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

205 Pages·2008·5.2 MB·English

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.
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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.
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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.
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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
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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
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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

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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
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