CITY UNIVERSITY OF HONG KONG 香港城市大學

Electrode Modification and Interface Optimization of Organic Light-Emitting

Devices 有機發光器件的電極改良及界面優化

Submitted to Department of Physics and Materials Science

物理及材料科學系 in Partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy 哲學博士學位

by

Lai Shiu Lun 黎肇倫

June 2004 二零零四年六月

Abstract

Since Tang et al. at the Eastman Kodak Company reported the first

multi-layered organic light-emitting device (OLED) with high efficiency, OLED has

attracted intense attention because of its enormous potential as flat panel displays (FPD).

Comparing to FPD using the conventional liquid crystal display (LCD) technology,

OLED has many merits including high brightness, low power consumption, high

contrast, wide viewing angles, broad operating temperature ranges, fast response time,

and potentially low manufacturing costs. In contrast to LCD displays, OLED itself is

light emitting and thus requires no backlighting. In short, OLED display has the

necessary attributes to compete or complement effectively with the LCD technology

and to become a major display technology in the new millennium. Thus, OLED has

attracted attention of researchers from academic institutes as well as from industries. On

the other hand, as the OLED is still a new technology, the processing techniques have

not been optimized and the underlying physical mechanisms are not well understood.

The present project aims to work along this direction to explore how performance of

OLEDs can be improved by modifying the device architectures and the physical

mechanisms involved.

Modifications and optimization have been carried out on both polymeric and

small-molecular OLEDs in this thesis. First of all,

poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) based polymeric

electroluminescent (EL) devices using glycerol-modified poly(styrene sulfonate)-doped

poly(3,4-ethylene dioxythiophene) (GPEDOT) films as anodes had been fabricated. By

comparing the devices made with unmodified commercial PEDOT anode, glycerol

doping effectively reduced the operating voltage. This was mainly attributed to the

significant enhancement of conductivity by incorporating a suitable amount of glycerol

into polymeric PEDOT layer. It was further found that the conductivity of PEDOT film

increased with the concentration of glycerol doping. Therefore, this highly conductive

polymeric anode could be finely tuned to have an optimized conductivity for obtaining a

better balance of electron and hole currents, and thus leads to a better device efficiency.

Novel cathode architectures were also explored for standard

α-naphthylphenylbiphenyl diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq3)

devices. One interesting stroke was the exploration of rare-earth metal, ytterbium (Yb).

By co-evaporating Yb and Ag to form a Yb:Ag alloy electrode, a highly transparent

cathode was formed. Both surface emitting and transparent OLEDs with low turn-on

voltage and high efficiency could then be realized.

The feasibility of using calcium (Ca) as a high performance cathode was also

studied. While Ca has a low work function, its detrimental reaction with organic

molecules such as Alq3 has hindered its applications as cathode in small-molecule based

OLEDs. Devices with the Ca electrode exhibited a poor operational stability, even

worse than the typical Mg:Ag one. Device performance, on the other hand,

demonstrated a striking contrast by inserting an ultrathin cesium fluoride (CsF) as

buffer layer. OLEDs with the CsF/Ca cathode outperformed those with the other two

electrodes in terms of both efficiency and lifetime. An important reason for the

enhancement was related to the formation of free Cs metal atoms at the interface.

In order to explore the mechanism responsible for the performance improvement,

the electronic structures of Alq3/Cs interface was carried out. Our ultraviolet

photoelectron spectroscopy (UPS) data suggest that the low work function of Cs

resulted in the substantial reduction of a cathodic barrier at the Alq3/Cs contact.

Through the UPS studies of the interface between Cs and Alq3, two remarkable

consequences were also obtained. First, a Cs-induced gap state was induced, regardless

of the deposition sequence of Alq3-on-Cs or Cs-on-Alq3. Second, the deposition

sequences definitely affected the interface. For the case of Alq3-on-Cs, none of gap state

was observed, as the underlying Cs coverage was low. At high Cs coverages, the Cs

might diffuse as neutral atoms and underwent oxidation into the Alq3 molecule, forming

a new electronic feature.

We also fabricated devices with new electron-transporting materials with the aim

to lower the operating voltage and power consumption. Two materials with superior

electron mobility, namely 2,- bis- (2’ ,2”- bipyridin- 6- yl)- 1, 1- dimethyl- 3, 4-

diphenylsiacyclopentadiene (PyPySPyPy), and bathophenanthroline (BPhen), were

utilized to replace Alq3 for electron transport. On account of their comparatively large

electron affinity, there was also essentially free of electron injection barrier. Devices

employing PyPySPyPy and BPhen, in turn, exhibited a very low threshold voltage for

light emission (2.15 V) and standard display brightness of 100 cd/m2 below 3 V.

ii

Table of Contents

Abstract

Certification of Approval by the Panel of Examiners

Acknowledgements i

Table of Contents ii

List of Figures vii

List of Tables xvii

List of Symbols and Abbreviations xviii

Chapter 1. Introduction 1

1.1 Historical development of OLED 1

1.2 Present status of OLED technology 2

1.3 OLED potential applications 5

1.4 Evolution of OLED display 10

1.5 Technical challenges in OLED technology 14

References 15

Chapter 2. OLED Structures, Fabrication, Operation and Performance

Characterization 16

2.1 Device structures 16

iii

2.1.1 Single layer device, why not? 18

2.2 Device fabrication 20

2.2.1 Substrate preparation 20

2.2.2 Small-molecule organic materials deposition at high vacuum 20

2.2.3 Polymer spin-casting 21

2.2.4 Encapsulation for testing operational lifetime 24

2.3 Device operation 24

2.4 Device performance characterization 26

2.4.1 Luminance/brightness 29

2.4.2 Luminance efficiency 29

2.4.3 Operational stability 30

References 34

Chapter 3. Electroluminescence Processes 36

3.1 Energy level alignment of metal/organic interfaces 36

3.2 Charge carrier transport 39

3.3 Energy levels for the organics 40

3.4 Quantum efficiency 41

References 48

Chapter 4. Materials for Organic Light-Emitting Devices 49

iv

4.1 Anode 49

4.2 Cathode 52

4.3 Hole transporting material 56

4.4 Electron transporting and emissive material 57

References 63

Chapter 5. Techniques for Interfacial Studies 66

5.1 X-ray photoelectron spectroscopy (XPS) 66

5.2 Ultraviolet photoelectron spectroscopy (UPS) 70

5.3 Instrumentation for surface analysis 76

Chapter 6. Concentration Effect of Glycerol on the Conductivity of PEDOT Film

and the Device Performance 79

6.1 Overview 79

6.2 PLED fabrication with the use of GPEDOT 81

6.3 Effect of glycerol concentration on device performance 82

6.4 Effect of glycerol concentration on conductivity of PEDOT film 86

6.5 Summary 89

References 91

Chapter 7. Applications of Ytterbium in Organic Light-Emitting Devices as High

Performance and Transparent Electrodes 93

v

7.1 Overview 93

7.2 Transparent, surface-emitting, and bottom-emitting devices 95

7.3 Characteristics of devices with Yb cathode 97

7.4 Characteristics of transparent devices with Yb:Ag cathode 100

7.5 Summary 110

References 111

Chapter 8. Investigation of Calcium as High Performance Cathode in

Small-Molecule Based Organic Light-Emitting Devices 112

8.1 Overview 112

8.2 Devices with CsF/Ca and Ca cathodes 113

8.3 J–V–L characteristics 114

8.4 Operational stability 118

8.5 Summary 122

References 124

Chapter 9. Reduction of Driving Voltage in Organic Light-Emitting Devices

127

9.1 Overview 127

9.2 Low driving voltage OLEDs 129

9.3 Effect of ETL 130

vi

9.4 Summary 143

References 145

Chapter 10. Interfaces between 8-Hydroxyquinoline Aluminum and Cesium as

Affected by Their Deposition Sequences 147

10.1 Overview 147

10.2 Experimental procedure 148

10.3 Electronic structure of Alq3 151

10.4 Cs-on-Alq3 and Alq3-on-Cs interfaces 151

10.5 Summary 163

References 165

Chapter 11. Conclusions 167

Appendix one: List of Publications 169

vii

List of Figures

Figure 1-1. A comparison of luminous intensity between LCD (left) and OLED (right)

screens at a glancing angle.

Figure 1-2. A 5.2” passive matrix full color display presented by Pioneer at Japan

Electronics Show 98”.

Figure 1-3. The first 2.5” active matrix full color panel from Sanyo and Kodak in 1999.

Figure 1-4. The first 2.85” polymer AMOLED prototype demonstrated by Toshiba at

Society for Information Display 2001.

Figure 1-5. A 15” AMOLED prototype demonstrated by Sanyo and Kodak in 2002.

Figure 1-6. The largest AMOLED screen with amorphous silicon TFTs fabricated by ID

Tech in 2003.

Figure 1-7. The first AMOLED product, Kodak EasyShare L633 zoom digital camera.

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Figure 1-8. (a) Dupont’s Olight 4” AMOLED display shown on a portable DVD player;

(b) Intel cellular phone “Universal communicator”; (c) a 2.2” full color 3G cellular

display from Sanyo; (d) Game Boy Advance with a Kodak’s AMOLED screen.

Figure 1-9. (a) A roll-up display; (b) an electronic newspaper; (c) a smart card.

Figure 2-1. Simple device configuration for (a) bi-layer and (b) tri-layer EL device.

Figure 2-2. Basic operation of an OLED.

Figure 2-3. Photographs of two polymeric green light-emitting devices with either a

rigid ITO glass (upper) or a flexible ITO PET (lower) as an anode substrate.

Figure 2-4. Cross-sectional view of an encapsulated OLED.

Figure 2-5. Schematic EL processes diagram of a bi-layer OLED.

Figure 2-6. (a) J-V, (b) L-V characteristics, and (c) power efficiency and luminance as a

ix

function of current density of standard OLED.

Figure 2-7. Typical operational stability curve of a standard ITO/NPB/Alq3/Mg:Ag

device.

Figure 3-1. Schematic energy level diagram of a metal/organic interface with and

without interface dipole. Φh and Φe denote the injection barriers for hole and electron.

Figure 3-2. Band diagram of an OLED with a structure of ITO/NPB/Alq3/Mg:Ag.

Figure 3-3. Schematic diagram of electroluminescent process.

Figure 3-4. Schematic diagram of exciton decay.

Figure 4-1. ITO pattern used for OLEDs fabrication.

Figure 4-2. The chemical structures of PEDOT, PSS and glycerol.

Figure 4-3. The chemical structures of hole transporting NPB and TPD.

x

Figure 4-4. The chemical structures of green emitting Alq3 and F8BT.

Figure 4-5. Absorption and photoluminescence spectra of (a) Alq3 and (b) F8BT.

Figure 4-6. The chemical structures of electron transporting BPhen and PyPySPyPy.

Figure 5-1. Schematic diagrams of the processes involved in XPS and AES. XPS

involves the removal of a single core electron while AES is a two-electron process.

Figure 5-2. A schematic diagram of the photoemission process. The sample is irradiated

with x-rays of known energy, hv, and electrons of binding energy, Eb are ejected. These

electrons have a kinetic energy, Ek, which can be measured in the spectrometer, and is

given by Fig. 5-1. Φs is the work function of the sample, and Φsp is the spectrometer

work function.

Figure 5-3. A XPS survey spectrum of ITO glass substrate.

Figure 5-4. A schematic diagram showing the photoexcitation of electrons from a

xi

valence band due to monochromatic photons of energy, hv. An idealized energy

distributed is also shown with primary emission distinguished from secondary electrons.

Figure 5-5. (a) Two He I UPS spectra corresponding to Cs deposited on pre-covered Ag

surface (upper), and Alq3 on Cs on pre-covered Ag surface (lower) samples. (b) The

energy level diagram of Cs/Alq3 is also plotted.

Figure 5-6. Schematic diagram of the photoemission system.

Figure 6-1. (a) J-V and (b) L-V characteristics of the devices using different

concentrations of glycerol in PEDOT anodes. The inset in part (a) shows the chemical

structure of F8BT.

Figure 6-2. A graph of luminance against the current density for the devices using

different concentrations of glycerol in PEDOT anodes.

Figure 6-3. I-V characteristics of pristine and glycerol doped PEDOT films. The inset

shows the chemical structure of PEDOT (upper) and glycerol (lower).

xii

Figure 7-1. (a) Current density–voltage, (b) luminance–voltage characteristics of

OLEDs made with Yb/Ag, Mg:Ag and pure Ag cathodes. (c) Power efficiency against

current density curves of OLEDs made with 3 different cathodes.

Figure 7-2. Normalized luminance [luminance (L)/initial luminance (Lo)] vs time for

OLED operation at room temperature, using dc driving at a constant current density of

20 mA/cm2. Lo values are in cd/m2, and the time values are in hours.

Figure 7-3. J-V-B characteristics of the surface-emitting devices Ag/ITO/NPB/Alq3 with

different Yb:Ag cathode thickness of 18.6, 24.8 and 37.2 nm. For comparison purpose,

data (dotted line) from a BE ITO/NPB/Alq3/Mg:Ag device is added.

Figure 7-4. (a) A photograph of biased OLED, showing a strong green

electroluminescent emission. (b) A photograph of the same OLED with no voltage

applied, showing the transparent devices.

Figure 7-5. Transmission spectra of the ITO glass, the transparency device with and

without a 18.6 nm thick Yb:Ag cathode.

xiii

Figure 7-6. (a) Current density, and surface emitted (measured from devices with silver

mirror deposited on the back of the glass slide) and total luminance (sum of luminance

from both sides of the transparent devices) at 7 V plotting against cathode thickness. (b)

Current efficiency – film thickness characteristics from both surface emitted and total

luminance devices, respectively.

Figure 8-1. (a) J–V–L characteristics of the devices with configurations of ITO/NPB (72

nm)/Alq3 (48 nm)/CsF (1.3 nm)/Ca (14.5 nm)/Ag (200 nm) (■), ITO/NPB (72

nm)/Alq3 (48 nm)/Ca (14.5 nm)/Ag (200 nm) (●), and ITO/NPB (72 nm)/Alq3 (48

nm)/Mg:Ag (200 nm) (▲).

Figure 8-1. (b) Power efficiency plotting against the current density for the devices with

CsF (1.3 nm)/Ca (14.5 nm)/Ag (200 nm), Ca (14.5 nm)/Ag (200 nm), and Mg:Ag (200

nm) electrodes.

Figure 8-2. (a) Relative electroluminescence as a function of operating time of standard

NPB/Alq3 encapsulated devices with CsF/Ca, Ca, and Mg:Ag cathodes, tested under dc

driving at a constant current density of 20 mA/cm2. Lo values are in cd/m2 and the time

values are in hours.

xiv

Figure 8-2. (b) Relative voltage as a function of operating time of standard NPB/Alq3

encapsulated devices with CsF/Ca, Ca, and Mg:Ag cathodes, tested under dc driving at

a constant current density of 20 mA/cm2. Vo values are in V and the time values are in

hours.

Figure 9-1. Normalized EL spectra of ITO/NPB/Alq3/ETL/CsF/Yb/Ag devices.

Figure 9-2. (a) J-V-L characteristics of the devices with a configuration of

ITO/NPB/Alq3 (48 nm) (● ), ITO/NPB/Alq3 (32 nm) (■ ), ITO/NPB/Alq3 (20

nm)/PyPySPyPy (12 nm) (▲), ITO/NPB/Alq3 (20 nm)/BPhen (12 nm) (◆) covered with

CsF (1.3 nm)/Yb (14.5 nm)/Ag (200 nm).

Figure 9-2. (b) Luminance plotting against the current density of the devices with a

configuration of ITO/NPB/Alq3 (48 nm) ( ● ), ITO/NPB/Alq3 (32 nm) ( ■ ),

ITO/NPB/Alq3 (20 nm)/PyPySPyPy (12 nm) (▲), ITO/NPB/Alq3 (20 nm)/BPhen (12 nm)

(◆) covered with CsF (1.3 nm)/Yb (14.5 nm)/Ag (200 nm).

Figure 9-3. (a) Current density–voltage, (b) luminance–voltage characteristics of the

xv

devices with a configuration of ITO/NPB (72 nm)/Alq3 (48 nm) using the CsF/Yb,

NaF/Yb, LiF/Yb and pure Yb cathodes.

Figure 9-4. (a) Energy level diagram showing Alq3/Cs interface. (Electron injection

barrier from Cs: -0.3 eV).

Figure 9-4. (b) Energy level diagram showing Alq3/PyPySPyPy/Cs interfaces. (Electron

injection barrier from Cs: -0.8 eV).

Figure 9-4. (c) Energy level diagram showing Alq3/BPhen/Cs interfaces. (Electron

injection barrier from Cs: -0.6 eV).

Figure 10. Schematic diagram depicting the sample structures.

Figure 10-1. He I UPS spectra of Alq3 on clean silver surface with different Alq3

overlayers.

Figure 10-2. UPS spectra of Alq3 as a function of incremental Cs deposition.

xvi

Figure 10-3. UPS spectra of Alq3 on clean and Cs pre-covered Ag surface with different

Alq3 overlayers. The solid, dot and dot-dash curves were obtained from the 0.0, 0.15

and 1.0 ML Cs pre-covered Ag surface, respectively. a, b and c show the He I spectra

after Alq3 deposition with 0.0, 1.0 and 10.0 nm thickness, respectively. d shows the He

II spectra deposited with 10.0 nm thickness of Alq3.

Figure 10-4. (a) Work function of Alq3 as a function of its film thickness on clean and

Cs pre-covered Ag surface.

Fig. 10-4. (b) Energy of the highest occupied state of Alq3 as a function of its film

thickness on clean and Cs pre-covered Ag surface.

Fig. 10-4. (c) Ionization potential of Alq3 as a function of its film thickness on clean and

Cs pre-covered Ag surface.

Figure 10-5. Energy level diagrams for Alq3/Ag and Alq3/Cs/Ag contacts.

xvii

List of Tables

Table 1-1. A comparison between camera-class OLED and LCD displays.

Table 7-1. Performance parameters of the present and several recently reported

transparent OLEDs. (*This brightness refers to the surface-emitting luminance only)

Table 9-1. Electron mobilities of PyPySPyPy, BPhen, and the most commonly used

electron transporting materials.