DISSERTAÇÃO DE MESTRADO · 2021. 7. 7. · centro de ciÊncias exatas e da terra (ccet) programa...
Transcript of DISSERTAÇÃO DE MESTRADO · 2021. 7. 7. · centro de ciÊncias exatas e da terra (ccet) programa...
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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
REFERENCES
ASHOKAN, A. et al. Impact of solution temperature-dependent aggregation on the solid-state packing and electronic properties of polymers for organic photovoltaics. Journal of Materials Chemistry C, v. 6, n. 48, p. 13162–13170, 2018. BÄSSLER, H.; KÖHLER, A. Charge transport in organic semiconductors. Topics in Current Chemistry, v. 312, p. 1–65, 2012. BHARTI, M. et al. Conductive polymers for thermoelectric power generation. Progress in Materials Science, v. 93, p. 270–310, 2018. BLACKBURN, J. L. et al. Carbon-Nanotube-Based Thermoelectric Materials and Devices. Advanced Materials, v. 30, n. 11, p. 1–35, 2018. BREDAS, J. L.; STREET, G. B. Polarons, Bipolarons, and Solitons in Conducting Polymers. Accounts of Chemical Research, v. 18, n. 10, p. 309–315, 1985. BUBNOVA, O.; CRISPIN, X. Towards polymer-based organic thermoelectric generators. Energy and Environmental Science, v. 5, n. 11, p. 9345–9362, 2012. CAMPOY-QUILES, M. Will organic thermoelectrics get hot? Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, v. 377, n. 2152, 2019. CHEN, Y.; ZHAO, Y.; LIANG, Z. Solution processed organic thermoelectrics: Towards flexible thermoelectric modules. Energy and Environmental Science, v. 8, n. 2, p. 401–422, 2015. CHU, W. et al. Experimental study on the energy harvesting of a cooktop via thermoelectric module assisted with phase change material. Energy Storage, v. 1, n. 2, p. e55, 2019. COWEN, L. M. et al. Review—Organic Materials for Thermoelectric Energy Generation. ECS Journal of Solid State Science and Technology, v. 6, n. 3, p. N3080–N3088, 2017. DING, L. C.; AKBARZADEH, A.; DATE, A. Performance and reliability of commercially available thermoelectric cells for power generation. Applied Thermal Engineering, v. 102, p. 548–556, 2016. DISALVO, F. J. Thermoelectric cooling and power generation. Science, v. 285, n. 5428, p. 703–706, 1999. DONGMIN KANG, S.; JEFFREY SNYDER, G. Charge-transport model for conducting polymers. Nature Materials, v. 16, n. 2, p. 252–257, 2017. GAO, X.; HU, Y. Development of n-type organic semiconductors for thin film transistors: A viewpoint of molecular design. Journal of Materials Chemistry C, v. 2, n. 17, p. 3099–3117, 2014. GIRI, G. et al. Effect of solution shearing method on packing and disorder of organic semiconductor polymers. Chemistry of Materials, v. 27, n. 7, p. 2350–2359, 2015. GOLDSMID, H. J. Introduction to Thermoelectricity (Springer Series in Materials Science). [s.l: s.n.]. HOFMANN, A. I.; KROON, R.; MÜLLER, C. Doping and processing of organic semiconductors for plastic thermoelectrics. Handbook of Organic Materials for Electronic and Photonic Devices, p. 429–449, 2019a. HOFMANN, A. I.; KROON, R.; MÜLLER, C. Doping and processing of organic semiconductors for plastic thermoelectrics. Handbook of Organic Materials for Electronic and Photonic Devices, p. 429–449, 2019b. HUANG, D. et al. Conjugated-Backbone Effect of Organic Small Molecules for n-Type Thermoelectric Materials with ZT over 0.2. Journal of the American Chemical Society, v. 139, n. 37, p. 13013–13023, 2017.
49
JACOBS, I. E. et al. Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: Effect of doping-induced aggregation on film morphology. Journal of Materials Chemistry C, v. 4, n. 16, p. 3454–3466, 2016. KIEFER, D. et al. Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics. ACS Energy Letters, v. 3, n. 2, p. 278–285, 2018. KIM, G. H. et al. Engineered doping of organic semiconductors for enhanced thermoelectric efficiency. Nature Materials, v. 12, n. 8, p. 719–723, 2013. KIM, N. et al. Highly conductive PEDOT:PSS nanofibrils induced by solution-processed crystallization. Advanced Materials, v. 26, n. 14, p. 2268–2272, 2014. KIM, N. K. et al. High-Performance Organic Field-Effect Transistors with Directionally Aligned Conjugated Polymer Film Deposited from Pre-Aggregated Solution. Chemistry of Materials, v. 27, n. 24, p. 8345–8353, 2015. KROON, R. et al. Thermoelectric plastics: From design to synthesis, processing and structure-property relationships. Chemical Society Reviews, v. 45, n. 22, p. 6147–6164, 2016. KROON, R. et al. Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p-Doped Polythiophene. Advanced Materials, v. 29, n. 24, p. 1–7, 2017. LEBLANC, S. Thermoelectric generators: Linking material properties and systems engineering for waste heat recovery applications. Sustainable Materials and Technologies, v. 1, p. 26–35, 2014. LETCHER, T. M. Why do we have global warming? [s.l.] Elsevier Inc., 2019. LIU, J. et al. Enhancing Molecular n-Type Doping of Donor–Acceptor Copolymers by Tailoring Side Chains. Advanced Materials, v. 30, n. 7, p. 1–9, 2018a. LIU, J. et al. N-Type Organic Thermoelectrics of Donor–Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States. Advanced Materials, v. 30, n. 44, 2018b. LIU, J. et al. N-Type Organic Thermoelectrics of Donor–Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States. Advanced Materials, v. 30, n. 44, 2018c. LU, Y.; WANG, J. Y.; PEI, J. Strategies to Enhance the Conductivity of n-Type Polymer Thermoelectric Materials. Chemistry of Materials, v. 31, n. 17, p. 6412–6423, 2019. LU, Z. et al. Silk fabric-based wearable thermoelectric generator for energy harvesting from the human body. Applied Energy, v. 164, p. 57–63, 2016. LÜSSEM, B.; RIEDE, M.; LEO, K. Doping of organic semiconductors. [s.l: s.n.]. v. 210 MA, W. et al. Enhanced Molecular Packing of a Conjugated Polymer with High Organic Thermoelectric Power Factor. ACS Applied Materials and Interfaces, v. 8, n. 37, p. 24737–24743, 2016. MEHDIZADEH DEHKORDI, A. et al. Thermoelectric power factor: Enhancement mechanisms and strategies for higher performance thermoelectric materials. Materials Science and Engineering R: Reports, v. 97, n. September 2019, p. 1–22, 2015. MILIĆ, D. et al. Characterization of commercial thermoelectric modules for application in energy harvesting wireless sensor nodes. Applied Thermal Engineering, v. 121, p. 74–82, 2017. MOHSENZADEH, M.; SHAFII, M. B.; JAFARI MOSLEH, H. A novel concentrating photovoltaic/thermal solar system combined with thermoelectric module in an
50
integrated design. Renewable Energy, v. 113, p. 822–834, 2017. NAAB, B. D. et al. Role of Polymer Structure on the Conductivity of N-Doped Polymers. Advanced Electronic Materials, v. 2, n. 5, 2016. NAVA, D. et al. Drastic Improvement of Air Stability in an n-Type Doped Naphthalene-Diimide Polymer by Thionation. ACS Omega, v. 1, n. 9, p. 4626–4634, 2018. NGUYEN, T. H.; O’LEARY, S. K. The dependence of the Fermi level on temperature, doping concentration, and disorder in disordered semiconductors. Journal of Applied Physics, v. 88, n. 6, p. 3479–3483, 2000. NORIEGA, R. et al. A general relationship between disorder, aggregation and charge transport in conjugated polymers. Nature Materials, v. 12, n. 11, p. 1038–1044, 2013. OLUWALOWO, A. et al. Electrical and thermal conductivity improvement of carbon nanotube and silver composites. Carbon, v. 146, p. 224–231, 2019. QI, Y. et al. Solution doping of organic semiconductors using air-stable n-dopants. Applied Physics Letters, v. 100, n. 8, 2012. QUINN, J. T. E. et al. Recent progress in the development of n-type organic semiconductors for organic field effect transistors. Journal of Materials Chemistry C, v. 5, n. 34, p. 8654–8681, 2017. RUSS, B. Design Rules for Solution-Processable n-type Organic Thermoelectric Materials By. 2015. RUSS, B. et al. Organic thermoelectric materials for energy harvesting and temperature control. Nature Reviews Materials, v. 1, n. 10, 2016. SCHLITZ, R. A. et al. Solubility-limited extrinsic n-type doping of a high electron mobility polymer for thermoelectric applications. Advanced Materials, v. 26, n. 18, p. 2825–2830, 2014. SHI, K. et al. Toward High Performance n-Type Thermoelectric Materials by Rational Modification of BDPPV Backbones. Journal of the American Chemical Society, v. 137, n. 22, p. 6979–6982, 2015. SHIN, Y. et al. Improving Miscibility of a Naphthalene Diimide-Bithiophene Copolymer with n-Type Dopants through the Incorporation of “Kinked” Monomers. Advanced Electronic Materials, v. 4, n. 10, p. 1–8, 2018. SUN, Y. et al. Organic thermoelectric materials and devices based on p- and n-type poly(metal 1,1,2,2-ethenetetrathiolate)s. Advanced Materials, v. 24, n. 7, p. 932–937, 2012. SUN, Y. et al. Advances in n-Type Organic Thermoelectric Materials and Devices. Advanced Electronic Materials, v. 1800825, p. 1–27, 2019a. SUN, Y. et al. Advances in n-Type Organic Thermoelectric Materials and Devices. Advanced Electronic Materials, v. 5, n. 11, p. 1–27, 2019b. TANG, J. et al. Recent Advances in n-Type Thermoelectric Nanocomposites. Advanced Electronic Materials, v. 5, n. 11, p. 1–15, 2019. THOMAS, E. M. et al. Role of Disorder Induced by Doping on the Thermoelectric Properties of Semiconducting Polymers. Chemistry of Materials, v. 30, n. 9, p. 2965–2972, 2018. UEBE, M. et al. A dendritic oligoarylamine-substituted benzimidazole derivative as a useful n-type dopant. Journal of Materials Chemistry C, v. 6, n. 24, p. 6429–6439, 2018. VENKATESHVARAN, D. et al. Approaching disorder-free transport in high-mobility conjugated polymers. Nature, v. 515, n. 7527, p. 384–388, 2014. WANG, C. et al. Optical measurement of doping efficiency in poly(3-hexylthiophene)
51
solutions and thin films. Physical Review B - Condensed Matter and Materials Physics, v. 91, n. 8, p. 1–7, 2015. WANG, C.; CALDERÓN, C.; WANG, Y. D. An experimental study of a thermoelectric heat exchange module for domestic space heating. Energy and Buildings, v. 145, p. 1–21, 2017. WANG, H.; YU, C. Organic Thermoelectrics: Materials Preparation, Performance Optimization, and Device Integration. Joule, v. 3, n. 1, p. 53–80, 2019. WANG, S. et al. A Chemically Doped Naphthalenediimide-Bithiazole Polymer for n-Type Organic Thermoelectrics. Advanced Materials, v. 30, n. 31, p. 1–6, 2018. WANG, Y. et al. Naphthodithiophenediimide-Benzobisthiadiazole-Based Polymers: Versatile n-Type Materials for Field-Effect Transistors and Thermoelectric Devices. Macromolecules, v. 50, n. 3, p. 857–864, 2017. WEI, P. et al. Use of a 1 H -benzoimidazole derivative as an n -type dopant and to enable air-stable solution-processed n -channel organic thin-film transistors. Journal of the American Chemical Society, v. 132, n. 26, p. 8852–8853, 2010. WELSH, T. A.; LAVENTURE, A.; WELCH, G. C. Direct (hetero)arylation for the synthesis of molecular materials: Coupling thieno[3,4-c]pyrrole-4,6-dione with perylene diimide to yield novel non-fullerene acceptors for organic solar cells. Molecules, v. 23, n. 4, 2018. YANG, C. Y. et al. Enhancing the n-Type Conductivity and Thermoelectric Performance of Donor–Acceptor Copolymers through Donor Engineering. Advanced Materials, v. 30, n. 43, p. 1–9, 2018. YAO, C. J.; ZHANG, H. L.; ZHANG, Q. Recent progress in thermoelectric materials based on conjugated polymers. Polymers, v. 11, n. 1, p. 1–19, 2019. YUAN, D. et al. Cholesteric Aggregation at the Quinoidal-to-Diradical Border Enabled Stable n-Doped Conductor. Chem, v. 5, n. 4, p. 964–976, 2019. ZHANG, S. et al. N-Dopants Based on Dimers of Benzimidazoline Radicals: Structures and Mechanism of Redox Reactions. Chemistry - A European Journal, v. 21, n. 30, p. 10878–10885, 2015. ZHANG, Y. et al. Recent advances in organic thermoelectric materials: Principle mechanisms and emerging carbon-based green energy materials. Polymers, v. 11, n. 1, 2019. ZHENG, Y. Q. et al. Strong Electron-Deficient Polymers Lead to High Electron Mobility in Air and Their Morphology-Dependent Transport Behaviors. Advanced Materials, v. 28, n. 33, p. 7213–7219, 2016. ZHENG, Y. Q. et al. Unraveling the Solution-State Supramolecular Structures of Donor–Acceptor Polymers and their Influence on Solid-State Morphology and Charge-Transport Properties. Advanced Materials, v. 29, n. 42, p. 1–8, 2017. ZUO, G. et al. High Seebeck Coefficient in Mixtures of Conjugated Polymers. Advanced Functional Materials, v. 28, n. 15, p. 1–7, 2018.