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Table of Contents
ABSTRACT i
摘要 ii
ACKNOWLEDGEMENTS iii
TABLE OF CONTENTS iv
LIST OF FIGURES vi
LIST OF TABLES viii
CHAPTER 1 INTRODUCTION 1
Background 1
Purpose 3
CHAPTER 2 OVERVIEW 4
Organic Semiconductor 4
Organic Semiconductor based on Dithienothiophene (DTT) Derivatives 10
Organic Thin Film Transistor (OTFT) Overview 13
CHAPTER 3 EXPERIMENTAL SECTION 17
Materials and Methods 17
Materials 17
Methods 18
Nuclear Magnetic Resonance (NMR) 19
Ultraviolet Visible Spectrometer (UV-Vis) 19
Differential Scanning Calorimetric (DSC) 19
Thermo Gravimetric Analysis (TGA) 19
Electrochemical Analyzer-Differential Pulse Parameter (DPV) 20
Synthesis 20
Synthetic Scheme Route 20
Syntethic of DTT 20
Synthetic of DDTT-SBT14 21
Synthetic of DDTT-SBT18 22
CHAPTER 4 RESULTS AND DISCUSSION 30
Synthesis 30
Molecular Characterization 31
CHAPTER 5 CONCLUSIONS 38
REFERENCE 39
CHAPTER 6 APPENDIX 42
LIST OF FIGURES
Figure 2.1 The application of Organic Semiconductor 4
Figure 2.2 Energy diagram, valence and conduction bands 7
Figure 2.3 Different possible molecular packing motifs existing in organic solid states 9
Figure 2.4 Chemical structure of some representative p-channel organic semiconductors 11
Figure 2.5 DP-DTT compound 12
Figure 2.6 Schematic OTFT 15
Figure 2.7 Illustration of OTFT application 16
Figure 4.1 Optical spectra of DDTT-SBT14 32
Figure 4.2 Optical spectra of DDTT-SBT18 32
Figure 4.3 The comparison of optical spectra between DDTT-SBT14 and DDTT-SBT18 33
Figure 4.4 (a) Wavelength shifted range; (b) Example ilsutration a comparison of the -*energy gap in series of polyenes of increasing chain length 34
Figure 4.5 TGA weigth loss temperature between DDTT-SBT14 and DDTT-SBT18 35
Figure 4.6 Comparison of electrochemically derived HOMO and LUMO energy level of DDTT-SBT14 and DDTT-SBT18 using DPV 37
Figure 6.1 1H NMR (300 MHz) Dithieno[3,2-b:2′,3′-d]thiophene (1a) 42
Figure 6.2 ¬1H NMR (300 MHz) 2,2′-bithiophene (1b) 42
Figure 6.3 ¬1H NMR (300 MHz) 3,3′,5,5′-tetrabromo-2,2′-bithiophene (2b) 43
Figure 6.4 ¬1H NMR (300 MHz) 3,3′-dibromo-2,2′-bithiophene (3b) 43
Figure 6.5 ¬1H NMR (300 MHz) Tetradecanethiol 44
Figure 6.6 ¬1H NMR (300 MHz) 3,3′-bis(tetradecylthio)-2,2′-bithiophene (4b) 44
Figure 6.7 ¬1H NMR (300 MHz) 5,5′-dibromo-3,3′-bis(tetradecylthio)-2,2′-bithiophene 45
Figure 6.8 1H NMR (300 MHz) 2,2′-(3,3′-bis(tetradecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT14 (6b) 45
Figure 6.9 ¬1H NMR (300 MHz) Octadecanethiol 46
Figure 6.10 ¬1H NMR (300 MHz) 3,3′-bis(toctadecylthio)-2,2′-bithiophene (4c) 46
Figure 6.11 ¬1H NMR (300 MHz) 5,5′-dibromo-3,3′-bis(octadecylthio)-2,2′-bithiophene 47
Figure 6.12 ¬1H NMR (300 MHz) 2,2′-(3,3′-bis(octadecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT18 (6c) 47
LIST OF TABLES
Table 2.1 Comparison of OTFTs and amorphous silicon (a-Si) TFTs 14
Table 3.1 Material Description of CAS NO. 18
Table 4.1 The comparison of thermal properties of DDTT-SBT14 and DDTT-SBT18 36
CHAPTER 1
INTRODUCTION
Background
Organic Semiconductor is an organic compound that possesses similar properties to inorganic semiconductors with hole and electron conduction layer and a band gap. Study of organic semiconductor in recent years has developed quite rapidly because of the potential in the future-generation ultra-thin, large area, low-end, lightweight, and flexible electronics applications. It has many advantages when compared to conventional semiconductors, such as can be fabricated by low-cost and low-energy, offer a unique physical features from those conventional semiconductors, and have appeal for a broad range of devices including transistors, diodes, sensors, solar cells, and light-emitting devices [10,20].
Basically, there are three classes of organic devices: OLEDs (Organic Light Emitting Diodes), solar cells and OTFTs (Organic Thin Films Transistors). OTFTs in recent years have attracted to be developed. Because they have many advantages, OTFTs technology is quite advanced, with devices with a higher efficiency than hydrogenated amorphous silicon, which is the standard material of the TFTs fabrication, used in screens. With a certain voltage tension applied to the gate and to the drain, a controlled current flows through the semiconductor [18].
OTFTs have been the subject of much attention in the scientific research community in recent decades. There have already been many review articles summarizing research into sensing devices in recent years that integrate OTFTs into their architecture because of their potential applications in large-area, flexible, and printed electronics. Many research has primarly focused on molecular design, dielectric-semiconductor interfacial engineering and device optimization [17]. The use of conjugated polymer blends as active materials has brought a new way to tune and optimize the electronic properties of devices, such as ambipolar filed-effect charge transport has been reported in binary blends of p-type and n-type conjugated polymers or oligomers. In p-type semiconductors the majority carriers are holes, while in n-type the majority carriers are electrons [24].
Most of the OTFTs semiconductors such as pentacene derivatives, oligo/polythiophenes, fused-thiophenes, and anthradithiophene are hole-transporting (p-type) materials. Fused thiophene-based materials (from two to seven fused rings) have a strong tendency to form - stack with a large overlapping area that is preferable for charge carrier [25]. Synthesized of fused thiophene substructures has attracted attention continuously due to their utility as bioactive materials, pharmaceuticals and intermediates in the manufacture of dyes. They were also employed as key units for the development of electronic materials, such as narrow band gap polymers and electron donors or acceptors for conductive charge transfer salts [22]. The fused thiophene, thienoacenes, is the most interesting alternative material for organic semiconductor, due their extensive conjugation, strong intermolecular S ••• S interactions, large band gap and high ambient stability [25].
There were several derivatives of fused thiophene, thienoacene, with three and four thiophene rings, have been explored and have demonstrated as a model of p-type charge transport performance. The mobility value of those compounds was very varieties, 0.1 cm2V-1s-1≥µ≥ 2 cm cm2V-1s-1.One example of organic semiconductors based on thienoacene derivatives which used DTT as a core, which is a DP-DTT. These compound have a high stability and good mobility value, which is 0.42 cm2 /Vs [19].
Long alkyl chains are widely used in solution processable organic semiconductors to improve their solubility and the compatibility with different solution processing techniques. Previously it has been believed that the long alkyl chain is an unfavorable unit in charge transport semiconductors to achieve a highly crystalline film, which is of great importance for high-performance OTFTs. Inspired by the recent reports on high-performance OTFTs based on semiconductors that bear long side chains, the application of long alkyl chains in solution processable organic semiconductors is now attracting increasing attention [3].
More recently, Pei et al. (2012)[11], reported significant influence of long alkyl chain on the p-channel FET performance of isoindigo-based conjugated polymers. These results demonstrated the critical role of long alkyl chain on charge transport properties of solution processable organic semiconductors. systematic investigation on long alkyl chain-dependent molecular packing and device performance is still a challenging topic. It therefore makes deep insight into the relationship between the nature of the long alkyl chain and device performance of great importance, but challenging because a series of representative organic semiconductors with a variety of sidechain lengths and branching points should be designed to investigate the influence of branching points on OTFTs mobility [11].
Based on these consideration we decide to synthesized the simple fused thiophene (two fused rings) as a core and will be conjugated with dithienothiophene (DTT) and modified with branched alkyl chains to improve their solubility. Additionally, it was explained that if DTT was conjugated with some compounds which presented short S ••• S interactions, such as various thiophene oligomers and fulvalene derivatives, as the backbone. It would provide an additional channel for charge carrier transport and if it’s happened, we can get the high performance of organic semiconductors for OTFTs application. We estimated that DDTT-SBT14 and DDTT-SBT18, would have a good performance as soluble p-type semiconductors.
Purpose
The purpose of this research is to study another alternative of organic semiconductor based on dithienothiophene derivative compounds with synthesize and characterize compounds, such as DDTT-SBT14 and DDTT-SBT18 that can be used as an alternative of organic semiconductor for OTFT application.
CHAPTER 2
OVERVIEW
Organic Semiconductor
The first metal-oxide-silicon field-effect transistor demonstrated by Kahng and Atalla in 1960, they introduced organic materials as an integral part of the semiconductor. Organic materials are attractive for many components of electronic devices, particulary the active semiconductor layer, due to many advantages over their inorganic counterparts. The low temperature required for these methods, combined with the mechanical flexibility of organic materials, offers compatibility with plastic substrates, leading to the possibility of flexible integrated circuits, electronic paper, and roll-up displays. Although organic materials are not currently suitable for applications requiring high switching speeds, their low material and fabrication costs make them ideal for large-area applications, such as displays or solar cells [14].
Figure 2.1 The application of Organic Semiconductor
A semiconductor is a material that has the electrical characteristics of an insulator, but for which the probability that an electron can contribute to an electric current, though small, is large enough. In other words, the electrical conductivity of a semiconductor is intermediate between that of metals and insulators. The electrical behavior of semiconductors is usually described using the theory of energy bands. According to this, a semiconductor material has a band gap small enough that electrons from the valence band can easily reach the conduction band. If an electric potential is applied to its terminals, low power appears, driven both by the movement of electrons and that such "holes" they leave in the valence band [21].
Organic materials used in the manufacture of OTFT can be classified into two groups: small molecules and polymers. The family of the small molecules includes the organic compounds made up of a small number of monomers whose total molecular mass does not exceed 1000amu (atomic mass unit) [21]. A monomer is either an atom or a small molecule that has the potential of chemically binding to other monomers of the same species to form a polymer. Consequently, the main difference between small molecules and polymers is the amount of monomers they contain. A second difference lies in the technological processes used to deposit a thin film. Polymers can be deposited from solution by spin-coating or printing, the small molecule in turn is deposited from thermal evaporation under vacuum, because almost all the small molecules used in OTFT are insoluble. Despite these differences, the physics is behind the small molecules and polymers is very similar [7].
The semiconductors are carbon based. The carbon atoms have single bonds or double bonds and this structure allows the properties of semiconductors. the most stable configuration of carbon which is 1s ² 2s ² 2px 2py, carbon is the chemical element with atomic number 6, normally 2 of these electrons are in condition to form a link because the lowest levels "s" of energy are filled (with 2 electrons each ). This is not quite right because it is known that carbon can form 4 bonds. This phenomenon is explained by the hybridization of carbon: an electron from the 2s orbital is excited and moves to the 2p orbital of higher energy, causing a mixture of 2s with 2p orbitals, which form the sp hybrid orbital, whose number depends on the molecular structure. In the case of organic molecules, the 2s orbital is combined with the 2px and 2py orbitals to form 3 sp, known as 3 sp2, leaving a pz orbital without hybridization [21].
The pz orbital which has not been hybridized, overlap with the p orbital of the neighboring carbon atom and creates a p bond. Thus, we have a double bond between two carbon atoms (s and p) and single bond between other atoms that can also be carbon giving rise to the conjugated structure [9]. The p bond is significantly weaker than the s bond and can be easily changed, allowing the electrons to participate in part to conduction. The inorganic semiconductors we speak of valence band and conduction band to describe the energy levels that electrons must acquire to become a free carrier, for organic semiconductors we use HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital). The highest occupied molecular orbital (or HOMO) is the energy level at the top of this continuous band of occupied states, whereas the lowest unoccupied molecular orbital (or LUMO) is the first available energy level in the unoccupied band.
The bound molecular orbitals are more stable but less energy than unbound molecular orbitals, so if an electron is present in a bond is excited, he can change and occupy an unbound orbital. Carbon, as presented above, forms a double bond with another carbon atom (s bond and p bond). The p orbital is higher in energy than the s electrons and can be easily excited to form part of the unbound p* orbital and belongs to the LUMO level, thus creating free carriers. The s orbitals can be excited at the s*, which is higher in energy than the p* and therefore will require more energy to break the link [7].
The energy required to move from a bound to an unbound state depends, among other things, of the interaction energy between the electrons of two atoms. The value of this energy is different for each link. When several atoms are combined, as in any molecular solid, the individual molecular orbital levels broaden into continuous bands, analogous to the valence and conduction bands which arise from band theory with crystalline semiconductors. The more electrons in the system, and the greater their overlap, the broader the bands and the narrower the band gap. This is the origin of bands in the organic semiconductor and the HOMO and LUMO levels which correspond to the p orbital of higher energy and p* orbital of less energy, respectively [7,21].
Figure 2.2 Energy diagram, valence and conduction bands
To realize stable PN-junctions, the study of the N-type and P-type conducting semiconductor was decisive. The basic principles of doping in organic semiconductors are similar to those in inorganic materials: Mobile carriers are generated by exciting electrons from donors into a conduction band or by capturing electrons with acceptors and thus creating holes in a valence band. In organics, one has to add constituents, which either donate electrons to the lowest unoccupied molecular orbitals (LUMO, n-type doping) or remove electrons from the highest occupied molecular orbitals (HOMO) to generate holes (p-type doping)[1]. The doping operation is very difficult and not yet completely controlled. LUMO and HOMO of organic materials are respectively equivalent of the conduction and the valence band of inorganic materials.
One of the key parameters to characterize charge-transport materials is the charge-
carrier mobility. In the absence of any external potential, the transport of charges is purely diffusive; whereas applying an external electric field can induce a drift of the charge carriers. In this circumstance, the mobility (µ, cm2/(Vs)) can be defined as the ratio between the carrier speed (V, cm/s) and the strength of electric field (E, V/cm) as expressed in Equation 2.1:
µ= v/E (2.1)
Since the intermolecular interaction in organic semiconductors, Van der Waals interactions are much weaker than the covalent interatomic bonds found in inorganic semiconductors, such as silicon, thermal fluctuations disrupt the molecular order and result in
lower mobility than the ones observed in their crystalline inorganic counterparts. The mobility
in organic semiconductor are found to be small (≤ 10 cm2/VS), because its band energy widths are typically smaller than ᴋBT. That if we were to literally calculate a mean-free path distance for scattering of the charge carriers, we would obtain values smaller than the intermolecular spacing. The mechanism of charge conduction in organic semiconductor is different with crystalline materials. In fact, it has to be a rate-limiting process where in each step, charge carriers must overcome an activated energy barrier, with the mobility given by a rate Equation 2.2:
µ exp ( (-EA)/ᴋBT ) (2.2)
Where EA is the activation energy, and κB is the Boltzmann constant. Therefore, the mobility
and hence the conductivity of (disordered) organic semiconductors exponentially increase with temperature, which is different from temperature depence of conductivity in crystalline
semiconductors such as Si or Ge [6, 15].
The mobility of charges was significantly determined by the overlap degree of neighboring molecular orbitals. In the other words, molecular packing also influences the charge carriers mobility in organic semiconductor materials. Molecular packing with strong intermolecular overlaps are favorable for efficient charge carrier transport and the availability for high field-effect mobility. Most organic semiconductor exhibit a herringbone packing motif without efficient face-to-face - overlap (Figure 2.3(a)), which is not beneficial to efficient charge transport. More recently, extensive work has been carried out on the design of judicious substitution at peri-position of acenes, introducing polarity, increasing the C/H ratio or adding heteroatoms to generate hydrogen bond, halogen-halogen interaction or chalcogen-chalcogen interactions to enhance the intermolecular overlaps, and thus improving the charge carrier transport properties. To date, slipped herringbone packing (Figure 2.3(b)), lamellar stacking (Figure 2.3(c)), brick wall (Figure 2.3(d)) and brickstone (Figure 2.3(e)) molecular arrangements with promoted - stacking have been controllably synthesized by rational design and synthesis. It was found that a close side by side and brick wallpacking with strong - interaction gave high mobility [4].
Figure 2.3 Different possible molecular packing motifs existing in organic solid states: (a) herringbone packing motifs face to face - overlap, (b) herringbone packing motifs with slipped face to face - stacking and (c) lamellar stacking motifs with one dimensional (1D) charge carrier channel, (d) brick wall and (e) brickstone arrangements providing two dimentional percolation paths for charge carrier transport in organic semiconductors
.
Organic semiconductor based on n-type or p-type materials are mainly characterized by their high electron affinity and low ionization potential. The most of organic semiconductor that has been investigated is p-type form, because p-type semiconductors are stable in air and have relatively high mobility when they are used in OFETs. On the other hands, most of n-type semiconductors are sentisitive to air and moisture. Furthermore, the n-type semiconductors have relatively low field-effect mobility [23].
Organic Semiconductor based on Dithienothiophene (DTT) Derivative
Xiang (2009), explained that p-type semiconductors are stable in air and have relatively high mobility when they are used in OFETs. The carrier mobility is the most important electrical parameter of organic transistors The largest mobility for each of six categories was taken from the following references: polymer p-channel transistors; polymer n-channel transistors; vacuum-processed small-molecule p-channel transistors; vacuum-processed small- molecule n channel transistors; solution-processed small-molecule p-channel transistors; solution-processed small-molecule n-channel transistors [23].
Recently, there were some kind of p-type small molecules which have been invetigated. Those kinds of p-type small molecules were belong to some group, such as:
Aromatic hydrocarbons and derivatives which are include acenes, pyrene, perylene, coronene, and other fused arenes, and oligoacenes;
Chalcogen-containing heterocyclic semiconductors which are include higher thienoacenes, oligomers of thienoacenes, fused aromatic chalcogen-containing compounds, tetrathiafulvalene and its derivatives, oligothiophenes;
Nitrogen-containing heterocyclic semiconductors which are include phtalocyanines and porphyrins, azaacenes, oligomers based on nitrogen-containing p systems.
Figure 2.4 Chemical structure of some representative p-channel organic semiconductors. (a) pentacene, (b) tetraceno[2,3-b]thiophene, (c) TIPS-pentacene, (e) oligothiophene-fluorene derivative, (f) poly3-hexylthiophene, (g) poly(3,3-didodecylquarterthiophene), (h) poly(2,5bis(3-decylthiophen-2-yl)thieno[3,2-b]thiophene).
Among these materials, there were oligo and polythiophenes which important representative systems because of their synthetic availability, widespread possibility and tuneable electronic properties. Contrast to well known organic semiconductors, such as polythiophene, anthradithiophene and pentacenes, there was fused-ring thienoacenes which have more rigid structures, offer the attraction of relatively higher ambient stability originating from large band gap, good charge transport properties as a reflection of a better molecular conjugation and strong intermolecular S•••S interactions which promote close molecular packing.
Molecular packing in sulfur-rich annelated oligomers was characterized by the cooperation of multiple S•••S interaction and - which may increase the effective dimesionality of the electronic structure, leading to enhanced transport properties. And sulfur decorated oligoacenes become the relevant examples for this kind of organic semiconductors.
Youn et al. (2012) and Qiao et al. (2012) found that nowadays there were several fused thiophene derivatives which have been demonstrated as a model of a good p-type charge transport performance. And the mobility value of those compounds were so varieties, between 0.1 cm2V-1s-1 ≥ µ ≥ 2 cm2V-1s-1. They also showed that dithienothiophenes (DTTs). DTT represents a sulfur-rich, planar, and rigid p-conjugated system, which has been widely used as a building block in functional materials. Due to the planar and rigid structure of DTT, analogues have been developed as p-type semiconductors for OFETs. DTT as the active semiconducting channel of OFETs, showing moderately high field-effect mobility (µFET) of up to 0.05 cm2/Vs and high on/off current ratios (up to 108) with sharp turn-on characteristic [16,25].
DTT is also known as an important building block in the synthesis of fused thiophenes. Its good conjugation has attracted the attention of researchers, and its derivatives have been widely used as organic semiconductors materials in TFTs. Due to their synthetic availability and widespread possibility for modifying, these compounds has a high possibility to have a good performance as p-type organic semiconductors. One of thienoacene derivative which have DTT as its core is DP-DTT. DP-DTT has been synthesized by Sun et al. (2006) and its known as a p-type organic semiconductor that have a high mobility value (0.42 cm2/Vs) and a good thermal also oxidation stability [19].
Figure 2.5 DP-DTT compound
Based on the information above, this research used DP-DTT as the reference of thienoacene derivative to synthesize a new thienoacene derivative. One of useful and practical
strategy to investigate the properties of an organic semiconductor materials for organic thin film transistor application depend on its chemical structure can be done either by varying the
number of units in the conjugated backbone of the molecule, or by varying the length of alkyl substituents.
Organic Thin Film Transistor (OTFT) Overview
Transistor is a fundamental building block for all modern electronics. A transistor it
self is a semiconductor device where the flow of electrical current between two terminals was controlled by an applied voltage to a third terminal. Nowadays, there were two type of transistors, thin film transistors (TFTs) and organic thin film transistors (OTFTs). Fundamentally, TFTs and OTFTs are the same things, these are a special kind of field-effect transistor where the semiconductor is deposited as a thin film on an insulating substrate, such as glass or plastic foil [18].
The arrangement of TFTs and OTFTs also the same, they are consist of a thin semiconductor layer which is separated from a gate electrode by a layer of an electronically insulating material, that commonly is referred to as the gate insulator, or the gate dielectric if it is an electrically insulating material. This stack of materials constitutes a capacitor, which is crucial for the function of the transistor. Moreover, a source (S) and a drain (D) electrode are in direct contact with the semiconductor. The device output current flowing between the S and drain D electrodes and collected at the drain terminal. IDRAIN is modulated and controlled by applying input voltages to the drain (VDRAIN) and gate (VGATE) electrodes both referred to the source electrode (VSOURCE). The region between these two separated electrode represents the channel, which has a width W and a length L given by the extensions and separation of the electrodes [15].
The difference between TFTs and OTFTs was take place just on theirs kind of semiconductor. TFTs as the conventional ones was based on inorganic semiconductor material as its active layer. And OTFTs was based on organic semiconductor material as an active layer to regulate the electric current flow. As a transistor, an OTFT also can be assemble in several configurations.
Table 2.1 Comparison of OTFTs and amorphous silicon (a-Si) TFTs[9]
OTFT a-Si TFT
Material Organic semiconductor as active layer (p-type, n-type, ambipolat) a-Si as active layer n-type
Processing Spin-coat, print, evaporation, low temperature Plasma enhanced chemical vapor deposition (PECVD)
T < 3500C
Mobility Can be comparable to a-Si ~ 1 cm2 V-1 s-1
Substrate and form factor Variety of substrates and flexible form factor Glass (most common) and plastic (in development)
Mechanical flexibility Bendable Fragile and brittle
Key applications Numerous: displays, RFID tags, sensors, disposable electronics Circuit for large-area display and sensors array backplane
Electrical stability Rapid degradation, but degradation stabilizes (maybe favorable for devices that turn on for a longer time) Slower bias-induced degradation, but degradation does not stabilize
Outlook New opportunities: smaller/flexible displays, disposable electronics, smart textiles Continue to excel in AMLCD, AMOLED, active-matrix sensor technologies
Figure 2.6 Schematic OTFT under applied gate and drain bias showing charge accumulation and transport. For n-type device VGS > 0V, VDS > 0V. For p-type VGS < 0V, VDS < 0V
The source and drain contacts are deposited on the semiconducting layer in the top-contact configuration or staggered geometry whereas they lay under the semiconductor in the bottom-contact configuration. Regarding to the gate, it can be deposited either over the substrate with the dielectric layer on top (bottom-gate configuration) or the gate dielectric at the top of the whole structure (top-gate configuration). Consequently, we can have top-contact bottom-gate configuration, bottom-contact bottom-gate configuration, bottom-contact top-gate configuration and finally top-contact top-gate configuration.
Device performance is greatly influenced by various deposition conditions due to the different resulting molecular structure and thin film morphology. To achieve high field effect mobility, the semiconducting molecules should have an orientation in which - stacking
direction between molecules is arranged in the same direction as that of the current flow. Moreover, larger grain sizes and smooth grains usually tend to give better mobility. It is known that the current flow in an organic field effect transistor (OFET) is mainly confined in the first 50Å of semiconductor layer away from the semiconductor or dielectric interface. The typical semiconductor thickness required for an OFET is in the range of 200 – 500 Å. A thinner film may have incomplete surface coverage. For bottom-contact devices, a thicker
semiconductor layer usually does not affect the charge carrier mobility even though the device
off-current may increase for materials with a higher unintentional doping. For top-contact
devices, a thicker layer may lead to higher resistance for the charge carriers to travel from the
source electrode to the semiconductor or dielectric interface, where the majority of the current flow occurs.
The unique processing characteristics and demonstrated performance of OTFTs suggest that they can be competitive candidates for existing or novel thin film transistor applications requiring large area coverage, structural flexibility, low temperature processing and especially low cost. Beside of that, simple manufacturing process of OTFT and also non-breakable to impacts that can be bended and folded, has been argued to be the most important research area. Figure 2.7 showed the simple illustration about some kinds of OTFT application [2].
Figure 2.7 Illustration of OTFT application
Nowadays, there were some applications of OTFT in our daily life, include switching
devices for active matrix flat panel displays (AMFPDs) based on liquid crystal pixels (AMLCDs), organic light emitting diodes (AMOLEDs), or electronic paper displays based on
pixel comprising either electrophoretic ink-contain microcapsules or twisting balls. Additionally, sensor, low-end smart cards, and radio frequency identification tags (RFIDs) consisting of organic integrated circuits have been proposed and prototype all-polymer integrated circuits have been demonstrated [3].
CHAPTER 3
EXPERIMENTAL SECTION
Materials and Methods
Materials
All chemicals and solvents were purchased from Sigma Aldrich as the commercial suppliers, the specification catalog number were showed in Table 3.1. Reaction, THF, dry ether was distilled under nitrogen to prevent it from water and air. Toluene and DMSO were dried with calcium hydride (CaH2) to prevent it from water and air.
Table 3.1 Material Description of CAS NO.
No. Material Formula CAS NO.
1. Acetone CH3COCH3 67-64-1
2. Bromine Br2 7726-95-6
3. Chloroform CDCl3 67-66-3
4. Hexane C6H14 110-54-3
5. Hydrochloric acid HCl 7647-01-0
6. Methanol CH3OH 67-56-1
7. N-Buthyllithium (2.5 M) C4H9Li 109-72-8
8. Sodium Acetate CH3COONa 127-09-3
9. Sodium Hydroxide NaOH 1310-73-2
10. Copper(II) Oxide CuO 1317-38-0
11. Tetrahydrofuran C4H8O 109-99-9
12. Thiophene C4H4S 110-02-1
13. Toluene C7H8 108-88-3
14. Zinc Powder Zn 7440-66-6
15. Sodium Sulfate Na2SO4 7757-82-6
16. Ammonium Chloride NH4Cl 12125-02-9
17. Calcium Hydride CaH2 7789-78-8
18. Iodine I2 7553-56-2
19. Diethyl Ether C4H10O 71-36-3
20. Dichloromethane CH2Cl2 75-09-2
21. Acetic Acid CH3COOH 64-19-7
22. Dichloro(1,3-bis-(diphenylphosphino)propane)Nickel Ni(dppp)Cl2 15629-92-2
23. Magnesium powder Mg 7439-95-4
24. 1-Bromotetradecane C14H29Br 112-71-0
25. Thiourea NH2CSNH2 62-56-6
26. Sulfur powder S¬8 7704-34-9
27. p-Toluenesulfonyl Chloride p-TsCl 98-59-8
28. 3-Bromothiophene C4H3BrS 872-31-1
29. 2-Bromothiophene C4H3BrS 1003-09-4
30. Dimethyl Sulfoxide C2H6OS 67-68-5
31. N-bromosuccinamide C4H4BrNO2 128-08-5
32. Tetrakis(triphenylphosphine)palladium Pd(PPh3)4 14221-01-3
Methods
Some of the methods used to test and characterize the properties of those two final compounds, such as NMR, UV-Vis spectrometer, differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), and differential pulse voltammetry (DPV). The description of those instrument were described below.
Nuclear Magnetic Resonance (NMR)
NMR is a method for qualitative analysis in the determination of the molecular structure of organic matter. More precisely the location of an atom in the molecule. In this case used 1H NMR, the spectras were recorded on Bruker Model 300 MHz spectrometer at 370C in CDCl3 solvent.
Ultraviolet Visible Spectrometer (UV-Vis)
Uv-Vis spectrometer is a spectrometer that is used for measurements in the ultra violet and visible region. The UV-Vis spectras of those two final compounds were determined with Hitachi UV-vis Spectrometer. Orto-dichlorobenzene, HPLC grade, was used as the solvent of the compounds and also as the blank solution. The cuvette was made from orz with 10 nm of thickness. Wavelength range which was used around 300-600 nm.
Differential Scanning Calorimetric (DSC)
The melting point value of those two final compounds were determined with Mettler Toledo DSC 822.
Thermo Gravimetric Analysis (TGA)
TGA measurement is a type of testing performed on samples that determines changes on weight in relation to temperature program by a controlled atmosphere. There are three important parameter on TGA measurement, such as weight, temperature, and temperature change. Three parameter should be have a precise value.
The TGA measurement for those two final compounds used 1.5 – 2 mg sample compounds. TGA measurement was applied between 25 – 6000C on temperature range, temperature change was measured every 200C/min.
Electrochemical Analyzer – Differntial Pulse Parameter (DPV)
DPV analysis a common technique to measure the electrochemical properties of compounds, include of reduction and oxidation properties, then also use to calculated the HOMO / LOMO level of a compound.
The solvent used in the analysis of DPV is ortho-dichlorobenzene which has been dried first and tetrabuthylammonium 0.1M hexafluorophosphate as an electrolyte solution. DPV analysis was done under nitrogen to prevent contamination, including water and air. DPV instruments was degassed with N2 and then calibrated with a solvent (ortho-dichlorobenzene). Furthermore, a standard measure containing solvents (o-dichlorobenzene) and ferrocene ( 0.1 mg). And then proceed to measure samples or the compounds diluted in solvent (o-dichlorobenzene).
Synthesis
The synthetic route and chemical structures of compound used in this study are shown in Scheme 1, 2, and 3. Synthetic details of the preparation some title compounds were described.
Synthetic Scheme Route
DTT
Scheme 3.1 Synthesis of DTT
DDTT-SBT18
Scheme 3.2 Synthesis of DDTT-SBT14
DDTT-SBT18
Scheme 3.3 Synthesis of DDTT-SBT18
Synthetic of Dithieno[3,2-b:2′,3′-d]thiophene or DTT
One-pot of Dithieno[3,2-b:2′,3′-d]thiophene (1a)
Under vacuum condition, 3-bromothiophene (11.97 g; 0.073 mol) in dry ether (60 mL) was added into the flask (I), then n-Buli (2.5 M; 29.36 mL) was added slowly at -780C and stirred for 40 minutes. After stir completely, the reaction mixture was warmed at 00C for remove the solvent using trap. S8 (2.35 g; 0.073 mol) dissolved in dry ether, added into the reaction mixture at -780C then stirred for 30 minutes. p-TsCl (13.99 g; 0.073 mol) reagent was added at 00C and stirred for 30 minutes, then warmed at 400C for 4 h. prepared for making anion, 3-bromothiophene (14.36 g; 0.088 mol) in dry ether (60 mL) was added into the flask (II) and added n-Buli (2.5 M; 35.24 mL) slowly, then stirred for 40 minutes. After stir completely, the reaction mixture was warmed at 00C for remove the solvent using trap. The reaction mixture in flask (II) was added into the flask (I) at -780C, then stirred for 1 h. After the reaction mixture was stirred completely, warmed to room temperature then stirred for 12 h. After the reaction mixture completed, n-Buli (2.5 M; 64.59 mL) was added at 00C and stirred for 30 minutes, then refluxed for 1.5. Copper(II) Chloride (22.69 g; 0.169 mol) was added into the reaction mixture at -780C and stirred for 30 minutes, then warmed to room temperature and stirred for 12 h. After the reaction completed, filtered the reaction and extracted with diethyl ether and evaporated completely for afforded the crude product. The crude product was purified by column chromatography (silica, hexane as eluent), then recrystallization using hexane for afford the pure compound Dithieno[3,2-b:2′,3′-d]thiophene 1a (2.3 g; 0.012 mol; 16.44 % yield) as a white solid. 1H NMR (300 MHz, CDCl3): 7.360 ppm (dd, 1H, J= 5.1 Hz), 7.289 ppm (dd, 1H, J= 5.4 Hz).
Synthetic of 2,2′-(3,3′-bis(tetradecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT14
2,2′-bithiophene (1b)
Magnesium (1.79 g; 0.074 mol) and some iodine was dissolved in dry diethyl ether with vacuum condition, then added a solution of commercial 2-bromothiophene. The reaction mixture was maintained at gentle reflux for a further 1 hour and cooled to 00C. The resultant intermediate Grignard reagent was added to a mixture of 2-bromothiophene, [1,3-Bis(diphenylphosphino)propane]nickel(II) chloride, and dry diethyl ether at 00C. After stirring 3 h at 00C, the reaction mixture was quenched with ammonium chloride solution (75 mL) and transferred to a separating funnel. Then, the mixtrure was extracted with ethyl ether solvent and water, the organic layer was dried with anhydrous sodium sulfate and subsequently evaporated the solvent under vacuum condition. The crude product was purified by distillation to afford 2,2-bithiophene 1b (8.37 g; 0.050 mol; 82.08 % yield) as a green solids. 1H NMR (300 MHz, CDCl3): 7.216 ppm (dd, 1H, J= 5.25 Hz), 7.185 ppm (dd, 1H, J= 3.6 Hz), 7.021 ppm (dd, 1H, J= 5.1 Hz).
3,3′,5,5′-tetrabromo-2,2′-bithiophene (2b)
2,2-bithiophene 1b (8.37 g; 0.050 mol) and acetic acid (40 mL) were mixed in chloroform, then added slowly at 00C of bromine (10.35 mL; 0.201 mol) into the reaction, the mixture was stirred for 1 h. After the mixture was dissolved completely, stirred at room temperature for 18 h. Filtered the reaction and washed with ethyl ether. Removing the solvent under vacuum condition and subsequently afforded 3,3′,5,5′-tetrabromo-2,2′-bithiophene 2b (16.1 g; 0.033 mol; 66.37 % yield) as a yellow powder. 1H NMR (300 MHz CDCl3): 7.048 ppm (s, 1H).
3,3′-dibromo-2,2′-bithiophene (3b)
3,3′,5,5′-tetrabromo-2,2′-bithiophene 2b (15 g; 0.031 mol) was dissolved in ethanol (77 mL), water (7.7 mL), acetic acid glasial (20 mL), and hydrochloric acid (3M; 1,5 mL) at room temperature. Zinc powder was addes slowly into the mixture and stirred at 00C, then reflux for 2 hours at 1200C. After the reaction completed, quenched was extracted with ethyl ether solvent and water, then the organic layer was dried with anhydrous sodium sulfate, filtered and concentrated under vacuum condition to remove the solvent which afford the product 3,3′-dibromo-2,2′-bithiophene 3b (8.01; 0.024 mol; 79.41 % yield) as a light yellow powder. 1H NMR (300 MHz, CDCl3): 7.407 ppm (dd, 1H, J= 5.4 Hz), 7.078 ppm (dd, 1H, J= 5.4 Hz).
3,3′-bis(tetradecylthio)-2,2′-bithiophene (4b)
Under ambient condition, tetradecanethiol (12.44 g; 0.054 mol) into the flask and dissolved in dry ethyl ether (50 mL), the added n-Buthyllithium in hexane solution (2.5 M; 12.7 mL) at 00C. The reaction mixture was stirred for 1 h, then the solvent dry diethyl ether was removed under nitrogen. After the solvent was removed, the reaction mixture was dissolved in DMSO (20 mL). The compound 3,3′-dibromo-2,2′-bithiophene 3b (5 g; 0.0154 mol), Copper (II) oxide (0.102 g), Potassium Iodide (0.102 g) was added into another flask and dissolved in DMSO (30 mL). The reaction mixture in flask (I) was added into the flask (II) and reflux at 1000C for overnight. After the recation completed, cooled at room temperature then quenched with water and extracted with ethyl ether solution. The organic layer was dried with anhydrous sodium sulfate, filtered and concentrated under vacuum condition to remove the solvent which afford the crude product. Then the crude product was purified by column chromatography (silica, hexane as eluent). The product 3,3′-bis(tetradecylthio)-2,2′-bithiophene 4b ( 5.6 g; 0.0089 mol; 58.26 % yield) as a white solid. 1H NMR (300 MHz, CDCl3) : 7.361 ppm (dd, 1H, J= 5.4 Hz), 7.078 ppm (dd, 1H, J= 5.4 Hz), 2.767 ppm (t, 2H, J= 9 Hz), 1.241 ppm (m, 27H).
5,5′-dibromo-3,3′-bis(tetradecylthio)-2,2′-bithiophene (5b)
The compound 3,3′-bis(tetradecylthio)-2,2′-bithiophene 4b (0.5 g; 0.0008 mol) in dichloromethane was added N-bromosuccinamide (0.31 g; 0.0017 mol) at 00C and stirred for 30 minutes. Then warmed at room temperature and stirred for 6 h. After the reaction completed, the reaction mixture was quenched with water and extraxted. The organic layer was dried with anhydrous sodium sulfate, filtrated and concentrated under vacuum condition to remove the solvent which afford the crude product. The crude product was purified by column chromatography (silica, hexane as eluent) . The product 5,5′-dibromo-3,3′-bis(tetradecylthio)-2,2′-bithiophene 5b afforded as a white solid (0.5 g; 0.00064 mol; 79.80 % yield). 1H NMR (300 MHz, CDCl3): 7.009 ppm (s, 1H), 2.771 ppm (t, 2H, J= 7.5 Hz).
2,2′-(3,3′-bis(tetradecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT14 (6b)
Under vacuum condition, the compound 5,5′-dibromo-3,3′-bis(tetradecylthio)-2,2′-bithiophene 5b (0.18 g; 0.0002 mol) and catalyst Tetrakis(triphenylphosphine)palladium (0.012 g; 0.00001 mol) was dissolved in dry toluene then stirred for a while. After the reaction mixture dissolved, tributyl(dithieno[3,2-b:2′,3′-d]thiophen-2-yl)stannane (0.2 g; 0.0004 mol) was added slowly. Then refluxed at 1200C for 12 h. After the reaction completed, the reaction mixture was cooled at room temperature. The solvent in the reaction mixture was concentrated under vacuum condition, then filtered using cellite for remove the catalyst Tetrakis(triphenylphosphine)palladium. The crude product was afforded by recrystallization using dichloromethane as solvent and washed with methanol. The product 2,2′-(3,3′-bis(tetradecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene 6b afforded as a red solid (0.25 g; 0.00032 mol, 50 % yield). 1H NMR (300 MHz, CDCl3): 7.417 ppm (s, 1H), 7.389 ppm (d, 1H, J= 5.1 Hz), 7.293 ppm (d, 1H, J= 5.4 Hz), 7.197 ppm (s, 1H), 2.887 ppm (t, 2H, J= 7.2 Hz), 1.251 ppm (m, 27H).
Synthetic of 2,2′-(3,3′-bis(octadecylthio)-[2,2′-bithiophene]-5,5′ diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT18
3,3′-bis(toctadecylthio)-2,2′-bithiophene (4c)
Under ambient condition, octadecanethiol (8.23 g; 0.024 mol) into the flask and dissolved in dry ethyl ether (50 mL), the added n-Buthyllithium in hexane solution (2.5 M; 12.7 mL) at 00C. The reaction mixture was stirred for 1 h, then the solvent dry diethyl ether was removed under nitrogen. After the solvent was removed, the reaction mixture was dissolved in DMSO (20 mL). The compound 3,3′-dibromo-2,2′-bithiophene 3b (2 g; 0.006 mol), Copper (II) oxide (0.102 g), Potassium Iodide (0.102 g) was added into another flask and dissolved in DMSO (30 mL). The reaction mixture in flask (I) was added into the flask (II) and reflux at 1000C for overnight. After the recation completed, cooled at room temperature then quenched with water and extracted with ethyl ether solution. The organic layer was dried with anhydrous sodium sulfate, filtered and concentrated under vacuum condition to remove the solvent which afford the crude product. Then the crude product was purified by column chromatography (silica, hexane as eluent). The product 3,3′-bis(octadecylthio)-2,2′-bithiophene 4c (1.95 g; 0.0026 mol; 42.95 % yield) as a white solid.1H NMR (300 MHz, CDCl3): 7.351 ppm (dd, 1H, J= 5.1 Hz), 7.065 ppm (dd, 1H, J= 5.1 Hz), 2.519 ppm (t, 2H, J= 10.8 Hz), 1.343 ppm (m, 35H).
5,5′-dibromo-3,3′-bis(octadecylthio)-2,2′-bithiophene (5c)
The compound 3,3′-bis(octadecylthio)-2,2′-bithiophene 4c (0.5 g; 0.00068 mol) in dichloromethane was added N-bromosuccinamide (0.27 g; 0.0015 mol) at 00C and stirred for 30 minutes. Then warmed at room temperature and stirred for 6 h. After the reaction completed, the reaction mixture was quenched with water and extraxted. The organic layer was dried with anhydrous sodium sulfate, filtrated and concentrated under vacuum condition to remove the solvent which afford the crude product. The crude product was purified by column chromatography (silica, hexane as eluent) . The product 5,5′-dibromo-3,3′-bis(octadecylthio)-2,2′-bithiophene 5c afforded as a white solid (0.5 g; 0.00056 mol; 82.35 % yield).1H NMR (300 MHz, CDCl3) : 7.010 ppm (s, 1H), 2.519 ppm (t, 2H, J= 10.8 Hz).
2,2′-(3,3′-bis(octadecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT18 (6c)
Under vacuum condition, the compound 5,5′-dibromo-3,3′-bis(octadecylthio)-2,2′-bithiophene 5c (0.18 g; 0.0002 mol) and catalyst Tetrakis(triphenylphosphine)palladium (0.012 g; 0.00001 mol) was dissolved in dry toluene then stirred for a while. After the reaction mixture dissolved, tributyl(dithieno[3,2-b:2′,3′-d]thiophen-2-yl)stannane (0.2 g; 0.0004 mol) was added slowly. Then refluxed at 1200C for 12 h. After the reaction completed, the reaction mixture was cooled at room temperature. The solvent in the reaction mixture was concentrated under vacuum condition, then filtered using cellite for remove the catalyst Tetrakis(triphenylphosphine)palladium. The crude product was afforded by recrystallization using dichloromethane as solvent and washed with methanol. The product 2,2′-(3,3′-bis(octadecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene 6c afforded as a red solid (0.17 g; 0.00016 mol, 79.6 % yield). 1H NMR (300 MHz, CDCl3) ): 7.417 ppm (s, 1H), 7.389 ppm (d, 1H, J= 5.1 Hz), 7.292 ppm (d, 1H, J= 5.4 Hz), 7.197 ppm (s, 1H), 2.887 ppm (t, 2H, J= 7.2 Hz), 1.229 ppm (m, 35H).
CHAPTER 4
RESULT AND DISCUSSION
This section will explain about all procedures that have been done on this research, such as the synthetic route to obtain the final compound to be used as an organic semiconductor material, and also the analysis and characterization of compounds to be applied in the OTFT.
Synthesis
DTT
The synthetic route of DTT was described in Scheme 3.1. In this synthesis used one-pot approachment, 3-Bromothiophene was first lithiated with n-Buli, followed first by S8 and then TsCl addition and make an anion of thiophene for forming di(thiophen-3-yl)sulfane then lithiated with n-Buli and ring closure was achieved with CuCl2 to afford DTT.
DDTT-SBT14
The synthetic route of DDTT-SBT14 was described in Scheme 3.2. In this synthesis approachment, 2-bromothiophene as the strating material first reacted with magnesium to get 2,2-bithiophene by Grignard reaction. Then brominated with bromine to get tetrabromobithiophene. Next, reduction the bromine on two sides of tetrabromobithiophene with zinc to get dibromo-bithiophene. Then lithiated with n-Buli, followed by addition C14H30S as an alkyl chain and using CuO and KI as catalyst, then SBT14 was formed. NBS was added by bromination reaction and the final compound DDTT-SBT14 was formed by Stille coupling reaction between dibromo-SBT14 and DTT-SnBu3 with Pd(PPh3)4 as catalyst.
DDTT-SBT18
The synthetic route of DDTT-SBT18 is almost same with DDTT-SBT14 and described in Scheme 3.3. In this synthesis, the difference between DDTT-SBT14 and DDTT-SBT18 is DDTT-SBT14 using C14H30S while DDTT-SBT18 using C18H38S as an alkyl chain to understand the effort of the performance those both as the organic semiconductor material.
The structure and purity of the intermediate and final compounds were proved by 1H- NMR spectroscopy analysis. The solubility difference can be observed both when the two compounds are dissolved in CDCl3 as solvent for NMR analysis. DDTT-SBT14 more soluble than the DDTT-SBT18 which requires raising the temperature when dissolved in a solvent (CDCl3, methanol, acetone). Then, those final compounds were characterized by UV-vis spectrometer, differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), and differential pulse voltammetry (DPV).
Molecular Characterization
Optical Properties
The optical properties for two final compounds DDTT-SBT14 and DDTT-SBT18 were analyzed used UV-visible spectrometer in o-dichlorobenzene solution. Figure 4.1 and Figure 4.2 described the absorption spectra for each of final compounds. According to the absorption spectra, the maximum absorption value for DDTT-SBT14 and DDTT-SBT18 are observed at max ~ 426 nm.
Figure 4.1 UV Spectrum absorption of DDTT-SBT14
Figure 4.2 UV Spectrum absorption of DDTT-SBT18
On this research DTT was conjugated with Bithiophene. When DTT was conjugated, these maximum absorbance wa shifted become higher or those maximum absorbance shifted to bathocromic range. These condition can be happened because the length of - molecular conjugation were increase as the addition of thiophenes.
Figure 4.3 The comparison of optical spectra between DDTT-SBT14 and DDTT-SBT18
Figure 4.3 shows the comparison of absorption spectra for DDTT-SBT14 and DDTT-SBT18. The absorption spectra of DDTT-SBT14 compound in o-dichlorobenzene solution has a very similar red-shift to that of DDTT-SBT18. This is because these compounds have the same molecular - conjugation length and the difference lies in the length of the alkyl chains.
The increasing extent of conjugation length in double-bonded system can result in bathochromic shift (Figure 4.4(a)). In the presence of extented conjugated double bonds, it will decrease the energy required for electronic excitation. In the other word, the electronic energy levels of a chromophore move closer together. As a result, the energy required to produce a transition from an occupied electronic energy level to an unoccupied level decreases, and the absorbance of the light becomes longer in wavelength. The ilustration of this condition is shown in Figure 4.4(b) [22].
Figure 4.4 (a) Wavelength shifted range; (b) Example ilsutration a comparison of the -* energy gap in series of polyenes of increasing chain length
The longer the - molecular conjugation, the energy level is from an occupied electronic energy level to an unoccupied level and the maximum absorbance value will be shifted. The absorption peak could be formed because of the electronic transition reaction. In order to promote an electron from HOMO to LUMO, the system should require energy equal to the difference between the two levels, or called the energy gap (Eg). Therefore, when the energy of the incident radiation is comparable to Eg the absorption signal will rapidly increase as a function of the wavelength. The other peaks detecable in the spectra can be due to the presence of deep or shallow traps, when the energy of incident radiation is lower than Eg or to transition between higher energy levels when it is higher than Eg. In fact, an organic molecule has several anti-bonding orbitals (LUMO representing the lowest one) and higher energetic peaks can be explained with higher energetic transitions.
Thermal Properties
Thermal analysis decribes the techniques used in characterizing materials by measuring a physical or mechanical property as a function of temperature or time at a constant temperaturevor as a function of temperature. There are some thermal analysis techniques, but in this research used TGA analysis and melting point measurement. In general thermal characterization, TGA analysis method is normally used to obtain the onset temperature of initial polymer or small molecule weight loss, as well as the extent of oxidative effects.
The main purposes of thermal analysis were to investigate the thermal stability of a compounds, it was important, since it is likely related to practical concerns such as ease of purification and device longevity.
Figure 4.5 TGA weigth loss temperature between DDTT-SBT14 and DDTT-SBT18
The detailed TGA and melting point measurements data about those thermal properties of DDTT-SBT14 and DDTT-SBT18 showed on Table 4.1.
Table 4.1 The comparison of thermal properties of DDTT-SBT14 and DDTT-SBT18
Compound
Tm (0C) TGA
5% weight loss
DDTT-SBT14 140 327
DDTT-SBT18 128.7 339
As the results on Table 4.1, DDTT-SBT18 has a slightly lower Tm and TGA weight loss temperature has a slightly higher than DDTT-SBT14. The reason of this condition can be explained by the differences of those intermolecular interaction. Although both of them has the same type of intermolecular interaction, which known as Van der Waals and - molecular conjugation. Melting point measurement is used to determine the resistance of a compound at high temperatures, DDTT-SBT14 more resistance in high temperature than DDTT-SBT18.
Nowadays, organic semiconductor materials with a high melting point and TGA weight loss temperature is needed. Because if those organic semiconductors material has both of requirements, hopefully it will have a high thermal stability and when it was fabricated to be a device, it will become a longevity device.
Electrochemical Properties
Differential Pulse Voltammograms (DPV) of the final compounds, DDTT-SBT14 and DDTT-SBT18 were analyzed in o-dichlorobenzene at 250C which resulted the HOMO/LUMO levels, reductive and oxidative potential data. Figure 4.6 showed the comparison of electrochemically properties that derived HOMO levels and LUMO levels for DDTT-SBT14 and DDTT-SBT18. Based on the result, there is a consideration about the relationship between - molecular conjugations length, HOMO and LUMO level. The - molecular conjugation length of DDTT-SBT14 and DDTT-SBT18 are similar.
Figure 4.6 Comparison of electrochemically derived HOMO and LUMO energy level of DDTT-SBT14 and DDTT-SBT18 using DPV
The comparasion of electrochemical properties for both DDTT-SBT14 and DDTT-SBT18, DDTT-SBT18 have a slightly higher HOMO level than DDTT-SBT14 and for the LUMO level, DDTT-SBT18 have a slightly lower value than DDTT-SBT14. Based on these results it can be seen that the alkyl chain length does not affect the value of Humo and LUMO, energy gap (Eg) values of the two did not differ significantly, i.e the difference is only 0.084 eV.
CHAPTER 5
CONCLUSIONS
According to the research results, DDTT-SBT14 and DDTT-SBT18 have almost the same results. Those are showed by the molecular characterization, include of optical properties using UV-visible spectroscopy, electrochemical properties using Differential Pulse Voltamogram (DPV), and thermal properties using Thermal Gravimetric Analysis (TGA) and DSC.
Based on UV-vis spectra, DDTT-SBT14 and DDTT-SBT18 have the same wavelength at 426 nm. Based on DPV result, it also showed that DDTT-SBT14 (2.495 eV) have a slightly lower energy gap value than DDTT-SBT18 (2.579 eV). This condition was caused by the same of - molecular conjugation length between DDTT-SBT14 and DDTT-SBT18. The longer molecular conjugation length could have a small different band gap value between HOMO / LUMO levels, the difference is only 0.084 eV. From thermal properties, DDTT-SBT14 have a higher Tm but for TGA weight loss temperature value have a slightly lower than DDTT-SBT18, which showed that DDTT-SBT14 have a higher thermal stability than DDTT-SBT18.
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CHAPTER 6
APPENDIX
DTT (NMR)
Figure 6.1 1H NMR (300 MHz) Dithieno[3,2-b:2′,3′-d]thiophene (1a)
DDTT-SBT14
Figure 6.2 ¬1H NMR (300 MHz) 2,2′-bithiophene (1b)
Figure 6.3 ¬1H NMR (300 MHz) 3,3′,5,5′-tetrabromo-2,2′-bithiophene (2b)
Figure 6.4 ¬1H NMR (300 MHz) 3,3′-dibromo-2,2′-bithiophene (3b)
Figure 6.5 ¬1H NMR (300 MHz) Tetradecanethiol
Figure 6.6 ¬1H NMR (300 MHz) 3,3′-bis(tetradecylthio)-2,2′-bithiophene (4b)
Figure 6.7 ¬1H NMR (300 MHz) 5,5′-dibromo-3,3′-bis(tetradecylthio)-2,2′-bithiophene (5b)
Figure 6.8 ¬1H NMR (300 MHz) 2,2′-(3,3′-bis(tetradecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT14 (6b)
DDTT-SBT18 (NMR)
Figure 6.9 ¬1H NMR (300 MHz) Octadecanethiol
Figure 6.10 ¬1H NMR (300 MHz) 3,3′-bis(toctadecylthio)-2,2′-bithiophene (4c)
Figure 6.11 ¬1H NMR (300 MHz) 5,5′-dibromo-3,3′-bis(octadecylthio)-2,2′-bithiophene (5c)
Figure 6.12 ¬1H NMR (300 MHz) 2,2′-(3,3′-bis(octadecylthio)-[2,2′-bithiophene]-5,5′-diyl)didithieno[3,2-b:2′,3′-d]thiophene or DDTT-SBT18 (6c) |
| 參考文獻 |
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