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UNIVERSIDADE FEDERAL DO RIO GRANDE DO NORTE CENTRO DE TECNOLOGIA (CT)

CENTRO DE CIÊNCIAS EXATAS E DA TERRA (CCET)

PROGRAMA DE PÓS-GRADUAÇÃO EM CIÊNCIA E

ENGENHARIA DE MATERIAIS

DISSERTAÇÃO DE MESTRADO

TUNING THE ELECTRICAL CONDUCTIVITY OF AN N-TYPE ORGANIC SEMICONDUCTOR BY MEANS OF SOLUTION DOPING

FOR THERMOELECTRIC APPLICATIONS

Angel Roberta Oliveira de Sousa

Advisor:

Prof. Dr. Carlos Alberto Paskocimas Co-advisor:

Prof. Dr. Derya Baran

Janeiro de 2020 Natal-RN

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TUNING THE ELECTRICAL CONDUCTIVITY OF AN N-TYPE ORGANIC SEMICONDUCTOR BY MEANS OF SOLUTION DOPING

FOR THERMOELECTRIC APPLICATIONS

Angel Roberta Oliveira de Sousa

Thesis presented to the Post-Graduation program in Materials Science and Engineering, in the Technology Center of the Federal University of Rio Grande do Norte, as part of the necessary requirements for the Master’s degree in Materials Science and Engineering. Research field: Polymeric Materials, Materials development, characterization and application in thermoelectrics.

Advisor: Prof. Dr. Carlos Alberto Paskocimas

Co-Advisor: Prof. Dr. Derya Baran

Janeiro de 2020 Natal-RN

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Universidade Federal do Rio Grande do Norte - UFRN

Sistema de Bibliotecas - SISBI

Catalogação de Publicação na Fonte. UFRN - Biblioteca Central Zila Mamede

Sousa, Angel Roberta Oliveira de.

Tuning the electral conductivity of an n-type organic

semiconductor by means of solution doping for thermoeletric

applications / Angel Roberta Oliveira de Sousa. - 2020.

68 f.: il.

Dissertação (mestrado) - Universidade Federal do Rio Grande do

Norte, Centro de Tecnologia, Programa de Pós-Graduação em Ciência

e Engenharia de Materiais, Natal, RN, 2020.

Orientador: Prof. Dr. Carlos Alberto Paskocimas.

Coorientadora: Profa. Dra. Derya Baran.

1. Organic thermoelectric - Dissertação. 2. Organic Doping -

Dissertação. 3. Electrical Conductivity - Dissertação. 4. N2200 -

Dissertação. 5. Dimer - Dissertação. 6. NDMBI - Dissertação. I.

Paskocimas, Carlos Alberto. II. Baran, Derya. III. Título.

RN/UF/BCZM CDU 621.311.23

Elaborado por Fernanda de Medeiros Ferreira Aquino - CRB-15/301

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To my beloved family My father, José Roberto, my mother, Mariza Oliveira, my

brother, Anderson Sousa and my sister, Alessandra Sousa Because they exist

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In the name of the best within you, do not sacrifice this world to

those who are its worst. In the name of the values that keep you alive, do not let your vision of man be distorted by the ugly, the

cowardly, the mindless in those who have never achieved his title. Do not lose your knowledge that man's proper estate is an

upright posture, an intransigent mind and a step that travels unlimited roads. Do not let your fire go out, spark by irreplaceable spark, in the hopeless swamps of the

approximate, the not-quite, the not-yet, the not-at-all. Do not let the hero in your soul perish, in lonely frustration for the life you deserved, but have never been able to reach. Check your road

and the nature of your battle. The world you desired can be won, it exists, it is real, it is possible, it is yours.

(Ayn Rand)

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AGRADECIMENTOS

Primeiramente a Deus por me entregar a uma família que me olha com Seus

olhos e me enxerga melhor do que sou. Agradeço também a capacidade de meus pais

e irmãos de me olharem devagar numa vida que exige tanta pressa. Obrigada pai,

José Roberto, e mãe, Mariza Oliveira, pela resiliência, pela força e pela orientação.

Obrigada, Ande e Alê, por acreditarem em mim.

À CAPES por financiar o início de meus estudos no Mestrado.

À KAUST, à Prof. Dra. Derya Baran e ao Diego Rosas pelos 6 meses de

aprendizado que resultou nessa dissertação. Gostaria de agradecer também ao Lab

Operations Team do KAUST Solar Center pelo trabalho diligente e pela paciência de

me treinar, explicar e reexplicar sempre que necessário.

Ao Prof. Dr. Carlos Alberto Paskocimas e ao corpo docente do PPGCEM pelos

incentivos durante essa Pós-Graduação. À Prof. Dra. Ana Paula, ao Prof. Dr. Eduardo

Martinelli e ao Prof. Dr. Rubens pelas contribuições na qualificação. Ao Dr. João Paulo

Grilo pela disponibilidade em participar e contribuir também com este trabalho.

À minha família de estágio – Ana Luíza Slama, Carlos Zan e Emily Alexandre –

que me ajudaram a não perder o prumo.

A Allan Menezes e Artur de Morais pela amizade e companhia desde a UFPB.

Aos amigos do PPGCEM/Natal: obrigada pelos açaís, milk-shakes, Outbacks,

Tá fluindos e saídas de última hora! Foi uma aventura muito boa!

À Jules Bertrandie que incansavelmente ouviu, leu, releu e criticou esse

trabalho com a disposição de quem o faz pela primeira vez, ainda que fosse a décima.

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ABSTRACT Organic thermoelectric devices are composed of conductive polymers capable of converting thermal energy into electrical energy, and vice versa, through Seebeck and Peltier Effects. Organic doping is one of the most important approaches used to improve and tune the electrical properties of polymers, especially N-type organic semiconductors, which are known to be the obstacle to improve the performance of thermoelectric devices, since their performance lags behind when compared to the P-type semiconductor polymers due to their inefficient doping process. Systems using Poly { [ N , N′ - bis ( 2 – octyldodecyl ) – naphthalene – 1 , 4 , 5 , 8 - bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) P(NDI2OD-T2), also known as N2200, as the host material, have been extensively investigated with 4 - ( 1 , 3 - Dimethyl - 2 , 3 - dihydro - 1 H - benzoimidazol - 2 - yl ) phenyl) dimethylamine (NDMBI), Mesitylene pentamethylcyclopentadienyl ruthenium dimer ((RuCp*mes)2), 4 - ( 1 , 3 - Dimethyl - 2 , 3 - dihydro - 1H - benzoimidazol - 2 – yl)-N,N-diphenylaniline (DPBI), Tetrabutilamonium fluoride hydrate (TBAF), among others, but nothing has been reported about the interaction of the NDMBI dimer with the P(NDI2OD-T2). Given this, the present work aimed to investigate the effect of the concentration increase of (NDMBI)2 in the N2200 used as a matrix, mainly evaluating the impact on the electrical conductivity and on the morphology of the produced film. The characterizations used in this study were Visible ultraviolet spectroscopy (UV-vis), Electronic Paramagnetic Resonance Spectroscopy (EPR), Atomic Force Microscopy (AFM), Kelvin Probe Microscopy (KPM), two-point probe station and the Thin Film Analyzer (TFA) from Linseis. The electrical characterizations were able to measure a 4 orders increase in the electrical conductivity for the 20% doped N2200-dimer sample when compared to the neat material N2200 and slightly doped samples. The Seebeck coefficient for the 10% doped sample was -80 uV/K and for the 20% was -66 uV/K at 30ºC. The AFM and the KPM helped to better understand the morphology and miscibility of the dopant in the host material. Keywords: Organic Thermoelectric; Organic Doping; Eletrical Conductivity; N2200, Dimer; (NDMBI)2.

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RESUMO Dispositivos termoelétricos orgânicos são polímeros condutores capazes de converter energia térmica em elétrica, e vice-versa, através dos efeitos Seebeck e Peltier. A dopagem desses materiais orgânicos é uma das mais importantes abordagens utilizadas para melhorar e controlar as propriedades elétricas dos polímeros, principalmente dos semicondutores tipo n, os quais são reconhecidamente o empecilho para o avanço da performance dos dispositivos termoelétricos, devido ao seu desempenho aquém ao dos semicondutores tipo p e sua dopagem ineficiente. Sistemas utilizando Poly{[N,N′-bis(2-octildodecil)-nafthaleno - 1 , 4 , 5 , 8 - bis ( dicarboximida ) - 2 , 6 - diil ] - alt - 5 , 5′ - ( 2 , 2′ -bitiofeno) P(NDI2OD-T2), também conhecido como N2200, como material da matriz foram extensivamente investigados com os dopantes 4-(1,3-Dimetil-2,3-di-hidro-1H-benzoimidazol-2-il)fenil)dimetilamina (NDMBI) dímero de mesetileno pentametilciclopentandienil rutênio ((RuCp*mes)2), 4-(1,3-Dimetil-2,3-di-hidro-1H-benzoimidazol-2-il)-N,N-difenilanilina (DPBI), hidrato de fluoreto de tetrabutilamônio (TBAF), entre outros, porém nada foi reportado acerca do dímero do NDMBI. Diante disto, o presente trabalho se voltou para o estudo do efeito do aumento da concentração do (NDMBI)2 na matriz do N2200, principalmente seu impacto nas propriedades elétricas e na morfologia do filme produzido. Para tanto, foram utilizados espectroscopia no ultravioleta visível (UV-vis), espectroscopia de ressonância paramagnética eletrônica (RPE), microscopia de força atômica (AFM), microscopia de potencial de superfície (KPFM), estação de medida elétrica pelo método de dois pontos e um equipamento especializado para medidas termoelétricas de filmes finos da Linseis. As caracterizações elétricas permitiram medir uma melhora na condutividade elétrica de até 4 ordens para a amostra de 20% N2200-(NDMBI)2 quando comparada ao N2200 sem a dopagem. O coeficiente Seebeck das amostras de 10% e 20% foram, respectivamente, -80 uV/K e -60uV/K a 30ºC. O AFM e o KPM auxiliaram na compreensão da correlação entre a quantidade de dopante presente, a condutividade elétrica e o aspecto morfológico do filme. Palavras-chaves: Termoelétrico Orgânico; Dopagem Orgânica; Condutividade Elétrica; N2200; Dímero; (NDMBI)2.

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LIST OF FIGURES

Figure 1: Energy band of several materials: (a) Energy band of Metal (left), (b) Energy band semi-conductor (middle), Energy band insulator (right) (Image elaborated by the author) ................................................................................. 06

Figure 2: A thermoelectric device is composed by a P-type and an N-type semiconductor connected electrically in series and thermally in parallel. a) Seebeck Effect. b) Peltier Effect (Image elaborated by the author) ................................. 07

Figure 3: Schematic correlating the TE properties with the charge carrier density. (BLACKBURN, FERGUSON et al, 2018) ......................................................... 09

Figure 4: Thermoelectric films easily bent by the touch of fingertips (CHEN, ZHAO et al, 2015) ....................................................................................................... 10

Figure 5: Most investigated N-type semiconductor materials in the thermoelectric field. At the top, the two most studied molecular dopants (HOFMANN, KROON et al, 2019) ........................................................................................................... 13

Figure 6: Band theory explaining the mechanisms of doping in inorganic materials ......................................................................................................................... 14

Figure 7: Organic semiconductor doping via ICT model (COWEN, ATOYO et al, 2017) ................................................................................................................ 15

Figure 8: The three main challenges needed to surpass to achieve high conductivity in N-type polymers; (1) N-doping efficiency; (2) charge transport mobility; (3) stability in air (LU, WANG et al, 2019) ........................................................................... 16

Figure 9: N-doping of conjugated polymer: (a) noN-doped polymer; (b) polaron states; (c) bipolaron states (LU, WANG et al, 2019) ......................................... 16

Figure 10: Steps to achieve efficiency in the charge transport of conductive polymers: (1) the design and selection of the host and dopant; (2) solution aggregates; (3) film microstructure formation (LU, WANG et al, 2019) ............ 17

Figure 11: Chemical structures of BDPPV derivatives and N-type dopant NDMBI (SHI, ZHANG et al, 2015) ................................................................................. 20

Figure 12: Molecular structures of a) P(NDI2OD-T2) and, the modified, b) P(NDI2TEG-T2), and simulations of NDMBI molecules dissolved c) in pure P-NDI2OD-T2 and d) pure P-NDI2TEG-T2 (LIU, QIU et al, 2018) ....................... 22

Figure 13: N-doping pathways of a) DMBI derivatives and b) Organic dimer dopants (UEBE, YOSHIHASHI et al, 2018) ................................................................... 24

Figure 14: Fluxogram exhibiting the methodology adopted in the present work ......................................................................................................................... 26

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Figure 15: Transistor mask used to evaporate the electrodes ......................... 27

Figure 16: EPR of four representative samples: pristine N2200, 10, 20 and 30% of (DMBI)2 ............................................................................................................ 32

Figure 17: UV-vis spectra of the 5, 10, 15, 20, 25 and 30% doped samples a) in solution and b) in film state ............................................................................... 34

Figure 18: Electrical conductivity (S/cm) in function of the dimer concentration (% mol) .................................................................................................................. 35

Figure 19: Electrical conductivity as a function of the Temperature ................ 36

Figure 16: Thermal conductivity as a function of the Temperature .................. 37

Figure 21: Seebeck coefficient as a function of the Temperature.................... 38 Figure 17: AFM of the 0, 10, 20, 30% doped samples. Morphology and Phase signals, first and second columns, respectively ................................................ 39

Figura 23: AFM of the 10, 20 and 30% aged samples without influence of the Temperature. Morphology and Phase signals, first and second columns, respectively ...................................................................................................... 41

Figure 18: Surface potential difference microscopy of the 30% doped sample. Morphology, Phase and KPM signals, from left to right .................................... 43

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POLYMER ACRONYMS BDPPV: Poly[[1,2-dihydro-1-(4-octadecyldocosy-1-(4-octadecyldocosyl)-2-oxo3H-indol-6-yl-3-ylidene]-(1E)-1,2-ethenediyl[5-chloro-1,2-dihydro-1-(4-octadecyldocosyl)-2-oxo-3H-Indol-6-yl-3-ylidene][2,6-dioxobenzo[1,2-b:4,5b’]difuran-3,7(2H,6H)-diylidene] CIBDPPV: (Poly[[5-chloro – 1 , 2 - dihydro - 1 -(4-octadecyldocosy-1-(4-octadecyldocosyl)-2-oxo3H-indol-6-yl-3-ylidene]-(1E)-1,2-ethenediyl [5-chloro-1,2-dihydro-1-(4-octadecyldocosyl)-2-oxo-3H-indol-6-yl-3-ylidene][2,6-dioxobenzo[1,2-b:4,5b’]difuran-3,7(2H,6H)-diylidene]] FBDPPV: Poly[[7-fluoro-1,2-dihydro-1-(4-octadecyldocosyl)-2-oxo-3H-indol-6-yl-3-ylidene]-(1E)-1,2-ethenediyl[7-fluoro-1,2-dihydro-1-(4-octadecyldocosyl)-2-oxo-3H-indol-6-yl-3-ylidene](2,6-dioxobenzo[1,2-b:4,5-b']difuran-3,7(2H,6H)-diylidene)] NDMBI: 4-(1,3-Dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine IDTBT: indacenodithiophene-co-benzothiadiazole copolymer P3HT: Poly(3-hexylthiophene-2,5-diyl PCBM: phenyl-C61-butyric acid methyl ester P(NDI2OD-T2): Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) P(NDI2TEG-T2): Poly {[N,N′-bis(2-triethyleneglycol)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} PP-PEDOT: Polypropylene poly(3,4-ethylenediocythiophene)

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GENERAL ACRONYMS AFM: Atomic Force Microscopy Bi2Te3: Bismuth Telluride C: Concentration in g/L DIW: Deionized water EPR: Electronic Paramagnetic Resonance Spectroscopy ESR: Electron spin resonance HOMO: Highest occupied molecular orbital ICT: Integer-charge transfer IPA: Isopropanol alcohol KPM: Kelvin Probe Microscopy LUMO: Lowest unoccupied molecular orbital Mx: Concentration in mol/L, where x can be ‘h’ or ‘d’, refering to the host or the dopant, respectively. Mwx: Molecular weight (g/mol), where x can be ‘h’ or ‘d’, refering to the host or the dopant, respectively OSC: Organic semiconductor OTE: Organic thermoelectric RPM: Rotation per minute (RuCp*mes)2: Mesitylene pentamethylcyclopentadienyl ruthenium dimer TBAF: Tetrabutilamonium fluoride hydrate TE: Thermoelectric TEG: Thermoelectric generators TFA: Thin Film Analyzer UV-vis: Visible ultraviolet spectroscopy V : Total volume in L

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V’x : Partial volume in L, where x can be either ‘h’ or ‘d’, refering to the host or dopant volume needed to correspond to the ratio desired in the study, respectively.

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CONTENTS

I - INTRODUCTION .......................................................................................... 01

II - LITERATURE REVIEW .............................................................................. 04 2.1. Fundamentals of semiconductors for thermoelectric devices .................... 05 2.2. Thermoelectric devices ............................................................................. 06 2.1.1. Principle ............................................................................................... 06 2.1.2. Characterization ................................................................................... 07 2.3. Materials for thermoelectrics ..................................................................... 09 2.3.1. Inorganic Material ................................................................................. 09 2.3.2. Organic Materials ................................................................................. 10 2.4. Organic Thermoelectric (OTEs) Development .......................................... 11 2.5. N-type Thermoelectrics ............................................................................. 12 2.5.1. Fundamentals of Organic Doping ......................................................... 13 2.5.2. N-type Doping ...................................................................................... 15 2.5.2.1. N-doping efficiency .......................................................................... 16 2.5.2.2. Charge carrier mobility ................................................................... 17 2.5.2.3. Stability ........................................................................................... 19 2.5.3. Strategies towards the enhancement of the N-doping efficiency .......... 20

III - EXPERIMENTAL ....................................................................................... 25 3.1. Substrate and Devices preparation ........................................................... 27 3.2. Solution preparation .................................................................................. 27 3.3. Spin Coating .............................................................................................. 29 3.4. Characterizations ...................................................................................... 29 3.4.1. Optical and Morphology Characterization and EPR .............................. 29 3.4.2. Electrical and Thermoelectrical measurements ..................................... 30

IV - RESULTS AND DISCUSSION .................................................................. 31 V - CONCLUSION AND FUTURE OUTLOOK ................................................. 44 5.1 Conclusions ................................................................................................ 45 5.2 Future Outlook ........................................................................................... 46 REFERENCES ................................................................................................. 47

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I - INTRODUCTION

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A thermoelectric (TE) device converts temperature gradients directly into

electric voltage and vice versa, being able to transform electricity into cooling and

heating according to the characteristics of the material used (CHU et al, 2019).

Currently, the efficiency of these modules used for heat recovery is around 5% to 10%

(DING; AKBARZADEH; DATE, 2016). Commercial thermoelectric modules are largely

applied in diverse applications such as converting human body heat into electricity,

heating and cooling systems, sensors, recovering the wasted heat of photovoltaic

power generation systems and others (DING; AKBARZADEH; DATE, 2016; LU et al,

2016; MILIĆ et al, 2017; MOHSENZADEH; SHAFII; JAFARI MOSLEH, 2017; WANG;

CALDERÓN; WANG, 2017).

The semiconductor materials are chosen according to the nature of the wasted

heat, they can be either organic or inorganic. Lately, the inorganic materials are the

reference in the field, as they have the best performances (COWEN, ATOYO et al,

2017). The doped inorganic semiconductors as thermoelectric (TE) materials, like the

ones based on the semimetals silicon and germanium, have been largely applied to

recover heat waste above 250°C (LEBLANC, 2014). The most used material for

thermoelectric application is Bismuth Telluride (Bi2Te3). However, the toxicity of this

compound has become one of the concerns around the use of inorganic materials as

thermoelectric devices. The contact with this compound can cause skin and eyes

irritation and its breathing can irritate the nose and the throat. Also, the dopants used

to improve the Bi2Te3 properties, like Se and Pb, are already known as capable of

damaging the human health (BLACKBURN et al, 2018). Adding to the toxicity problem,

because of their rigid characteristic, the inorganic materials also demand a high cost

processing that burdens the final product. As a solution for these problems, the organic

semiconductor materials have emerged in an attempt to provide a non-harmful device

and a flexible way of processing the TE materials and the TE devices (CAMPOY-

QUILES, 2019).

To perform as an ideal thermoelectric, the semiconductor must have a low

thermal conductivity and a high electrical conductivity, so the temperature gradient is

maintained while the electrical current flows. The organic semiconductors (OSCs) are

intrinsically bad thermal conductors, but the electrical conductivity is still a parameter

that needs improvement (LU, WANG et al, 2019). The P-type OSCs, when doped,

already present electrical conductivities comparable to those of the inorganic

semiconductors, whereas the N-type OSCs are still lagging behind ascribed to their

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low lowest unoccupied molecular orbital (LUMO) energy (KIM et al, 2014). The deep

LUMO energy restricts the performance of the N-type OSCs to atmospheres without

oxygen or water, being materials of very delicate handling that require N2 working

atmosphere. Since the P-type and N-type semiconductors are both required in the TE

device due to their complementary functions, it is a priority to optimize the electrical

conductivity of the N-type OSCs. One strategy used to tune the electrical conductivity

is the doping (SUN, DI et al, 2019a).

The doping involves the intentional insertion of an impurity in a material aiming

to improve its performance. The impurity is called dopant and the material that receives

the dopant is called host. One of the most studied N-type organic thermoelectric (OTE)

host is a high electron mobility polymer, the Poly{[N,N′-bis(2-octyldodecyl)-

naphthalene-1,4,5,8 - bis (dicarboximide) - 2, 6 - diyl ] - alt - 5 , 5′ - (2, 2′ bithiophene)

P(NDI2OD-T2), also known as N2200. The dopants reported with this host material

are several like the 4-(1,3-Dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)

dimethylamine (NDMBI), Mesitylene pentamethylcyclopentadienyl ruthenium dimer

((RuCp*mes)2), 4 - ( 1 , 3 - Dimethyl - 2 , 3 - dihydro - 1H - benzoimidazol - 2 – yl)-N,N-

diphenylaniline (DPBI), Tetrabutilamonium fluoride hydrate (TBAF), among others

(YANG et al, 2018). The highest electrical conductivity achieved for the N2200 was

reached with the monomer of the NDMBI, exhibiting a value of 8 . 10-3 S/cm (SCHLITZ

et al, 2014). The NDMBI monomer has also a dimer version, which was not reported

as a dopant of the N2200 for thermoelectric application, but it is a promising dopant,

since it proposes to inject two electrons (dimer) instead of only one (monomer) and it

also carries high reactivity.

Taking into account what was described previously, it is relevant and necessary

comprehend how the dimer influences the properties of the N2200, more specifically

how it affects the electrical conductivity. This understanding will contribute to the

advance of the Materials Science and the Organic Thermoelectric field, being a

building block towards the future of the Energy Conversion.

In this sense, the present work aimed to further understand the doping effect on

the electrical conductivity of the system N2200-(NDMBI)2. In order to achieve the

proposed goal, the specific paths were taken: tune the electrical conductivity of the

N2200 by means of solution doping; investigate the doping effect in the Seebeck

coefficient; correlate the dopant ratio with the surface morphology and the electrical

conductivity.

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II - LITERATURE REVIEW

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2.1. Fundamentals of semiconductors for thermoelectric devices

Population growth and, consequently, the demand for food, energy, water

resources, among others, has been worrying governments around the world. High

demand for energy is only a fraction of this problem (WANG; YU, 2019). Petroleum-

based fuels are widely used as energy sources in the everyday life of society.

Fossil fuels are used in a variety of industries, from automotive to cosmetics and

daily life tasks such as cooking (BUBNOVA; CRISPIN, 2012). The burning of these

fuels is responsible to emit gases known as NOx, SOx and CO/CO2 (LETCHER, 2019).

These gases damage human health and contribute to global warming.

In an attempt to reduce the damage caused by the dependence on fossil fuels,

clean energy devices offer the possibility of meeting the needs of the population without

causing damage to human health and nature. Energy captured for conversion can

come from various sources: solar, Wind, heat, biomass, etc. Devices that are able to

turn heat into electricity are known as thermoelectrics (CHU et al, 2019).

In early 1820s, Thomas Johann Seebeck discovered the first thermoelectric

effect while looking for the relationship between heat and electricity (WANG; YU,

2019). In order to make devices with thermoelectric properties, special materials are

used: semiconductors. A semiconductor is a material capable of conducting electricity

under specific stimuli, making it possible to control its electrical current. These specific

stimuli can be: potential difference, light or heat among others. The semiconducting

behavior is only possible because of the bandgap size of these materials, which is

intermediate to that of an insulator and a metal (COWEN, ATOY et al, 2017).

The metals do not have a bandgap, which means that the valence band (where

the electrons are in their ground state) and the conduction band (where the electrons

conduct electricity) are overlapping, so there is no need to provide power to the

electron leap from one band to the other and become free to conduct electricity.

Meanwhile, the insulators have a wide bandgap, requiring a large amount of energy

for the valence band electron reach the conduction band; it is theoretically possible to

cause these materials to conduct electricity, but the amount of energy to be supplied

is not suitable for daily applications and can also result in the material degradation.

The semiconductors are classified as intermediate to metals and insulators, exhibiting

a small bandgap, which can be overcome when properly stimulated. For a better

understanding of this concept, see Figure 1 below. The semiconductors can be divided

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according to the nature of the charge carriers responsible for generating the electric

current. These charge carriers can be either positively charged (holes) or negatively

charged (electrons). They are also called P-type (holes) and N-type (electrons)

semiconductors (BHARTI, SINGH et al, 2018; KANG, SNYDER, 2017).

2.2. Thermoelectric devices

2.2.1. Principle

The ideal thermoelectric device is a combination of a P-type semiconductor

(hole transporting) and an N-type semiconductor (electron transporting) connected

electrically in series and thermally in parallel, as shown in Figure 2. The Figure 2a

refers to the Seebeck effect, which means that under a thermal gradient, it is possible

to obtain an electrical potential difference caused by the flow of charge carriers from

the hot to the cold side, thus generating electricity (MA, SHI et al, 2016; RUSS,

GLAUDELL et al, 2016; ZUO, LIU et al, 2018). Thermal conductivity can be either

phonon or electron dominated. Since the phonons occur only in crystalline materials,

for conducting polymers, the thermal conduction mechanism is electron dominated.

This means that by heating one edge of the device, the electrons (or holes) absorb

enough energy to jump into the conduction band; like any heat conduction process,

Figure 1: Energy band of several materials: a) Energy band of Metal (left), b) Energy band semi-

conductor (middle), c) Energy band insulator (right) (Image elaborated by the author)

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heat flows from hot to cold, favoring the flow of electrons and thus a potential difference

responsible for generating electricity (KROON et al, 2016). This process is yet

reversible, which means that if a potential difference is applied to the device, the

thermoelectric material will generate a temperature gradient that could be used for

heating or cooling applications (Peltier coolers, Figure 2b) (BHARTI et al, 2018).

Figure 2: A thermoelectric device is composed by a P-type and an N-type semiconductor connected electrically in series and thermally in parallel. a) Seebeck Effect. b) Peltier Effect (Image elaborated by

the author)

2.2.2. Fundamentals of Thermoelectrics

In each science field a figure of merit is needed to indicate the efficiency of the

material studied. In the thermoelectric field, the figure of merit dimensionless ZT

determines the device performance (KROON, MENGISTIE et al, 2016; LEBLANC,

2014; TANG, CHEN et al, 2019). It can be calculated from the equation below.

ZT =𝑺𝟐𝝈𝑻

𝒌

Where S is the Seebeck coefficient (uV/K), σ is the electrical conductivity

(S/cm), T is the operating temperature and so the average between the hot and cold

side of the device (K) and k is the thermal conductivity (W/mK) (SUN, SHENG et al,

2012). The Seebeck coefficient is given by the potential difference generated by the

thermal gradient (dV/dT) and its sign indicates the nature of the charge carriers

responsible for the electrical conduction. The product in the numerator, S2 σ, is also

called Power Factor (PF). The PF indicates the efficiency of the thermoelectric

8

conversion of the material, correlating two extremely important parameters (electrical

conductivity and Seebeck), giving clues as how the trade-off between the parameters

happens, as they are anti-correlated (LIU, YE et al, 2018; NGUYEN, O’LEARY, 2000;

THOMAS, POPERE et al, 2018).

In possession of the ZT equation, it is readily apparent that low thermal

conductivity is highly desired for thermoelectric applications in order to maximize the

thermal gradient across the semiconductors and, as a result, increase the amount of

heat flow converted in electricity. However, when the device operates under a

temperature difference, part of the heat is lost through two mechanisms: thermal

conduction (k.𝛥T) and Joule heating (I2R) due to the resulting electrical current

(HOFMANN; KROON; MÜLLER, 2019b). The outcome is that highly electrically

conductive materials need to reduce these losses. Therefore, ideally, the best material

for this application is the one with very low k and high σ, also described as electron-

crystal and phonon-glass (COWEN, ATOYO et al, 2017; GOLDSMID, 2009; RUSS,

2015).

It is almost intuitive to affirm that to achieve a higher ZT, one simply needs to

increase the electrical conductivity, which can be done by inserting more charge

carriers, for example, but this becomes more complex as S and σ are anti-correlated

and coupled with k. All this trade-off can be seen in the Figure 3 (BLACKBURN,

FERGUSON et al, 2018). Consequently, the insertion of charge carriers arouses a

decrease of the Seebeck coefficient and an increase in the thermal conductivity.

Therefore, the optimization of thermoelectric performance is nothing less than a trade-

off between increasing the electrical conductivity and decreasing the Seebeck

coefficient. The most commonly used thermoelectric materials are the doped

semiconductors with balanced thermal and electrical properties.

9

Figure 3: Schematic correlating the TE properties with the charge carrier density (BLACKBURN,

FERGUSON et al, 2018)

2.3. Materials for thermoelectrics

2.3.1. Inorganic Materials

The inorganic materials, such as skutterudites, silicides, half Heusler alloys,

inorganic cathrates, and oxides, allow high operating temperatures, recovering wasted

heat from vehicle exhaust and steam pumps, for example. For lower operating

temperatures (below 200°C), Bismuth Telluride (Bi2Te3) and its alloys are the reference

for thermoelectric conversion with efficiency (ZT) of nearly 1 (DISALVO, 1999;

LÜSSEM, RIEDE et al, 2013; RUSS, GLAUDELL et al, 2016). The electronic transport

mechanism obeys the band theory, which happens in crystalline semiconductors; the

underlying mechanism is the delocalization of the wavefunctions over the entire

volume of the crystal. In this case, the charge is intramolecular transferred, hence, the

distance that the charge carrier needs to overcome is the length of the chemical bond

(less than 1nm) (SUN, DI et al, 2019b). In addition, the semiconductor crystal is very

well organized, which makes it favorable for the electron to flow more freely, without

any unexpected barrier, making the mean free path larger and contributing to high ZTs

(COWEN et al, 2017).

10

2.3.2. Organic Materials

However, despite the good performance of inorganic materials, they are

considered toxic, not Earth abundant, fragile, rigid, expensive and require a demanding

process of obtaining and confining, limiting the architecture of thermoelectric devices

(BHARTI, SINGH et al, 2018; YANG, JIN et al, 2018). On the other hand, conductive

polymers are a quite attractive earth abundant alternative for low temperature

applications, with the addition of their flexibility, light weight, costless, solution

processability and consequent process scalability associated with their intrinsic low

thermal conductivity. In Figure 4 is shown a Polypropylene poly(3,4-

ethylenediocythiophene) PP-PEDOT film, with Gold electrodes, exempliflying the

flexibility that can be achieved by using organic materials for thermoelectric devices

(WANG, DUONG et al, 2015).

Figure 4: Thermoelectric films easily bent by the touch of fingertips: a) bending and b) twisting (CHEN, ZHAO et al, 2015)

The transport in organic semiconductors consists of intermolecular hopping

transport. This directly affects the mobility of the charge carriers, because the transition

mechanism is between localized sites via tunneling, making the process more difficult

ascribable to the need to overcome a potential barrier (QI, MOHAPATRA et al, 2012).

To make this mechanism more efficient, it is necessary to facilitate the electron hopping

from one molecule to another without being trapped or scattered. Mobility in organic

semiconductors is, therefore, strongly affected by the molecular packing, presence of

impurities, temperature, electric field, charge carrier density, among others (GIRI,

DELONGCHAMP et al, 2015; MA, SHI et al, 2016).

11

What prevents an organic material to become a reference in the TE field is the

Power Factor. Unfortunately, it is still very difficult to modulate the Seebeck coefficient

in organic semiconductor materials because it involves deep understanding of

quantum confinement, electron energy filtering, resonance levels and other concepts

that are not yet well clarified. The modulation of Seebeck coefficient will not be the

focus of this work. For further information, the reader can look into the paper

Thermoelectric power factor: Enhancement mechanisms and strategies for higher

performance thermoelectric materials (MEHDIZADEH DEHKORDI, ZEBARJADI et al,

2015).

2.4. Organic Thermoelectric (OTEs) Development

Around 1970s, a research involving conjugated polymers brought new concepts

to the organic materials field, leaving behind the exclusively insulating characteristics

of these materials and giving birth to the possibility of making polymers conductive,

achieving conductivities comparable to metals such as aluminum and copper –

discovery that received the 2000 Nobel Prize (BREDAS; STREET, 1985). Conjugated

materials have sp2 hybridized carbon atoms that results in delocalized п-orbitals

perpendicular to the plane of the sigma bonded molecular skeleton (BUBNOVA;

CRISPIN, 2012). To stabilize the п-bonding, some torsional penalties are created,

making the molecule more rigid. These kinds of molecules generally present small

bandgaps (1-4eV) and can receive charges that stay delocalized over the п-orbital

system without harming the structural integrity of the backbone established by the σ-

bond (BÄSSLER; KÖHLER, 2012).

This organization of the (conjugated) bonds favors the stacking of the п-п bond,

and this might result in the appearance of crystallites and aggregates. The stacking

benefits the mobility of the charge carriers by assisting in the intermolecular coupling

and with the charge delocalization (NAAB, GU et al, 2016, VENKATESHVARAN,

NIKOLKA et al, 2014). Nevertheless, overstacking leads to low solubility in solution as

a reflection of the heterogeneity of the polymer dispersion, which harms the doping

efficiency since it hampers the interaction with the dopant. What can be done to

improve the processability of these materials is to synthetically introduce solubilizing

side chains, which will contribute to a better homogeneity without necessarily harm the

packing. Organized N-type and P-type organic materials were already reported with

12

high intrinsic mobility (approximately 1-10 cm2/Vs) (BÄSSLER, KÖHLER, 2012;

KANG, SNYDER, 2017).

Because there are countless ways to improve the performance of the organic

materials, they have become the subject of study throughout the field of

thermoelectricity and optoelectronics, being considered as promising materials. This

reflects in the researches developed ever since their discovery, which consists of field-

effect transistors, sensors, solar cells and light emitting diode applications (LÜSSEM,

RIEDE, LEO, 2013; KIM et al, 2015; WELSH, LAVENTURE, WELCH, 2018). Although

thermoelectrical properties (S and σ) were being measured at the time the conductive

polymers were discovered, the main purpose was still to collect information about these

materials. However, about 10 years ago, the interest in organic materials for

thermoelectric applications has grown (SUN, DI et al, 2019b).

The P-type organic thermoelectric (OTE) are already well established owing to

their interesting features such as high electrical conductivity, low band gap energy,

environmental and thermal stability and easy processability. Compared to P-type

OTEs, the N-type are lagging behind, exhibiting poorer TE behaviors assignable to

inefficient doping, lower electrical conductivity and lower stability, with the latter being

the main disadvantage (LIU, QIU et al, 2018; LIU, YE et al, 2018; SHIN, MASSETTI et

al, 2018). The inefficient doping is explained by the low miscibility between host and

dopant materials. What makes this challenge even more complicated is the deep

lowest unoccupied molecular orbital (LUMO) energy level (around -4,0 eV), which

restricts amount of compatible N-dopant materials (WANG; NAKANO et al, 2017).

Conductive polymers operate only in a narrow window around room

temperature. In addition, the N-type organic semiconductors (OSCs) are also

susceptible to rapid degradation when exposed to air (O2 and H2O), which makes them

intrinsically stable organic materials extremely sought-after, since the operation of the

thermoelectric devices relies on the complementary performance of p and N-type

semiconductor materials (GAO, HU, 2014; LU, WANG et al, 2019; NAVA, SHIN et al,

2018).

2.5. N-type Thermoelectrics

The stability of the N-type TEs is directly related to the LUMO. To achieve good

stability under ambient conditions, the LUMO energy must be decreased, so the

13

material will not be susceptible to oxidation when exposed to O2 and H2O. Introducing

electron deficient atoms or side groups into the backbone has been widely used for the

design of high-mobility N-type polymers as a strategy for improving the stability of these

materials (GAO; HU, 2014). In Figure 5 is shown an overview of the most studied host

materials and their possible dopants at the top.

Figure 5: Most investigated N-type semiconductor materials in the thermoelectric field. At the top, the

two most studied molecular dopants (HOFMANN, KROON et al, 2019)

2.5.1. Fundamentals of Organic Doping

Doping is indispensable for maximizing the thermoelectric Power Factor (S2σ),

as this is a way to increase the free-carrier concentration and the charge carrier

mobility (μ), and, furthermore, the σ (KIM, SHAO et al, 2013). Basic Ohmic contact and

P-N junction devices relies on the doping efficiency and its precise profile control to

tailor the properties for the designed application. Controlled doping is the basis of the

modern electronic industry (QUINN, ZHU et al, 2017).

The mechanisms by which OSCs conduct the charge carrier are not simply

explained using the band theory as in inorganic materials. In band theory, the electrical

conductivity of a semiconductor is a result of the small bandgap that allows the electron

to move from the highest occupied energy level to the lowest unoccupied energy level,

14

also known as valence and conduction band, respectively (YAO, ZHANG et al, 2019;

ZHANG, HEO et al, 2019b). In doping process, it is possible to introduce defects by

adding energy levels between the valence band and the conduction band and, as a

consequence, reducing the bandgap. Figure 6 brings a scheme ilustrating the doping

process in inorganic semiconductors, following the band theory and describing how

the charge carriers are generated for these materials.

Unlike inorganic semiconductors, which are doped by introducing an impurity

into their crystal structure to decrease the bandgap, organic doping requires the

addition of strongly electron donating (N-type) or strongly electron withdrawing (P-type)

molecular species capable of inducing mobile charge carriers along the polymer

backbone, also called polarons and bipolarons, through a redox reaction (LU, WANG

et al, 2019).

A P-type dopant can be understood as an oxidizing agent, which has the

function of removing an electron from the highest occupied molecular orbital (HOMO)

of the OSCs, creating posite mobile charge carries along the polymer backbone. While

the N-type dopant is a reducing agent which donates electrons to the LUMO of the

host material, introducing negative charge carriers (KIEFER, GIOVANNITTI et al,

2018; NAAB, GU et al, 2016). Figure 7 clarifies the processes of charge transfer for

doped organic semiconductors for both P and N-type OTEs.

Figure 6: Band theory explaining the mechanisms of doping in inorganic materials for a) n-type semiconductors and b) p-type semiconductors

15

Figure 7 : Organic semiconductor doping via ICT model: on the left, the p-type doping; on the right,

the n-type doping (COWEN, ATOYO et al, 2017)

Doping mechanisms are still not well understood, but several novels were

published on this subject; two possible mechanisms were formulated: either host-

dopant electronic wave-function hybridization or ground-state integer-charge transfer

(ICT) from donor (D) to acceptor (A) (COWEN, ATOYO et al, 2017). Other steps are

only qualitatively understood. For the present work the ICT mechanism will be taken in

consideration.

The effect of dopants is particularly large on the mobility of the OSCs, because

dopants in Van der Waals bonded solids generally modify the conformation and

packing of the host molecules, altering the carrier transport properties by the increasing

the tunneling distance, thereby reducing the rate of thermally activated hopping (KIM,

SHAO et al, 2013).

2.5.2. N-type Doping

There are three main challenges to surpass in order to achieve high

conductivities in N-type OSCs: low N-doping efficiency that limits the charge carrier

density; low charge carrier mobility after doping, in turn, damaging the intra- and

interchain charge transport; and poor air and operation stabilities. The Figure 8

ilustrates the three most important drawbacks to be overcome: the low doping

efficiency that leads to charge carrier generation; the inefficient delocalization of the

polaron due to the microstructure of the final film; the low stability in air that leads to

oxidation and, thus, the loss of the highly desired electrical properties.

16

Figure 8: The three main challenges needed to surpass to achieve high conductivity in N-type

polymers; N-doping efficiency; charge transport mobility; stability in air (LU, WANG et al, 2019)

2.5.2.1. N-doping efficiency

As explained above, conjugated polymers are doped by intermolecular charge

transfer, involving a chemical redox reaction between dopant and host (LÜSSEM,

RIEDE et al, 2013). Effective N-doping occurs when the dopant transfers electrons

from the HOMO to the host LUMO. Moreover, the transfer may also not be only about

electrons, but also hydrides (H-) or anions, depending on the dopant and the host in

the reaction, so the mechanisms may change (KROON, MENGISTIE et al, 2016).

When an organic material is doped, the process can generate two kinds of

charge carriers named polaron (radical anion) and bipolaron (dianion) represented in

the Figure 9. When these two quasi-particles arise in the polymer backbone, metallic

transport may be observed at room temperature in conducting polymers.

Figure 9: N-doping of conjugated polymer: (a) noN-doped polymer; (b) polaron states; (c) bipolaron

states (LU, WANG et al, 2019)

17

2.5.2.2. Charge carrier mobility

Following the injection of the charges into the polymer backbone by doping, the

second challenge arises: the charge transport – given by the charge carrier mobility μ.

The transport mechanism can be understood as a combination of ultrafast wavelength

motion created by the (bi)polaron wave function overlapping localized on the same

polymeric chain and its rate-determined hopping motion from site to site is created by

the interaction of the (bi)polarons on neighboring chains (BÄSSLER; KÖHLER, 2012).

Unlike charge transport in undoped organic materials, the movement of the carriers in

doped conducting polymers requires taking into account the Coulombic effects of

counterions. Since the dielectric constant of polymers is low, the Coulombic traps are

large in size and suppress the intra- and interchain transport of charge carriers. With

the increase of the dopant concentration, these traps begin to overlap; as a result, the

hopping barrier is increasing the mobility. The highly doped semiconductors start

presenting a “band” character owing to a complete overlap of Coulomb traps, which

helps to boost the mobility and by consequence the conductivity (BUBNOVA;

CRISPIN, 2012).

In addition, the interchain transport is also deeply influenced by the morphology

and microstructure in film state. The Figure 10 shows a correlation between materials

selection, how it affects the aggregation in solution and, consequently, the film

microstructure.

Figure 10: Steps to achieve efficiency in the charge transport of conductive polymers: (1) the design and selection of the host and dopant; (2) solution aggregates; (3) film microstructure formation (LU,

WANG et al, 2019)

18

As far as is known, conjugated polymers feature long п-conjugated backbones

responsible for causing strong interchain п-п stacking, resulting in the formation of

aggregates when solubilized in the solvents used (JACOBS, AASEN et al, 2016,

KROON, KIEFER et al, 2017). This harms the microstructure of the film at the end of

the process because it is a consequence of the solution-state micro-nanoassembly

structures (ASHOKAN, WANG et al, 2018). The features in the solution aggregation

have been reported and appear to affect directly the morphology and performance of

TE devices (ZHENG, LEI et al, 2016). By tuning the solution-state structure, films with

high crystallinity and good interdomain connectivity were obtained from cosolvents

casting (ZHENG, YAO et al, 2017). This optimization led to higher mobilites and

highlighted the importance of properly choose the solvent in which the polymer and the

dopant will be dissolved and mixed.

Conductive polymer films consist of crystalline domains connected by

amorphous chains with microscale heterogeneity (ZHENG, YAO et al, 2017).

Conjugated polymers can be classified into three types according to their solid state

aggregation behavior and the paracrystallinity disorder: semicrystalline, disorder

aggregated, and completely amorphous polymers (NORIEGA, RIVNAY et al, 2013).

For polymers such as Poly(3-hexylthiophene-2,5-diyl) P3HT, considered

semicrystalline, there is a large volume ratio of ordered regions in their films with

crystallites/ordered aggregates. For disorder aggregated polymers, like donor-

acceptor polymers (PNDI2OD-T2, for example) ̶ which means that there is a group

with electrons-π present in the structure of these polymers (attributed to the donating

character) and another part, in the same molecule, that has high electron affinity (EA)

(responsible for the accepting character) ̶ regions of short-range ordering were

observed in the polymeric films, which may be the reflection of their close п-п distance,

large conjugated planes and strong electronic coupling between two repeating units.

With this, it, it was possible to infer that the efficient charge transport happens in an

interconnected network of ordered regions.

In contrast to the two organizations described above, there are also amorphous

polymers, which has low mobility attributable to their high level of disorder throughout

the film – the indacenodithiophene-co-benzothiadiazole copolymer (IDTBT) is one

exception, exhibiting high mobility and little crystalline microstructures than

semicrystalline polymers (VENKATESHVARAN, NIKOLKA et al, 2014). Simulations in

this paper justify this behavior as being as a result of a planar and torsion-free

19

backbone that is amazingly resilient to side-chain disorder, which facilitates the charge

carrier mobility in the material. Another important aspect is that, after doping, the

organizational aspects of chains and morphology, tend do be damaged by the increase

in the dopant ratio in the film, affecting the charge carrier transport. In general, the

moment morphology of the ordered regions in the film is destroyed, the carrier transfer

efficiency decreases notably (SCHLITZ, BRUNETTI et al, 2014). This is also known as

a disruption of the film.

2.5.2.3. Stability

Stability is the property of the material that allows to infer its resistance to

properties loss when exposed to operating conditions. In case of N-type

semiconductors, efficiency may drop when the material begins to degrade owing to air

exposure or to unstable operation at higher temperatures than the ones endured by

the material. Differing from the P-type polymers, most N-type semiconductors are

stable only under nitrogen atmosphere, because the carbanions are very unstable in

air, restricting the stability of these conductive polymer. When the N-doped polymers

are exposed to air, a redox reaction occurs with water and oxygen as follows

2H2O + 2e- H2 + 2OH-

2H2O O2 + 4H+ + 4e-

causing rapid reduction of electrical conductivity of N-doped polymers (SCHLITZ,

BRUNETTI et al, 2014). One way to prevent this redox reaction from happening is to

find or tailor the LUMO level of the N-type polymers so that it reaches values below -

4.7 eV, which is extremely challenging.

Two other factors contributing to the poor stability of these materials are the

dopant diffusion and the dopant escaping. The dopant migration creates spatially

dedoped regions responsible for drastically decrease the electrical properties. The low

stability of the N-type doped organic materials severely limits their application as OTE

devices.

20

2.5.3. Strategies towards the enhancement of the N-doping efficiency

As mentioned before, LUMO energy directly influences doping efficiency.

Decreasing the LUMO level in an N-type conductive polymer by means of halogenation

and how it affects the thermoelectric properties was studied by LIU et al, 2018c,

inserting Cl and F into the Poly[[1,2-dihydro-1-(4-octadecyldocosy-1- ( 4 -

octadecyldocosyl) – 2 – oxo 3 H - indol - 6 - yl - 3 - ylidene ] - ( 1 E ) - 1 , 2 - ethenediyl

[ 5 - chloro - 1 , 2 - dihydro - 1 - ( 4 -octadecyldocosyl)-2-oxo-3H-Indol-6 - yl - 3 - ylidene

] [ 2 , 6 - dioxobenzo [ 1 , 2 - b : 4 , 5 b ’ ]difuran-3,7(2H,6H)-diylidene] (BDPPV)

structure (Figure 11).

Figure 11: Chemical structures of BDPPV derivatives and N-type dopant NDMBI (SHI, ZHANG et al,

2015)

The energy levels of BDPPV derivatives were investigated using cyclic

voltammetry to elucidate the effect of the halogenation. This characterization showed

that both HOMO and LUMO were lowered in the derivative versions (Poly[[5-chloro –

1 , 2 - dihydro - 1 -(4-octadecyldocosy-1- ( 4 - octadecyldocosyl) – 2 – oxo 3H -indol-

6-yl-3-ylidene]-(1E)-1,2-ethenediyl [5-chloro - 1,2 – dihydro – 1 - (4-octadecyldocosyl)

– 2 – oxo - 3H – indol – -Indol-6 - yl - 3 - ylidene ] [ 2 , 6 - dioxobenzo [ 1 , 2 - b : 4 , 5

b ’ ]difuran-3,7(2H,6H)-diylidene]] (ClBDPPV) and Poly[[7-fluoro-1,2-dihydro - 1 - ( 4 -

octadecyldocosyl ) - 2 - oxo - 3 H - indol - 6 - yl - 3 - ylidene ] - (1E)-1,2-ethenediyl [ 7

- fluoro - 1 , 2 - dihydro - 1 - ( 4 - octadecyldocosyl ) - 2 - oxo - 3 H - indol - 6 - yl - 3 -

21

ylidene](2,6-dioxobenzo[1,2-b:4,5-b']difuran-3,7(2H,6H)-diylidene)] (FBDPPV)). The

ClBDPPV and FBDPPV LUMO levels reached -4.30 eV and -4.17 eV, respectively,

0.29 eV and 0.16 eV lower than that of BDPPV. In addition, the electrical conductivity

of BDPPV is 0.26 S/cm, while both ClBDPPV and FBDPPV showed surprising

conductivities above 4 S/cm. When the derivative FBDPPV was doped with NDMBI,

its conductivity reached a value of 14 S/cm, roughly two times higher than that shown

by ClBDPPV, indicating that the decrease in the LUMO level positively influenced the

doping efficiency which corroborates the XPS results in the work of SHI et al, 2015.

The good electrical performance resulted in a high Power factor of 28 uW/mK2 for the

FBDPPV at room temperature.

Another extremely promising material is the NDMBI-doped P(NDI2OD-T2)

reported by (SCHLITZ et al, 2014). In his work it was possible to notice the phase

segregation between polymer and dopant, making difficult to increase density of

charge carriers, since it hinders the homogenization of the dopant distribution and, in

turn, the injected charges. To overcome this issue, polar groups were inserted into the

polymer backbone. Due to this modification, a 200-fold increase in electrical

conductivity was achieved for the Poly { [ N , N′ - bis ( 2 - triethyleneglycol ) -

naphthalene - 1 , 4 , 5 , 8 - bis ( dicarboximide ) - 2 , 6 - diyl ] - alt - 5 , 5′ - ( 2 , 2′ -

bithiophene )} P(NDI2TEG-T2)) by replacing the alkyl side chains of the (P(NDI2OD-

T2) with polar triethylene glycol-based side chains (P(NDI2TEG-T2) that can be seen

in the Figure 12).

a)

) b)

)

22

Figure 12: Molecular structures of a) P(NDI2OD-T2) and, the modified, b) P(NDI2TEG-T2), and simulations of NDMBI molecules dissolved c) in pure P-NDI2OD-T2 and d) pure P-NDI2TEG-T2 (LIU,

QIU et al, 2018)

Although the pristine P(NDI2OD-T2) has a higher electron mobility compared to

the P(NDI2TEG-T2), the latter exhibits much higher charge carrier density in

consequence of its doping efficiency with NDMBI. The molecular dynamic simulation

corroborates the concept developed by the authors that the polar side chains improves

the dispersion of dopants in the host matrix in comparison with the alkyl chains (Figure

12, c and d, respectively). The logic of this method was effectively reproduced by other

groups, extolling the efficiency of this strategy (KIEFER, GIOVANNITTI et al, 2018).

Another strategy used to enhance the doping efficiency is the selection of N-

dopants. The N-type dopant design is much more difficult compared with P-type

dopants owing to their high HOMO level, which makes them prone to oxidation. As an

attempt to overcome this hindrance, air-stable precursor molecules that may become

intermediates for N-doping after thermal- or photoactivation have become quite

promising in this function (LÜSSEM, RIEDE et al, 2013).

Benzimidazole derivatives have been widely used, proving to be capable of

doping various organic semiconductor,s such as C60 and phenyl-C61-butyric acid

methyl ester (PCBM) (WEI, OH et al, 2010). NDMBI and its derivatives are examples

of N-dopants that are able to dope efficiently several polymers and small molecules

(SCHLITZ et al, 2014; SHI et al, 2015; LIU et al, 2018b; YANG et al, 2018). What is

intriguing about this is that the NDMBI HOMO level is approximately -4.45 eV, which

c)

))

d)

))

23

means it is deeper than most N-type polymers LUMO (~-4.0 eV) (YUAN et al, 2019).

In order to explain the high efficiency of NDMBI in the organic doping, (HUANG, YAO

et al, 2017) demonstrated that NDMBI undergoes different doping mechanisms when

reacting with organic molecules with different LUMO energy levels.

The ability to react according various mechanisms is why the NDMBI is

extensively used as an N-dopant. In addition to the benzimidazole dopants, there is

also the possibility of using their dimers. The dimer is the junction of two identical or

very similar molecules; it is a special case of polymer. Provided that it is two monomers

together, rather than contributing with only one electron, the dimer contributes with two,

making doping more pronounced when compared to its monomer, such as (DMBI)2

compared with NDMBI. This high reactivity is a reflection of its very particular doping

mechanism, which can occur according to a reversible endergonic cleavage of the

dimer followed by a rapid exergonic electron transfer or an endergonic electron transfer

followed by a rapid cleavage of the dimer cation and a second electron-transfer

reaction (ZHANG, NAAB et al, 2015) (Figure 13).

It is also the high reactivity of the dimer that makes it attractive as an N-dopant,

but one needs to know that not only the dopant efficiency matters, but also how the

same dopant interacts with the host material, how it fits in the morphology of the host

and how the Coulombic forces behave between them, among other aspects.

Therefore, even though these dopants became very attractive for their reactivity, little

is known about how the dimers fit into the polymeric matrix and until which point they

are beneficial to its properties.

24

Figure 13: N-doping pathways of a) DMBI derivatives and b) Organic dimer dopants (UEBE,

YOSHIHASHI et al, 2018)

25

III - EXPERIMENTAL

26

Below is exhibited a Fluxogram (Figure 14) with simplified steps followed

throughout this work from substrate and solution preparation to the characterization of

the samples.

Figure 14: Fluxogram exhibiting the methodology adopted in the present work

27

3.1. Substrate and Devices preparation

Initially the glass wafer was cut using a diamond pen into a 20 x 20 mm

substrate. The following step was to clean it with an ultrasonic bath using deionized

water, acetone and isopropanol alcohol (IPA), one solvent at a time, subsequently, for

10 minutes at ambient temperature. After cleaning, the substrate was treated in plasma

for 10 minutes to eliminate the presence of any impurities from the cleaning solvents

and also to promote better homogeneity of the film to be coated later.

Subsequently, the substrate was taken to the Resistive Thermal Evaporator

from Angstrom for the deposition of electrodes made of 0.30 nm of gold, approximately,

according to the mask shown in Figure 15. Electrode pairs of 30, 40, 50, 80 and 100

um in channel length made up the evaporation mask, with 10 pairs each length,

resulting in 50 electrode pairs in total. The structure used was bottom contact, since

the electrodes stay in direct contact with the substrate and the film is coated on top of

it.

3.2. Solution preparation

Since the N-type organic semiconductors are not stable in air, the preparation

of solutions and devices took place inside a glove box under nitrogen atmosphere. The

host material, N2200 (P(NDI2OD-T2)) supplied by 1-Material, was weighed on a

balance inside the glove box and then dissolved in Chlorobenzene. The same

Figure 15: Transistor mask used to evaporate the electrodes

28

procedure was repeated for the dopant, (NDMBI)2. These two solutions became stock

solutions, both with 10 g/L concentration, for the preparation of the doped solutions.

In 1 mL vials, N2200 solutions with 5, 10, 15, 20, 25 and 30% in mol of the dimer

(NDMBI)2 were prepared at room temperature inside a glove box under Nitrogen

atmosphere. The calculations were made in Excel, taking into account the equations

below, step by step.

First it is calculated the Molar Concentration (Mx) in mol/L – where x can be ‘h’

refering to the host or ‘d’ refering to the dopant – dividing the Normal Concentration

desired (C), in g/L, by the Molecular Weight of the compound (Mwx) – same case for x

– in g/mol.

𝑀𝑥 =C

Mwx

Then, the percentage aimed to be investigated in the study (%) multiplies the

Molar concentration (Mx) of each individual compound (either the host or the dopant),

divided by 100, since it is used a percentage in the calculus. In this way, the Ratio (Rx)

is obtained.

𝑅𝑥 =% ∗ Mx

100

The next step is to calculate the quantity of solution needed. For this purpose,

the Volume of solution desired (V) in L multiplies the Mwx times Rx, divided by C results

in the Partial volume V’x.

𝑉′𝑥 =V ∗ Mwx ∗ Rx

C

The ‘x’ can be taken as ‘h’ or ‘d’ when calculating the host or the dopant amount,

respectively, in all the formulas.

29

3.3. Spin Coating

The films were spin coated (40 μL of the doped solution, 10 g/L in

chlorobenzene) on the glass substrates with the electrodes already deposited. The

speed used was 1000 RPM for 30 seconds with 5 seconds of acceleration ramp. All

the process was held under nitrogen atmosphere and without temperature contribution

during the coating. After spin coating, the samples went through a consolidation step

that is the thermal annealing at 130ºC for 10 minutes to ensure the solvent exit. Similar

parameters have already been reported for the N2200-NDMBI system (LIU et al,

2018b).

3.4. Characterizations

3.4.1. Optical and Morphology Characterization and EPR

The Electron Paramagnetic Resonance (EPR) or electron spin resonance

(ESR) spectroscopy is a technique used to investigate materials with unpaired

electrons. In this case, the EPR was applied mostly as a sensitive method to the

formation of the radicals (polarons) formed by the chemical reaction of the dopant with

the host, attesting the doping.

The Cary 5000 UV-Vis-NIR spectrometer was used to measure the UV-NIR

absorption of the solutions and films produced in order to ensure the doping as well.

The measurement in solution was performed by a highly diluted solution, in which the

10 g/L solutions were slowly diluted in 4 mL Chlorobenzene in a 4 mL vial, and then

transferred to quartz cubets to continue the characterization. The same equipment was

used for the spin coated thin films on 20 x 20 mm glass substrates without electrodes.

The AFM was of the NT-MDT model, performed also the Kelvin Probe

Microscopy (KPM) modality – also known as surface potential microscopy. These

techniques allowed to elucidate not only facts about the morphology of the films, but

also about the stability of the proposed material.

30

3.4.2. Electrical and Thermoelectrical measurements

Electrical measurements were obtained using a two-point probe station in

nitrogen atmosphere. Tungsten-tipped micromanipulators were used to pierce the films

and make electrical contact with the Au electrodes at the bottom.

The Thin Film Analyzer (TFA) from Linseis was used to measure thermoelectric

properties such as Seebeck coefficient, thermal conductivity and electrical

conductivity. A 30 g/L solution of N2200 was doped with a 10 g/L solution of (DMBI)2

and then drop casted in a microchip membrane for the measurement.

31

IV - RESULTS AND DISCUSSION

32

The EPR spectrum can be observed below. As expected, the N2200 pristine

sample shows no response to the EPR excitation, however, as the doping occurs, the

stimulus response arises and becomes increasingly intense as the dopant ratio

increases. It is important to note that the more intense the signal, the more polarons

were created during the doping process. In the EPR, it is perceived that the the dimer

contributes to a greater amount of delocalized charges (polarons) in the 30% doped

sample than in the 20% and 10% doped samples, which in turn present quite similar

intensities.

To gain insight in the doping processes, the UV-vis absorption spectrum was

obtained in solution and in film state. Both spectra can be seen in Figure 19. The

pristine P(NDI2OD-T2) shows two characteristic neutral features centered at 400 nm

and 700 nm which are assigned to the π-π* transition and an intramolecular charge

transfer band, respectively (LIU et al, 2018c). As the N2200 is doped with (NDMBI)2,

the neutral spectrum transition peaks gradually decrease in intensity. This is

accompanied by the appearance of additional peaks at 500 nm and 820 nm. These

Figure 16: EPR of four representative samples: pristine N2200, 10, 20 and 30% of (NDMBI)2

33

new spectrum features are attributed to polaron-induced transitions, and are

considered evidence that the polymer was effectively doped (WANG et al, 2018).

Surprisingly, contradicting what has been reported in the literature on the monomer,

the dimer is able to dope the N2200 when they are still in the solution state (SCHLITZ

et al, 2014), and not just when the film is consolidated after solvent removal.

However, with the N2200-(NDMBI)2 system, due to the strong electron donating

character of the dimer as previously discussed in the literature review, the doping could

already be observed in solution, becoming noticeable because of the solution color

change from light blue to grey. The 550 nm and 820 nm peaks began to appear,

followed by the quenching of the 700 nm peak, characterizing the doping process and,

thus, the polaron formation. The trend followed by the concentrations is not continuous,

but this may be justified by the sample preparation, since this method need to be

performed using a extremely diluted solution. The error possibly arose from the use of

a fraction of the doped solution which, in consequence to the heterogeneities, did not

correspond to the expected dopant/matrix ratio.

After solution analysis, the films were spin coated. The UV-vis spectrum for the

films can be seen in Figure 19b. The concentration increase of the dimer in the N2200

was shown to be accompanied by a sharp decrease of the peak intensity around 700

nm, along with a much more pronounced rise of the two previously described features

at 550 nm and 820 nm, reaching an optimal point, with very distinct peaks, in the 20%

doped sample (SCHLITZ et al, 2014). The dopant concentration increase was also

followed by a dramatic change in the film colors going from blue to grey. The red shift

observed in the UV-vis spectrum explained by a change in the electronic structure that

reflects electronic transitions between the LUMO and the HOMO (LIU et al, 2018c).

34

Thanks to the UV-Vis and EPR results, it was possible to ensure that the doping

process was successful, but it was not yet possible to confirm which concentration

would correspond to the best electrical conductivity. With this, it was decided to perform

the electrical characterization. This has made it possible to measure which ratio would

be considered as the optimal point for the electrical conductivity using the two-point

probe station under nitrogen atmosphere.

Using the two-point probe station under nitrogen atmosphere, the graph in

Figure 20 was obtained. Up to a certain dopant concentration, the Coulombic forces

are so strong that they hinder the charge carriers delocalization and flow (LU; WANG;

PEI, 2019). Thereunto, it is necessary to increase the dopant amount until it is possible

to overcome the potential barrier created by the Coulombic forces and thus increase

the electrical conductivity. Looking at the graph it is easy to see that arguably the 20%

doped sample is the one with largest amount of dopant incorporated with the highest

electrical conductivity, henceforth being considered the optimum dimer concentration

point in the N2200 due to the 4 orders increase in the electrical conductivity when

compared to slightly doped samples, which are comparable to the neat material N2200

(SCHLITZ et al, 2014). A drastic decay in the electrical performance of the sample is

also noted with the concentration increase beyond 20%, demonstrating a deterioration

of the electrical properties occasioned by excess of dopant (HOFMANN; KROON;

MÜLLER, 2019b). This phenomenon is reported in the literature as segregation of

dopant and matrix, causing the formation of dopant islands that eventually disrupt the

Figure 17: UV-vis spectra of the 5, 10, 15, 20, 25 and 30% doped samples a) in solution and b) in film state

a) b)

35

continuity of the film, compromising the electronic mobility and, therefore, the electrical

conductivity (SCHLITZ, BRUNETTI et al, 2014).

The Thin Film Analyzer (TFA) returned the data of three parameters in order to

characterize the thermoelectric properties: electrical conductivity, thermal conductivity

and Seebeck coefficient. The samples chosen for this characterization were the best

ones shown by the previous methos: 10, 15 and 20%. However, the 15% doped

sample was discarded owing to inconsistent results. This inconsistency attributable to

to the solvent evaporation of the stock solutions, resulting in a different concentration

from the expected and, therefore, not corresponding to the expected dopant/host ratio.

By measuring the electrical conductivity in the Linseis, it was possible to

confirm the average values of electrical conductivity obtained using the two-point probe

station, proving the reliability of both methods. At 25ºC, the 20% shows an electrical

conductivity of 6 . 10-3 S/cm corroborating with the electrical conductivity obtained by

the two-point probe method, higher than the value for 10% but, still, very similar. The

Figure 18: Electrical conductivity (S/cm) in function of the dimer concentration (% mol)

36

conductivity increase with the temperature is assignable to the higher amount of

injected electrons owing to the energy supplied by the system.

The thermal conductivity as a function of the temperature is shown in the graph

in Figure 22. This result confirms the electron dominated conductivity and not a

phonon dominated, as expected. A phonon dominated thermal conductivity enhances

with the temperature increase as the heat supplied causes the crystal lattice to vibrate

more intensely. On the other hand, when the thermal conductivity is electron

dominated, the temperature increase has no influence on this coefficient, which

remains unchanged (OLUWALOWO et al, 2019). The latter behavior can be observed

in this graph, meaning that even though the host polymer is highly packed and

paracrystalline, it still does not present the phonon dominated thermal conductivity.

The 20% doped sample has a higher thermal conductivity in response to the presence

of more charge carriers descendant from the doping process than the 10% doped

sample.

Figure 19: Electrical conductivity as a function of the Temperature

37

Figure 20: Thermal conductivity as a function of the Temperature

The Seebeck coefficient is shown in Figure 23. From the negative sign of the

Seebeck it can be confirmed the N-type character of the material after doping. Some

materials may change their character after doping, becoming P-type or the reverse,

justifying the importance of this confirmation (LIU et al, 2018b). Another notable aspect

is the decrease, in modulus, of the Seebeck with increasing temperature. This is a

result of thermally activated charge carrier injection and Seebeck anticorrelation with

electrical conductivity and charge carriers density enhancement. At 30ºC, the 10%

doped sample exhibited a -80 uV/K Seebeck and the 20% a -66 uV/K. The reason why

the 20% doped sample showed a lower Seebeck modulus is the higher charge carrier

density compared to the 10% doped sample.

38

Figure 21: Seebeck coefficient as a function of the Temperature

To better understand the doping impact, the AFM was used to elucidate the

surface morphology of the film (Figure 24). As can be seen in the following images,

four representative samples were chosen - pristine (N2200), 10%, 20% and 30% - and

analyzed immediately after spin coated and annealed. The pristine sample (Figures

24 a and b) gives an idea that the material exhibits a tendency for a fibrillar

microstructure with a very smooth surface and a scale ranging from 0 to 5 nm without

any aggregates.

Once the 10, 20 and 30% doped samples were analyzed, it was possible to

observe the formation of aggregates in the morphology image (Height) of the 30%

doped sample, but not in the Phase image, in which the surface shows a homogeneous

coloration, indicating that there are no different types of materials heterogeneously

distributed on the film surface (YANG et al, 2018). But even in this case, it is possible

to see some aggregates that appear clearly in the 30% doped sample, but are not

perceived in either the 10% or the 20% doped sample. These aggregates are also

called dopant islands, based on the nature of the material being segregated (SCHLITZ,

BRUNETTI et al, 2014). In the 30% doped sample, even though the aggregates did

39

not show to be above the surface, their presence in the bulk and close to the surface

give a clue that instead of boosting the electrical conductivity with an effective doping

process and good miscibility, the material segregation was taunting the exact opposite

effect (HOFMANN; KROON; MÜLLER, 2019b). This explains why the 30% doped

sample was becoming more resistive.

Figure 22: AFM of the 0 (a and b), 10 (c and d), 20 (e and f), 30% (g and h) doped samples.

Morphology and Phase signals, first and second columns, respectively

a b

c d

e f

h g

40

In an attempt to prove that the aggregates seen in the 30% doped sample were

islands of dopant, the same doped samples were left to age without temperature

influence. After aging, AFM was performed again resulting in Figure 25. This time all

the samples exhibited clearly segregated areas, the supposedly dopant islands;

however, once again nothing conclusive was observed in the Phase signal (second

column), which exhibits homogeneous colors, making it impossible to infer that the

composition of the aggregates and the film was different. Arguably the aggregates

appeared in larger amounts when the dopant concentration increased, with the 30%

doped sample showing the largest amount of aggregates than the two other samples.

This result supports the need to optimize the miscibility and compatibility of the system.

Knowing this, some strategies have already been reported in the literature

aiming to enhance the compatibility of the materials used and, in this sense, their

miscibility. The miscibility is influenced by side chains, and the modification of these is

one of the most effective ways to improve the miscibility between host and dopant.

Some studies have shown that replacing the alkyl side chains with polar side chais

promotes increased miscibility in the system and, thus, higher efficiency of doping; this

is assignable to a better interaction between the benzoimidazole dopant and the host

N2200-glycolated (KROON, KIEFER et al, 2017). After the characterization of the aged

samples, it was still necessary to understand the composition of the aggregates in

order to confirm their nature. Thus, a new 30% doped sample was exposed to a

temperature high enough for it to degrade (200ºC) to promote a dopant bloom above

the surface and make it possible to detect its presence performing the AFM with the

Phase signal.

41

Figure 23: AFM of the 10 (a and b), 20 (c and d) and 30% (e and f) aged samples without influence of

the Temperature. Morphology and Phase signals, first and second columns, respectively

a b

c d

e f

42

The 30% doped sample was chosen because it was the sample with the highest

dopant content among the others, contributing to identify the effect of dopant migration

and escape more easily. The sample was exposed to 200⁰C for 20 minutes and taken

to AFM shortly thereafter. This time, besides the Height and Phase signals, the Kelvin

Probe Microscopy (KPM) was included in the analysis. This latter technique is also

called Surface Potential Microscopy. The scan area chosen was as small as possible

given the presence of aggregates. The reason for that is the distance between the tip

and the surface in the KPM - which is a non-contact scan – is very small, and at the

moment that the tip touches the surface due to the presence of aggregates, the image

is ruined and, if the sample is too soft, the tip can also scratch the surface of the film.

This time, with the 5 x 5 μm scan area, it was possible to notice small dots higher

than the aggregates in the Height that appeared in a different color in the Phase image

(Figure 26 a and b), meaning that these dots had a different nature from the rest of

the film and, as they were blooming from the aggregates, it was possible to afirm that

those were indeed the dopant islands.

Going further, since these dots were dimer aggregates, when performing the

KPM it would be possible to observe a darker area surrounding the dots, since the

dopant was responsible for injecting electrons into the host and, as a result of the

aggregation, the process was not so efficient, retaining electrons in these regions.

Once they aggregate, the dopant island gets negatively charged in comparison to the

host material, that should appear lighter in the image. The Figure 26 c brings the KPM

of the 30% doped sample. As expected, what was explained previously was observed

in the KPM image with the color scale translated in 200 mV of potential difference and

dark areas - highlighted with a red circle - corresponding to the dopant islands circling

the dots, confirming the hypothesis of dopant blooming.

43

Figure 24: Surface potential difference microscopy of the 30% doped sample. Morphology, Phase and KPM signals, from left to right

a

b

c

44

45

V - CONCLUSIONS AND FUTURE OUTLOOK

46

5.1 Conclusions

Recently, the development of technologies for green energy harvesting and

generation has become a priority. Organic thermoelectric devices meet the

requirement using a temperature gradient that is converted into electricity and vice

versa. To achieve na optimal performance, a complementary P-type (hole transporting)

and N-type (electron transporting) architecture is required. However, while high

performance P-type organic systems are already well-stablished by having achieved

comparable values of conductivity and Seebeck comparable to the inorganic

semiconductors within the same application, high performing electron transporting

materials (N-type) are still scarce, hindering the advance of thermoelectric devices.

Throughout this work, it was possible to tune the electrical conductivity of the N-type

organic semiconductor N2200 by means of solution doping introducing the (NDMBI)2.

The EPR and the UV-vis ensured the doping process under the conditions

adopted. The appearance of an intense EPR signal demonstrated the occurrence of

doping after the dimer incorporation to the host (N2200), this addition provoked the

formation of delocalized charge carriers, also known as polarons, that in practical ways

are the unpaired electrons detected by the EPR; the more the dimer content was

increased, the more intese was the signal for the 10%, 20% and 30% doped samples,

highlighting the last one. The UV-vis showed a peak growth around 550 and 820 nm,

in solution and film state, corresponding to polaron transition peaks; the 20% doped

sample showed the most well defined polaron peaks.

The two-point probe station revealed an average conductivity of 6 . 10-3 S/cm

for the 20% doped sample. This reveals a lower electrical conductivity than the one of

the N2200-NDMBI system, contradicting the expectation of better performance due to

the injection of two electrons with the dimer instead of only one as in the monomer, but

the N2200-dimer still exhibits a 4 orders improvement when compared to N2200 neat

material and its slightly doped samples, praising the success of the doping with the

intention of tuning the electrical conductivity of the N-type semiconductor N2200.

The TFA provided the Thermoelectrical parameters such as electrical

conductivity, thermal conductivity and Seebeck coefficient for the 10% and 20% doped

samples, all these parameters as a function of the Temperature (ºC). As expected, the

20% doped sample exhibited higher values of thermal and electrical conductivity in

comparison with the 10% doped sample because of the higher charge carrier density

47

of the first. The electrical conductivity increases when the temperature increases, but

the thermal conductivity reamins unchanged. In addition, the electrical conductivity

holds the same value as the average obtained with the two-point probe station 6 . 10-

3 S/cm. The modulus of the Seebeck coefficient measured for the 10% doped sample,

-80 uV/K, is higher than the one for the 20%, -66 uV/K. This can be explained by the

anticorrelation between Seebeck and electrical conductivity, and thus, the charge

carrier density.

Finally, the AFM and KPM were successfully used to confirm and justify the poor

performance of the samples with a dimer content above 20%, elucidating the

morphological aspects of the N2200-(NDMBI)2 system as proposed at the beginning

of this work. The dopant islands were identified through different signals: height, phase

and surface potential difference. The aggregation reflects the poor miscibility of the

dopant in the matrix, being the cause of the lower performance when compared to the

system N2200-NDMBI.

5.3 Future Outlook

The findings exposed here craft promising paths for future enhancements in the

thermoelectric performance of N-type organic semiconductor materials. In the future,

the influence of additives such as compatibilizers and antioxidants need to be focused

in order to improve the electrical conductivity by increasing the stability and the doping

efficiency. Additionally, a rational synthetic modification of the side chains substituting

the alkyl side chains for glycolated side chains could also provide a better miscibility of

the dopant in the matrix.

With the data presented in this thesis, it is still necessary the Transmission

Electron Microscopy in order to comprehend the effect of the doping in the

microstructure within the film, below the surface, so it would be possible to explain

deeply the organization attributed as an effect of the doping process.

48

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