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White-Light Generation and OLED Lifetime Issues by Aaron R. Johnson A dissertation submitted in ... PDF

205 Pages·2008·5.2 MB·English
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White-Light Generation and OLED Lifetime Issues by Aaron R. Johnson A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Macromolecular Science and Engineering) In The University of Michigan 2008 Doctoral Committee: Professor Jerzy Kanicki, Chair Professor David C. Martin Assistant Professor Jinsang Kim Assistant Professor Jamie Dean Phillips © Aaron R. Johnson 2008 To my family and friends, frogs, and three additional ladies who know their names. ii Acknowledgements It is a pleasure to thank the many people who have made the completion of this thesis possible. Foremost among them, I would like to thank my adviser, Professor Jerzy Kanicki, who has, for many years been a teacher, a counselor and a friend. It is with no small amount of gratitude that I credit his persistent encouragement through the tribulations of my graduate years with my continuance in the program and the completion of this body of work. Professor Kanicki is first among a long list of professors, researchers and informal advisers at the University of Michigan who have influenced and guided me and who deserve a hearty thank- you. Thank-you to my committee, Prof. David Martin, Prof. Jamie Philips and Prof. Jinsang Kim. Thank-you to Dr. Sandrine Martin for her guidance in my first years at UofM. I am most excited to thank my family and friends. Whatever joy I have had in Michigan can be traced, in one way or another, directly to them. From the long conversations about the esoteric nature of the universe to crass jokes about flatulence, they have been prolific. They have arrived with ice cream in the dead of night at the precise moment when ice cream was needed. They have provided long- and short-distance encouragement with a gusto and certainty. Mostly, they have been never-ending wells of love and deep pools of solace and I swim in their kindness daily. I have also received a tremendous amount of aid from the staff at the Georgia Tech Microelectronics Research Center, most notably from Dr. Gregory Book whose skill with choosing lunch locations is surpassed only by his technical and scientific acumen. iii In addition, I am fortunate to have been funded and aided by a number of generous agencies starting with the National Science Foundation, though an IGERT fellowship. In this vein, I must also thank IBM for awarding me with the IBM Ph.D. Fellowship and bringing me to the hallowed halls of the T. J. Watson Research Center in Yorktown Heights, NY for a tremendous experience I will not forget. A large thank you goes to the Office of Naval Research for funding through our partners at eMagin corp. eMagin, was of tremendous aid for the work discussed in Chapter 6 as they provided OLED samples and testing results. I wish to especially thank Dr. Fridrich Vazan for his help, conversation and efforts. And finally, thank-you to Korea for Bi Bim Bob. Your unassuming mixture of rice, vegetables, beef and spicy pepper sauce has nourished me through the late hours of research and writing and has developed within me a mild obsession I will coddle for all my remaining days. iv Table of Contents Dedication .................................................................................................................................................. ii Acknowledgements................................................................................................................................... iii List of Figures .......................................................................................................................................... viii List of Tables ........................................................................................................................................... xiii Abstract....................................................................................................................................................xiv Chapter 1: Background and Introduction ................................................................................................... 1 Introduction ........................................................................................................................................... 1 White-Emitting OLEDs ............................................................................................................................ 3 Lifetime .................................................................................................................................................. 4 Packaging ............................................................................................................................................... 8 Organization of Thesis .......................................................................................................................... 11 Bibliography ......................................................................................................................................... 13 Chapter 2: Electrochemical Characterization of Organic Materials .......................................................... 16 Introduction: AC voltammetry .............................................................................................................. 16 Electrochemical Instrumentation and Operation .................................................................................. 18 Ferrocene/Ferrocenium Standard ........................................................................................................ 22 Measurement of Organic Semiconductors ........................................................................................... 25 Conclusions .......................................................................................................................................... 30 Bibliography ......................................................................................................................................... 32 Chapter 3: PL Quantum Efficiency of Polymer Thin Films ........................................................................ 34 v Introduction ......................................................................................................................................... 34 Calibrating the system.......................................................................................................................... 36 Measuring the input flux ...................................................................................................................... 40 Measuring the Sample Emission ........................................................................................................... 40 Self Absorption Correction ................................................................................................................... 41 Quantum Efficiency Measurement ....................................................................................................... 45 Measurement Uncertainty ................................................................................................................... 46 System Validation and Sample Measurement ...................................................................................... 49 Conclusions .......................................................................................................................................... 51 Bibliography ......................................................................................................................................... 52 Chapter 4: OLED Fabrication .................................................................................................................... 53 Introduction ......................................................................................................................................... 53 Structure and Materials ....................................................................................................................... 55 Fabrication Procedure .......................................................................................................................... 60 Characterization ................................................................................................................................... 61 CIE calculation ...................................................................................................................................... 67 OLED Fabrication and Discussion .......................................................................................................... 69 Conclusions .......................................................................................................................................... 72 Bibliography ......................................................................................................................................... 74 Chapter 5: White Light Emission from Emissve Polymer Blends .............................................................. 77 Introduction ......................................................................................................................................... 77 Förster Transfer ................................................................................................................................... 79 Förster transfer in PFO-MEHPPV systems ............................................................................................. 83 White Light Emission from PLEDs Using PFO-MEHPPV blends .............................................................. 92 Energy transfer in PFAT-PFBTB systems................................................................................................ 94 White light Emission from PLEDs using PFAT-PFBTB blend ................................................................. 100 vi Conclusions ........................................................................................................................................ 102 Bibliography ....................................................................................................................................... 103 Chapter 6: Low Temperature, Thin Film Encapsulation for OLEDs ......................................................... 105 Introduction ....................................................................................................................................... 105 Low-Temperature Plasma-Enhanced Chemical Vapor Deposition....................................................... 112 Encapsulation Failure Mechanisms .................................................................................................... 125 Development of Low Temperature Encapsulation Schemes ............................................................... 126 Conclusions and Future Work ............................................................................................................ 162 Bibliography ....................................................................................................................................... 164 Chapter 7: Dark Spot Growth Rate of Pulsed OLEDs .............................................................................. 168 Introduction ....................................................................................................................................... 168 Experimental ...................................................................................................................................... 171 Results and Discussion ....................................................................................................................... 173 Combination of Pulsed Driving Scheme and Thin Film Encapsulation ................................................. 176 Conclusions and Future Work ............................................................................................................ 177 Bibliography ....................................................................................................................................... 179 Chapter 8: Conclusions and Future Work ............................................................................................... 180 White Light Generation and Förster Blends ........................................................................................ 180 Packaging by Thin Film Encapsulation ................................................................................................ 183 Pulsed Driving Methods ..................................................................................................................... 186 vii List of Figures Figure 1-1: Lighting energy consumption borken down by sector. .............................................................. 2 Figure 1-2: Rapid efficiency increases in white OLEDs shown in reference to inorganic LED progress.......... 3 Figure 2-1. Three electrode electrochemistry cell with glassy carbon working electrode, Ag/Ag+ reference electrode and platinum counter electrode. All electrode potentials are controlled by a potentiostat. ........................................................................................................................................................ 18 Figure 2-2. This diagram shows the cell with a sufficiently negative bias (a) to reduce the polymer films and a sufficiently positive bias (b) to oxidize the films. The dashed line indicates the potential across the cell. The potential is flat in the electrolytic solution because it is assumed that the cell has zero resistance. ....................................................................................................................................... 19 Figure 2-3. Comparison of Cyclic voltammetry and AC voltammetry. Cyclic voltammetry scan the constant potential from a start (S) to an end (E) potential and then back to S at a given rate. AC voltammetry superimposes a small sinusoidal signal on a DC sweep. ................................................................... 22 Figure 2-4. CV (a) and ACV (b) curves for a reversible redox reaction (ferrocene/ferrocenium). The dashed line in (a) represents the difference between the forward and reverse scan currents for each sampled potential. ........................................................................................................................... 23 Figure 2-5. CV (a) and ACV (b) curves for red light-emitting polymer. The ACV curve was fitted to multiple Gaussian peaks in order to extract the estimated HOMO and LUMO potentials. ............................. 26 Figure 3-1: Measurement setup for photoluminescent quantum efficiency measurements ...................... 35 Figure 3-2: Calibration setup for spectroradiometer ................................................................................. 37 Figure 3-3: Calibration factor (inverse of the detection system response) plotted with its relative uncertainty ...................................................................................................................................... 39 Figure 3-4: Self absorption setup for measuring the sample outside the sphere. Measuring the sample inside the sphere makes use of the normal measurement setup described above........................... 43 Figure 3-5: Effect of self absorption correction on the photoluminescent emission spectrum of MEH-PPV- POSS ................................................................................................................................................ 45 Figure 4-1: Schematic of bottom emitting PLED structure. ........................................................................ 55 Figure 4-2: Chemical structure of F8BT ..................................................................................................... 56 Figure 4-3: Chemical structure of PFO-POSS (a) and MEHPPV-POSS (b)..................................................... 58 viii

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Aaron R. Johnson Microelectronics Research Center, most notably from Dr. Gregory Book whose skill Chapter 1: Background and Introduction .
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