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1 TRANSPARENT THERMOSETS BASED ON MULTIFUNCTIONAL THIOLS AND THEIR APPLICATIONS By SANGJUN LEE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2012
2 2012 S angjun L ee
3 To my family and all my friends
4 ACKNOWLEDGMENTS Above all, I would like to express my deepest gratitude and respect to Dr. Douglas for his support. Your brilli ant advices and guidance has led me to successfully attain a PhD degree in Materials Science and Engineering. It is such an honor to graduate from your research group. The experiences I have gained and the moments spent with my research colleagues are cert ainly unforgettable. To Dr. So, Dr. Xue, Dr. Singh, and Dr. Miller, it is an honor to have such admirable people as part of my committee. I want to especially recognize Dr. Xue for the great advice you gave in co project. To my research colleagues: Than k you Sungwon Choi for being a good friend and giving me great advices from your research experiences. Yuping Li and Jei Le for helping me get started in the lab. Andrew Steward and Changhwa Lui for all the discussions we had of the experiments. My since re thanks to the people back home in Korea. My wonderful friends Guisun Lee, Minsoo Kim, and Hyekyung Yoon for the kindness and compassion they have shown to my family and I. I cannot express how grateful I am especially after they have visited and looked after my parents multiple times ever since my move to the states. To the people who started at the University of Florida with me: Sanghyun Eom, Inkook Jeon, Dongwoo Song, Byungwook Lee, Kangtaek Lee, Jinwoo Kwak, and Seonhoo Kim You have all been extreme ly helpful when I had difficulties adjusting to my new life here in Gainesville.
5 Jinhyung Lee, Hyuksoo Han, Sungwook Mhin, Jiho Ryu, Wooram Yoon, and Hyunggeon You. You guys were also a great help and made my stay in Gainesville a fun experience I will c herish all the moments we have shared. I want to express my unchanging love and appreciation to my girlfriend Elaine for making each and everyday bliss. A final thanks to my parents I love and respect. Thank you for always believing in me supporting and encouraging me. Words cannot express my gratitude towards the both of you.
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 14 CHAPTER 1 INTRODUCTION TO THIOL ENE AND YNE PHOTOPOLYMERIZATION ............ 16 Thiol Enes Click Reactions ................................ ................................ ..................... 16 Kinetics of Thiol Ene Polymerization ................................ ................................ ...... 18 Initiation ................................ ................................ ................................ ............ 18 Propagation ................................ ................................ ................................ ...... 20 Applications of Thiol Ene Thermosets ................................ ................................ .... 22 Thiol Yne Click Polymerization ................................ ................................ ............... 24 Kinetics of Thiol Yne Polymerization ................................ ................................ 24 Materials Properties of Thiol Yne Thermosets ................................ ................. 27 Outline of Dissertation ................................ ................................ ............................. 28 2 HIGH REFRACTIVE INDEX POLYMERS ................................ .............................. 30 Introduction ................................ ................................ ................................ ............. 30 Strategies to Increase Refractive Index of Polymers ................................ .............. 30 HRIP Inclu ding Halogens ................................ ................................ ................. 31 HRIP Including Phosphorus ................................ ................................ ............. 32 HRIP Including Sulfur atoms ................................ ................................ ............ 33 Organic inorganic nanocomposites ................................ ................................ ........ 34 3 LIGHT EXTRACTIONS OF ORGANIC LIGHT EMITTING DIODES ....................... 38 Introduction to OLEDs ................................ ................................ ............................. 38 Efficiency Limitations of OLEDs ................................ ................................ .............. 40 Methods to Improve Low Out coupling Efficiency ................................ ................... 42 F abrication Processes for Microlens Array on OLEDs ................................ ............ 45 4 ENCAPSULATIONS OF LIGHT EMITTING DIODES ................................ ............. 51 Introduction to Encapsulations of LE D ................................ ................................ .... 51 Encapsulation Materials ................................ ................................ .......................... 52
7 Epoxy Silicone hybrid resin ................................ ................................ .............. 53 Inorganic Particle Loaded Nanocomposites for LED Encapsulations ............... 55 5 UV CURABLE HIGH REFRACTIVE INDEX AND TRANSPARENT T i O 2 LOADED THIOL YNE NANOCOMPOSITES ................................ .......................... 58 Introduction ................................ ................................ ................................ ............. 58 Experiment ................................ ................................ ................................ .............. 60 The Refractive Index of Thiol Yne Thermosets ................................ ....................... 62 Suface Treatment of TiO 2 by Silane Coupling Agent ................................ .............. 64 TGA Measurement of Pure TiO 2 and Surface Modified TiO 2 ............................ 66 FTIR Analysis of Pure TiO 2 and Surface Modified TiO 2 ................................ .... 66 Nanocomposites ................................ ................................ ................................ ..... 67 Composition and Thermal Properties ................................ ............................... 67 Optical Transparency of Nanocomposites ................................ ........................ 69 Refractive Indices of Nanocomposites ................................ ............................. 71 Conclusion ................................ ................................ ................................ .............. 73 6 MICROLENS ARRAYS CREATED BY A DIRECT PRINTING TECHINQUE FOR LIGHT OUT COUPLING EFFICIENCY ENHANCEMENT OF ORGANIC LIGHT EMITTING DEVICES ................................ ................................ .................. 74 Introduction ................................ ................................ ................................ ............. 74 Experiment ................................ ................................ ................................ .............. 76 Surface Treatment on Substrates of OLEDs by Silane Coupling Agent ................. 79 Zisman plots of hydrophobic SCAs ................................ ................................ .. 7 9 Vapor phase deposition of SCA ................................ ................................ ....... 81 Optical Properties of Thiol Ene Len s Materials ................................ ....................... 83 Microlens array fabrication by direct printing technique ................................ .......... 84 Enchnement of light out coupling efficiency ................................ ............................ 86 Microlens array fabrication by inkjet printing technique ................................ .......... 88 Conclusion ................................ ................................ ................................ .............. 90 7 MULTI LAY ER ENCAPSULANT FOR LIGHT EMITTING DIDOES ........................ 92 Introduction ................................ ................................ ................................ ............. 92 Experiment ................................ ................................ ................................ .............. 94 Optical properties of encapsulants ................................ ................................ .......... 95 Optical Total Transmission of Graded Refractive Index Encapsulations ................ 96 Thermal Stability and d iscoloration ................................ ................................ ......... 98 Conclusion ................................ ................................ ................................ ............ 103 8 CONCLUSION S ................................ ................................ ................................ ... 104 LIST OF REFERENCES ................................ ................................ ............................. 107 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 115
8 LIST OF TABLES Table page 1 1 Summary of Tg, fwhm, elastic modulus, and crosslink density for PETMP/DDY and PETMP/BDDVE networks. ................................ ..................... 28 2 1 Molar refraction of atoms and organic groups. ................................ ................... 31 2 2 High Abbe nu mber and high refractive index sulfur containing polymers. .......... 34 7 1 Total transmittance of no encapsulation, conventional encapsulation (n=1.5), dual encapsulations, and multi layer encapsulations. ................................ ......... 97
9 LIST OF FIGURES Figure page 1 1 General thiol ene coupling ................................ ................................ .................. 17 1 2 General thiol ene p olymerization process. ................................ ......................... 18 1 3 Mechanism of the initiation process of thiol ene radical polymerization by DMPA. ................................ ................................ ................................ ................ 19 1 4 Reversible proce ss of the thiyl radical addition to the internal ene. .................... 20 1 5 Several surface modification approaches by thiol reactions .............................. 23 1 6 Reacti on mechanism for sequential addition and hydrogen abstraction process. ................................ ................................ ................................ .............. 25 1 7 stoichiometrically balanced DDY and PETMP photopolymerization. .................. 26 2 1 Chemical structures and refractive index of poly carbonates and polyphosphonates. ................................ ................................ ............................. 32 2 2 Chemical structure of poly(amic acid) and SEM images of polyimide titania nanocomposites and the pattern by photolithography. ................................ ....... 36 3 1 Illustrations of flexible OLEDs.. ................................ ................................ ........... 39 3 2 Exemplar waveguide modes in bottom emitting OLEDs (BOLEDs) ................... 41 3 3 Schematic illustration of optical ray trajectories of light generated from OLEDs with substrate modifications.. ................................ ................................ 42 3 4 Schematic illustration of OLEDs with ordered arrays of silica mi cro spheres. .... 43 3 5 Light output scheme. ................................ ................................ .......................... 44 3 6 Coupling enhancement factor versus lens height for different lens diameter. .... 46 3 7 Schematic illustration for hemispherical microlens array formed by the soft lithography method assembled the PS array. ................................ ..................... 47 3 8 Schematic i llustration of fabrication process via a self assembly approach. ....... 48 3 9 SEM images of microlens array fabricated by inkjet printing process. ................ 49 4 1 Light extraction of LEDs by the encapsulation ................................ ................... 52
10 4 2 Fabrication process of cycloaliphatic epoxy hybrimer bulk. ................................ 54 4 3 Three dimensional ray tracing simulation of light extraction efficiency as a function of scattering coefficient of the encapsulation with various absorption coefficients. ................................ ................................ ................................ ......... 56 5 1 Chemical structur es and abbreviation of dithiols and dialkynes. ......................... 62 5 2 Dependence of the refractive index on concentration of sulfur in the photopolymerized networks ................................ ................................ ................ 63 5 3 Measured refractive index of the film made of TDET HptDY .............................. 64 5 4 General structure of silane coupling agent ................................ ......................... 65 5 5 TGA analysis of pure TiO 2 and MPTMS modified TiO 2 ................................ ...... 66 5 6 FT IR spectra of untreated nanoparticle and MPTMS modified nanoparticle ..... 67 5 7 TGA results of matrix polymer and nanocomposites. ................................ ......... 68 5 8 UV VIS transmittance spectra of the TDET HptDY film and nanocomposite films ................................ ................................ ................................ .................... 69 5 9 Refractive indices of the TDET HptDY film and nanocomposites at 580 nm wavelength ................................ ................................ ................................ ........ 72 5 10 Sample species of the ellipsometer ................................ ................................ .... 72 6 1 Deposition of microlens array by the direct write instrument .............................. 78 6 2 Zisman plot and the critical surface tension ................................ ........................ 80 6 3 SEM cross section images of microlenses on the modified surfaces resulting from various silane vapor deposition times ................................ ......................... 82 6 4 The refractive index of lens material ................................ ................................ ... 83 6 5 T ransmittance of thiol ene lens material ................................ ............................. 84 6 6 SEM images of a printed microlens array ................................ ........................... 86 6 7 The normalized spectral intensity of both the bare OLED side and the lens enhanced side of a large area device. ................................ ................................ 87 6 8 The angular emission pattern of a lens enhanced 4 mm 2 device compared to both a bare device and an ideal Lambertian light emitting pattern. ..................... 87 6 9 Microdrop inkjet printer with 3D movement actuators. ................................ ...... 88
11 6 10 Microlens array images created by inkjet printing system ................................ .. 89 6 11 Mea surement of lens diameter and height by Wyko optical profilometer ............ 90 7 1 Transmittance of encapsulation materials and the refractive index of FG (inset image) ................................ ................................ ................................ ....... 96 7 2 Optical total transmittance versus refractive indices of the encapsulation layers following the semiconductor in dual encapsulations. ............................... 98 7 3 Optical transmittance spectra of the TY and FG (inset image) samples before and after thermal aging at 120 C for 120hours ................................ .................. 99 7 4 Optical transmittance spectra of the NCTY sample before and after thermal aging at 120 C for 120hours. ................................ ................................ ........... 100 7 5 Optical transmittance spectra of the double layers sample before and after aging at 120 C for 120hours ................................ ................................ ............ 101 7 6 Yellowness indices during thermal aging for 0 120 hours for FG, TY, NCTY and FGNC during thermal aging ................................ ................................ ....... 102 7 7 C hange in yellowness index and transmittance at 450 nm (lower graph) for FG, TY, NCTY and FGNC during thermal aging ................................ ............... 103
12 LIST OF ABBREVIATION S Alq3 tris (8 hydroxyquinoline) BDDVE 1, 4 butanediol divinyl ether BDT buthanedithiol BP benzophenone CVD Chemical vapor deposition DDDY dodecadiyne DDT decanedi thiol DDY 1,9 dodecadiyne DMPA dimethoxyphenyl acetophenone DPSD diphenylsilanediol ECTS 2 (3,4 epoxycyclohexyl)ethyltrimethoxysilane EDT ethanedithol EMDS 2 (3,4 epoxycyclohexylethyl) methyldiethoxysilane FG faux glass FS Flory Stockmayer fwhm full width at half maximum HHPA hexahydrophthalic anhydride HptDY heptadyne HRIP High Refractive Index Polymer ITO indium tin oxide LED Light Emitting Diode MBE Molecular beam epitaxy MeHHPA methylhexahydrophthalic anhydride MPTMS 3 methacryloxypropyltrimethoxysil ane
13 NDY nonadiyne NPB N,N¡odi(naphth 2 yl) N,N¡ndiphenyl benzidine ODY octadiyne OLED Organic Light Emitting Diode PAA poly(amic acid) PCE power conversion efficiency PDMS poly dimethyl siloxane PETMP pentaerythriol tetra(3 mercaptopropionate) PS polyst yrene PVO phenyl vinyl oligosiloxane RI Refractive Index SSL Solid state lighting TBPM tetrabutylphosphonium methanesulfonate TDET thiodiethanethiol TGA thermal gravimetric analysis TIR Total internal reflection UV Ultraviolet v Abbe number VTMS vinyltrime thoxysilane IQE I nternal quantum efficiency out O utcoupling efficiency EQE External quantum efficiency
14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degr ee of Doctor of Philosophy TRANSPARENT THERMOSETS BASED ON MULTIFUNCTIONAL THIOLS AND THEIR APPLICATIONS By S angjun L ee May 2012 Chair: Elliot Douglas Major: Materials Science and Engineering Click rea ctions using thiols provide highly crosslinked netw orks with rapid reaction rate in ambient environment such as oxygen and moisture Thermosets based on thiols show the most ideal homogeneous network arrangement with narrow glass transition regions and extremely low polymerization shrinkage. Click reaction s empl o ying thiols can be applied to a variety of applications such as surface science, coatings, optical components, adhesives, photolithography, microdevice fabrication and so on In this dissertation we developed various optical applications by using c lick reactions with thiols. First, we studied UV curable high refractive index and transparent polymers In order to overcome limitation of low refractive index of intrinsic polymers, organic inorganic nanocomposites have been widely used. However, nanopa rticles loaded nanocomposites have some limitations such as optical loss and difficult processability which is not compatible with usual processing for polymers such as UV curing and molding. We utilized alkyl dithiol and dialkyne monomer mixtures as a mat rix and surface modified TiO 2 nanoparticles as a filler. Thiol yne nanocomposites have the high refractive index ~1.69 and high transmittance over 88 %. Furthermore, the approach
15 presented herein can easily produce the desired shape by printing or molding methods without side products It does not require materials clean up and does not use solvents or high temperature processes. Microlens arrays are effective methods to improve low out coupling efficiency of OLEDs. We demonstrated simple fabrication proce ss via printing techniques by using mutifunctional thiol and ene as a lens material. Trimethylolpropane tris(3 mercaptopropionate) and tri(ethylene glycol) divinyl ether monomer mixture has low viscosity, high transpar e ncy and ~1.5 refractive index. Theref ore it is suitable for the lens material and printing process. In order to obtain high contact angle of liquid lens droplets, surface energy of glass substrates was modified with hydrophobic silane coupling ag en ts. One of the fundamental limitations in the light extraction efficiency of LEDs is the total internal reflection at the interface between high refractive index semiconductors (n=2.5 3.5) and low refractive index encapsulations. Here, we proposed multi layer encapsulation and studied graded refracti ve index LED encapsulation for minimizing Fresnel reflection and scattering losses. Yellowness index of each layer is calculated after thermal agin g to identify thermal stability I n this dissertation, we presented optical applications such as high refrac tive index polymer, microlens array for enhancement of outcoupling efficiency for OLEDs and graded refractive index LED encapsulation by us ing thermosets based on thiols in order to provide homogeneous networks with high refractive inde x and ease of proce ssing for the se applications.
16 CHAPTER 1 INTRODUCTION TO THIO L ENE AND YNE PHOTOPOL YMERIZATION 1.1 Thiol Enes Click Reactions The use of thiols in chemical reactions has been known for 100 years, including various fields ranging from polymer science to biochemistry. Early work on thiol ene free radical polymerization started the late 1930s. 1 The first large sca l e use of thiol ene radiation curing in the United S tates w as successfully done by Morgan, Ketley and their coworkers. 2 3 However, there were objections to thiol ene based ultraviolet (UV) curable resins because of the odor and incorrect belief that all thiol ene coatings were prone to yellowing and discoloration upon weatherin g. Unfortunately, a large quantity of benzophenone (BP) was used as a photoinitiator and significant light stability problems occurred such as yellowing and discoloration in the cured thermoset network since a portion of BP remained unconsumed at the end o f the polymerization and acted as plasticizer and photoreactive species in the network. Therefore, colored byproducts were produced upon exposure to interior or exterior light. However, this problem was overcome by employing cleavage type photoinitiator s I t was possible to use conventional cleavage type photoinitiators to initiate thiol ene polymerization and thiols could be effectively combined into acrylate formation to reduce oxygen inhibition and enhance the final film properties. In 2001, researchers described a new concept for accompanying organic reactions which focus ed consideration on highly selective, simple orthogonal reactions that do not have side products and give heteroatom linked molecular systems with high efficiency under various mild reac tion environments by the Sharpless et al 4 Several effect ive reactions have been grouped under the term click reactions. These reactions are able to
17 produce a variety of organic materials and functional synthetic molecules. Copper catalyzed azide/alkyne click reactions have received particular attention with app lications extending to the synthesis of biomedical libraries, dendrimer preparation, cross linking of adhesives for metal substrates among others. 5 7 Hoyle et al. 8 focus ed on the comparably weak bondin g between sulfur and hydrogen of thiols because of a n excess of chemical reactions with nearly quantitative yields with a capability to initiated by various methods under the mild condition. Two main classifications of thiol reactions have been noted durin g the last century. Figure 1 1 shows both thiol ene free radical addition to carbon carbon double bond and the catalyzed thiol Michael addition to electron deficient carbon carbon double bond. 8 Figure 1 1. General thiol ene coupling by a) free radical and b) Michael addition reactions. A single thiol reacts with a single ene in the idealized reaction. ( Adapted from  ) Both thiol ene reactions provide a quantitative yield, require little or no catalysts, are not susceptible to ambient environment such as oxygen and moisture, have rapid reaction rate s and a wide range of thiols and enes are readily available There are four major thiol types including alkyl thiols, thio phenols, thiol propionates, and thiol glycolates in the literature reports. Free radical and Michael addition reactions are very efficient with these thiols. Any non sterically blocked terminal ene is able to participate in the radical mediated thiol ene r eaction. E lectron rich and strained enes such as vinyl ether and norbornene are capable of reacting more rapidly than electron deficient ene s. These four types of thiols and electron rich and strained enes systems can form ideal
18 homogeneous network arrange ment s with narrow glass transition regions and extremely low polymerization shrinkag e. 9 10 1.2 Kinetics of Thiol Ene Polymerization 1.2.1 Initiation Figure 1 2 shows general process of thiol ene polymerization Polymerization is initiated by generating thiyl radicals from thiol groups. An initial addition of the generated thiyl radical to the carbon carbon double bon d (propagation 1) and a consecutive hydrogen abstraction of a thiol group by a carbon centered radical provide a thiyl radical (propagation 2). Radical radical coupling causes a termination process 10 Figure 1 2. General thiol ene polymerization process. ( Adapted from  )
19 On e early thiol ene reaction, diarylketone such as BP employed as an initiator but there were significant issues with long term stability of fi lms when they were exposed to interior or exterior light due to the remaining unconsumed a diarylketone. Figure 1 3. Mechanism of the initiation process of thiol ene radical polymerization by DMPA. ( Adapted from  ) Thiol ene polymerization can also be initiated by the excitation of cleavage type photoinitiators, which give a benzoyl radical and a tertiary carbon radical by the exposure of light. Figure 1 3 describes the initiation of thiol ene polymerization by dimethoxyphenyl acetophenone (DMPA). 11 Initiation by cleavage type photoinitiators is more effi cient than hydrogen abstraction type initiator like BP due to higher quantum yield s for the generation of reactive radicals. Researchers showed that the rate of the
20 initiation process is proportional to the square root of concentration of thiol and ene fun ctional groups in a 1:1 molar ratio mixture. 12 R initiation is the rate of initiation. (1) 1.2.2 Propagation Propagation of thiol ene polymerization consists of the alteration between thiyl radical propagation across the ene functional groups and chain transfer reactions by hydrogen abstractions from thiol by the carbon cent ered radicals. In an ideal thiol ene radical reaction, only step growth reactions, no homopolymeriztion, occur and the conversion rate reaches 100 %. Hence, the combination of thiol and ene functional groups is the net reaction of the ideal thiol ene polym erization. 13 T he addition of thiols to carbon carbon double bond s is an exothermic pro cess and there is a reaction enthalpy difference depending on the ene species. For electron rich vinyl ether and electron poor N alkyl maleimide, the reaction enthalpies are 10.5 kcal/mol and 22.6 kcal/mol, respectively. 14 Propagation rate and conversion reduction is observed at the polymerization of 1,2 substituted internal enes system which is probably due to the reversible addition of the thiyl ra dical to the disubstituted ene 15 16 which is shown in Figure 1 4. Figure 1 4. Reversible process of the thiyl radica l addition to the internal ene. ( Adapted from  ) Processes of propagation 1 and propagation 2 in F igure 1 2 have the revolving features and their overall rates are required to be equal. If one of the steps is naturally slower
21 than another, that reaction step becomes the rate limiting step in the reaction process and concentration differences between the two radical species occurs The anticipated overall reaction rate (Rp) behavior s are shown in Eq. (2) (4) in the cases for which 1) the kinetic constants of two reactions are almost equal [Eq. (2)], 2) chain transfer is the slow reaction [Eq. (3)], and 3) the thiyl radical propagation is slow reaction [Eq. (4)]. 17 k ct k p (2) k ct >> k p ( 3 ) k ct << k p (4) In th ses equation s k ct is the chain transfer rate constant, k p the thiyl radical propagation rate constant, [R SH] is the thiol concentration, [R S ] is the thiyl radical concentration, [R S C C R ] is the carbon centered radical concentration and [R C=C] is the ene concentra tion. In each case, the reaction process is first order overall in the concentration of monomers of thiol and ene. However, the characteristic s and reactivity of the radicals and the chemical nature of the thiol and ene functional groups determine the deta iled dependence in the reaction steps. Thiols such as alkyl thiols have fewer abstractable hydrogen atoms and will have a tendency to reduce chain transfer rates [Eq. (2)]. Whereas the reaction of allyl ethers is chain transfer limited and first order in [ R SH], vinyl silazanes are propagation limited and first order in [R C=C]. In the ideal case of radical polymerization, norbornene and vinyl ethers have very similar
22 propagation and chain transfer rate s and result in a half order dependence on both the th iol and ene concentration s. 18 In contrast to conventional free radical chain growth polymerization kinetics of thiol ene free radical polymerization are very simple in most thiol and ene systems. 1.3 Applications of Thiol Ene Thermosets Thiol ene click reactions can be applied to a variety of applications such as surface science, coatings, optical components, adhesives, photolithography, microdevice fabrication among others These applications are primarily based on thiol ene free radical chemi stry the extent of Michael addition reactions is limited The thiol ene reactions is a popular surface modification process due to the ability to use pattern s to alter surfaces simply by exposure to light. General approaches show in Figure 1 5, contain 1) a grafting to approach that uses two different thiol ene polymerization processes, a free radical polymerization and Michael addition polymerization (Figure 1 5 a), 19 2) a grafting from approach that employs initiator s including thoil ene substrates to make the grafting process (Figure 1 5 b), 20 21 3) a combination of the above two approaches. Either p hoto induced free radical or Michael addition polymerizations are performed in bonding with surfaces (Figure 1 5 c,d). 22 23 The thiol ene reaction has several advantages such as delay in gel point, homogeneous network and low oxygen inhibition. Those benefits make thiol ene systems ideal for photolithography and microdevice fabrications. In direct photolithographic a pplications, thiol ene reactions are quite effective in the manufacture of nanoscale devices via diverse methods of nanoimprint lithography. 24 A Thiol ene polymer produced with nanoscale patterns is consequently exposed to a further thiol ene grafti ng reaction that further decreases feature sizes.
23 Because of the high polarizability of sulfur atom s compared to other organic components such as carbon, hydrogen and oxygen, thiol has been incorporated into organic materials and used for optoelectronics. Since 2000, numerous articles and patents applications, especially from Norland Optics photocurable thiol ene systems, have arisen in a wide range of journals. The applications in optics and optoelectronics of thiol ene system are very broad and include l ens components, flexible display components, photonic crystals, optical waveguides, among others Figure 1 5. approaches that utilize both thoil ene and thiol Michael addition coupling reactions, b) a photoinduced grafting from approach employing thiyl radicals to initiate acrylate polymerization c) and d) combinations of grafting to and grafting from radical reactions and thiol Michael addition reactions, respectively. ( Adapted from  )
24 1.4 Thiol Yne Click Polymerization In the previous explanation of thiol ene polymerization, several advantages of thiol ene crosslinked polymer networks such as nearly idea l, and homogeneous structure s were introduced. However, since eac h ene is able to connect with only a single thiol in the thiol ene reactions, in order to produce a crosslinked network it is essential to react monomers with more than two enes or thiols. Therefore, the degree of monomer functionality determines the maxim um obtainable crosslink density. Likewise, the extent of small molecular replacement or change is also limited by the one thiol to one ene nature of the reaction. 25 Alkynes are relatively easy to synthesize in various structural configurations and are usually stable until given the chance to react. Therefore, alkynes have been selected as ideal substrates for a wide range of materials applications through Cu catalyzed Huisgen alk yne azide reactions. As an extension of other types of highly efficient reactions with alkyne s certain radical meditated thiol yne reactions have been investigated by several researchers. 26 29 Thiol yne reactions have been explored as a chemical platform for materials synthesis including network film formation, polymer functionalization to control solution properties in water, and synthesis of new highly functio nal chemical species. 1.4.1 Kinetics of Thiol Yne Polymerization Figure 1 6 shows the reaction mechanism of the thiol yne system. A Yne functional group reacts with a single thiol to produce a vinyl sulfide. Unlike the thiol ene reaction, a vinyl sulfide c an react with a second thiyl radical to form a dithioether. Therefore each yne moiety i s difunctional in thiol yne radical polymerization.
25 Figure 1 6. Reaction mechanism for sequential addition and hydrogen abstraction process of (1) primary alkyne and ( 2) a vinyl sulfide during thiol yne step growth polymerization. ( Adapted from  ) Fairbanks and colleagues researched the kinetics and mechanism of the thiol yne photopolymerizatoin by using 1, 9 dodecadiyne (DDY) and pentaerythriol tetra(3 mercaptopropi onate) (PETMP). 26 Almost 80% of thiol functional groups react within the first minute. Consumption of alkyne groups is also relatively fast while the intermediary vinyl sulfide species is generated and consequently consumed during the polymerization (Figure 1 7) During the reaction process, the concentration of vinyl sulfide reaches a maximum at ~0.2 min of UV irradiation after 0.2 min consumption of vinyl sulfide exceeds its generation due to the thiol addition to alkyne. The addition of thiol to alkyne and the consequent addition of thiol to vinyl sulfide can be understood by the reactive species balances, (5) (6)
26 (7) where k p,1 and k p, 2 are the propagation rate constants for addition of thiyl radical to alkyne and vinyl sulfide respectively. If k p, 2 /k p,1 <<1, the rate of vanishing alkyne is the same as the rates of thiol loss and vinyl generation. Reversely, for k p, 2 /k p,1 >>1, vinyl sulfides are consumed instantly upon generation. Therefore, significant vinyl sulfides concentration would not be detected. In the stoichiometrically balanced DDY and PETMP system, the rate constant rat io (k p, 2 /k p,1 ) is decided to be ~3. 25 Figure 1 7. Concentration of reactive thiol ( ), alkyne ( stoichiometrically balanced DDY and PETMP photop olymerization. ( Adapted from  ) The Flory Stockmayer (FS) equation is generally utilized to predict the gel point conversion for a step growth polymerization between two monomers with degrees of
27 functionality In the case of assuming the same reactivity of the alkyne and vinyl sulfide, the gel point conversion of a stoichiometrically balanced polymerization between tetrathiol and dialkyne is 33%. There is a report demonstrating deviation from the gel point conversion predicted by FS equation for the step growth process where subsequent addition to a reactive component has different reactivity. 30 In this reference, a higher gel point conversion was obtained the rate of initia l addition was faster than the subsequent addition (k p, 2 /k p, 1 <1). The opposite behavior was observed when the rate of subsequent addition was more rapid than initial addition (k p, 2 /k p, 1 >1). The observed rate constant ratio (k p, 2 /k p, 1 ) for thiol yne system was 3 and a gel point conversion somewhat lower than the Flory Stockmayer theory. However the rate constant was still of the same order of magnitu d e and the deviation from the value of the Flory S tockmayer was likely small. The gel point conversion from th e theory was 33% and it is much higher than methacrylate system of below 5%. Shrinkage stress did not start to grow until the conversion exceeds the gel point conversion. Therefore thiol yne network systems were likely to show alike low shrinkage stress be havior as well as thiol ene system. 1.4.2 Materials Properties of Thiol Yne Thermosets Fairbanks and coworkers compared materials properties of thiol yne to thiol ene thermosets. In order to make thermosets, t hey employed tetra functional thiol, pentaeryth riol tetra(3 mercaptopropionate) (PETMP), dialkyne, 1, 9 dodecadiyne (DDY) and diene, 1, 4 butanediol divinyl ether (BDDVE). As a result, the crosslink density of fully polymerized PETMP/DDY is 6 times higher than that of fully cured PETMP/BDDVE network T he full width at half maximum (fwhm) of PETMP/DDY thermoset s is almost twice than that of PETMP/BDDVE thermoset s However, this width is still quite narrow compared to network formed by chain growth polymerization. Materials properties of
28 PETMP/DDY and PET MP/BDDVE networks are presented in Table 1 1. 26 Thiol yne networks showed higher glass transition and crosslink density than the thiol ene network. The crosslinked network of thiol yne system is a promising click po lymerization system as i s that of thiol ene. Table 1 1 Summary of Tg, fwhm, elastic modulus and crosslink density for PETMP/DDY and PETMP/BDDVE networks. ( Adapted from  ) system T g ( C) T an fwhm ( C) E at 65 C (MPa) c alculated cross link density (M) PETMP/DDY, 80 C 48.9 0.9 17.7 0.2 80 1 9.6 0.2 PETMP/DDY, 25 C 40.7 0.2 17.7 0.5 69 2 8.4 0.2 PETMP/BDDVE 22.3 0.5 9.3 0.3 13 1 1.5 0.1 1.5 Outline of Dissertation This dissertation aims to deliver a fundamental background of thermoset materials based on multi functional thiol as w ell as their important applications It consists of three chapters about literature reviews on applications and three experimental chapters are followed by reviews. First, Chapter 2 will describe high refractive index polymers, approaches to develop them a nd their important applications Chapter 3 will detail general information about organic light emitting diode and light extraction technique s for enhancing low out coupling efficiency of OLEDs. A range of fabrication techniques of OLED microlens array s will be depicted in Chapter 3 as well. In Chapter 4 encapsulation of light emitting diode will be delineated based on encapsulation materials and their thermal and optical properties In Chapter 5 UV curable low viscosity high refractive index and transparen t TiO 2 loaded dithiol dialkyne nanocomposites will be exhibited T he surface modification of nanoparticles for
29 dispersion and optical properties of nanocomposites will be discussed as well. In order improve the low out coupling efficiency of OLEDs a light extraction method using direct printed microlens arrays will be applied to the OLEDs in Chapter 6 Both the s urface treatment of the glass substrate to establish low surface free energy for obtaining high contact angle of thiol ene liquid lens droplets fi rst and microlens array fabrication via direct printing technique will be explained. In Chapter 7, graded refractive index LED encapsulation will be demonstrated. In order to extract light in LEDs, high refractive index thiol yne nanocomposites will be emp loyed and low refractive index materials based on room temperature curing silicone will be coated on the high refractive index thiol yne nanocomposites for index grading and for protecting the thiol yne film from thermal oxidation Conclusions and future w ork will be discussed in Chapter 8.
30 CHAPTER 2 HIGH REFRACTIVE INDE X POLYMERS 2.1 Introduction The development of photonic devices has led to increasing interest in high refractive index materials, especially high refractive index polymer s High refractiv e index polymer materials can be obtained either by substituting low molar refraction to high molar refraction or introducing high refractive index inorganic nanoparticles to the polymer matrix. Applications of high refractive index polymers range from hig h performance substrates, encapsulation of lighting devices, antireflection coating, photoresists, and microlenses for charge coupled devices. 31 33 Typical refra ctive indices of conventional polymers are around 1.30~1.70. However, for example, the encapsulation of LED requires very high refractive index to match the refractive index of the semiconducting layer (n: 2.50~3.50). Epoxy and silicone compound materials for the encapsulation have refractive indices around 1.45~1.55. Therefore, there is significant total internal reflection at the interface between the encapsulation layer and semiconducting layer. Therefore, it is desirable to achieve the refractive index range from 1.80 to 2.50 for this application. In addition to the refractive index, polymer materials must have high transmittance at visible wavelengths for lighting applications. 2.2 Strategies to Increase Refractive Index of Polymers The Lorentz Lorenz equation is widely used to anticipate the refractive index of polymers. This equation expresses the refractive index in terms of the molecular refraction R, molecular weight M, and molecular volume V of the polymer repeating units. 34 R M the molar fraction can be expressed R/M and M/V can be expressed as the reciprocal of molar volume V M
31 ( 8 ) ( 9 ) From the above equations employing the moiety of high molar refractions and low molar volumes can be an effective approach to wards increas ing the ref ractive index of polymers. Table 2 1 shows the molar refraction of atoms and organic groups of common polymers. 35 Table 2 1. Molar refraction of atoms and organic groups. ( Adapted from  ) Group R M Group R M H 1.100 Phenyl (C6H5) 25.463 C 2.418 Naphthyl (C10H7) 43.00 Double bond (C=C) 1.733 Cl 5.967 Triple bond (C C) 2.398 Br 8.865 O (carbonyl) (C=O) 2.211 I 13.900 O (hydroxyl) (O H) 1.525 S( thiocarbonyl) (C=S) 7.97 O (ether, ester) ( C O ) 1.643 S (thiol) (S H) 7.69 F 0.95 S (dithia) ( S S ) 8.11 2.2.1 HRIP Including Halogens Halogen materials were used as the substituting components in the early development of HRIP. A series of polymethacrylates containing carbazole rings with lateral substituent of bromine and iodine were studied in an effort to increase the refractive index of polymethacrylates. 36 These halogen substituted methacrylates exhibited a range of refractive index from 1.67~1.77 at 589 nm wavelength. However, the optical properties of polymers including halogen compone nts occasionally deteriorate and are not suitable for applications in optical devices. Furthermore, recently, the use of halogen materials has been significantly restricted in electronic devices by the European Union due to the potential environmental poll ution. 37
32 2.2.2 HRIP Including Phosphorus Phosphonates and phosphazenes groups show high mola r refractions by modifying functional groups. Allcock and coworkers developed high refractive index polyphosphonates which have phosphorus nitrogen backbones 38 39 They were able to obtain a high refractive index by attaching side groups with iodinated aromatic groups and the refractive index was reported to be great er than 1.70. Figure 2 1. Chemical structur es and refractive index of polycarbonates and polyphosphonates. ( Adapted from  ) Polyphosphonates are very similar to polycarbonates in terms of chemical structures. However, by introducing phosphor s and attaching aromatic group s the refractive index c an be increased and other properties such as melt stability and fire retardancy were also improved. 40 Figure 2 1 shows the chemical structures and refractive index of polycarbonates and polyphosphonates.
33 2.2.3 HRIP Including Sulfur atoms Sulfur atoms havehigh molar refraction and sulfur containing polymers have been devel oped for HRIPs. The most common high refractive index sulfur containing polymer is polyimide. High molar refraction groups including sulfur atoms and aromatic groups were substituted to the repeating group of polyimide. They have refractive indices greater than 1.720 and the refractive index increases with increased sulfur concentration of substituting groups. Although sulfur concentration in the polyimide is an important factor for improving refractive index, the degree of molecular packing is also a cruci al component. For example, polyimide s having bulky moiety such as sulfonyl groups have lower refractive index than polyimide s containing linear moiety consisting of sulfur atoms and aromatic groups despite having higher sulfur concentration. This can be ex plained by the loose molecular packing of bulky sulfonyl groups. 41 44 The Abbe number is described as the optical dispersion of the refractive index depending on wav elength In optic devices, low Abbe number, high dispersion of refractive index, is avoided because it can alter the color of the image and the focal length of lenses. The Abbe number (v) is determined by the refractive index (n), molecular refraction (R), and molecular dispersion ( R) according to Equation ( 10 ) 45 ( 10 ) From E quation (10) a large refractive index induces a small Abbe number. Typical high refractive index polymers (refracitve indices greater than 1.70) have smaller than 20.0 Abbe number.
34 Table 2 2. High Abbe number and high refractive index sulfur containing polymers. ( Adapted from  ) Sulfur containing polymers having high Abbe number and refractive index are listed in Table 2 2. Researchers have reported improve d Abbe number s while maintaining high refractiv e index using for examp le condensed sulfur containing alicyclic ring s polymethacrylates including thiophene, and brominated poly(thiophene methacrylate). 46 49 These polymers do not reach n=1.70 but have relatively low dispersion with the Abbe number greater than 30. 2.3 Organic inorganic nanocomposites In Section 2.2, intrinsic methods for improving the refractive index of polymers were discussed. Although refractive indices of polymers c an be improved significantly by substituting high molar refraction components, the highest value is limited to less
35 than 1.80. Therefore, hybrid approaches which integrate organic polymers as matrixes with high refractive index inorganic nanoparticles as f illers have been introduced in order to achieve much higher refractive indices The refractive index of nanocomposites is determined by three factors including the properties of matrixes and nanoparticles and the technique used to incorporating nanopartic les to the matrixes. The refractive index of composites can be predicted by the Maxwell Garnett effective medium theory. 50 According to E quation (11) the dielectric constant of the nanocomposite sample can be expl ained from the dielectric constants of the host and inclusion where c h and i are the dielectric constants of the composite, matrix polymer and nanoparticle, respectively and is the volume fraction of nanoparticles. ( 11 ) It is clear that the higher concentration of high refractive index nanoparticles the higher refractive index of nanocomposites. However, an overload of nanoparticles has harmful influence to optical properties such as opti cal losses. Moreover, direct dispersion of nanoparticles into the polymers can induce aggregation of nanoparticles. Therefore, in practice, surface modification of nanoparticles is frequently employed to fabricate well dispersed nanocomposites. A variety o f nanoparticles such as TiO 2 ZrO 2 PbS, and ZnS have been incorporated into polymer matrixes TiO 2 being the most widely applied in the nanocomposite fields due to beneficial characteristics including nontoxicity, high refractive index, good thermal stabi lity and environmental stability. Ueda research group developed TiO 2 loaded sulfur including polyimide nanocomposites and were able to obtain a refractive index value of greater than 1.80. 51 In this work,
36 silica was used for enhancing affinity with the polymer matrix and poly(amic acid) (PAA) was selected as the matrix for the photolithography process. Figure 2 2. Chemical structure of poly(amic aci d) and SEM images of polyimide titania nanocomposites and the pattern by photolithography. ( Adapted from [5 1 ] ) The Chen research group also developed high refractive index polymers with a series of TiO 2 loaded polyimide nanocomposites. In their study a so luble PI containing carboxyl endcaps underwent an esterification reaction with titanium butoxide to bond between matrix and fillers. They acquired a 1.82 refractive index with 40 w t% concentration of TiO 2 52 Other conventional polymers such as epoxy, PMMA, and polycarbonate h ave been used as the polymer matrix to obtain high refractive index values. 50 53 54 However, those matrixes required much higher degrees of nanoparticle loading due to the nature of the low refractive index of the matrix polymers. Thermosets based on thiol can inherently have high refractive index due to sulfur atom. In a ddition, both thiol ene and thiol yne networks provide high transparency and
37 easy processability such as UV curing. In Chapter 5, we will discuss high transparent and high refractive index nanocomposites by using TiO 2 nanoparticle as a filler and thiol yne monomer mixture as a matrix. This approach can easily produce the desired shape by printing or molding methods without side products.
38 CHAPTER 3 LIGHT EXTRACTIONS OF ORGANIC LIGHT EMITTING DIODES 3.1 Introduction to OLEDs Glo bal e nergy consumption is regul arly increased every year and approximately ~22% of the total electricity consumed is utilized as lighting in the United States. 55 Conventional light sources such as incandescent light bulb s consume 95% of supplied electricity as a heat and only 5% of the supplied energy is converted into light. The fluorescent tube, which is another traditional light source, has ~20% power conversion efficiency (PCE) but fluorescent tubes contain a certain amount of mercur y which is an environmentally hazardous materials and in increased temperature working conditions around 35 C, power efficiency is reduced by as much as 40~60% compared with the condition at room temperature. 56 If the light source can convert 50% of the supplied electricity into light, it is possible to save up to 650 billion kilowatt hours per year in the United States. This is equ ivalent to the amount of energy generated from almost 70 nuclear plants per year in the US. Therefore, there is high demand for efficient lighting source s in order to reduce energy consumption and protect the environment. Solid state lighting (SSL) is a highly efficient light emitting device based on semiconducting materials. C onventiona l LEDs utilize inorganic semiconducting materials and have almost 100% internal quantum efficiency. LEDs have many advantages such as low energy consumption, longer lifetime, and smaller size compared with traditional light sources. Therefore, the market fo r LEDs has grown rapidly in the past few years. LED application s range from flat panel display appliances, to signals, to automo biles to illumination. 57
39 On the other hand, organic light emitting diodes convert electric power into light by employing organic semiconducting materials rather than inorganic components. In c omparison to inorganic LEDs, OLEDs have several benefits although low efficiency and device lifetime and stability issues must still be overcome. Figure 3 1. Illustrations of flexible OLEDs. (above picture) Samsung 4 5 inch AMOLED (below picture) Univ ersal Display Incorporation flexible OLED c ollaborated with LG ( Adapted from http://www.oled info.com and http://www.oled display.net ) First, deposition processes of organ ic materials are very simple by employing spin coating, inkjet printing, vacuum thermal evaporation and roll to roll processes
40 compared to inorganic film growth processes such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Organic ma terials are more economical than inorganic materials. Moreover, the extremely thin and flexible nature of organic materials make OLEDs applicable for flexible electronic device applications. From the point of view of display performance, there are also ben eficial characteristics of OLEDs compared to liquid crystal display such as a quick respon se time, a wide viewing angle, a high contrast and low power consumption. 58 59 Therefore, OLEDs are a very competitive candidate for flat panel display and the next generation of SSL devices 3 .2 Efficiency Limitations of OLEDs External quantum efficiency is the number of photon s emitted per injected electron. On the other hand, internal quantum efficiency ( EQE ) is defined as the ratio of the total number of photons generated in the device to the total number of electrons injected into the device. EQE is calculated by multiplyi ng the internal quantum efficiency ( IQE ) by outcoupling efficiency ( out ) Although tailoring of the OLED design by for example introducing phosphorescent materials makes the internal quantum efficiency achieve 100% the external quantum efficiency of the typical bottom emitting type of OLED is generally limited around 20% due to total internal reflection losses (TIRs) which result from index mismatching at multiple interfaces. 60 61 The structure of an OLED has a multiple layer s consisting of different refractive indices such as a metal reflector, organic layers, indium tin oxide ( ITO ) and a glass substrate. Therefore, the light generated in the emissive organic layer must pass through multiple interfaces in order to finally escape into the air and during this process the light confronts TIRs as depicted in Fig 3 2
41 Figure 3 2 Exemplar waveguide modes in bottom emit ting OLEDs (BOLEDs); (1) the external modes (0 1), ( 2 ) substrate waveguiding modes ( 1 2), and ( 3 ) ITO/organic waveguiding modes ( 2 90 ) There are three different types of modes depend ing on the angle of the emitted light The light c an escape through the glass substrate into the air in the external mode. In substrate waveguding mode, generated photons undergo TIR losses at the glass substrate/air interface and these losses are around 20~30%. The third mode is the ITO / organic waveguidi ng modes In this mode, almost 50 ~ 60% of the generated photons are confined since TIR losses occur at the interface between ITO and glas s substrate and photons reflected by TIR are trapped near the metal cathode due to the strong localized electric field on the metal cathode 62 64 Therefore, it is trusted that only around ~ 20% of total generated photons can contribute to the out in a planar structure BOLED system and this low out coupling efficiency is main limitation of device performance.
42 3. 3 Methods to Improve Low Out coupling Efficiency Several techniques to improve low out coupling efficiency are proposed by many researchers First, researchers have suggested structured and shaped substrates. OLEDs fabricated on 2.2 mm high square glass mesas with a 3 mm top edge length and a 34 wall angle were developed by Gu et al. 65 and this structure effectively increased the portion of light scattered into the range of critical angle as shown in Figu re 3 3 (left image) They also proposed the insertion of a thin film of low loss, high refractive index dielectric material on the glass substrates before ITO deposition followed by mesa etching. Figure 3 3. Schematic illustration of optical ray traject ories of light generated from OLEDs with substrate modifications. T he glass mesa substrate (left image) and a thin film having mesh structure (right image). (Adapted from  and ) Spherically shaped patterns of the glass substrate are suggested to ex tract substrate waveguiding mode. 6 6 Macrolens with refractive index matching materials such as polyfluorene (RI ~ 1.55) is also placed on the glass substrates to improve low out coupling efficiency. 67 The textured meshed structure fabricated using poly dimethyl
43 siloxane (PDMS) was proposed by Cheng et al. 68 for low cost and large scale production (Figure 3 3 right image) The s econd approach to enhance low out coupling efficiency is the introduction of a sca ttering medium for light extraction into the inside of devices. Yamasaki and coworkers inserted an ordered monolayer of silica microspheres with a 550nm diameter as a scattering medium in the conventional OLED structure. Hexagonally closed packed silica mi crospheres behave as two dimensional diffraction lattices which behave as strong scattering medium Silica micro spheres array were fabricated both inside the device and on the backside of glass substrates as shown in Figure 3 4 and waveguided modes into g lass and ITO/organic can be efficiently extracted. 69 Figure 3 4 Schematic illustration of OLEDs with ordered arrays of silica micro spheres (Adapted from ) The microlens array is also an effective way to improve low out coupling efficiency for OLEDs. 70 73 Theoretically nearly 85% enhancement of out coupling efficiency is possible by employing a microlens array and a 70% of improvement has
44 been achieved experimentally with an optimized pattern. The microlens array can be fabricated outside of the devices and there is no harmful effect on the electrical device performance. The microlens array can cover large area and fabrication processes can be simple compared to other extraction techni que which modifies device structures. Figure 3 5 illustrates light output scheme with and without microlens. 74 Reflected light at the interface between the glass substrate and air can be extracted by microlens. Figure 3 5 Light output scheme (a) with lens and (b) without lens. (Adapted from ) OLEDs can be considered as a one dimensional micro cavity because the structure of OLEDs is designed to have a total organic film thickness on the order of the wavelength of emitting light. 75 Weak micro cavities are constructed with conventional OLEDs structure s because of the metal cathode and high refractive index anod e. 76 On the other hand, a strong micro cavity has a metal mirror a nd highly reflective dielectric layer such as Si x N y 77 78 OLEDs having tandem structure which is a vertica l stack of multiple active regions can yield high enhancement s in efficiency. Photonic crystals are regular dielectric structures which have an energy band gap and thereby prohibit the propagation of a certain frequency range of light. Fujita and coworker s utilized two dimensional photonic crystal structures into OLEDs to improve
45 ITO/organic wave guiding modes. 79 Other researchers introduced two dimensional photonic crystal structures between ITO and glass substrates by deposition of alternative layers of SiO 2 and SiN x OLEDs with photonic crystals shows 50% enhancement of light extraction efficiency compared to conventional OLEDs. 80 81 Sun and Forrest developed a method for extracting wave guiding modes into the external mode by embedding a low index grid into the OLED active organic layer. The lig ht trapped at the interface between ITO/glass substrate is able to enter into the low index grid and is refracted into the substrate normal direction. There is no effect on the initially emitted photons into the forward viewing cone. 82 3. 4 Fabrication P rocesses for M icrolens A rray on OLEDs In the previous section several methods were introduced to improve out coupling efficiency of OLEDs. Although these methods can efficiently increase out coupling efficiency, they require complicated and expensive fabrication processes and modifications of device structures by, for example, the insertion of additional layers into the device In contrast, m icrolens arrays can be created through a simple and reliable process to reduce TIR losses over a large area at the glass substrate/air interface. Several researchers utilized lithography processes in order to fabricate PDMS molding for microlens array s Various technique s were employed to produce mold s with microlens array pattern s such as conventional photolithography, imprint lithography, three dimensional lithography, and soft lithography using polystyrene (PS) beads. 73 82 86 The p hotoresist was spun on the silicone wafer and via a typical photolithography process a square plate pattern was produced on the wafer. Thes e plate shaped photoresist s were thermally reflowed to have spherical curvature and PDMS was poured on the wafer and thermally cured. However, due to a limitation of the thermal reflow
46 method, individual microlens were not hemispherical in shape and showed 0.28:1 ratio of height to diameter. 85 Fig ure 3 6 illustrates the calculated coupling enhancement factor as a function of the lens height for various lens diameters. T he highest value is obtained when the lens has is a hemisphere structure. 87 Figure 3 6 Coupling enhancement factor versus lens height for different lens diameter (Adapted from ) Sun and Forrest suggested imprint lithography to make the PDMS microlens array mold. The pattern of hexagonal arrays with a small distance between two photoresist plates was produced by conventional photolithography. The glass substrate with patterned photoresist was wet etched in buffered oxide to obtain an approximately hemispherical shape. PDMS was poured on the glass sub strate after removing residual photoresist. I n comparison to reflow method, the microlens array avoided gaps between adjacent microlenes and the height to diameter ratio (0.33:1) was improved as well. 86 The three dimensional diffuser lithography technique, which utilizes randomized light,
47 was employed in order to produce various shapes of microlens es by Chang et al. By controlling parameters of the lithograph y process, vari ant shapes of microlenses were able to be fabricated such as conventional convex, hemispherical, and ellipsoidal design. 83 Although microlens array fabricated by the above methods showed efficient extr action of rays trapped at interface between substrate/air, expensive and complex conventional photolithography and wet etching processes are regarded as inefficient fabrication methods for generating a reproducible microlens array mold. Therefore, Eom et a l. and Nam et al. proposed a soft lithography process using PS beads as shown in Figure 3 7 84 88 Figure 3 7 Sc hematic illustration for hemispherical microlens array formed by the soft lithography method assembled the PS array. (Adapted from )
48 A PS colloidal solution was spin coated on the glass or SiO 2 substrates and PDMS was poured on the substrates containi ng PS beads array s After curing PDMS, a concave PDMS mold was separated by removing the PS array using adhesive tapes. This allowed them to avoid complex photolithography processes for producing the microlens array mold. Figure 3 8 Schematic illustrat ion of fabrication process via a self assembly approach (Adapted from ) Yu Lu et al. suggested a self assembly approach to fabricate two dimensional array s of microlenses. Desired patterns were formed on the glass substrate by typical photolithograph y and aqueous solution containing monodispersed PS beads was confined within a packing cell consisting of two glass substrates. PS beads were trapped in the cylindrical holes on the patterned glass during capillary flow. By heating PS beads above the glass transition temperature, mushroom shaped microlenses were
49 obtained and after removing photoresist, hemispherical microlenses were formed by annealing. 89 Jun Xia et al. studied a self assembly polymer microlens array as well. They employed a hydrophobic layer on the patterned photoresist by an imprinting techni que to prohibit the mixing of prepolymers from adjacent lens. 90 Inkjet printing processes were introduced to produce the microlens array as well. Compared with other method s inkjet printing has several advantages such as large scalable, simplicity cost effective ness and environmentally friendliness 91 93 Howeve r, such attempts have produced microlenses with very low diameter to height ratios of approximately 1:0.075. Microlenses with such low contact angles are not suitable for light extraction. Furthermore, sag can occur at the center of the microlenses due to solvent evaporation during fabrication. Consequently, enhancement of light out coupling efficiency by printed microlens arrays applied to OLEDs has not been demonstrated. Figure 3 9 SEM images of microlens array fabricated by inkjet printing process. (A dapted from ) Thiol ene monomer mixtures are suitable for the microlens material for light extraction of bottom emitting OLEDs due to high transparency; the refractive index is close to glass substrates. In addition, its low viscosity provides thiol en e compatibility to
50 inkjet and direct printing processes. In Chapter 6, we will explain microlens array created by direct printing technique with thiol ene monomer mixture as lens material for light out coupling efficiency enhancement.
51 CHAPTER 4 E NCAPSULAT IONS OF LIGHT EMITTI NG DIODES 4 .1 Introduction to Encapsulations of LED The light emitting diode (LED) has received significant interest for illumination applications and displays since it has several beneficial properties such as extended lifespan, low e nergy consumption, high luminescence efficiency and heavy metal free. Therefore, the LED market has been dramatically increas ed in the last few years. 94 Current applications of LEDs are extended to wide areas such as automotive forward lighting, backlight of LCD displays, indicator lamps, signs, and equipments displays. With increasing demands for high performance, high brightness, of LED, the encapsulation of LED has aroused interest as well. High brightness LED is inevitably exposed to a high intensity UV and a high working temperature. These conditions can occur the a ccelerated degradation of encapsulation materials usually polymers. Thermal stability of the LED encapsulation requires stable transparency and resistance against to discoloration such as yellowing by thermal aging and thermal degradation. In addition, one of the basic limitations of LED is the light extraction efficiency which is resulted from the huge refractive index mismatch between semiconductors such as GaN or GaP and air. This refractive index mismatch makes the angle of light escape cone for the sem iconductor air interface to be very restricted. For instance, the light escape cone of AlGaInP is limited by ~17 and trapped light inside of the semiconductor is likely lost by absorption. 95 The light extraction efficiency of both GaN and GaP LEDs was dramatically increased as the refractive index of encapsulant increased as seen in Figure 4 1 T he light extraction efficiency of GaN LED s increased more rapidly than that of GaP due to a lesser refractive index mismatch with the encapsulation layer.
52 Figure 4 1 Light extraction of LEDs by the encapsulation (a) Light escape cones of LED with and without encapsulation (b) light extraction efficiency ratio for GaN and GaP LEDs as a function of refractive index of encapsulation. (Adapted from ) Therefore, in order to achieve high performance LEDs, the materials for encapsulation of LEDs are required to have the properties such as thermal stability, UV re sistance, optical clarity and high refractive index. In addition, it is also required to have good hardness to protect LED chips. 4 2 Encapsulation Materials Epoxy materials have been employed in an extended range of applications such as coating s adhesi ve s industrial tooling, and biology. They are also used in electronics and LED encapsulation is a considerable application for transparent epoxy resins. In order to make transparent epoxy, hexahydrophthalic anhydride (HHPA) and methylhexahydrophthalic anh ydride (MeHHPA) are utilized as curing agents. 96 These
53 anhydride epoxy system has several advantages s uch as manageable treatment, ease of processing due to relatively low viscosity, and long term storage stability. However, acceleration catalysts such as tertially amine, imidazole, or phosphine are used for epoxy curing due to low reactivity of the anhydr ide system T hese catalysts have a deleterious influence on the optical propert ies of epoxy including discoloration by either thermal or UV exposure. In addition, anhydride evaporation causes volume shrinkage which leads to internal stress within the packa ges. 97 Another common material for LED encapsulations is polysiloxane resin. Compared to the epoxy system, cured polysiloxane encapsulation shows impressive thermal resistance to yellowing at high temperature. 98 PDMS is the most common product from silicone manufacturer s It has excellent thermal resistance to discoloration even when aging at temperatures up to 200 C. However, PDMS has very low re fractive index ~1.4 and the light extraction of LEDs can be limited by the PDMS encapsulation due to the large refractive index mismatch. In order to increase the refractive index, high molar refraction phenyl groups are introduced to polysiloxane. Althoug h phenyl groups can help increase the refractive index up to 1.52, phenyl groups are susceptible to thermal oxidation when in air for long period. 99 Therefore, the development of the high refractive index, transparent, and highly thermally stable materials is very important for the betterment of LED encapsulation technology. 4 2 1 Epoxy Silicone hybrid resin Several research groups have investigated silicone epoxy hybrid resins in order to overcome the limitations of epoxy and silicone materials. Bae and coworkers developed epoxy hybrimers in order to obtain both thermal stability and high refractive index. They synthesized cycloaliphatic epoxy oligo siloxane resin by a simple sol gel
54 condensation reaction between 2 (3,4 epoxycyclohexyl)ethyltrimethoxysilane (ECTS) and diphenylsilanediol (DPSD) In order to make the bulk sample for the application of LEDs encapsulations m ethylhexahydrophthalic anhydri de (M e HHPA, 97%, Aldrich) and tetrabutylphosphonium methanesulfonate (TBPM) were added as a hardener and a catalyst, respectively. Epoxy hybrimer bulk showed good transmittance during thermal aging at 120 C and compared to conventional epoxy resin whose re fractive index ~1.52, Figure 4 2. Fabrication process of cycloaliphatic epoxy hybrimer bulk (Adapted from [ 100 ]) cycloaliphatic epoxy hybrimer bulk exhibited higher refractive index ~1.55 at 632 nm wavelength. 100 Figure 4 2 shows the fabrication process of cycloaliphatic epoxy hybrimer bulk. The researchers developed inorganic organic hybrid resin as well. Phenyl vinyl oligosiloxane (PVO) was synthesized by a sol gel condensation reaction between vinyltrimethoxysilane (VTMS) and diphenylsilanediol (DPSD) Phenyl hybrimer s showed a slight change in transmittance ~6% and yellowness index ~12 after
55 thermal aging at temperature s up to 200 Although phenyl groups are susceptible to thermal aging, the strong bonding of the siloxane network and the branched structure in PVO restrict s phenyl group s from cleavage from backbone chain. PVO also showed a higher refractive index ~1.56 due to its high phenyl group concentration compared with other polysiloxane materials. 101 The silicone epoxy resin consisting of 2 (3,4 epoxycyclohexyleth yl) methyldiethoxysilane (EMDS) and dimethyldiethoxysilane was developed by Yang et al. In their work the silicone epoxy resin was fabricated with different silicone contents by controlling dimethyldiethoxysilane reactions They found that thermal stabili ty is dependent on both the silicone content in the network and crosslink density. They reported silicone epoxy resin showed better thermal propert ies than the commercial LED encapsulation material, 3,4 Epoxycyclohexylmethyl 3,4 epoxycyclohexane carboxylat e 102 The Morita research group reported on the extension of flexibility of epoxy siloxane. Although epoxy siloxane has promising properties such as photo thermal stability, its brittleness is a significant issue for encapsulation a pplication s In order to make longer siloxane segment length, they employed hydroxyl terminated hydrogenated polybutadiene and were able to obtain better mechanical properties for the packaging application. 103 104 4 2 2 Inorganic P article L oaded N anocomposites for LED E ncapsulations Inorganic particles such as ZnO and TiO 2 are utilized to improve the refractive index of the encapsulation materials. A high ly transparent thermolytic epoxy silicone which was synthesized by polymerization between the silicone matrix with diglycidyl ether bisphenol A epoxy as reinforcing materials and ZnO nanowires as a filler to impr ove the refractive index and modify conductivity. 105 PFPA silane (N (3 trimethoxys ilylpropyl) 4 azido 2,3,5,6 tetrafluorobenzamide) was employed to modify
56 the surface of ZnO nanowires to increase their affinity to matrix packaging materials. The refractive index of nanocomposites was increased from 1.47 to 1.56 by increasing ZnO nanowir es concentration of (0.025~0.200%). Below 0.175% concentration of ZnO, the transmittance of hybrid material was greater than 85% which is adequate for LED packag ing Figure 4 3. Three dimensional ray tracing simulation of light extraction efficiency as a function of scattering coefficient of the encapsulation with various absorption coefficients. (Adapted from ) High refractive index TiO 2 nanoparticle loaded epoxy was introduced by Frank et al. 95 They obtained a refractive index of ~1.67 with 10 w t% TiO 2 nanoparticle content at 500nm wavelength. They also simulated the scattering effect of nanoparticles on light extraction efficiency. For strongly s cattering nanoparticles in the encapsulation layer, the optical scattering length is short but light traveling distance is long. If the light traveling distance is longer than the absorption length, photon s are absorbed in the
57 encapsulant resulting in a re duction in the light extraction efficiency. An optimized degree of scattering for improved light extraction efficiency is shown in Figure 4 3. Although the study showed promising results using nanocomposite s for increased refractive index, they did not dem onstrate optical transparency which is significantly decreased by agglomeration of nanoparticles and thermal stability which is related with discoloration of packaging materials. Materials for the encapsulation of LEDs are required to be transparent and ha ve high refractive indices. In general, the encapsulation of LEDs is a potting process and thermosetting and UV curable resins are frequently used. UV curable thermosets based on thiol and yne provide a high refractive index and high transmittance. In Chap ter 7, we will explain the multi layer LED encapsulation by utilizing high refractive index thiol yne nanocomposites.
58 CHAPTER 5 UV CURABLE HIGH REFRACTIVE INDEX AND TRANSPARENT T I O 2 LOADED THIOL YNE NANOCOMPOSITES 5.1 Introduction The refractive index i s one of the most important properties of materials used in optical designs and applications. High refractive index materials are needed for various applications such as ophthalmic lenses, optical adhesives, antireflection coatings, LED encapsulations as w ell as microlens arrays for CMOS censors. 106 110 In c hapter 2 several strategies to enhance the refractive index of polymers have been reviewed Although these meth ods can efficiently increase the refractive index of polymers, there are many limitations to optical waveguiding lenses or encapsulations at the outmost layer of devices. In order to utilize these applications, high transmittance without absorption at all visible regions and processability similar to the usual polymers such as UV curing and molding for obtaining desired shapes are necessary. Since halogens such as bromine and iodine normally lead to colors in the polymer, high refractive index polymers with halogens are restricted for transparent applications. Furthermore, many of the recent regulations for environmental protection limit the use of halogen components due to potential pollution of the environment. Phosphorus containing high refractive polymer has halogen substituent in order to increase the refractive index. Polyimides which are known for good thermal stability, chemical resistance, and mechanical properties can increase the refractive index by 1.76 by including aromatic groups and sulfur moie ty However, because of its orange/yellow characteristic color it is difficult to use it for waveguiding lenses or encapsulations. Inorganic organic nanocomposites are developed to overcome the limitation of low refractive index of intrinsic polymers. Seve ral inorganic nanoparticles, such as TiO 2
59 51 52 111 113 ZrO 2 111 113 Nb 2 O 5 114 and ZnS, 115 116 have been incorporated into various polymer matrixes to obtain high refract ive index nanocomposites. Although high content of nanoparticles in the matrix can deteriorate optical properties of nanocomposites such as optical losses, inorganic nanoparticles loaded composites can obtain a high refractive index of over 1.80. In order to prevent agglomeration of nanoparticles, typical surface treatments are used to functionalize metal oxide nanoparticles. Silane coupling agents are chemically bonded to the oxide surfaces through condensation reactions with surface hydroxyl groups. Surfa ce treated nanoparticles and polymer matrix are mixed into appropriate organic solvents Nanocomposite films are obtained after baking and drying. Currently, there are many references regarding the synthesis, characterization, and applications of nanocompo sites. However, there are only a few references on nanocomposites with processability like polymers high transparency and high refractive index. TiO 2 9 5 ZnO, 105 and ZnS 117 nanoparticles are dispersed into the epoxy, epoxy silicone and polycarbonate in order to achieve an easy processable high refractive index nanocomposites. T hese nanocomposites can be processed by the molding method to obtain desired shapes and an increase in refractive indices of nanocomposites However, particle concentration was limited to 0.2 w t% due to optical losses from aggregation of particles in the ZnO loaded epoxy silicone system and the refr active index was restricted to 1. 58. ZnS polycarbonate nanocomposite showed high particle contents up to 20 w t% but the refractive index was 1.61 due to low refractive index of the matrix polymer.
60 As introduced in Chapter 1, thiol yne networks are densely linked and highly unifor m with rapid polymerization in mild condition as well as in the thiol ene system. In order to form a network, thiol ene system needs greater than two of average functionalities of thiols and enes. Therefore, typically tri or tetrafu nctional thiols and enes are utilized to produce networks. On the other hand, in thiols and ynes system, dialkynes are technically tetrafunctional with respect to the addition of thiols and the reaction of dithiols and dialkynes can produce highly crosslin ked polymers. A series of dithiol and dialkyne networks were fabricated by Chan et al. 27 In this chapter, UV curable transparent high refra ctive index nanocomposites with 2,2 thiodiethanethiol and 1,6 heptadiyne monomer mixtures as the matrix and TiO 2 nanoparticles as the filler will be demonstrated. 3 M ethacryloxypropyltrimethoxysilane was used for the surface treatment of TiO 2 nanoparticle s to prevent agglomeration. Increase of particle concentrations result in the increase of refractive indices of nanocomposites and the slight decrease in the transmittance of nanocomposites. TiO 2 loaded thiol yne nanocomposites have the high refractive in dex ~1.683 and high transmittance of 88 %. Furthermore, the approach presented herein can easily produce the desired shape by printing or molding methods without side products It does not require the clean up of materials, and does not u tilize solvents or high temperature processes. 5. 2 Experiment A ll dithiols, dialkynes and the photoinitiator, 1 hydroxy cyclohexyl1 phenyl ketone, were purchased from Sigma Aldrich and used as received. Dithiols and dialkynes were mixed with 2:1 thiol to alkyne molar ratio or 1:1 functional group ratios for network
61 formation b ecause dialkyne is technically tetrafunctional. 2 w t% of ultraviolet active radical generating photoinitiator was added to the monomer mixtures. Solution of TiO 2 nanoparticles (anatase, 5 to 30 nm) in water was obtained from Nanoamor, Inc. Silane coupling agent, 3 methacryloxypropyltrimethoxysilane (MPTMS), was purchased from Fisher Scientific. The d eposition from aqueous solution method was employed for surface treatment of TiO 2 nanoparticles Silane coupling agents were dissolved in ethanol and solution of TiO 2 nanoparticles Solutions were stirred for 10 minutes The mixture was rinsed with ethanol 3 times, isolated by centrifugation and dried. Dried particles were grinded, dispersed in ethanol and filtered. Ethanol evaporated in the oven at 60 C. Particles were mixed into 2,2 thiodiethanethiol and 1,6 heptadiyne monomer mixtures and ultrasonicated. Solution w as placed between the cover glass and hydrophobic glass substrate and exposed to UV light (365nm, 100mW/cm3) in the duration of 1 minute for polymerization. By removing hydrophobic glass, nanocomposite film remained on the cover glass. Refractive indices of pure thiol yne films were measured by the ellipsometer (J. A. Woolam) at visible wavele ngth s and the refractometer ( Reichert Abbe Mark II Plus Refractometers ) at a wavelength of 580 nm. Thermogravimetric analysis (TGA) was performed on a thermal analysis system (Mettler Toledo TGA/DSC). Samples of approximately 8mg were loaded and heated up to 800 C at a rate of 10 C/minute under air flow of 60 ml/minute. The transmittance of nanocomposite films was measured in UV VIS spectrometer ( PerkinElmer Lambda 750 ) at a wavelength region from 300 nm to 800 nm.
62 5. 3 The Refractive Index of Thiol Yne T hermosets The four dithiols and four dialkynes in Figure 5 1 were utilized to produce a series of thiol yne networks. The dithiols and dialkynes networks consist of only carbon, hydrogen and sulfur atoms and the refractive index of each polymer can be cont rolled by changing the weight percentage of sulfur atoms in the networks. The higher Figure 5 1. Chemical structures and abbreviation of dithiols and dialkynes. sulfur content results in a higher index of refraction due to the characteristic of high atomic refraction of sulfur atoms. The network consisted of TDET and HptDY has the highest weight percent age of sulfur with 48 %. Among the listed dithiols and dialkynes in Figure 5 1, DDT and DDDY have the longest chain length and the highest molecular weight. Hence, the DDT DDDY network shows the lowest sulfur content with 22.3 %. Figure 5 2 shows the refractive index versus the weight percent sulfur plots for photopolymerized networks produced from 2:1 alkyl dithiol and dialkyne m onomer mixtures. The refractive index of TDET and EDT(1,2 ethanedit hi ol) BDT(1,4 buthanedit hi ol) DDT(1,10 decanedit hi ol) TDET(2,2 thiodiethanedit hi ol) DDDY(1,11 dodecadiyne) HptDY(1,6 Heptadiyne) ODY(1,7 octadiyne) NDY(1,8 nonadiyne)
63 HptDY film was measured by both ellipsometer and refractometer and their measured values were 1.623 and 1.621, respectively. Figure 5 3 shows the refractive index of TDET HptDY film from the ellipsom e ter. Figure 5 2. Dependence of the refractive index on concentration of sulfur in the photopolymerized networks The Abbe number is defined as optical dispersion of t he refractive index depending on the wavelength s. For the application of optics, high dispersio n of the refractive index depending on wavelengths is refrained because it can change the color of the image and the focal length of lenses. (1 2 ) The Abbe number is determined by E quation (1 2 ). As discussed in Chapter 2, a large refractive index induces a small Abbe number. Typical high refractive index
64 polymers have smaller Abbe number of 20.0 than the ones with high refractive indices over 1.70. However, the film made using TDET and HptDY showed refractive i ndices 1.6351, 1.6237, and 1.6196 at 486 nm, 589 nm, and 656 nm wavelength. Calculated Abbe number was 40.37 and it is relatively higher than polymers reported as high Abbe number materials. 46 48 Figure 5 3. Measured refractive index of the film made of TDET HptDY 5 4 Suface Treatment of TiO 2 by Silane Coupling Agent The direct mixing of nanoparticles into the polymer matrix promotes agglomeration s of nanoparticle s. Therefore, virtually, the surface of the particle is modified to be suitable for mixing processes. Organic groups attached to the surface of nano particles prohibit aggregat ions and cause a surface polarity adjustable to the medium
65 Silane coupling agent s have been utilized to form a durable bond between organic and inorganic materials. The typical structure for a silane coupling agent is shown in Figure 5 4. X is a hydrolyzable group typically consisting alkoxy, acyloxy, halogen or amine. H ydrolysis re action forms a reactive silanol group, which can condense with hydroxyl group on the surface of substrates or fillers. The R group has a functionality that imparts desired propertie s. In this work, polymerizable re a gent, 3 methacryloxypropyltrimethoxysilan e (MPTMS), was used to stabilize nanoparticles and to allow them to be compatible with thiol yne monomer mixtures. Figure 5 4. General structure of silane coupling agent (upper image) and chemical structure of MPTMS (lower image)
66 5.4.1 TGA Measurement of Pure TiO 2 and Surface Modified TiO 2 The amount of attached MPTMS to nanoparticles was estimated from the TGA measurement in Figure 5 5. The weight loss values of untreated TiO 2 and MPTMS modified TiO 2 were 7.1 % and 15.3 %, respectively. Weight loss of pure TiO 2 is possibly due to adsorbed moisture molecules. It is assumed that the percentage of residue from MPTMS molecules anchored on nanoparticles was approximately 8 %. Figure 5 5. TGA analysis of pure TiO 2 and MPTMS modified TiO 2 5.4.2 FTIR Analysi s of Pure TiO 2 and Surface Modified TiO 2 TiO 2 nanoparticles were well dispersed in solution of water and ethanol due to electrostatic repulsion. After adding MPTMS into the solution, the translucent solution changed to opaque This indicates that the surfa ce conditions of nanoparticles were altered by the reaction between nanoparticles and the surface modifier, MPTMS. The
67 change of surface states of nanoparticles was analyzed using FT IR. Figure 5 6 shows FT IR spectra of pure MPTMS, untreated TiO 2 and surf ace modified TiO 2 by MPTMS. After applying silane coupling agent treatment on nanoparticles, C O stretc hing absorption peaks were observed at 1319 cm 1 C=O vibration of unsaturated ester and C=C double bond peaks were also observed at 1718 cm 1 and 1636 cm 1 respectively. Th e se peaks indicate organofunctional group of MPTMS adsorbed on the nanoparticles. At 1180 cm 1 Si CH2 R stretching peaks were observed as well. Figure 5 6. FT IR spectra of untreated nanoparticle and MPTMS modified nanoparticle 5 5 Nanocomposites 5.5.1 Composition and Thermal Properties Nanocomposite films were prepared from MPTMS modified TiO 2 nanoparticles and TDET HptDY with various particle weight percents. Untreated TiO 2 nanoparticles
68 were also dispersed into the TDET HptDY monomer mixtures for comparison. The amount of nanoparticles incorporated into the nanocomposites was measured by the TGA results. Figure 5 6 shows TGA thermograms of t he TDET HptDY polymer matrix and nanocomposites where TY and NCTY are pure TDET HptDY film and nanocomposite TDET HptDY film, respectively. 0.7 w t% of cured thiol yne monomer mixtures remained under the employed TGA condition due to the buoyancy effect whi ch can be observed that a specimen heated in thermobalance. This is, in part, due to the differences in thermal conductivity, density, and heat capacity for the gas and the sample/crucibl e. At employed condition of TGA measurement, NCTY1 and NCTY2 had part icle concentration of 3.33 % and 6.4 4 %, respectively. NCTY3 and NCTY4 had relatively higher particle content of 8.87 % and 10.67 %, respectively. Figure 5 7. TGA results of matrix polymer and nanocomposites.
69 A slight increase in thermal stability of the nanocomposites was demonstrated by TGA analysis. Decomposition temperatures of nanocomposites increases with particle content. Decomposition temperatures of 1 0% weight loss was 228 C, 275 C, 283 C for TGA samples of TY, NCTY1 and NCTY4, respectively. Th e increase in thermal stability was attributed to restrict molecule mobility imposed by nanoparticles. 118 The formation of strong siloxane bonds between silane coupling agents and nanoparticles and the polymerization between thiols and C=C bonds of functional group of silane coupling agent may further enhance the increase in decomposition temperatures 5.5.2 Optical Transparency of Nanocomposites Figure 5 8. UV VIS transmittance spectra of the TDET HptDY film and nanocomposite films Optical transparency of nanocomposites is a good measure of dispersibility of nanoparticles in the matrix. The dispersability of TiO 2 nanoparticles in the TDET
70 HptDYmonomer mixtures was estimated from the transmittance measurement of nanocomposite films with thicknesses of approximately 30 35 m. The agglomer ation of nanoparticles to a size lager than several tens of nanometers is a major determinant in possible inducement of the scattering loss of visible lights traveling through a nanocomposite film. Furthermore, a large mismatch of the refractive index betw een polymer matrix and nanoparticles can result in a large scattering efficiency. T he nanocomposite with untreated TiO 2 nanoparticles showed macroscopic phase separation A n analysis of this sample was not performed. Figure 5 8 shows transmittance of the T DET HptDY film and nanocomposite films. TEDT HptDY film had very high transmittance ~95% at visible regions and there was almost no change of transmittance from 650nm to 480nm. Although introduction of surface treatment on nanoparticles improved dispersion the optical clarity of nanocomposites with TiO 2 nanoparticles was reduced with increasing weigh t percent of particles. However, samples with low particle content, NCTY1 and NCTY2, still had approximately 90 % transmittance at 650 nm while NCTY3 showed 88 % transmittance with a relatively high particle content. Transmittance of NCTY1 3 showed 2 or 3 % difference from 650 nm to 480 nm measurement s NCTY4 including the highest particle concentration showed the largest scattering loss at whole visible regions and had 81 % transmittance at 650 nm. The occurrence of scattering loss was stronger at short wavelength regions in this sample and the transmittance difference from 650 nm to 480 nm reached to 8 %. Unfortunately, TiO 2 nanoparticles act as a photocatalyst by UV light R efractive ind ex difference s can occur through oxidation of polymer materials and particle aggregation.
71 5.5.2 Refractive Indices of Nanocomposites The refractive index of composite materials can be predicted by Maxwell Garnett effective mediu m theory 119 and Lorentz Lorenz approximation. 120 Maxwell Garnett effective medium theory is described as Equation ( 13 ) and the refractive index of the composite can be calculated by di electric constants of host and inclusion materials and the Maxwell relation n 2 = ( 13 ) c h, and i are dielectric constants of the composite, host, and inclusion, respectively and is the vol ume fraction of the inclusion in Equation ( 13 ). Solid line in figure 5 9 is based on the Equation ( 14 ) where h, and i are 1. 62 2 and 2.53 2 121 respectively. Lorentz Lo renz effective medium approximation is shown in the Equation ( 14 ) ( 14 ) where n c n np n sca and n m are refractive indices of the composite, nanoparticle, silane coupling agent (MPTMS) and matrix polymer, respectively np SCA and m are volume fractions of the nanoparicle, silane coupling agent and matrix polymer. Maxwell Garnett equation and Lorentz Lorenz equation were plotted with solid and dashed line in Figure 5 9. Samples were measured by the A bbe refractomet er at 580nm wavelength. TDET HptDY had the refractive index ~ 1.621 and it matched with the value (1.623) measured by an ellipsometer. Thiol yne films were regarded as a bulk substrate sample O nly the top reflected lights on the surface were collected by t he detector of an ellipsometer. In this case, films were d e posited onto the rough side of a single side polished silicon wafer and t he measurement only contain ed information fr om the front
72 surface reflection while light reaching the back surface of film s w ere scattered rather than reflected as shown in Figure 5 10 (left image). Therefore, film thickness and Figure 5 9. Refractive indices of the TDET HptDY film and nanocomposites at 580 nm wavelength. Black solid line and red dash line are theoretical ass umptions of Maxwell Garnett equation and Lorentz Lorenz equation, respectively. Figure 5 10. Sample species of the ellipsometer; (left image) a bulk substrate and (right image) multi layer films uniformity were not important for ellipsometer measurement of a bulk substrate. However, in order to measure the refractive index of nanocomposite by an ellipsometer,
73 it is necessary to probe inside the sample like multi layer samples as shown in Figure 5 10 (right image). Unfortunately thicknesses of nanocomposi te films a nd surface un iformity were not good enough to be measured by the ellipsometer. Therefore, refractive indices of nanocomposite were measured only with a refractometer. Refractive indices of nanocomposites were increased with increasing particle co ntents as expected by theories. Refractive indices of NCTY3 and NCTY4 showed 1.6835 and 1.6981 respectively. 5 6 Conclusion UV curable high transparent and refractive index nanocomposites were successfully fabricated with surface modified TiO 2 nanoparticl es and high refractive index TDET HptDY matrix. T he polymerizable silane coupling agent, MPTMS, was introduced to improve dispersion of nanoparticles and nanocomposite films which showed high transmittance with particle content up to ~ 8.8 w t% Nanocomposit e with highest particle content s showed lowest transmittance. Larger transmittance dispersion depending on wavelengths was also observed than that of other nanocomposites which had less than 9 wt% particle contents. It is assumed both particle aggregation and polymer oxidation by TiO 2 nanoparticles influence scattering losses and absorptions. Refractive indices of nanocomposites gradually increased with particle contents in the composites. The refractive index of the nanocomposite with 8.8 wt% nanoparticle content was 1.6835 which is relatively higher than reported values such as 1.58 and 1.61.
74 CHAPTER 6 MICROLENS ARRAYS CRE ATED BY A DIRECT PRI NTING TECHINQUE FOR LIGHT OUT COUPLING EFFICIENCY ENHANCEMENT OF ORGAN IC LIGHT EMITTING DEVICES 6 .1 Introduction Organic light emitting diodes (OLED) are electroluminescent devices which have experienced rapid development in the past two decades and have become competitive solutions in illumination tasks requiring a large area light source. 122 Although nearly 100% internal quantum efficiency of such devices ha s been achieved using phosphorescent emitting materials, 60 123 low out coupling efficiency of around 20% due to tota l internal reflection (TIR) losses result from refractive index mismatches between the multiple layers of such devices. 124 Various methods have been proposed to i mprove out coupling efficiency such as shaping devices into mesa structures, 65 making use of a microcavity structure, 125 126 employing a photo nic crystal, 127 or embedding a low index gri d between the indium tin oxide (ITO) and the organic layer. 82 Although these methods can increase the out coupling efficiency, they requ ire complicated and expensive fabrication processes and modifications of device structures such as the insertion of additional device layers. In contrast, microlens arrays can be created through a simple and reliable process to reduce TIR losses over a lar ge area at the glass substrate/air interface. Several different methods for fabricating microlens arrays have been reported, such as using imprint lithography and wet etching of a silicon substrate to create a microlens mold, 70 three dimensional diffuser lithography utilizing randomized light to form various shapes, 83 liquid crystal droplet microlens array, 128 microlens array mold from microporous polymer film by casting of polymer solutions under humid conditions, 129 melting a self assembled two dimensional polystyrene array,
75 89 and a soft lithography based method in which a colloidal monolaye r of polystyrene spheres serves as a mold template. 84 These approaches are all technically complicated and/or expensive. A direct printing technique is an attractive alternative method for fabrication of microlens arrays due to its relative simplicity, low consumption of lens materials, and the ability to control array pattern and positioning. Printing techniques are familiar and relatively mature and a well supported industrial infrastructure makes printing techniques a promising method of simple and cost e ffective microlens array fabrication. 130 Several research groups have attempted to fabricate microlens arrays using the inkjet printing technique. 91 93 However, such attempts have produced microlenses with a very low diameter to height ratio of approximately 1:0.075. Microlenses with such low contact angles are not suitable for light extraction. Effective microlenses should be as nearly hemispherical as possible. 87 Furtherm ore, sag at the center of microlenses due to solvent evaporation during fabrication has been a problem. Consequently, enhancement of light out coupling efficiency by printed microlens arrays applied to OLEDs has not been demonstrated. In this Chapter we p resent a process for the fabrication of large area microlens arrays having a high diameter to height ratio (1:0.41). These arrays were printed using a substrate surface modification technique employing a hydrophobic silane coupling agent to control the mic rolens shape. Several hydrophobic silane coupling agents were tested to find one that would achieve a high contact angle for the lens material. The surface properties of a glass substrate were modified by three different silane coupling agents in order to find the lowest critical surface tension, which can be determined using a
76 Zisman plot. 131 We employed a solventle ss multifunctional thiol and ene monomer mixture as a lens material to obtain a nearly hemispherical lens shape without sag at the lens centers. The photopolymerization of a mixture of multifunctional thiol and ene is an efficient method for rapid producti on of film and thermoset plastics. A thiol and ene monomer mixture is suitable as a microlens material due to its transparency, similar refractive index to a glass substrate, and adaptability to printing techniques because of its low viscosity. Using such a printed microlens array we have been able to obtain a 30% enhancement of light out coupling efficiency without observing changes to the emission spectrum. 6. 2 Experiment (Heptadecafluoro 1, 1, 2, 2, tetra hydrodecyl) trimethoxysilane, polydimethylsiloxan e(methoxy terminated), and octyl trichlorosilane were obtained from Gelest as hydrophobic silane coupling agents. Deposition from aqueous solution was employed for surface treatment of glass substrates. Silane coupling agents were dissolved at 2 w t% concen trations in water. Cleaned glass substrates were dipped into the solution and removed after 2 minutes of agitation. for 10 minutes. DI water (surface tension of 73 mN/m), glycerol (64 mN/m), ethylene glycol (48 mN/m), 1,2 dichloroethane (33 mN/m), acetone (23 mN/m), hexane (18 mN/m) (Fisher) were selected as test liquids for a wetting e xperiment to determine the lowest surface critical tension that could be obtained for modified surfaces. A goniometer ( Rame Hart ) was used to measure contact angles prod uced by the six test liquids on glass surfaces modified by each of the silane coupling agents. The silane coupling agent having the lowest critical surface tension determined by a Zisman plot was applied on an ITO sputtered glass substrate using a chemica l
77 vapor deposition method to achieve better uniformity than deposition from an aqueous solution. In a closed chamber, substrates were supported above a silane reservoir with utes to 8 hours. The lens material was a monomer mixture of trimethylolpropane tris(3 mercaptopropionate) and tri(ethylene glycol) divinyl ether. Multifunctional thiol and ene were mixed in a 2:3 molar ratio to match the number of functional groups for com plete conversion. This solution was formulated with a 2 w t% ultraviolet active radical generating photoinitiator, 1 hydroxycyclohexylphenyl ketone (acros organics). The refractive index and transmittance of the film of the thiol and ene mixture were measur ed by ellipsometry ( J.A. Woolam ) and with a UV VIS spectrometer (PerkinElmer Lambda750) respectively. Deposition of the microlens array was conducted with a model 3Dn 450 HP direct write instrument (nScrypt, Inc.) as shown in 6 1. Materials dispensing was controlled by air pressure fed to a syringe. The liquid lens material was pushed into a chamber and then pushed out through a dispensing tip. The chamber was connected to a gantry which contained a series of motors allowing movement in the x, y, and z dimensions. A software interface was used to convert CAD or other design specifications into a series of commands that stepped the motors in sequence to draw des ired lens array patterns. Controllable variables included the size of the tip, the chamber valve opening distance relative to the sealed position, air pressure, and deposition speed. The rheology of the material being dispensed also affects deposition qual ity. For microlens array deposition, a tip with an inner di
78 pressure of 1.5 psi. Figure 6 1 Deposition of microlens array by the direct write instrument Printed microlens arrays were exposed to UV ligh t (365nm, 100mW/cm 3 ) for 10 seconds for polymerization. Scanning electron microscopy (SEM, JEOL 6335F) was employed to analyze the geometry of the resulting microlens arrays and the contact angles of individual lenses. In order to measure the optical effec ts these lenses would provide, fluorescent organic light emitting diodes (OLED) were fabricated on glass substrates with the microlenses preprinted on the opposite side, allowing for no change to the standard OLED fabrication. All devices were fabricated u sing vacuum thermal evaporation at a pressure of 3.0 x 10 6 Torr or less. Devices consisted of tin doped indium oxide (ITO)
79 pre deposited on glass with 50 nm N N 2 yl) N N benzidine (NPB) and 50 nm tris (8 hydroxyquinoline) (Alq 3 ) as h ole transport and emission layers respectively. A 1 nm CsCO 3 interlayer followed by 80 nm Al was used for electron injection into the device. The device area defined by the cathode was either 4 mm 2 or 1 cm 2 Devices were characterized with and without micr olenses using an Ocean Optics Jaz spectrometer to measure enhancement and emission spectra. A 400 mm diameter optical fiber was used to capture the emitted light. All measurements were conducted at ambient room conditions. Lens array d eposition was conduct ed using Microdrop MD series inkjet printing methods as well as direct printing technique s A dispenser head nozzle with a diameter of 50 m, which dispensed approximately droplets with a liquid volume of 90 pl, was used at a nozzle temperature of 40 C, a driving v oltage of 200 V and a distance from substrate to nozzle of 75 m. Labview software was employed to automat e the process of microlens array fabrication. In order to measure the diameter and height of printed lens es a Wyko optical profilometer was utilized. 6. 3 Surface Treatment on Substrates of OLEDs by Silane Coupling Agent 6 3 1 Zisman plots of hydrophobic SCAs Critical surface tension is useful in determining wettability of solid surfaces with a range of liquids. A liquid with a surface tension below the critical surface tension of a The critical surface tension is determined by plotting the cosine of the contact angle versus surface tension of different test liquids and extrapolating t o 1. Typically hydrophobic behavior is observed for surfaces with a critical surface tension less than 35 mN/m Aliphatic hydrocarbon substituents or fluorinated hydrocarbon substituents are the hydrophobic components
80 which enable silanes to introduce surf ace hydrophobicity. Surfaces with a critical surface tension below 20 mN/m resist wetting by hydrocarbon oils and are regarded as oleophobic as well as hydrophobic. Figure 6 2 Zisman plot and the critical surface tension of (a) (heptadecafluoro 1, 1, 2, 2, tetra hydrodecyl) trimethoxysilane (b) polydimethylsiloxane(methoxy terminated ) (c) octyl trichlorosilane In order to determine the critical surface tension of a surface modified by a silane coupling agent, Zisman plots were constructed after meas uring the contact angles of six test liquids. Water has the highest surface tension, 73mN/m, among the test liquids and the test liquids decrease as the surface tension of the liquids increased, as described by 132 Hexane, having the lowest surface tension, 18mN/m, among the test liquids was not able to resist surface wetting on the surface modified with octyl trichlorosilane. However, hexane showed contact angles of 14.48 1.14 and 34.4 2.06 polydimethylsiloxane(methoxy terminated) and (heptadecafluoro 1, 1, 2, 2, tetra hydrodecyl) trimethoxysilane, respectively. Figure 6 2
81 shows Zisman Plots constructed from measured contact angles of test liquids. The calculated critical surface tension of a surface with (heptadecafluoro 1, 1, 2, 2, tetra hydrodecyl) trimethoxysilane is 9.29 mN/m polydimethylsiloxane(methoxy terminated), 11.68 mN/m and octyl trichlorosilane, 20.29 mN/m. All selected sil ane coupling agents introduce very hydrophobic as well as oleophobic surfaces with critical surface tensions below 20mN/m. 6 3 2 Vapor phase deposition of SCA (Heptadecafluoro 1, 1, 2, 2, tetra hydrodecyl) trimethoxysilane, having the lowest critical sur face tension, was selected for the device fabrication. We can visually observe slight non uniformities on the surface of a glass substrate after coating with a silane coupling agent from an aqueous solution because defects can result from solvent evaporati on. Although a wetting experiment with droplets a few millimeters in diameter is not affected by small surface defects, these defects could alter the lens shape or contact angle of a microlens thus damaging the microlens array geometry. In order to obtain a uniform surface on glass substrates, the silane coupling agent was applied using a chemical vapor deposition method in a heated vacuum chamber. A hydrophobic surface is achieved if the hydroxyl groups of a substrate are capped by silane and surface is e ffectively shielded by the hydrophobic entities of the silane coupling agent. If residual hydroxyl groups are exposed then the surface will not be completely shielded, decreasing surface hydrophobicity. Vapor phase deposition time was varied in order to co nfirm that the silane coupling agent had effectively screened the surface of the glass substrate. Figure 6 3 shows the change of contact angle of the microlenses on the modified surfaces resulting from various silane vapor deposition times. On a bare glass substrate
82 a multifunctional thiol ene monomer mixture does not have resistance to wetting and individual microlenses have a diameter to height ratio of 1:0.19. Figure 6 3 SEM cross section images of microlenses on the modified surfaces resulting from various silane vapor deposition times (a) no treatment, (b) 5 minutes, (c) 15 minutes, (d) 30 minutes, (e) 1 hour, (f) 4 hours, (g) 8 hours
83 The silane coupling agent effectively shields the hydrophilic glass surface even after treatments of short dur ation; after 5 minutes of deposition the diameter to height ratio increased to 1:0.33. The diameter to height ratio increased slightly to 1:0.39 after 30 minutes surface treatment. A maximum diameter to height ratio of 1:0.41 was measured after a one hour vapor phase deposition and no further increases in the ratio were measured with dep osition times of 4 and 8 hours. 6.4 Optical Properties of Thiol Ene Lens Materials A film of a multifunctional thiol and ene monomer mixture was deposited on a silicon subst rate for measuring the refractive index of the lens material. In order to avoid the effects of film non uniformity, the film was deposited on the backside of the silicon substrate and the ellipsometry detector only collected light reflected from the top of the film. Figure 6 4 shows the refractive index measured by ellipsometry. At a wavelength of Figure 6 4. The refractive index of lens material
84 486nm, the index of refraction is 1.492, at 589nm it was 1.480, and at 656nm it was 1.473. The Abbe number wa s calculated as 25.26 using the equation below: (1 5 ) A film of a multifunctional thiol and ene monomer mixture was also deposited on a glass substrate to measure the transmittance of the lens material. The transmittance measured using a UV VIS spectrometer was over 98% ac ross the visible spectrum as shown in Figure 6 5 Figure 6 5 T ransmittance of thiol ene lens material 6. 5 Microlens array fabrication by direct printing technique For device fabrication, the silane coupling agent was deposited on the ITO sputtered glas s substrate for 4 hours using the chemical vapor deposition method. The thiol ene monomer mixture was then printed on the surface of the modified glass
85 substrate using direct printing. The outer diameter of the dispensing tip initially interfered with the formation of adjacent lenses in the array resulting in a very low fill
86 Figure 6 6 SEM images of a printed microlens array, (a) a top view, (b) an individual lens (c) factor of the microlens array. High air pressure had to be applie d so that enough liquid lens materials would flow from the tip in order to obtain larger microlens diameters, thus resulting in a higher fill factor. The distance between adjacent microlenses was set at array of 30 lenses by 20 lenses was printed, covering half of the emission area of a large area device. Figure 6 6 shows scanning electron microscope (SEM) images of a printed microlens array: a top view (Fig 6 6 ividual lens (Fig. c). The diam e ter of an %. 6. 6 Enchnement of light out coupling efficiency The light extraction enhancement of the microlenses was measured on devices 1 cm x 1 c m with half the emitting area covered in the microlens array while the other half remained bare. Figure 6 7 shows the normalized spectral intensity of both the bare
87 Figure 6 7 The normalized spectral intensity of both the bare OLED side and the lens e nhanced side of a large area device. OLED side as well as the enhanced lens side. An enhancement factor of f= (1.31 + 4) was observed, and while lower than other reported methods 73 this enhancement is likely limited by the spacing between lenses which is highly dependent on the printing system used. Further enhancement is likely with finer mechanical control o f drop position. Figure 6 8 shows the angular emission pattern of a lens enhanced 4 mm 2 device compared to both a bare device and an ideal Lambertian light emitting pattern. The lens enhanced device is shown to be more forwardly directional in its emission pattern than the other two sources. This is believed to be due to redirection of wide angle light into the forward direction by other lenses in the array. Smaller micro lens heights (7 m), as shown by Sun and Forrest, 86 do not exhibit this effect, but rather show expanded emission at high angles. Figure 6 8 The angular emission pattern of a lens enhanced 4 mm 2 device compared to both a bare device and an ideal La mbertian light emitting pattern.
88 6. 7 Microlens array fabrication by inkjet printing technique W e have fabricate d microlens arrays using an inkjet printer in addition to the direct printing system described in Section 6. 5 In order to deposit microlens arr ay s the glass substrate was first treated with a hydrophobic silane coupling agent using the chemical vapor deposition technique with a deposition time of 4 hours The substrate was then placed on the sample stage of the inkjet printer and the liquid rese rvoir was filled with a thiol ene. Figure 6 9 shows the inkjet printing system consisting of printer head, liquid reservoir, 3D movement actuators and printer controller Figure 6 9. Microdrop inkjet printer with 3D movement actuators. The distance from the substrate to the dispensing nozzle tip was set by moving the movement actuator 75 in the upward z direction after the nozzle tip had carefully contacted the substrate. If the distance between the substrate and the end of nozzle tip is too far away, an excessive potential energy of liquid droplets prevents stable deposition onto the s ubstrate and air flow influences the positioning of droplets. The
89 d ispensing head consists of a glass capillary which is surrounded by a tubular piezo actuator. T he piezo actuator contracts and produce s a pressure wave by applying a voltage resulting in a pressure propagat ing into the liquid. A small droplet of the thiol ene monomer mixture l eft the nozzle by the pressure generated from the piezo device and was deposited on the substrate The p iezoelectric device in the printer head and the movement actuat or were controlled using L abview software allowing automatic creation of lens arrays. Figure 6 10 shows SEM (left) and optical microscope images (right) of printed microlens array s Figure 6 10. Microlens array images created by inkjet printing system. 1 5 SEM image (left) and optical microscope image (right). The d iameter and height of the printed lens es were measured with a Wyko optical profilometer which is an optical profiler providing three dimensional surface profile measurements without conta ct. The m easured diameter and height were 73.6 and 32 respectively as show n in Figure 6 11. The s ize of individual microlens created by the inkjet printing technique can be altered by employing various nozzle diameters. In this work, we obtained mic rolens es having diameter s of 73.6 with a 50 nozzle diameter which generated droplets with a volume of 90 pl. In comparison with direct printing methods, inkjet printing can produce microlens arrays with better
90 uniformity because in the direct printi ng system the dispensing tip must have contact with the substrate In addition, inkjet printing system s can control lens size using various nozzle diameters. However, the narrow nozzle of inkjet printer s can be easily blocked by materials that are easily c ured and thus become insoluble due to crosslinking. Therefore, a robust cleaning system is necessary. In this work it was not possible to consistently deposit microlens arrays on OLED devices because of this cleaning issue Figure 6 11. Measurement of lens diameter and height by Wyko optical profilometer 6. 8 C onclusion In summary, the microlens array was successfully fabricated on the OLED by a direct printing technique. Surface treatment by hydrophobic silane coupling agent had a significant effect on increase of the contract angle of lens droplets. T hiol ene monomer mixture was a very suitable for a lens material because it had the refractive index
91 matched with the glass substrate and high transmittance. In order to dispense lens droplets, dispensing t ip must have contact with the substrate in this direct printing system This created slight non uniformities on the lens array. Further enhancement will be possible to the employ more precise mechanical positioning system and inkjet printing system for bet ter uniformity of the lens array.
92 CHAPTER 7 MULTI LAYER ENCAPSUL ANT FOR LIGHT EMITTI NG DIDOES 7.1 Introduction White LEDs have received extensive attention for applications of lighting and displays due to many advantages such as low energy consumption, l ow driving voltage, long lifespan, and high luminescence efficiency. In the application of LEDs, the roles of encapsulation materials have a great impact on the brightness and life span of LEDs. High brightness LED generates more heat and shortwave radiati on which may induce discoloration of encapsulations. 133 Thermal stability of the LED encapsulation requires stable tr ansparency and resistance against discoloration such as yellowing of the material by thermal aging and thermal degradation. In addition, one of the basic limitations of LED is the light extraction efficiency which results from huge refractive index mismatc h between semiconductors such as GaN (n=2.5) or GaP (n=3.45) and encapsulation materials. Total internal reflections at semiconductor encapsulation interface confine lights inside the LED, which is most likely lost by absorption and heat. Therefore, the li ght extraction efficiency of LEDs dramatically increased as the refractive index of encapsulant increased. 95 Epoxy resins have been used for encapsulat ion and packaging materials. 96 Anhydride epoxy system has several advantages such as ease of processin g relatively low viscosity, and long term storage stability. However, acceleration catalysts used for epoxy curing due to low reactivity of the anhydride system leads to a harmful influence on the optical properties of epoxy, including discoloration by ei ther thermal or UV exposure. In addition, anhydride evaporation causes volume shrinkage which leads to internal stress within the packages. 97 Another common material for LED encapsulations
93 is silicone resin. Compared to the epoxy system, cured polysiloxane encapsulation, for example, PDMS shows excellent thermal resistance to discoloration such as yellowing during thermal aging at high temperature even up to 200C. 98 However, the light extraction of LEDs can be limited by the PDMS encapsulation due to the low refractive index ~1.4 of PDMS. In order to increase the refractive index, high molar refraction phenyl groups were introduced to polysiloxane. 134 Although phenyl groups can help increase the refractive index up to 1.52, phenyl groups are susceptible to thermal oxidation when in air for a long period. 99 Several research groups have investigated silicone epoxy hybrid resins or inorganic organic hybrid resin to overcome the limitations of epoxy and silicone materials. 100 104 They developed thermally stable epoxy hybrimer or phenyl hybrid resin but was limi ted to the refractive index of ~1.56. Inorganic particles such as ZnO and TiO 2 were utilized to improve the refractive index of the encapsulation materials. 95 105 The refractive index of nanocomposites increased from 1.47 to 1.56 with an increase of ZnO nanowires concentration (0.025~0.200%). Below 0.175% concentration, trans mittance of hybrid material was greater than 85% which is adequate for LED packaging. High refractive index TiO 2 nanoparticle loaded epoxy was introduced by Frank et al. 95 Although they did not evaluate transparency of nanocomposites, they obtained a refractive index of ~1.67 with 10 w t% TiO 2 nanoparticle content at 500nm wavelength. In Chapter 5, we proposed UV curable high refractive index TiO 2 loaded n anocomposites with high transparency The refractive index of nanocomposite was 1.68 with 88 % transmittance at 600 nm. The thiol yne monomer mixture as a polymer matrix of inorganic organic nanocomposites provided high refractive index and eas e of process ing
94 In this Chapter, we will explain the multi layer LED encapsulation by utilizing high refractive index TiO 2 loaded thiol yne nanocomposite and silicone resin. Graded refractive index multi layer encapsulation was employed to minimiz e Fresnel reflection losses at the interface between air and encapsulation layer. Thermal stability of silicone resin, pure thiol yne film, nanocomposite, and dual encapsulation layers consisting of nanocomposite and silicone resin was evaluated by transmittance measurement a fter thermal aging at 120 C for 120 hours under the air environment Materials discol oration was calculated based on transmittance results. Both light extraction efficiency and thermal stability can be improved by using double layer encapsulation due to d ecrease of Fresnel reflection and reducing the chance for nanocomposite to contact oxidation species. In addition, UV curable thiol yne nanocomposite and room temperature vulcanizing tin based silicone resin are suitable for a potting process which is a ge neral encapsulation of LEDs. 7.2 Experiment The thiol yne film and TiO 2 loaded thiol yne nanocomposite were prepared using the method described in Chapter 5. The high refractive index nanocomposite was thiodiethanethiol and 1,6 heptad iyne monomer (Sigma Aldrich) mixture as a matrix and TiO 2 nanoparticles ( Nanoamor, Inc ) as a filler. 3 M ethacryloxypropyltrimethoxysilane (Fisher Scientific) was used for the surface treatment of TiO 2 nanoparticles to prevent agglomeration. Room temperatur e vulcanizing silicone resin, also known as Faux Glass, was purchased from Silicone Inc. Dual encapsulation layers were prepared by coating the nanocomposite with the silicone resin. Addition cure system using Pt catalyst is susceptible to attack from cert ain chemical compounds such as nitrogen, sulfur, phosphorous and arsenic.
95 Contact with these compounds during mixing and manufacturing will result in inhibition of cure. Therefore, condensation curing system using tin catalyst was employed for coating on t he thiol yne nanocomposite film. Solutions of the silicone resin, thiol yne monomer mixture and TiO 2 nanoparticle loaded thiol yne nanocomposite were placed between the cover glass and hydrophobic glass substrate and exposed to UV light (365nm, 100mW/cm3) in the duration of 1 minute for polymerization. By removing hydrophobic glass, nanocomposite film remained on the cover glass. In order to investigate thermal stability and discoloration by thermal aging, films were placed in the oven at 120 C for 120 hou rs under the air environment Refractive indices of pure thiol yne film and silicone resin were measured by the ellipsometer (J. A. Woolam) at visible wavelengths and the refractometer (Reichert Abbe Mark II Plus Refractometers) at a wavelength of 580 nm. Transmittance of samples was measured in UV VIS spectrometer (PerkinElmer Lambda 750) at a wavelength region from 300 nm to 800 nm. 7.3 Optical properties of encapsulants In order to study the light transmittance of encapsulation materials, we have prepare d film s of the silicone compound, thiol yne network, and nanocomposite on glass substrates. Transmittance of these films was measured by ultraviolet visible spectrometer in the wavelength of 300 800 nm with the reference as a glass FG, TY and NCTY were th e F aux G lass (silicone resin) TDET HptDY film and TiO 2 loaded nanocomposite, respectively. FG showed very high transmittance of 99 % and it is assumed that lower refractive index of FG than that of the glass may reduce Fresnel reflection by graded refract ive index. TY and NCYT had transmittance of 95 % and 88 %, respectively. There were no transmittance dispersion depending on wavelengths
96 in FG and TY but there was a 4 % transmittance difference between 650 nm and 480 nm in NCTY as shown in Figure 7 1. As discussed in Chapter 5, transmittance dispersion occurred due to oxidation of the polymer and particle aggregations. The refractive index of FG was measured by the ellipsometer and its measure d value was 1.397 at 580 nm. The refractive index of TY was 1.62 1 from the measurement result of Chapter 5. NCTY had an index of refraction of 1.682 measured by the refractometer Figure 7 1. Transmittance of encapsulation materials and the refractive index of FG (inset image) 7.4 Optical Total Transmission of Gra ded Refractive Index Encapsulations Light extraction efficiency of LEDs can be improved by using high refractive index encapsulants. Further enhancement is possible by minimizing Fresnel reflection losses at the interface between the air and encapsulation layer. In this study the low refractive index silicone resin was employed to reduce Fresnel reflection losses at the
97 nanocomposite air interface. Fresnel transmission coefficient of normal incidence can be described by Equation (1 5 ) for two media, (15) where n 1 and n 2 are the refractive index of media 1 and 2, respectively. For graded refractive index encapsulation layers, multiple transmissions and reflections occur at each interface. Total transmittance of grade d encapsulation layers can be expressed by Equation (16), 135 (16) whe re n 0 and n i+1 are refractive indices of the semiconductor and air, respectively, and n 1 n 2 n i are the refractive indices of encapsulation layers following n 0 Table 7 1. Total transmittance of no encapsulation, conventional encapsulation (n=1.5), dual encapsulations, and multi layer encapsulations. GaN n=2.5 GaP n=3.45 No encapsulation 81.6% 69.7% n=1.5 single encapsulation 90.2% 81 .6% n=1.68 & 1.4 dual encapsulations 92.8% 85.0% n=1.68, 1.62, & 1.4 multi layer 93.0% 85.2% By utilizing multi layer for encapsulations, total transmittance calculated with Equation (16) can be increased by 2.8 % and 3.6 % for GaN and GaP, respectively, compared with the conventional encapsulation as shown in Table 7 1 These enhancements are only for normal incide nce of light. Thus, it is assumed that there may be further improvements for light extraction efficiency because light is emitted to all directions. Figure 7 2 shows calculated transmittance as a function of change in
98 refractive indices of the encapsulatio n layer following the semiconductor in dual encapsulation layers, which have the refractive index of 1.4 for the second layer. Optical transmittance is increased as the index of refraction increases due to reduction of Fresnel reflection. Figure 7 2. Op tical total transmittance versus refractive indices of the encapsulation layers following the semiconductor in dual encapsulations. 7.5 Thermal Stability and discoloration Thermal discoloration behaviors of encapsulation materials were evaluated. Figure 7 3 shows transmittance spectrum of TY and FG, before and after thermal aging. There is no change of transmittance after thermal aging at 120 C for 120 hours compared to the FG sample before thermal aging. Silicone resins are well known for excellent therma l stability against yellow discoloration at high temperatures. 134 They have high
99 thermal stability against discoloration even up to 200 C, although their applications for the encapsulation of LEDs are lim ited due to the low refractive index. Figure 7 3. Optical transmittance spectra of the TY and FG (inset image) samples before and after thermal aging at 120 C for 120hours Transmittance decreased as aging time increased in the TY sample Although UV abs orption in the sample increased incrementally with aging time, transmittance of thiol yne network did not change significantly at visible regions after thermal aging for 120 hours. Figure 7 3 presents transmittance variations of the TY film depending on th ermal aging time from 0 to 120 hours. There was a slight transmittance change of approximately 3 % at 450 nm between samples of 0 and 120 hours. Figure 7 4 presents transmittance variations of NCTY depending on the thermal aging times. TiO 2 loaded nanocom posite showed 7 % more transmittance change than the pure thiol yne film at 450 nm. It can be seen that the photocatalytic degradation of
100 thiol yne network was initiated by active oxygen species such as O 2 HO 2 were produced from O 2 photocatal ytic reaction of TiO 2 while UV polymerization 136 and thermal aging enhanced the further degradation of the polymer matrix. NCTY also showed much lower transmittance at UV regions which may be explain ed by both light scattering by nanoparticles and UV absorption by the degradation of polymers resulting in low transmittance. Figure 7 4. Optical transmittance spectra o f the NCTY sample before and after thermal aging at 120 C for 120hours. Dual encapsulation layers were prepared using silicone resin and NCTY. Silicone resin was coated on the NCTY for graded refractive index. Thermal stability of these layers was investi gated in the same condition. Transmittance of dual encapsulation layers decreased as aging times increased, similar to other samples which had thiol yne network as a polymer matrix. However, the decrease in transmittance at 450 nm of dual
101 encapsulation lay ers was 4 % and less than that of a single NCTY film. Like in a nitrogen environment, i t is assumed that the coating of silicone resin can reduc e the chance of reacting with oxidation species In comparison with other films, there was a slight transmittanc e change of this sample during the 48 120 hours of thermal aging. It is implied that the small amount of oxygen that was dissolved or trapped in the film contribute to fast thermal oxidation following slow thermal oxidation, due to both depletion of oxygen in the film and prevention of exposure to air by the silicone resin. Figure 7 5. Optical transmittance spectra of the double layers sample before and after aging at 120 C for 120hours Yellowness index was determined based on transmittance spectra of e ach sample. Equation ( 17 ) was utilized to calculate yellowness index where T 420 T 560 and T 680 had transmittance of 420, 560, and 680 nm, respectively. 137 (17)
102 (18) YI is the change in yellowness index. YI before aging is the yellowness index of initial state before aging and YI after aging is the yellowness index after thermal aging for 120 hours Figure 7 6. Yellowness indices during thermal aging for 0 120 hours for FG, TY, NCTY and FGNC during thermal aging Figure 7 6 show s yellowness indices as a function of the aging time for silicone resin, thiol yne film, TiO 2 loaded nanocomposite and du al encapsulation layers (FGNC). Figure 7 7 shows the changes of yellowness index before and after aging and changes of transmittance at 450 nm of each sample. NCTY exhibited the most serious c ase of showed the most resistance to discoloration from an almost identical transmittance and no change in YI during thermal aging. Although YI of FGNC (~13.1) was higher than that of TY (~9.4) due to initial high yellowness index of the first layer (NCTY), change in YI of FGNC (~7.6) was slightly smaller than that of TY (~8.3).
103 Figure 7 7 C hange in yellowness index and transmittance at 450 nm (lower graph) for FG, TY, NCTY and FGNC during thermal aging 7 6 Conclusion In order to reduce Fresnel reflection losses at the interface between air and the encapsulation layer, double layers encapsulation for LEDs was fabricated using high refractive index TiO 2 loaded nanocompos ite and low refractive index silicone resin. By employing multi layer encapsulation, t otal transmittance for normal incidence light can be enhanced by 2.8 % and 3.6 % for GaN and GaP, respectively, compared with the conventional encapsulation. Thus, furthe r enhance ment for light extraction efficiency will be possible for light emitted in all directions. Although the nanocomposite showed the due to the both photocatalytic and thermal degradation of the polymer matrix, double layers encapsulation had better thermal stability than pure thiol yne sample because coating nanocomposite by the si licone resin reduced the chance of reacting with oxidation species
104 CHAPTER 8 CONCLUS ION S The photopolymerization of mixtures of multifunctional thiols enes and thiols ynes is an efficient method for the rapid production of films and thermoset plastics. R eactions of t hiol ene and thiol yne provide delayed gelation, low shrinkage, high conversion, and uniform crosslink densities with rapid reaction rate in ambient environment s such as oxygen and moisture. Click reactions empl o ying thiols can be applied to v ariety of applications with our focus being on optical applications. First, high refractive index polymers have received significant attention with t he development of photonic device s. There have been many attempts to increase the refractive index of polym ers. Among them, the hybrid approaches integrate s organic polymers as matrixes with high refractive index inorganic nanoparticles as fillers ; the inorganic organic nanocomposites have been introduced in order to achieve much higher refractive indices compa red to intrinsic polymers In Chapter 5, UV curable thiodiethanethiol and 1,6 heptadiyne monomer mixtures as a matrix and TiO 2 nanoparticles as a filler were fabricated Silane coupling agents, 3 m ethacryloxypropyltrimethoxysilane, was used for surface treatment of TiO 2 nanoparticles to prevent agglomeration. As predicted from effective medium approximations increases in refractive indices of nanocomposites result from an increase in particle conce ntration but also a slight decrease in the transmittance of nanocomposites. TiO 2 loaded thiol yne nanocomposites have a high refractive index of ~1.683 and high transmittance of 88 %. Furthermore, UV curable nanocomposite can easily produce the desired sha pe by printing or molding methods without side products.
105 Second, i n planar structure OLEDs, the light outcoupling efficiency is generally believed to be ~ 20% due to t otal internal reflection losses result ing from refractive index mismatches between multip le layers of devices. The microlens array is an effective way to improve low out coupling efficiency for OLEDs by extracting substrate air waveguide mode. In Chapter 6, microlens array was created by a direct printing technique and monomer mixture of trime thylolpropane tris(3 mercaptopropionate) and tri(ethylene glycol) divinyl ether was employed as a lens material due to its characteristics such as high transparency and similar refractive index to a glass substrate In addition, its low viscosity provides thiol ene compatibility to inkjet and direct printing processes. In order to obtain hemispherical shape of lenses, hydrophobic silane coupling agents were employed to decrease the surface free energy of glass substrates. The surface properties of a glass s ubstrate were modified by three different silane coupling agents in order to find the lowest critical surface tension and it was determined by a Zisman plot. The glass substrate modified with (heptadecafluoro 1, 1, 2, 2, tetra hydrodecyl) trimethoxysilan e showed the lowest critical surface tension of 9.29 mN/m. In order to obtain a uniform surface on glass substrates, the silane coupling agent was applied using a chemical vapor deposition method in a heated vacuum chamber. Diameter to height ratio of lens es increased from 1:0.19 to 1:0.41 after a one hour vapor phase deposition. The light extraction enhancement of the microlenses was measured on devices with half the emitting area covered in the microlens array while the other half remained bare and a n enh ancement factor of f= 1.31 + 4 was observed. In order to dispense lens droplets, dispensing tip must have contact with the substrate in this direct printing system. This created slight non uniformities on the lens array. Further
106 enhancement will be possible to employ more precise mechanical positioning system and inkjet printing system for better uniformity of the lens array. Lastly, i n the application of LEDs, the roles of encapsulation materials have a great impact on the brightness and life span of LEDs. Thermal stability of the LED encapsulation requires stable transparency and resistance against discoloration and high refractive in dex is an important factor for the light extraction efficiency of LEDs In Chapter 7, multi layer encapsulation of LEDs was proposed to reduce Fresnel reflection losses at the interface between air and the encapsulation layer. In order to produce double la yers encapsulation, high refractive index TiO 2 loaded nanocomposite and low refractive index silicone resin were utilized. By employing multi layer encapsulation, calculated t otal transmittance for normal incidence light can be enhanced by 2.8 % and 3.6 % for GaN and GaP, respectively, compared with the conventional encapsulation. Thus, further enhance ment for light extraction efficiency will be possible for light emitted in all directions. In order to evaluate thermal stability, transmittance of samples wa s measured after thermal aging at 120 C for 120 hours under the air environment. Transmittance of thiol yne network did not change significantly at visible regions after thermal aging. Although the nanocomposite showed the most serious discoloration with change in yellowness index of 15.1 due to both photocatalytic and thermal degradation of the polymer matrix, double layers encapsulation had better thermal stability than pure thiol yne sample because coating nanocomposite by the silicone resin reduced the chance of reacting with oxidation species
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115 BIOGRAPHICAL SKETCH Sangjun Lee was born in Seoul, South Korea He received his B.S in advanced materials science and engineering at Sungkyunkwan University from 1998 to 2005 H e served in the Korean Army for 26 months after his fresh man year He joined magnetic materials laboratory as an undergraduate research student. In 2006, he entered the department of Materials Science and Engineering in University of Florida. He joined to polymer research group in 2007 and spent the next 5 years contributing to the development of applications for transparent thermosets based on thiols such as high refractive index polymers, microlens arrays on OLEDs, and encapsulations for LED. Upon completion of his Ph.D. program, he is planning to work a t t he central research center for LG chemicals in South Korea, where he is going to continue light extraction of OLEDs and inkjet printing processes.