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
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
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
To my beloved family My father, José Roberto, my mother, Mariza Oliveira, my brother, Anderson Sousa and my sister, Alessandra Sousa Because they exist
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)
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.
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.
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.
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
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
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)
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
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.
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
I - INTRODUCTION
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
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.
II - LITERATURE REVIEW
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
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)
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
=
𝑺𝟐𝝈𝑻𝒌
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
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.
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).
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).
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
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
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,
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
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.
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)
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)
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
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.
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 -
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) )
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) ))
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.
Figure 13: N-doping pathways of a) DMBI derivatives and b) Organic dimer dopants (UEBE, YOSHIHASHI et al, 2018)
III - EXPERIMENTAL
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
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
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.
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.
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.
IV - RESULTS AND DISCUSSION
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
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).
