Thiophenol-modified CdS nanoparticles enhance the luminescence of

Figure 1. Normalized UV–Vis absorption spectra recorded in DMF for S-CdS nanoparticles having three different diameters. ...84 Figure 2. (a) Photoluminescence spectra of PF [poly-2,7-(9,9-dioctylfluorene)], excited by a xenon lamp at λmax = 393 nm, and PF-G0, PF-G1, and PF-G2, all recorded in THF at the same concentration (5 × 10^–6 M). (b) Photoluminescence spectra recorded in DMF of (i) PF-G1 in the solid state, using a xenon lamp as the excitation light source, (ii) PF-G1 in the solid state, using a GaN diode laser, and (iii) S-CdS nanoparticles having three different diameters. ...85 Figure 3. Photoluminescence spectra of thin films of (a) S-CdS3nm/PF-G1, (b) S-CdS4nm/PF-G1, and (c) S-CdS7nm/PF-G1, normalized with respect to the PL intensity of PF-G1. ...86 Figure 4. Photoluminescence spectra of thin films of (a) pure PF-G0, (b) PF-G0 containing 4 wt% S-CdS, (c) pure MEHPPV, (d) MEHPPV containing 4 wt% S-CdS, and (e) PMMA containing 4 wt% S-CdS. ...87 Figure 5. A. X-ray diffraction spectra of S-CdS/ PF-G1 nanocomposite. (a) PF-G1 (b) PF-G1 containing 3 wt% S-CdS, (c) PF-G1 containing 4 wt% S-CdS, and (d) PF-G1 containing 8 wt% S-CdS. B. The effect of the amount of S-CdS on the Bragg d spacing of PF-G1. C. FTIR spectra of (a) PF-G1 and (b) PF-G1 containing 4 wt% S-CdS. D. Cyclic voltammogram of the oxidation of polymer. ...88 Figure 6. Transmission electron microscopy images of PF-G1 films containing (a) 3 wt% and (b) 4 wt% of S-CdS. ...90 Figure 7. Normalized electroluminescence spectra of devices prepared from

S-CdS/PF-G1 in the configuration ITO/PEDOT/polymer/Ca/Al. ...90 Figure 8. (a) I–V and (b) L–V curves of devices prepared from S-CdS/PF-G1 in the configuration ITO/PEDOT/polymer/Ca/Al. ...91

Chapter 5: Polyfluorene Copolymer Incorporating Side-Chain-Tethered Gold Nanoparticles

Figure 1. ¹H NMR spectra of (a) tris(4-bromophenyl)amine and (b) DBMS. ...102

Figure 2. XPS spectra [S(2P) region] of PF-DBMS adsorbed onto Au NPs. ...103 Figure 3. Normalized UV–Vis absorption spectra and PL emission spectra of

PF-DBMS recorded in solution (THF) and from a thin film. The inset displays the thin film after thermal treatment at 100^oC for 2 h. ...103 Figure 4. (a) Transmission electron microscopy images of Au/PF-DBMS films. The inset displays the lattice image (lattice spacing: ca. 2.3Å) of the Au NPs. (b) Size distribution of Au NPs in the PF-DBMS polymer matrix. (c) TEM images of cross-sections of the device. ...104 Figure 5. (a) I–V and (b) L–V curves of devices prepared from Au/PF-DBMS in the configuration ITO/PEDOT/polymer/Ca/LiF/Al. ...105 Figure 6. EL spectra of devices prepared from Au/PF-DBMS in the configuration ITO/PEDOT/polymer/LiF/Ca/Al. ...106 Figure 7. (a) ¹H NMR spectra of the chemical structure of all polymers. (b) The chemical structure of the PF-DBMS. ...107

Scheme & Table Lists

Chapter 1: Introduction

Scheme 1. (a) The Wessling-Zimmerman precursor route to PPV. (b) End-capping

modification of the Gilch polymerization. ...24

Chapter 2. Polyfluorenes Incorporating Side-Chain-Tethered Polyhedral Oligomeric Silsesquioxane Units Scheme 1. Synthesis of D-POSS-diAF (4) (i) CrO3, acetic anhydride, HCl(aq). (ii) aniline, aniline hydrochloride. (iii) K2CO3, KI, DMF/THF (5:4) ...40

Scheme 2. Synthesis of PFO-POSS copolymers ...41

Table 1. Physical properties of the PFO-POSS copolymers ...42

Table 2. Optical properties of the PFO-POSS nanocomposites. ...42

Chapter 3: Polyphenylenevinylene Copolymer Presenting Side-Chain-Tethered Silsesquioxane Units Table 1. Physical Properties of the POSS-PPV-co-MEHPPV Copolymers. ...60

Table 2. Optical Properties of the POSS-PPV-co-MEHPPV Nanocomposites. ...60

Scheme 1. Synthesis of POSS-PPV-co-MEHPPV copolymers. Reagents and conditions: i, trichloro[4-(chloromethyl)phenyl]silane, HNEt3Cl; ii, K2CO3, DMF/THF; iii, N-bromosuccinimide (NBS)/ AIBN/ CCl4; iv, tert-BuOK/ THF. ..61

Chapter 4: Thiophenol-modified CdS nanoparticles enhance the luminescence of benzoxyl dendron-substituted polyfluorene copolymers Scheme 1. Synthesis of (a) S-CdS and (b) PF-GX and (c) a schematic drawing of the architecture of S-CdS/PF-GX (X = 1, 2). ...82

Table 1. Absorption and photoluminescence data for S-CdS/polymer nanocomposites in the solid state. ...83

Table 2. Thermal behavior of S-CdS/polymer nanocomposites. ...83

Chapter 5: Polyfluorene Copolymer Incorporating Side-Chain-Tethered Gold Nanoparticles

Table 1. Absorptions and Quantum Yields for Au NP/Polymer Nanocomposite Solid Films. ...100 Table 2. Absorptions and Quantum Yields for Polymer Solid Films. ...100 Table 3. Molecular Weights of the Polymers. ...101 Scheme 1. Synthesis of (a) PF-DBMS copolymers (b) A schematic drawing of the architecture of Au/ PF-DBMS nanocomposites. ...101

The list of abbreviations

BuLi n-butyllithium

CDT Cambridge Display Technology

CdS Cadmium Sulfide

DMF N,N-dimethylformami

DSC differential scanning calorimetry FWHM full width at half maximum, [nm]

GPC gel-permeating chromatography HOMO highest occupied molecular orbital

HTL hole-transport layer

PLED polymeric light emitting diode

PMMA poly(methyl methacrylate)

POSS polyhedral oligomeric silsesquioxane

PPP poly(p-phenylene)

PPV poly(p-phenylene vinylene)

PT polythiophene

Q.E. quantum efficiency

T^g glass transition temperature TGA thermal gravimetric analysis

THF tetrahydrofuran

Φ^PL quantum yield of photoluminescence XPS X-ray photoelectron spectroscopy

Abstract

The main objective of this dissertation is to study the performance of polymer light emitting diodes involving luminescent polymers incorporating different kinds of inorganic segment in their side chains. In the introduction of this dissertation, we gave an explanation on the historical evolution of polymer nanocomposites light emitting diodes and summarized the literatures in the recent years. In the chapter 2, we have synthesized polyhedral silsesquioxane-tethered polyfluorene copolymers, poly(9,9´-dioctylfluorene-co-9,9´-bis[4-(N,N-dipolysilsesquioxane)

aminophenyl]fluorene) (PFO-POSS), that have well-defined architectures using Suzuki polycondensation. This particular PFO-POSS molecular architecture increases the quantum yield of polyfluorene significantly by reducing the degree of interchain aggregation; in addition, these copolymers exhibit a purer and stronger blue light by preventing the formation of keto defects. The PPV-POSS molecular architecture also increases the quantum yield significantly by reducing the degree of interchain aggregation were discussed in Chapter 3. This particular molecular architecture of POSS-PPV-co-MEHPPV copolymers possesses not only a larger quantum yield (0.85 vs. 0.19) but also higher degradation and glass transition temperatures relative to those of pure MEHPPV. The maximum brightness of a double-layered-configured light emitting diode (ITO/PEDOT/emissive polymer/Ca/Al) incorporating a copolymer of MEHPPV and 10 mol% PPV-POSS was five times as large as that of a similar light emitting diode incorporating pure MEHPPV (2196 vs.

473 cd/m²).

The presence of a low percentage of thiophenol-modified cadmium sulfide

(S-CdS) nanoparticles in the benzoxyl-dendritic structure of a copolyfluorene (PF-GX)

substantially improves the efficiency of its light emission were discussed in Chapter 3.

The enhancements in photoluminescence and electroluminescence arise mainly from a reduction in the degree of energy transfer from the excited polymer chains to their neighboring polymer chains in the ground state; i.e., there is an increase in the

inter-polymer chain distance when CdS nanoparticles are present. We have prepared highly luminescent dendron-substituted copolyfluorenes that incorporate

surface-modified cadmium sulfide nanoparticles. A small percentage of these nanoparticles can be incorporated into the dendritic structures upon tailoring the interfaces between the ligands on the nanoparticles and the dendritic structures in the copolyfluorene. Both the photoluminescence and electroluminescence efficiencies of the polymer nanocomposites are dramatically enhanced. Moreover, in order to know the effect to some other nanoparticles, we have tethered gold nanoparticles (Au NPs) to the side chains of poly{2,7-(9,9´-dioctylfluorene)-co- 4-diphenylamino-4´- bipenylmethylsulfide} (PF-DBMS) through its ArSCH3 anchor groups. The presence of 1 wt% of the tethered gold NPs led to a reduction in the degree of

aggregation of the polymer chains, resulting in a 50% increase in its quantum yield.

The electroluminescence of a 1wt% Au/ PF-DBMS device was three times higher in terms of its maximum brightness and its full-width-at-half-maximum emission peak was much narrower than that of a pure PF-DBMS device. These phenomena arise from the photooxidation suppression, hole blocking, and electron transport enhancing effects of the Au NPs were also demonstrated in Chapter 5.

摘要

高分子發光二極體(PLED)是未來發展成大平面的顯示器的重要技術，而大部份的電激發光高分子，由於具有豐富的π電子，因此電洞注入特性和傳輸電洞的能力遠比電子注入特性和傳輸電子的能力來的有效率。近期不少研究著重於開

發高效率且穩定的發光材料，其中又以藍光材料最受重視。聚茀（Polyfluorene）

及其衍生物由於包含一剛性且共平面的雙苯環結構，所以表現出特殊的物理和化學性質; 然而，無論是PPV 或者是PF系列元件在製成薄膜時，由於材料累積濃度過高造成分子堆疊或產生excimer，嚴重影響光色以及降低發光效率; 此外，PF 系列由於C9位置容易產生氧化現象(keto defect)，也會改變元件原有穩定的發光

光色。為了改善這些缺點，選擇導入多立面體聚矽氧烷(POSS) 於高分子材料之

側鏈期望以防止氧化及減少分子堆疊，使得高分子的光色及熱穩定性進一步改良，可廣泛地應用在顯示器的發光材料上。此方法所合成之高分子奈米複合材料可提升發光高分子之發光效率、元件效率並提升其耐熱度及穩定性。在本文的第二及第三章節，我們將分別討論POSS在Polyfluorene(PF) 與 Polyphenyl vinylene (PPV)中所扮演的角色。另外一部分，有鑒於高分子內部螢光發光效率最高僅達25%的物理限制，高分子與無機材料的結合便受到了矚目﹔雖近期有磷光高分子發光二極體之開發，但合成時必須使用重金屬，來源恐不穩定。因此，

本研究嘗試以膠體化學法合成半導體材料量子點 (其直徑小於10 奈米)製備一系列S-CdS/PF-GX (X=1, 2)之奈米複合發光材料，並藉由實驗證實，導入少量改質的量子點(S-CdS)，不但可有效地提昇螢光及電激發光效率至原來之兩倍至三倍，同時也增強原本材料在製程元件後的穩定度及電性。在第五章中我們選擇用金奈米粒子並對其特性作進一步的探討。含有1wt %的金奈米粒子之聚茀高分子共聚物提高了原本的量子效率與光學穩定性；同時在元件部份，與純聚茀高分子共聚物比較，高分子藉由鍵結金奈米粒子之元件也具有較優異的表現。

Chapter 1

Chapter 1: Introduction

1-1 Introduction of Light Emitting Polymer and History

Since the discovery of PLEDs in 1989^[1], significant effort has been directed into the development of red, green and blue materials that exhibit high efficiency and stability under normal operating conditions, and to enable integration into flat panel display (FPD) applications (Figure 1). From 1989 until now, LEDs is probably the most important application maintaining the researchers’ interest towards conjugated (conducting) polymers, although in recent years we witness a growing interest towards other relevant applications such as sensors and photovoltaics. Hundreds of academic research groups around the worlds have contributed to the development of electroluminescent polymers. An even more pronounced research activity is being held in industry. Several newly born R&D companies such as Cambridge Display Technologies (CDT, spin-off from Cambridge University), Covion Organic

Semiconductors and UNIAX (spin-off from UCSB), are targeted at development of high efficiency, long life-time EL polymers. A huge commercial potential, connected with the possibility of solution fabrication of EL devices, and, particularly, flat and/or flexible displays, attracted in the business such industrial giants as Dow Chemical, DuPont, IBM, Kodak and Philips.^[2] PLEDs utilize the same physic principles as LEDs but use polymers as the active light-emitting layer. It has many

advantages when compared to normal inorganic LEDs. Simple and costefficient manufacturing and the ability to generate a uniform area of light demonstrate that PLEDs exhibit excellent promise for current and future electronic and optical applications. The first PLEDs used poly(phenylene vinylene) PPV as the emitting layer. PPV is an undoped conjugated polymer, which has a molecular structure given

Chapter 1

in Figure 2. Today many other polymers such as polythiopenes, polypyridine, poly(pyridyl vinylenes) and polyphenylene have been used to emit light (Figure 3).

Light emitting diodes consist of active or emitting layers placed between a cathode (typically aluminum or calcium) and an anode (ITO, indium tin oxide). A diagram of a typical PLED is shown in Figure 4. When the two electrodes are connected electrons are injected from the cathode into the p* -band semiconducting polymer and holes are injected from an electrode into the p –band. The oppositely charged carriers in the two bands meet within the polymer films and recombine (return to their ground state) radiatively to give off light.^[3] (Figure 5)

1-2 Research Motivation

1-2-1. The Original of Green Emission in Polyfluorene-based Conjugated Polymers: On-Chain defect Fluorescence.

Conjugated polymers have been studied in great detail as electroluminescent materials for use in organic light emitting diodes. Significant progress has been made in understanding the fundamental working processes involved in generating efficient, reliable light across the entire visible spectrum. Polyfluorene-based materials have been investigated extensively because of the many attractive properties they possess.

Additional experimental results are needed to discern the nature of the unwanted green emission that often appears during device operation and quantum efficiencies need to be significantly improved. The currently most challenging topic for conjugated polymers applications is blue emission color stability. All available poly(para-phenylene) (PP)-type materials, which are the most promising family of

Chapter 1

blue light emitters, are prone to degradation, resulting in an unwanted change in color due to the emergence of low energy green emission peak. However, vital to overcome the problem and to provide sutible future synthetic stragies. The low-energy

emission band has been mostly attributed to recordering of the polymer chains and subsequent aggregate^[4] or excimer formation.^[5] It will be shown in the following that both excimer and aggregate formation cannot explain the presented experimental observations and can therefore be ruled out as the main origins of the energy emission band of polyfluorene-type polymers as suggested previously.^[6] Instead, the

experimental observations show fluorescence from an on-chain oxidative defect to be the source of this emission band.

1-2-2. keto defect

Low-energy emission bands have been identified in all PPP-type polymers upon thermal,^[7] photo-, and/ or electrical degradation. Recently, the occurrence of this emission band has been correlated to presence of ketonic defects incorporated into the polymer backbone in the form of 9-fluorenones. Furthermore, fluorene-fluorenone copolymers has established a quantitative correlation between the 9-fluorenone content and the low-energy emission band intensity. However, no unambiguous verdict about the nature of the emission (emission from the aggregation or excimer) could be found. This identification is, however, crucial for determining future synthetic approaches: Influencing the solid state packing of the polymer chains by bulky side chains,^[8] spiro-linked compounds,^[9]or disorder induced by the

copolymerization of fluorine with e.g., anthracene^[10] could effectively hinder aggregate or excimer formation. Fluorescence emission from oxidative (on-chain) defects, on the contrary, can only be excluded by improving the oxidative stability of

Chapter 1

the polymers.

1-2-3. Luminescence Enhancement in Polymer/Nanoparticle Composite Electro-Optic Devices

Polymer light emitting diodes have a good chance to become the main display system in the near future since diodes have many advantages concerning preparation and operation over other display systems. However, material with single species often can not meet all the stringent criteria of industrial applications. The major drawbacks of the PLEDs have been operating life times and insufficient device radiances.

Recently, it has been shown that incorporating oxide nanoparticles into a PPV

derivative enhances the PLED current density and radiance by an order of magnitude.

In this thesis, we have to improve or tailor material properties with composite and develop finely tailored nanocomposite materials for emissive or non emissive display technology. An efficient way to obtain promising new materials is to modify existing potential materials with doping of semiconductor nanoparticles are described.

1-3 Materials

1-3-1. Polyfluorene (PF)

Fluorene is a polycyclic aromatic compound, which received its name due to strong violet fluorescence which arises from highly conjugated planar π-electron system. The 2,7-Positions in fluorene are the most reactive sites towards

electrophilic attack, which allows construction of a fully conjugated rigid-rod polymer chain by substitution reactions, whereas the methylene bridge provides an opportunity

Chapter 1

to modify the processability of the polymer by substituents, without perturbing the electronic structure of the backbone. The varieties, excellent optical and electronic properties, and high thermal and chemical stability of polyfluorenes (PFs) make them an attractive class of materials for polymer light-emitting diodes (PLEDs). Different aspects of syntheses, properties and LED applications of fluorene-based conjugated polymers and co-polymers have been highlighted in several recent reviews.^[11] In fact, polyfluorenes are the only class of conjugated polymers that can emit a whole range of visible colors with relatively high quantum efficiency.

It is well known that PFs are the most promising class of blue-emitting materials.

The original problem associated with undesirable “green emission band” was shown to be a result of exciton trapping on the electron deficient fluorenone defect sites. The color purity can be reestablished via 1) careful purification of the monomer (complete elimination of mono-substituted units), 2) inserting a protecting layer between the PF and reactive cathode material, 3) introducing hole-trapping sites (most commonly, triarylamine units), which would compete with fluorenone defects, minimizing the excitone formation on the latter, 4) introducing bulky substituents into PF backbone, which would minimize the exciton trapping on fluorenone defects. Furthermore, introducing of different conjugated moieties into PF backbone allows for efficient color tuing in these materials.

1-3-2. Poly(p-phenylene vinylene) PPV

Conjugated polymers are organic semiconductors in which the п-molecular orbitals are delocalized along the polymer backbone. Polymer-based OLEDs are attractive due to their excellent film forming properties and their ease of application

Chapter 1

over large surfaces through simple, economically viable coating techniques such as spin coating or ink-jet printing. Small molecule emissive materials are typically coated as thin films via vacuum-deposition which is difficult over large areas and is not as cost effective. As previously stated, PPV was the first polymer that was shown to display electroluminescence. Since direct synthesis of PPV produces an insoluble material, an alternative route was developed to allow the spin-coating of a soluble precursor polymer from solution (Scheme 1). Poly(p-phenylene vinylene) (PPV) is a highly stable conjugated polymer. Its yellow color is due to an absorption band centered at ~400–420 nm (depending on the method of synthesis) with an on-set corresponding to a band gap of ~2.5 eV.^[12] The HOMO and LUMO levels in PPV

在文檔中側鏈含有奈米結構之高分子電激發光二極體 (頁 7-109)