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Enhancement of Light Extraction from Organic Light Emitting Diodes by Direct Fabrication on Buckling Structure Substrates

Permanent Link: http://ufdc.ufl.edu/UFE0044324/00001

Material Information

Title: Enhancement of Light Extraction from Organic Light Emitting Diodes by Direct Fabrication on Buckling Structure Substrates
Physical Description: 1 online resource (72 p.)
Language: english
Creator: Yin, Zhe
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: buckling -- extraction -- oled -- uvo
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic electronics have many advantages such as compatibility with flexible substrates, low cost of manufacturing processes and eligibility of large area applications. As one of the major devices in organic electronics, organic light emitting diodes (OLEDs) have become very promising devices for display applications and solid-state light sources due to their fast response time, wide viewing angle and low power consumption. However, the conventional OLEDs have limited light extraction efficiency and the previous methods to extract light cannot enhance the outcoupling of guided light at a wide range of emission wavelengths, which make them hard for the application of white OLEDs. In our research, the buckling structure is used to enhance the light extraction in OLEDs. A new route of direct fabrication of OLEDs on top of buckling structure substrates is put forward. The buckles were generated spontaneously by the deposition of Al thin film on the PDMS substrates. The thickness of Al layer and the UVO treatment time before Al deposition were changed to control the periodicity and depth of the buckles. The buckling structure substrates with a periodicity of 390 nm and depth of 70 nm were obtained. Alq3 OLED devices were fabricated directly on the top of modified buckling PDMS substrates. The modification of the OLED devices included the adjustment to the thicknesses of ITO, NPB and cathode layers. Buffer layer was chosen and added before the deposition of ITO layer to release the compressive stress which could cause the cracks on the surfaces of the substrates. UVO treatment was used before and after the deposition of buffer layer to further release the compressive stress and enhance the current efficiency of the devices. Without the usage of buffer layer and UVO treatment, a total enhancement of 3.1 times in current efficiency was obtained in the buckling devices compared with the reference devices. With 20-nm-thick LiF buffer layer and UVO treatment, the current efficiencies of both the grating and reference devices were enhanced and the total enhancement for the grating devices was 2.4 times.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Zhe Yin.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: So, Franky.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-11-30

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Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044324:00001

Permanent Link: http://ufdc.ufl.edu/UFE0044324/00001

Material Information

Title: Enhancement of Light Extraction from Organic Light Emitting Diodes by Direct Fabrication on Buckling Structure Substrates
Physical Description: 1 online resource (72 p.)
Language: english
Creator: Yin, Zhe
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2012

Subjects

Subjects / Keywords: buckling -- extraction -- oled -- uvo
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic electronics have many advantages such as compatibility with flexible substrates, low cost of manufacturing processes and eligibility of large area applications. As one of the major devices in organic electronics, organic light emitting diodes (OLEDs) have become very promising devices for display applications and solid-state light sources due to their fast response time, wide viewing angle and low power consumption. However, the conventional OLEDs have limited light extraction efficiency and the previous methods to extract light cannot enhance the outcoupling of guided light at a wide range of emission wavelengths, which make them hard for the application of white OLEDs. In our research, the buckling structure is used to enhance the light extraction in OLEDs. A new route of direct fabrication of OLEDs on top of buckling structure substrates is put forward. The buckles were generated spontaneously by the deposition of Al thin film on the PDMS substrates. The thickness of Al layer and the UVO treatment time before Al deposition were changed to control the periodicity and depth of the buckles. The buckling structure substrates with a periodicity of 390 nm and depth of 70 nm were obtained. Alq3 OLED devices were fabricated directly on the top of modified buckling PDMS substrates. The modification of the OLED devices included the adjustment to the thicknesses of ITO, NPB and cathode layers. Buffer layer was chosen and added before the deposition of ITO layer to release the compressive stress which could cause the cracks on the surfaces of the substrates. UVO treatment was used before and after the deposition of buffer layer to further release the compressive stress and enhance the current efficiency of the devices. Without the usage of buffer layer and UVO treatment, a total enhancement of 3.1 times in current efficiency was obtained in the buckling devices compared with the reference devices. With 20-nm-thick LiF buffer layer and UVO treatment, the current efficiencies of both the grating and reference devices were enhanced and the total enhancement for the grating devices was 2.4 times.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Zhe Yin.
Thesis: Thesis (M.S.)--University of Florida, 2012.
Local: Adviser: So, Franky.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-11-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2012
System ID: UFE0044324:00001


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1 ENHANCEMENT OF LIGHT EXTRACTION FROM ORGANIC LIGHT EMITTING DIODES BY DIRECT FABRICATION ON BUCKLING STRUCTURE SUBSTRATES By ZHE YIN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Zhe Yin

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3 To my family and my friends

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4 ACKNOWLEDGMENTS This work cannot be done without the help of many people. Firstly, I would like to give my sincere thank to my advisor Dr. Franky So. At my first year in UF, Dr. So took me in his group and guided me into the organic electronic field. Every time I had questions Dr. So would give me detailed instructions and helped me solve the problems. I really appreciate his help during my application to PhD degree and during my job interview. I am very proud to graduate from Dr. So s group. Also, I want to express my appreci ation to Dr. Nino and Dr. Gila for being my committee members and providing me many useful advices. I also thank my colleagues in Dr. So s group for their k indly help and teaching. I w ant to show my thank to Do Young Kim, Wonh oe Koo, Stephen Tsang, Dong Woo Song, Cephas Small, Michael Hartel Song Chen, Jesse Manders Jae Woong Lee, Castiello Xiang, Tzung Han Lai Wooram Youn, Shuyi Liu, Jiho Ryu, Iordania Constantinou, David Yu Sujin Baek and the graduated students Fred Steffy Chie h Chun Chiang and Jegadesan Subblah. Without their help, I c ould never finish this work and learned so much. Among the colleagues, I need to give my special thank s to Wonhoe Koo. I finished all the experiments under his instruction. As an experienced resea rcher in buckling structure OLED, he taught me a lot in both theoretical principles and experimental operations. I would like to express my best wish to him and his family. Finally, I want to thank my parents. Without their support, I even cannot come to t he United States and meet so many brilliant people. I want to show my greatest gratitude and appreciation to them for their love, understanding and encouragement throughout my college study.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF FIGURES .......................................................................................................... 7 LIST OF ABBREVIATIONS ............................................................................................. 9 ABSTRACT ................................................................................................................... 11 CHAPTER 1 INTRODUCTION TO ORGANIC ELECTRONICS .................................................. 13 1.1 Organic Electronics ........................................................................................... 13 1.2 Physical B asis of O rganic E lectronic s ............................................................... 15 1.2.1 Electronic Structures ............................................................................... 15 1.2.2 Charge C arriers T ransport ....................................................................... 16 1.2.3 Excitons ................................................................................................... 17 1.2.4 Energy Trans fer ....................................................................................... 17 1.3 Advantages and Disadvantages of Organic Electronic s ................................... 19 2 INTRODUCTION TO ORGANIC LIGHT EMITTING DIODES ................................ 24 2.1 Organic Light Emitting Diodes .......................................................................... 24 2.2 Physics and Device Structure of OLED ............................................................ 25 2.2.1 S tructure of OLED ................................................................................... 25 2.2.2 Operation Principles of OLED ................................................................. 27 2. 3 Fabrication of OLED ......................................................................................... 27 2.4 Characterization of OLED ................................................................................. 28 3 BUCKLING STRUCTURE IN OLED ....................................................................... 35 3.1 Introduction to L ight E xtraction in OLED ........................................................... 35 3.2 Problems of Conventional OLED and Advantages of Buckling Structure ......... 36 3.3 Techniques to G enerate B uckling S tructure ..................................................... 38 3.4 Effects of Al Thickness and UVO Treatment on Buckling Structure .................. 38 3.5 Direct F abrication of OLED D evice on B uckling S ubstrate ............................... 40 4 MODIFICATION OF BUCKLING STRUCTURE ..................................................... 43 4.1 Background ....................................................................................................... 43 4.2 Experimental Details ......................................................................................... 44 4.3 Results and D iscussion ..................................................................................... 45 4.3.1 Transmittance of B uckling S tructure S ubstrates ...................................... 45

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6 4.3.2 AFM A nalysis of B uckle s ......................................................................... 45 4.4 Conclusion ........................................................................................................ 47 5 OLED FABRICATION AND STRUCTURE MODIFICATION .................................. 50 5.1 Background ....................................................................................................... 50 5.2 Experimental D etails ......................................................................................... 50 5.3 Results and Discussion ..................................................................................... 52 5.3.1 Adjustment of B uckling S tructure after ITO D eposition ............................ 52 5.3.2 Modifica tion of OLED S tructure ............................................................... 53 5.3.3 Effects of B uffer L ayer ............................................................................. 55 5.3.4 Effects of UVO T reatment b efore and/or a fter D eposition of B uffer L ayer ............................................................................................................. 56 6 CONCLUSION AND FUTURE WORK .................................................................... 64 6.1 Conclusion ........................................................................................................ 64 6.2 Future Work ...................................................................................................... 67 LIST OF REFERENC ES ............................................................................................... 69 BIOGRAPHICAL SKETCH ............................................................................................ 72

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7 LIST OF FIGURES Figure page 1 1 Molecular structures of several organic small molecules (top row) and polymers (bottom row) ........................................................................................ 21 1 2 bond in organic semiconductors ... 21 1 3 Schematic diagram of HOMO and LUMO energy level in organic molecule ...... 22 1 4 Schematic representation of different types of excitons ..................................... 22 1 5 Schematic diagram of fluorescence and phosphorescence process where ISC is intersystem crossing ................................................................................ 23 1 6 The emission spectrum of cyclomated platinum compl exes for phosphorescent OLEDs with different molecular structures ............................... 23 2 1 OLED applications: A) 11 OLED display, (Sony, XEL1) B) 5 .3 active matrix OLED display (AMOLED, Samsung Mobile Display, SMD) ................................ 31 2 2 Demonstration of typical OLED structure. .......................................................... 31 2 3 The structure of BOLED and TOLED ................................................................. 32 2 4 Schematic energy band diagrams during OLEDs operation. Energy band diagram. ............................................................................................................. 32 2 5 A schematic of vacuum thermal evaporation (VTE) system ............................... 33 2 6 Demonstration of the process of spin coating ..................................................... 33 2 7 A schematic of sputtering process ...................................................................... 34 3 1 Light extraction modes in OLEDs ....................................................................... 41 3 2 Demonstration of light escape cone ................................................................... 41 3 3 Demonstration of light extraction mechanis m in buckling structure devices ....... 42 3 4 Structure of PDMS and surface of buckles. A) Molecular structure of PDMS. B) The structure and surface morphology of buckling structure substrates. ....... 42 4 1 Struc ture of the buckling substrates ................................................................... 48 4 2 Transmittance of the buckling structure substrates. ........................................... 48 4 3 AFM analysis of buckling patterns ...................................................................... 49

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8 4 4 AFM analysis of buckling patterns. ..................................................................... 49 5 1 Structure of Alq3 OLED devices ......................................................................... 58 5 2 Optical microscope image demonstrating the cracks on the surface of the buckling structure substrates af ter the deposition of ITO layer ........................... 58 5 3 AFM analysis of buckling patterns ...................................................................... 59 5 4 The characterization of OLED with the structure of PDMS/Al 15 nm/ITO 70nm/NPB 50 nm/Alq3 63 nm/LiF 1 nm/Al 90 nm ............................................... 60 5 5 The characterization of OLED with the structure of PDMS/Al 15 nm/ITO 7 0nm/NPB 70 nm/Alq3 63 nm/LiF 1 nm/Al 90 nm ............................................... 61 5 6 Photo of cracks on the surface of 40 nm LiF layer sample after ITO deposit i on ........................................................................................................... 61 5 7 AFM analysis of buckling patterns after 120 nm ITO deposition with different buffer layers ........................................................................................................ 62 5 8 The characterization of OLED with the structure of PDMS/Al 15 nm/UVO 20min/LiF 20 nm/UVO 20 min/ITO 70 nm/NPB 50 nm/Alq3 63 nm/LiF 1 nm/Al 120 nm ............................................................................................................... 63

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9 LIST OF ABBREVIATIONS AFM Atomic f orce m icroscopy Al Aluminum A lq3 Tris(8 hydroxy quinolinato)aluminium BOLED Bottom emitting organic light emitting diodes CCT C orrelated color temperature CIE Commission Internationale de l CRI C olor rendering index CT Chargetransfer CuPc C opper phthalocyanine EBL Electron blocking layer EF Fermi level EIL Electron injecting layer EL E lectroluminescence EML Emissive layer EQE E xternal quantum efficiency ETL Electron transport layer FFT F ast Fourier transform HBL Hole blocking layer HIL Hole injecting layer HOMO H ighest occupied molecular orbitals HTL Hole transport layer ITO Indium tin oxide LCD L iquid crystal display LED Light emitting diode

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10 LiF Lithium fluoride L I V L uminance C urrent V oltage LUMO L owest unoccupied molecular orbitals MO Molecular orbital MoOx Molybdenum oxide NPB N, N' bis(naphthalen1 yl) N,N' bis(phenyl) benzidine OLED Organic light emitting diode OPV Organic p hotovoltaic P3HT P oly(3 hexylthiophene) PCPDTBT Poly[2,1,3benzothiadiazole4,7diyl[4,4 bis(2 ethylhexyl) 4H cyclopenta[2,1b:3,4b']dithiophene2,6diyl]] PDMS P oly(dimethylsiloxane) PEDOT:PSS Poly (3,4 ethylenedioxythiophene): poly(styrenesulfonate) SSL Solid state light TIR T otal internal reflection TOLED Topemitting organic light emitting diode UVO U ltraviolet/ozone VTE V acuum thermal evaporation

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11 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ENHANCEMENT OF LIGHT EXTRACTION FROM ORGANIC LIGHT EMITTING DIODES BY DIRECT FABRICATION ON BUCKLING STRUCTURE SUBSTRATES By Zhe Yin May 2012 Chair: Franky So Major: Materials Science and Engineering Organic electronics have many advantages such as compatibility with flexible substrates, low cost of manufacturing processes and eligibility of large area applications. As one of the major devices in organic electronics, organic light emitting diodes (OLED s) have become very promising devices for display applications and solidstate light sources due to their fast response time, wide viewing angle and low power consumption. However the conventional OLEDs have limited light extraction efficiency and the previous methods to extract light cannot enhance the outcoupling of guided light at a wide range of emission wavelengths, which make them hard for the application of white OLEDs. In our research, the buckling structure is used to enhance the light extraction in OLEDs. A new route of direct fabrication of OLED s on top of buckling structure substrates is put forward. The buckles were generated spontaneously by the deposition of Al thin film on the PDMS substrates. The thickness of Al layer and the UVO treatment time before Al deposition were changed to control the periodicity and depth of the buckles. The buckl ing structure substrates with a periodicity of 390 nm and depth of 70 nm were obtained. Alq3 OLED devices were fabricated directly on the top of modified buckling

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12 PDMS substrates. The modification of the OLED devices include d the adjustment to the thickness es of ITO, NPB and cathode layers. Buffer layer was chosen and added before the deposition of ITO layer to release the compressive stress which c ould cause the cracks on the surfaces of the substrates. UVO treatment was used before and after the deposition of buffer layer to further release the compressive stress and enhance the current efficiency of the devices. Without the usage of buffer layer and UVO treatment, a total enhancement of 3.1 times in current efficiency was obtained in the buckling devices compared with the reference devices. With 20nm thick LiF buffer layer and UVO treatment, the current efficiencies of both the grating and reference devices were enhanced and the total enhancement for the grating devices was 2.4 times.

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13 CHAPTER 1 INTRODUCTION TO ORGA NIC ELECTRONICS Ha ving many advantages over conventional inorganic electronic materials such as compatibility with flexible substrates, low cost of manufacturing processes and eligibility of large area applications, organic electronic materials are now being given more and more attentions and bringing revolutions in many aspects of society. In this chapter, the readers are provided an introduction to the field of organic electronic materials. Electrical and optical properties of the organic electronics are discussed for a better understanding of the applications to optoelectronic devices. This introduction also highlight s the advantages and possible disadvantages of organic electronic materials. 1.1 Organic Electronics In general, organic compounds are defi ned as the materials which contain carbon. According to this broad definition, about two million organic compounds have been made that constitute nearly 90% of all known materials.1 However, not all organic materials are viewed to be organic semiconductors Organic semiconductors are characterized as the materials that exhibit semiconducting properties. Only the organic materials with semiconducting properties can be used to fabricate electronic devices. On the contrary to conventional covalently bonded inorganic semiconductors, organic semiconductors are composed of discrete molecules held together by van der Waals forces.1 Because of the weak van der Waals interactions, organic semiconductors exhibit electronic and optical properties different from inorganic semiconductors. More physical principles of organic electronics are covered in Section 1.2 Organic materials can be divided into three categories: small molecules, polymers and biological molecules. However, the most complex biological molecules are not

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14 normally used as organic electronic materials. Therefore, in fact, the organic electronic materials are classified into two classes: low molecular weight small molecules and polymers. Small molecular weight materials typically refer to molecules with several to a few hundred atoms while polymers consist of a long chain of monomer repeating units.4 Typical small molecules used in organic electronic devices include copper phthalocyanine (CuPc), C60, and Tris(8hydroxy quinolinato)aluminium (Alq3). Polymers normally used in organic electronic devices include p ol y (3,4 ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), poly(3hexylthiophene) (P3HT), and p oly[2,1,3benzothiadiazole4,7diyl[4,4 bis(2 ethylhexyl) 4H cyclopenta[2,1b:3,4b'] dithiophene2,6diyl]] (P CPDTBT ). Figure 11 shows the structures of some small molecules and polymers commonly used for organic electronic devices. One of the main differences between small molecules materials and polymer materials is the thin film proces sing method. Small molecules are usually deposited by simple vacuum thermal evaporation (VTE) and polymer thin films are formed from solution using the techniques like spin coating, drop casting and ink jet printing. 59 For decades, inorganic semiconductors were in the dominant position in electronic industry. Silicon, germanium and gallium arsenide were some of the typical inorganic materials for the applications of transistors, solar cells and light emitting diodes. However, the urge of a revolution in electronic industry makes it important to find new materials which can improve the performance of electronic devices. Having the advantages of low cost, eligibility of large area applications and flexibility, organic semiconductors have made this kind of revolution possible and shaped the structures and properties of the electronic devices considerably during these years. The study of

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15 organic electronic materials can date back to the later half of 20th century, when Bernanose et al. found the organic electroluminescence phenomenon using molecularly di spersed polymer films in 1950s.10 From then on, more and more researches were done in the field of organic electronics. In the 1960s, the research on the carrier injection based electroluminescence was carried out by Pope11, 12 and Helfrich13, 14 using a single crystal anthracene. Then in 1983, Partridge introduced the polymer organic electroluminescence by using poly(vinylcarbazole) thin films doped with fluorescent molecules as emission centers15 and it was sinc e then that polymers were used as organic semiconductors. Another landmark in the research of organic electronics was the invention of organic light emitting diodes (OLEDs) by Tang et al. in 1987.16 In order to improve the stability and efficiency of organic electronic devices, continuous studies are still under way conducting by the scientists all over the world. Nowadays, organic electronics have taken up a large proportion in industries. Some of t he main applications include organic solar cells, organic light emitting diodes organic photodetectors and organic transistors. 1.2 Physical B asis of O rganic E lectronics 1.2.1 Electronic Structures Defined as compounds that contain carbon atoms, organic semiconductors gain conjugated systems. C arbons in organic semiconductors form sp2 bonds) and pzorbitals which are perpendicular to the plane of sp2 orbitals The parallel d umbbell shaped pzorbitals around each carbon atom form those sobonds. Figure 1 bond in organic semiconductors The overlaps of t he sp2 orbitals l ead to the strong occupied unoccupied lar orbitals (MOs) Meanwhile the

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16 parallel pzsmallest possible energy needed to generate electronic excitation within the molecules. In most cases, there are the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) in the organic semiconductors, which are similar to the valence bands and conduction bands in inorganic semiconductors. This is schematically demonstrated in Figure 1 3. 1.2.2 Charge C arriers T ransport electrons are free t o move and enable the charge transport within the molecule. However, the weak van der Waals interaction restricts the efficiency of charge transport due to the low carrier mobility. Two types of charge carriers transport mechanisms are studied in organic s emiconductors: band transport model and hopping transport model. Band transport model is formed in the solid thin films with highly crystalline structure. The bands in the highly crystalline organic semiconductors are similar to that in the inorganic semic onductors. Therefore the carrier mobility can be relatively high and reach 1~ 102 cm2/V s in a pentacene crystal.17 H opping transport model usually shows in amorphous organic solids and is the dominant model for the intermolecular transport in organic semiconductors. In hopping model, the charge carrier needs to overcome an energy barrier to move from one molecule to the next and the mobility of the carrier values just 106~103 cm2/V s.18, 19 The mobility in hopping transport model is a function of energy barrier height, electric field, and temperature, which can be expressed by E quation1 1 : 20

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17 T k F T k E T F exp exp ) ( (1 1) 1.2.3 Excitons A bound electronhole pair is defined as exciton in organic electronics. Due to the strong tendency of localization of charge carriers in organic semiconductors, i nstead of generating free electrons and holes, the excitation leads to the creation of excitons.21 In o rganic photovoltaic ( OPV) the absorption of photon leads to the excitation of one electron from HOMO level on to LUMO level, leaving one hole in the HOM O level. In OLEDs, electrons and holes are injected into LUMO and HOMO level from cathode and anode and form excitons in an emissive layer, which then recombine to emit a photon.2 There are three types of excitons as shown in Figure 1 4: Frenkel, chargetransfer (CT), and Wannier Mott. The highly delocalized Wannier Mott excitons have low binding energy (a few meV) and are found only in inorganic semiconductors. In the organic semiconductors, the typical excitons are Frenkel excitons and CT excitons. In Frenkel exciton, the electronhole pair is bounded by relatively high energy (0.52 eV)1, 23, 24 within one molecule. CT excitons exist when excitonic state delocalizes over adjacent molecules such as in the donor/acceptor intersurface. 30 1.2.4 Energy Transfer Excitons diffuse in organic semiconductors through band transport or hopping process, similar to the transport of charge carriers. The energy transfer between molecules occurs in company with the diffusion of excitons. When the excitation energy i s larger than the bandgap energy, the absorption process occurs causing the formation

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18 of excitons T he two carriers in an exciton can have four different spin states. T hree of the spin states are symmetric and have the spin quantum number of 1. Those spin sta tes are defined as triplet. The other one state is anti symmetric and has the spin quantum number to be 0, which is defined as singlet. D ue to different spatial symmetry, the singlet state has a higher energy than the triplet state. After absorption of the energy, the excitons relax from the excited singlet state to the ground state by emitting a photon, which is defined as fluorescence. Some energy may transfer through intersystem crossing, from the lowest excited singlet state to the excited triplet s tate. This process is a nonradiative transition involving vibrational coupling. Once the molecule reaches a triplet state, it co ntinues loose some energy by stepping down in triplet ladder. T he radiative decay of excited triplet state to the ground state is defined as p hosphorescence. Typically, the life time of phosphorescence ( 104 s ~ 1 s ) is much longer than that of fluorescence ( 109 s to 107 s).4 Figure 15 demonstrates the process of fluorescence and phosphorescence. Inter molecular energy transfer consists of tw o types of mechanisms: the long range Frster transfer and the short range Dexter transfer. The Frster transfer is due to the dipoledipole interaction and no photon is emitted in this process. The donor molecules dipole relaxes first with the emission of photon, and then the energy is absorbed by the acceptor molecule. This process depends on both the spectral absorption overlap of the two molecules and the distance between them, usually tens of Angstroms. In Dexter transfer, the electron moves directly to the neighboring molecules, when there is a strong overlap between the orbitals of donor and acceptor molecules, usually in a short distance.

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19 1.3 Advantages and Disadvantages of Organic Electronics Organic electronics are widely used recently because they have many advantages over the conventional inorganic electronics. Most of the organic semiconductors have attractive properties such as flexibility, low cost manufacturing processes and the capability of largearea fabrication. The techniques used to manufacture organic thin film are cheap, simple and require relatively low temperatures. Small molecular thin films can be deposited using sim ple vacuum thermal evaporation.5 Polymer thin films are usually formed by spincoating or ink jet printing.69 Some of those techniques, especially the ones used to make polymer thin films, are compatible with largearea process which allow high efficiency and low cost to come. Another impressive advantage is that the optical and electrical properties of the organic semiconductors can be simply controlled by changing their chemical structures. For example, the OLEDs can emit blue, green or red light when modifying the molecular structures, as for iridium or platinum complexes.2528 Figure 1 6 shows the emission spectrum of cyclomated platinum complexes for phosphorescent OLEDs with different molecular structures. However, organic electronics do have some disadvantages despite all those advantages above. As has been mentioned before, due to the weak intermolecular interactions, the carrier mobility in organic semiconductors is low. Besides, the localized molecular nature causes the intrinsic carrier density to be low, which then restricts the conductivity of the organic semiconductors. Also, the purificat ion of such materials can be a problem because the small molecules can contain high concentrations of chemical impurities. Finally, organic materials often degrade in ambient environment when exposing to oxygen, water vapor or other contaminants, which can decrease the

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20 stability and shorten the lifetime. Therefore, further researches still need to be done to solve all the problems and to enlarge the applications of organic electronics

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21 Figure 1 1. Molecular structures of several organic small molecules (top row) and polymers (bottom row) [ Adapt ed with permission from J.D. Myers 2011. PhD dissertation (Page 23 Figure 1 2) a nd J. Lee. 2009. PhD dissertation (Page 152, Appendix A ) University of Florida, Gainesville, Florida. ] Figure 1 2 Diagram of the formation of bond in organic semiconductors. [Source: http://www.orgworld.de/] PCPDTBT CuPc

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22 Figure 1 3 S chematic diagram of HOMO and LUMO energy level in organic molecule. [Adapted with permission from J.D. Myers 2011. PhD dissertation (Page 25 Figure 1 4 ) University of Florida, Gainesville, Florida. ] Figure 1 4 Schematic representation of different types of excitons : A ) Frenkel B ) chargetransfer and C ) Wannier Mott, with different delocalization of excitons. [Reprinted with permission from J. Lee. 2009. PhD dissertation (Page 52, Figure 16 ) University of Florida, Gainesville, Florida. ] B A C

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23 Figure 1 5 Schematic diagram of fluorescence and phosphorescence process where ISC is intersystem crossing. [ Adapt ed with permission from J. Kalinowski. 2005. Organic Light Emitting Diodes : principles, characteristics, and processes (Page 24, Figure 10). Marcel Dekk er, New York ] Figure 1 6 T he emission spectrum of cyclomated platinum complexes for phosphorescent OLEDs with different molecular structures [ Reprinted with permission from N. Chopra. 2009. PhD dissertation (Page 39, Figure 16 ) University of Florida, Gainesville, Florida. ]

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24 CHAPTER 2 INTRODUCTION TO ORGANIC LIGHT EMITTI NG DIODES Holding the advantages of fast response time, wide viewing angle and low power consumption, organic light emitting diodes (OLEDs) have become one of the most promising devices for display applications and solidstate light sources. In this chapter, a brief introduction to OLED including their history, application, advantages and disadvantages is given. The structure and physics of OLED are also explained. Finally, the readers are provided the fabrication processes and char acterization techniques of OLED. 2.1 Organic Light Emitting Diodes An OLED is an LED with organic light emitting layer. It was first invented by C. W. Tang from Kodak in 1987.16 They report ed an OLED based on heterojunction structure using Alq3 as the light emitting layer and aromatic diamine as the hole transport layer. This research initiated tremendous development s in OLED and the OLED technology has become the most mature technology in organic electronics. Another important research was about phosphorescence OLED done by Frrest et al., which reached the internal quantum efficiency higher than the theoretical limit of fluorescent OLED s.29 Now, as the most mature devices in organic electronics, OLEDs are used widely in industry. OLEDs are commercialized out for mobile display s and TV display s by Samsung, LG and Sony. There are two main application areas for OLED : fla t panel display and solidstate light source. The most traditional and popular display device is liquid crystal display (LCD). However, its efficiency is limited by the color filters used to emit different color of light. One of the advantages of OLED over traditional LED is its short radiative lifetime,

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25 usually in the s cale of nanoseconds, which leads to fast response time. Also in order to be used as display devices, the devices need to have a wide viewing angle which is another great advantage of OLED. I n 2007, Sony released the first commercialized 11 OLED display, XEL 1, as is shown in Figure 2 1 A. Samsung also introduced OLED in the fabrication of mobile display as is shown in Figure 2 1 B The OLED used for solidstate light (SSL) source is not as popular as the application of display device, but it is expected to have significant growth by 2015.31 A good SSL source requires higher luminance, better color rendering index (CRI), matching of Commission Internationale de l with that of a blackbody radiator, which is on the Planckian locus, and a proper correlated color temperature (CCT) between 2500 K and 6000 K.32 OLEDs use very low voltages compared with typical incandescent light bulbs, however, the efficiency can reach c lose to 100 lm/W when incandescent light bulbs have the efficiency of 1217 lm/W.21 Therefore, OLED is a great energy saving light source which has a promising future. In spite of all the advantages mentioned above, there are still some challenges needed t o be overcome for OLEDs. Although OLED s have relatively high current and power efficienc ies they are low compared with florescent lighting. So the optimization of device structure and development of new organic materials are urgently needed for better per formance of OLED devices until they can be commercialized in a larger scale. 2.2 Physics and D evice S tructure of OLED 2.2.1 S tructure of OLED B asically, an OLED consists of an organic layer which functions as light emitting laye r between two electrodes, like the sandwich structure. C harge carriers are injected into the organic layer and recombine to form excitons then radiatively decay to emit li ght.

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26 However, in most cases, an OLED has multiple layers which serve different functions. The structure of a conventional OLED includes substrate, anode, hole transport layer (HTL), emissive layer (EML), electron transport layer (ETL) and cathode layer as is shown in Figure 2 2 Hole i njecting l ayer (HIL) electron i njecting l ayer ( E IL) hole blocking layer (HBL) and electron blocking layer (EBL) are sometimes added to improve the performance of OLED s. G lass or plas tics is often used as substrate to support the OLED device. A node layer is used to inject holes into the device. Indium tin oxide (ITO) is the most popular material for the anode because of its high transmit tance and low sheet resistance. HTL is to transport the injected holes to the EM L and similarly, ETL is to transport electrons to the EML. EML is where the electrons and holes recombine and generate excitons, then emit light by radiative relaxation. C athode layer is used to inject electrons into the device and highly reflective metals such as aluminum or silver are usually used in this layer. HIL is sometimes added between anode and HTL to facilitate hole injection. HBL is a layer between EML and ETL to block the holes from entering ETL in order to prevent any emission from ETL. EIL and EBL serve the similar function as HIL and HBL. The structure of those layers is always overlapped therefore one material can be used for several layers. OLED devices can be divided into two types: the bottom emitting OLED (BOLED) and the topemitting OLED (TOLED). In BOLED, the light is emitted from EML and goes through the glass substrate into the air while in TOLED, the emitted light comes out direct ly to the air through transparent cathode. The structures of BOLED and TOLED are shown in Figure 2 3

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27 2. 2.2 Operation P rinciples of OLED In order to explain the operation principles of OLED, a simple three layer model is used. When the electrical contact happens between the electrodes, the Fermi levels (EF) of the electrodes and organic layer are aligned. However, the charge carriers cannot simply be injected into the organic layer due to the energy barrier which is defined as built in potential. The energy band diagram is show n in Figure 24 A and B When the applied bias equals the built in potential th e electrons and holes are ready to be injected into the organic layer W hen the voltage bias continues increasing, the electrons are finally injected from cathode into ETL and move through HOMO level Meanwhile holes are injected from anode into HTL and mo ve through LUMO level of the organic layer. T he energy band diagram during this process is shown in Figure 24 C and D Once the charge carriers reach the EML, they will form excitons by bounding each other and emit light through the radiative relaxation. 2. 3 Fabrication of OLED As has been mentioned before, small molecules and polymers can both be used to make OLEDs. Small molecule thin films are deposited by vacuum thermal evaporation. Polymer thin films are usually formed using the techniques like spin coating, drop casting and ink jet printing. In our research, vacuum thermal evaporation, spin coating and sputtering processes are used. Figure 2 5 shows the schematic of vac uum thermal evaporation system. A vacuum thermal evaporation system usually consists of the deposition boats, a quartz crystal monitor, a shutter and some patterned shadow masks. The source materials are loaded into the boats and heated to their evaporation temperatures. The vapor of organic materials travels ballistically and deposits on the cold substrates. To accurately control

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28 the thickness of the thin film, a quartz crystal monitor and a shutter are used to check the deposition rate and real time thickness. Different masks are used to deposit certain patterns of the thin film. Spin coating is used to fabricate the PDMS substrates in our research. Figure 2 6 demonstrat es the process of spin coating. The polymer solution is dispensed on the substrate, and then the substrate is accelerated. The centrifugal force of the spin coating helps to spread the polymer solution evenly on the substrate and the excess solution is thrown off the edge of the substrate. The thickness of the film is controlled by the spin speed and the concentration of the solution. A hot plate is used after the s pin coating to heat the substrate so as to completely evaporate the solvent residue. In our research, the anode ITO layer is deposited by sputtering process. Figure 2 7 shows the s chematic of sputtering process In the sputtering process, a target plate is bombarded by energetic ions generated in g low discharge plasma, situated in front of the target.34 The target atoms are sputtered and deposit on the substrates. Secondary electrons can also be generated from the target surface, and help to maintain the pl asma. In our research, magnetron sputtering is used, which applies a magnetic field to get a denser plasma and higher deposition rate. 2.4 C haracterization of OLED I n our research, the luminance current voltage (L I V), e lectroluminescent (EL) spectra and angular emission patterns are measured to test the performance of OLED devices. Luminance is defined as the luminous intensity per unit area in the unit of cd/m2. Luminance can be expressed by E quation 2 1 :

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29 detI L (2 1) Where is the conversion factor and Idet is the measured photocurrent T he current efficiency is the ratio of luminance to the current density in the unit of cd/A, which can be expressed by E quation 2 2 : J LL (2 2) Where L is the current efficiency (luminance efficiency), and J is the current density of the device (mA/cm2). Power efficiency is defined as luminous power emitted in the forward direction divides by total electrical power needed to operate the OLED device, in the unit of (lm/W). The power efficiency can be expressed by E quation 2 3 : L OLED PV V I lm osity Lu )( min (2 3) Here, is the solid angle, V is the applied bias and the luminosity or luminous flux is defines as 21 d V Km) ( (2 4) is the luminance flu x in lumens (lm), wavel m is the maximum spectral luminous efficacy, which is 683 lm/W and 1754 lm/W for photopic and scotopic vision, respectively. T he external quantum efficiency (EQE) is the ratio of the number of photons emitted from OLED to the number of charge carriers injected.

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30 q I d hc SEQE/ / ) ( (2 5) Where S( ) is the optical power of the OLED, I is the current injected into the OLED, h is Plancks constant and q is an electric charge.

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31 Figure 2 1. OLED applications: A ) 11 OLED display, (Sony, XEL 1) [Source: http://www.oled display.net/sony xel 1 oledtv/ ] B ) 5.3 active matrix OLED display (AMOLED, Samsung Mobile Display, SMD) [Source: http://www.samsung.com/us/news/newsRead.do?news_seq=20057] Figure 2 2. Demonstration of typical OLED structure. A B Cathode Electron Transport Layer Emissive Layer Hole Transport Layer Anode Substrate

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32 Figure 2 3. The structure of BOLED and TOLED [Reprinted with permission from S. Eom 2010. PhD dissertation (Page 48, Figure 3 1 ) University of Florida, Gainesville, Florida. ] Figure 2 4. Schematic energy band diagrams during OLEDs operation. Energy band diagram : A ) before the electric contact, B ) after electrical contact, C ) with the applied bias to be the same as built in voltage and D ) with the applied bias larger than the built in voltage. HO MO LUMO Cathode Anode E vac B LUMO Cathode HO MO Anode E F E vac A D h + e Cathode LUMO Cathode HO MO Anode E vac C h + e HO MO LUMO Anode E vac

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33 Figure 2 5. A schematic of vacuum thermal evaporation (VTE) system [Reprinted with permission from J.D. Myers 2011. PhD dissertat ion (Page 34, Figure 1 7 ) University of Florida, Gainesville, Florida. ] Figure 2 6. Demonstration of the process of spin coating. [Reprinted with permission from J.D. Myers 2011. PhD dissertation (Page 38, Figure 1 9 ) University of Florida, Gainesville, Florida. ]

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34 Figure 2 7. A schematic of sputtering process [Source: http://ia.physik.rwth aachen.de/research/sputtering/www sputter eng.pdf ]

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35 CHAPTER 3 BUCKLING STRUCTURE I N OLED T he conventional OLEDs have low light extraction efficiency of about 20%.35, 36 Many techniques were used to solve this problem but they all had certain limitations. A quasi periodic buckling structure was proved to be effective in the enhancement of light extr action efficiency in wide range of spectra.37 In this chapter, a brief introduction of light extraction modes is provided. The problem s of conventional OLED s are put forward and the advantages of buck ling structure are introduced. Besides, t he readers are provided the fabrication process of buckles. This chapter also highlights the meaning of our research that is to giv e a new method to fabricate OLED device s on buckling structure substrates. 3.1 I ntroduction to L ight E xtraction in OLED There are three modes of the light emitt ing from an OLED device: the extracted mode, the substrate mode and the ITO /organic mode as is shown in Figure 3 1. Because of the mismatch of the refractive indices between air and glass, and between glass and ITO some of the emitted light is trapped in the glass substrate, ITO and organic layers due to total internal reflection (TIR) The percentage of the light trapped is determined by the angle of incidence of the light ray undergoing TIR at different interface.38 In extracted mode, the light is extracted from the substrate to air; in substrate mode, the light is trapped in the substrate; in ITO/organic mode, which is also called waveguide mode, the light is trapped in the ITO and organic layers. T he definition of light extraction efficiency can be explained as following. Define the angle of incidence in the semiconductor at the semiconductor air interface as and the angle of the refracted ray as then according to the Snell s Law:

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36 sin sinair sn n (3 1) w here ns and nair are the refractive indices of the semiconductor and air, respectively The critical angle for TIR is got ten when the refracted ray is parallel to the semiconductor air interface that is, =90 s air Cn n / sin (3 2) The internal reflection can be demonstrated as a light escape cone, as is shown in Figure 3 2 The light emitted into the cone can escape from the OLED while the light emitted outside the cone is trapped in each layer. The surface area of the calotteshaped surface is given by: ) cos 1 ( 22 Cr A (3 3) Therefore, the power of escaped light will be: 2 24 ) cos 1 ( 2 r r P PC source escape (3 4) w here Psource is the total power of emitted light from the OLED. T he light extraction efficiency is then defined as the ratio of Pescape to Psource. 3.2 P roblems of Conventional OLED and Advantages of Buckling Structure I n conventional OLEDs, since great differences between the refractive indices of the ITO /organic layer (n = 1.72), glass substrate (n = 1.5) and air (n = 1) exist the emitted light is trapped in ITO/organic layer and glass The amount of trapped light in ITO/organic layer and glass can be 4060% and 2040% of the total emission, respectively.36 Therefore the total light ext raction efficiency can be relatively low (~20%) I n order to solve this problem, many methods were used to extract more light out of the OLEDs. Those methods can be divided into two kinds, one is to extract light from glass

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37 substrate mode and the other is to extract light from waveguide mode. Surface modification such as macrolens or microlens arrays fabricated by imprint lithography40, 41 is always used to extract glass substrate mode. Bragg diffraction grating using photonic crystal layers fabricated by h olographic lithography42 or nanoimprint lithography 43, or using nanoimprinted low refractive index material44, as well as lowindex grids45 is used to extract ITO/organic mode. However, the Bragg diffraction grating can enhance the outcoupling of guided light just at specific emission wavelengths because of the directionality of the periodic structure T herefore it is not possible for the application of white OLED devices. Besides, the cost for large periodic pattern is high and the procedures are complicated. In order to reach higher light extraction efficiency in wider range of wavelengths a new technology introducing buckling structure into the OLED device is presented. Compared with Bragg diffraction grating, buckling structure OLEDs have many advantages such as broad periodicity distribution and the randomly oriented wave vectors As is shown in Figure 33, the grating vector of the buckles can shorten the x axis component of the wave vector, which consequently reduces the incident angle of the emitted light and extract s the light from waveguide mode. T he grating vector is in inverse proportion to the periodicity of the buckles. Therefore, shorter periodicity can lead to large grating vector and smaller incident angle, which will finally increase the possibility of light extraction. B ecause of the broad periodicity distribution and the randomly oriented wave vectors of the buckles, the light can be extracted from a broad range of angles corresponded to the entire emissive spectr a

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38 3.3 Techniques to Generate Buckling Structure The buckles can be generated by thermally evaporating aluminium thin film on poly(dimethylsiloxane) (PDMS) substrate. Due to the difference between the thermal expansion coefficients of the PDMS and aluminium film by releasing the compressive stress, the buckles are spontaneously generated. Figure 34 shows the structure of PDMS and the buckling substrate. T he depth of the buckling structure depends on buckling periodicity and the imposed compressive str ain .46, 47 2 / 1~ W A (3 5) W here 3 / 1~ s f fE E t (3 6 ) is the periodicity (characteristic wavelength) of the buckling structure, /W is the compressive strain, tf is the thickness of the thin film, Ef is the elastic modulus of the thin film and Es is the elastic modulus of the substrate. For the higher efficiency, larger depth of buck les is needed. In order to get light emission at certain spectra, the periodicity of the buckling structure also needs to be controlled. For example, to get green light with wavelength around 530 nm, the periodicity of the buckling structure should be in the range from 300 nm to 400 nm .48 3.4 Effects of Al Thickness and UVO Treatment on Buckling Structure In order to get effective buckling structure, the periodicity should be as short as 300 to 400 nm and the depth should be as deep as possible. To meet those requirements, two factors were controlled in our research: the thickness of Al thin film and ultraviolet/ozone (UVO) treatment before the deposition of Al

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39 T he thickness of Al layer has effects on both the periodicity and the depth of the buckles. According to E quation 36, the periodicity of the buckles is in proportion to the thickness of Al thin film. Besides, based on E quation 35, the depth of the buckles is a function of periodicity. Therefore, when increasing the thickness of Al layer, the periodicity and depth of the buckles can both increase. UVO treatment was used to decrease the periodicity of the buckles. I t was proved that UVO treatment could modify the surface of the PDMS to the silica like structure. A short time of UVO treatment can convert the density of the first ~5 nm PDMS to ~50% density of pure silica and the PDMS density at the depth ~10 nm below the surface to ~10% density of pure silica.50 It was raised by Kovacs and Vincett39, 51, 52 that a metal subsurface c ould be formed under the polymer surface, which in our case was the PDMS surface, when the metal wa s thermally evaporated on the polymer substrate above its glass transition temperature. The subsurface consists of spherical clusters, which are embedded below the surface with a distribution of several tens of nanometers The Al adatoms can sink and coalesce under the PDMS surface due to the low surface energy a nd glass transition temperatur e of PDMS. Besides, as a reactive metal thin film, th e Al film can react with PDMS and be oxidized. However, the UVO treated silicalike PDMS surface can prevent the Al adatoms from being sank and oxidized beneath surface. Therefore the thickness of the modified composite layer decreases, so does the elastic modulus due to a lower elastic modulus of Al ( ~ 69 GPa) compared with that of alumina (~370 GPa).33 According to E quation 3 5 and 3 6 the periodicity and depth can decrease with the increase of the UVO treatment time

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40 3. 5 D irect F abrication of OLED D evice on B uckling S ubstrate As has been mentioned above, the buckling structure requires large depth and a relatively small periodicity. In the research done by WH Koo et al.,48, 49 in order to increase the depth of the buckles without changing their periodicit y, imprinting process and multiple deposition were used. PDMS replica was obtained from buckled PDMS master and was used as the substrate for the second deposition. The procedure was repeated once to get the third deposition substrate Although effective, this fabrication method has its own limitations. First of all, during the imprinting process, it has no guarantee that the added PDMS can have a thoroughly contact with the buckled PDMS master. Therefore, the periodicity and depth of th e buckles can possibly change from the master to the replica. Besides, the fabrication of PDMS replica and the second and third deposition are timeconsuming as well as complicated. In our research, a more direct way to fabricate OLED devices on buckling s tructure substrates was put forward. Buckling structure was spontaneously generated by thermally evaporating aluminum films on PDMS substrates like the conventional way. Then the anode ITO layer was deposited directly on the buckling substrates, followed b y the deposition of organic and cathode layers. T he periodicity and depth of the buckles were controlled by adjusting the thickness of Al film and applying UVO treatment before the deposition of Al. This new technique abandoned the imprinting and multiple deposition processes which m ade the whole process simpler and avoided the possible changes in the buckling structure.

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41 Figure 3 1. Light extraction modes in OLEDs [Reprinted with permission from J. Lee. 2009. PhD dissertation (Page 5 8 Figure 113 ) University of Florida, Gainesville, Florida. ] Figure 3 2. Demonstration of light escape cone. A ) Definition of the escape cone by the critical angle. B ) Area element dA. C ) Area of calotteshaped section of the sphere defined by radius and critical angle. [ Adapt ed with permission from J. Lee. 2009. PhD dissertation (Page 5 8 Figure 11 2 ) University of Florida, Gainesville, Florida. ] A B C

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42 Figure 3 3. Demonstration of light extraction mechanism in buckling structure devices. Black line represents the light trapped in the waveguide mode. When introducing the buckling structure, the grating vector of the periodic structure (blue line) will change the direction of wave vector and make the light to meet the requirement of extract ion. Red line represents the modified wave vector by buckling structure. Figure 3 4. Structure of PDMS and surface of buckles A ) Molecular structure of PDMS B ) The structure and surface morphology of buckling structure substrates. Waveguide mode Substrate mode Air mode W ave vector y c x Grating vector A B

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43 CHAPTER 4 MODIFICATION OF BUCK LING STRUCTURE 4.1 Background In order to get higher light extraction efficiency of the OLED devices, the periodicity and depth of the buckles need to be modifie d to get most favorable structure before the fabrication of the O LED devices. In previous research,49 it was demonstrated that the green light with a wavelength of 525 nm c ould be obtained from the buckling structure OLED with a periodicity of 300 to 400 nm. The periodicity of the buckles can be changed by depositing different thickness of Al thin film based on E quation 3 6 However, according to the previous research, it wa s hard to generate buckles with the periodicity lower than 400 nm only by changing the Al thickness. So the UVO treatment wa s applied before the deposition of Al thin film to further decrease the periodicity of the buckles. Based on the research done by WH Koo et al.,48 the buckling structure generated by depositing 10nm thick Al layer on PDMS substrate with 3 min UVO treatment could reach a peak characteristic wavelength as low as ~300 nm. Based on the above background, in this chapter, the relationship between the periodicity of the buckles and the thickness of the Al thin film wa s studied. Besides, UVO treatment wa s used and the time for UVO treatment wa s changed so as to observe the effect of UVO treatment on the buckling structure. Transmittance and AFM test wa s carried out to check the transmittance of the substrates, the periodicity and depth of the buckling patt erns respectively The structure of buckling substrates is shown in Figure 41.

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44 4.2 Experimental D etails T he thin glass substrates were cleaned by acetone and isopropanol in ultrasonic cleaner for 15 minutes respectively. After blown by nitrogen gas, the substrates were treated by UVO treatment (Jelight UVO cleaner model 42) for another 15 min utes C ommercial PDMS material (Wacker ELASTOSIL RT 601) w as mixed with the curing agent in a weight ratio of 9:1, and then the mixed solution was spin coated on precleaned glass substrates at a spin rate of 1000 rpm for one second and 4000 rpm for 50 seconds. The PDSM substrates were cured in a hotplate at 100 C for 1 h. 10 20 and 50nm thick Al layer was deposited on the cured PDMS substrates by thermal evaporation at the pressure around 3106 Pa. The deposition rate wa s manually controlled to be 1.7 /s. The substrates were cool ed to ambient temperature by keeping in the chamber for 30 minutes and venting to atmosph ere. UVO treatment was conducted on the PDMS substrates before Al deposition. For 20and 50nm thick Al layer substrates, 0, 1, 3 and 5 minutes UVO treatment was used. For 10nm thick Al layer substrates, 0, 20, 40 and 60 seconds UVO treatment was conducted. Transmittance of the buckling structure substrates was measured by the spectrophotometer for the wavelength from 250 nm to 850 nm, with a r eference of bare thin glass. A tomic force microscopy (AFM) images of the buckling structure substrates were obtained using tapping mode and were analyzed to get the information about the characteristic wavelength, depth, area ratio and fast Fourier transfo rm (FFT) pattern of the buckles.

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45 4.3 Results and D iscussion 4.3.1 Transmittance of B uckling S tructure S ubstrates As is shown in Figure 4 2 A when the UVO treatment time before Al thin film deposition stay ed the same (in this case, 0 min UVO treatment wa s used for the comparison), the transmittance of the substrates had an obvious relationship with the thickness of Al layer. The buckl ing structure substrates with 10 nm thick Al layer ha d the transmittance abov e 90% in visible wavelength region; the buckl ing structure substrates with 20 nm thick Al layer ha d the transmittance increasing from 30% to 72% with increasing wavelength; the buckl ing structure substrate s with 50 nm thick Al layer ha d the transmittance increasing from 10% to 38% with increasing wavelength. Therefore, it is obvious that with increasing thickness of Al layer, the transmittance of the buckl ing structure substrates decreases sharply. This is because the solid Al is opaque metal, and increasing thickness of the thin film can make its proper t ies more like the solid bulk. In order to get high luminance of the OLED devices, t he substrates need to have high transmittance above 90%. Thus the buckl ing structure substrates with 10nm thick Al layer are the best for making the OLED devices consideri ng the transmittance. The relationship between the transmittance and the UVO treatment time is shown in Figure 4 2 B The 10nm thick Al layer substrates with 0, 20, 40, 60 seconds UVO treatment were measured and it c ould be seen that the UVO treatment nearly ha d no effect on the transmittance of buckl ing structure substrates. 4.3.2 AFM A nalysis of B uckles The AFM images and FFT patterns under different conditions are shown in Figure 4 3 and 44 The symmetric rings in the FFT patterns indicate that the buckles have characteristic wavelengths with wide distribution and random orientations.48 The AFM

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46 images in Figure 43 demonstrate the effect of Al layer thickness on the structure of buckles. It could be analyzed that with increas ing Al thickness, the characteristic wavelength and depth of the buckles also increased The characteristic wavelengths for the 10, 20and 50nm thick Al layers without UVO treatment we re 390 to 460 nm, 630 to 780 nm and 1300 to 1600 nm, respectively. This result corresponded with E quation 3 6 showing that the periodicit y of buckles depends on the thickness of the thin film. Figure 44 exposes the relationship between the UVO treatment time and the buckling structure. T he characteristic wavelengths for the 10nm thick Al layer with 0, 20 and 40 seconds UVO treatment we re 390 to 460 nm, 390 nm and 320 to 360 nm, respectively, showing that the characteristic wavelengths decreased with the increase of UVO treatment time. As has been introduced in Section 3.4, UVO treatment can modify the surface of PDMS to the silica like structure. This silica like PDMS surface can prevent the Al adatoms from being sank and oxidized beneath the surface. It can t herefore decrease the thic kness of the metal subsurface, as well as the elastic modulus According to E quation 3 6 the characteristic wavelengths of the buckles may decrease with increas ing UVO treatment time. The depth of the buckles can also be analyzed from the AFM images T he depths of the buckles for 0, 20 and 40 seconds UVO treatment samples we re 65, 70, 60 nm, respectively. Based on E quation 3 5 the depth of the buckles slightly decreased with the decrease of periodicity However, the buckled substrate with 20 seconds UVO t reatment had the largest depth among all the substrates. Considering both the characteristic wavelength and the depth of the buckles, 10nm thick Al layer buckling

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47 structure substrate s with 20 seconds UVO treatment we re the most suitable ones for the OLED device fabrication. 4.4 Conclusion In this chapter, the most favorable condition for the fabrication of buckling structure substrates among all the conditions under testing wa s explored. Two factors we re considered when making the buckled substrates: the thickness of Al thin film and the UVO treatment time. T he transmittance and AFM images of the substrates we re analyzed. Based on the transmittance measurement, the increasing Al thickness caused the tra nsmittance of the substrates to decrease, while the UVO treatment did not have obvious effect on the transmittance. The AFM images show ed that the periodicity and depth of the buckles may decrease with the decrease in the thickness of Al layer and with the increase in UVO treatment time. Considering all the factors, the buckling structure substrate with 10nm thick Al layer and 20 seconds UVO treatment, which ha d periodicity of 390 nm and depth of 70 nm, wa s the most suitable one for the OLED device fabrication.

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48 Figure 41. Structure of the buckling substrates. Figure 42. Transmittance of the buckling structure substrates. A ) Buckling structures formed by depositing different thickness of Al layer on PDMS without UVO treatment B ) Buckling structures formed by depositing 10nm thick of Al layer on the PDMS with different UVO treatment time. A B

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49 Figure 43. AFM analysis of buckling patterns A ) Buckling structures formed by depositing 50nm thick Al layer on PDMS without UVO treatment (Dimension 20 mx20 m) B ) Buckling structures formed by depositing 20nm thick Al layer on PDMS without UVO treatment (Dimension 10 mx10 m) C ) Buckling structures formed by depositing 10nm thick Al layer on PDMS without UVO treatment (Dimension 5 mx5 m) Figure 44. AFM analysis of buckling patterns (Dimensions 5 mx5 m) A ) Buckling structures formed by depositing 10nm thick Al layer on PDMS without UVO treatment B ) Buckling structures formed by depositing 10nm thick Al layer on PDMS with 20s UVO treatment C ) Buckling structures formed by depositing 10nm thick Al layer on PDMS with 40s UVO treatment A B C A B C

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50 CHAPTER 5 OLED FABRICATION AND STRU CTURE MODIFICATION 5.1 Background In this chapter, the green light emitting Alq3 OLEDs were fabricated directly on the top of buckling struct ure PDMS substrates made in former step s. Although it wa s shown in the previous experiments that the buckling structure substrates with 10 nm thick Al layer and 20 seconds UVO treatment had the optimum buckling structure, after the deposition of ITO layer, some cracks we re found on the surface of the substrates with 10nm thick Al layer. Therefor e the buckling structure substrates with 15nm thick Al layer and different UVO treatment time we re prepared and examined. The OLED devices ha d the structure of ITO/NPB/Alq3/LiF/Cathode and the thickness es of ITO, NPB and cathode Al layer we re changed to improve the performance of the OLED devices. The structure of OLED device is demonstrated in Figure 51. In order to further solve the problem of cracks on the PDMS substrates and improve the efficiency of the devices, a buffer layer before the ITO deposition wa s added and the UVO treatment wa s used before and after the deposition of the buffer layer to improve the device performance. AFM images we re measured to analyze the buckling structure of 15nm thick Al layer substrates. T he OLED devices performance wa s expressed by LI V ch aracteristics, electroluminescence (EL) spectra and angular emission patterns 5 .2 Experimental D etails The buckling structure PDMS substrates were prepared in the process introduced in Section 4.2. After UVO treatment for 5 min utes the substrates were loaded into the DC sputter to deposit ITO layer. After another 5 minutes UVO treatment, all the

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51 following layers were thermally evaporated at a pressure around 3106 Pa: N, N' bis(naphthalen1 yl) N,N' bis(phenyl) benzidine (NPB) lay er as hole transport layer, tris (8 hydroxyquinoline) aluminium (Alq3) as light emissive layer and electron transport layer, LiF as the electron injection layer and Al as cathode layer. The deposition rate for LiF layer wa s 0.1 /s and for other layers wa s 2 /s. T he thickness of ITO, NPB and cathode layer was adjusted to improve the device performance. The buffer layers of LiF, MoOx and Alq3 with different thickness were thermally evaporated before the deposition of ITO layer. 5 and 20 minutes UVO treatmen t before or after the deposition of buffer layer was carried out to find its relationship to the current efficiency of the OLED devices. The devices were encapsulated in a glove box under an argon atmosphere with cover glass es and 5 min utes UV curable epoxy The epoxy should only cover the edge of the cover glass and avoid touching the pixels. The l uminancecurrent voltage (L I V) characteristics of the devices were measured by a sourcemeter ( Keithley Series 2400) a picoammeter ( Keithley Series 6485) and a luminance meter ( Konica Minolta LS 100). The applied bias was from 5 V to 10 V. All the four pixels on one sample were measured for the LI V characteristics and the pixels with highest current efficiency on the reference and grating devices were te sts for the EL spectra and angular emission patterns. E lectroluminescence (EL) spectra were measured by a spectrometer (Ocean Optics USB 2000) under a constant current density (5 mA / cm2). The angular emission patterns were measured by integrating the EL sp ectra of the devices according to emission angles from 0o to 70o. During the EL spectra and angular emission patterns measurements, a glass hemisphere lens with 5 mm diameter w as used to enhance the outcoupling of light from

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52 the glass substrate. Then the lens was removed to compare the enhancement of light extraction efficiency without extraction from the substrate mode. 5.3 Results and Discussion 5.3.1 Adjustment of B uckling S tructure after ITO D eposition It had been studied that the best buckling struc ture wa s the one with 10nm thick Al layer under 20 seconds UVO treatment for the fabrication of OLED devices based on its periodicity and depth of the buckles. However, after 120nm thick ITO layer deposition, some cracks were generated on the surface of the substrates because of the compressive stress in ITO layer caused by the squeezing of buckles and the low glass transition temperature of PDMS as is shown in Figure 5 2 Meanwhile, the substrates with 20nm thick Al layer showed much less cracks after ITO deposition. Therefore, 15nm thick Al layer substrates with 0, 20, 40, 60 seconds UVO treatment were made to get acceptable buckling structure as well as to reduc e the cr acks. T he AFM measurement was carried out to check the buckling structure. The AFM result of the 15nm thick Al layer samples is shown in Figure 53 The samples with 0, 20, 40, 60 seconds UVO treatment had characteristic wavelengths of 630 nm, 560 nm, 390 nm and ~390 nm, respectively. After 70nm thick ITO deposition, the substrates with 40 and 60 seconds UVO treatment show ed cracks on the surface, then after another 50 nm thick ITO deposition, all substrates had cracks on the surface. Among all these subs trates, the one without UVO treatment show ed the least number of cracks on the surface. Therefore, considering both reasonable buckling structure and the number of cracks, the OLED devices were fabricated on the buckling structure substrates with 15 nm thi ck Al layer and without UVO treatment before Al deposition.

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53 5.3.2 Modification of OLED S tructure T he sputtering condition and thickness of anode ITO layer control the conductivity of the OLED device; the increase in the thickness of organic layers can generate lager buckles even second buckles and the thickness of cathode Al layer can affect the reflection of the emitted light and the proportion of back emitted light. Thus, to get higher current effic iency and light extraction efficiency, the structure of each layer needs to be modified. In our research, the thickness es of ITO, NPB and Al layers were changed to see their effects on the device efficiency. T h e basic structure of OLED in our research wa s PDMS/Al 15 nm/ ITO 70 nm /NPB 50 nm /Alq3 63 nm /LiF 1 nm/ Al 90 nm. 120nm thick ITO layer, 70 85and 90nm thick NPB layer and 70nm thick cathode Al layer were also applied to make the OLED devices. LI V characteristics show ed that the highest enhanceme nt of grating devices in the light extraction efficiency compared with reference devices without buckling structure c a me from the OLED device with the basic structure. T he LI V characteristics, EL spectra and the angular dependence pattern of the devices are shown in Figure 5 4 From the graphs, it c ould be told that the current efficiency of the reference devices without buckling structure wa s 1.65 cd/A, while the current efficiency of the grating devices wa s 3.7 cd/A, which ha d an enhancement of 120%. Also, there wa s no leakage current based on the result in low bias area. It was studied49 that since the buckling structure ha d a broad distribution in periodicity and random directionality the modes guided in all azimuthal directions could be extracted over the entire emission wavelength range. T he enhancement of light extraction is the result of the increase in the outcoupling efficiency. The EL spectra show ed that there wa s a broad enhancement over the entire spectrum indicating that the entire em ission wavelength range in a white

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54 OLED c ould be outcoupled by only one grating structure. The angular dependence pattern show ed that the strongest intensity emerged in normal direction (except for the grating device with hemisphere lens which show ed high intensity from 030 ). Moreover, with hemisphere lens the buckling structure devices ha d another 39% enhancement of light extracted from substrate mode which ma d e the total enhancement in the light extraction efficiency to be 12.21.393.1 times When increasing the thickness of ITO layer from 70 nm to 120 nm, more cracks were generated because the increasing thickness created more compressive stress inside the ITO layer. By increasing the thickness of NPB layer, the current efficiency of the refer ence devices increased however, the total enhancement for the grating devices decreased The L I V characteristics of 70nm thick NPB layer device is shown in Figure 5 5 It was studied that the increase in the current density of grating devices wa s due t o the partially reduced organic layer thickness of the sinusoidal patterned gratings .3, 22 It can be deducted from this result that the decrease in the thickness of organic layers can increase the current density J, which then decreases the current efficiency based on E quation 2 2 This gives an explanation to the increase in current efficiency of the reference devices H owever the increase in current efficiency of the grating devices wa s not as much as that for the reference devices so the total enhancement m ight experience a decrease. As for the thickness of cathode layer, when decreasing, the Al layer bec a me sort of translucent which c ould allow some light emit from the back side.

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55 5.3.3 Effects of B uffer L ayer According to previous publications, the Alq3 OLED devices have higher efficiency when the thickness of ITO layer is ~120 nm. Nevertheless, the experiments above show ed that the cracks on the surface of the substrates c ould not be eliminated simply by changing the thickness of Al layer or the UVO treatment time. Therefore a buffer layer wa s added before the deposition of ITO layer in order to increase the possibility for a thicker ITO layer and to enhance the efficiency of both grating and reference devices. 20, 40, 60 and 80nm thick LiF, 10and 20nm thick MoOx, and 20and 40nm thick Alq3 were used to be the candidates for the buffer layer. By releasing the compressive stress of ITO buffer layer wa s expected to suppress the generation of cracks and large buckles. Because the effects of buffer layer on the buckling structure were not clear, the AFM measurement was used to check the periodicity and depth of the buckles before making the devices. T he AFM results are shown i n Figure 5 7 From the AFM results, it could be seen that the buffer layers did not have much effects on the buckling structure, so the ITO layer was deposited to see the number of cracks on the surface. For LiF buffer layer, after 70nm thick ITO depositi on, the samples with the LiF layer thicker than 40 nm show ed cracks on the surface and the number of cracks increased dramatically with the increase of LiF thickness Figure 56 shows the photo of cracks on the surface of 40 nm LiF sample. After 70 nm thic k ITO deposition, the samples with MoOx or Alq3 buffer layer show ed cracks more than the substrates without buffer layer. In sum, the most effective buffer layer to avoid the generation of cracks wa s 20nm thick LiF layer. The mechanism of why LiF buffer layers work effectively in this situation instead of MoOx or Alq3 is still unknown and needs future research.

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56 5.3.4 Effects of UVO T reatment b efore and/or a fter D eposition of B uffer L ayer As has been discussed above, the UVO treatment can be used t o modify the surface structure of the device. In order to assist the buffer layer to reach better performance and to eliminate the cracks, the UVO treatment wa s applied before and/or after the deposition of buffer layer. OLED devices we re made on the UVO t reated substrates to observe the performance. In this section, the 20nm thick LiF wa s used as buffer layer. 5and 20min utes UVO treatment wa s introduced before the deposition of LiF buffer layer and/or after the deposition of LiF layer. After trying to use the UVO treatment merely before LiF deposition, merely after LiF deposition and both before and after LiF deposition, the result c ould be demonstrated from the LI V characteristics that the device with 20 minutes UVO treatment both bef ore and after the buffer layer had the best performance. The complete structure of the OLED device wa s PDMS/Al 15 nm/UVO 20 min/LiF 20nm/ UVO 20 min / ITO 70 nm /NPB 50 nm /Alq3 63 nm /LiF 1 nm/ Al 120 nm. T he LI V characteristics, EL spectra and the angular dependence pattern of the devices are shown in Figure 5 8 The current efficiency of the reference device without buckling structure wa s 2.22 cd/A, and the current efficiency of the bucking device with the structure listed above wa s 4.04 cd/A, which ha d a n enhancement of 82%. T he EL spectra and the angular dependence pattern show ed a broad enhancement over the entire spectrum and an extra enhancement of 34% with hemisphere lens which ma d e the total enhancement of the device to be 1 1.821.342.44 times The total enhancement wa s not as high as the OLED device with the structure of PDMS/Al 15 nm / ITO 70 nm /NPB 50 nm /Alq3 63 nm /LiF 1 nm/ Al 90 nm, but the UVO treatment before

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57 and after the deposition of buffer layer did increase the current efficiency of both grating and reference devices.

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58 Figure 51. Structure of Alq3 OLED devices Figure 52. Optical microscope image demonstrating the cracks on the surface of the buckling structure substrates after the deposition of ITO layer.

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59 Figure 53. AFM analysis of buckling patterns (Dimensions 5 mx5 m) A ) Buckling structures formed by depositing 15nm thick Al layer on PDMS without UVO treatment B ) Buckling structures formed by depositing 15nm thick Al layer on PDMS with 20 s UVO treatment C ) Buckling structures formed by depositing 15nm thick Al layer on PDMS with 40 s UVO treatment. D ) Buckling structures formed by depositing 15 nm thick Al layer on PDMS with 60 s UVO treatment A B C D

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60 Figure 54. The characterization of OLED with the structure of PDMS/Al 15 nm/ITO 70nm /NPB 50 nm /Alq3 63 nm/LiF 1 nm/Al 90 nm. Black line represents the reference devices without buckling structure. Red line represents the grating devices with buckling structure. In Figure 54 A and B, each line with same color and different symbol represents one of the four pixels in the same sample. A ) Current density voltage characteristics B ) Current efficiency luminance characteristics C ) EL spectra. D ) A ngular dependence patterns A B C D

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61 A B Figure 55. The characterization of OLED with the structure of PDMS/Al 15 nm/ITO 70nm /NPB 70 nm /Alq3 63 nm/LiF 1 nm/Al 90 nm. Black line represents the reference devices without buckling structure. Red line represents th e grating devices with buckling structure. Each line with same color and different symbol represents one of the four pixels in the same sample. A ) Current density voltage characteristics B ) Current efficiency luminance characteristics Figure 56. Photo of cracks on the surface of 40 nm LiF layer sample after ITO deposit i on.

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62 Figure 57. AFM analysis of buckling patterns after 120 nm ITO deposition with different buffer layers. (Dimensions 5 mx5 m) A ) no buffer layer B ) 20 nm LiF layer C ) 40 nm LiF layer D ) 60 nm LiF layer E ) 80 nm LiF layer F ) 20 nm MoOx layer G ) 20 nm Alq3 layer H ) 40 nm Alq3 layer Depth: 82 nm A 62 7 nm Depth: 75 nm B 557 nm Depth: 95 nm C 02 nm Depth: 8 5 nm D 57 nm Depth: 93 nm E ~ 627 nm Depth: 100 nm F 57 nm Depth: 70 nm G 57 nm Depth: 95 nm H

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63 Figure 58. The characterization of OLED with the structure of PDMS/Al 15 nm/UVO 20min/LiF 20 nm/UVO 20 min/ITO 70 nm/NPB 50 nm/Alq3 63 nm/LiF 1 nm/Al 120 nm Black line represents the reference devices without buckling structure. Red line represents the grating devices with buckling structure. In Figure 58 A and B, each line with same color and different symbol represents one of the four pixels in the same s ample. A ) Current density voltage characteristics B ) Current efficiency luminance characteristics C ) EL spectra. D ) A ngular dependence patterns C D B A

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64 CHAPTER 6 CONCLUSION AND FUTUR E WORK 6.1 Conclusion OLEDs have become one of the most promising organic electronic devices now. With fast response time, wide viewing angle and low power consumption, OLEDs can absolutely be the next generation of display and SSL source. However, the conventional OLEDs have l imited light extraction efficiency due to the difference between the refractive indices of the ITO /organic layer, glass substrate and air Many techniques we re invented to enhance the light extraction efficiency including surface modification and Bragg dif fraction grating, but none of them c ould enhance the outcoupling of guided light at a wide range of emission wavelength, which m ade it hard for the application of white OLEDs. Recently, the buckling structure OLEDs were fabricated by thermally evaporating Al thin film on the PDMS substrates to generate buckles. Those buckling structure devices have the advantages such as broad distribution in periodicity and r andom directionality which can make it possible to enhance the light extrac tion at the entire spect ra. In our research, a new route to fabricate buckling structure OLEDs wa s put forward. A ll the layers of the OLEDs we re deposited directly on the top of buckled PDMS substrates without imprinting process and the use of PDMS replica. Both the conditions in generating the buckles and the structure of OLED devices needed to be modified in order to get the highest light extraction efficiency. First of all, the buckling structure wa s modified. Two factors we re considered: the thickness of the Al thin film and t he UVO treatment time. From the transmittance measurement, it wa s obviously that the larger thickness of Al layer decreased the transmittance of the buckling structure substrates, but the UVO treatment time had

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65 negligible effect on the transmittance. The A FM images show ed that the increase in the thickness of Al thin film c ould increase both the periodicity and depth of the buckles, while the increase in the UVO treatment time c ould cause a decrease of the periodicity due to the silicalike surface modification. Because the goal of our research wa s to make green OLED devices which needed the periodicity from 300 to 400 nm and large depth, considering all the results of different conditions, the most suitable buckling structure wa s generated by the deposition of 10nm thick Al thin film on PDMS substrate pretreated by UVO treatment for 20 seconds. After the modification of buckling structure, the Alq3 OLED devices we re fabricated and the optical and electrical properties of th ose devices we re measured. After the deposition of ITO layer, some cracks appear ed on the surface of the substrates due to the compressive stress. Therefore, the conditions for making the buckling structure substrates needed to be changed to avoid the generat ion of cracks. It wa s found that the buckling structure substrate s with 15 nm thick Al layer and without UVO treatment had the least possibility to generate the crack s and c ould still keep the periodicity and depth of the buckles in a reasonable range (characteristic wavelength of ~600 nm and depth ~80 nm). The thickness of ITO, NPB and cathode layer wa s changed based on the LI V characteristics, EL spectra and the angular dependence pattern of the devices. The increase in the thickness of ITO layer gener ated more cracks due to the increasing compressive stress. T he increase in the thickness of NPB layer c ould improve the current efficiency for the both buckling structure devices and the reference devices without buckling structure, but it could not get a high total enhancement in the light

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66 extraction efficiency for the grating devices. When the thickness of cathode layer decreased the transmittance of the cathode layer may inc rease, which c ould allow some light emit from the back side. The optimum OLED structure in the first part of our research wa s PDMS/Al 15 nm/ ITO 70 nm /NPB 50 nm /Alq3 63 nm /LiF 1 nm/ Al 90 nm. The current efficiency of the reference devices without buckling structure wa s 1.65 cd/A, while the current efficiency of the grating devices wa s 3.7 cd/A, which had an enhancement of 120%. Besi d es, with hemisphere lens the buckling structure devices had another 39% enhancement from substrate mode, which mad e the total enhancement in the light extraction efficiency to be 1 2.21.393.1 times Buffer layer wa s used before the deposition of ITO layer to suppress the generation of cracks and large buckles. LiF, MoOx and Alq3 we re used as the buffer layers to see their effects on the buckling structure. T he MoOx and Alq3 could cause more cracks on the surface so they we re not proper for the buffer layer. When using the LiF as the buffer layer, the periodicity of the buckles slightly decreased while the depth of the buckles slightly increased which wa s perfect for the enhancement of light extraction. However, after 70nm thick ITO deposition, the samples with LiF layer thicker than 40 nm show ed cracks on the surface and the number of cracks increased dramatically with the increase of LiF thickness. T herefore, the most effective buffer layer wa s 20nm thick LiF layer. T he UVO treatment used before and after the deposition of the buffer layer wa s to modify the surface structure of the substrates and eliminated the cracks. The optimum stru cture of the OLEDs in our research with UVO treatment wa s PDMS/Al 15 nm/UVO 20 min/LiF 20 nm/UVO 20 min / ITO 70 nm /NPB 50 nm /Alq3 63 nm /LiF 1 nm/ Al 120 nm.

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67 The current efficiency of the reference devices wa s 2.22 cd/A, and the current efficiency of the bucking devices wa s 4.04 cd/A, which ha d a n enhancement of 82%. T he EL spectra and the angular dependence pattern show ed a broad enhancement over the entire spectrum and an extra enhancement of 34% with hemisphere lens which ma de the total enhancement in light extraction efficiency of the grating devices to be 11.82 1.342.44 times 6.2 Future Work In our research, the method of direct deposition of OLED devices on the buckling structure PDMS substrates wa s proved to be effective. H owever, future work s still need to be done. First of all, although a total enhancement in light extraction efficiency of 3.1 times wa s reached till now, the individual efficiencies of the reference and buckling devices we re not high enough. I t must be sure that the current efficiency of reference devices, buckling devices and the total enhancement can be improved at same time. So the structure of the OLED s still need s to be optimized and the cracks must be eliminated. Secondly, some of the mechanisms of the interactions between certain layers are not clear till now. One of the examples is the function of buffer layer. It is known that the buffer layer plays the role of releasing the compressive stress and suppress ing the generation of large buckles. B ut this cannot explain the reason why LiF can help to reduce the cracks while MoOx and Alq3 may increase the number of cracks. Therefore more theoretical physic al principles need to be researched to get more information which can help to choose the buffer layer materials more accur ately.

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68 Finally, our research only focused on the Alq3 OLED devices, however, the buckling structure substrates can be used in many kinds of organic electronic devices theoretical ly. Thus more researches should be done to fabricate OLED devices with differ ent range of emitting wavelengths in order to get a wider application and further enhance the light extraction.

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69 LIST OF REFERENCES 1. M. Pope and C.E. Swenberg, "Electronic Processes in Organic Crystals and Polymers" 2nd Ed. (Oxford University Press, Oxford, 1999) 2. J.D. Myers Ph. D. Thesis, University of Florida, ( 2011) 3. M Fujita T Ueno, K Ishi hara, T Asano, S Noda, H Ohata, T Tsuji H Nakada, and N Shimoji Appl. Phys. Lett. 85, 5769 (2004) 4. D. Y. Kim, Ph. D. Thesis, University of Florida, (2009) 5. S. R. Forrest, Chem. Rev. 97, 1793 (1997). 6. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature (London) 347, 539 (1990). 7. C. J. Drury, C. M. J. Mutsaers, C. M. Hart, M. Matters, and D. M. de Leeuw, Appl. Phys. Lett. 73, 108 (1998) 8. T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu, and J. C. Sturm, Appl. Phys. Lett. 7 2 519 (1998). 9. D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991). 10. A. Bernanose, M. Comte, and P. Vouaux, J. Chem Phys. 50, 65 (1953) 11. M. Pope, H. Kallmann, and P. Magnate, J. Chem. Phys. 38, 2042 (1963) 12. M. Sano, M. Pope, and H. Kallmann, J. Chem. Phys. 43, 2920 (1965) 13. W. Helfrich and W.G. Schneider, J. Chem Phys. 14, 229 (1965) 14. W.G. Schneider and W Helfrich, J. Chem. Phys 44, 2902 (1966) 15. R. H. Partridge, Polymer 24, 733 ( 1983) 16. C. W. Tang and S. A. VanSlyke, App. Phys. Lett. 51, 913 ( 1987) 17. S. R. Forrest, M. L. Kaplan, and P. H. Schmidt, J. Appl. Phys. 56, 543 (1984). 18. A. J. Campbell, D. D. C. Bradley, and H. Antoniadis, Appl. Phys. Lett. 79, 2133 (2001). 19. P. W. M. Blom, M. J. M. deJong, and M. G. vanMunster, Phys. Rev. B 55, R656 (1997). 20. J. D. Wright, "Molecular Crystals" 2nd Ed. (Cambridge University Press, New York, 1995).

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70 21. N C hopra, Ph. D. Thesis, University of Florida, (2009) 22. S. M Jeong, F Araoka, Y. Machida, Y Takanishi, K Ishikawa, H Takezoe, S Nishimura, and G Suzaki Jpn J. Appl. Phys. 47, 4566 (2008). 23. G. Hill, A. Kahn, Z. G. Soos and R. A. Pascal, Chem. Phys. Lett. 327, 181 (2000). 24. M. Chandross, S. Mazumdar, S. Jeglinski, X. Wei, Z. V. Vardeny, E. W. Kwock, and T. M. Miller, Phys. Rev. B 50, 14702 (1994). 25. J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau, and M. E. Thompson, Inorg. Chem. 41, 3055 ( 2002) 26. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel Razzaq, H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest, and M. E. Thompson, J. Am. Chem. Soc. 123, 4304 ( 2001) 27. S. Lamansky, P. I. Djurovich, F. Abdel Razzaq, S. Garon, D. L. Murphy, and M. E. Thompson, J. Appl. Phys. 92, 1570 ( 2002) 28. C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, J. Appl. Phys. 90, 5048 ( 2001 ) 29. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R. Forrest, Nature 395, 151 (1998). 30. X Y. Z hu, Q. Y ang, and M. Muntwiler, Acc. Chem. Res. 42, 1779 (2009) 31. W.D. Callister, Materials Science and Engineering An Introduction 3 rd E d. ( John Wiley and Sons, New York, 1994) 32. S E om, Ph. D. Thesis, University of Florida, (2010) 33. J. Lee, Ph. D. Thesis, University of Florida, (2009) 34. P.J. Kelly and R.D. Arnell, Vacuum 56, 159 (2000). 35. C. F. Madigan, M. H. Lu, and J. C. Sturm, Appl. Phys. Lett. 76, 1650 (2000). 36. A. Chutinan, K. Ishihara, T. Asano, M. Fujit a and S. Noda, Org. Elect. 6, 3 (2005). 37. M Fujita, K Ishihara, T. Ueno, T. Asano, S Noda, H Ohata, T Tsuji, H Nakada, and N Shimoji Jpn J. Appl. Phys. 44, 3669 (2005). 38. J. S. Kim, P. K. H. Ho, N. C. Greenham, and R. H. Friend, J. Appl. Phys. 88, 1073 ( 2000 )

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72 BIOGRAPHICAL SKETCH Zhe Y i n was born in Beijing, China. Growing up with a desire to make contribution to human society, she chose to dedicate herself to engineering fields. Graduated from Beijing No. 4 High School in 2006, she entered Beijing University of Technology for her bachel or s degree in materials science and engineering. During her undergraduate studies she chose the major of electronic materials and finished her graduation thesis on the study of nanocrystalline SmCo alloy. She graduated from Beijing University of Technology in 2010 with the honor of Highest Distinction in Beijing In 2010, she came to the United States and began her master s stud ies in University of Florida. She joined Dr. Franky So s o rganic e lectro nic m aterials and d evices g roup doing the research on buckling structure OLED. She graduated with the degree of Master of Science in May 2012 and plans to continue her studies in the United States