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Effect of Pressure on the Performance of Organic Light Emitting Devices Fabricated by Cold Welding

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Effect of Pressure on the Performance of Organic Light Emitting Devices Fabricated by Cold Welding

ABSTRACT

Analytical and finite element models were used to simulate additive cold welding patterning processes. The effects of adhesion, pressure, thin film (layer) thickness and dust particle modulus (between the contacting surfaces) are examined.

A simple model is developed and used to determine the contact profiles around impurities between cold-welded thin films. These are computed as a function of adhesion, particle modulus, and layer thickness.

The models are shown to provide new insights into the significance of adhesion, layer thickness and particle modulus in the surface contacts that occur during cold welding.

The implications of the results are then discussed for the design and fabrication organic electronics via cold welding.

TABLE OF CONTENTS

DEDICATION………………………………………………………………………………i
ACKNOWLEDGEMENT…………………………………………………………………..ii
ABSTRACT……………………………………………………………………………..…iv
TABLE OF CONTENT……………………………………………………………………..v
LIST OF FIGURES…………………………………………………………………….….viii
LIST OF TABLES…………………………………………………………………………..ii

CHAPTER 1 BACKGROUND AND INTRODUCTION ………………………………1
1.1 Introduction ………………………………………………………………..…… 1
1.2 Research Objectives ………………………………………………………..……3
1.3 Scope and Organization ……………………………………………………..…..3

CHAPTER 2 LITERATURE REVIEW…………………………………….……………7
2.1 Structure of Small Organic LEDs ………………………………….……………………7
2.2 The Development of Organic Light-Emitting Diodes ……………………………………9
2.3 Cold Welding ………………………………………………………………………… …15
2.4 Dust Particles and Interfaces …………………………………………………………….20
2.5 Material Transport across the Interface ……………………………………….. . 22
2.6 Theory of Adhesion ………………………………………………………………23

CHAPTER 3 ANALYTICAL AND COMPUTATIONAL MODELINGS OF CONTACT
AND ADHESION ………………………………………………………………………….38
3.1 Device Architecture ……………………………………………………………. 38
3.2 Analytical Modeling of Contact and Adhesion …………………………………40
3.3 Finite Element Modeling ……………………………………………………….45

CHAPTRER 4 RESULTS AND DISCUSSIONS……………………………………….52
4.1 Analytical Results and Discussions …………………………………………….52
4.1.1 Plot of Contact Length as a function of Adhesion Energy for
Different Thicknesses ………………………………………………52
4.1.2 Plot of Void Length as a function of Adhesion Energy Different
Moduli ………………………………………………………………54
4.1.3 Plot of Contact Length as a function of Adhesion Energy for
Different Moduli…………………………………………….………54
4.2 Finite Element Analyses (FEA) Results and Discussions ………………………55
4.2.1 Effect of Pressure on the Contact Length……………………………..55
4.2.2 Effect of dust particles’ moduli……………………………………….56
4.2.3 Effect of Layer Thickness ……………………………………………57
4.3 Implications …………………………………………………………………….58

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS ………………………61
5.1 Conclusion ……………………………………………………………………61
5.2 Recommendations ……………………………………………………………62
APPENDICES ………………………………………………………………….……….63

Introduction

Light-emitting diodes (LEDs) are optoelectronic devices that generate light when they are electronically biased in the forward direction.

The early commercial LEDs devices, in 1960s, were based on inorganic semiconductors such as gallium arsenide phosphide (GaAsP) as an emitter and their efficiencies were very low [1].

After 40 years of development, the efficiencies of inorganic LEDs have been significantly improved and they are used in a wide range of applications such as telecommunications, indicator lights, and more recently in solid-state lighting.

The applications of LEDs have been very limited for flat panel displays. High-resolution pixelated LED arrays are very expensive to fabricate and the application of LEDs in displays has been limited to bill board displays

where individual LEDs are manually mounted on the display boards. The light-emitting diodes that are made with organic materials are called organic light emitting diodes (OLEDs).

Prior to the invention of OLEDs, organic-based devices could only be operated in electroluminescence mode. The first organic electroluminescence device was demonstrated in 1960s, and very high operating voltages were required [1].

These devices were made with anthracene single crystals doped with tetracene (a blue-emitting fluorescence dye) sandwiching between two electrodes.

Very high voltages were required and the efficiencies were very low. In the 1980s, a major breakthrough was made as low-voltage OLEDs were demonstrated.

In contrast to the first electroluminescence devices, the new OLEDs devices were based on a multilayer structure and they consisted of a transparent anode, a hole transporting layer, an electron emitting layer, and a cathode.

During operation, electrons and holes are injected from a cathode and an anode, respectively, and recombination of electrons and holes leads to efficient light generation.

References

Sam-Shajing Sun, “Introduction to Organic Electronics and Optoelectronic Materials and Devices” (2008). 2.

Du, T. Tong, W. Akande, A. Tsakiridou and W. Soboyejo “Pressure effects on the lamination of organic light emitting devices,” under review. 3.Kim, P. E. Burrows, and S. R. Forrest, “Micropatterning of Organic Electronic Devices by Cold-welding,” Science, vol. 288, no. 5467, pp. 831-833, May (2000). 4.Akande et. al. “Adhesion and the cold welding of gold-silver thin films,” J. Appl. Phys. 107, 043519 (2010). 5.

Zong, Z. et al. (2006), “Nano- and microscale adhesion energy measurement for Au– Au contacts in microswitch structures”- Journal Of Applied Physics 100, 104313 6.

M. Moreau, Semiconductor Lithography: Principles, Practices, and Materials. New York: Plenum Press, (1988). 7.International Organization for Standardization, ISO 14644-1:1999 Cleanrooms Of, and Associated Controlled Environments-Part I: Classification of Air Cleanliness, vol. 1999. Switzerland: ISO, (1999). 8.

Kim and S. Forrest, “Fabrication of organic light-emitting devices by low-pressure cold welding,” Advanced Materials, vol. 15, no. 6, pp. 541-545, (2003). 9.

Cao, C. Kim, S. R. Forrest, and W. Soboyejo, “Effects of dust particles and layer properties on organic electronic devices fabricated by stamping,” Journal of Applied Physics, vol. 98, no. 3, pp. 033713-033713-6, (2005). 10.

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