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Theses
8-26-2009
Fabrication and characterization of organic light emitting diodes Fabrication and characterization of organic light emitting diodes
for display applications for display applications
Omkar Vyavahare
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1
Fabrication and Characterization of Organic Light Emitting Diodes for Display
Applications
By
Omkar Vyavahare
A Thesis Submitted in Partial Fulfillment
of the Requirements for the Degree of
Master of Science
in
Materials Science and Engineering
Approved On: 08/26/2009
CENTER FOR MATERIALS SCIENCE AND ENGINEERING
COLLEGE OF SCIENCE
ROCHESTER INSTITUTE OF TECHNOLOGY
Rochester, NY
2
Fabrication and Characterization of Organic Light Emitting Diodes for Display
Applications
By
Omkar Vyavahare
APPROVAL
Approved by Thesis Committee:
Prof. Richard Hailstone ________________________
(Associate Professor, Center for Imaging Science)
Dr. T. K. Hatwar
________________________
(Senior Principal Scientist, Eastman Kodak Company)
Prof. Surendra Gupta ________________________
(Professor, Department of Mechanical Engineering)
Accepted by Center for Materials Science and Engineering:
Prof. K. S. V. Santhanam _______________________
(Director, Center for Materials Science and Engineering)
3
Fabrication and Characterization of Organic Light Emitting Diodes for Display
Applications
By
Omkar Vyavahare
I, Omkar Vyavahare, hereby grant permission to the Wallace Memorial Library of the Rochester
Institute of Technology to reproduce this document in whole or in part that any reproduction will
not be for commercial use or profit.
_______________________ _______________________
Omkar Vyavahare Month, Date, Year
4
ROCHESTER INSTITUTE OF TECHNOLOGY
This volume is the property of the Institute, but the literary rights of the author must be
respected. Please refer to the permission statement in this volume for denial or permission by the
author to reproduce. In addition, if the reader obtains assistance from this volume, he must give
proper credit in his own work.
The following persons, whose signatures attest to their acceptance of the above restrictions, have
used this thesis.
Name Address Date
5
Fabrication and Characterization of Organic Light Emitting Diodes for Display
Applications
By
Omkar Vyavahare
A Thesis Submitted to the Department of Center for Materials Science and Engineering in Partial
Fulfillment of the Requirements for the Degree of Master of Science in
Materials Science and Engineering
ABSTRACT
Organic Light Emitting Diodes (OLEDs) based on the principle of electroluminescence,
constitute a new and exciting emissive display technology for flat panel displays. In order to
attain high quantum efficiency for electroluminescence, it is necessary to achieve three
attributes: efficient charge injection from the electrodes at low drive voltage, good charge
balance, and confinement of the injected charge carriers within the emitting layers. The purpose
of this research work was to fabricate, measure and analyze OLEDs based on these fundamental
principles using different cathode materials, injection layers and buffer layers in order to
determine the best possible configuration. Starting from a simple bi-layered device, multilayered
heterojunction OLEDs were built by employing energy band engineering. Since it was the first
time that these imaging devices were being built in our Laboratory, developing tools and
techniques to get reproducible OLEDs was a prerequisite to the realization of this goal. Thus,
through this process, the Lab’s capability was realized from the fabrication and characterization
perspective, and fundamental knowledge regarding the operation of OLEDs was gained. The
OLEDs fabricated were of high efficiency and brightness, and their properties match well with
the published literature.
6
To my loving mother
Dr. Vinaya Vyavahare
And to the memory of my father
Dr. Vidyadhar Vyavahare
7
ACKNOWLEDGMENTS
First and foremost, I want to thank my research advisor, Prof. Richard Hailstone for his guidance
during my research and study at RIT. I am thankful to him for not only giving me an opportunity
to work in his laboratory but also for being a great mentor. He was always accessible and willing
to help. He unhesitatingly provided all the resources and assistance required for successful
execution of this project. As a result life became smooth and rewarding for me.
Special thanks to my thesis committee members and advisors, Dr. T. K. Hatwar and Prof.
Surendra Gupta. I owe much gratitude to Dr. Hatwar for his time and exceptional support of this
research work. Many times, he went out of his way to ensure we got everything we needed. My
most sincere thanks go to Prof. Gupta, for his strong support and encouragement. I was delighted
to interact with Prof. Gupta, having him as my co-advisor, and attending his classes.
I would like to thank my lab-mate Trisha Slyker for training me on the tools and providing very
fine technical details at an early stage of the research. I also want to thank Gary DiFrancesco for
helping me in every way. The friendly discussions with him made my lab experience enjoyable. I
am grateful to Professor C.W. Chang, of University of Rochester and my friend Dr. Kalyan
Garre for their valuable suggestions.
My deepest gratitude goes to my family; my parents Dr. Vinaya Vyavahare, and Dr. Vidyadhar
Vyavahare and my brothers, Kedar and Tushar, for their unflagging love and support throughout
my life. I cannot ask for more from my mother as she is simply perfect. I am proud of my
brother, Kedar. He has always been there for me and has been my best counselor.
Finally, I am grateful to all my colleagues and friends whose suggestions and encouragement
were invaluable throughout my graduate career at RIT.
8
TABLE OF CONTENTS
Chapter I – Introduction to Display Technology
1.1 Introduction to OLEDs ……………………………………………………………………...12
1.2 Overview of Display Technology ………………………………………………………….. 13
1.3 Liquid Crystal Displays (LCDs) …………………………………………………………… 15
1.4 Inorganic LEDs (ILEDs) …………………………………………………………………... 17
Chapter II – Organic Light Emitting Diodes
2.1 History of OLEDs ………………………………………………………………………….. 18
2.2 Device Configuration and Operation ………………………………………………………. 19
2.3 Types of OLED Technology ……………………………………………………………….. 22
2.4 Advantages and Disadvantages of OLEDs ………………………………………………….23
2.5 Degradation Mechanisms of OLEDs ………………………………………………………. 25
2.6 Challenges Faced by OLEDs ………………………………………………………………. 27
2.7 Applications of OLEDs …………………………………………………………………….. 28
Chapter III – OLED Fabrication and Characterization Techniques
3.1 Common OLED Materials …………………………………………………………………. 29
3.2 OLED Deposition Techniques ……………………………………………………………... 32
3.3 OLED Analysis Techniques ……………………………………………………………….. 35
II. Research Work …………………………………………………………………………….. 37
Chapter IV –
Goal 1: To Develop Procedures for Reproducibly
Fabricating Organic Light Emitting Diodes
4.1 Theory ……………………………………………………………………………………… 39
4.2 Experimental ……………………………………………………………………………….. 41
4.3 Results and Discussion …………………………………………………………………….. 43
4.4 Conclusions ………………………………………………………………………………… 47
9
Chapter V –
Goal 2: To Study Performance of OLEDs
Based on Different Cathode Materials
5.1 Theory ……………………………………………………………………………………… 48
5.2 Experimental ……………………………………………………………………………….. 49
5.3 Results and Discussion …………………………………………………………………….. 50
5.4 Conclusions ………………………………………………………………………………… 53
Chapter VI –
Goal 3: To Study Effect of LiF on the Performance of OLEDs
6.1 Theory ……………………………………………………………………………………… 54
6.2 Experimental ……………………………………………………………………………….. 55
6.3 Results and Discussion …………………………………………………………………….. 56
6.4 Conclusions ………………………………………………………………………………… 58
Chapter VII –
Goal 4: To Study Effect of Hole Blocking
Layer (HBL) on the performance of OLEDs
7.1 Theory ……………………………………………………………………………………… 59
7.2 Experimental ……………………………………………………………………………….. 61
7.3 Results and Discussion …………………………………………………………………….. 62
7.4 Conclusions ………………………………………………………………………………… 66
Chapter VIII – Summary and Future Work …………………………………………………67
Chapter IX – Appendix ………………………………………………………………………..70
References ………………………………………………………………………………………74
10
LIST OF FIGURES
Figure 1.1: Typical OLED structure……………………………………………………………..12
Figure 1.2: Liquid Crystal Display………………………………………………………………15
Figure 2.1: OLED Progress………………………………………………………………………18
Figure 2.2: Typical Bilayerd OLED…………………………………………………………......19
Figure 2.3: Schematic of Charge Transport by Hopping………………………………………...19
Figure 2.4: a) Energy Level Diagram and b) Operation of a bi-layered OLED…………………20
Figure 3.1: Chemical Structure of TPD and NPB………………………………………………..31
Figure 3.2: Chemical Structure of Alq3………………………………………………………….31
Figure 3.3: Schematic of Spin Coating Process………………………………………………….32
Figure 3.4: Schematic Depicting Vacuum Thermal Evaporator…………………………………33
Figure 4.1: Schematic of Oxygen Plasma Treatment Mechanism……………………………….40
Figure 4.2: Chemical Structure of PEDOT:PSS…………………………………………………41
Figure 6.1: Depiction of exciton quenching due to imbalance in carrier transport……………...54
Figure 7.1: The Desired Electron-hole Recombination Region…………………………………59
Figure 7.2: Chemical Structure of BCP and Energy Level Diagram of OLED with BCP………60
Figure 8.1: Summary of Drive Voltages of OLEDs from Goals 1 through 4……………………67
Figure 8.2: Summary of Luminance Characteristics of OLEDs from Goals 1 through 4……….68
Figure 8.3: Summary of Luminous Efficiency of OLEDs from Goals 1 through 4……………..68
Figure 9.1: Response of Human Eye to Brightness……………………………………………..71
Figure 9.2: Luminous flux efficiency of various light sources…………………………………..73
11
LIST OF TABLES
Table 1.1: Summary of the Three Current Display Technologies…………………………….....14
Table 1.2: Comparison between OLED and LCD Technologies………………………………..16
Table 1.3: ILED Configurations for Different Colors…………………………………………...17
Table 2.1: Types of OLED Technology…………………………………………………………22
Table 3.1: Optoelectronic Properties of OLED……………………………………………...35, 36
Table 4.1: Effect of Oxygen Plasma Treatment on the Drive Voltage…………………………..45
Table 7.1: Effect of BCP Thickness on EL Spectrum of OLED………………………………...62
12
Chapter I: Introduction to Display Technology
1.1 Introduction to OLEDs
Organic electroluminescent devices have been one of the most attractive research topics in
materials science over the last two decades. The attraction and fascination of this field stems
mainly from the interdisciplinary nature of this research topic, which includes synthetic and
physical chemistry, device physics and electrical engineering [1].
Organic Light Emitting Diodes (OLEDs) constitute a new and exciting emissive display
technology. These electroluminescent devices have the advantages of being self emitting,
consuming low power, having a wide viewing angle, and having a faster switching speed.
Organic electroluminescence is the electrically driven emission of light from non-crystalline
organic materials. OLEDs have been extensively investigated for improving their performance
owing to their potential applications in flat-panel displays and solid-state lighting [2, 3].
An OLED consists of one or more semiconducting organic thin films sandwiched between two
electrodes, one of which must be transparent. A simplified schematic diagram of a typical OLED
is shown in Fig. 1.1. Indium tin oxide (ITO) is commonly utilized as the transparent anode, and a
low work function metal is utilized as the cathode. The device is fabricated by sequentially
depositing organic thin films followed by a shadow-mask-defined thin metal cathode onto a
transparent substrate such as glass or a flexible plastic. When a forward bias is applied, injected
electrons and holes recombine in the organic layers to generate light. The two major classes of
organic semiconductors used in fabricating OLEDs are: low molecular weight materials (small
molecules) and higher molecular weight materials (polymers). Due to their immense potential
nearly every global electronics firm now has an effort in OLED technology [4].
Light
Figure 1.1: Typical OLED structure
ITO
Substrate
Organic Layers
Cathode
13
1.2 Overview of Display Technology
In today’s world of technology, increasing amounts of time are spent interacting with display
screens. Through computer displays, by watching projections or viewing TV, we handle day-to-
day office information, process research, view e-learning content and engage with multimedia
materials. For this reason the functionality, design and technology of display systems profoundly
affects our everyday working lives. Display technology is in a period of transformation, driven
by the recent introduction of flat panel display (FPD) technologies such as LCD and Plasma,
which are rapidly replacing the traditional cathode ray tube (CRT) displays[5, 6].
Until relatively recently the TV and computer displays have been largely static technologies
based on the CRTs. The current state-of-the-art is thus characterized by evolution from bulky
CRT displays to flatter screens known as Flat Panel Displays (FPD). It is based around two
technologies: Liquid Crystal Display (LCD) and Plasma; triggered from the desire for space-
saving, lower power usage and portability, general attractiveness, and the need for larger,
brighter screens capable of handling new, higher definition content. The CRT is inherently bulky
as its operation requires a relatively large space for the vacuum tube, and FPDs have challenged
CRT displays in both market segments by exploiting this key weakness. LCD and Plasma
technologies continue to improve, owing to strong consumer demand for such portable displays.
Despite this momentum, new technologies in development are aimed at replacing the LCD,
continuing the trend to flatter, more powerful displays. These developments include nanotube-
based field emission displays and Organic and Polymer LEDs [7]. Large numbers of researchers
and technology companies are competing to take these developments out of the prototype stage
and into commercial products. All of these technologies promise greatly improved capabilities
with regard to resolution and brightness. A summary of these display technologies is given in the
following table:
14
Display
Type Main Uses Mechanism Notes
LCD Passive
Transmissive
or
Passive:
Reflective
or
Passive:
Transflective
Mobile phone
Laptop screens
Computer displays
TVs up to 42"
Digital watches
and calculators
Mobile/laptops
Liquid crystal light gates
control transmission of a
backlight through polarised
light filters.
A detailed
description in
the following
section.
Plasma Emissive Large TVs
Public information
displays
Cells of neon gas are
ionised by high voltage to
release UV photons which
hit a phosphor screen
Can only be
manufactured in
large sizes.
CRT Emissive Traditional TV
Traditional
computer monitor
Electrons generated by
thermionic emission by
high voltage at a cathode
are swept by magnets into
a focused beam which
strikes a phosphor screen
Sweeps in a
raster pattern,
right to left and
up to down
across screen –
has implications
for flicker.
Table 1.2: Summary of the three current display technologies
15
Backlight
Horizontal
filter
Transparent
Electrodes
Twisting
LCD crystals
Vertical
Light
1.3 Liquid Crystal Displays (LCDs)
Since the 1970’s, liquid crystal display (LCD) has been the dominant FPD technology. Being the
biggest competitor technology, it is important to describe and compare them with OLEDs.
Liquid crystals are partly ordered materials, somewhere between their solid and liquid phases.
Their molecules are often shaped like rods or plates or some other forms that encourage them to
align collectively along a certain direction. The order of liquid crystals can be manipulated with
mechanical, magnetic or electric forces. Liquid crystals are temperature sensitive since they turn
into solid if it is too cold and into liquid if it is too hot. Liquid crystals have two main phases
which are called the nematic phase and the smectic phase. The nematic phase is the simplest of
liquid crystal phases and is close to the liquid phase. The molecules float around as in a liquid
phase, but are still ordered in their orientation. The smectic phase is close to the solid phase. The
liquid crystals here are ordered in layers. Inside these layers, the liquid crystals normally float
around freely, but they cannot move freely between the layers. Still, the molecules tend to
arrange themselves in the same direction.
Figure 1.2: Liquid Crystal Display
A conventional liquid crystal display (see Fig.1.2) basically consists of a package of two crossed
polarizers with a liquid crystal in between. If the molecules lie perpendicular to the plane of the
polarizers, i.e. along the direction of the light ray, they have no influence on the state of
polarization. Thus, the package of crossed polarizers lets no light through. The cell appears
black. On the other hand, if the molecules are arranged to lie parallel to the plane of the
polarizers, presence of the liquid crystal will strongly affect the state of polarization. In a twisted
16
nematic display, the molecules are arranged in this way. More specifically, the glass surfaces are
treated such that the molecular direction is parallel to the admitting direction of each neighboring
polarizer. Because these directions are crossed, the molecular direction is confined to a 90° twist
from one side of the cell to the other. In this case, the light vibration follows this twist from one
polarizer to the other, so that all light in fact passes the cell, without being absorbed, in spite of
the fact that the polarizers are crossed. Hence, the cell appears bright.
As mentioned earlier, liquid crystals are sensitive to electric forces. If a strong enough electric
field is applied across a liquid crystal, the molecules arrange themselves parallel to the electric
field. So, by applying a voltage across the liquid crystal cell, i.e. along the light direction, you
destroy the twist and instead force the molecules into the direction in which they do not affect
the polarization state of the light. All light is now absorbed by the crossed polarizers and the cell
appears black when the electric field is turned on. By creating a matrix of cells (pixels) that
locally control the state of the twist in their respective area, a liquid crystal display containing a
large number of individual picture elements (pixels) is obtained [8].
Table 1.2 compares OLED and LCD technologies:
Table 1.2: Comparison between OLED and LCD Technologies (values courtesy: DuPont)
Factors OLED LCD
Brightness
Excellent (70–600+ cd/m
2
) Good (400-600 cd/m
2
)
Contrast
Varies: 100:1 to 5,000:1 100-600:1
Field of view
>160
°
140-150
°
Colour gamut
80% NTSC 65-75% NTSC
Motion perception
Very fast: 10µ sec Typical: 16ms
Low Cost
Potential Great
Light-Weight, Compact
Excellent Poor
Lifetime
OK Excellent
17
1.4 Inorganic LED
Inorganic light emitting diodes are an important background technology to this work. ILEDs
comprise a unique category of non-coherent light sources that are capable of producing
continuous and efficient illumination from a simple twin-element semiconductor diode (termed
a chip or die) encased in clear epoxy housing. One of the two semiconductor regions in the chip
is dominated by negative charges (the n region), while the other is dominated by positive charges
(the p region). When sufficient voltage is applied to the electrical leads, a current is created as
electrons transition across the junction between the two semiconductors from the n region into
the p region where the negatively charged electrons combine with positive charges. The
intermediate area or junction between the two semiconductors is known as the depletion region.
Each recombination of charges that occurs in the depletion region is associated with a reduction
in energy level (equal to the charge times the band gap, of the semiconductor), which may
release a quantum of electromagnetic radiation in the form of a photon having an energy equal to
the band gap energy. Photon-emitting diode p-n junctions are typically based on a mixture
of Group III and Group V elements, such as gallium, arsenic, phosphorous, indium, and
aluminum. The wavelength bandwidth of emitted photons is a characteristic of the
semiconductor material (see Table 1.3), therefore, different colors can readily be achieved by
making changes to the semiconductor composition of the chip.
Color Wavelength
(nm)
Semiconductor
Composition
Blue 470 GaN/SiC
Green 520 InGaN/Sapphire
Yellow
585 GaAsP/GaP
Red 633 AlGaInP
Table 1.3: ILED Configurations for Different Colors
ILEDs can be extremely bright, efficient, and stable emitters of light at many wavelengths of
interest. However ILEDs have been limited to point source applications due to cost of producing
such emitters. These devices find wide range of applications; from indication lights, computer
components, watches, medical devices to mobile applications and backlight for small area LCD
FPDs [1, 9].
18
Chapter II: Organic Light Emitting Diodes
2.1 History of OLEDs
Organic electroluminescence was first discovered by Martin Pope et al. in 1963 [10, 11]. They
observed luminescence when a voltage of about 400 was applied to an anthracene crystal.
However, the development of devices based on organic electroluminescence was very slow,
because of the high voltage required and the low efficiency. In 1987, Ching W. Tang and Steve
Van Slyke developed a novel electroluminescent device at Eastman Kodak Company [2]. This is
considered the first organic light-emitting diode.
The device was fabricated by vapor deposition
using Tris(8-hydroxyquinolinato)aluminium (Alq
3
) and diamine in a double layer structure. This
structure made the electron and hole recombination effective. The device had 1% external
quantum efficiency, 1.5 lm/W luminous efficiency, brightness of more than 1000 cd/m
2
and a
driving voltage of about 10 V. In 1990 Richard Friend’s group at Cambridge University
developed a poly(p-phenylenevinylene) (PPV) based OLED, which is called polymer-LED or
PLED [3]. The light emission was in the green-yellow part of the spectrum, and the efficiency
was about 0.05%. Since then, there have been increasing interests, and research activities in this
new field. Enormous progress has been made in the improvements of color gamut, luminance
efficiency and device reliability. The growing interest is largely motivated by the promise of the
use of this technology in flat panel displays.
Figure 2.1: OLED progress (Originally from Sheats et al. Science 273, 884, 1996)
19
2.2 Device configuration and operation
An OLED has an organic EL medium consisting of extremely thin layers sandwiched by two
electrodes. In a basic two-layer OLED structure, one organic layer is specifically chosen to
transport holes and the other organic layer is specifically chosen to transport electrons. The
interface between the two layers provides an efficient site for the recombination of the injected
hole–electron pair and resultant electroluminescence. Figure 2.2 shows typical bi-layer OLED
structure and the thicknesses of different layers.
Figure 2.2: A typical bilayerd OLED
When an electrical potential difference is applied between the anode and the cathode such that
the anode is at a more positive electrical potential with respect to the cathode, injection of holes
occurs from the anode into the highest occupied molecular orbital (HOMO) of hole-transport
layer (HTL), while electrons are injected from the cathode into the lowest unoccupied molecular
orbital (LUMO) of electron-transport layer (ETL) (refer to appendix for HOMO and LUMO).
Holes are transported via hopping transport within HTL until they reach the ETL interface,
where they buildup at the band edge mismatch. Simultaneously electrons injected into the ETL
are transported via hopping transport to the same heterojunction, where they also accumulate.
Figure 2.3: Schematic of charge transport by hopping (courtesy: Franky So research group)
Cathode (150 nm)
HTL (40 nm)
ETL (60 nm)
Anode (150 nm)
20
Statistically a fraction of the built-up holes or electrons can cross the heterojunction interface
leading to the creation of tightly bound electron-hole (e-h) pairs on individual molecules of either
the HTL or the ETL. These tightly bound e-h pairs are referred to as excitons and may be
thought of as single particles. They can relax either radiatively, emitting light characteristic of
the optical band gap of whichever material that they were residing on, or non-radiatively, losing
the energy as heat. In a two-layer device, all the excitons will transfer their energy to the ETL
molecules prior to relaxing, and hence no HTL emission will be observed.
The heterojunction should be designed to facilitate hole-injection from the HTL into the ETL and
to block electron injection in the opposite direction in order to enhance the probability of exciton
formation and recombination near the interface region. As shown in Fig. 2.4, the HOMO of the
HTL is slightly above that of the ETL, so that holes can readily enter into the ETL, while the
LUMO of the ETL is significantly below that of the HTL, so that electrons are confined in the
ETL. The low hole mobility in the ETL causes a buildup in hole density, and thus enhances the
collision capture process. Furthermore, by positioning this interface at a sufficient distance from
the contact, the probability of quenching near the metallic surface is greatly reduced [12].
(a) (b)
Figure 2.4: a) Energy Level Diagram and b) Operation of a bi-layered OLED.
21
The simple structure can be modified to a three-layer structure, in which an additional
luminescent layer is introduced between the HTL and ETL to function primarily as the site for
hole–electron recombination and thus electroluminescence. In this respect, the functions of the
individual organic layers are distinct and can therefore be optimized independently. Thus, the
luminescent or recombination layer can be chosen to have a desirable EL color as well as high
luminance efficiency. Likewise, the ETL and HTL can be optimized primarily for the carrier-
transport property.
The extremely thin organic EL medium offers reduced resistance, permitting higher current
densities for a given level of electrical bias voltage. Since light emission is directly related to
current density through the organic EL medium, the thin layers coupled with increased charge
injection and transport efficiencies have allowed acceptable light emission to be achieved at low
voltages [13].
22
2.3 Types of OLED Technology:
Small Molecule Materials Polymer Materials
•
Emissive layer based on small molecule
materials
• Require vacuum processing
• More manufacturing experience already
gained
• More mature materials with longer
lifetimes
• Phosphorescent materials are available
•
Emissive layer (EML) based on large
molecule/polymer materials
• Can be deposited at atmospheric pressure
• More compatible with roll-to-roll
processing
• Lower operating voltages
• Phosphorescent materials are in early
development
Bottom Emitting Top Emitting
•
Cathode is reflective; anode is transparent
• It is a mature technology
• Pixel electronics (Bus lines, TFTs,
capacitors) reduce aspect ratio
•
Cathode is transparent; anode is reflective
• It is still a developing technology
• Pixel electronics do not block light
enabling more complicated TFT structure.
Passive Matrix Active Matrix
•
Simpler electronic array structures
• High voltage & power needed for high
resolution
• suited for small-area display applications,
such as cell phones and automotive audio
•
Challenging TFT array fabrication
• Lower voltage & power needed for high
resolution
• suited for portable electronics where
battery power consumption is critical and
for displays that are larger than 2” to 3” in
diagonal
Glass Substrate Plastic Substrate
•
Greater use of traditional LCD processes
• Glass protects OLED materials
• Limited benefits of thinness & light
weight
• Fragile if struck or dropped
•
Low temperature processes needed
• Plastic is porous to harmful elements
• Reduced weight & thickness
• Resistant to dropping and striking
• Enables conformal displays
Table 2.1: Types of OLED Technology
23
2.4 Advantages and Disadvantages of OLEDs
OLEDs are already available commercially and they are making inroads in the display markets.
Currently OLEDs are used in low information displays with limited size such as mobile phones,
PDAs, MP3 players, digital cameras and some laptop displays. The driving forces behind this
success is due to following advantages of the OLEDs listed below:
Advantages
• Self-luminous: OLEDs are self-luminous and thus do not require backlight, diffusers, or
polarizers.
• Low Power: 2-10 Volts (DC)
• Low cost and easy fabrication: Roll-to-roll manufacturing process, such as inkjet printing and
screen printing, are possible for polymer OLEDs.
• Color selectivity: There are abundant organic materials to produce a whole spectrum of
visible light.
• Light weight, compact and thin devices: OLEDs are generally very thin.
• Flexibility: OLEDs can be easily fabricated on plastic substrates paving the way for flexible
electronics.
• High brightness and high resolution: OLEDs are very bright at low operating voltage e.g.
white OLEDs can be as bright as 150,000 cd/m
2
.
• Wide viewing angle: OLED emission is lambertian and so the viewing angle is as high as
160 degrees.
• Fast response: OLEDs electroluminescence decay time is less than one microsecond.
Impact / Benefits
• Enormous energy saving for the society
• Environmental impact associated with the reduction of the need for electricity (less air
pollution, depletion of non-renewable sources of energy, less greenhouse effect)
• Creation of new lighting (fixture) industry. New methods of power distribution and conduits
• New architectural designs enabled (lower ceilings, contour lighting, wall / ceiling panel
lighting, space saving in airplanes, and tall buildings, etc.).
24
• Easy to read
• Design flexibility
• Images, graphics, video
• Product differentiation
• Long battery life
The following are some of the disadvantages:
Disadvantages
• Highly susceptible to degradation by oxygen and water molecules: Organic materials are
very sensitive to oxygen and water molecules which can degrade the device very fast. So the
main current disadvantage of an OLED is the short lifetime.
• Low glass transition temperature T
g
for small molecular devices (>70
o
C). So the operating
temperature cannot exceed the glass transition temperature.
• Low mobility of holes and electrons due to amorphous nature of the organic molecules.
• Low stability at high brightness levels.
• Low device efficiency.
• Device complexity: may affect the cost of manufacturing.
• Difficulty in fabricating uniform, large-area lighting sources.
• Non-existent infrastructure.
25
2.5 Degradation Mechanisms of OLEDs [13]
Reliability of OLEDs is a critical factor in commercialization of this technology. Extensive
research work has been aimed at understanding the degradation mechanism of small-molecule
based OLEDs. Operational as well as storage instability leads to loss of efficiency and
degradation of these devices. Operational instability is a long term intrinsic decay in luminance
intensity leading to uniform loss of efficiency over the device emitting area. It leads to two
commonly observed features: A) Luminance decay is initially rapid, followed by a slower
period; B) At constant current, the operating voltage increases gradually.
Several causes of OLED degradation have been suggested
• Anode Contact
With a large energy barrier at the anode interface, large joule heat is produced which results in
the local aggregation of molecules. The improvement of an ITO anode contact via oxygen
plasma treatments has been well recognized. Enhanced hole-injection dramatically improves the
performance of OLEDs, including voltage reduction, efficiency enhancement, and improved
reliability. Another approach to anode modification is the use of a hole-injecting layer overlying
the ITO anode [14].
• Excited State Reactions
Excited state formation is intrinsic to the operation of OLEDs. Irreversible chemistry of the
excited states can remove emissive species from the device. These reaction products may form
quench centers for excitons formed on nearby unaffected sites. So the selection and purification
of materials are an important factor in the rate of emitter degradation, and the operational
stability of OLEDs can be substantially improved by introducing a stable dopant. Rubrene and
DMQA are commonly used dopants to improve device durability [18].
• Crystallization
Since organic thin films prepared by vapor evaporation are glassy and amorphous, crystallization
is considered as one of the dominant degradation mechanisms. Since most of the HTLs have
26
relatively low glass transition temperatures (T
g
), the thermal stability of the OLEDs with an Alq3
emitting layer is dominated by their crystallization.
• Mobile Ion Impurities
Injection of holes in Alq3 is the main factor responsible for device degradation. Possible mobile
ion species include In, Sn, Mg, etc. from the electrodes, or other contaminants incorporated
during the fabrication process. Although the mobile ionic impurities can be used to explain the
early rapid decay of luminance, it may originate from the reaction of OLEDs with contamination
introduced into the package at the time of construction: either water/oxygen adsorbed on to the
surface of the devices or out-gassed products of the epoxy cure. Once this initial contamination is
scavenged by reactions at the contact, the subsequent degradation is slowed [19].
• Non-emissive sites
OLEDs are extremely sensitive to moisture. Without encapsulation the evolution of device
failure can be clearly observed. Non-emissive spots or dark spots develop initially and
continuously grow with time. Dark spots result from the formation of metal oxides or hydroxides
at the Alq3/metal interface initiated by moisture through the pre-existing pinholes on the
cathode. A particle or an asperity may exist on the surface of ITO/glass before device
construction, or an organic chunk may be deposited on the substrate by spitting of the organic
material upon vacuum deposition. Both types of particles may have sizes exceeding the thickness
of the organic layer, thus causing shadowing effects during deposition. The interruption of
complete coverage will give rise to entry points for water and oxygen. Since the formation of
dark spots are related to the growth of metal oxide and hydroxide at cathode/organic interfaces,
the growth rates are expected to have a dependence on the chemical stability of the metals.
Increasing the metal thickness can decrease the density of dark spots [20, 21].
27
2.6 Challenges faced by OLEDs [22]
Many obstacles must be overcome before the potential of this technology can be fully realized.
These include:
• Device Stability: OLEDs have relatively short lifetime. Exposure to humidity and heat can
be particularly damaging to these devices. Although encapsulation can reduce the impact of
hostile environments, it is still difficult to preserves the advantages of low weight, thin
profile and flexibility. The performance of the device must not deteriorate markedly with
age, either through extended storage or operation. Differential aging between the RGB
pixels, or between pixels that are used at different frequencies, must be kept low.
• Voltage: The voltage needed to provide adequate current in direct drive pulsed mode is too
high for inexpensive CMOS electronics and efficient operation. For active-matrix devices,
drift in threshold voltages can lead to loss of control in operation, and so must be minimized
or compensated for.
• Fine patterns with vivid colors: Human perception of luminous intensity peaks sharply in
the green, making blue and red devices much more difficult to create at the same efficiency.
Although great progress has been made with respect to the active organic materials, better
blue, green and red emitters are needed to establish clear superiority over the competing
technologies.
• Light extraction: With the present planar structures, most of the light emitted by the organic
molecules remains trapped in the diode and does not reach the viewer. An easily
manufacturable structure is needed that directs more light forward without increasing the
reflection of ambient light.
• Fabrication costs: Fabrication cost must be reduced so that OLED technology can compete
with more mature and well developed technologies.
28
2.7 Applications of OLEDs
Readily achieved by OLEDs (2002 – 2005)
• Monochrome applications: Small monochrome displays for hand held electronic devices
(cell phones, PDAs, digital cameras, GPS devices etc.).
• Two or multicolor applications: Car electronics (radios, GPS displays, maps, warning
lights, etc.), instrument electronics, heads-up instrumentation for aircraft and automobiles,
and rugged PDAs.
• Full color application: LCD backlights, small full color displays such as high-resolution
personal communicators.
Nearly-readily achieved by OLEDs (2005 -2010)
• Large Displays: Wall-hanging TV monitors, large screen computer monitors
Applications Convertible to OLEDs
• General White light applications (to replace incandescent / halogen, fluorescent)
• Lighting panels for illumination of residential and commercial buildings.
• Lighting panels for advertising boards, large signs, etc.
• Ultra-lightweight, wall-size television monitors.
• Office windows, walls and partitions.
• Color-changing lighting panels and light walls for home and office, etc.
• Large displays, "smart panels".
New Applications that could be enabled by OLEDs
• Applications benefiting from programmable performance (intensity, color, direction)
• Applications capitalizing on integration with displays, vehicles, architecture, military
equipment, etc.
• Smart lights [23]
29
Chapter III: OLED Fabrication and Characterization Techniques
3.1 Common OLED Materials
• Anode:
Indium tin oxide (ITO) is the most widely used anode material. It has a high work function (Φ
0
=
4.3-5.1 eV), high transparency (90%) to visible light, good electrical conductivity, excellent
adhesion to the substrates, and easy patterning ability. ITO surface properties are critical in the
performance OLEDs. Oxygen plasma treatment combined with chemical treatment is effective in
increasing hole-injection efficiency of ITO [15-17].
• Cathode:
The use of low work function metals, such as lithium, calcium, and magnesium, results in
unreliable OLEDs, mainly due to the reactive nature of these materials, especially in ambient
atmosphere. More stable materials, such as aluminum (Al), silver (Ag) are preferable as
cathodes. However, OLEDs based on a cathode made from these materials are inefficient, and
their light output is very low compared to OLEDs with a reactive metal cathode. An alternative
is to use an Al:Li alloy (~0.1% Li) to fabricate efficient and stable OLEDs. The reproducibility
of these devices, however, is rather poor. This is mainly due to the difficulty in controlling Li
content in Al during the co-evaporation process. To achieve better reproducibility and retain a
performance comparable to that of OLEDs with Al:Li alloy cathode, a thin film of an insulator,
such as lithium fluoride (LiF) deposited between the organic layer and the Al cathode have been
used. Beside LiF, other materials such as silicon dioxide, magnesium fluoride, calcium fluoride,
sodium chloride, hexatriacontane, and cesium carbonate have been used as insulating layers in
OLEDs fabrication. However, these materials do not usually lead to as great an enhancement in
device performance and stability as is the case with devices using a LiF buffer layer [24].
• Organic Layers:
Amorphous molecular materials function as hole-transporting, hole-blocking, electron
transporting, or emitting materials, mainly depending upon their ionization potentials and
electron affinities. There are no lattice matching requirements between layers in organic LEDs as
30
in inorganic LEDs [1, 9]. Materials having low ionization potential together with low electron
affinities usually function as hole transporting materials by accepting hole carriers with a
positive charge and transporting them. While materials having high electron affinities together
with high ionization potentials usually function as electron transporting materials by accepting
negative charges and allowing them to move through the molecules.
General OLED material requirements are as follows:
• High luminescence efficiency (PL, EL)
• Adequate conductivity
• Good oxidative stability (water, oxygen)
• Good radical cation/anion stability
• Good temperature stability (T
g
)
• Coatability (thin, uniform films with no pinhole defects or impurities)
• OLED: Does not degrade during sublimation
• No catastrophic film crystallization
• Color saturation/purity
• Narrow spectra and correct CIE coordinates
Hole-Transporting Amorphous Molecular Materials:
Materials with electron-donating properties serve in OLEDs as hole-transporting materials. The
hole transporting layer (HTL) plays the role of facilitating hole injection from the anode,
accepting holes, and transporting injected holes to the emitting layer. The hole-transport layer
also functions as the electron-blocking layer that stops electrons from escaping from the emitting
layer.
The common hole transport materials are N,N'-diphenyl-N,N-bis (3-methylphenyl) l,l'-biphenyl-
4,4'-diamine (TPD), 4,4’-bis-1-naphthyl-N-phenylamino1-biphenyl (NPB), N,N-di (naphthalene-
1-l)-N, N-diphenylbenzidine (NPD). The chemical structure of TPD and NPB is shown in Fig.
3.1. However both TPD and α-NPD are not thermally stable. TPD is not morphologically stable
either, tending to crystallize easily. A number of thermally stable hole transporting amorphous
materials with higher T
g
’s have been developed.
31
Figure 3.1: Chemical Structure of TPD and NPB
Electron-transporting Amorphous Molecular Materials
Materials with electron-accepting properties serve in OLEDs as electron-transporting materials.
Analogous to HTL, the electron transporting layer (ETL) plays the role of facilitating electron
injection from the cathode, accepting electrons, and transporting injected electrons to the
emitting layer. ETL functions at the same time as hole-blocking layer that blocks holes from the
emitting layer.
As compared with hole transporting materials, fewer electron transporting materials have been
reported. A well known green emitter, Tris(8-hydroxyquinolinato)aluminium (Alq3) (Fig. 3.2)
has been widely used as a good electron transporter. It is thermally and morphologically stable to
be evaporated into thin films, robust, and easily synthesized and purified [1].
Figure 3.2: Chemical Structure of Alq3
32
3.2 OLED Deposition Techniques [25, 52]
Deposition techniques involving OLED materials can be classified as wet or dry techniques. Dry
techniques such as vacuum thermal evaporation (VTE) and organic vapor phase deposition
(OVPD) have been predominantly used for small molecular organic material deposition. This is
because of the ease of depositing large area uniform and homogenous film using these dry
techniques, and because the solubility of small aromatic molecules tends to be too small for
solution processing of sufficiently thick films.
Polymer organic materials are deposited using wet techniques such as spin coating, ink jet
printing, and contact stamping. Typical dry techniques such as vacuum thermal evaporation are
not viable due to their large molecular weight which causes their evaporation temperature to be
far in excess of their decomposition temperature.
The following section describes spin coating and the vacuum thermal evaporation in detail:
• Spin coating:
Spin coating is a fast and easy deposition technique as depicted in Fig. 3.3, in which a quantity of
solid (usually polymer) is dissolved into an organic solvent. This solution is placed onto a
substrate, allowed to wet the entire area to be coated, and then the spinning speed is typically set
at 1000-10000 rpm.
Figure 3.3: Schematic of Spin Coating Process
33
The centrifugal force of spinning causes majority of the solution to expel; however a fraction of
the solution is left behind. The exact thickness of this liquid film is difficult to predict, but is
controlled by a combination of adhesion forces at the substrate/liquid interface, solution
viscosity, and friction at the air/liquid interface. For a low pressure solvent, this thin liquid film
can exist indefinitely on the spinning substrate surface. However, for typical organic solvents
used in spin coating, the vapor pressure is quite high, and the solvent begins to evaporate
immediately upon exposure to any unsaturated environment. Thus, this thin liquid film
eventually decreases in thickness (time scales are typically 1-60s), leaving behind an even
thinner (1-1000nm), flat solid film of the initial solvated liquid. It is important to note that with
the spin coating deposition technique, patterned deposition is not possible.
• Vacuum Thermal Evaporation:
Figure 3.4: Schematic Depicting Vacuum Thermal Evaporator
VTE is the simplest deposition technique as depicted in Fig. 3.4. A boat made out of a resistive
metal (typically tungsten, molybdenum or tantalum) is heated by passage of electric current. The
boat contains organic material, which upon heating evaporates or sublimes. The evaporation
takes place in a low pressure (typically 10
-6
torr) vacuum chamber so that the evaporated
material is unlikely to undergo any collisions along its path towards the substrate, and also to
keep the deposited materials as pure as possible. Each molecule that is liberated from the solid
34
has some initial speed and direction that is not changed until it contacts a substrate or chamber
surface, which is typically cold enough to cause it to condense on contact. Because of the even
distributions of the initial molecular trajectories, the resulting deposition is essentially of uniform
thickness for any solid angle intersected by a substrate. Using this technique, 70-99% (depending
on throw distance) of the material in the boat is deposited on the walls of the vacuum chamber
rather than the substrate.
Materials deposited by VTE are most commonly patterned using shadow masking. Shadow
masking allows for patterning of a material by preventing deposition in the areas where it is not
desired. Typically, a thin metal foil with a pattern of through apertures across its surface is used
as a shadow mask.
35
3.3 OLED Analysis Techniques
• Electrical and optical properties:
Current-voltage characteristics are measured using digital voltmeter while absorption, excitation
and luminance spectra/characteristics are measured using spectrophotometer or radiometer. The
opto-electrical properties of OLEDs are summarized in the following table:
Characteristic Description Graph
1
Diode
Characteristics
Similar to inorganic
LEDs, OLEDs
conduct in forward
bias and do not
conduct under
reverse bias. The
drive voltage is very
low; 2-4 volts.
2
Cell Resistance
The impedance
drops exponentially
with increase in
voltage.
36
3
Luminescence
Characteristic
Light output is
proportional to
current flow, so as
impedance drops,
light output
increases rapidly.
There is virtually no
delay between
generation of current
flow and generation
of light output.
Table 3.1: Optoelectronic Properties of OLED
• Interfacial chemistry (metal-organic interfaces): Interfacial chemistry of metal-organic
interface is studied using ultraviolet photoelectron spectroscopy (UPS) and X-ray
photoelectron spectroscopy (XPS). Samples are illuminated with light, and the kinetic energy
distribution of photogenerated electrons is analyzed. With a relatively low photon energy of
21.2 eV, UPS is employed to measure the ionization potential of organic materials and the
work function of metals. With relative high photon energy in the range of 1 keV, XPS is
employed to determine the elements present in the near surface region and provides subtle
information on chemical bonding [26].
• Carrier mobility: Both time-of-flight (TOF) and transient EL are utilized to measure carrier
mobilities [9, 10]. The TOF method determines the flight time, which is needed for single
sign charges generated near one surface of a sample to move across the sample to the other
side. A short pulse of strongly absorbed light generates the free charge carriers. The sample
is sandwiched between two electrodes (one semitransparent) to allow application of a
constant electrical field. In transient EL analysis, EL from a testing OLED is investigated
with the materials of interest as a carrier-transport layer. Using a voltage pulse as an
excitation source, the delay time for the EL is measured and interpreted as the carrier transit
time across the carrier-transport layer [27, 28].
37
II. Research Work
Research Overview
The research direction of the Nanoimaging and Nanomaterials Laboratory of RIT is to fabricate
hybrid organic/inorganic quantum dot LEDs in which inorganic nanocrystal quantum dots are the
light-emitting element and the organic layers are used strictly for electron and hole transport. A
necessary prerequisite to this goal is building devices that use the organic material to emit light.
Since it was the first time such imaging devices were fabricated in this lab, the goals of this
research work were:
• To develop tools and techniques for fabricating reproducible OLEDs based on small
molecules;
• Optimize their performance to match with those published in the scientific literature;
• And thus through this process, realize lab’s capabilities from fabrication and characterization
perspective and gain fundamental knowledge of their operation.
As discussed previously, the operation of organic light emitting diodes involves charge injection
from electrodes, transport of charge carriers, recombination of holes and electrons to generate
electronically excited states called excitons, followed by their deactivation by emission of either
fluorescence or phosphorescence. The main factors that determine luminous and external
quantum efficiencies (refer to appendix for definition) are: efficiency of charge injection from
electrodes, charge balance, spin multiplicity of the luminescent state, emission quantum yield,
and light output coupling factor. In order to attain high quantum efficiency for
electroluminescence (refer to appendix for definition), it is necessary to achieve three things:
efficient charge injection from the electrodes at low drive voltage, good charge balance, and
confinement of the injected charge carriers within the emitting layers to increase the probability
of desired emissive recombination [1]. The goals of this research work were based on these three
fundamental phenomena.
The first goal of the research was to develop procedures to fabricate reproducible OLEDs. It
involved tools/technology setup and development. Before we could hope to optimize OLEDs, it
was necessary to understand the scientific fundamentals behind their operation. Hence, the
38
logical step to begin was by fabricating simple bi-layered devices. Once we were able to
reproducibly get light emission at a moderate luminance and efficiency, we then proceeded to
improving the performance by device optimization.
The second goal was studying performance OLEDs based on different cathode materials.
Selection of cathode material is critical for effective electron injection. This study was based
primarily on Al, Al/LiF, Mg:Ag and Ca as cathode materials. The device with Al/LiF as cathode
yielded best results.
The next goal of the research was based on studying effect of LiF device performance as a
function of layer position and thickness.
Finally, we focused our attention on improving the efficiency of these devices by incorporating
hole blocking layer (HBL) in these multilayered devices; which formed the fourth goal. HBL is
expected to control confinement of electron-hole within the desired recombination zone thus
preventing hole leakage to cathode and hence improving the efficiency.
It should be noted that the primary objective of the research was to set up the Lab and build the
devices with limited resources. Thus, the device results were generated within the constraints
which matched early published results. In following sections, each research goal is documented
in detail.
39
Chapter IV
Goal 1: To Develop Procedure for Reproducibly Fabricating Organic Light Emitting
Diodes
Major Objectives:
1. Develop tools and techniques to fabricate, measure and analyze OLEDs
2. To study effect of oxygen plasma treatment on the performance of OLEDs
3. To study effect of PEDOT:PSS as a hole injection layer (HIL)
Desired Result: Reproducible OLEDs with moderate luminance and efficiency
4.1 Theory
The preliminary and a major goal of the research was to develop a procedure for reproducibly
fabricating organic light emitting diodes and then characterizing them. The biggest challenge
was to develop all the tools (fabrication and characterization) and techniques from scratch.
During the initial phase, a lot of attention was dedicated to the following factors:
• Tool Set-up for Thin Film Deposition: The heterojunction devices were formed by
sequential high-vacuum vapor deposition of organic and metal layers. Fabrication tools such
as vacuum coater, and spin coater and thickness monitor were set up with heating parameters,
thickness parameters, and selection of proper evaporator accessories (boats, baskets) for each
material, so as to get uniform layers of required thicknesses at preferred deposition rate. All
the deposited layers have to be very uniform since non-uniformities may lead to localized
surges of electric current, localized overheating and gradual destruction of the device. A
spectrometer for luminance characterization (luminance, electroluminescence spectrum and
color co-ordinates) was calibrated too.
• Cleaning Procedure and Surface Treatment of ITO: Since the organic thin film is directly
deposited on the ITO, its surface properties are expected to affect the performance of the
device [15-17]. Unclean and untreated ITO slide results in abnormal device behavior,
shorting, unstable I-V characteristics and damage on the surface of the top cathode contact
40
after continuous operation of the device. Thus, cleaning ITO coated substrate with chemicals
is essential. Various chemicals treatments were tried to optimize the procedure.
• Oxygen Plasma Treatment: Plasma cleaning is suitable for removing very thin films,
especially hydro-carbonates and oxides, which remain after conventional cleaning. In this
treatment oxygen removes contaminants by oxidation and reduction (see Fig. 4.1). Hence,
after the chemical cleaning procedure, to further enhance the surface properties of ITO,
oxygen plasma treatment was carried out. In this study, effect of oxygen plasma treatment on
the performance of the device has been investigated.
Figure 4.1: Schematic of oxygen plasma treatment mechanism (Originally from Sebastian
Deiries, et al. “Plasma Cleaning”)
• Device Structure and Architecture: Correct thickness of each layer is critical to the
performance of OLEDs. Thus devices were prepared by varying the thicknesses of ETL,
HTL and cathode to find out the optimum device structure. From our experiments it was
established that OLED with 60 nm of Al cathode, 60 nm of Alq3 and 40 nm of TPD is the
optimum device configuration. Un-patterned ITO coated glass slides were available for
device building. Once a complete device is built, a negative potential to the cathode and a
positive potential to the anode needs to be provided. The negative end cannot be attached
directly on the cathode, since that could puncture the layer. In order to facilitate the
attachment, an insulating groove is created at around the middle portion of the ITO slide.
With the insulating groove, both positive and negative connectors can be attached to the bare
41
portion of ITO, on each side of the groove. A diamond tipped cutter can be employed for this
purpose which, although crude, is a very easy and effective method.
• Anode Buffer Layer
PEDOT:PSS abbreviated for poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (Fig.
4.2) is a very promising candidate for application in OLEDs as an anode buffer layer to improve
hole injection [29]. This buffer layer has a high morphological and redox stability, and very good
film forming properties. The aqueous dispersion of PEDOT:PSS can form a uniform, conductive,
and a transparent thin film, which can smooth the surface of the indium tin oxide (ITO) anode,
enhance adhesion to the organic layer, and decrease the hole injection barrier owing to its high
work function (5.1 eV)
Figure 4.2: Chemical structure of PEDOT:PSS
4.2 Experimental
The device configurations of the OLEDs fabricated are as follows:
Device 1: ITO/TPD (40nm)/Alq3 (60nm)/Al (60nm)
Device 2: ITO/PEDOT:PSS (60nm)/TPD (40nm)/Alq3 (60nm)/Al (60nm)
Schematics of these devices are shown in Fig. 4.3.
42
Device 1 Device 2
Figure 4.3: Schematics of the devices fabricated for this study
A detailed description of fabrication procedure is as follows:
Materials Used: OLEDs were based on Aluminum (Al) as cathode, Tris (8-hydroxyquinolinato)
aluminium (Alq3) as both electron transporting (ETL) and emissive layer (EL), N,N′-Bis(3-
methylphenyl)-N,N′- diphenylbenzidine (TPD) as hole transporting layer (HTL), Poly(3,4
ethylenedioxythiophene): Poly(styrenesulfonate) (PEDOT: PSS) as anode buffer layer, and
indium tin oxide (ITO) as anode. Following materials were purchased from Sigma-Aldrich with
the mentioned initial percent purity: Al pellets (99.99%), Alq3 (99.995%), TPD (99%) and
PEDOT:PSS (1.3 weight % dispersion in H
2
O).
Pre-Cleaning Procedure: Commercial ITO-coated glass with film thickness and sheet
resistance of 150 nm and 15 ohm respectively was cut into a 1” × 1” plates and an insulating
groove was created using a diamond cutter (see Fig. 4.4). The ITO slides were used as received
without any characterization or quality check. The slides were cleaned as described below:
Cleaning Procedure: Prior to their use, ITO substrates were routinely cleaned by 20%
ethanolamine at 80
o
C for 20 min with constant stirring, followed by rinsing in DI water and
finally drying in oven for 15 min at 80
o
C.
Oxygen Plasma Treatment: After chemical cleaning, the ITO slides were transferred into a
vacuum chamber and exposed to oxygen plasma. The slides were treated for 5, 10, 20 and 30
minutes at low (4W), medium (10W) and high (18W) settings to find out the optimum treatment
duration and plasma RF power.
Deposition and Patterning: Immediately after oxygen plasma treatment PEDOT:PSS was spin
coated on to the substrate. The deposition of remaining layers was done by vacuum thermal
ITO
PEDOT:PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
Al (60 nm)
Al (60 nm)
TPD (40 nm)
Alq3 (60 nm)
ITO
43
evaporation (VTE) technique while patterning was done using aluminum shadow masking. The
evaporator has two ports with only one port available for evaporation at a time. Thus, vacuum
had to be broken after evaporation of any two layers. The organic layers (TPD and Alq3) and
cathode (Al) were sequentially deposited by resistive heating under a pressure of ~5 x 10
-6
Torr.
Thermal deposition rates for organic layers (TPD and Alq3), and Al were around 5 Å/s and 1
Å/s, respectively. Deposition thickness and rates were controlled by the quartz oscillating
thickness monitor. The thickness and uniformity of the layers was confirmed using a
profilometer. The emission area of the each device was 0.25 cm
2
. After the fabrication, the
device was kept idle in the vacuum chamber for 20 minutes.
Figure 4.4: Schematic of Device architecture
Characterization: Current-voltage measurements were performed using a Keithley 2400
sourcemeter and luminance characteristics, including electroluminescence spectra, were
measured using an Ocean Optics USB2000 Spectrometer. L-I-V (luminance-current-voltage)
measurements were recorded simultaneously. All measurements were done in ambient air at
room temperature without any encapsulation.
4.3 Results and Discussion
1. Effect of PEDOT:PSS Buffer Layer:
Light emission was not obtained and very poor I-V characteristics were observed for device 1 in
spite of performing routine chemical cleaning procedure, oxygen plasma surface treatment, and
having uniform layers. When PEDOT:PSS was used as an anode buffer layer, the device (device
ITO Coated Insulting
Glass Groove
Substrate
PEDOT:PSS coating on ITO
Organic Layers
Aluminum
44
2) emitted light. Thus it indicates existence of a large charge injection barrier at anode/HTL
interface in absence of the PEDOT:PSS buffer layer.
Thus, it confirms that PEDOT:PSS serves as a critical and effective buffer layer for OLED
operation. It forms a clear and smooth layer on the transparent ITO electrode, improves the
contact between ITO and HTL. Having a higher work function of 5.2 eV, it decreases the hole
injection barrier, as the energy level diagram (fig. 4.5) illustrates.
Figure 4.5: Energy level diagram of OLEDs with and without PEDOT:PSS layer. Values for
LUMO and HOMO energy levels of organic layers and work functions of electrodes are in eV.
From the diagram it is evident that the anode buffer layer, PEDOT:PSS enables effective
transport of the holes from ITO to HTL due to reduced energy barrier. The following sections
describe characteristics of device 2.
2. Effect of Oxygen Plasma Treatment:
Table 4.1 summarizes the effect of oxygen plasma treatment on drive voltage (voltage required
for 20mA/cm
2
current density) of OLEDs when employed for 0, 5, 10, 20 and 30 minutes at a
high RF power (18 W) setting.
45
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
0 200 400 600 800 1000
Intensity (counts)
Wavelength (nm)
EL Spectrum
Peak Wavelength
at 530 nm
Treatment Duration
(min)
Drive Voltage
(V)
0 10.5
5 9.6
10 9.5
20 8
30 8.1
Table 4.1: Effect of Oxygen Plasma Treatment on the Drive Voltage
From Table 4.1, it is evident that oxygen plasma treatment does enhance performance of OLEDs.
The decrease in the drive voltage can be attributed to the enhancement of hole injection due to an
increase in the work function of the ITO [15]. The improvement of device performance suggests
that the surface chemical composition might play a role in increasing the hole injection ability at
the ITO/organic interface [16-17]. The removal of the organic residue from the ITO surface
could therefore also be responsible for the device improvement. Hence oxygen plasma treatment
for 20 min at 18W RF power was identified as an optimum treatment setting.
3. Device Characteristics:
Figure 4.6: Electroluminescence spectrum
46
1.0
10.0
100.0
1000.0
0.10
1.00
10.00
100.00
0.00 5.00 10.00 15.00
Current Density (mA/cm
2
)
Voltage (V)
L-I-V Characteristics
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0.00 5.00 10.00 15.00
Luminance Yield (cd/A)
Voltage (V)
Luminous
Efficiency
Electroluminescence spectrum shows peak wavelength at 530 nm indicating emission of pure
green light from Alq3. L-I-V characteristics plot is shown in Fig. 4.7. From the plot it can be
seen that the device shows a typical OLED L-I-V trend. The device yielded highest luminance of
678.10 cd/m
2
and maximum luminous efficiency of 1.42 cd/A.
Luminance (Cd/m
2
)
Figure 4.7: Semi-log plot of L-I-V characteristics and luminous efficiency of device 2
Table 4.2 summarizes significant device properties:
Property Value
Turn on Voltage 5.5 V
Drive Voltage (voltage at 20 mA/cm
2
) 8 V
Current Density at 12 V 47.78 mA/cm
2
Luminance at 12 V 678.10 cd/m
2
Maximum Efficiency 1.42 cd/A
Table 4.2: Device Properties
47
4.4 Conclusions
1. A procedure for fabricating reproducible OLEDs has been developed.
2. Anode buffer layer PEDOT:PSS is essential for device operation.
3. Treating the ITO slide with oxygen plasma clearly enhances the performance of OLEDs.
4. L-I-V characteristics of the OLED with configuration ITO/PEDOT:PSS/TPD/Alq3/Al are
moderate and need to be improved.
48
Chapter V
Goal 2: To Study Performance of OLEDs Based on Different Cathode Materials
Main Objective: To fabricate, measure and analyze devices using Ca, Ca/Al, Mg:Ag, Al and
Al/LiF cathode materials.
Desired Result: Enhanced Electron Injection
5.1 Theory
As described in Section 2.2, the electroluminescence mechanism involves injection of holes from
anode into the HOMO of HTL and injection of electrons from cathode into the LUMO of ETL,
on application of forward bias. These charges then recombine to form excitons which decay
radiatively resulting in light emission. The process of charge injection is of fundamental
importance since it can control the electrical characteristics and hence the efficiency of devices.
For devices involving vacuum deposited functional materials such as Alq3 and TPD, the energy
barrier for electron injection is more than that for hole injection [30]. Thus, electron injection is a
limiting factor for overall device performance.
To facilitate electron injection, energy barrier between cathode and LUMO of adjacent organic
electron transporting layer (ETL) needs to be lowered. Reasonable choice of cathode material
and cathode buffer layer is important to achieve efficient charge injection. For our study, various
cathode materials such as Ca , Ca/Al, Mg:Ag, Al and Al/LiF which cover a range from 2.87 eV
up to 4.26 eV work function, have been considered and investigated [31, 32].
Ca and Ca/Al: Ca has a work function of 2.87 eV. The low work function should facilitate
electron injection. It is of great interest to find out how well electron injection is correlated with
the work function of the cathode metal. Ca has a low corrosion resistance and a high chemical
reactivity. Hence it is often capped with metals such as aluminum.
Mg:Ag: Mg has a work function of 3.7 eV. Addition of Ag considerably improves both,
stability of Mg in the atmosphere and Mg sticking coefficient onto Alq3 upon deposition [33].
49
Al/LiF: Being more stable and resistant to oxidation, Al is a highly desired cathode material.
However, it has a higher work function (4.3 eV). To reduce injection barrier, inserting ultrathin
layer of LiF has stimulated a great deal of interest [34, 35].
5.2 Experimental
The devices were fabricated using the same cleaning and deposition procedure as described in
section 4.2. Figure 5.1 shows the schematics of the device architecture. Keeping the thicknesses
of PEDOT :PSS, TPD and Alq3 same as those in goal 1, thicknesses of Al/LiF, Mg:Ag, and
Ca/Al were varied to determine optimum configuration.
• Al and Al/LiF: Al thickness was kept at 60 nm. LiF thickness was varied from 0.5 to 2.0 nm.
Device with 0.5 nm of LiF demonstrated best I-V and luminance characteristics.
Figure 5.1: Schematics of device configuration having Al and Al/LiF as Cathodes
• Ag/Mg/Ag: From the literature, it has been recognized that Mg and Ag needs to be co-
evaporated at a ratio of 10:1. But, since we have a two-port evaporation system wherein only
one material can be evaporated at a time, co-evaporation was not possible. Thus, Mg and Ag
were deposited in series one after another. Different devices with varying Mg and Ag film
thicknesses were fabricated to find out the optimum configuration. The device with 6 nm of
Ag followed by 60 nm of Mg and finally capped by 6 nm of Ag showed the best I-V and
luminance characteristics. Since Mg has low sticking coefficient on Alq3, initial 6 nm of Ag
provides a base for Mg deposition, while the capping prevents Mg from oxidation.
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
Al (60 nm)
50
• Ca and Ca/Al: Devices with 60, 90 and 120 nm of Ca as cathode were fabricated. Devices
having same configuration with Al capping of 60 nm were also fabricated. These devices are
illustrated in Fig. 5.2.
Figure 5.2: Schematics of device configuration using Mg:Ag and Al/Ca as Cathodes
5.3 Results and Discussion
All the devices had electroluminescence spectrum peak wavelength of 530 nm confirming
emission of pure green light from Alq3 layer.
The devices with Ca as cathode did not emit light. This can be attributed to high reactivity of Ca
to atmosphere. Capping Ca with Al also showed poor I-V characteristics and extremely low light
emission. Thus, even though Ca has a very low work function, it makes a poor cathode for
OLEDs.
The L-I-V plots are shown in Fig. 5.3. From the I-V plot it can be seen that the metal-alloy
cathode, Mg:Ag and the pure metal Al, have higher current densities initially. But after 6V, the
bilayered cathode, Al/LiF yielded higher current density compared to other cathodes. Highest
luminance was also obtained from Al/LiF OLED with maximum luminance of 5927.5 cd/m
2
.
Ag (6 nm)
Mg (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
Ag (60 nm)
Ca (60/90/120
Al (0/60 n
m)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
51
1
10
100
1000
10000
3 8 13
Luminance (cd/m
2
)
Voltage (V)
Luminance Characteristics
Ag/Mg/Ag
Al/LiF
Al
0.1
1
10
100
1000
0 5 10 15
Current Density (mA/cm
2
)
Voltage (V)
I-V Characteristics
Ag/Mg/Ag
Al/LiF
Al
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15
Luminance Yield (cd/A)
Voltage (V)
Ag/Mg/Ag
Al/LiF
Al
Figure 5.3: L-I-V Characteristics
Figure 5.4: Luminous Efficiency
Luminance yield is shown in Fig. 5.4. Al/LiF OLED yielded highest efficiency of 3.16 cd/A at 6
V. Among all metals under investigation in this work, no metal other than Al in combination
with a thin LiF layer showed a comparable rise in efficiency. Energy level diagram of Al/LiF
OLED and Mg:Ag OLED is as depicted in Fig. 5.5.
52
Figure 5.5: The energy level diagram of devices Al and Al/LiF cathodes
Significant device properties obtained from the experiment are listed in Table 5.1.
Property Al Mg:Ag Al/LiF
Turn on Voltage (V) 5.5 4 4
Voltage at 20 mA/cm
2
8.4 8 7.4
Current Density (mA/cm
2
)
at 12 V
47.78 64.44 224
Maximum Luminance
(cd/m
2
)
678.10 at 12 V 2427.54 at 14 V 5927.5 at 12 V
Maximum Luminance
Efficiency (cd/A)
1.42 at 12 V 2.35 at 13 V 3.16 at 6 V
Table 5.1: Device Properties
Thus, a very bright and efficient OLED was obtained using Al/LiF cathode. A relatively low
performance of OLED with Ag/Mg/Ag cathode can be attributed to the fact that co-evaporation
of Mg:Ag was not feasible with the given evaporator setup.
53
5.4 Conclusions
1. Using a low work function metal such as Ca does not enhance device performance.
2. Ca is extremely sensitive to oxidation resulting in a non-performing device.
3. OLED with Ag/Mg/Ag cathode yields higher brightness and efficiency compared to Al and
Ca OLEDs.
4. LiF with Al used as a metal layer enhances the performance of the device tremendously.
54
Chapter VI
Goal 3: To Study Effect of LiF on the Performance of OLEDs
(Published Paper: Omkar Vyavahare and Richard Hailstone, “Enhanced Performance of
Organic Light Emitting Diodes Using LiF Buffer Layer”, Mater. Res. Soc. Symp. Proc, 1154
B05-84 (2009).)
Major Objective: From the previous study it has been established that ultrathin layer of LiF
when inserted at Al/Alq3 interface, greatly enhances the performance of OLEDs. The objective
of this study is to determine whether LiF has any effect when deposited at other interfaces such
as anode-organic and organic-organic interfaces of the device.
Desired Result: Hole-electron balance due to enhanced electron injection and suppressed hole
injection.
6.1 Theory
Figure 6.1: Depiction of exciton quenching due to imbalance in carrier transport (courtesy:
Franky So research group)
As illustrated in Fig. 6.1, uneven rates of injection result in charge accumulation at interfaces
between the charge transport layers, and consequently, to very high electric fields across some
layers. This leads to localized breakdowns due to uneven thicknesses and thus degradation of the
device. Imbalance in carrier transport also causes exciton quenching. Since the majority of
carriers in OLEDs are holes due to their higher mobility [36] and smaller injection barrier [37], it
55
is necessary to enhance electron injection, suppress hole injection, and thus shift electron-hole
recombination zone close to the organic-organic interface.
From the previous goal (goal 2), it was demonstrated that LiF improves device performance due
to enhanced electron injection. A number of mechanisms have been proposed to explain this
effect, such as electron tunneling through a thin insulator layer [38], band bending at the
metal/organic interface [38], lowering of the work function of Al [42], the presence of interfacial
dipoles [43] and LiF dissociation with released Li atoms reacting with Alq to form Alq2 anions
[39]. Understanding the structure and electronic properties of the metal/organic interface is a
complicated phenomenon since the interface is not abrupt, extending for several nanometers; that
the interface is located deep inside the device; and that the 0.5 nm thick LiF layer cannot form a
continuous interlayer [45]. To completely understand this phenomenon, aspects of surface
chemistry have to be taken into account. The investigation of these interfaces on a microscopic
scale is beyond the scope of this contribution.
Since LiF at cathode-organic interface has been shown to control electron injection, it would be
interesting to find out if it does so when inserted at other interfaces such as anode-organic and
organic-organic interfaces. In some of research papers [46, 47], it has been reported that the LiF
buffer layer at these interfaces improves device performance. The purpose of this study is to
verify these claims. This study is mainly concentrated on performance evaluation of these
devices with little emphasis on interfacial/surface chemistry studies.
6.2 Experimental
The following devices (Fig. 6.2) were fabricated for this study. LiF was evaporated at a rate of
0.5 Å/s. The fabrication and characterization procedure as described in Section 4.2 was followed
for deposition of remaining layers. Device with configuration ITO/PEDOT:PSS (60nm)/TPD
(40nm)/Alq3 (60nm)/LiF (0.5nm)/Al (60nm) (Device 1) is the control device of the experiment.
Other devices were fabricated by depositing 0.5 nm of LiF was deposited at 3 different
interfaces:
56
Device 2: ITO/TPD Interface
Device 3: Al/Alq3 and Anode/ TPD Interface
Device 4: Alq3/TPD and Al/Alq3 Interfaces
Device 1 Device 2 Device 3 Device 4
Figure 6.2: Schematics of devices fabricated for the study
6.3 Results and Discussion
Electroluminescent spectra of all the devices showed peak wavelength at 530 nm.
Current-voltage characteristics of all the devices are plotted in Fig. 6.3a. Device 1 with LiF at
cathode/Alq3 interface has steeper I-V characteristics with drive voltage (voltage at 20 mA/cm
2
current density) of 7.4 V. Device 3, with LiF both at anode/TPD and cathode/Alq3 interfaces,
shows even lower drive voltage with 20 mA/cm
2
at 6.7 V. Devices 2 and 4 did not show any
improvement compared to the device without the LiF layer.
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
LiF (0.5 nm)
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
LiF
(0.5 nm)
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
57
0.01
0.1
1
10
100
1000
0 2 4 6 8 10 12
Current Density (mA/cm
2
)
Voltage (V)
Device 1
Device 2
Device 3
Device 4
1
10
100
1000
10000
3 5 7 9 11 13
Luminance (cd/m
2
)
Voltage (V)
Device 1
Device 2
Device 3
Device 4
0
0.5
1
1.5
2
2.5
3
3.5
3.00 8.00 13.00
Luminous Efficiency (cd/A)
Voltage (V)
Device 1
Device 2
Device 3
Device 4
Figure 6.3: a) Semi log I-V Plot b) Semi log Luminance Plot
Luminance characteristics of all the devices are plotted in plot 6.3b. Device 1 shows the highest
luminance of 6000 cd/m2 at 12V. Luminance decreases with deposition of LiF at any other
interface. Device 2 again shows no improvement in luminance compared to device 1.
Figure 6.4: Luminous Efficiency and Energy Level Diagram
58
Luminance yield is plotted in Fig. 6.4. Maximum luminance efficiencies of devices 1 to 4 are
3.16, 2.04, 2.23 and 2.44 cd/A, respectively. Fig. 6.4 also shows the energy level diagram. These
results do not agree with those in [9, 10]. Presence of PEDOT:PSS as anode buffer layer may
account for this difference.
6.4 Conclusion
Inserting LiF at cathode/Alq3 interface clearly enhances performance of the device. The device
shows steeper I-V characteristics, improved luminance and luminance yield due to enhanced
electron injection, carrier balance, and recombination efficiency. Device 3, with LiF at both
electrode interfaces, shows lowest drive voltage, possibly due to reduced hole injection shifting
the recombination zone closer to Alq3/TPD interface. By comparing the performance of all the
devices, this study rules out the possibility of improvement in the performance of the devices
with structures 3 and 4, as proposed in the research papers [46, 47]. LiF is most effective only as
a cathode buffer layer, enhancing OLED performance tremendously.
59
Chapter VII
Goal 4: To Study Effect of Hole Blocking Layer (HBL) on the performance of OLEDs
Major Objective: To fabricate, measure and analyze OLEDs having BCP as a HBL
Desired Result: Confinement of the injected charge carriers within the emitting layers to
increase the probability of desired emissive recombination
7.1 Theory
For a device structure with TPD as HTL and Alq3 as ETL, mobility of holes is more than the
mobility of electrons [36]. Also, the injection barrier at anode/HTL interface is lower than that of
cathode/ETL interface [37]. These two factors result in charge imbalance in the recombination
region. The holes that do not recombine with electrons travel though ETL to the cathode. Such a
device causes hole leakage, which decreases the efficiency. In addition, once electrons and holes
recombine to form excitons, the charge accumulation at the interface may lead to exciton
quenching. Thus it is required to confine these holes at the emitting zones, as illustrated in Fig.
7.1.
Figure 7.1: The desired recombination region is the interface of ETL and HTL
Hence, to achieve improvement in the efficiency and purity of electroluminescent spectrum for
OLEDs, bandgap engineering is applied by inserting a hole blocking layer (HBL) at HTL-ETL
interface. HBL has a large ionization potential which causes hole blocking effect and because of
its wide band gap it also acts as a barrier to exciton diffusion. With the confinement of holes and
excitons at the emitting zone, optimum efficiency can be reached.
60
In this work we have used bathocuproine (BCP) abbreviated for 2,9-Dimethyl-4,7-diphenyl-
1,10-phenanthroline as HBL. It is a derivative of phenanthroline, a heterocyclic organic
compound and a bidentate ligand in coordination chemistry. As illustrated in Fig. 7.2, BCP has
HOMO of 6.2 eV which is much larger than that of TPD (5.4 eV) and LUMO of 3.2eV matching
closely with that of Alq3 (3.1 eV). Thus BCP forms an excellent HBL material.
Figure 7.2: Chemical Structure of BCP and Energy Level Diagram of OLED with BCP
We report performance of OLEDs having TPD as a hole transporting layer (HTL), BCP as a hole
blocking layer (HBL) and Alq3 as an electron transporting layer (ETL). By adjusting the film
thickness of BCP layer, the hole blocking effect of BCP has been investigated. The EL behavior
of these heterojunction devices is discussed in detail.
61
7.2 Experimental
Figure 7.3 illustrates the devices fabricated for this study. Devices with 20 nm, 5 nm, 1 nm and
0.5 nm of BCP at Al/LiF-Alq3 interface were built. BCP (Purity: 99.99 %) was purchased from
Sigma-Aldrich. BCP was evaporated at a rate of around 3 Å/s. The fabrication procedure as
discussed in section 4.2 was followed for the deposition of the remaining layers. The device with
configuration ITO/PEDOT:PSS (60nm)/TPD (40nm)/Alq3 (60nm)/LiF (0.5nm)/Al (60nm)
(Device 1) was the control device of the experiment.
Device 1 (control device) Device 2
Device 3 Device 4 Device 5
Figure 7.3: Schematics of Fabricated Devices
The device with BCP of 10-20 nm had the same characteristics as 20 nm.
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
BCP (20 nm)
Alq3 (60 nm)
LiF (0.5
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
BCP (5 nm)
Alq3 (60 nm)
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
BCP (1 nm)
Alq3 (60 nm)
LiF (0.5 nm)
Al (60 nm)
ITO
PEDOT/PSS
(60 nm)
TPD (40 nm)
BCP (0.5 nm)
Alq3 (60 nm)
LiF (0.5 nm)
Al (60 nm)
LiF (0.5
Al (60 nm)
ITO
PEDOT/PSS (60 nm)
TPD (40 nm)
Alq3 (60 nm)
62
0
10
20
30
40
50
60
70
80
90
100
350 400 450 500 550 600 650 700
Normalized Intensity
Wavelength (nm)
EL Spectrum
Device 2 (Green)
Device 3 (Blue)
Device 5 (Blue
-
Green)
Device 3
Device 2
Device 5
7.3 Results and Discussion
• Electroluminescence Spectrum:
Figure 7.4: EL spectra of devices with varying BCP thickness
Electroluminescence spectra of the device 2 (BCP 20nm), device 3 (BCP 5nm) and device 5
(BCP 0.5nm) are shown in Fig. 7.4. The EL spectrum of device 4 was same as device 5 and the
EL spectrum of device 1 was same as device 2. From EL spectra it can be seen that as BCP
thickness at HTL-ETL interface was varied from 0 to 20 nm the, light emission with different
spectrum peaks were obtained. The results are tabulated in Table 7.1.
BCP Thickness at ETL-HTL Interface Peak Wavelength Color of Emitted Light
0 nm (Device 1) 530 Green
20 nm (Device 2) 470 nm Blue
5 nm (Device 3) 470 nm and 530 nm Bluish Green
1 nm (Device 4) 530 nm Green
0.5 nm (Device 5) 530 nm Green
Table 7.1: Effect of BCP Thickness on EL Spectrum of OLED
When the thickness of BCP is between 0 to 1 nm, it does not form a homogenous layer instead
the molecules are just dispersed at the surface of TPD and Alq3 [49]. A few holes are blocked at
63
0.01
0.1
1
10
100
1000
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00
Current Density (mA/cm
2
)
Voltage (V)
I-V Characteristics
Control Device
Device 2
Device 3
Device 4
Device 5
TPD-BCP interface. The remaining holes tunnel through BCP layer to reach Alq3 interface,
resulting in emission of pure green light from Alq3 layer. EL spectrum confirms this with
spectral peak at 530 nm.
When BCP thickness is 5 nm, many holes are blocked and excitons are formed at TPD interface
while the remaining holes manage to tunnel through BCP layer to form excitons at Alq3
interface. Hence the EL spectrum shows two spectral peaks both at 470nm (blue) and 530nm
(green).
As the thickness of BCP is further increased, the tunneling effect gradually weakens while the
blocked holes gradually increase. At 20 nm BCP forms a homogenous layer. In this case it is
very difficult for the holes to tunnel through this layer to reach Alq3 interface. A pure blue light
is emitted at spectral peak of 470 nm. Hence it can be postulated that the exciton and radiation
are from TPD layer.
• Current-Voltage Properties:
Figure 7.5: Semi log plot of I-V Characteristics
64
1.0
10.0
100.0
1000.0
10000.0
3.00 8.00 13.00 18.00
Luminance (cd/m2)
Voltage (V)
Luminance Characteristics
Device 1
Device 4
Device 5
Figure 7.5 shows I-V characteristics of the devices 1 through 5. When 20 nm of HBL is inserted
at cathode-ETL interface, its I-V plot matches closely with that of control device up to 8 V but
drops significantly afterwards. Device 2 with 20 nm of HBL at ETL-HTL interface shows poor I-
V characteristic. Device 3 with 5 nm HBL shows improved I-V only up to 7 V compared to
control device. Devices 4 and 5 (BCP 1 nm and 0.5 nm respectively) have drive voltage (voltage
at 20 mA/cm
2
) of 10.1 V and 9.9 V respectively which is much higher compared to drive voltage
of the control device (7.4 V). Thus addition of HBL at ETL-HTL interface increases the
operating voltage significantly. This could be because of low electron mobility in HBL compared
to that in ETL [51]. It was also observed that at a constant voltage the current drop was very low
compared to the devices without BCP layer.
• Luminance Properties:
Figure 7.6: Semi log Plot of Luminance Characteristics
Luminance characteristics of the devices are shown in Fig. 7.6. Due to extremely low luminance
from Device 2 (blue emission) and Device 3 (bluish green emission), no luminance data could be
taken. The best luminance was obtained from devices 4 and 5 but still much lower than the
control device.
65
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
3.00 5.00 7.00 9.00 11.00 13.00 15.00 17.00
Luminance Yield (cd/A)
Voltage (V)
Luminous Efficiency
Control Device
Device 4
Device 5
• Luminous Efficiency:
Figure 7.7: Luminous Efficiency
As illustrated in Fig. 7.7, luminous efficiency is maximized at a thickness of 0.5 nm. Devices 4
and 5 showed maximum efficiencies of 3.51 cd/A and 2.48 cd/A respectively. The improvement
of current efficiency by inserting a HBL can be explained in terms of hole-carrier blocking by
the BCP leading to balanced electron and hole currents. Further increase in the thickness causes
most of the holes to be blocked at the HIL/HBL interface resulting in an uneven balance of
mobilities. Right after the luminescence onset, a constant saturation value is achieved for device
efficiency. The luminous efficiency trend is in good agreement with the published literature as
compared to the random nature of the plots obtained for goals 1 through 3.
66
A summary of results is given in Table 7.2.
Table 7.2: Significant Device Properties
7.4 Conclusions
We studied the effect of BCP on the performance of OLEDs when deposited at different
interfaces. Although devices did not show improvement in luminance and current-voltage
properties compared to the control device, the following advantages were obtained:
1. Luminance efficiency of 3.51 was obtained which is highest compared to all the devices
fabricated so far.
2. The device’s chromaticity depends only on the thickness of BCP layer. Hence it is very easy
to tune the color of the device
3. Very stable devices were obtained.
Property Control Device 1 nm BCP 0.5 nm BCP
Turn on Voltage (V) 4 4 5
Drive Voltage 7.4 10.1 9.9
Current Density (mA/cm
2
)
at 12 V
224 54.58 52.84
Maximum Luminance
(cd/m
2
)
5927.54 at 12V 2427.591 at 14V 1757.91 at 15V
Maximum Efficiency
(cd/A)
3.09 at 10 V 2.48 at 10V 3.51 at 7V
67
Chapter VIII
Summary and Future Work
• We were able to successfully develop tools and techniques for fabricating reproducible
OLEDs. The characteristic trends and the device properties matched well with the published
literature [2, 12, 13, 15, 19, 20, 33, 34, 44, 49]. The following summarizes the results
obtained from the experiments:
Figure 8.1: Summary of Drive Voltages of OLEDs from Goals 1 through 4
• Fig. 8.1 shows the lowest drive voltage of 6.7 V was obtained for the device with LiF both at
cathode-ETL and anode-HTL interface (goal 3), which is significantly lower compared to the
device fabricated in goal 1. Due to addition of BCP at ETL-HTL interface (goal 4), the drive
voltage increased significantly. Goal 2 yielded reasonably low drive voltage of 7.4 V.
68
Figure 8.2: Summary of Luminance Characteristics of OLEDs from Goals 1 through 4.
• Fig. 8.2 shows the device with LiF at cathode-ETL interface (goal 2) yielded highest
luminance of 5927.54 cd/m
2
.
Figure 8.3: Summary of Luminous Efficiency of OLEDs from Goals 1 through 4.
• Fig. 8.3 shows the device with BCP as HBL yielded highest efficiency of 3.51 cd/A.
69
• If each factor; drive voltage, luminance and luminance efficiency, is considered individually,
goal 2 yielded highest luminance, goal 3 yielded lowest drive voltage while goal 4 yielded
highest luminance efficiency. Considering these three factors collectively, we can say that
device fabricated in goal 2 with a configuration ITO/PEDOT:PSS (60nm)/TPD (40nm)/ Alq3
(60nm)/LiF (0.5nm)/Al(60nm) was the best device of this work. In the future, there is a scope
for determining a configuration that would have all these advantages in a single device.
• These four goals give us clues as to the fundamental mechanism of OLEDs. We started the
research work from scratch and by the end of this research, we are much more confident
about the Lab’s capability for fabricating these electroluminescent devices. With the
knowledge we have gained from these four goals collectively, we can now make even
stronger statements about device operation. From the summary of goals 1 through 4, it can be
seen that we have made a significant progress in improving device performance and have
moved closer to the realization of ultimate goal of fabricating hybrid organic/inorganic
quantum dot light emitting devices.
70
Chapter IX
Appendix
• Electroluminescence
There are two main ways of producing light: incandescence and luminescence. In incandescence,
electric current is passed through a conductor (filament) whose resistance to the passage of
current produces heat. The greater the heat of the filament, the more light it produces.
Luminescence, in contrast, is the name given to all forms of visible radiant energy due to causes
other than temperature.
There are a number of different types of luminescence, including (among others):
electroluminescence, chemiluminescence, cathodoluminescence, triboluminescence, and
photoluminescence. Electroluminescence is the production of visible light by a substance
exposed to an electric field. EL devices include light emitting diodes, which are discrete devices
that produce light when a current is applied to a doped p-n junction of a semiconductor. EL is
also used in lamps and backlights. There are four steps necessary to produce electroluminescence
in ELDs:
1. Electrons tunnel from electronic states at the insulator/phosphor interface;
2. Electrons are accelerated to ballistic energies by high fields in the phosphor;
3. The energetic electrons impact-ionize the luminescent center or create electron-hole pairs
that lead to the activation of the luminescent center; and
4. The luminescent center relaxes toward the ground state and emits a photon.
• Molecular Orbitals: HOMO and LUMO
Orbital states can be described with several terms:
1. Filled: An orbital that contains the maximum number of electrons it can hold.
2. Empty: An orbital that contains no electrons.
3. Occupied: An orbital that contains at least one electron.
4. Unoccupied: An orbital that contains at least one open space for an electron.
71
HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied
molecular, respectively. The difference between the energies of HOMO and LUMO is termed as
the band gap. The HOMO and LUMO levels in organic semiconductors are analogous to
the valence band and conduction bands in inorganic semiconductors. The HOMO is the orbital
that could act as an electron donor, since it is the outermost (highest energy) orbital containing
electrons. The LUMO is the orbital that could act as the electron acceptor, since it is the
innermost (lowest energy) orbital that has room to accept electrons.
• Measurement Units
Photometric Units:
The aim of photometry is to measure light in such a way that the results correlate with human
vision. While radiometry covers all spectral regions from ultraviolet to infrared, photometry
deals with only the spectral region from 360 nm to 830 nm (the visible region) where human
eyes are sensitive. Photometry is essential for evaluation of light sources and objects used for
lighting, signaling, displays, and other applications where light is seen by the human eye.
Figure 9.1: Response of human eye to brightness
Luminous Intensity:
Candela is defined as the luminous intensity of a source that emits monochromatic light of
wavelength 555 nm and that has a radiant intensity of 1/683 watts per steradian in a specified
direction. One candela will radiate one lumen in all directions, so, 1 cd = 1 lm/sr
72
Luminance:
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in
a given direction. It describes the amount of light that passes through or is emitted from a
particular area, and falls within a given solid angle. The SI unit for luminance is candela per
square metre (cd/m
2
).
• Device Efficiency
Fluorescent emission of singlet excitons is the main mechanism of OLED light emission. As the
probability of forming spin singlet states and spin triplet states are 25% and 75% respectively,
the ideal maximum fluorescent yield is, therefore, limited to 25% by spin statistics. To overcome
this theoretical limit M. A. Baldo et al. [12] fabricated and demonstrated phosphorescent
OLEDs, by doping phosphorescent molecules, where the EL is due to triplet emission, into a
fluorescent host layer.
Internal Quantum Efficiency (IQE): Ratio of number of photons produced within a device to
the number of charges injected into the device. IQE can be written as
η
φ
= γ β φ
f
Where, γ = Charge balance factor (fraction of injected charges producing excitons)
β = efficiency of emissive excitations (the fraction of excitons that are formed as singlets)
φ
f
= Fluorescence quantum yield (number of photons released as fluorescence relative to number
of photons used to create excited state.)
External EL quantum efficiency: Ratio of number of emitted photons outside a device divided
by number of charges injected into the device. It can be written as:
η
φ
(ext) = χ η
φ
Where, χ = Light output coupling factor
73
Luminous Efficiency (cd/A)
Converting external quantum efficiency requires taking into account response of human eye to
light of various wavelength. Luminous efficiency can be obtained by taking the ratio of
luminance (cd/m
2
) to current density (mA/cm
2
).
Figure 9.2: Luminous flux efficiency of various light sources
74
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