Spectral stability study and molecular modeling of fluorence based conjugated polymers 1

1 CHAPTER GENERAL INTRODUCTION 1-1 Scope of thesis The work of this thesis could be roughly categorized into two parts. The first part examines spectral and thermal spectral stability in films and aggregation in solutions for fluorene-based conjugated polymers. The second part reports influence of donor and acceptor substituents on the electronic characteristics of poly(fluorene-phenylene). The following sections in this chapter give a description of stability and processing, as well as optical and electronic properties of conjugated polymers, followed by a discussion on the development of polymer light-emitting diodes. Polyfluorene derivatives and light-emitting diodes are also discussed. Chapter discusses experimental and calculation methods used in this work. Ultraviolet-visible absorption spectroscopy, photoluminescence spectroscopy and differential scanning calorimetry are described. It is then followed by a discussion on semiempirical molecular orbital calculation. Chapter discusses in details spectral and thermal spectral stability of seven fluorene-based conjugated polymers in film states. Ultraviolet-visible absorption and fluorescence spectra of these polymers were presented. Their differential scanning calorimetry and crystallization analysis results were discussed. Chapter investigates aggregation behavior of polyfluorene derivatives in solutions. Five representative polyfluorene derivatives were examined with respect to their absorption and emission spectra in chloroform/methanol mixtures. Chapter investigates theoretically influence of presence of acceptor or donor group(s) along poly(9,9-dihexylfluorene-1,4-phenylene) backbone. The present quantum chemistry calculations report on changes in geometric and electronic properties of poly(9,9-dihexylfluorene-1,4-phenylene) unit cell induced by substitution with cyano, methoxy or amino group(s). 1-2 Introduction 1-2-1 Conducting polymers There is hardly an aspect of our lives that is not touched by synthetic polymers. The role of polymers in the electronics industry has been traditionally associated with insulating properties, whether these are for isolating metallic conductors or for use in photoresist technology. From that starting point it was the pioneering work of MacDiarmid, Heeger, and Shirakawa et al. that inspired chemists and physicists to consider the opportunity of using polymers as conductors. The report in 1977 from the University of Pennsylvania of high conductivity in charge-transfer complexes formed with a polymer, polyacetylene, which exhibited extended π conjugation along the polymer chain, provoked considerable excitement.1 Conjugated polymers derive their conducting properties by having delocalized π -electron bonding along the polymer chain. The π (bonding) and π * (antibonding) orbitals form delocalized valence and conduction wavefunctions, which support mobile charge carriers. As the length of the conjugated sequence is increased, the energy gap between the filled π and empty π* states falls, though in the long chain limit the gap remains finite, and takes a value of about 1.5 eV. Polyacetylene has served as the prototypical conjugated polymer; the simplicity of its structure has allowed theoretical modeling. This was the first demonstration of metallic behavior within the intramolecular π electron system along the polymer chain, and the significance of these results was quickly picked up by many other groups worldwide. Conducting polymers have tremendous potential for innovation. After 20 years of progress, these unusual polymeric materials can now be used as transparent antistatic coatings, electromagnetic shielding, superconductors, modified electrodes, electrochromic windows, supercapacitors, transistors, light-emitting diodes, lasers, conducting photoresists, photovoltaic cells, biosensors, and so forth.2,3 The significance of this class of polymers was recently highlighted by the awarding of the 2000 Nobel Prize in Chemistry to H. Shirakawa, A. G. MacDiarmid, and A. J. Heeger, the three scientists who pioneered this novel materials field. Whereas conventional polymers are readily processed in solution or in the melt and can be cheaply manipulated into desirable forms, this is not in general possible for conjugated polymers. The delocalized π electron system makes the molecular chains rigid, with resultant high melting points and low solubilities. The dilemma of a potentially attractive material that cannot be processed is not new in materials science in general or polymer science in particular. Two well-established lines of attack on such problems involve either modifying the molecular structure so as to retain the property of interest while rendering the material processible, or carrying out the processing stages with a more tractable precursor, which can be converted subsequently to the desired material. Both of these approaches, have been successfully applied to the processing of most classes of conjugated polymers, and the methods adopted are summarized in Section 1-3 for the major structural classes. Conjugated polymers behave as “molecular materials,” and there is a considerable reorganization of the local π electron bonding in the vicinity of extra charges added to the chains. This results in self-localization of the added charge, to form, in general, polarons, though for the particular symmetry of the trans isomer of polyacetylene, these take the form of bond-alternation defects, or solitons. The theoretical models developed to describe these processes are discussed in Section 1-4, along with a description of fluorescence from conducting polymers. 1-2-2 Polymer light-emitting diodes The availability of film-forming conducting polymers in the late 1970s resulted in attempts to fabricate a range of semiconductor devices, principally two-terminal diodes formed as sandwich structures with metallic electrodes to either side of a film of polymer. The first report on polyacetylene formed by the Shirakawa route, of in situ polymerization of acetylene gas onto the bottom electrode, revealed that Schottky barriers could be formed against metals with appropriate work functions (here the polyacetylene was functioning as a p-type semiconductor),4 However, these early experiments were constrained by the poor processibility of the polymers then available. Thin-film electroluminescence (EL), that is the emission of light when excited by flow of an electric current, in conjugated polymers has provided the other major area of device-related activity for display. The discovery that conjugated polymers could act as both transport and emissive layers, reported in 1990,5 has generated a very high level of interest. Device fabrication can be very straightforward, with a layer (or layers) of polymer sandwiched between two electrodes, one of which is transparent. This is illustrated in Figure 1.1 for the case of the first EL diodes.5 The polymer was prepared as a thin film, of thickness of order 100 nm, by spin-coating a “precursor” polymer from solution, using a standard photoresist spin-coater, and subsequently converting the “precursor” polymer to the semiconducting PPV by heating. Spin-coating from solution has been demonstrated to be capable of producing highly uniform layer thickness, with a thickness variation of no more than a few ångströms spread over several cm2. This polymer layer was formed on a glass substrate coated with indium-tin oxide (ITO), which has a relatively high workfunction and is therefore suitable for use as a hole-injecting electrode, and the other electrode then formed by thermal evaporation of the selected low workfunction metal such as Al, Mg or Ca, which are suitable for injection of electrons, as shown in Figure 1.1. Organic electroluminescent displays represent an alternative to the well-established display technologies based on cathode-ray tubes and liquid-crystal displays (LCDs), particularly with respect to large-area displays for which the existing methods are not well suited. Rapid progress has since been made, and EL diodes with a wide range of emission colors and with quantum efficiencies (photons/electron) of several percent are now reported. The development of polymer light-emitting diodes is discussed in Section 1-5. Figure 1.1 Structure of an electroluminescent diode based on a conjugated polymer. The polymer film is formed on a glass substrate coated with indium-tin oxide, and the top electrode is then formed by thermal evaporation. 1-2-3 Polyfluorenes Polyfluorenes are an important class of electroactive and photoactive materials. In the last few years this research field has literally exploded because of polyfluorenes’ exceptional electrooptical properties for applications in light-emitting diodes. This is the only family of conjugated polymers that emit colors spanning the entire visible range with high efficiency and low operating voltage. The development of this area is discussed in Section 1-6. 1-3 Chemical structures, stability and processing of conducting polymers The chemical structures of some common conjugated polymers are shown in Table 1.1. Table 1.1 Polymer Some common conjugated polymers Chemical Name Bandgapa Formula (eV) PA trans-polyacetylene PDA polydiacetylene 1.7 R n R PPP 1.5 n poly(p-phenylene) 3.0 n PPV poly(p-phenylenevinylene) 2.5 n RO-PPV poly(2,5-dialkoxy-p-phenyl 2.2 OR enevinylene) n RO PT polythiophene P3AT poly(3-alkylthiophene) S S n R 2.0 2.0 S S n R PTV poly(2,5-thiophenevinylene) S 1.8 n PPy polypyrrole 3.1 N H PAni polyaniline n 3.2 NH n The band gap is taken as the energy at the maximum slope, ∂α/∂E, of the spectrum of the optical absorption (α). a In order to successfully exploit the properties of conducting polymers in commercial applications, it is imperative that the candidate materials exhibit good environmental stability and be amenable to a wide variety of processing techniques. A polymer with poor environmental stability is essentially unstable in its doped state under normal atmospheric conditions. Compared to polyacetylene, polyheterocycles (such as polythiophene and polypyrrole) have demonstrated much better environmental stability6. For example, polypyrrole displays only minor changes in its conductivity state even after exposing in air to temperatures as high as 200 °C for extended periods of time. In general, the stability of a conducting polymer depends on a number of factors including its susceptibility and accessibility to external chemical species, the nature and type of counterion present in the material, the reactivity of its doped sites to surrounding chains, and the flexibility and conformational states of its backbone. A great deal of progress has been made towards the development of stable conducting polymers; however, this issue continues to be of paramount importance to the successful utilization of these materials in commercial applications. Many of the initially prepared conducting polymers were formed as intractable, insoluble films or powders that, once synthesized, could not be further manipulated into forms with more ordered, controllable structures. In fact, the structural attributes that give rise to the interesting electrical and optical properties of the conducting polymers, namely their rigid, planar conjugated backbones, severely limit the ways in which the polymer can be processed. To overcome these limitations, a number of structurally modified polymers and novel processing schemes have been developed that allow substantially more control over the state of the final product. These processing schemes can be conveniently divided into four categories. The first category is the manipulation of soluble precursor polymers. This scheme is based on the synthesis and manipulation of a processible, nonconducting precursor polymer that, once fabricated into a suitable form using conventional polymer 10 processing techniques (usually by thermal treatment), can be converted into an insoluble electrically conducting polymer. This route has been successfully utilized to prepare highly oriented thin films and fibers of polyacetylene7, poly(phenylene vinylene)8, poly(thienylene vinylene)9 and some other similar polymers10. The second processing scheme is the manipulation of soluble conducting polymer derivatives and copolymers. This is to modify the structure of the polymer in such a way to improve the processibility without compromising its electrical or optical properties. For example, it is possible to dramatically modify the processibility of the polythiophenes without severely compromising the electrical properties. By simply substituting the hydrogen atom attached to the three position of the thiophene ring with an alkyl group containing at least four carbons, conjugated polythiophenes that are both solution and melt processible can be achieved11. Meanwhile, the conductivities of the doped derivatives are also comparable to the parent polymer and generally range from 1-200 S/cm. The third processing scheme is the in-situ polymerization of conducting polymers in insulting matrix polymers. This processing scheme focuses on the growth of insoluble and intractable conjugated polymers within a performed polymer matrix. In this case, a processible, insulating polymer impregnated with a catalyst system is fabricated into a desired form such as a thin film or fiber. This activated polymer matrix is then exposed to the monomer, usually in the form of a gas or vapor, resulting in a blend typically comprised of an isolated or semi-continuous conjugated polymer phase dispersed throughout a continuous phase of the host polymer. For example, stretched aligned blends of polyacetylene/polybutadiene exhibit conductivities at least one order 20 states that pin the Fermi level22. When a positive bias is applied to the LED, the Fermi level of the cathode is raised relative to that of the anode. Carriers tunnel across the barrier primarily by Fowler-Nordheim field emission tunneling, and also by thermionic emission if the barriers are small and the temperature is relatively high. Since the rate of injection due to Fowler-Nordheim is determined by the electric field strength, it is important to keep a thinner polymer layer that is around 100-200 nm. In this case, high electric fields can be obtained at low applied voltages. To optimize the performance of LEDs, it is important to minimize the barriers for charge injection by choosing the right electrodes whose work function are well matched to the energy bands of the polymer. Indium tin oxide (ITO), polyaniline23, polypyrrole24 and poly(3,4-ethylenedioxythiophene) (PEDOT)25 are the most commonly used anode materials because they have high work functions and are transparent. Transparency is a very important factor for the anode because it allows light to escape from the device. Calcium is widely chosen as the cathode material because of its low work function. But due to its high reactivity, PLEDs must be hermetically sealed to prevent the degradation when using calcium as cathode. Recently, Campbell et al. and Cao et al. made a great progress in improving electron injection by coating the aluminum (which is more stable) with a polar self-assembled monolayer. This treatment effectively shifts the electrode work function26,27. If the electrodes are well matched to the bands of the polymer, the barrier for charge injection is small and therefore the current that passes through the LED is not limited by injection. 21 1-5-2 Efficiencies From the schematic energy-level diagram of the polymer light-emitting device (Figure 1.3), one can note that once the current has been applied, electrons and holes will be injected into the polymer, they must encounter each other and recombine radiatively to give off light. But there are several factors that will determine the efficiency of the PLEDs. The first factor is the degree of the carrier balance. If one carrier type is injected much more efficiently than the other one, the majority carriers traverse the entire polymer layer without recombining with the minority carrier. This problem can be solved by either choosing the appropriate electrodes so that both carriers are injected efficiently or by adding a hole or electron blocking layer so that the blocking layer creates a barrier at the interface of two polymers that blocks the flow of the majority carrier. Table 1.2 shows the range of efficiencies achievable with the standard PPV EL device using an ITO anode and a variety of metals as cathodes, with and without the electron transporting/hole blocking (ETHB) layer.28 It is noteworthy that useful efficiencies are already accessible using metals other than calcium. Table 1.2 Comparison of efficiencies achieved in the low voltage regime for an ITO-PPV-Metal EL device with an analogous device containing an extra PBD ETHB layer Metal Contact Work Function Efficiency Efficiency (eV) (PPV) (PPV+PBD) Ca 2.9 0.1% 1% Mg 3.7 0.05% 0.35% Al 4.2 0.002% 0.06% Au 5.3 0.00005%  22 As the density of the majority carrier increases at the blocking interface, the electric field at the minority carrier injecting electrode increases, which will enhance the minority carrier injection. But some times it is very difficult to spincast multiple layers of polymers onto the glass substrate because the first layer may be dissolved during the spincasting of the subsequent layers. Onitsuka et al. found a way to prevent this problem by depositing the polymers one monolayer at a time using a polyelectrolyte self-assembly technique in which alternating layers have opposite charge and are coulombically bound to each other29. Ho et al. have demonstrated that depositing a monolayer as thin as 10-20 Å thickness can sinnificantly improve carrier balance and enhance the performance of the PLEDs30. The second factor that can determine the efficiency of PLEDs is the photoluminescence (PL) efficiency. The photoluminescence is the fraction of photoexcited states that recombine radiatively. Since the radiative lifetime of most conjugated polymers is less than ns and there are few non-radiative channels for relaxation, the PL efficiency can be relatively high. Many conjugated polymers have PL efficiencies higher than 60%31. It is generally accepted that when an electron and a hole are combined, approximately 25% of the electron-hole pair in an LED form singlet and 75% of the pair form triplet states. Since triplet excitations are non-emissive, EL efficiency can be no greater than 25% of the PL efficiency32. Interestingly, Cao et al. have made LEDs with an EL efficiency of 50% PL efficiency, and demonstrated that all electron-hole pairs are potentially emissive33. The third factor that determines the luminescence efficiency of LEDs is the quenching behavior by the metal electrode. This behavior can be caused when electrons 23 and holes recombine too close to one of the metal electrodes34. The primary mechanism for the quenching is interference between the radiation field from virtual image oscillators in the metal. Metal quenching is a serious problem for extremely thin devices or for polymers whose electron mobility is so low that the electrons are not able to travel away form the cathode. The quenching behavior can be avoided by using an electron-transporting layer that also acts as a hole-blocking layer and thereby moves the emission region away from the metal region. 1-5-3 Control of color One of the attractive features of polymer EL is that the color of emission can be controlled by altering the chemical structure of the polymer. PPV gives emission in the yellow-green; the emission color can be moved toward the red by the substitution of electron-donating groups such as alkoxy chains at the 2- and 5- positions on the phenyl ring.35 Substituents can also cause changes in energy gap through steric, rather than electronic effects by disrupting the conjugation along the chain. The substitution of bulky cholestanoxy groups, for example, has been used to obtain green emission in soluble polymers.36 Other polymer systems, not based on PPV, can also be used for EL; the poly(alkylthiophenes), for example, conveniently give emission in the red region of the spectrum.37-40 An alternative strategy to obtain blue emission is the use of completely different conjugated polymer systems. Blue EL has been reported in poly(p-phenylene) (PPP),41 poly(alkylfluorene),42 fluorinated polyquinoline,43 and PPP-based ladder copolymers.44,45 One problem that is found for the larger gap polymers is that although 24 the luminescence from isolated polymer chains (in solution or in solid solution) may be blue, solid films often show red-shifted emission. This is observed for the PPP-ladder polymers, for which the dominant emission band is in the yellow part of the spectrum,46 and it has been established that this is due to formation of aggregates in the solid film, which support excitons that extend over more than the single chain.47 The tendency to form aggregates can be controlled by introduction of disorder in the ladder-PPP copolymers, which can produce good blue emission.44,45 Soluble copolymers of PPP and alkyl- or alkoxy-substituted PPP have also been used to produce blue EL.48-50 1-5-4 Operating lifetime The operating lifetime of PLEDs means the time needed for light emission to degrade to half of the initial value at constant current. Since the novel conjugated polymers are regarded as the new generation light emitting materials, there is one question existed: can commercial display products with sufficient lifetime be achieved with polymers processed from solution? Until recently, there was doubt that the level of purity required for semiconductor applications could be achieved. However, recent progress at UNIAX Corporation has demonstrated that high performance PLEDs can be fabricated with long operating life. At 400-500 Cd/m2 initial brightness, room temperature operating lifetime of several thousand hours can be achieved. Accelerated lifetime studies at 85 °C (initial brightness of 100 Cd/m2) have demonstrated in excess of 400 hours to half brightness, indicative of more than 40,000 hours at room temperature (independent measurements give an acceleration factor of approximately 100 between 85 °C and room temperature). Thus, PLEDs fabricated with materials processed from solution and spincast onto substrates can meet the requirements for commercial products 25 with operating lifetime in excess of 10,000 hours at display-level brightness. 1-5-5 Towards applications Polymer LEDs are expected to have commerical applications as backlights for liquid crystal displays and as the emissive material in alphanumeric displays within the near future. Extension to a full-colour graphic display (for computer monitors and for video display) is very attractive. However, this required red, green and blue colours with appropriate chromaticity, methods for colour patterning, and also new addressing schemes. Rapid progress is being made with these three problems. Development of full colour has been reported for polymers51. Solution-processing of polymers offers new methods for colour patterning, among which there is particular interest in ink-jet printing, to place separated pixels of red-, green- and blue-emitting polymers onto the prepared substrate. This is being developed by Seiko-Epson and Cambridge Display Technology51, and the use of ink-jet printing has also been reported by other groups52,53. Active-matrix transistor arrays, modified from those at present used for liquid-crystal displays, can now provide sufficient current-driving capability to meet the requirements of polymer LEDs. Demonstrator active-matrix polymer displays have been made51. 1-6 Polyfluorene derivatives and light-emitting diodes Poly(9,9-dialkylfluorene)s have recently received a lot of attention that can be attributed to a major issue: the possibility that they could be used to develop all plastic, full-color, light-emitting diodes. An important driving force for this research is the dream of building ultrathin and flexible screens for computers and televisions. The first 26 blue-emitting (λmax = 470 nm) polymer LED was made by Yoshino et al.42 using poly(9,9-dihexylfluorene) by oxidative coupling of the fluorene monomer with ferric chloride.54,55 Polyfluorene derivatives are a particularly suitable class of materials because they contain a rigid biphenyl unit (which leads to a large band gap with efficient blue emission), and the facile substitution at the remote C9 position provides the possibility of improving the solubility and processibility of polymers without significantly increasing the steric interactions in the polymer backbone. However, the first experiment on diode that emitted blue light were carried out with oxidatively prepared polyfluorene derivatives. The nonspecific oxidation reaction produces some partially crosslinked materials, and NMR studies on the low molecular weight soluble fraction of these polymers showed some evidence of irregular couplings along the backbone.55 In parallel, and mainly on the basis of studies on polyacetylene56 and polythiophenes,57 it became quite clear that the synthesis of well-defined (hopefully defect-free) conjugated polymers should lead to a significant improvement in the performance of electroactive and photoactive conjugated polymers. Some chemists accepted the challenge; in attempts to bring more reliable synthetic procedures to the field of electronic materials, a variety of synthetic tools (Grignard, Stille, Yamamoto, Heck, and Suzuki couplings, etc.) were utilized that allowed significant advances in this research field. For instance, investigations by Pei and Yang on nickel-catalyzed Yamamoto couplings of 2,7-dibromo-9,9-disubstituted fluorenes [Figure 1.4(A)] led to the synthesis of well-defined, poly[2,7-(9,9-dialkylfluorene)]s.58 highly This first conjugated, report and on processible regioregular poly[2,7-(9,9-dialkylfluorene)]s was rapidly followed by investigations on the 27 polymerization of well-defined fluorene-containing conjugated polymers using palladium-catalyzed Suzuki coupling reactions between 2,7-dibromofluorene derivatives and 2,7-diboronylfluorene derivatives [Figure 1.4(B)].59 The utilization of a phase transfer catalyst gives higher molecular weight (a number-average molecular weight of ca. 50,000 instead of ca. 15,000).59,60 Figure 1.4 Schematics of the polymerization of fluorenes from (A) Yamamoto, (B) Suzuki, or (C) Stille coupling reactions. Unfortunately, poly[2,7-(9,9-dialkylfluorene)]s and poly[2,7-(9-alkylfluorene)]s generally show some excimer emission (the formation of dimerized units in the excited 28 state that emit at lower energies) in the solid state that affects the color emission and the lifetime of light-emitting devices.58,61 This formation of excimers is surprising because the remote substituents at the C9 position should limit interchain interactions. Nevertheless, as is usual in polymer chemistry, the preparation of random copolymers and alternating copolymers perturbs short- and long-range organization in the materials. A variety of copolymers derived from 2,7-fluorenes were thus recently reported59,60,62 (Figure 1.5) and tested in different electrooptical devices. Most copolymers were obtained from a Yamamoto [Figure 1.4(A)], Suzuki [Figure 1.4(B)], or Stille [Figure 1.4(C)] coupling reaction. In most cases excimer formation was suppressed and a fairly good correspondence was observed between solution and solid-state fluorescence spectra (usually, a slight redshift is observed in the solid state that is due to an extended delocalization length), as well as between solid-state fluorescence and electroluminescence spectra. In addition, the development of such copolymers permitted the preparation of a variety of fluorene-containing copolymers that emit colors spanning the entire visible range (red, green, blue). In many cases luminance between 100 and 10,000 cd/m2 was obtained at only few volts. Clearly, polyfluorenes are seen as one of the most promising classes of electroluminescent polymers. However, as is the case for all polymer-based light-emitting diodes, the problem of stability remains and hinders their industrial applications. 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[...]... 12 Rubner, M F.; Tripathy, S K.; Georger, S K.; Cholewa, P., Macromolecules, 19 83, 16 , 870 13 Su, W.-P.; Schrieffer, J R.; Heeger, A J., Phys Rev Lett., 19 79, 42, 16 98 14 Su, W.-P.; Schrieffer, J R.; Heeger, A J., Phys Rev B, 19 80, 22, 2099; erratum, 31 1983, 28, 11 38 15 Rice, M J., Phys Lett., 19 79, 71A, 15 2 16 Heeger, A J.; Kivelson, S.; Schrieffer, J R.; Su, W.-P., Rev Mod Phys., 19 88, 60, 7 81 17... emphasize the importance of achieving high fluorescence efficiency of the conducting polymers The basic conception of band gap is also described 13 1- 4-2 Electronic structure – Ground state 1- 4-2a Tight-binding models The band structure of trans-polyacetylene has been modeled by several groups15 ,16 ; the most widely used model was developed by Su, Schrieffer and Heeger (SSH) ,13 ,14 and involves a tight-binding... Band gap can be determined by the bond alternation and torsion angles between rings in the polymers backbone Also, the extent of electron delocalization is the main factor that may determine the band gap of the conjugated polymers By tuning these factors, the band gap of conjugated polymers can be fluctuated in fine increments from 1. 1 eV to 3.3 eV Polyparaphenylene (PPP) has one of the largest band... structures and film thickness that are controllable at the molecular level The true promise of the LB processing technique is its unique ability to allow control over the molecular architecture of conducting polymer thin films 1- 4 Optical and electronic properties of conjugated polymers 1- 4 -1 Introduction The electronic structure of conjugated polymers can be conveniently described in terms of σ bonding... gaps (3.0 eV) of all conjugated polymers because its excited state 18 wavefunctions are localized to one repeat unit, while polyisothianaphene (PITN) has the smallest band gap (1. 1 eV) because its wavefunctions are highly delocalized and it has minimal bond alternation 1- 5 Polymer light-emitting diodes 1- 5 -1 Conjugated polymer electroluminescence Conjugated polymers exhibit a number of attractive properties... efficiencies of light emitting diodes can be achieved by encapsulating the conjugated polymers whose side chains will prevent neighboring backbones from forming excimers in an inert atmosphere and a hermetic packaging 1- 4-4c Band gap Band gap of the conjugated polymers can be tuned by modifying the polymers structures to obtain emission colors of the polymers in the visible and near infrared regions of spectra... 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Figure 1. 3 LUMO IP ∆Εe HOMO PPV Al Schematic energy-level diagram for an ITO/PPV/Al LED, showing the ionization potential (IP) and electron affinity (EA) of PPV, the work functions of ITO and Al (φITO and φAl), and the barriers to injection of electrons and holes (∆Ee and ∆Eh) [Handbook of Conducting Polymers, 2nd ed.; Skotheim, T.; Reynolds, J R.; Elsenbaumer, R L., Eds.; Marcel Dekker: New York, 19 98.]... the major factors that limit the stability of these electrooptical devices.63 29 Figure 1. 5 Examples of alternating and random copolymers derived from fluorenes and other aromatic monomers 30 References 1 Chiang, C K.; Fincher, C R.; Park, Y W.; Heeger, A J.; Shirakawa, H.; Louis, E J.; Gau, S C.; MacDiarmid, A G., Phys Rev Lett., 19 77, 39, 10 98 2 Handbook of Conducting Polymers, 2nd ed.; Skotheim, T.; . structures, stability and processing of conducting polymers The chemical structures of some common conjugated polymers are shown in Table 1. 1. 8 Table 1. 1 Some common conjugated polymers. the molecular architecture of conducting polymer thin films. 1- 4 Optical and electronic properties of conjugated polymers 1- 4 -1 Introduction The electronic structure of conjugated polymers. 1 CHAPTER 1 GENERAL INTRODUCTION 2 1- 1 Scope of thesis The work of this thesis could be roughly categorized into two parts. The first part examines spectral and thermal spectral stability