Dissertation, No. 1491

Thermoelectric properties of conducting polymers

Olga Bubnova

Organic Electronics Department of science and technology (ITN)

Linköping University, SE-601-74 Norrköping, Sweden

Thermoelectric properties of conducting polymers

Olga Bubnova

ISBN: 978-91-7519-741-8 ISSN 0345-7524

Copyright ©, 2013, Olga Bubnova olgbu@itn.liu.se

Linköping University Department of science and technology

SE-601-74 Norrköping Sweden

Printed by LiU-Tryck, Linköping 2013.

Abstract According to different sources, from forty to sixty percent of the overall energy generated in the world today is squandered in waste heat. The existing energy conversion technologies are either close to their efficiency limits or too costly to justify their implementation. Therefore, the development of new technological approaches for waste heat recovery is highly demanded. The field of thermoelectrics can potentially provide an inexpensive, clean and efficient solution to waste heat underutilization, given that a new type of thermoelectric materials capable of meeting those requirements are available. This thesis reports on strategies to optimize a thermoelectric efficiency (ZT) of conducting polymers, more specifically poly(3,4-ethylenedioxythiophene) (Pedot). Conducting polymers constitute a special class of semiconductors characterized by low thermal conductivity as well as electrical conductivity and thermopower that can be readily modified by doping in order to achieve the best combination of thermoelectric parameters. Conducting polymers that have never previously been regarded as hypothetically compatible for thermoelectric energy conversion, can exhibit promising thermoelectric performance at moderate temperatures, which is a sought-after quality for waste heat recovery. A rather substandard thermoelectric efficiency of Pedot-Pss can be markedly improved by various secondary dopants whose addition usually improves polymer’s morphology accompanied by a drastic increase in electrical conductivity and, consequently, in ZT. In order to enable further enhancement in thermoelectric properties, the optimization of the charge carrier concentration is commonly used. The oxidation level of Pedot-Pss can be precisely controlled by electrochemical doping resulting in a tenfold increase of ZT. In contrast to Pedot-Pss, another conducting polymer Pedot-Tos exhibits superior thermoelectric performance even without secondary doping owning to its partially crystalline nature that allows for an improved electronic conduction. With the aid of a strong electron donor, positively doped Pedot-Tos gets partially reduced reaching the optimum oxidation state at which its thermoelectric efficiency is just four times smaller than that of Be2Te3 and the highest among all stable conducting polymers. The downsides associated with chemical doping of Pedot-Tos such as doping inhomogeneity or chemical dopants air sensitivity can be surmounted if the doping level of Pedot-Tos is controlled by acidity/basicity of the polymer. This approach yields similar maximum thermoelectric efficiency but does not necessitate inert conditions for sample preparation. Optimized Pedot-Tos/Pedot-Pss can be functionalized as a p-type material in organic thermogenerators (OTEG) to power low energy electronic devices. If printed on large areas, OTEGs could be used as an alternative technique for capturing heat discarded by industrial processes,

households, transportation sector or any natural heat sources for electricity production.

Acknowledgments

To my own surprise, this part of the thesis is the hardest one to put into

words, since it has nothing to do with research and science, but solely concerns with the people to whom I owe a debt of gratitude for their being by my side when I needed it most. So, let me publically acknowledge those remarkable individuals who gave me the courage to carry on with my PhD.

The biggest “merci” goes to my SUPERvisor Xavier Crispin – a true scientist and mentor. I will probably never know why he had given me this opportunity 4 years ago, but for that I am more than grateful. One can hardly imagine how much patience it takes over the years to guide, teach and constantly cheer up a student. Xavier has done it all and I am more certain than I’ve ever been that this PhD wouldn’t be accomplished without his creative mind and positive attitude.

Next big “tack” is for Magnus Berggren, my co-supervisor whose boundless optimism and belief in science will never cease to amaze me. Thanks to his personality and incredible energy, our group today is stronger than ever and I am proud to have been a part of this big scientific family.

Now, it’s time for my friends in chronological order: Hiam: We’ve shared rooms, we’ve sang, we’ve played numerous board

and card games, we’ve been kayaking and killing spiders together we survived a volcano eruption and came back home alive. Sounds like a serious commitment, doesn’t it? At least I know that after all we’ve been through I can call you my friend!

Artsem: The man of remarkable personal integrity who has been a true inspiration and a great source of strength for me. I know you will be with me on that special day, and just for that I am looking forward to my defense.

Loig: for teaching me french, taking care of my rabbit and being such a good company. Special thanks for rillettes de canard and brioches.

Jiang: for great atmosphere in the office and just for being such a nice guy

Hui: for your help and support in the lab and outside. Amal: for your friendship and kind heart Zia: for all sleepless nights in the lab because of me Negar: for great company and lots of fun (especially on the plane from

Marseille) Amanda: for singing, skating, “afro-dancing” and partying with me Many thanks to Henrik, Simone, Jun, Xiadong, Maria, Kristin, Ek,

Darius and the entire Orgel group for being a part of this journey. Special thanks to Sophie for her indispensible help in administrative

issues and great Orgel on tour trips (especially the last one!) To my family in Russia and France, especially to my mom and dad. You

haven’t missed a thing even being so far away from me.

To Brice: I don’t know who or what I have to thank for having you in my life, but I feel very fortunate.

To my little brother: I hope you know how much you mean to me. Special thanks to Franz-Renan for a wonderful cosmo-design of the

cover. And finally, to Lucia: for being мой пушистик.

List of included papers Paper I: Towards polymer-based organic thermoelectric generators O. Bubnova, X. Crispin, Energy & Environmental Science, 2012, 5, 9345-9362. Contributions: Wrote the first draft and contributed to the final editing of the manuscript. Paper II: Advantageous thermoelectric properties of a semimetallic polymer O. Bubnova, Z. Ullah Khan, H. Wang, D. Dagnelund, Y. Geerts, M. Berggren, X.Crispin, submitted to Contributions: Conductivity and thermopower measurements, temperature dependence measurements. Wrote the first draft. Paper III: Tuning the thermoelectric properties of conducting polymers in an electrochemical transistor O. Bubnova, M. Berggren and X. Crispin, Journal of the American Chemical Society, 2012, 134 (40), 16456–16459. Contributions: All experimental work. Wrote the first draft and contributed to the final editing of the manuscript. Paper IV: Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene) O. Bubnova, Z. U. Khan, A. Malti, S. Braun, M. Fahlman, M. Berggren and X. Crispin, Nature Materials, 2011, 10, 429-433. Contributions: Conductivity and thermopower measurements, design and fabrication of the thermogenerators. Wrote the first draft and contributed to the final editing of the manuscript. Paper V: Ph effect on thermoelectric properties of poly(3,4-ethylenedioxythiophene):tosylate O. Bubnova, M. Berggren and X. Crispin, manuscript Contributions: All experimental work. Wrote the first draft of the manuscript

Table of Contents CHAPTER 1 – INTRODUCTION .................................................................... 1 CHAPTER 2 - THERMOELECTRIC POWER CONVERSION ................. 3

2.1 THERMOELECTRIC EFFECTS ........................................................................... 3 2.1.1 Seebeck effect ........................................................................................ 3 2.1.2 Peltier effect .......................................................................................... 5 2.1.3 Thomson Effect ...................................................................................... 6 2.1.4 Thermal conduction and Joule heating ................................................. 7

2.2 PHYSICS OF THERMOELECTRICS .................................................................... 8 2.2.1 Thermodynamics of thermoelectrics ..................................................... 8 2.2.2 Solid-state physics of thermoelectrics ................................................. 12

2.3 THERMOELECTRIC MATERIALS .................................................................... 17 2.4 THERMOELECTRIC APPLICATIONS ............................................................... 20

CHAPTER 3 – CONDUCTING POLYMERS .............................................. 25 3.1 ELECTRONIC AND CHEMICAL STRUCTURE OF CONJUGATED POLYMERS ....... 25 3.2 CHARGE CARRIERS IN CONJUGATED POLYMERS .......................................... 27 3.3 DOPING OF CONDUCTING POLYMERS ........................................................... 29 3.4 CHARGE TRANSPORT IN CONJUGATED POLYMERS ....................................... 31

3.4.1 Role of morphology in charge transport ............................................. 31 3.4.2 Density of states and sources of localization in disordered materials33 3.4.3 Role of insulator-to-metal transition in charge transport .................. 34 3.4.4 Nearest-neighbour hopping and variable range hopping .................. 35

3.5 POLY(3,4-ETHYLENEDIOXYTHIOPHENE) ..................................................... 36 CHAPTER 4 - EXPERIMENTAL TECHNIQUES ...................................... 39

4.1 ELECTRICAL CONDUCTIVITY MEASUREMENTS ON THIN FILM ....................... 39 4.2 SEEBECK COEFFICIENT MEASUREMENTS ..................................................... 40 4.3 THERMAL CONDUCTIVITY MEASUREMENTS ................................................. 42

CHAPTER 5 - GOAL OF THE THESIS ....................................................... 45 CHAPTER 6 – CONCLUSIONS AND PERSPECTIVE .............................. 47 REFERENCES: ................................................................................................ 48 Paper 1 Paper 2 Paper 3 Paper 4 Paper 5

1

Chapter 1 – Introduction Despite the energy resources limitation problem, roughly a two third of

the energy consumed in industrial processes and private households is lost to the environment in the form of low-grade heat commonly referred to as waste heat. Inefficiency of the waste heat recovery techniques is mostly associated with low quality of waste heat making its harvesting economically impractical and expensive. Besides anthropogenic heat, there are numerous natural sources of energy such as solar, wind or geothermal energy, which is underutilized given its great potentials. The inexorable depletion of oil and gas dictates a strong need for “green” energy generation technologies as well as more thrifty use of the energy that is being produced in a conventional way.

Thermoelectric energy conversion is one possible solution that could efficiently address these problems as it offers extremely reliable, small in size, light and silent in operation power sources with no moving parts and no need for maintenance. Thermoelectrics that has been known for nearly two centuries is far from being considered as a possible alternative to fossil-fuel power plants. As it turn out, thermoelectric generation fails in that respect due to very low efficiencies achieved in thermogenerators (TEGs)1 and high production cost that further diminishes the already low dissemination of this technology on the electricity production market. Thermoelectric generation only requires a small temperature difference across a TEG to recover some portion of the waste heat even at low temperatures, meaning that thermoelectrics can be used there where most other energy conversion technologies fail.

Obviously, if thermoelectric installations are too bulky and rigid there is no way to effectively adapt them for a given industrial or natural environment. Instead, a miniaturized version of TEGs on a flexible substrate could do exceptionally well covering large heat exchanger areas of whatever shape and size is required. As for now, the main challenge has been to find materials capable of highly efficient thermoelectric energy conversion. Another problem to tackle is materials scarcity. If this condition is not satisfy a higher production cost results. Hence natural abundance along with good processibility is essential.

After a period of oblivion in the field of thermoelectrics, the research into this matter has been intensified anew as a result of significant advancements in material science. Regardless of the material complexity and dimensionality the overwhelming majority of the thermoelectrics are highly doped inorganic semiconductors. High temperature solid-state synthesis used to form more complex materials out of single elements represents a big limitation on the way of creating multinary compounds. Low temperature processing from

2

solution is a great option that enables patterning of thermoelectric elements by means of printing techniques. Moreover, flexible substrates equally imply low temperature synthesis. For that reason, organic conducting polymers can offer a surprisingly good alternative provided their relatively high electrical conductivity in doped state, extremely low thermal conductivity and Seebeck coefficient that can be altered by redox chemistry typical in these materials.

Many conducting polymers have already been tested as potentially appropriate materials for thermoelectric applications. Thermoelectric properties of polyaniline (PANI)2, polythiophene (PTH), poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate)/tosylate (PEDOT:PSS, PEDOT-Tos)3, polyacetylene (PA)4, polypyrrole (PPY)5 6, polycarbazoles (PC)7, polyphenylenevinylene (PPV) and their derivatives show a lot of promise and certainly worth further investigation. The research on the thermoelectric performance of Pedot and its derivatives is currently underway, but the question for now is what are the main strategies one can use in order to improve its thermoelectric efficiency (zT). For that a better understanding of charge transport in organic semiconductors is required. In many ways it’s been a hard subject to study due to polymers morphological complexity8 and anisotropy of their physical properties. In spite of these obstacles, the thermoelectric properties of PEDOT based polymers are easily tunable in practice by doping, which has been a good and straightforward method for zT enhancement capable of inducing a dramatic rise of this value (mainly due to thermopower increase). The material is equally characterized with high environmental stability, nontoxicity and great technological adaptability and flexibility. It can be functionalized with inorganic substances including various types of nanostructured materials in order to create better performing organic-inorganic hybrids9 10.

In this thesis some of zT optimization approaches including different doping and chemical treatment techniques are shown on the example of PEDOT-PSS and PEDOT-Tos. In both cases thermoelectric efficiency is drastically changed to yield higher resulting zT. Nonetheless, the PEDOT-based polymers exhibit rather dissimilar thermoelectric properties with PEDOT-Tos being a much better thermoelectric potentially capable of efficient room T energy conversion.

3

Chapter 2 - Thermoelectric power conversion

2.1 Thermoelectric effects Any transport phenomenon that implicates an exchange of heat and

electrical potential energy can be referred to as a thermoelectric effect. The process in which system’s final state can be restored to its initial state without entropy production or dissipation of energy is defined as thermodynamically reversible. There exist three reversible thermoelectric effects: the Seebeck and Peltier effects used for thermoelectric generation and electronic refrigeration respectively and the Thomson effect that holds no practicality. At the same time, the performance of a real thermoelectric device always remains lower than Carnot efficiency due to two present irreversible processes that include Joule heating and thermal conduction. Since it is virtually impossible to separate reversible and irreversible processes in thermoelectrics it can only be treated within non-equilibrium thermodynamics.

2.1.1 Seebeck effect The discovery of the first thermoelectric effect belongs to Estonian-

German physicist Thomas Johann Seebeck11, which was made in early 1820 as a result of his experimental investigations into a possible relation between electricity and heat. In that experiment he used an open circuit consisting of two dissimilar materials (standard thermocouple configuration) that is depicted in fig 1 where the cold junction is kept at temperature T1 and the hot junction at T2 creating a temperature gradient ΔT. Under these conditions an electromotive force (Seebeck voltage) is induced in the circuit. The measured voltage ΔV is directly related to ΔT by a proportionality coefficient, which is best known as Seebeck coefficient or thermopower12:

4

Figure 1. a) Seebeck effect illustration in a circuit comprising of two dissimilar materials b) Effect of a temperature gradient on the charge carrier diffusion in a single conductor

∆! = !! ! − !! !                (2.1)!!

!!

The Seebeck coefficient of a thermocouple can be expressed as:

!! − !! =∆!∆!                      (2.2)

Practically it is not possible to measure directly an absolute Seebeck

coefficient value of an individual homogeneous conductor since evidently the resulting emf will be zero. For that reason the combination of two different materials is always used in Seebeck coefficient measurements. It is very important to mention herein that the Seebeck effect is in no way a contact phenomenon. Individual coefficients can be calculated if at a given T the Seebeck coefficient of one of the materials (reference) forming a thermocouple is known with good precision or negligibly small as compared to the second material13. One obvious solution is to use a superconductor whose Seebeck coefficient is zero, however it can be only done at low temperatures while the operating temperatures for thermoelectrics are significantly higher. Hence, the limitation comes from the fact that the Seebeck coefficient is a T dependent parameter and this dependence is nonlinear and can vary appreciably subject to material nature. Charge carriers available for conduction in the material under the influence of temperature gradient start to diffuse from the region with higher energy to the cooler region of a conductor that is kept at lower T (fig 1). This process continues until the steady state is reached characterized by a stable Seebeck voltage, which arises as a force acting against further migration of charge carriers. If the majority of charge carriers are holes the Seebeck coefficient is positive whereas for electrons it is obviously negative. In a junction between two dissimilar materials charge carriers of a conductor with higher electronic pressure will tend to diffuse into a lower pressure region of the other conductor and once again the Seebeck voltage will build up. This voltage difference is of course dependent on a material type (n- or p-type) and temperature gradient. At higher T the charge carriers’ velocity is substantially increased, which corresponds to a greater “electronic pressure”. In this situation, the voltage difference measured in a circuit will be further raised. As it is seen from (2.1) one needs to use two conductors with opposite signs of Seebeck coefficient to acquire high ∆!. As we shall see later, the materials with highest

5

thermopower are doped semiconductors14, while metals15 possess lowest Seebeck coefficients and are only used in thermocouples16 for temperature sensing. In the early years of thermoelectrics, thermocouples based on metals and their alloys were the only practical implications of the Seebeck effect. In 1949 A. F. Ioffe developed the modern theory of semiconductor thermoelectrics17 using the concept of the 'figure of merit' zT, which gave the green light to a new Seebeck effect application, namely thermoelectric generation.

2.1.2 Peltier effect The second thermoelectric effect, which is complementary in nature to

the Seebeck effect, was discovered in 1834 by a French watchmaker Jean Peltier. In his experiment, the same set-up as for the Seebeck effect demonstration was used. However, no temperature gradient was established between the junctions. Instead, the current was passed through the loop consisted of two conductors producing heating or cooling at the junction18 (fig 2).

Figure 2. Peltier effect in a circuit of two different materials Similar to Seebeck, Peltier didn’t completely understand the physics of

this phenomenon, and it was Lenz who recognized the reversibility of this effect and demonstrated that the rate of heat ejection or absorption was proportional to the electrical current with the proportionality coefficient that was subsequently named after J. Peltier19. At a constant temperature the absolute Peltier coefficient of an individual material is defined as:

6

Π =!!                (2.3)

Analogous to the Seebeck coefficient, for a circuit made up of two

dissimilar materials A and B the heat absorbed at the junction is given by:

! = Π!"! = (Π! − Π!)!            (2.4), where I is the electrical current and ΠAB is the Peltier coefficient of the

junction between two conductors. The Peltier coefficient is measured in volts and can be defined for an individual conductor using the approach previously discussed for the Seebeck coefficient. The same rule applies for the Peltier coefficient sign: p-type material would have a positive Peltier coefficient and n-type typically would be characterized by a negative Peltier coefficient. Physical representation of the Peltier effect can be interpreted as follows: In an n-type material in the presence of an electric field electrons tend to leave the negative side carrying away energy in a form of heat that is subsequently released at the positive side. In a p-type semiconductor the holes drift towards the negative side instead, opposite to a current flow and so does the heat. Hence, reversing the polarity of the electrical current will change the direction of heat transfer.

2.1.3 Thomson Effect The last of the three thermoelectric effects was discovered by Thomson

(Lord Kelvin) in 1851 and referred to as Thomson effect. This effect represents a combination of the Seebeck and Peltier effects and describes the generation of heat when an electrical current flows through an unequally heated single conductor20 (fig 3).

Figure 3. Thomson effect in a single conductor

7

The heating/cooling rate in the material is proportional to the electrical current and the temperature gradient through a proportionality coefficient ! (Thomson coefficient):

!!!!"#!$ = !"(−!!)      (2.5) The sign of the Thomson coefficient depends on whether an electrical

current flows from warmer to cooler side of the conductor or, in opposite, from cooler to warmer. The total heat production within the material is given by:

!!"#$% = !" −!! + !!! − ! k −!!     2.6 , where !!! is irreversible Joule heating and the term in 2.6 is the heat

conduction rate in the material. The first term in the equation is the reversible Thomson heat that changes sign whenever the direction of the electrical current is reversed. The Thomson coefficient is related to the Seebeck coefficient by the first Kelvin relation:

! = !!"!"          (2.7)

Essentially, the Thompson effect represents the heat generated (or

absorbed) due to the fact that the Peltier heat changes with temperature. Similar to the Seebeck and Peltier coefficients, the Thomson coefficient can be positive or negative depending on the material’s type, if ! is temperature independent ! is zero. The Thomson effect offers no interest in the operation of thermoelectric devices, however it shall not be neglected in experimental analysis that requires good accuracy.

2.1.4 Thermal conduction and Joule heating Along with the aforesaid thermoelectric effects there are two irreversible

processes that have to be mentioned: Joule heating and thermal/heat conduction that always tend to lower the thermoelectric device performance21. Their presence precludes device efficiency from attaining its thermodynamic limit better known as Carnot efficiency.

Joule heating inevitably occurs as a consequence of any electric current and in case of thermoelectrics is particularly undesirable as it converts useful electric energy into heat that dissipates resulting in so-called ohmic loss. Joule

8

heat is given by the second term in 2.6, where i is the electrical current density and ρ is the resistivity.

In a conductor subject to a temperature gradient the increase of its cold end temperature is always observed as a result of heat conduction, which is expressed as following:

! = −!"!"!"           2.8 ,

where A is the cross-section area of the conductor and k is material’s thermal conductivity, the expression is known as Fourier law. The thermoelectric efficiency loss caused by heat conduction is directly proportional to k, hence the materials with low thermal conductivity are required for efficient energy conversion.

2.2 Physics of thermoelectrics

2.2.1 Thermodynamics of thermoelectrics Despite the reversibility of thermoelectric effects, the processes taking

place in a thermoelectric system are not in equilibrium and therefore beyond the scope of classical thermodynamics. Still it can be assumed that the transformations persisting in the macroscopic system over time are quasi-static and the problem can be treated as near equilibrium. In other words, the system tends to restore the state as close as possible to equilibrium and is said to be in steady state, which is conditioned by minimum entropy production rule22. This special case of non-equilibrium is generally treated by Onsager theory of irreversible thermodynamics23 that establishes linear relations between conjugate “flows” and “driving forces” through proportionality coefficients24. The most straightforward examples that illustrate this principle are: Ohm’s law, where the electrical flow is proportional to the electrical potential and Fourier’s laws (the heat flow is proportional to the temperature gradient). In a thermoelectric system the situation gets more complicated as both electrical and heat currents are present concomitantly and interact with one another. Furthermore, both forces can cause either of two flows or both of them simultaneously25.

By virtue of the first law of thermodynamics the expression for the total energy flux JE in terms of heat flux JQ and particle flux JN is given by:

9

JE = JQ +µeJN (2.9), where µe is the electrochemical potential. The corresponding set of

conjugated thermodynamic potentials for energy and particles are 1/T and µe/T. Finally, using kinetic coefficient matrix the linear combination between fluxes and forces can be written as:

JNJE

!

"#

$

%&=

LNN LNELEN LEE

!

"##

$

%&&

'((µe /T )

'(1 /T )

!

"

##

$

%

&& (2.10)

The off-diagonal terms in the matrix are equal by virtue of Onsager

reciprocal relations that imply the symmetry of the transport coefficients in the system, which is asymptotically close to equilibrium26.

In order to thermodynamically express the thermoelectric coefficient, we need to rewrite (2.10) in terms of particle and heat flux density:

(2.11) In the absence of thermal gradient (2.11) yields for the electrical current

density:

J = !eLNNT

"µe (2.12),

where the proportionality coefficient is nothing but the electrical

conductivity:

! T =e2

TLNN (2.13)

Analogously, in the absence of the electrical current (2.10) the heat

density is given by:

JQ =1/T 2 LQNLNQ ! LNNLQQLNN

"

#$

%

&'(T = kJ(T (2.14)

Here, kJ is the thermal conductivity in zero electrical current limit and

2.14 is simply Fourier’s law. If instead the electrochemical gradient is zero (JN=0), then the thermal conductivity is:

JNJQ

!

"#

$

%&=

LNN LNQLQN LQQ

!

"##

$

%&&

'(1 /T )(µe

((1 /T )

!

"

##

$

%

&&

10

JNJQ

!

"#

$

%&=

Te2!

!"T 2

e!"T 2

eT 3!" 2 +T 2kJ

!

"

####

$

%

&&&&

'(1 /T )(µe

((1 /T )

!

"

##

$

%

&&

kE=LQQ /T2 (2.15) The thermoelectric effects are so-called coupled processes since

different irreversible transport phenomena are involved simultaneously and may interfere with each other. In a circuit consisting of two dissimilar materials an electric current will generate heat absorption/evolution at the junctions (Peltier effect), or the electromotive force will be established in a circuit if the junctions are maintained at different temperatures. Moreover, heat by itself can transport electrically charged particles or these particles under electrical force can diffuse along a conductor carrying some heat.

The Seebeck effect is a coupled thermoelectric process that requires a presence of temperature gradient in an open circuit created by two dissimilar conductors. The Seebeck coefficient is then represented by a ratio between the electrochemical gradient and the temperature difference at J=0:

! = 1eT

LNQLNN

(2.16)

To define the Peltier coefficient, one needs to consider an isothermal

case ( !T=0), then from (2.11) and provided that J=eJN we get:

JQ =1eLNQLNN

J =!J (2.17)

It follows immediately that the Seebeck coefficient is related to the

Peltier coefficient by a well-known relation: Π=αT.

Finally, using 2.14 and 2.15 we can we can express kE and kJ:

(2.18) Since the kinetic coefficients are known, the Onsager expressions now

can be rewritten: (2.19)

kE = T!2" + kJ

11

JJQ

!

"#

$

%&=

! "!

!"T kE

!

"##

$

%&&

E'(T

!

"#

$

%&

or in terms of J and JQ:

(2.20)

Under zero electric field condition 2.20 reduces to JQ. The heat flux

divergence is given by:

!JQ =!(!TJ " kJ!T ) = TJ!! +!!TJ +!T!J +!("kJ!T ) (2.21), where TJ!! is the Peltier–Thomson term, which under temperature

gradient leads to the Thomson contribution:

TJ!! = TJ! "T#

$%

&

'(= J(!")!!T ) = J d"

dT)!

#

$%

&

'(!T = " J!T (2.22)

The Thomson coefficient τ is defined via the well-known Kelvin

relation. The Peltier contribution can be obtained setting !T =0. Finally, substituting 2.18 in 2.19 we get in terms of J and JQ: (2.23) Note, that this result defines α as a coupling term between heat and

electrical conduction and in absence of this term the matrix reduces to Ohm’s law and Fourier’s law that are decoupled.

The thermoelectric effects are characterized by the coupling between heat flux and electrical flux represented by the off-diagonal coefficients in 2.23. Therefore, in a thermoelectric device in open circuit configuration, the voltage is high, but the heat flux is too small, so that no power can be generated. If instead the current (electrical flux) is increased, the voltage drops as the temperature gradient is diminished due to improved thermal conduction. Therefore, neither of two configurations provides a satisfactory solution and hence, subject to certain provisions, there is a specific condition such that the highest thermoelectric efficiency is found. The thermal conductivity is expected to be maximal in the short-circuit configuration (E=0) and minimal in the open-circuit one (i=0). Hence kE/kJ must be maximized to get the highest possible power output. Using 2.18 we get the expression:

12

(2.24)

The first term of the right hand side of the equation is so-called Ioffe

formula that introduces the central concept in the theory of thermoelectrics: dimensionless thermoelectric figure of merit zT. This expression defines the thermoelectric efficiency of a given thermoelectric material. Even though temperature is not an intrinsic property of the material, it represents certain working conditions that are of a great importance in practice and, besides, the three thermoelectric parameters are temperature dependent. The figure of merit results from the band structure, lattice dynamics, and scattering mechanisms of charge carriers of a particular material, thus zT optimization is beyond the scope of thermodynamics and shall be considered in the frame of solid-state physics.

2.2.2 Solid-state physics of thermoelectrics In the following section electrical and heat transport in thermoelectric

materials will be briefly discussed with the aid of classical Boltzmann transport equation, which is commonly used in transport properties analysis in the systems with non-uniform particle density and temperature27. A non-equilibrium distribution function reflects the probability of a particle to be found at some specific location within the system associated with some local thermal velocity. Globally, the system is described by the non-equilibrium distribution, which is given by a sum of local equilibrium distributions. Equivalently, equilibrium systems properties provide a basis for a study of an overall non-equilibrium system. Since the external forces are steady, the average rate of change of the distribution function must be zero. In other words the “in-and-out” effects of scattering, forces and drift have to be balanced out as shown in Figure 5.

Under the steady state conditions and the relaxation time approximation, the linear Boltzmann transport equation28 can be written as:

(2.25)

kE / kJ =! 2"kJ

T +1

v !"r f +F!"k f = #

f # f0!

13

Figure 4. Boltzmann’s Equation Determines the Non-Equilibrium

Occupation of an Energy State by Balancing the Effects of Scattering (both phonons and charge carriers can be involved), Forces (any external forces that act on a system causing a change in the momentum of a wave) and Drift (waves "drifting" due to differences in the distribution function) on a Small Group of Energy States, in a Small Region of the Material

Here f (r,k) represents the non-equilibrium distribution function where r

and k are coordinates and wavevector in phase space. The right hand side is given by f0 - the local equilibrium distribution defined by Fermi-Dirac function for electrons and by Bose-Einstein function for phonons and τ is the relaxation time or an amount of time it takes the non-equilibrium distribution to restore its equilibrium upon removal of all driving forces. Note, that in Boltzmann’s equation each particle has its momentum and position that are precisely described and the uncertainty principle does not apply here.

k k

f(k) f0(k)

E(k) E(k)

14

Figure 5. Illustration of equilibrium and non-equilibrium distribution function for electrons at the conduction band

In a conductor subject to a small electric field, temperature gradient, and concentration gradient the local equilibrium distribution function for electrons in given by the Fermi-Dirac distribution f0=1/exp((E- ! )/kBT)+1. Here ! is the Fermi level, kB is the Boltzmann constant. Provided that the external forces are small, no big deviation from equilibrium is expected and the solution of the non-equilibrium distribution function is:

f = f0 +!v !E !EF

T"rT + q(" +

1q"rEF )

#

$%

&

'( !

)f0)E

#

$%

&

'( (2.26)

In order to derive the expressions for thermoelectric parameters, we shall

first recall that current density and heat flux carried by electrons can be written as:

(2.27) Substituting the expression 2.26 in the above equations gives:

J(r) = q2L0 (F!"!1q"!"

rEF )+qTL1(!"!"

rT )

JQ (r) = qL1 !1q"µe

#

$%

&

'(+1TL2 (!"T )

(2.28),

where Φ is the electrochemical potential (-! µe/q=! + ! EF/q) and

transport coefficients Ln are defined by the integral:

(2.29)

J(r) = 14! 3 qv(k) f (r,k)d3k!!!

JQ (r) =14! 3 (E(k)"EF (r))v(k) f (r,k)d

3k!!!

Ln =14! 3 " (k)v(k)v(k)!!! (E(k)"EF )

n "#f0#E

$

%&

'

()d3k

15

From the expressions 2.28 thermoelectric parameters such as the electrical conductivity, electronic thermal conductivity and the Seebeck coefficient can be calculated. Assuming that the current flow and the temperature gradient are both in the x-direction we get:

! = Jx / (!"µe / q ) "xT=0= q2L0

" = (!"µe / q) /"x T Jx=0=1qT

L0!1L1

kE = JQx/"x T Jx=0

=!1TL1L0

!1L1 +1TL2

(2.30)

Or in the form of integrals:

! ! ! (E) "#f0#E

$

%&

'

()* dE

" !kBq

! (E) (E "EF )kBT

"#f0#E

$

%&

'

()* dE

! (E) "#f0#E

$

%&

'

()* dE

+ E "#

(2.31)

In 2.31 σ(E) is the differential conductivity that evaluates the

contribution of electrons with energy E to the total conductivity. This parameter can be expressed by the product of the density of states D(E) (see Figure 6):

! (E) = q2" (E) vx

2 (E,ky,kz )!! dkydkz " q2" (E)vx

2 (E)D(E) (2.32) As it follows from the above equation, the electrical conductivity is

limited by a derivative of the Fermi-Dirac distribution !ƒ0/!E known as the Fermi window factor, is a bell-shape function centered at E= EF with a width of kBT. This function is nonzero only in the region of several kBT near the Fermi level (see Figure 6). This condition arises from the fact that at finite temperature among all available charge carriers in the system, only those close to the Fermi surface contribute to the conduction process. Therefore, electrical conductivity requires large area under differential conductivity while the magnitude and sign

16

of the Seebeck coefficient are related to an asymmetry of the electron distribution around the Fermi level. The Seebeck coefficient is the average energy transported by the charge carriers corresponding to a diffusion thermopower. The factor (E-EF) appears in the thermopower equation because

Figure 6. Schematic illustration of the Fermi-Dirac probability

distribution and its derivative (light green curves), the density of states D(E) (purple curve), and their product (shaded blue area) for a semiconductor thermoelectric material

heat is nothing but the difference between the total energy and the internal energy. Therefore, increasing the electron density of states D(E) and electron energy E near the Fermi level can result in a high zT.

The electron thermal conductivity is given by the third expression in 2.30 and in the simplest can be calculated using Wiedemann-Franz law that relates the electrical conductivity and electronic part of thermal conductivity (kE/σ=LT). The Lorenz factor (L=π2kB

2/3e2) is constant for metals and can vary in semiconductors depending on the charge carrier concentration. In order to determine the second contribution to the thermal conductivity the knowledge about the phonon distribution is required. Here again we turn to the Boltzmann transport equation. Phonon transport properties cannot be affected by external forces such as electric or magnetic field, hence 2.25 reduces to29:

v !"r f = #f # f0! (2.33),

where f and f0 once again represent the non-equilibrium and equilibrium

distributions except that phonons are described by the Bose-Einstein statistics30

17

31, which provides the average number of phonons in a system at equilibrium. The distribution is given by:

f0 =1

exp (E !EF ) / kBT[ ]!1 (2.34)

The solution of the non-equilibrium function has the form: f = f0 !!v "#r f0 (2.35) And the phonon heat flux is given by:

Jp = !"T !vv #f0#T$

%&

'

()*** d3k = !kp"T (2.36)

Finally for the thermal conductivity contribution due to lattice vibration

in homogeneous materials we get:

kJ =13

C(!)! v2p(!)" (!)d! (2.37),

where C is the specific heat of phonons at frequency ω, vp the phonon group velocity, and τ the phonon relaxation time.

2.3 Thermoelectric materials According to the definition of zT, a good thermoelectric material should

have a large Seebeck coefficient, high electrical conductivity and low thermal conductivity32 33. These parameters are uniquely defined for each specific material.

18

Figure 7. Seebeck coefficient, electrical and thermal conductivity as a function of free carrier concentration for different classes of materials, after 34

The choice of materials with proper thermoelectric performance is based upon the precise identification of the specific electronic makeup that can only be provided by the theory of solids that connects the structure of solids to their thermoelectric properties. Several criteria that the best thermoelectric materials must have include35 36:

• Many valleys electronic bands near the Fermi level for large Seebeck coefficient and low average electronegativity between the elements for high mobility

• Large unit cell size ternary, quaternary compounds made of elements with large atomic number and large spin orbit coupling for low thermal conductivity

• Energy gaps equal to 10kBT for higher Seebeck coefficient.

Figure 7 provides an overview of existing material types and their respective appropriateness for thermoelectric energy conversion. Among them, metals have best electrical conductivities but their Seebeck coefficients are way too low for their being even remotely considered as decent thermoelectrics. In general, the carriers with energy above the Fermi level tend to increase the Seebeck coefficient and those with lower energy contribute to making it smaller. Consequently, in metals that typically have half-filled bands the thermopower is relatively small.

19

The resulting thermoelectric efficiency is further decreased by high thermal conductivities. Therefore the average zT does not exceed 10-3 (see Table 1). Thus, metals are only used in thermocouples for temperature sensing but certainly not for power generation or cooling.

Likewise, insulators with their gigantic thermopowers (up to several mV/K) and lowest among all known materials thermal conductivities (less than 1W/mK) are equally unfit for thermoelectric applications due to poor electronic conduction. In principle, there is only one class of materials that can truly meet the requirements for high zT - highly doped semiconductors. If their doping level is properly adjusted high electrical conductivity may result, however it should always go hand in hand with Seebeck coefficients as large as several hundreds of µV/K and acceptably low thermal conductivity. In semiconductors the latter parameter can be extraordinarily large exceeding typical metallic values as in case of diamond37 or, to the contrary, be as small as that of glasses, one example is clathrates38. The best material for room temperature thermo-generation and cooling is Bi2Te3

39 40. Its thermoelectric efficiency of zT=1 for

bulk and 1.2 for nanostructured Bi2Te341has been a longstanding benchmark

which, despite numerous studies that have been performed on various mixed-crystal semiconductors, remains unbeaten.

Metals Semiconductors Insulators S≈5µV/K S≈200µV/K S≈1mV/K

σ=neµ≈108S/m σ ≈105S/m σ ≈10-10S/m k≈10-1000Wm-1K-1 k≈1-100Wm-1K-1 k≈0.1-1Wm-1K-1

zT≈10-3 zT≈0.1-1.0 zT≈10-14 Table 1 - Comparison at room temperature of the thermoelectric

properties of metals, semiconductors and insulators, after 42 Because of renewed interest in thermoelectrics in recent years, the

search for efficient thermoelectric materials is on the fast track again. The list of candidates is long and plentiful, among them the most studied thermoelectrics are: Bismuth chalcogenides43, inorganic clathrates44, Half Heusler alloys45, skutterudite materials46, silicides47, oxides48. Organic conducting polymers represent a rather overlooked class of thermoelectrics that is nonetheless suitable for room T energy conversion49 50. It is not only on account of their generally low thermal conductivities and satisfactory thermopower and electrical conductivity can they be considered for low T thermoelectric generation, but also due to their technological compliance, reduced production cost, nontoxicity and abundance. The detailed overview of their thermoelectric

20

performance as well as some examples of organic thermo generators are given in Paper 1 of the present dissertation.

2.4 Thermoelectric applications It was mentioned in the preceding sections that there exist three

thermoelectric effects: the Seebeck, Peltier and Thomson effects. However, only the first two effects are of practical interest. The Seebeck effect is at the basis of thermoelectric generation51 52, whilst the Peltier effect is used in refrigeration53 and can be equally adopted for another convenient albeit less common application i.e. thermoelectric heating. Regardless of the application, the appropriate device architecture is that represented by a typical thermoelectric module (Fig 8).

The thermoelectric module consisting of many thermocouples connected electrically in series and thermally in parallel does not have any moving parts making it durable, silent and can be produced in a variety of sizes and shapes depending on the specific application. Among the elements designated in Fig 8, the thermoelectric legs of two types (n-type for electron conduction and p-type for hole conduction) largely determine the efficiency of the device. Ideally, their thermoelectric performance should be balanced such that the all three parameters constituting zT are matched. This arises because the thermoelectric legs should be ideally of the same size to facilitate the fabrication process and enable a better performance. The thermoelectric figure-of-merit for a single thermocouple consisting of two dissimilar materials is given by54:

21

Figure 8. Thermoelectric module can be used in two different modes: 1) for thermoelectric generation if an external load is connected creating a close circuit while a temperature gradient is established across the device; 2) for cooling if operated by a DC power source turning the surface where heat is absorbed cold and heating the opposite side where heat is rejected.

Z =(! p !!n )

2

(kp"p )1/2 + (kn"n )

1/2"# $%2 =

(! p !!n )2

KR(2.38)

The denominator KR can be equally expressed using the geometrical

parameters of the TEG:

KR =kpAp

Lp

+knAnLn

!

"##

$

%&&!pLp

Ap

+!nLnAn

!

"##

$

%&& (2.39)

Here L is the length, A – cross-sectional area and ρ is the resistivity, the

subscripts n and p have their usual meaning. Since the Seebeck coefficient is not a geometry dependent parameter, in

order to maximize Z the denominator of 2.38 shall be minimized. This condition is satisfied when:

Ln / AnLp / Ap

=!pkn!nkp

!

"##

$

%&&

1/2

(2.40)

In the thermoelectric generation (TEG) mode, the heat flow can be

directly converted into electrical power. The temperature difference across thermoelectric elements controls the output voltage of the device (Fig 9).

As discussed previously the conversion of heat into electricity is associated with some irreversible processes that cause heat losses and lower TEG’s efficiency. If for simplicity we consider a TEG consisting of only one

22

Figure 9. Two operational modes of a thermoelectric module: a)

thermoelectric generation mode b) thermoelectric cooling/heating mode.

thermocouple, the incoming thermal power at the hot side of the TEG is expressed as following55:

(2.41)

where ! is the total Seebeck coefficient of a thermocouple (!n,p), K is

the thermal conductance similarly given by both materials, TH is the hot side temperature and !T is the temperature gradient, R is the internal resistance of the TEG. Hence there are three terms associated with the heat flow at the hot side i.e. heat due to the Seebeck effect, the half of Joule heating and the thermal conduction. The last two inputs are hugely detrimental for the TEG’s performance and have to be minimized.

When the load with a resistance Rload is connected to the TEG the current flowing in the closed circuit will be equal to:

(2.42) and the useful power produced by the TEG is simply Pload=I2Rload or

equivalently: (2.43) It follows that the maximum power output is reached when the load

resistance and the device resistance are matched, which corresponds to the condition !P/! (Rload/R)=0. Therefore the Pmax is V2

open/4R56

QH =!TH I +K!T "1/ 2RI2

I =Vopen / (R+ Rload )

Pload = I2Rload =!

2!T 2Rload / (R+ Rload )2

23

The TEG’s efficiency is given by the ratio between the generated power and the supplied power (incoming heat) φ=Pload/QH or equivalently:

(2.44), The efficiency of the maximum power output mode can be found setting

R=Rload and bearing in mind that the thermocouple figure-of-merit Z=α2/RK: (2.45), On the other hand R=Rload condition does not lead to the maximum

efficiency of the TEG. Instead, its maximum performance is obtained setting to zero the derivative of the efficiency with respect to the internal-to-load resistance ratio, the result for the maximum conversion efficiency is:

(2.46) T is the average temperature of the TEG and !T /TH is the reversible

Carnot efficiency (ϕC) and the rest of the expression is the material-related contribution since it depends on Z. Hence ϕmax ! ϕC when Z!∞.

Consequently, there are two possible modes of operation for a TEG, one that provides the maximum efficiency and the other that enables its maximum power output. If the ratio R/Rload= 1+ ZT , the TEG is in the maximum efficiency mode, while R/Rload=1 (matched load condition) corresponds to the maximum power generation. The discrepancy between the maximum power efficiency and maximum conversion efficiency at ZT≈1 is within 3%. Therefore the maximum power output mode shall be used as an optimum TEG configuration. However, as soon as ZT ≥ 3, the deviation becomes larger than 10% and the higher heat transfer rate needs to be maintained at the hot junction as the efficiency degrades.

! =I 2Rload

"ITH +K!T "12I 2R

!P =!T

3 / 2TH +1/ 2TC +1/ 4Z"1

!max =!TTH

1+ ZT "1

1+ ZT + TCTH

24

25

Chapter 3 – Conducting Polymers

3.1 Electronic and chemical structure of conjugated polymers

Polymers are carbon-based compounds consisting of numerous repeating

units (monomers). Conjugated conducting polymers constitute a special class of organic materials whose ability to conduct electrical charge originates uniquely from π –conjugation.

In regular saturated polymers, that are practically insulators, all the available electrons are bound in strong covalent bonds and are characterized by sp3 hybridization. In contrast to this situation, in conjugated polymers planar σ-bond arises from hybridization of one s and two p orbitals (Fig10a). That is to say, there is a region where p-orbitals overlap, bridging the adjacent single bonds and creating a π collective orbital in which p-electrons do not belong to one single bond, but rather delocalized over a group of atoms.

Figure 10. sp2 hybridization in conjugated π-system where electron

density can be delocalized over many carbon atoms. Since the interaction between the parallel 2pπ-atomic orbitals is

weaker than between the hybrid 2pσ-atomic orbitals, the resulting occupied π-

26

molecular orbitals are less stable than the occupied σ-molecular orbitals. Since σ-orbital is at lower energy, the highest occupied molecular orbital (HOMO) is a π-orbital. As a result of bonding (in-phase interaction) and antibonding (out-of-phase interaction) atomic orbital overlap. A system of N 2pπ-atomic orbitals, leads to N/2 π-molecular orbitals with a predominant bonding character that are stable and occupied with N π-electrons. There are N/2 unoccupied π*-molecular orbitals with a predominant antibonding character. The most stable of those orbitals is called the lowest unoccupied molecular orbital (LUMO). In the solid phase HOMO and LUMO rearrange to form bands somewhat analogous to the valence band and the conduction band in inorganic semiconductors. The existence of bond alternation in conjugated polymers is in fact responsible for the band gap formation and semiconducting properties in these materials. According to the Peierls theorem, the metallic state with one electron per ion is unstable in (quasi) one-dimensional material with the tendency to distort the geometrical structure and lower the symmetry.

A typical conjugated polymer consists of a large number of repeating units. In a system containing many polymer chains that tend to intertwine creating spaghetti-like structures it is virtually impossible to maintain a long-range order. Therefore, conjugated polymers are rather amorphous in nature with the conjugation length strongly dependent on intra- and intermolecular structure. Their physical properties are very sensitive to any modifications in chemical architecture and can vary significantly subject to the morphological complexity of these materials. Generally, conjugated polymers are shorter in length than saturated polymers and characterized by stiffness with respect to chain twisting and bending. Besides, their solubility is very poor unless specific side-chain groups are introduced. Some examples of conjugated polymers are shown in Figure 11.

27

Figure 11. Chemical structure of some most commonly used conjugated polymers

3.2 Charge carriers in conjugated polymers Depending on the ground state of a conjugated polymer, different exited

states can occur. A polymer whose ground state energy remains unaffected when interchanging single and double bonds is referred to as a degenerate ground system. In trans-polyacetylene, for example, it is of no importance for its energetics on what side the double bond is (see Fig. 12)57. Conversely, in polyphenylene the change of the double bond position transforms the aromatic lower energy state (three double bonds in the ring) to the quinoid state with much higher energy (two double bonds in the ring)58.

Figure 12. a) Total energy curve for an infinite trans-polyacetylene chain as a function of the degree of bond length alternation b) The energy levels of two poly(para-phenylene) states with stable aromatic configuration that can be excited to quinoid state.

A phase kink in a single-double bond sequence in trans-polyacetylene

made of odd number of carbon atoms results in a formation of an in-band defect in a form of an unpaired electron called a neutral soliton59 (see Fig 13a). In doped trans-polyacetylene positive or negative spinless solitons can be created60. However, most of the conjugated polymers have a non-degenerate

28

ground state, which impedes the soliton formation. It is suggested that in these polymers polarons and bipolarons are created upon doping61 (Fig.13). The oxidation (an electron removal) of the polymer leads to a formation of a radical cation. Its presence causes an attenuation of neighbouring bond alteration amplitudes and a half-filled electronic level above the valence band along with a new anti-bonding level below the bottom of the conduction band results (Fig. 13b). Polaron is a single charge excitation that can be either positive as discussed above or negative in case if an electron is added to a polymer chain. A polaronic distortion can spread over three to four monomers depending on polymer chemical structure. A removal/addition of the electron from a polaron produces a bipolaron i.e. a radical ion pair associated with a strong local lattice distortion.

29

Figure 13. a) Chemical structure of trans-polyacetylene with tree types of solitons and the corresponding electronic structures. b) A schematic illustration of the different excited states in polythiophene and the polymer’s electronic structure corresponding to (from left to right) a neutral chain, a chain carrying a polaron, a chain carrying a bipolaron, a chain or assembly of chains with a high concentration of polarons (intra chain or interchain polaron network), a chain or assembly of chains with a high concentration of bipolarons (intrachain or interchain bipolaron bands). The relaxation of the lattice around a doubly charge bipolaron is larger than in case of a polaron with only one charge, hence the bipolaron in-gap states are moved even further away from the band edges. It is suggested that a bipolaron formation is more thermodynamically favorable than a formation of a polaron pair since the energy required for their creation is nearly identical while in case of bipolaron the decrease in ionization energy is greater.

3.3 Doping of conducting polymers

The presence of π–conjugated chain is the first fundamental requirement for a polymer to become conducting. However, the band gaps of conjugated polymers are ranging from 1 to 3eV62, which is consistent with semiconducting or even insulating properties. In order to acquire metallic electronic characteristics, a doping process is used. In contrast to inorganic doping involving atoms replacement, doping in conducting polymers is due to an oxidation (p-type doping) or reduction (n-type doping) process. The oxidation level in conducting polymers can be extremely high, sometimes up to 50%63 exceeding that of inorganics by several orders of magnitude. Oxidation/reduction can be achieved by removing/adding an electron from/to a polymer via an electron transfer reaction with an chemical species (redox reaction) or with an electrode (electrochemical reaction). The doping charge on the polymer is stabilized by a counter ion (cation/anion) to ensure electroneutrality of the materials. Polymer can either be synthesised either in their doped state or their neutral state. The oxidation level can be controlled in a number of ways.

Chemical doping is a method that involves an exposure of a polymer to a solution or vapor of the dopant64. A suitable dopant (donor or acceptor) used in this process should have a proper redox potential and the polymer’s ionization potential (electron affinity) shall be such as to facilitate the doping process. Throughout the exposure chemical dopants can physically penetrate into or move out of the polymer putting a strain on the polymer film, which can cause its swelling, shrinking or other morphology change. Increase in a charge carrier

30

concentration manifests in a tremendous change in electrical conductivity that can be nearly metallic in some highly ordered polymers65.

Indeed, a choice of chemical dopant is rather limited since in each case the requirement for a dopant would be dictated by a unique polymer electronic structure. Typical dopants used in chemical doping of conjugated polymers and the achieved oxidation levels are listed in the Table 2. Dopant ion Formula Chloride Perchlorate Tetrafluoroborate p-toluene sulfonate Trifluoromethane sulfonate Hexafluorophosphate Polystyrene sulfonate Bromide Sulfate Iodide

Cl− ClO4

−

BF4−

CH3-C6H5-SO3−

CF3SO3−

PF6−

(-CH2CH(C6H4SO3)-)n Br- SO4

2- I-

Polymer Poly(pyrrole) Poly(thiophene) Poly(aniline) Poly(p-phenylene)

Maximum doping level 33% (ClO4

−) 30% (ClO4

− ) 6% (PF6

−)

42% (Cl− )

44% (Li+) Table 2 - Anionic and cationic dopants commonly used for chemical doping in conducting polymers. Some examples of maximum doping level that can be achieved in certain conducting polymers with the aid of chemical doping (after66).

Chemical doping takes place via redox reaction (electron transfer with a dopant species), this is a straightforward and technologically simple charge concentration modification method. However, there are some known downsides to this approach such as a limited choice of dopants and chemical stability issues of the dopant due to their high reactivity. Another problem arises from the homogeneity of the doped samples, which can be high in heavily doped polymers but fails to be so at intermediate doping levels67. In some applications where semiconducting properties of conjugated polymers are requested it becomes crucial to ensure homogeneity of moderately doped samples. This problem can be successfully tackled by electrochemical doping.

In electrochemical doping an electrolyte is used between a conducting polymer and the electrode that supplies the charge to the polymer while ions

31

from the electrolyte diffuse in and out to maintain polymer’s charge neutrality. In case of n-doping the polymer is reduced and the electrolyte cations are introduced into the polymer as counterions. Upon p-doping, the polymer is oxidised and the anions penetrate into the polymer film as charge balancing ions. This method provides precise control of the doping level, which is set by the applied potential between the electrode and the polymer film. Almost any conjugated polymer can be electrochemically doped but the stability of the obtained samples strongly depends on the applied voltage. Compared to chemical doping the electrochemical doping procedure is technologically more complicated to perform as it necessitates an electrolyte and a counter electrode. Consequently, it is not ubiquitously used. This doping method is employed in electrochemical batteries, light-emitting electrochemical cells68 and electrochemical transistors69.

Other doping methods include photodoping70 based on photo-absorption and charge separation (photovoltaic devices), doping by charge injection without counterions involved (OFET)71, doping by acid-base treatment72.

3.4 Charge transport in conjugated polymers

3.4.1 Role of morphology in charge transport

A conducting polymer film can be described as large macroscopic assemblies of charged polymer chains and ions. The cohesion in the solid is maintained by electrostatic (dominant in doped polymers) and van der Waals forces. Clearly, the structural order can’t be effectively maintained over large distances provided a significant length of polymer chains. Most of the conducting polymers appear to be amorphous, sometimes with some degree of crystallinity73, as in the case of highly conducting samples. The latter example pertains to the situation where the three-dimensional highly conducting inclusions and disordered quasi one-dimensional regions with strong localization effects coexist74 (Fig 14). The polymer chain packing is the only difference between amorphous and crystalline regions as both of them are obviously made of the same material.

High degree of disorder in amorphous materials translates into strong charge localization. These systems are featured with low charge carrier mobility and electrical conductivities even at high charge carrier concentrations. These so-called “dirty” conductors are extremely disordered and can be attributed to the Fermi glass concept75.

32

In “metallic” conducting polymers the interchain order and crystalline coherence length are expected to be large while the defect density has to be low to enable delocalization of charge carriers with mean free paths significantly exceeding one repeat unit76.

The charge transport in conducting polymers can proceed in three possible ways: 1) propagation of a charge carrier along a polymer backbone, 2) transfer across the polymer chains by hopping, 3) tunnelling between conducting segments separated by amorphous regions. The delocalization of π-

Figure 14. Schematic representation of conducting polymer

morphology represented by metallic highly ordered regions randomly distributed in an amorphous host made from chains of the same polymer.

electrons along the polymer backbone alone is not a sufficient condition for conduction due to intrinsic localization in a one-dimensional system. For this reason the interchain coupling is essential77. Usually the transfer integral for interchain coupling represents a very small fraction of the one-chain transfer integral. Nonetheless, even being weak it can still encourage a charge carrier to hop from site to site diffusing across the polymer chains instead of being trapped by a defect or having to move in the opposite direction. The interchain transfer is further disrupted when side groups are introduced into initial conjugated polymers. This modification of the chemical structure improves the solubility of the polymer but may corrupts the backbone rigidity resulting in the reduced intermolecular overlap78.

Another important structural aspect affecting the charge transport in the conducting polymers is the molecular weight of polymers. Polymers with low molecular weight have shorter chain lengths and are more crystalline in nature

33

than high molecular weight polymers. Yet, the mobility in these polymers is lower possibly because the connections between the crystals are obstructed by insulating boundaries. In high molecular weight polymers bends and twists interrupt the conjugation, but at the same time longer polymer chains that may spread trough several material segments connect more effectively ordered regions between one another74.

3.4.2 Density of states and sources of localization in disordered materials As previously discussed, the conduction process in disordered systems is

generally limited by the localization of the electronic states. Conducting polymers are commonly described by the density of states (DOS) where delocalized-states band and smooth band-tails with localized states at the band edges are separated by a "mobility edge"79 (see Fig 15a). When the Fermi level is found below the mobility edge all the charge carriers are localized at zero temperature and the conduction is ceased. An abrupt transition from insulating to metallic state is predicted once the Fermi level overpasses the mobility edge. This transition is known as Mott Transition80. Another type of localization observed in organic materials is Anderson transition81, which may take place if the mobility edge shifts deeper into the band. Once the random disorder potential exceeds the bandwidth all states become localized and the system turns into an insulator with continuous DOS even in the absence of the band gap (Fig 15b). The material is then called a Fermi glass as mentioned previously.

Figure 15: Density of states types in conjugated polymers including: a) band of fully localized states, b) band with localized states in a tail

and free states above the mobility edge, c) in-gap band of localized states that can be represented by polaron or bipolaron bands as shown in Figure 13. The Fermi level position can vary, if, for example, the doping level is changed.

34

In conducting polymers the in-gap localized states are induced by doping in the form of (bi)polaron states/bands. In the lack of better knowledge about the DOS shape, the distribution of these states is commonly assumed by Gaussian DOS82 as shown in Figure 15c. If the distortion of lattice due to polarization is strong (bi)polarons become self-trapped. A deep potential well is created around a localized carrier when the lattice acquires a more stable equilibrium state. Hence, the (bi)polaron transport can be carried out by means of phonon assisted hopping between available states (Fig 16). Unlike the band regime with strong electron coupling, a coherent transport is not enabled in hopping transport since the charge carriers do not have a “memory” of where they were previously and the process is virtually random83.

Figure 16. Hopping transitions from occupied (full circles) states to

available sates above the Fermi level.

3.4.3 Role of insulator-to-metal transition in charge transport The mechanism of the charge transport in conducting polymers largely

depends on to what side of the metal-to-insulator transition the polymer belongs84. Upon doping the (bi)polaron lattice can be formed progressively filling the band gap. Eventually a distinct band structure can emerge. In reality even the best conducting polymers are only partially crystalline suggestive to inhomogeneous doping with highly conducting regions separated by insulating boundaries85. The conductivity in these materials grows rapidly once the percolation threshold is surmounted.

To give a rough estimation to whether a polymer is in its insulating (semiconducting) or metallic state, the reduced activation energy is calculated:

W =d(ln! )d(lnT )

(3.1)

35

To a first approximation negative slope of W versus T is suggestive of

the insulating regime, positive coefficients of W imply a metallic nature of a sample, and finally, a weak temperature dependence of W is an indication of the critical regime86.

3.4.4 Nearest-neighbour hopping and variable range hopping With the exception of metallic polymers with the resonant quantum

tunneling transport among the chain-linked network of nanoscale grains, hopping is considered as a basic transport mechanism for polymers87. For the conjugated polymers on the insulating side of IMT the conduction of charges is governed by thermally activated hopping. Subject to the localization degree two types of hopping process can be distinguished i.e. 1) nearest-neighbor hopping88 and 2) variable-range hopping89 90. The probability of hopping is generally given by:

Phop =! phonon exp(!2"R!WkT) (3.1)

where, vphonon is the phonon frequency; ! is the decay rate of the

wavefunction or the inverse of the localization length; R and W are the distance and the energy difference between two sites. If the condition ! R0>>1 (R0 is the nearest neighbor distance) is satisfied it implies a strong localization with the mobility edge shifted in the higher band where only nearest-neighbor hopping within kT is allowed. This is to say, the charge transfer between localized states is solely defined by spatial factors, which is valid at high temperature limit. Once the energy distribution of the localized states becomes comparable or larger than kT, the energy dependent term comes into play. At some point, the energy difference between two closest states can happen to be larger than that between more remote states lowering the nearest hop probability and increasing the chance of a longer hop event. Therefore, at lower temperatures the variable-range hopping (VRH) first suggested by Mott shall be assumed. The most effective transport of charge carriers shall be expected in the vicinity of the Fermi energy where many empty states with matching energy can be available for occupation.

For the hopping conduction, the electrical conductivity is zero at T=0K since the charge carriers are localized. At finite temperatures, charge carriers hop by absorbing or emitting phonons. The typical temperature dependence of conductivity is expressed by91:

36

! =! 0 exp !T0T

"

#$

%

&'1/d(

)**

+

,--

(3.2),

where d=n+1, where n is the system’s dimensionality, T0 is the

characteristic temperature given by:

T0 =!" 3

kg(EF )(3.3)

In this expression β is the numerical constant and g(EF) is the DOS in the

vicinity of the Fermi level. This model, however, does not take into account the long-range electron-electron interactions assuming constant DOS.

VRH is consistent with d=4, while d=2 is generally explained in the framework of the charging-energy limited tunneling model. It is been suggested that conduction proceeds through tunneling between small conducting grains separated by insulating barriers (Sheng’s model of granular metals)92. According to this model, highly doped metallic-like islands are generated by heterogeneities in the doping distribution, bridging neighboring chains and improving the charge transport.

The best fit to the experimentally observed temperature dependent conductivity can be attained using the heterogeneous model proposed by Kaiser73, where based on complex polymer’s morphology the electrical conductivity can be represented in terms of the fractions of ordered metallic regions, insulating barriers and disordered metallic barrier regions. A more detailed discussion of the temperature dependent conductivity and thermopower behavior is provided in the Paper 1.

3.5 Poly(3,4-ethylenedioxythiophene) Poly(3,4-ethylenedioxythiophene) better known as (PEDOT) does not need a long introduction as over the course of many years this truly remarkable conducting polymer has been used far and wide. A long list of PEDOT applications includes photovoltaics93, light-emitting diodes94, sensors95, actuators96, thin-film transistors97, bioactive coatings98 etc. In addition to a very high conductivity (ca. 200 S/cm)99, PEDOT forms transparent and stable films in its oxidized state, has moderate band gap ofapproximately 1.6 eV100 and low redox potential101. Yet, it is virtually insoluble and unstable in its neutral state. To circumvent the solubility problem, polystyrene sulfonate (PSS) is commonly used as a supplement yielding an aqueous dispersion of PEDOT:PSS (see

37

figure17). The introduction of a bulky insulating counterion cannot but affect the electrical conductivity of the polymer, which drops tenfold.

Subject to counterion choice, PEDOT can exhibit conductivities ranging from 10-2 to 3500 S/cm. PEDOT-PSS typically has low conductivity (<10S/cm)102, but it can be readily improved using various organic and inorganic solvents treatment. Secondary doping with polar solvents such as dimethyl sulfoxide (DMSO)103, sorbitol, dimethyl sulfate104 and ethylene glycol (EG)102 or acidic treatment105 with HCl or H2SO4 have proven to be very effective in electrical conductivity enhancement. Besides, there are some commercially available aqueous dispersions of PEDOT-PSS under the trademark Clevios that are highly conductive (up to 800S/cm). Insoluble films of PEDOT doped with tosylate106 (Fig.17b) prepared by chemical polymerization yield much higher conductivities (up to 1000S/cm) and unlike amorphous PEDOT-PSS are partially crystalline.

Figure 17. Chemical structure of PEDOT doped with a) polystyrene sulfonate and b)

tosylate

Environmental stability and high conductivity of these polymers is a good starting point for investigating their thermoelectric performance. Similar to other conjugated polymers the oxidation level of PEDOT can be modified to a

38

large extent, providing an additional flexibility to its electronic properties. The thermal conductivity in PEDOT derivatives is low as expected from typical polymers (ca. 0.2W/mK), slightly higher for PEDOT-TOS. The Seebeck coefficient is positive and small (almost metallic) in highly doped films. Moderate thermopower is the major concern and the blocking point for high thermoelectric efficiency. Once again, doping can be a relevant approach to tackle this issue. As previously stated, various doping procedures are appropriate for conducting polymers and can be adopted for optimization of thermoelectric properties, which is the ultimate goal of this thesis.

39

Chapter 4 - Experimental techniques

4.1 Electrical conductivity measurements on thin film Among all parameters required for thermoelectric characterization of

materials, the electrical conductivity is perhaps the easiest to measure. A four-point probe method used for this purpose is known to be straightforward and simple. In this measurement four equally spaced electrodes are coated on a substrate whether it’s glass, silicon or another appropriate material. In order to exclude possible errors due to the parasitic contact resistance between the thin film surface and the probe one set of leads is used to source an electrical current, while the other one is intended for the measurements of a resulting voltage. The electrical conductivity of a thin film with the cross sectional area A and the length l determined by the distance between two inner electrodes is given by:

! =IVlA=IV

lf !w

(4.1)

Here I is the electrical current injected through two outer leads and V is

the voltage measured between two inner leads. The cross sectional area is found as a product of the film thickness f and its width w.

Presence of electrical current in a thermoelectric material can cause the emergence of the Seebeck voltage due to the Peltier effect. The better the material’s thermoelectric performance the larger this contribution may become. Therefore, it is important to minimize this effect so that only the resistive component of the measured voltage is used in the electrical conductivity estimation. In order to eliminate this source of errors a fast DC or AC measurements are used. The DC current should not be applied for more that 2-3 seconds and its direction can be reversed to obtain V by averaging the results of measurements in both directions. Besides, all four ohmic contacts should consist of the same material.

In the samples with arbitrary shapes the electrical conductivity can be estimated by the van der Pauw technique107 that was designed for two-dimensional system where the sample thickness is much smaller than its length and width. The four contacts are chosen such that they are located at the extremities of the sample, which by itself should preferably be as symmetrical

40

as possible. The resistances between the points (one along a horizontal edge and the other along the vertical edge) are measured similar to the four-point probe method and can be corrected by comparing them with their reciprocal values. The resistance of the sample is found using the van der Pauw formula:

e!!RA /RSample + e!!RB /RSample =1 (4.2) The schematic illustration of both methods is depicted in figure 17.

Figure 17. Schematic display of a) van der Pauw method and b) four-point probe measurement technique used for thin film electrical conductivity estimation

4.2 Seebeck coefficient measurements

It follows from 2.2 that the Seebeck coefficient can be readily calculated from the voltage and the temperature difference measured between the junctions made of two dissimilar conductors. If the Seebeck coefficient of one of the materials is precisely known as a function of temperature or negligibly small compared to the total measured Seebeck coefficient, the absolute thermopower of the second conductor can be estimated108. Despite the apparent simplicity of the measurement, in practice it is associated with many pitfalls. Since the Seebeck coefficient is not geometry dependent, the error coming from the wrong estimation of the sample dimensions is not a concern. Caution should be exercised when calculating the absolute Seebeck value of the unknown material as it usually has an opposite sign as compared to the total Seebeck coefficient sign109. The accuracy of the measurements can be impaired due to the contact potential, sometimes not only the value but also the sign of the thermopower can

41

be misinterpreted. Thus, a good ohmic contact is required between the sample and the probes; the contact resistance can be measured separately taking into account different temperatures at which probes are kept.

The Seebeck coefficient is a temperature dependent parameter that rarely has a linear dependence on T. It is important to know its variation in a working T range as it can have a big effect on the thermoelectric efficiency. To estimate the thermopower at a given T, the measurement has to be done at different ∆T to make sure that ∆V plotted vs. ∆T is linear.

Providing a good thermal contact between a thermoelectric material and a temperature sensor is another serious issue as far as the accuracy of the Seebeck coefficient measurements is concerned. The heat exchange area between the sample and the thermocouple has to be maximized to ensure the uniformity of temperature profile. Thermally conducting pastes or other adhesives as well as soldering can be used to improve the thermal contact, however it still not sufficient in case of the measurements on thin films. A regular temperature sensor of spherical shape cannot be implanted into the sample due to its limited thickness. Moreover, the probes for voltage measurement and the thermocouples have to be placed as close to each other as possible since by definition both ∆V and ∆T are supposed to be estimated at the same points, which requires a good electrical insulation between them.

Figure 18. Schematic illustration of the set-up used for Seebeck coefficient measurements on thin films.

In order to avoid errors in temperature gradient estimation, the set-up shown in Fig 18 can be used. The main idea is based on the utilization of thin metal thermistors for accurate temperature measurements. The sample comprises two symmetrical parts: the reference and the polymer film under

42

investigation. The whole structure is fixed between two metallic blocks mounted on an electrically and thermally insulating platform and separated from one another by a distance of several centimeters. One block is heated and therefore acting as a hot side, while the other one can be cooled serving as a heat sink. The sample is kept between the blocks and the four golden electrodes are aligned and have the same separation distance. The two golden thermistors on the reference sample are used for T measurements. The T of each line can be determined from the calibration slope (T vs. resistance) specific to each thermistor. The width of the lines is so small (≈35µm) that its resistance is very sensitive to any changes in temperature. In addition, the thermistors are covered with an insulating material with the thermal conductivity comparable to polymer’s thermal conductivity to imitate the heat transfer processes on the real film. The other pair of identical electrodes coated underneath the polymer film is used for the Seebeck voltage measurements. Since the sample is symmetrical and the separation between the electrodes as well as the material they are made of are the same, one can assume that the T profile along these lines is constant as long as the heating and cooling rate is uniform on both sides. The generated thermoelectric voltage (ΔV) can be measured with nanovoltmeter keithley 2200, with one probe on the hot and cold Au lines joined by the polymer film in between. Knowing the T of each thermistor, the Seebeck coefficient is calculated as ΔV/ ΔT.

Even though, the thermopower is conceptually easy to estimate, potentially it represents a big challenge, especially in thin films.

4.3 Thermal conductivity measurements Thermal conductivity measurement are considered to be the most

difficult to execute as far as high accuracy is concerned. In amorphous materials, which is what conducting polymers are, the thermal conductivity is often very small (≤2 W/mK). Consequently, the measurement becomes even harder to perform, provided that among all the elements constituting the set-up the measured material is the least preferable for the heat to flow through.

The 3ω technique introduced by Cahill in late 80’s is commonly used for the thermal conductivity measurements in thin films, bulk materials and liquids110. An AC current at frequency ω applied to a thin heater causes joule heating at 2ω. The fluctuation in the resistance of the heater creates an oscillating Joule heat of frequency 2ω. The metallic heater is calibrated similar to the thermistors in the Seebeck coefficient measurements described in the previous section.

43

Figure 19. a) Circuit diagram for the 3ω voltage measurement. The

input signal is applied from a function generator; a bridge circuit is used to minimize first harmonic voltage signal. RS represents the gold heater while R1, R3 are standard resistors and R2 is a variable resistor. Differential inputs (A & B) of the lock-in- amplifier are used to extract the 3ω voltage signal after comparing with frequency of a reference signal from function generator. b) Cross-section of the device used to measure the thermal conductivity. It is composed of a glass substrate partially coated with a polymer layer having SiNx insulation layer and gold heater (H1) on top. Gold heater (H2) of the same size on top of glass substrate with SiNx serves as reference.

Thus, it is used as a heater and a thermometer simultaneously. The

fluctuation in the resistance of the heater at the 2nd harmonic creates a 3ω component in the voltage. This 3ω voltage provides information about the amount of heat dissipated through the underlying layers and can be used to calculate thermal conductivity (k) of those layers.

For a metal heater with width 2b, the temperature oscillation (∆T) can be found by Cahill’s simplified of heat diffusion equation:

!T = PL!k

sin2(kb)(kb)2 (k2 + q2 )1/2

dk0

"

# (4.3)

q = i2"D

44

1/q is thermal penetration depth, D is thermal diffusivity and P/L is power per unit length of heater. Experimentally ∆T can be found as:

!T = 2dTdR

RV1!

V3! (4.4)

Where R, V1ω, V3ω are resistance, first and 3rd harmonic voltages across

the heater. Differential 3ω method has been frequently used to investigate thermal conductivity of thin films. Two heaters of the same dimensions are patterned on top of reference sample without film and another one having the film under test. Temperature drop in thin film (∆Tf ) can be found by:

!TfP1

=!T1P1

"!T2P2

(4.5)

∆T1 and ∆T2 are the temperatures on the substrate and the film and the

substrate only, P1 and P2 are the respective power dissipation. An insulation layer is added if the film is conducting.

The thermal conductivity is estimated using the equation111:

k =P1hf

2b 'L!Tf(4.6)

where hf and L are the thickness and the length of the film,

2b’=2b+0.76 hf is the effective line width of the heater. Thermal conductivity characterization with 3ω technique is an

advantageous method for a number of reasons. Firstly, it is an AC measurement where radiation effects are minimized and secondly, it readily provides the thermal conductivity temperature dependence. In amorphous materials, such as conjugated polymers, associated with high anisotropy of physical properties, it is crucial to measure both vertical and lateral thermal conductivity. With 3ω technique it can be done using the set-up shown in figure 19b.

45

Chapter 5 - Goal of the thesis This thesis provides an insight into the emerging field of organic

thermoelectrics, more specifically, thermoelectric power generation based on conducting polymers as functional materials. Starting from the basic principles of thermoelectrics, containing thermodynamics, solid-sate physics and thermoelectric energy conversion aspects, the discussion is followed by the section describing physical and electronic properties of conducting polymers as well as the charge transport mechanisms characteristic to these materials. Paper 1 serves as the main introduction to organic thermoelectrics by merging together conducting polymers and thermoelectrics. The review provides a brief discussion of electronic and thermoelectric properties of various conducting polymers along with an overview of the progress in the field of polymer-based thermoelectrics. The potentials of organic thermoelectric materials as well as the strategies of thermoelectric efficiency improvement are examined. Paper 2 attempts to compare and comprehend thermoelectric properties of some PEDOT derivatives by looking into their morphology and structure as the origin of different conduction mechanism in seemingly similar materials. Besides, the effect of secondary doping on thermoelectric properties is studied. Paper 3 concerns with optimization of the thermoelectric efficiency of PEDOT-PSS used as an active material in an electrochemical transistor. Applied gate potential acts as a tuning parameter that induces a certain oxidation state in the organic semiconductor and ultimately enables to obtain an optimum zT. In Paper 4 another PEDOT derivative i.e. PEDOT-Tos is gradually reduced with Tetrarkis(dimethylamino)ethylene (TDAE) from its highly doped state (ca. one charge per three monomers) down to lower doping levels. The thermoelectric efficiency reaches 0.25 at room T, which is almost comparable with the best inorganic thermoelectrics. Finally Paper 5 shows another PEDOT-Tos zT optimization method, this time using acid/base chemistry. This approach can be seen as an alternative to chemical doping. The doping level in the latter process can be challenging to control precisely, moreover it may involve highly reactive and therefore unstable in air dopants. Changing pH of the solution with which the polymer is treated, the thermoelectric properties of PEDOT-Tos can be altered in a manner similar to chemical doping. Acidic pH increases electrical conductivity without affecting thermopower, while basic pH reduces the polymer and promotes higher thermopower values. As a result zT is optimized at a certain pH value of the used solution.

The ultimate goal of the thesis is to demonstrate high potentials of conducting polymers in thermoelectric energy conversion applications. PEDOT derivatives, especially PEDOT-Tos are one of those materials that can exhibit fairly high thermoelectric efficiency if properly modified. The polymers can be

46

easily functionalized in different sizes and shapes to pattern organic TEGs on a variety of different substrates. Being a p-type material, PEDOT cannot be used for both types of thermoelectric element and a stable n-type polymer is required to complete the converter. The solution can be perhaps found in the organic-inorganic symbiosis, not only in order to obtain n-type materials but also for the sake of higher zT.

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Chapter 6 – Conclusions and perspective

Organic thermoelectrics is a rapidly emerging field that is yet to evolve into a strong, independent and clearly oriented research area. In the “department” of conventional thermoelectrics that is overly inorganic, conducting polymers have not been recognized as potentially suitable materials for thermoelectric applications and rarely mentioned in the related literature. But things are certainly changing, as the advantages offered by this new approach are considerable as compared to inorganic thermoelectrics. Loosing the battle for high efficiency, organic materials can compensate for it with their good processibility, mechanical flexibility, low material and manufacturing cost, variety of synthesis techniques and light weightiness of the polymer based devices. The efficiency problem can be taken care of by controlling the structure and morphology of polymers in order to achieve higher crystallinity and therefore improve the charge transport in the materials. Optimizing the oxidation level in conducting polymers enables to achieve the balance of thermoelectric properties and acquire favorable DOS at the Fermi level. Extra improvement can be obtained by decreasing lattice thermal conductivity in conducting polymers. It can be done by incorporating scattering centers into the polymer structure in the form of voids, pores or any other inclusions of nanosize. Equivalently, inorganic nanostructures with high Seebeck coefficient or electrical conductivity can be functionalized within polymer matrix in order to enhance the overall zT. Polymers themselves can form nanostructures, which allows for DOS engineering and much higher thermopower can be expected in such materials.

All in all, there are numerous directions in which organic thermoelectrics can potentially progress. There is no doubt that in the years to come it shouldn’t be and won’t be abandoned because of the constantly growing need in energy. Based on today’s energy consumption patterns, it’s clearly just a matter of time before the renewable energy resources become a substantial part of the total energy supply and at that point organic thermoelectrics shall not be overlooked.

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