– Adhesion and Optoelectronic Properties of Organic Light Emitting Devices (Oleds) and Hybrid Organic-inorganic Light Emitting Devices (Hoileds) with Tio2 Nano-particles –
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ABSTRACT
This study examines the basic operation of an Organic Light Emitting Device (OLED) and Hybrid Organic-Inorganic Light Emitting Devices (HOILED) by looking at the individual characteristics of each layer which makes up the entire device. Adhesion analysis and different types of layer characterization were carried out.
First of all, the material of interest was deposited on AFM tips, clean 1 sq. inch glass substrates and ITO-coated glass with subsequent final fabrication of the device. Characterization was carried out to determine the Surface, Electrical and Optical properties.
Results obtained showed improvements in all layer properties when annealed. Deposition techniques and parameters used also affected the final device properties. Atomic Force Microscopy (AFM) measurements of adhesion between interacting layers were carried out for both normal and hybrid layers on glass and ITO.
Results obtained were quantified in terms of average pull-off forces and corresponding adhesion energies were gotten by incorporating forces into theoretical models. This forms a path to further establishing devices with suitable design structures and materials combinations.
TABLE OF CONTENTS
ACKNOWLEDGEMENT ………………………………………………………………………………………………..iii
ABSTRACT ………………………………………………………………………………………………………………….. v
TABLE OF CONTENTS ………………………………………………………………………………………………… vi
LIST OF TABLES ………………………………………………………………………………………………………….ix
LIST OF FIGURES ………………………………………………………………………………………………………… x
CHAPTER ONE …………………………………………………………………………………………………………… 13
1.0 Introduction …………………………………………………………………………………………………….. 13
1.1 Background ………………………………………………………………………………………………….. 13
1.2 Scope of Work …………………………………………………………………………………………………. 15
1.3 Summary and Layout of Thesis …………………………………………………………………………… 15
CHAPTER TWO ………………………………………………………………………………………………………….. 16
2.0 Background Theory and Literature Review …………………………………………………………… 16
1.4 2.1 Organic Light Emitting devices (OLEDs) ………………………………………………………… 16
2.1.1 Introduction ……………………………………………………………………………………………… 16
2.1.2 Physical Structure of OLED and Theory of Operation ………………………………………. 16
2.1.3 Trends of OLED Development in the Future and its Applications………………………. 20
2.2 Hybrid Organic-Inorganic Device (HOILED)………………………………………………………… 22
2.3 Theory of Thin-film Deposition and Processing Techniques …………………………………….. 23
2.3.1 Introduction ………………………………………………………………………………………………. 23
2.3.2. Anode Deposition by R.F Magnetron Sputtering and Annealing Methods ……………. 23
2.3.2.1 Annealing Methods ………………………………………………………………………………. 25
2.3.3 Spin Coating of Organic Layers ……………………………………………………………………. 25
2.3.4 Thermal Evaporation of Cathode Layer ………………………………………………………….. 27
2.4 Adhesion Theory………………………………………………………………………………………………. 28
2.4.1 Adhesive Contact Models ……………………………………………………………………………. 29
2.5 Charge Transport in OLEDs and HOILEDs …………………………………………………………… 37
2.5.1 Introduction ………………………………………………………………………………………………. 37
2.5.2. Fowler-Nordheim tunneling …………………………………………………………………………. 37
2.5.3. Schottky Barrier …………………………………………………………………………………………. 39
2.6 Earlier research work and contributions to present OLED technology ……………………….. 40
CHAPTER THREE ……………………………………………………………………………………………………….. 45
3. Characterization and Adhesion Analysis in OLEDs and HOILEDs ……………………………… 45
3.1 Experimental Methods ……………………………………………………………………………………… 45
3.1.1 Materials …………………………………………………………………………………………………… 45
3.1.2 Deposition of ITO Thin-Film layer on glass Substrate and AFM tips …………………… 46
3.1.2.1 Cleaning of glass slides ………………………………………………………………………… 46
3.1.2.2 Preparation of AFM tips ………………………………………………………………………… 47
3.1.2.3 Deposition of Indium Tin-Oxide by Sputtering………………………………………….. 47
3.1.2.4 Annealing of ITO Thin-Film layer and AFM tips ………………………………………. 48
3.1.3 Deposition of PEDOT:PSS Thin-Film layer on glass substrate and ITO- coated glass
3.1.4 Deposition of MEH-PPV Thin-film layer on Glass and ITO-coated glass …………………. 49
3.1.4.2 Spin Coating of MEH-PPV on glass and ITO-coated glass …………………………. 50
3.1.5 Deposition of Aluminum thin-film on glass substrates ………………………………………….. 51
3.1.6 Deposition of MEH-PPV+TiO2 hybrid layer ……………………………………………………….. 51
3.1.7 Fabrication of Organic Light Emitting Device……………………………………………………… 52
3.1.8 Fabrication of Hybrid OLEDs …………………………………………………………………………… 52
3.2 Thin-Film Layer and Device Characterization ……………………………………………………….. 52
3.2.1 Surface Morphological Characterization ………………………………………………………… 53
3.2.2 Optical Characterization………………………………………………………………………………. 54
3.2.3 Adhesion Measurements with Atomic Force Microscopy ………………………………….. 54
3.2.4 Electrical Characterization of Layered Structures and Device…………………………….. 56
3.3 Results and Discussions …………………………………………………………………………………….. 58
3.3.1 Surface Morphological Characterization of Thin-Film Layers ……………………………. 58
3.3.2 Electrical Characterization and Optical Analysis of Thin-Film Layers …………………. 66
3.3.3. Optical Characterization of Thin-Film layers…………………………………………………… 69
3.3.4 I-V Characterization and Spectral Analysis of light emitted from device ……………… 74
3.3.5 Spectral Analysis of light emitted from device ………………………………………………… 79
3.3.5.1 Device Spectral Analysis ………………………………………………………………………. 79
3.3.6 Adhesion Measurements ……………………………………………………………………………… 81
3.3.7 Implications ………………………………………………………………………………………………. 84
CHAPTER FOUR …………………………………………………………………………………………………………. 86
4.0 Summary and Suggestions for Future Work…………………………………………………………… 86
5.0 References ………………………………………………………………………………………………………. 88
INTRODUCTION
Since their original development by Friend et al. about twenty years ago, organic light emitting devices (OLEDs) have received considerable attention due to their potential for low cost applications in ―soft‖ lighting and displays. Furthermore, the potential to pattern and stamp them into micron- and nano-scale patterns has made them potential candidates to replace liquid crystal displays that are based on inorganic silicon-based light emitting devices.
However light emitting devices are limited by their relatively low lifetimes due to the limited stability in the presence of oxygen and water vapor. They also have limited quantum efficiencies and undergo a range of degradation mechanisms during exposure to light and water vapor/oxygen in the environment. Along with the general advancement and optimization of organic light emitting devices, demand for solutions to the limitations of this technology is fueling an in-depth research.
These include detailed adjustment and optimization of various electrical, spectral properties, design of device structure layout, charge transport techniques etc. to boost efficiency and lifetime of these devices. Most recently, inorganic materials have been doped into the layers within OLEDs in an effort to promote charge and light transport. Such doping has also been suggested as a way of improving charge transport in hybrid organic-inorganic light emitting devices in which charge recombination can occur before electron/hole pairs reach the electrodes.
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