addition, the change in the film roughness is small.
While for P12, of which the 2nd step post treatment is with ammonia solution, there are also elon-
gated PEDOT-rich grains observed in the phase image. However, these grains seem to diffuse
Figure 4.13 Topology of PEDOT:PSS thin films post treated with different solvents. . Scan size 1 𝜇𝑚 × 1 𝜇𝑚.
59
Figure 4.14 Phase images of PEDOT:PSS thin films post treated with different solvents. . Scan size 1 𝜇𝑚 × 1 𝜇𝑚.
together, reflecting in a smaller surface roughness. The water content in the ammonia solution could
possibly attribute to this phenomenon. From the morphological information, it is difficult to differ-
entiate the effect of MEA and ammonia solution from DMSO and EG.
Next, attention is paid to the chemical structures of PEDOT chains. Crispin et al. presented that the
Seebeck coefficient of PEDOT chains indirectly depends on their oxidation level. [33] Chemical pro-
cess, [33] acid-base chemistry [109] and electrochemical process [110] can be used to modify their oxi-
dation level. For example, when the oxidation level is reduced (dedoping), PEDOT chains are given
with electrons thereby they are gradually reduced from bipolaron to polaron and neutral state as it is
depicted in Figure 4.15. These three states attribute to the absorptions at different wavelengths in
Sample P0 P1 P2
Rq [nm] 1.49 2.05 2.03
Sample P3 P11 P12
Rq [nm] 1.83 2.10 1.78
Table 4.6 Surface roughness of PEDOT:PSS thin films after the post treatment with different organic solvents.
60
the UV-Vis spectra: the neutral polymer chains have absorption at ~ 600 nm; chains in the polaron
state show absorption at ~ 900 nm and chains in the bipolaron state are with a broad absorption
near infrared. [111–114] Among these three states, the highest oxidative state, bipolaron, has high elec-
trical conductivity but low Seebeck coefficient. In contrast, PEDOT chains in the neutral state (the
lowest oxidative state) have the highest Seebeck coefficient and the lowest electrical conductivity.
During the reduction of PEDOT chains, the peaks at ~ 900 nm and ~ 600 nm start to appear. Con-
currently, the electrical conductivity decreases and the Seebeck coefficient increases. The spectra of
both P1 and P2 resemble that of P0 (see Figure 4.16), indicating DMSO or EG post treatment does
not alter the chemical structure of PEDOT chains. This result is in agreement with the trivial change
in the Seebeck coefficient. DMSO and EG are only secondary dopants which trigger the phase sepa-
ration for a higher electrical conductivity of the PEDOT:PSS film. MEA post treated film, in con-
trast, demonstrates a strong absorption band at ~ 900 nm and a weak absorption band at ~ 600 nm,
revealing the reduction ability of MEA on the PEDOT chains. As a result, the corresponding film
P3 exhibits a much higher Seebeck coefficient in comparison P1 and P2. The broader absorption at
~ 900 nm and the lower Seebeck coefficient of P12 than P11 indicates that the structure of used
amines determines the reduction degree of the PEDOT chains. Hojati-Talemi et al. proposed that
the covalent bonding of amine to PEDOT chains neutralizes the delocalized positive charges in the
Figure 4.15 Sketch of a PEDOT chain transition from bipolaron to polaron and neutral states.
Figure 4.16 UV-Vis spectra of PEDOT:PSS films post treated with different solvents.
61
PEDOT chains, thus, interrupts the conjugation of double bonds. [115] Compared to ammonia solu-
tion, the nitrogen lone pair electrons in MEA are easier to be donated as it is evidenced by the high-
er degree of reduction of PEDOT chains of P11. The presence of the hydroxyl group in MEA possi-
bly attributes to its stronger reduction ability than the ammonia solution. Furthermore, the two-
steps-treated film P11 has a slight difference in the spectra than P3: the absorption band at ~ 600
nm is barely observable for P11. As DMSO pre-treated film and the pristine film have different film
morphologies, the difference in the absorption band at ~ 600 nm might be due to the film topology
to which the amine post treatment is applied. The film (P1), which is with much higher degree of
phase separation, allows only the bonding of amine on limited surface areas of PEDOT chains. On
the other hand, there is a lower degree of phase separation in the pristine film P0. Thereby, the hy-
drophilic amine could have access to more PEDOT chains and reduce the chains.
Further evidence of the structure change is provided by the Raman spectra in Figure 4.17. The simi-
larity among the spectra of P0, P1 and P2 further confirms that DMSO and EG have only effect on
the film morphology. Instead, when MEA is used during the post treatment, the intensity of Raman
spectra is greatly enhanced. This increase in the Raman intensity (laser 514 nm) is correlated with
the increase of the optical absorption, in particular by the appearance of the ~ 600 nm and ~ 900
nm absorption bands in the UV-Vis spectra. MEA modified films (P3 and P11) exhibit 2 dominant
sharp peaks at ~ 1430 𝑐𝑚−1 and ~ 1516 𝑐𝑚−1 . This Raman spectrum is similar to a electro-
chemically reduced PEDOT:PSS thin film. [116] It is indicated that MEA is able to reduce the
PEDOT chains, inducing the conflicting phenomenon between the electrical conductivity and the
Seebeck coefficient. In contrast to MEA, the additional post treatment with ammonia solution does
Figure 4.17 Raman spectra of PEDOT:PSS films post treated with different solvents.
62
not introduce large modification to the Raman spectra. The lowering of wave number for C = C
symmetric stretching vibration and the increased peak intensity at 1510 𝑐𝑚−1 evidences the mild
reduction ability of ammonia solution on PEDOT chains.
4.4 Conclusions
The DMSO addition, as it is confirmed, initiates the phase separation between PEDOT-rich grains
and PSS-rich grains, leading to the formation of an interconnected network with better electrical
conductivity. DMSO post treatment, on the other hand, is found to be much more efficient regard-
ing the improvement of the electrical conductivity. It is for the first time that DMSO post treatment
is compared to DMSO addition method to modify the thermoelectric properties of PEDOT:PSS thin
films. DMSO post treatment is proven to be a more efficient method to increase the power factor of
PEDOT:PSS thin films. The analysis confirms that DMSO post treatment does not only lead to the
phase separation, but also removes the insulating PSS chains from the film surface for higher elec-
trical conductivity. DMSO and EG, as secondary dopants for PEDOT:PSS, are confirmed not to
change the chemical structure of PEDOT chains.
MEA and ammonia solution, on the other hand, work as reductants which donate electrons to
PEDOT chains and neutralize the positive charge. In turn, conflicting phenomenon between the
electrical conductivity and the Seebeck coefficient is yielded. This work also confirms that the abil-
ity of amines (MEA and ammonia solution) bonding to the PEDOT chains is determined by the
nature of amine. At the same time, the film morphology is also a crucial factor which determines
the reduction degree when amines affect the polymer chains. Compared to films with highly segre-
gated PEDOT-rich grains, films with lower degree of phase separation give rise to more accessible
surface areas of PEDOT chains, leading to a higher possibility of amine bonding. As a result, more
PEDOT chains in dedoped forms, which are with higher Seebeck coefficient and lower electrical
conductivity.
63
5 Thermoelectric Properties of PEDOT:PSS
Films by the Application of Ionic Liquid
(This work is published in Journal of Materials Chemistry A and featured as a cover page,
DOI: 10.1039/C3TA11209H)
5.1 Introduction to Ionic Liquids (ILs)
Ionic liquids (ILs) contain large, organic cations (see Figure 5.1) and different types of anions.
Many ILs are liquid over a wide temperature range (often> 200°𝐶). ILs have negligible vapor pres-
sure, high thermal and chemical stability, high ionic conductivity, high electrochemical window,
low flammability, moderate viscosity and ability to dissolve compounds of widely varying polari-
ties. [117,118,118,119] In particular, room temperature ionic liquids (RTILs) are of high interest because
they are in liquid phase at ambient temperature. With their unique characteristics, their potentials in
diversified technological fields such as electro-deposition and energy managements are being ex-
plored.
In the field of electro-deposition, the deposition of metals and semiconductors is usually limited by
the common aqueous or organic media which have high flammability, low chemical stability, low
electrochemical window and etc. For example, the electro-deposition of aluminum occurs below -
1.67 V vs. normal hydrogen electrode, which excludes aqueous solutions as the deposition media.
The other organic solvents based electro-deposition of aluminum, however, involves combustible
and explosive compounds. [120] The application of ILs as the electro-deposition medium makes it
possible for a deposition process without the usage of any flammable organic solvents.
Figure 5.1 Formulation of cations in ionic liquids. [121]
64
In the energy management field, the research focus is to apply ILs in lithium batteries. The present
lithium batteries consist of a positive electrode, a negative electrode and a liquid solution of lithium
salt between these two electrodes. A mixture of organic solvents (alcoholic solvents) is usually used
to dissolve the lithium salt, which could also be trapped within a gel. Due to the high flammability
of organic solvents, the application temperature of lithium batteries ranges only from -20°C to
50°C. With their unique high thermal stability and the low vapor pressure, ILs ensures the reliability
by preventing the thermal runaway and the pressure build up by replacing organic solvents in lithi-
um batteries.
At the same time, the integration of ionic moieties into functional polymers could result in a new
class of polymeric materials with a wide range of applications. [117,122,123] Döbbelin et al. showed
that the addition of a series of ILs results in high electrically conducting PEDOT:PSS films and
high ionic characteristics are observed in these films. [124] By using 1-ethyl-3-methylimidazolium
tetracyanoborate (EMIMTCB) as the additive for PEDOT:PSS solution, Badre et al. extended the
electrical conductivity of PEDOT:PSS thin films up to 2084 S/cm. [125] Besides, these high trans-
parent thin films could be used as transparent electrode materials. Through ionic exchange, ILs are
being able to modify PEDOT:PSS films with higher hydrophobicity in order to achieve better water
resistance. [126] ILs also offer a new strategy to synthesize new materials for electronic and optoelec-
tronic devices through the integration of the ionic moiety inside the polymer chains or as
counterions for the polyelectrolyte complex. [127]
A preliminary research of tuning the thermoelectric properties of PEDOT:PSS by ILs addition was
done by Liu et al., in which a simultaneous increase in the electrical conductivity and the Seebeck
coefficient was recorded and a maximum power factor of ~ 10 𝜇𝑊/(𝑚𝐾2) was obtained. [128]
Nevertheless, the role of ILs on the physical and chemical structures of polymer chains and its cor-
relation to the thermoelectric properties are unknown. Taken into consideration the unique proper-
ties of ILs, this chapter focused on the application of IL, 1-ethyl-3-methylimidazolium
tetrafluoroborate ( 𝐸𝑀𝐼𝑀𝐵𝐹4 ), as the post treatment medium to tune the TE properties of
PEDOT:PSS thin films. DMSO was used together as the post treatment medium with the attempt to
improve the electrical conductivity.
5.2 Sample Preparation and Materials Characterization
Badre et al. reported that transparent thin PEDOT:PSS films with high electrical conductivity could
be obtained by spin coating homogeneous IL/PEDOT:PSS mixtures, in which the weight fraction of
65
ILs went up to 2 wt%. [125] However, when 𝐸𝑀𝐼𝑀𝐵𝐹4 (Sigma Aldrich) was added into the
PEDOT:PSS solution (Clevios, PH 1000) in this work, no homogeneous mixture was obtained even
with 0.23wt% of 𝐸𝑀𝐼𝑀𝐵𝐹4. Instead, PEDOT:PSS solution starts to gel as soon as 𝐸𝑀𝐼𝑀𝐵𝐹4 was
added as shown in Figure 5.2. No PEDOT:PSS films could be spin coated from this gel mixture.
Therefore, post treatment of PEDOT:PSS films with 𝐸𝑀𝐼𝑀𝐵𝐹4 as the medium was used instead.
For the film preparation, the as received PEDOT:PSS solution was spin coated on pre-cleaned glass
and silicon substrates (20 mm × 20 mm) with fixed spin parameter at 3000 rpm and 20 s. After spin
coating, samples were then dried on a hot plate at 130°C for 10 min at ambient atmosphere to obtain
pristine films. 𝐸𝑀𝐼𝑀𝐵𝐹4/DMSO mixture solutions (with different concentration of 𝐸𝑀𝐼𝑀𝐵𝐹4 in
the mixture) were prepared by the homogenization of the mixtures with a shaker at 300 rpm for 20
min. Then ~ 150 𝜇𝐿 of each 𝐸𝑀𝐼𝑀𝐵𝐹4/DMSO mixture were dropped on top of the pristine films to
cover the whole surface. These samples were left inside the fume hood at ambient atmosphere for
30 min. Afterwards films were thoroughly rinsed with deionized water and dried on a hot plate at
ambient atmosphere at 120°C for 10 min. To exclude the effect of rinsing and thermal annealing
used for 𝐸𝑀𝐼𝑀𝐵𝐹4 post treated films, samples named as P_DMSO'' were prepared by dropping
DMSO on top of the film surface for 30 min. Rinsing with deionized water and hot plate baking at
120°C for 10 min were followed to prepare solid thin films. The nomenclature of samples is shown
in Table 5.1.
The electrical conductivity, Seebeck coefficient, morphological information, UV-Vis spectra, Ra-
man spectra and XPS were characterized as in Section 4.2.1.
Figure 5.2 Demonstration of the gelation of a PEDOT:PSS solution after the addition of 1.8 vol% (0.23 wt% in solu-tion) EMIMBF4 (the vial is put upside down).
66
Nomenclature EMIMBF4/DMSO mixture
(v/v), post treatment
1st DMSO post treatment +
2nd EMIMBF4 post treatment
1st DMSO post treatment +
rinsing + thermal annealing
pristine film NO NO NO
P_DMSO YES, 0/10 (v/v) NO NO
P_DMSO’’ NO NO YES
P_ 1/9 Mix YES, 1/9 (v/v) NO NO
P_5/5 Mix YES, 5/5 (v/v) NO NO
P_IL YES, 10/0 (v/v) NO NO
P_DMSO_IL NO YES NO
Table 5.1 Sample nomenclature of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures.
5.3 Influence of EMIMBF4/DMSO Mixtures on the Thermoelectric Properties
The effect of 𝐸𝑀𝐼𝑀𝐵𝐹4 on the electrical conductivity is shown in Figure 5.3. Post treatments with
pure DMSO, pure 𝐸𝑀𝐼𝑀𝐵𝐹4 or 𝐸𝑀𝐼𝑀𝐵𝐹4/DMSO mixtures all increase the electrical conductivity
of PEDOT:PSS films. Compared to the high electrical conductivity brought by DMSO post treat-
ment, the effect with only 𝐸𝑀𝐼𝑀𝐵𝐹4 is much smaller. Including 𝐸𝑀𝐼𝑀𝐵𝐹4 in the post treatment
mixture or performing an additional 𝐸𝑀𝐼𝑀𝐵𝐹4 post treatment on the DMSO pretreated films results
in a lower electrical conductivity than that with only DMSO treated films. The dependence of the
Seebeck coefficient on 𝐸𝑀𝐼𝑀𝐵𝐹4 is different as shown in Figure 5.4. Despite of the reduction in
the electrical conductivity, films post treated with the presence of 𝐸𝑀𝐼𝑀𝐵𝐹4 display higher See-
beck coefficient, which depends on the volume ratio of 𝐸𝑀𝐼𝑀𝐵𝐹4 in the post treatment mixture.
The additional 𝐸𝑀𝐼𝑀𝐵𝐹4 post treatment for DMSO pretreated film does not show high Seebeck
coefficient (for sample P_DMSO_IL) as those post treated with 𝐸𝑀𝐼𝑀𝐵𝐹4/DMSO mixture. As DI
water rinsing is used during the post treatment with 𝐸𝑀𝐼𝑀𝐵𝐹4/DMSO mixture, it might affect the
film properties. To clarify the effect of used water rinsing and thermal annealing, a sample
P_DMSO”, of which the DMSO post treatment and the DI water rinsing were used (same procedure
as those treated with 𝐸𝑀𝐼𝑀𝐵𝐹4), is analyzed. The DI water rinsing deteriorates the electrical con-
ductivity. But no increase in the Seebeck coefficient of P_DMSO” is observed compared to
P_DMSO. This phenomenon suggests that the DI water rinsing only affects the electrical conductiv-
ity. Consequently, it could be concluded that the enhancement in the Seebeck coefficient of films
treated with 𝐸𝑀𝐼𝑀𝐵𝐹4 is only due to 𝐸𝑀𝐼𝑀𝐵𝐹4. Due to the intercoupling phenomenon between
the electrical conductivity and the Seebeck coefficient, an optimized power factor of 38.46 𝜇𝑊/
(𝑚𝐾2) (shown in Figure 5.5) for the film post treated with 5/5(v/v) (𝐸𝑀𝐼𝑀𝐵𝐹4/DMSO) mixture is
obtained.
67
Figure 5.3 Electrical conductivity of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures.
Figure 5.4 Seebeck coefficient of PEDOT:PSS films post
treated with EMIMBF4/DMSO mixtures.
Figure 5.5 Power Factor of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures.
5.4 Physical and Chemical Properties of Films Modified with EMIMBF4/DMSO
Mixtures
To analyze the influence of 𝐸𝑀𝐼𝑀𝐵𝐹4 post treatment on PEDOT:PSS thin films, the morphologies
of different samples were recorded in Figure 5.6 and 5.7. It is known that PEDOT:PSS films are
composed of conducting, hard PEDOT-rich grains and insulating, soft PSS-rich grains. Compared
to the featureless pristine film, DMSO post treatment triggers strong phase separation, leading to an
elongated, well connected network of PEDOT-rich grains with high electrical conductivity (see
Figure 5.6). Post treatment with the presence of 𝐸𝑀𝐼𝑀𝐵𝐹4, however, results in different morpholo-
gies. Relative circular and large PEDOT grains are observed. Even for the film in which elongated
PEDOT-rich grains are formed by the DMSO post treatment, the following post treatment with
𝐸𝑀𝐼𝑀𝐵𝐹4 turns the elongated PEDOT-rich grains into large and circular structures (film
68
P_DMSO_IL). The 𝐸𝑀𝐼𝑀𝐵𝐹4 seems to constrain the elongation of PEDOT-rich grains and swells
the grains. The other effect induced by 𝐸𝑀𝐼𝑀𝐵𝐹4 is on the film roughness as it is listed in Table
5.2. The films with elongated PEDOT grains demonstrate larger surface roughness than the films
with circular, large PEDOT-rich grains. With the increasing amount of 𝐸𝑀𝐼𝑀𝐵𝐹4 in the post treat-
ment mixture, a smaller surface roughness is observed, indicating that the possible interaction be-
tween 𝐸𝑀𝐼𝑀𝐵𝐹4 and PEDOT:PSS film does not favor the formation of rough and elongated
PEDOT grains. Rather, the constrained growth of PEDOT-rich grains gives rise to smoother films.
The surface roughness of DMSO pretreated film is reduced after the additional 𝐸𝑀𝐼𝑀𝐵𝐹4 post
treatment, indicating EMIMBF4 has strong effect on the PEDOT grains.
Figure 5.6 Morphology evolutions of films treated with EMIMBF4/DMSO mixtures Part I. Upper row: Height images; down row: Phase images. Scan size:1 𝜇𝑚 × 1 𝜇𝑚.
69
Figure 5.7 Morphology evolutions of films treated with EMIMBF4/DMSO mixtures Part II. Upper row: Height images; down row: Phase images. Scan size: 1 𝜇𝑚 × 1 𝜇𝑚.
Sample pristine film P_DMSO P_5/5 Mix P_IL P_DMSO_IL
Rq [nm] 1.36 2.00 1.67 1.28 1.64
Table 5.2 Surface roughness of films subjected to post treatment with EMIMBF4/DMSO mixtures.
Next, the possible interaction of 𝐸𝑀𝐼𝑀𝐵𝐹4 with PEDOT:PSS thin films was investigated by XPS
analysis. The XPS survey of films is shown in Figure 5.8. Both pristine and DMSO post treated
films display a small peak of the atomic sodium (Na 1s), which is probably attributed to the residue
of oxidizing agent, sodium persulfate (Na2S2O8), during the polymerization process of PEDOT:PSS
solution. [129] Post treatment with the presence of 𝐸𝑀𝐼𝑀𝐵𝐹4 for the pristine film or previously
DMSO post treated film greatly increases the peak intensity of atomic nitrogen (N 1s). Moreover,
the atomic sodium atom (Na 1s) signal observed in the pristine film and the DMSO post treated film
disappears after the post treatment with 𝐸𝑀𝐼𝑀𝐵𝐹4 . This phenomenon indicates that the
imidazolium cation (𝐸𝑀𝐼𝑀+) and sodium cation (𝑁𝑎+) exchange with each other during the post
treatment, which is in accordance with the observation by Döbbellin. [126] Despite of the thorough
70
Figure 5.8 XPS survey of PEDOT:PSS films treated with EMIMBF4/DMSO mixtures.
Figure 5.9 Sulphur signals of pristine film and DMSO
post treated film.
Figure 5.10 Sulphur signals of films post treated with the
presence of EMIMBF4.
rinsing with DI water, the imidazolium cations (𝐸𝑀𝐼𝑀+) still remains in the post treated films with
the evidence of the strong peak intensity of N 1s. Besides, the XPS general scan shows that no 𝐵𝐹4−
anion remains in films. Furthermore, the characteristic atomic sulphur S(2p) peaks of PEDOT and
PSS were recorded. As it is discussed in previous chapter, the DMSO post treatment removes the
insulating PSS layer from the film surface which increases the sulphur atom ratio of PEDOT to PSS
on the surface. The peak form of both sulfur signals remains as shown in Figure 5.9. In contrast, the
application of 𝐸𝑀𝐼𝑀𝐵𝐹4 renders the convergence of the sulphur signals of PEDOT and PSS in such
a way that a different peak form is detected (see Figure 5.10). The smaller energy difference be-
tween the S(2p) signals for PEDOT and PSS implies that 𝐸𝑀𝐼𝑀𝐵𝐹4 does not only exchange ions
with the PEDOT:PSS film during the post treatment. At the same time the remaining of 𝐸𝑀𝐼𝑀+
cations in the film actively react with PEDOT chains and PSS chains to minimize the difference
between the chemical environment for the sulphur atoms in PEDOT and PSS chains. However, an
71
exact analysis of the surface ratio of PEDOT to PSS is not possible because of the complexity of the
S(2p) peak.
The Seebeck coefficient of a PEDOT:PSS film is sensitive to the chemical structure of PEDOT
chains, which have characteristic absorption peaks in the UV-Vis spectra. The evidence of the reac-
tion between 𝐸𝑀𝐼𝑀+ cations with PEDOT:PSS films gives rise to another question: whether this
reaction changes the chemical structure that is crucial for the Seebeck coefficient? The PEDOT
chains in the commercial product PH 1000, as it is given by the supplier, are in its high oxidative
state for high electrical conductivity. The absorption spectrum of the pristine film (see Figure 5.11)
shows broad absorption, indicating the high oxidation level of PEDOT chains. No noticeable
change is visible in the spectra of P_DMSO, indicating there is no effect of DMSO on the oxidation
level of PEDOT chains. However, the presence of 𝐸𝑀𝐼𝑀𝐵𝐹4 during the post treatment introduces a
polaron absorption band in the region of ~ 900 nm, suggesting the dedoping (or reduction) of
PEDOT chains. Compared to the film which is partially dedoped to the neutral state (reference ma-
terial with absorption from 600 nm to 700 nm), 𝐸𝑀𝐼𝑀𝐵𝐹4 only dedopes PEDOT chains from the
bipolaron to the polaron state. P_DMSO” also exhibits slight difference compared to the pristine
film. Nevertheless, it does not have the strong distinguishable absorption band at ~ 900 nm, sug-
gesting the dedoping of PEDOT is mainly attributed to 𝐸𝑀𝐼𝑀𝐵𝐹4.
Together with the XPS analysis and the UV-Vis absorption spectra, it could be concluded that the
remaining of 𝐸𝑀𝐼𝑀+ after ion exchange dedope the PEDOT chains and increase the polaron
Figure 5.11 UV-Vis absorption spectra of PEDOT:PSS thin films post treated with EMIMBF4/DMSO mixtures.
72
density, as it is confirmed by the enlargement of Seebeck coefficient and the decrease in the electri-
cal conductivity.
Additionally, Raman spectroscopy (see Figure 5.12) of thin films was conducted. The spectra of
DMSO post treated film resembles with that of pristine film, in which a complex series of peaks
from 1400 𝑐𝑚−1 to 1600 𝑐𝑚−1 with a characteristic band at 1450 𝑐𝑚−1 are shown. However, films
post treated with 𝐸𝑀𝐼𝑀𝐵𝐹4 demonstrate three distinct sharp peaks at 1436 𝑐𝑚−1, 1515 𝑐𝑚−1 and
1560 𝑐𝑚−1. As it is reported in Kok's work, the Raman peaks from 1200 𝑐𝑚−1 to 1400 𝑐𝑚−1 are
assigned to the C-C stretch vibration. The peaks from 1400 𝑐𝑚−1 to 1600 𝑐𝑚−1 are associated with
the C=C stretch vibration. The high sensitivity of C=C stretch vibration to the degree of (de-
)localization of holes can be used to detect the possible change of the state of PEDOT chains. The
shifting of the C=C stretch vibration from 1450 𝑐𝑚−1 of the pristine film to 1436 𝑐𝑚−1 for films
post treated with the presence of 𝐸𝑀𝐼𝑀𝐵𝐹4 is similar to the modification of PEDOT:PSS films by
typical reductants. [100,116,130] Therefore, it could be further concluded that the post treatment with
𝐸𝑀𝐼𝑀𝐵𝐹4 dedopes the PEDOT chains, reflecting in the higher Seebeck coefficient and the com-
promised electrical conductivity.
The other effect brought by 𝐸𝑀𝐼𝑀𝐵𝐹4 post treatment is on the film thickness (shown in Figure
5.13). After the DMSO post treatment, it is found that the film thickness is reduced more than 40%,
confirming the removal of PSS chains. On the other hand, the presence of 𝐸𝑀𝐼𝑀𝐵𝐹4 leads to the
Figure 5.12 Raman spectra of PEDOT:PSS thin films post treated with EMIMBF4/DMSO mixtures.
73
Figure 5.13 Film thickness variations regarding the post treatment medium.
film thickening compared to the film P_DMSO even after rinsing with DI water. The higher amount
of 𝐸𝑀𝐼𝑀𝐵𝐹4 in the post treatment mixture, the thicker is the film. The influence on the film thick-
ness is especially noticeable for the sample P_DMSO_IL with comparison to P_DMSO. This ob-
servation suggests the constrained growth of elongated PEDOT chains during the post treatment
with 𝐸𝑀𝐼𝑀𝐵𝐹4. The 𝐸𝑀𝐼𝑀𝐵𝐹4 swells PEDOT chains, forming large and circular grains. In turn
the film thickness is increased.
5.5 Conclusions
The application of 𝐸𝑀𝐼𝑀𝐵𝐹4 for the post treatment triggers the formation of a network with large,
circular PEDOT grains. These PEDOT grains are dedoped which is indicated by the presence of a
higher intensity of polaron in the UV-Vis spectra. This is also reflected in the improved Seebeck
coefficient and the degraded electrical conductivity. By controlling the ratio between DMSO and
𝐸𝑀𝐼𝑀𝐵𝐹4, a high power factor of 38.46 𝜇𝑊/(𝑚𝐾2) is found for the film post treated with 50 vol%
loading of 𝐸𝑀𝐼𝑀𝐵𝐹4 in the mixture. This value is much higher than the one reported by Liu et al. [128] Assuming a thermal conductivity of 0.17 W/(mK), this film (P_5/5 Mix) shows a ZT of
~ 0.068, which is better than the electrochemical method. [110] This work demonstrates the possibil-
ity of using 𝐸𝑀𝐼𝑀𝐵𝐹4 for tuning the TE properties of PEDOT:PSS films and 𝐸𝑀𝐼𝑀𝐵𝐹4 is found
for the first time to be able to modify the chemical structure of PEDOT chains. As the used ionic
liquid and DMSO are non toxic chemicals, it provides the opportunity for a green fabrication of
thermoelectric materials.
74
75
6 Hybrid Composites of Polymer with
Nanoinclusions
The intrinsic low thermal conductivity of polymers is ideal for the application as thermoelectric
materials. To develop efficient devices, the low Seebeck coefficient and the low electrical conduc-
tivity of polymers are still big issues. The modification of the oxidation level of polymers could
enlarge the Seebeck coefficient that is comparable to inorganic TE materials. [33] However, the same
intercoupling phenomenon between the electrical conductivity and the Seebeck coefficient as ob-
served in traditional inorganic TE materials takes place. The power factor of pure polymers is still
not comparable to the commercial inorganic TE materials. Besides, the progress in the technology
of hybrid composites based on polymer and nanoinclusions has made it possible to take advantages
of both phases in such type of hybrid composites. In an ideal case, a hybrid composite could com-
bine the low thermal conductivity as for the polymer matrix and a large Seebeck coefficient as well
a high electrical conductivity as for the nanoinclusions.
Tellurium (Te) is an interesting inorganic material for thermoelectric applications. Its alloys, bulk
tellurides especially bismuth telluride (Bi2Te3), have been known as the best TE materials for appli-
cations near room temperature. The ZT of Bi2Te3, for example, is as high as 1. Bulk Te also has a
high Seebeck coefficient (185 𝜇𝑉/𝐾) at room temperature. [131] Nevertheless, the figure of merit is
low due to its high thermal conductivity (~ 2 W/(mK)) in the bulk phase. Nanostructuring of mate-
rials into confined geometries is widely applied to minimize the lattice thermal conductivity for a
better TE performance. High Seebeck coefficients of 330 𝜇𝑉/𝐾 and 408 𝜇𝑉/𝐾 are reported for Te
in monocrystalline [131] and nanorod forms [71], respectively. The ZT in bulk alloys has remained
around 1 for more than 50 years. With nanostructuring, the ZT of the bismuth antimony telluride
alloys could be increased to up to 1.2 at room temperature.
One typical approach to prepare nanoparticles is based on the ball milling method. Subsequent hot
pressing is required to compact the nanoparticles. [132] This approach has been successfully em-
ployed to prepare many TE composites, such as Bi2Te3, silicon germanium (SiGe) alloys and
76
skutterudites based on cobalt triantimonide (CoSb3). [132] Through the combination of ball milling
and hot pressing, large amount of composites can be synthesized. Nevertheless, the ball milling
requires a high energy input. And hot pressing should be done at high temperature in order to obtain
materials with better mechanical and electrical properties. An alternative approach to prepare nano-
particles is the solution synthesis which is either done by a wet chemical reaction or a hydrothermal
reaction. In comparison to the ball milling and hot pressing, the solution synthesis of nanoparticles
is more efficient in turns of time and energy. [133] Furthermore, it provides the possibility to inte-
grate the synthesized nanoparticles with the polymers solution in order to employ the intrinsic low
thermal conductivity and the flexibility of the polymers.
Crystalline Te nanowires, nanotubes, nanobelts and nanorods can be synthesized by hydrothermal
synthesis, in which poly(vinyl pyrrolidone) (PVP) acts as the morphology directing agent and hy-
drazine hydrate (N2H4, acute toxic) as the reductant.[134,135] Zhu et al. developed a microwave assist-
ed synthesis of single crystalline Te nanorods and nanowires in ionic liquid, in which N2H4 is used
as the reductant. [136] Xi et al. employed formamide (HCONH2) as the reductant and sodium tellurite
(Na2TeO3) as the Te source to synthesize Te nanotubes with a hydrothermal method. [137] Besides
the hydrothermal synthesis, the wet chemical synthesis is another common approach to synthesize
Te nanostructures in the solution phase. Segalmann's group demonstrated that ascorbic acid (LAC)
could reduce Na2TeO3 into Te nanorods (300 nm to 100 nm length). [71] In their work, PEDOT:PSS
is used as the surfactant to mediate nanorods growth through a wet chemical reaction at 90°C for
overnight.[18,70,71] The synthesized composite is composed of Te nanorods, which is coated with a
layer of PEDOT:PSS film. Yang et al. applied N2H4 to reduce the tellurium dioxide (TeO2) at ambi-
ent temperature to obtain Te nanostructures. [64] Yue's group utilized N2H4 to reduce TeO2 with PVP
as the surfactant to prepare Te nanowires, which are further used as the template for the formation
of ball-bell Bi2Te3 nanostructures. [138]
In both hydrothermal and wet chemical syntheses, either a strong reductant N2H4 is used to reduce
TeO2 or a mild reductant to reduce Na2TeO3. Both of N2H4 and Na2TeO3 are acutely toxic. Alterna-
tives are to be searched for a relative green synthesis method in which non toxic Te source and non
toxic reductant are used. For example, telluric acid (H6TeO6), instead of Na2TeO3, was employed to
synthesize Te nanostructure, in which starch was applied as the reductant and the morphology di-
recting agent during a hydrothermal synthesis (160°C for 15 hours). [139] Wei et al. reported that the
fructose is able to reduce TeO2 into Te nanostructures in an alkaline aqueous solution. [140]
77
This work focused on a more green synthesis method for Te nanostructures. Instead of the acutely
toxic Na2TeO3, TeO2 was chosen as the Te source. Different non toxic reductants based on LAC
and LAC/MEA mixture were selected to control the morphology of synthesized Te nanostructures.
Afterwards, the synthesized dispersions of Te nanostructures were further combined with a polymer
solution to fabricate polymer/Te nanostructures hybrid composites in order to explore their ther-
moelectric properties.
6.1 Synthesis and Morphology of Te Nanostructures
6.1.1 Wet Chemical Synthesis of Tellurium Nanostructures All wet chemical reactions were conducted under air atmosphere. In a typical experiment, TeO2,
PVP and ethylene glycol (EG) were added into a vial. Next, sodium hydroxide (NaOH, 2mol/L)
were added. This mixture was magnetically stirred (800 rpm) and kept at 120°C for 10 min until the
mixture color changed into transparent. Then certain amount of LAC and ethanolamine (MEA)
were added into the prepared transparent mixture. As soon as the LAC and MEA were added into
the vial, the mixture color changed from transparent into light brown. The reaction condition was
maintained at 800 rpm and 120°C) for defined time period. In the next step, the obtained dark slurry
was centrifugated (10000 rpm, 1 hour). The supernatant was discarded and the puck was washed
with absolute ethanol and centrifugated again at 10000 rpm for 20 min. The final silver gray prod-
ucts were then collected and redispersed in absolute ethanol by a bath sonication for 20 min. It
should be mentioned, LAC/MEA mixture is proven to have stronger reduction ability than LAC
alone (see Appendix A.1 about the reduction ability of LAC/MEA mixture). The detailed conditions
for the sample preparations are listed in Table 6.1.
6.1.2 Hydrothermal Synthesis of Tellurium Nanostructures Hydrothermal synthesis was carried out in a 23 mL Teflon lined stainless steel autoclave in order to
perform the reaction at high vapor pressure. In a typical experiment, 96 mg TeO2 and 120 mg PVP
were added into 15 mL EG. Then this mixture was magnetically stirred at 800 rpm. After 10 min
mixing, 480 mg LAC was added and the mixture was stirred further for 5 min. Next the reaction
mixture was poured into the autoclave which was then placed in a hot air oven. The oven was pre-
heated to 180°C. After a defined period of heating, the autoclave was taken out of the oven and left
to cool to room temperature naturally. The obtained reaction products were centrifugated at 10000
rpm for 1 hour. The supernatant of centrifugated reaction mixture was disposed and the punk was
washed with absolute ethanol and centrifugated again at 10000 rpm for additional 20 min. The reac-
78
tion products, which were silver gray, were redispersed in absolute ethanol for further usage.
LAC/MEA mixture was used also as the reductant in order to compare its reduction ability to LAC.
In particular, the influence of NaOH addition was evaluated. In the hydrothermal synthesis, the total
added liquid amount was fixed to 15 mL. The detailed amount of the reaction conditions and no-
menclatures are listed in Table 6.2.
Sample Solution t [hour]
C-Te-0 TeO2 (96 mg) + LAC (200 mg) + PVP (120 mg) + EG (6 mL)
+ NaOH (2 mL) + MEA (2 mL)
3
C-Te-1 TeO2 (96 mg) + LAC (480 mg) + PVP (0 mg) + EG (6 mL)
+ NaOH (2 mL) + MEA (2 mL)
6
C-Te-2 TeO2 (96 mg) + LAC (480 mg) + PVP (60 mg) + EG (6 mL)
+ NaOH (2 mL) + MEA (2 mL)
6
C-Te-3 TeO2 (96 mg) + LAC (480 mg) + PVP(120 mg) + EG (6 mL)
+ NaOH (2 mL) + MEA (2 mL)
6
C-Te-4 TeO2 (96 mg) + LAC (480 mg) + PVP (120 mg) + EG (6 mL)
+ NaOH (0.6 mL) + MEA (2 mL)
6
C-Te-5 TeO2 (96 mg) + LAC (480 mg) + PVP (120 mg) + EG (6 mL)
+ NaOH (0 mL) + MEA (2 mL)
6
Table 6.1 Samples prepared by the wet chemical synthesis at 120°C.
Sample Solution t [hour]
Hydro-Te-1 TeO2 (96 mg) + PVP (120 mg) + EG (15 mL) 5
Hydro-Te-2 TeO2 (96 mg) + PVP (120 mg) + EG (15 mL) + LAC (480 mg) 5
Hydro-Te-3 TeO2 (96 mg) + PVP (120 mg) + EG (13 mL) + LAC (480 mg)
+ MEA (2 mL)
5
Hydro-Te-4 TeO2 (96 mg) + PVP (120 mg) + EG (13 mL) + NaOH (2 mL)
+ LAC (480 mg)
2
Hydro-Te-5 TeO2 (96 mg) + PVP (120 mg) + EG (11 mL) + NaOH (2 mL)
+ LAC (480 mg) + MEA (2 mL)
2
Table 6.2 Samples prepared by the hydrothermal synthesis at 180°C.
6.1.3 Characterization of Tellurium Nanostructures XRD is used to characterize the crystalline structure of the synthesized tellurium. The position and
intensity of peaks in a XRD diffraction pattern are determined by the crystal structure. Both Cop-
per-K𝛼 (Cu-K𝛼) and Cobalt-K𝛼 (Co-K𝛼) were used as the radiation sources (Same d-spacing are
observed for samples with different radiation sources, see Appendix A.2 of JCPDS pattern of Tellu-
rium with different radiation sources). For the XRD characterization with the Co-K𝛼 radiation
79
source, the measurements were performed in a back scattering geometry on a X'pert pro
(PANalytical) powder diffractometer. As for the XRD with Cu-K𝛼 as the radiation source, diffrac-
tion measurements were carried out using a Seifert XRD3000PTS XRD in the grazing incidence
mode. The XRD samples were prepared by dropping synthesized Te dispersions on top of silicon
substrates, which were then heated on a hot plate for drying. Furthermore, TEM imaging was per-
formed using PHILIPS CM 20 FEG instrument operated at 200 kV. To prepare TEM samples, the
dispersion of Te nanostructures was diluted and drop-casted on a carbon coated grid in order to im-
age single Te nanostructure.
6.1.4 Morphology of Tellurium Nanostructures Test Te nanotubes (named as C-Te-0) were synthesized with a LAC/MEA (200mg/2mL) mixture at
120°C for 3 hours. These nanotubes are with monodispersed sizes and bird beak opening as it is
observed in Figure 6.1a. These nanotubes exhibit a length of ~ 200 nm and a surface coated with
small particles. The clear lattice fringes in the HRTEM images taken in a tube end (Figure 6c) and
the middle (Figure 6d) of a tube indicate that the nanotube is structurally uniform and single crystal-
line. The selected area electron diffraction (SAED) pattern (Figure 6b) of one nanotube further con-
firms the single crystallinity of the synthesized Te nanotubes.
The crystal structure of the obtained nanotubes was studied with XRD. Figure 6.2 shows a typical
XRD pattern of the obtained C-Te-0 nanotubes. All the peaks can be perfectly indexed to the
trigonal crystal structure of Te, which are in agreement with the standard literature data (JCPDS file
no: 36-1452). From the analytical results, it is proven that the LAC/MEA mixture is able to replace
the toxic reductant to reduce TeO2 under air atmosphere to produce single crystalline Te nanostruc-
tures.
Since the test reaction to synthesize C-Te-0 was carried out with the presence of EG, it might influ-
ence the reduction process. To get a better understanding of EG in the reduction process, two mix-
tures with the same amount of LAC, TeO2 and MEA were prepared and magnetically stirred at
room temperature. To one mixture vial, 1 mL EG was additionally added. The reaction mixtures
after 3 days stirring are shown in Figure 6.3. With the presence of EG (Vial B in Figure 6.3), the
mixture with the reduced products is composed of much more dark particles. When there is no EG
in presence (Vial A in Figure 6.3), less dark particles are suspended in the mixture after 3 days reac-
tion. The formation of these dark particles is an evidence of the reduction from TeO2 to Te
80
nanostructures. This observation suggests that EG promotes the reduction process. Therefore, in the
following discussion, EG was used during the wet chemical synthesis.
(a)
(b)
(c)
(d)
Figure 6.1 Morphology and grain characterization of wet chemically synthesized Te nanostructures (C-Te-0).
Figure 6.2 Corresponding XRD pattern of C-Te-0 with Cu-K𝛼 as the radiation source.
81
Figure 6.3 Observation of the reduction of TeO2 without (vial A) or with (vial B) EG at room temperature after 3 days reaction.
In the wet chemical synthesis, the amount of directing agent PVP and NaOH as listed in Table 6.1 is
differed to illustrate their influence. Higher amount of LAC and longer reaction time were applied
in order to obtain larger Te nanostructures. The morphology and size of Te nanostructures are re-
vealed in Figure 6.4. The surfactant PVP is shown to have an important effect on the morphology of
Te nanostructures. When no PVP is used, the reaction products are nanorods and they tend to ag-
glomerate together (see Figure 6.4a). Upon the PVP addition, more distinct Te nanorods are formed.
These Te nanorods are thinner and their surface becomes smooth. When the PVP amount is in-
creased to 120 mg, Te nanotubes with hexagonal opening (Figure 6.4c and 6.4d) become visible.
NaOH is also found to be a crucial factor in the morphology formation of Te nanostructures. When
there is no NaOH or small amount of NaOH (0.6 mL) is added, trifold nanostructures of Te are
formed (see Figure 6.4e and 6.4f). And the surface of these nanostructures is not smooth. When the
NaOH amount is increased to 2 mL, homogeneous Te nanotubes with smooth surface are observed.
The crystalline information of selected Te nanostructures was given by XRD (see Figure 6.5). The
XRD peaks of C-Te-2, which is obtained with Co-Kα as the radiation source, can be perfected in-
dexed into JCPDS file No. 36-1452 (The JCPDS file with Co-Kα as the radiation source can be
found in the appendix A2.2). The XRD peaks of C-Te-3 and C-Te-5, obtained with Cu-Kα as the
radiation source, can also be indexed into JCPDS file No. 36-1452. The XRD analysis indicates that
the phase of the synthesized Te nanostructures is trigonal. Therefore, it is concluded that the wet
chemical synthesis with TeO2 as the Te source and LAC/MEA mixture as the reductant can synthe-
size Te crystals in trigonal phase.
82
(a) C-Te-1 with 0 mg PVP and 2 mL
NaOH
(b) C-Te-2 with 60 mg PVP and 2 mL
NaOH
(c) C-Te-3 with 120 mg PVP and 2 mL
NaOH
(d) C-Te-3 with 120 mg PVP and 2
mL NaOH
(e) C-Te-4 with 120 mg PVP and 0.6
mL NaOH
(f) C-Te-5 with 120 mg PVP and 0 mL
NaOH
Figure 6.4 Te nanostructures from wet chemical synthesis with varying PVP and NaOH amount.
Figure 6.5 XRD of the wet chemically synthesized Te nanostructures.
83
Compared to the N2H4 reduced nanowires reported by Yang et al., [64] the synthesis of Te nanotubes
here (C-Te-3) yields a better size control and homogeneity. The relative shorter nanotubes suggest
that LAC/MEA mixture used in this work has weaker reduction ability than the strong reductant
N2H4. However, LAC/MEA mixture could replace the toxic N2H4 to reduce TeO2 into defined sin-
gle crystalline Te nanostructures from a more environmental friendly point of view. During the
formation of Te nanostructures, NaOH affects the morphology. However, it is effect on the for-
mation mechanism of Te nanostructures is not well understood. The following discussion is based
on the impact of NaOH (C-Te-3 vs. C-Te-5). With the reduction of LAC/ MEA mixture, trifold Te
nanostructures (C-Te-5) are formed. On the other hand, with a sufficient amount of NaOH, it is able
to synthesize Te nanotubes (C-Te-3) with hexagonal opening. When NaOH was added, the milky
white dispersion changed to a transparent solution. This change indicates that TeO2, a kind of weak
acidic oxide, first reacts with NaOH to form TeO32-. The TeO3
2- is then disproportionated to form
Te in the basic reaction system. The Te formation may be formulated as follows
The formation of Te nanostructures is reported as: a small amount of amorphous-Te particles are
formed in the beginning and they are crystallized into trigonal-Te seed particles. Te nanowires
formed on the seeds of trigonal-Te at the expense of dissolution of amorphous-Te particles. [141] It is
well known that the reduction rate of 𝑇𝑒𝑂32− strongly depends on the pH value, which is decreased
with increasing pH value. [142] The reduction rate for C-Te-5 is much higher as no NaOH was add-
ed. At this high reduction rate, nanostructures grow both in the lateral and longitudinal directions.
As a result, trifold nanostructures are formed. With NaOH addition, the slower reduction rate leads
to the growth of Te nanotubes (C-Te-3).
In the hydrothermally synthesis, a test with only TeO2, PVP and EG in the reaction mixture was
investigated. After 5 hours reaction in the autoclave at 180°C, no change could be observed in the
mixture. This indicates that EG alone is not able reduce TeO2. Next, LAC and LAC/MEA mixture
were added as the reductants. In comparison to the wet chemical synthesis, the hydrothermal syn-
thesis results in much larger Te nanostructures (see Figure 6.6). When only LAC is used as the
reductant, Te nanostructures with large feather like feature (𝜇m range) were observed. Replacing
LAC with LAC/MEA mixture as the reductant leads to the formation of both small nanorods (~ 500
nm length) and long nanorods (~ 10 𝜇𝑚). On the other hand, when NaOH was added into the reac-
tion mixture, together with LAC as reductant, well controlled nanowires in the range of 𝜇𝑚 length
84
were produced even with 2 hours of reaction in the autoclave (see Figure 6.6c). The combination of
LAC/MEA mixture and NaOH results in 𝜇𝑚 long nanowires coated with small particles. In the re-
action where only LAC is used as the reductant, the difference between the morphology for Hydro-
Te-2 and Hydro-Te-4 is attributed to the reduction rate, which is determined by NaOH. When no
NaOH is used, the reduction rate is much higher. As a result, a feather like structure is yielded,
which is similar to the work done by Zhu et al. [143] LAC/MEA mixture is a stronger reductant com-
pared to LAC. Panahi-Kalamuei et al. reported than stronger reductant leads to an increase in the
nucleation rate and decrease in the growth rate of Te. [144] Thus, under the same condition, more Te
nanoparticles could be generated with LAC/MEA as the reductant. As a result, shorter Te nanowires
(Hydro-Te-5) are synthesized with comparison to Hydro-Te-4.
(a) Hydro-Te-2 with LAC as the reductant
(b) Hydro-Te-3 with LAC/MEA mixture as
the reductant
(c) Hydro-Te-4 with LAC as the reductant
and with NaOH
(d) Hydro-Te-5 with LAC/MEA mixture as
the reductant and with NaOH
Figure 6.6 Morphology of the hydrothermally synthesized Te nanostructures.
The composition and phase of the hydrothermally synthesized Te nanostructures were examined by
XRD (see Figure 6.7). All the peaks in these patterns can be indexed to the trigonal phase of Te
(JCPDS 36-1452). The TEM image in Figure 6.8 also shows feather like feature of Hydro-Te-2. In
85
addition, HRTEM images of different areas display only one type of grain, confirming the single
crystallinity of Hydro-Te-2. Te nanowires (Hydro-Te-4), fabricated with LAC as the reductant and
with NaOH, are with a diameter of ~ 30 nm (see Figure 6.9). The corresponding HRTEM images of
Hydro-Te-4 reveal also only one type of grain, confirming its single crystalline structure.
Figure 6.7 XRD of the hydrothermally synthesized Te nanostructures. Co-K𝛼 was used as the radiation source.
Figure 6.8 TEM observation of Hydro-Te-2.
Figure 6.9 TEM observation of Hydro-Te-4.
86
On the other hand, when the reductant LAC/MEA mixture and NaOH were used, Te nanowires
(Hydro-Te-5) with nanoparticles coating around are observed, which is in accordance with the SEM
images. The HRTEM image in Figure 6.10 gives evidence to the single crystalline structure of the
Te nanowires. However, there is no information given for the nanoparticles in the TEM images,
indicating that these particles are amorphous. These nanoparticles are similar to the report by Wei et
al. [140] Amorphous Te nanoparticles are formed in the first stage, which are further crystallized into
Te nanowires during the formation of Te nanostructures.
Figure 6.10 TEM observation of Hydro-Te-5.
6.2 PEDOT:PSS/Te nanostructures Hybrid Composites
6.2.1 Preparation of Hybrid Composites Selected Te dispersions were mixed with as received PEDOT:PSS solution (Clevios, PH 1000). The
detailed amount of the used Te dispersion and the PEDOT:PSS solution is listed in Table 6.3. Bath
sonication of 15 min was employed to create homogeneous composite dispersions. Afterwards, 200
𝜇𝐿 of the composite mixture were drop coated on precleaned substrates (20 𝑚𝑚 × 20 𝑚𝑚). The
samples were then baked at 50°C for 60 min, followed by a baking at 120°C for 15 min to prepare
solid films. With its well controlled morphology, Hydro-Te-4 nanowires were chosen for compo-
sites synthesis, with a varying volume fraction of the Te dispersion. The sample nomenclature is
shown in Table 6.3.
87
Sample Te dispersion Polymer solution
P-test 0.2 mL C-Te-0 1.8 mL PH1000
P-C-Te-2 1 mL C-Te-2 1 mL PH1000
P-Hydro-00 1 mL Hydro-Te-4 1 mL PH1000
P-Hydro-01 1.5 mL Hydro-Te-4 0.5 mL PH1000 (5wt%EG)
P-Hydro-02 1.8 mL Hydro-Te-4 0.2 mL PH1000 (5wt%EG)
P-Hydro-03 1.9 mL Hydro-Te-4 0.1 mL PH1000 (5wt%EG)
Te 2mL Hydro-Te-4 0 mL
PEDOT 0 mL 2mL PH 1000
Table 6.3 Nomenclature of polymer/Te nanostructures hybrid composites.
6.2.2 Thermoelectric Properties of Hybrid Composites
To study the influence of Te nanostructure inclusions on the TE properties, a test composite film (P-
test) with small amount loading of C-Te-0 dispersion was prepared. DMSO post treatment was ap-
plied in order to increase the electrical conductivity. In this composite, small Te nanotubes are ran-
domly dispersed inside the polymer matrix (shown in Figure 6.11). Despite of the high electrical
conductivity 82880 S/m, composite P-test exhibits only a Seebeck coefficient of 15.55 𝜇𝑉/𝐾. The
Seebeck coefficient and electrical conductivity of this composite is similar to that of the
PEDOT:PSS thin film post treated with DMSO (93000 S/m, 17.98 𝜇𝑉/𝐾) in Chapter 4. The solu-
tion synthesized Te nanowires (~ 𝜇𝑚 in length, 30 nm in diameter) is reported to have a Seebeck
coefficient of 408 𝜇𝑉/𝐾 and an electrical conductivity of 8 S/m at room temperature. [71] Yee et al.
pointed out that those longer Te nanowires have higher Seebeck coefficient and lower electrical
conductivity than shorter Te nanowires. [18] Furthermore, the thermoelectric properties of hybrid
composites are determined by the mass loading of Te nanostructures. [70] At low content of Te load-
ing, a composite model as function of the amount of polymer present is plausible, in which the See-
beck coefficient of composites demonstrates monotonic change as the concentration of Te
nanostructures is increased. [70] The low Seebeck coefficient of composite P-test here could be pos-
sibly attributed to the low content of Te nanotubes, which is in accordance with others work. [145–147]
In order to obtain higher Seebeck coefficient, composites loaded with higher volume ratio of Te
nanostructures dispersion were prepared. To compare the influence of different Te nanostructures,
the as received PEDOT:PSS solution was used for the composites preparation and no post treatment
was applied. The thermoelectric properties of composites with both wet chemically and hydrother-
mally synthesized Te nanostructures are given in Table 6.4. Compared to pure polymer, the See-
beck coefficients of hybrid composites are improved (P-Hydro-00, P-C-Te-2 compared to PEDOT).
88
For the composite loaded with Hydro-Te-4, the electrical conductivity is slightly increased. Howev-
er, the composite containing C-Te-2 displays a reduced electrical conductivity. In these two compo-
sites, the volume ratio of the PEDODT:PSS solution to the Te dispersion is the same. The differ-
ence is the morphology of Te nanostructures. The wet chemically synthesized Te nanostructures (C-
Te-2) are much smaller than the hydrothermally synthesized 𝜇𝑚 size nanowires (Hydro-Te-4). This
morphology difference results in different interfacial area between the polymer and the Te
nanostructures: larger interfacial area is expected in the composite P-C-Te-2. This interface reduces
the conductivity. [18] Consequently, reduced electrical conductivity is observed in the composite P-
C-Te-2. Nevertheless, the Seebeck coefficients of both composites are still low, which is possibly
due to the low concent of used Te dispersion.
Figure 6.11 SEM images of the composite P-test film with C-Te-0.
Sample Electrical conductivity
[S/m]
Seebeck coefficient
[𝜇𝑉/𝐾]
Power factor
[𝜇𝑊/(𝑚𝐾2)]
PEDOT 101±10% 16.55±5% 0.028
P-C-Te-2 10±10% 28.54±10% 0.008
P-Hydro-00 198±5% 22.41±5% 0.099
Table 6.4 TE properties of composite films embedded with different Te nanowires.
As longer Te nanostructures has higher Seebeck coefficient, [18] Te long nanowires (Hydro-Te-4)
was selected to prepare hybrid composites with varied volume fractions of Te dispersion. In addi-
tion, 5wt% EG was added into the PEDOT:PSS solution to improve the electrical conductivity. To
further improve the Seebeck coefficient, higher volume of Te dispersion is used for the composites
fabrication. When VTe:VPEDOT ≥ 9:1, composites films (P-Hydro-02 and P-Hydro-03) are mainly
consisted of Te nanowires (shown in Figure 6.12). The polymer generally coat around the Te nan-
owires, of which the morphology is similar with the composite loaded with 84.5wt% Te reported by
See et al. [71]
89
The dependence of thermoelectric properties on the volume fraction of Hydro-Te-4 dispersion is
shown in Figure 6.13. With comparison to pure Te or pure PEDOT film, composite P-Hydro-00 has
an improved electrical conductivity. The low Seebeck coefficient of composite even with the inclu-
sion of Te nanostructures is mainly due to the low concentration of Te dispersion. The small im-
provement in the electrical conductivity could be attributed to the effect of ethanol which was used
to disperse the Te nanostructures. Ethanol is reported to modify the electrical conductivity of
PEDOT:PSS by direct mixing at small extent. [98] In order to improve the electrical, 5 wt% EG is
added into the PEDOT:PSS solution. With the increased volume fraction of Te dispersion, the See-
beck coefficients of composite films are enhanced. Especially when VTe:VPEDOT=9:1, the composite
film demonstrates a Seebeck coefficient higher than 200 𝜇𝑉/𝐾, approaching the Seebeck coeffi-
cient of pure Te. The electrical conductivity of the composites, on the other hand, decreases with
the Te dispersion volume fraction due to the interfacial electron scattering.
Figure 6.12 SEM characterizations of composite films P-Hydro-02 and P-Hydro-03.
90
(a) Electrical conductivity
(b) Seebeck coefficient
(c) Power factor
Figure 6.13 Thermoelectric properties of PEDOT:PSS/Te nanostructures (Hydro-Te-4) composite films with varied Te concentrations.
6.2.3 Conclusions
Te nanostructures were successfully synthesized by a green synthesis approach (both with a wet
chemical synthesis and a hydrothermal synthesis), in which TeO2 was chosen as the Te source and
LAC/MEA mixture as the reductant. In the wet chemical synthesis, through the variation of the
used amount of PVP, NaOH, different Te nanostructures could be synthesized, including Te
nanorods, Te trifold nanostructures and Te nanotubes with hexagonal opening. Te nanostructures
are typically with length of ~ 150 nm. On the other hand, the hydrothermal synthesis could synthe-
size much longer Te nanostructures. Long Te nanowires in 𝜇𝑚 length are synthesized with PVP as
the surfactant, LAC as the reductant and NaOH in presence during the hydrothermal synthesis. All
the Te nanostructures from both wet chemical synthesis and hydrothermal synthesis are single crys-
talline and in trigonal phase (JCPDS file No. 36-1452) as it is indicated by the TEM and XRD anal-
ysis. During the formation of Te nanostructures, the reduction rate is reduced by the addition of
91
NaOH. Furthermore, stronger reductant, e.g. LAC/MEA, results in an increase in the nucleation rate
and a decrease in the growth rate of Te.
Polymer/Te nanostructures hybrid composites have been fabricated. Their thermoelectric properties
depend on the morphology and the volume fraction of the Te dispersion. When 1.9 mL hydrother-
mally synthesized Te nanowires (383.5±54% 𝜇𝑉/𝐾), which are dispersed in ethanol, were added
into 0.1 mL PEDODT:PSS solution, the composite yields a high Seebeck coefficient up to
209±30% 𝜇𝑉/𝐾. Even though the relative low electrical conductivity leads to only a small im-
provement in the power factor with comparison to the pure polymer, it points out the possibility to
integrate Te nanostructures into polymer for the fabrication of hybrid composites with high Seebeck
coefficient.
92
93
7 Summary and Outlook 7.1 Summary
In this thesis, thermoelectric properties of polymers are investigated in two separated systems: a
pure polymer system and a hybrid composite system with polymer and nanoinclusions. For the pure
polymer, this thesis considers a more efficient method to tune the power factor of PEDOT:PSS
films as an alternation to the traditional addition of secondary dopant DMSO into the polymer solu-
tion. Post treatment of films is studied in detail, of which the post treatment medium is differed. The
differences between secondary dopants and amines during the modification of the TE properties of
PEDOT:PSS films are demonstrated. Ionic liquid, 𝐸𝑀𝐼𝑀𝐵𝐹4, was for the first time discussed re-
garding its “reduction” ability on the chemical structure of PEDOT chains. For hybrid composites
with polymer and nanoinclusions, this thesis handles the fabrication of composites with Te
nanostructure inclusions. The objective is to develop a green synthesis method in which non toxic
chemicals are used during the formation of Te nanostructures and to understand the effect brought
by Te nanostructures in the hybrid composites.
Post treatment with organic solvents or ionic liquid of polymer film
For secondary dopants (e.g. DMSO or EG), post treatment, as used in this thesis, is proven to be
much more efficient in improving the power factor of the PEDOT:PSS thin films with comparison
to the traditional addition method. Post treatment of PEDOT:PSS films with secondary dopants in-
duces the phase separation between PEDOT-rich grains and PSS-rich grains, leading to the for-
mation of an interconnected network of elongated PEDOT-rich grains and in turn to a better electri-
cal conductivity. The underlying mechanism is similar to that by the direct addition of secondary
dopants into the polymer solution. The high electrical conductivity remains even when the second-
ary dopants are removed after the processing. Furthermore, post treatment depletes the insulating
PSS chains from the film surface, which contributes to the enhancement in the electrical conductivi-
ty. The small change in the Seebeck coefficient by the usage of secondary dopants suggests that
they do not modify the chemical structure of PEDOT chains. Amines, which have similar dielectric
94
constant and boiling point as the secondary dopants, on the other hand, could modify the chemical
structure of PEDOT chains by donating electrons. Both ethanolamine and ammonia have nitrogen
lone pair electrons, which could form a covalent bonding to PEDOT chains. The delocalized posi-
tive charges in the polymer chains are neutralized and the conjugation length of the double bonds in
PEDOT chains is interrupted. As a result, the oxidation level of PEDOT chains is decreased, lead-
ing to the increase in the Seebeck coefficient. Other aspect regarding the interaction between ethan-
olamine and PEDOT chains is the morphology of the PEDOT:PSS films, to which this post treat-
ment of amines is performed. Due to the limited surface area, films with segregated PEDOT-rich
grains (e.g. film post treated with DMSO) are less susceptible to ethanolamine than the films with
randomly distributed PEDOT chains (e.g. pristine PEDOT:PSS film).
It is for the first time that the ionic liquid, 𝐸𝑀𝐼𝑀𝐵𝐹4, is used as a post treatment medium for
PEDOT:PSS films in order to tune their TE properties. Additionally it is evidenced for the first time
that 𝐸𝑀𝐼𝑀𝐵𝐹4 constrains the elongation of PEDOT-rich grains and swells the grains, leaving large,
circular PEDOT-rich grains instead of the elongated PEDOT-rich grains by the post treatment with
secondary dopants. The analysis confirms that there is ionic exchange between 𝐸𝑀𝐼𝑀𝐵𝐹4 and the
PEDOT:PSS films during the post treatment. The 𝐸𝑀𝐼𝑀+ cations, which retain in the films, at the
same time, react with the PEDOT chains and decrease the oxidation level of PEDOT chains. This
effect leads to the observed enhancement in the Seebeck coefficient.
Polymer/Te nanostructures hybrid composites
In the wet chemical synthesis, LAC/MEA mixture is confirmed to be able to reduce TeO2 into Te
nanostructures, which are single crystalline in trigonal phase (JCPDS 36-1452). Through control-
ling the reaction mixtures (e.g. PVP and NaOH), it is able to synthesize Te nanotubes, nanorods and
trifold nanostructures. Hydrothermal synthesis used in this work, on the other hand, is shown to be
able to create much larger Te nanostructures in 𝜇𝑚 length. Feather like Te nanostructures, long Te
nanowires are synthesized by changing the reductants and conditions. With the introduction of
NaOH into the reaction mixture with LAC as the reductant, long Te nanowires (single crystalline,
JCPDS 36-1452) with high Seebeck coefficient can be synthesized. Compared to other literature
reports, this thesis work proposes a much greener synthesis method with a non toxic Te source and
a non toxic reductant. First investigation regarding hybrid composites of polymer and Te nanostruc-
tures was carried out. Both wet chemically and hydrothermally synthesized Te nanostructures were
involved. The Seebeck coefficient of hybrid composites displays monotonic increase with the vol-
95
ume fraction of Te nanostructures dispersion. The hybrid composite, in which 𝑉𝑇𝑒:𝑉𝑃𝐸𝐷𝑂𝑇 = 9: 1,
is given with high Seebeck coefficient up to 209 ± 30% 𝜇𝑉/𝐾.
7.2 Outlook
Due to the low electrical conductivity, the power factor of hybrid composites based on hydrother-
mally synthesized long nanowires is still low. Since the Seebeck coefficient of Te nanostructures is
dependent on the morphology, Te nanostructures with better Seebeck coefficient need to be
searched for. In addition, the interfacial design of the organic-inorganic composites is presented to
be unique opportunity to optimize ZT. [70] Further work has to be carried out to understand the in-
teraction between the Te nanostructures and the PEDOT:PSS matrix in order to improve the power
factor of composites.
96
Appendix
A.1 Reduction Ability of LAC/MEA Mixture
As it is reported by Zhang et al., LAC is able to reduce the graphite oxide (GO) dispersion into
graphene. The reduction is evidenced by the color change from yellow brown of the GO dispersion
into dark color of graphene dispersion. [148] To analyze the reduction ability of LAC/MEA mixture,
GO dispersion (1 mg/mL) was chosen.
GO dispersion was reduced by MEA, LAC and LAC/MEA mixture, respectively (the used amount
is indicated inside the figures). Magnetic stirring at 800 rpm was used. A reference sample of GO
dispersion without any reductant was magnetically stirred under the same condition. As it is shown
in Figure A.1.1, there is no change for the GO dispersion after 48 h magnetically stirring. When
MEA is used as the reductant, no change in GO dispersion is observed after 48 h mixing. Only after
48 h mixing and 72 h non-mixing with MEA in the GO dispersion, the color is slightly darker, indi-
cating the weak reduction ability of MEA. When LAC is used as the reductant, the GO dispersion is
changed into dark dispersion after 24 h mixing. At this stage there are aggregates of reduced
graphene. Up to 48 h mixing and 72 h non-mixing time with LAC as the reductant, more graphenes
are reduced and there is stronger sedimentation. On the other hand, when LAC/MEA mixture is
used as the reductant, the GO dispersion is reduced and this reduced graphene dispersion remains
(a) 0 min
(b) 48 h mixing with followed
72 h non mixing
Figure A.1.1 Observation of graphite oxide with different mixing time.
97
stable as it is evidenced in Figure A.1.2: a stable dark dispersion is formed after 24 h mixing. When
the reaction time is extended to 48 h mixing and 72 h non-mixing, the dark reduced graphene dis-
persion is still stable. And the color of this reduced graphene dispersion is much darker than that
obtained by using LAC as the reductant. This observation indicates that LAC/MEA mixture has
stronger reduction ability than LAC or MEA.
Figure A.1.2 Reduction of graphite oxide with different reductants after 24 h mixing.
Figure A.1.3 Reduction of graphite oxide with different reductants after 48 h mixing.
Figure A.1.4 Reduction of graphite oxide with different reductants after 48 h mixing and 72 h non-mixing.
98
A.2 JCPDS Patern with Cu-Kα and Co-Kα as Radiation Sources
A.2.1 JCPDS 00-036-1452 Card with Cu-Kα as Radiation Sources
00-036-1452 Status Primary QM: Star (S) Pressure/Temperature: Ambient Chemical Formula: Te Empirical Formula: Te Weight %: Te100.00 Atomic %: Te100.00 Compound Name: Tellurium Mineral Name: Tellurium, syn Radiation: CuKα1 : 1.5406Å Filter: Graph Mono d-Spacing: Diff. Cutoff: 17.70 Intensity: Diffractometer SYS: Hexagonal SPGR: P3121 (152) Author's Cell [ AuthCell a: 4.4579(3)Å AuthCell c: 5.9270(6)Å AuthCell Vol: 102.01ų AuthCell Z: 3.00 AuthCell MolVol: 34.00 ] Author's Cell Axial Ratio [ c/a: 1.330 ] Dcalc: 6.232g/cm³ SS/FOM: F(30) = 86.5(0.0116, 30) Crystal (Symmetry Allowed): Centrosymmetric CAS: 13494-80-9 Pearson: hP3.00 Prototype Structure: Se Prototype Structure (Alpha Order): Se LPF Prototype Structure: Se,hP3,152 LPF Prototype Structure (Alpha Order): Se Mineral Classification: Tetradymite (Supergroup), 1H (Group) Subfile(s):
Mineral Related (Mineral , Synthetic), Forensic, Inorganic, NBS Pattern, Educational Pattern, Primary Pattern, Metals & Alloys, Common Phase
Last Modification Date: 01/11/2011 Cross-Ref PDF #'s: 00-004-0554 (Alternate), 00-004-0555 (Deleted), 04-001-3623, 04-002-8634, 04-003-2449, 04-003-
6029, 04-004-6797, 04-006-5470, 04-007-4717, 04-007-5290
References: Type
Reference
Primary Reference Additional Pattern Structure
McMurdie, H., Morris, M., Evans, E., Paretzkin, B., Wong-Ng, W., Ettlinger, L., Hubbard, C. Powder Diffr. 1, 76 (1986). 2. Swanson, H., Tatge, E. Natl. Bur. Stand. (U. S. ), Circ. 539 1, 26 (1953). 1. Bradley, A. Philos. Mag. 48, 477 (1924).
Database Comments:
Additional Patterns: To replace 00-004-0554 (2). Color: Gray metallic. General Comments: QDF-2. The structure was determined by Bradley (1). On synthetic material. Hexagonal close packed. Color values in air C illuminant: x .311 .308, y .318 .317, Y% 59.9 69.4, ëd 560 495, Pe% 0.7 0.6. Polymorphism/Phase Transition: There are several high pressure polymorphs of tellurium. Reflectance: R%(air): 60-70(546nm), 60-70(470nm), 60-69(589nm), 59-67(650nm). Sample Source or Locality: The sample was from the Fisher Scientific Co., Fair Lawn, NJ, USA. It was ground and annealed at 300 C for 1/2 hour. Temperature of Data Collection: The mean temperature of data collection was 298.41 K. Vickers Hardness Number: VHN25(on 00.1) 117. Unit Cell Data Source: Powder Diffraction.
d-Spacings (34) - 00-036-1452 (Fixed Slit Intensity) - Cu K1 1.54056Å 2 d(Å) I h k L * 2 d(Å) I h k l * 2 d(Å) I h k l * 23.0434 27.5623 38.2601 40.4450 43.3308 45.8998 47.0447 49.6287 51.2417 51.9397 56.8764 62.8093
3.856430 3.233560 2.350470 2.228390 2.086430 1.975450 1.930010 1.835400 1.781350 1.759040 1.617520 1.478240
16 100 36 25 8 9 3 14 4 2 8 7
1 1 1 1 1 0 2 2 1 1 2 1
0 0 0 1 1 0 0 0 1 0 0 1
0 1 2 0 1 3 0 1 2 3 2 3
63.7513 65.8831 67.6571 67.7978 72.0902 73.5276 75.5360 77.2409 81.4779 81.9129 82.0197 85.6662
1.458650 1.416520 1.383630 1.381100 1.309060 1.286980 1.257670 1.234110 1.180300 1.175130 1.173870 1.132990
5 4 4 4 3 1 2 1 1 4 3 1
2 2 1 2 2 3 3 1 3 2 2 1
1 1 0 0 1 0 0 1 0 0 1 0
0 1 4 3 2 0 1 4 2 4 3 5
87.4248 89.3761 91.1772 91.9973 93.9726 94.7569 95.1671 95.6205 99.3907 99.7899
1.114670 1.095320 1.078320 1.070840 1.053460 1.046800 1.043370 1.039620 1.010050 1.007080
<1 <1 <1 <1 1 <1 <1 1 <1 1
2 2 3 3 3 1 2 2 2 3
2 2 0 1 1 1 2 1 0 1
0 1 3 0 1 5 2 4 5 2
99
A.2.2 JCPDS 00-036-1452 Card with Co-Kα as Radiation Sources
Name and Formula Reference code : 00-036-1452 Mineral name : Tellurium, syn Compound name : Tellurium Empirical formula : Te Chemical formula : Te Crystallographic parameters Crystal system : Hexagonal Space group : P3121 Space group number : 152 a (Å) 4.4579 b (Å) 4.4579 c (Å) 5.9270 Alpha (°) 90.0000 Beta (°) 90.0000 Gamma (°) 120.0000 Calculated density (g/cm^3) 6.23 Volume of cell (10^6 pm^3) 102.01 Z : 3.00 PIR : - Subfiles and quality Subfiles : Alloy, metal or intermetalic Common Phase Educational pattern Forensic Inorganic Mineral NBS pattern Quality : Star (S) Comments Color : Gray metallic Creation Date : 01.01.1970 Modification Date : 11.01.2011 Additional Patterns : To replace 00-004-0554 (2) Color : Gray metallic General Comments QDF-2. The structure was determined by Bradley (1). On synthetic
material. Hexagonl close packed Color values in air C illuminat : x.311 .308, y .318 .317, Y% 59.9 69.4, ed 560 495, Pe% 0.7 0.6 Polymorphism/Phase Transition: There are several high pressure polymorphs of tellurium Reflectance : R%(air) : 60-70(546nm), 60-70(470nm), 60-69(589nm), 59-
67(650nm). Sample Source or Locality : The sample was from the Fisher Scientific Co., Fair Lawn, NJ, USA. It was ground and annealed at 300°C for ½ hour. Temperature of Data Collection. The mean tem-perature of data collection was 298.41K. Vickers Hardness Number : VHN25(on 00.1) 117. Unit Cell Data Source : Powder Diffraction.
References Primary reference : McMurdie, H., Morris, M., Evans, E., Paretzkin, B., Wong-Ng, W.,
Ettlinger, L., Hubbard, C., Powder Diffr., 1, 76, (1986) Structure : 1.Bradley, A., Philos. Mag., 48, 477, (1924) Peak list
100
Structure No. Name Elem. X Y Z Biso sof Wyck. 1 TE1 Te 0.22540 0.00000 0.33333 0.5000 1.0000 3a
Stick Pattern
101
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List of Figures Figure 1.1 An example of a simple wireless sensor network. ......................................................................... 13 Figure 1.2 A smart city concept with wireless sensor networks. [8] ................................................................. 14 Figure 2.1 Schematic description of Seebeck effect. [10] ................................................................................. 17 Figure 2.2 Thermoelectric refrigeration (a), power generation (b) and a practical TEG (c). .......................... 18 Figure 2.3 Demonstration of the energy flows through a thermal leg upon a temperature gradient. [12] ......... 19 Figure 2.4 The dependence of device efficiency on the Z of the thermal couples. ......................................... 22 Figure 2.5 Hypothetical DOS with a) a large slope dlnσE/dE and b) a small slope near EF. [16] ................... 23 Figure 2.6 Optimization of ZT through tuning the charge carrier concentration. ........................................... 23 Figure 2.7 Figure of Merit, ZT, of the state of the art inorganic TE materials regarding the temperature. [15] 26 Figure 2.8 Electronic densities of states of materials with different dimensions. [26] ...................................... 26 Figure 2.9 Schematic cross section and SEM image (surface) of QDSL. [25] .................................................. 27 Figure 2.10 The correlation of thermoelectric properties regarding the oxidation level of PEDOT chains. [33]
......................................................................................................................................................................... 30 Figure 2.11 Nanotubes are coated by PEDOT:PSS particles, making nanotube-PEDOT:PSS-nanotube junctions in the composites. [48] ....................................................................................................................... 32 Figure 2.12 Demonstration of a flexible TEG based on PVDF/CNTs hybrid composites. [51]........................ 32 Figure 2.13 Illustration of the dispenser printed flexible polymer/inorganic nanoinclusion composites. [62].. 33 Figure 2.14 SEM picture of a PEDOT:PSS solution casted over a layer of ball milled Bi2Te3 particles. (a) Cross section; (b) Top view. [69]....................................................................................................................... 34 Figure 2.15 Demonstration of a composite casted from PEDOT:PSS directed synthesized tellurium nanorods. [71] ..................................................................................................................................................................... 34 Figure 3.1 Schematic diagram of a 2-Point Probe method. ............................................................................. 35 Figure 3.2 Schematic diagram of a 4-Point Probe method. ............................................................................. 36 Figure 3.3 Sketch of a Seebeck coefficient measurement based on the differential method. [9] ...................... 37 Figure 3.4 Sample holder for Seebeck coefficient measurement of the platform in SRX. [74] ........................ 38 Figure 3.5 Schematic views of thermal conductivity measurements. .............................................................. 39 Figure 3.6 Basic principles of AFM. [79] .......................................................................................................... 40 Figure 3.7 Atomic interaction force regardingthe tip distance from sample. [78] ............................................ 40 Figure 3.8 Monitoring the reduction of a polymer film on a glass substrate by UV-Vis spectroscopy. [81] .... 41 Figure 3.9 Spectroscopic transitions underlying several types of vibrational spectroscopy. ν0 indicates laser frequency, while ν is the vibrational quantum number. The virtual state is short-lived distortion of the electron distribution by the electric field of the incident light. [82] .................................................................. 42 Figure 3.10 Bragg’s diagram. [83] ..................................................................................................................... 43 Figure 3.11 Instrumentation of a X-ray diffractometer. [83] ............................................................................. 43 Figure 3.12 Schematic diagram of XPS working principle. [84] ....................................................................... 44 Figure 4.1 Seebeck coefficient of a pristine film by spin coating. .................................................................. 47 Figure 4.2 Seebeck coefficient of a drop casted pristine thick film. ............................................................... 47
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Figure 4.3 Electrical conductivity of PEDOT:PSS thin films by DMSO addition and DMSO post treatment. ......................................................................................................................................................................... 48 Figure 4.4 Seebeck coefficient of PEDOT:PSS thin films by DMSO addition and DMSO post treatment. .. 48 Figure 4.5 Power factor of PEDOT:PSS thin films by DMSO addition and DMSO post treatment. ............. 48 Figure 4.6 AFM images of films with only DMSO addition with scan size 1 µm × 1 µm. Upper row: topology; down row: phase image. .................................................................................................................. 50 Figure 4.7 AFM images of films with DMSO addition and DMSO post treatment. Scan size 1 µm × 1 µm. Upper row: topology; down row: phase image................................................................................................ 51 Figure 4.8 Evolution of film thickness regarding DMSO addition and DMSO post treatment. ..................... 52 Figure 4.9 Surface composition of the sulphur atom for PEDOT:PSS thin films by XPS. ............................. 52 Figure 4.10 Raman spectra of PEDOT:PSS thin films under different fabrication conditions. ...................... 54 Figure 4.11 Seebeck coefficient and electrical conductivity of PEDOT:PSS films regarding the post treatment medium. ........................................................................................................................................... 57 Figure 4.12 Power factor of PEDOT:PSS thin films regarding the post treatment medium. .......................... 57 Figure 4.13 Topology of PEDOT:PSS thin films post treated with different solvents. . Scan size 1 µm ×1 µm. ............................................................................................................................................................... 58 Figure 4.14 Phase images of PEDOT:PSS thin films post treated with different solvents. . Scan size 1 µm ×1 µm. ............................................................................................................................................................... 59 Figure 4.15 Sketch of a PEDOT chain transition from bipolaron to polaron and neutral states. .................... 60 Figure 4.16 UV-Vis spectra of PEDOT:PSS films post treated with different solvents. ................................ 60 Figure 4.17 Raman spectra of PEDOT:PSS films post treated with different solvents. .................................. 61 Figure 5.1 Formulation of cations in ionic liquids. [121] ................................................................................... 63 Figure 5.2 Demonstration of the gelation of a PEDOT:PSS solution after the addition of 1.8 vol% (0.23 wt% in solution) EMIMBF4 (the vial is put upside down). ..................................................................................... 65 Figure 5.3 Electrical conductivity of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures. ...... 67 Figure 5.4 Seebeck coefficient of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures. ........... 67 Figure 5.5 Power Factor of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures. ..................... 67 Figure 5.6 Morphology evolutions of films treated with EMIMBF4/DMSO mixtures Part I. Upper row: Height images; down row: Phase images. Scan size:1 µm × 1 µm. .............................................................. 68 Figure 5.7 Morphology evolutions of films treated with EMIMBF4/DMSO mixtures Part II. Upper row: Height images; down row: Phase images. Scan size: 1 µm × 1 µm............................................................... 69 Figure 5.8 XPS survey of PEDOT:PSS films treated with EMIMBF4/DMSO mixtures. ............................... 70 Figure 5.9 Sulphur signals of pristine film and DMSO post treated film. ....................................................... 70 Figure 5.10 Sulphur signals of films post treated with the presence of EMIMBF4. ........................................ 70 Figure 5.11 UV-Vis absorption spectra of PEDOT:PSS thin films post treated with EMIMBF4/DMSO mixtures. .......................................................................................................................................................... 71 Figure 5.12 Raman spectra of PEDOT:PSS thin films post treated with EMIMBF4/DMSO mixtures. .......... 72 Figure 5.13 Film thickness variations regarding the post treatment medium. ................................................. 73 Figure 6.1 Morphology and grain characterization of wet chemically synthesized Te nanostructures (C-Te-0). ......................................................................................................................................................................... 80 Figure 6.2 Corresponding XRD pattern of C-Te-0 with Cu-Kα as the radiation source. ................................ 80 Figure 6.3 Observation of the reduction of TeO2 without (vial A) or with (vial B) EG at room temperature after 3 days reaction. ........................................................................................................................................ 81 Figure 6.4 Te nanostructures from wet chemical synthesis with varying PVP and NaOH amount. ............... 82 Figure 6.5 XRD of the wet chemically synthesized Te nanostructures. .......................................................... 82 Figure 6.6 Morphology of the hydrothermally synthesized Te nanostructures. .............................................. 84
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Figure 6.7 XRD of the hydrothermally synthesized Te nanostructures. Co-Kα was used as the radiation source. .............................................................................................................................................................. 85 Figure 6.8 TEM observation of Hydro-Te-2. .................................................................................................. 85 Figure 6.9 TEM observation of Hydro-Te-4. .................................................................................................. 85 Figure 6.10 TEM observation of Hydro-Te-5. ................................................................................................ 86 Figure 6.11 SEM images of the composite P-test film with C-Te-0. .............................................................. 88 Figure 6.12 SEM characterizations of composite films P-Hydro-02 and P-Hydro-03. ................................... 89 Figure 6.13 Thermoelectric properties of PEDOT:PSS/Te nanostructures (Hydro-Te-4) composite films with varied Te concentrations. ................................................................................................................................. 90
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List of Tables Table 4.1 Sample nomenclature regarding DMSO addition and DMSO post treatment................................. 46 Table 4.2 Surface roughness of PEDOT:PSS thin films subjected to DMSO addition and DMSO post treatment. ......................................................................................................................................................... 51 Table 4.3 The PEDOT to PSS ratio on the surface of PEDOT:PSS thin films. .............................................. 53 Table 4.4 Sample notation of PEDOT:PSS thin films treated with vary organic solvents. ............................. 56 Table 4.5 Physical properties of the chemicals. .............................................................................................. 57 Table 5.1 Sample nomenclature of PEDOT:PSS films post treated with EMIMBF4/DMSO mixtures. ......... 66 Table 5.2 Surface roughness of films subjected to post treatment with EMIMBF4/DMSO mixtures. ............ 69 Table 6.1 Samples prepared by the wet chemical synthesis at 120°C. ............................................................ 78 Table 6.2 Samples prepared by the hydrothermal synthesis at 180°C. ........................................................... 78 Table 6.3 Nomenclature of polymer/Te nanostructures hybrid composites. ................................................... 87 Table 6.4 TE properties of composite films embedded with different Te nanowires. .................................... 88
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Versicherung Hiermit versichere ich, dass ich die vorliegende Arbeit ohne unzulässige Hilfe Dritter und ohne Benutzung
anderer als der angegebenen Hilfsmittel angefertigt habe; die aus fremden Quellen direkt oder indirekt über-
nommenen Gedanken sind als solche kenntlich gemacht.
Bei der Auswahl und Auswertung des Materials sowie bei der Herstellung des Manuskripts habe ich Unter-
stützungsleistungen von folgenden Personen erhalten:
-keine-
Weitere Personen waren an der Abfassung der vorliegenden Arbeit nicht beteiligt. Die Hilfe eines Promo-
tionsberaters habe ich nicht in Anspruch genommen. Weitere Personen haben von mir keine geldwerten
Leistungen für Arbeiten erhalten, die im Zusammenhang mit dem Inhalt der vorgelegten Dissertation stehen.
Der Arbeit wurde bisher weder im Inland noch im Ausland in gleicher oder ähnlicher Form einer anderen
Prüfungsbehörde vorgelegt.
Chemnitz, November 10, 2014 ……………………………….
Jinji Luo
110
Theses of the dissertation
“Investigation of Polymer Based Materials in
Thermoelectric Applications”
for the attainment of the title “Dr.-Ing.” at Chemnitz University of Technology,
Faculty for Electrical Engineering and Information Technology,
presented by M. Sc. Jinji Luo
Chemnitz, November 10, 2014
1. The state of the art inorganic thermoelectric materials demand high cost production tech-
niques to reduce the thermal conductivity for better thermoelectric performance. Further-
more, the rigidity of these materials and the limited production output limit their wide
spread applications.
2. Polymers have intrinsic low thermal conductivity, high electrical conductivity, high flexibil-
ity and are light weight. They can be fabricated by printing techniques to reduce the fabrica-
tion cost and improve the production output. With these features, polymers are proposed as
the alternative for traditional inorganic thermoelectric materials.
3. The addition of secondary dopants, such as DMSO, only induces phase separation, leading
to the formation of an interconnected network of elongated conducting PEDOT-rich chains
and increases the electrical conductivity.
4. Compared to conventional addition method with DMSO, post treatment of PEDOT:PSS
films with DMSO as the medium is more effective to enhance the electrical conductivity.
111
5. Post treatment with secondary dopants does not only trigger the phase separation as in the
addition method. It also depletes the insulating, featureless PSS chains from the film surface,
which diminishes the film thickness and increases the electrical conductivity.
6. Secondary dopants only affect the film morphology not the chemical structure of PEDOT
chains. Amines (MEA and ammonia solution), on the other hand, reduce the oxidation level
of PEDOT chains. The change in the oxidation level of PEDOT chains is reflected as the
intercoupling phenomenon between the electrical conductivity and Seebeck coefficient of
PEDOT:PSS films. The reduction degree of PEDOT chains is determined by the structure of
amines and the film morphology to which amines donate electrons.
7. As a post treatment medium, EMIMBF4 constrains the growth of elongated PEDOT-rich
grains. Instead, large and circular PEDOT-rich grains are generated.
8. EMIMBF4 exchanges ions with PEDOT:PSS films during the post treatment. The remaining
𝐸𝑀𝐼𝑀+ cations reduce the PEDOT chains, which decreases the intensity of bipolarons and
increases the intensity of polarons.
9. With an EMIMBF4/DMSO mixture as the post treatment medium, a high power factor as
high as 38.46 𝜇𝑊/(𝑚𝐾2) is observed.
10. Green synthesis methods based on wet chemical synthesis and hydrothermal synthesis are
proposed to synthesize Te nanostructures. Here, LAC and LAC/MEA mixture are confirmed
as the non toxic reductants to reduce non toxic Te source, TeO2, to synthesize Te nanostruc-
tures.
11. In the wet chemical synthesis, using LAC/MEA mixture as the reductant, small Te
nanostructures (e.g. nanorods, nanotubes and trifold nanostructures) are synthesized. In the
hydrothermal synthesis, 𝜇𝑚 long nanowires with smooth surface can be synthesized with
LAC as the reductant with the presence of NaOH.
12. The morphology of Te nanostructures strongly depends on the addition of NaOH and the
amount of PVP.
13. Addition of NaOH into the reaction mixture leads to lower reduction rate.
14. LAC/MEA mixture has higher reduction ability than LAC alone.
112
15. Hydrothermally synthesized Te nanowires yield a Seebeck coefficient of 383.5±54% 𝜇𝑉/𝐾.
16. The Seebeck coefficient of composites with embedded long Te nanowires is found to be as
high as 209±30% 𝜇𝑉/𝐾 in this work.
113
Curriculum Vitae Contact Information Name: Jinji Luo Date of Birth: 20.09.1985 Address: Am Kalkwiesenteich 14
09117 Chemnitz Germany
Phone: +49 0176 9588 8921 Email: [email protected] Education Master of Science (September, 2010)
Polymer Science Master Program hold by Freie Universität Berlin, Humboldt-Universität zu Berlin, Technische Universi-tät Berlin and Universität Potsdam
Bachelor of Science (July, 2008)
Materials Science and Engineering, Xiamen University, China
Experience Zentrum für Mikrotechnologien, Technische Universität Chemnitz 10.2010-09.2014 Developing Polymer/nanoinclusions (carbon nanotubes and
tellurium nanostructures) composites as light weight and flexible thermoelectric materials (energy harvester)
Institut fur Werkstoffwissenschaften Fachgebiet Polymertechnik /-physik Techni-sche Universität Berlin 10.2009-09.2010 Rheological and theoretical characterization of thermally
and thermooxidatively degraded low density polyethylene BAM- Bundesanstalt für Materialforschung und -prüfung 08.2009-09.2009 Dielectric spectroscopy characterization of
PolyhedralOligomeric Silsesquioxane (POSS) based films Department of Materials Science and Engineering, Xiamen University, China 09.2007-07.2008
Characterization of inorganic-organic hybrid materials based on Polyhedral Oligomeric Silsesquioxane (POSS) films
114
Languages English TOEFL iBT 99 German B2 Chinese Mother Tongue
115
Publications Peer reviewed journal publications and conference proceedings related to this thesis
1 Luo, J.; Billep, D.; Blaudeck, T.; Sheremet, E.; Rodriguez, R. D.; Zahn, D. R. T.; Toader, M.; Hietschold, M.; Otto, T.; Gessner, T.: Chemical post-treatment and ther-moelectric properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) thin films, Journal of Applied Physics, vol. 115, pp. 054908, 2014. DOI: 10.1063/1.4864749 2 Luo, J.; Billep, D.; Waechtler,T.; Otto, T.; Toader, M.; Gordan, O.; Sheremet, E.; Martin, J.; Hietschold, M.; Zahn, D. R. T.; Gessner, T.: Enhancement of the thermoe-lectric properties of PEDOT :PSS thin films by post treatment , Journal of Materials Chemistry A, vol.1, pp. 7575-7583, 2013 (Cover Page). DOI: 10.1039/C3TA11209H 3 Luo, J.; Billep, D.; Waechtler, T.; Sheremet, E.; Otto, T.; Gessner, T.: Influence of DMSO addition and post treatment on the thermoelectric properties of PEDOT:PSS thin films, Poster, 3rd International Conference of Organic Electronics, Grenoble (France), 18.-20. June 2013 4 Luo, J.; Streit, P.; Billep, D.; Otto, T.; Gessner, T.: Influence of dimethyl sulfoxide and carbon nanotubes on the thermoelectric properties of PEDOT:PSS, Smart System Integration, Amsterdam (Netherland), 13.-14. March 2013 Other Publications 5 Rolón-Garrido, V. H.; Luo, J.; Wagner, M. H.: Enhancement of strain-hardening by thermo-oxidative treatment of low-density polyethylene ; Rheologica Acta, vol. 50, pp. 519-535, 2011 6 Rolón-Garrido, V. H.; Luo, J.; Wagner, M. H.: Increase of Long-chain Branching by Thermo-oxidative Treatment of LDPE, AIP Conference Proceedings, vol. 173, pp. 1375, 2011
116
Acknowledgements Without the support of many peoples, this thesis would not be completed. I would like to
gratefully acknowledge their help.
First of all, I want to thank my supervisors Dr. Detlef Billep, Prof. Thomas Otto and Prof. Thomas
Gessner for giving me the opportunity to work in Center for Microtechnologies in Chemnitz. I also
would like to thank Prof. Carsten Deibel for being the co-supervisor of this work. Thanks to Dr.
Billep for giving me enough freedom to realize my research ideas.
Next I would like to thank Dr. Thomas Wächtler, who is the responsible person for the IRTG pro-
gram. In the beginning of my work, he helped me characterizing the samples with SEM and XPS.
From him, I have gained experience on the characterization techniques. Moreover, he supported me
with precious comments on my first publication. Here, I am also grateful with the help from Dr.
Jörg Martin, who provides me with expert suggestions.
I would like to thank Dr. Thomas Blaudeck. Thomas is a very open mind person. He supported me
with precious suggestions when I had some new ideas. Thank you for introducing me to Prof. Xavi-
er Crispin. Thank you for building up the connection between Chemnitz and Sweden for the BMBF
networking project. Thank for helping with the project writing.
I would like to thank Evgeniya and Raul in Prof. Zahn's group for their support with the Raman
characterization and analysis.
I would like to thank Ralf Zichner from Printing Functionality in Fraunhofer ENAS. Thank you
Ralf for teaching me how to use van der Pauw measurement platform, for introducing me to the
printing experts from the university and for helping me with the test printing of thermoelectric legs
by screen printing.
I would like to thank Dr. Ramona Ecke for administrative help and supporting me with the financial
allowance for the Seebeck coefficient and XRD measurements.
117
I would like to thank Dr. Stefan Schulz for the TEM measurement. Thanks for teaching me how to
comprehend the results.
I would like to thank Cornelia Kowol for taking the time for the SEM characterization.
I would like to thank Dr. Christian Kaufmann, Rene Reich and Dr. Sven Zimmermann for creating
a nice working environment in the clean room. Thank you for helping maintain the Omnimap and
Profilometer in working status.
I would like to thank Carmen Schulz and Katrin Träber for the administrative help.
I would like to thank my friends here and back in China. Thanks for your company to left me not
feel alone. Thanks for being the person I could share joy and sad together.
In the end but not the least, to my boyfriend Lutz, for always believing in me and encouraging me.
Thanks for taking care of me when I was sick. Without his support, there would be definitely not
this thesis. And thanks to my parents, aunt and families in China and Germany for their uncondi-
tional love.