High Efficiency Organic Light Emitting Diodes with MoO Doped … · 2012-11-03 · Figure 4-5: Device structure for doping thickness study, a) is the device structure of the control

High Efficiency Organic Light Emitting Diodes with MoO3 Doped Hole Transport Layer

by

Jacky Qiu

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Materials Science & Engineering University of Toronto

© Copyright by Jacky Qiu 2012

ii

High Efficiency Organic Light Emitting Diode

with MoO3 Doped Hole Transport Layer

Jacky Qiu

Master of Applied Science

Materials Science & Engineering

University of Toronto

2012

Abstract

Organic Light Emitting Diodes (OLEDs) are widely viewed as next generation platform for

flat panel displays and solid state lighting. Currently, OLED efficiency is not high due to high

driving voltage. Molybdenum trioxide (MoO3) is ideal for p-type doping of the wide bandgap

organic semiconductor 4,4’-bis-9-carbozyl biphenyl (CBP). With p-type doped CBP layer as

Hole Transport Layer (HTL), driving voltage can be significantly reduced. Effective design for

doped OLED structure consists of a HTL with doped layer from 20nm to 40nm and MoO3

concentration above 5%, the optimized OLED with doped CBP HTL present an 18%

improvement over a standard device with CBP HTL at 100mA/cm2.

Injection is found to be the principle cause of the reduction of driving voltage and shows close

relations to doped layer thickness. Also charge balance is an important factor for high current

efficiency, doped layer can be used as tools to promote charge balance.

iii

Acknowledgements

In my two years of study towards my master of applied science degree, I had the great honor

to work with some of the best and brightest minds in the field of organic optoelectronics. I

consider these people more than just colleagues but great friends in the pursuit for greater

understanding in OLEDs and beyond.

First of all, I would like to thank Prof. Zheng-Hong Lu who has provided me with his patience

and guidance. Without his guidance, I would not have been able to bring this work to fruition. I

would especially wish to thank him for the creative environment he has fostered which allowed

me the freedom to pursue the topics that interest me most.

Equally as important, I would like to thank my mentors, Zhibin Wang and Michael Helander.

With their continued guidance and support, I was able to learn so much from them, which

allowed me to acquire the knowledge necessary to understand the finer details of OLED

fabrication and operations, as well as develop the drive for my future scientific endeavors.

Lastly, I would like to thank all my lab mates, past and present. Gordon Yip, Dr. Daniel

Grozea, Mark Greiner, Dr. Wing Men (Stella) Tang, Dong Gao, Yi-Lu (Jack) Chang and Lily

Chai. I have enjoyed working with all of you, and hope in the future to be able to learn much

more from all of you.

iv

Table of content List of Figures ........................................................................................................................... vi

List of Abbreviations .................................................................................................................. x

1. Introduction and motivation ............................................................................................. 1

2. Physics and theory of p-type doped OLEDs .................................................................... 7

2.1 Organic Semiconductors ............................................................................................... 7

2.2 Organic Light Emitting Diode (OLED)........................................................................ 8

2.2.1 A simplistic picture ............................................................................................... 8

2.2.2 Charge injection .................................................................................................... 9

2.2.3 Charge transport .................................................................................................. 10

2.2.4 Recombination and emission .............................................................................. 11

2.3 Doped transport layer in OLEDs ................................................................................ 12

2.3.1 Doping Fundamentals ......................................................................................... 12

2.3.2 Increased conductivity......................................................................................... 13

2.3.3 Shift in effective work function of the HTL ....................................................... 15

2.4 Molybdenum Trioxide (MoO3) as p-type dopant for CBP ......................................... 17

2.4.1 MoO3, p-type dopant for deep HOMO level organic semiconductors ................ 17

2.4.2 Improved conductivity for MoO3 doped CBP..................................................... 18

2.4.3 Energy level alignment in MoO3 doped CBP ..................................................... 19

3. Materials and Experimental Setup ................................................................................. 22

3.1 Materials ..................................................................................................................... 22

3.1.1 CBP (4,4'-N,N'-dicarbazole-biphenyl) ................................................................ 22

3.1.2 MoO3 (Molybdenum Trioxide) and MoO3 doped CBP ...................................... 23

v

3.1.3 C545T doped Alq3 green florescent emitter system .......................................... 24

3.2 Fabrication Procedures ............................................................................................... 25

3.2.1 Substrate preparation ........................................................................................... 25

3.2.2 OLED Fabrication ............................................................................................... 26

3.3 Characterization Procedure......................................................................................... 27

4. MoO3 doped CBP as p-type HTL in OLEDs ................................................................. 29

4.1 Introduction ................................................................................................................ 29

4.2 Doping concentration ................................................................................................. 29

4.3 Doping Length ............................................................................................................ 33

4.3.1 Doping length study with 5% CBP:MoO3 doped HTL ....................................... 33

4.3.2 Control devices with varying doping length and fixed HTL thickness............... 35

4.3.3 Variations of control set with varying intrinsic CBP thickness and injection .... 38

4.4 Conclusion .................................................................................................................. 41

5. Variable hole injection with doped HTL towards charge balance engineering ............. 43

5.1 Introduction ................................................................................................................ 43

5.2 Doped layer thickness dependent injection ................................................................ 43

5.3 From current efficiency to charge balance ................................................................. 47

5.3.1 Decrease in current efficiency from improved hole injection ............................. 47

5.3.2 Charge imbalance through decrease in HTL thickness ....................................... 48

5.3.3 Charge balance with doped HTL for stable current efficiency ........................... 51

5.4 Conclusion - towards engineering charge balance ..................................................... 53

6. Conclusion and outlook .................................................................................................. 55

6.1 Conclusion .................................................................................................................. 55

6.2 Outlook ....................................................................................................................... 56

7. Bibliography ................................................................................................................... 57

vi

List of Figures

Figure 1-1: NASA - Earth at Night ................................................................................................. 1

Figure 1-2: OLED power efficiency (in red) relative to competing lighting technology, DOE

target for SSL the line in green (reprinted from DOE-SSL Multi Year Plan FY’09-FY’15) ......... 2

Figure 1-3: Development of OLED power efficiency (lm/W) 1987-2005,(reprinted from Ref 17)

a major shift occurred in 1999 when phosphorescent materials were first used as dopants by the

Forrest Group in Princeton, significant progress has since been made in efficiency and

lifetime[17] ..................................................................................................................................... 3

Figure 1-4: Light out-coupling scheme (reprinted from the pioneering work by the Forrest

group[24]) in an standard OLED device only 17.5% of light is out-coupled ................................ 4

Figure 1-5: Efficiency of WOLED (reprinted from work of Leo group[23]) efficiency at

luminance of 1000 cd/m2 is superior to fluorescent lights and comparable to inorganic LED light

source, this allows OLEDs to be competitive to traditional technology ......................................... 5

Figure 2-1: Schematic of semiconductor and effect of doping on energy levels ............................ 7

Figure 2-2: A simple schematic of light emission from injected charge carrier ............................. 9

Figure 2-3: Energetic charge injection process of electron from cathode to ETL (reprinted from

Scott et al.)[39] ............................................................................................................................. 10

Figure 2-4: Recombination and emission process for fluorescent emitter ................................... 12

Figure 2-5: Charge Transfer (CT) complex formation in p-type doping system .......................... 13

vii

Figure 2-6: Conductivity (a) with conductivity versus doping concentration in inest, and Seebeck

coefficient (b) with VOPc and F4-TCNQ structure temperature dependence plot for various

doping ratio, (reprinted from Pfeiffer et al [43]) .......................................................................... 14

Figure 2-7: Luminance - Voltage and current - voltage plot for OLED devices with p-type doped

HTL based on F4-TCNQ:VOPc system with multiple doping ratios (reprinted from Blochwitz et

al[32]) ........................................................................................................................................... 15

Figure 2-8: UPS spectra of intrinsic (left) and F4-TCNQ doped (right) ZnPc organic layer, p-type

doped layer has a significantly reduced EF-HOMO difference, reprinted from Ref 44 ............... 16

Figure 2-9: Fermi energy and HOMO level of intrinsic and F4-TCNQ doped ZnPc, significant

band bending and low EF-HOMO energy difference can be seen for the doped layer, reprinted

from Ref 44 ................................................................................................................................... 16

Figure 2-10: Energy level schematic of intrinsic and F4-TCNQ doped ZnPc, reprinted from Ref

44................................................................................................................................................... 17

Figure 2-11: Current - Voltage cureve of MoO3 doped (filled) and intrinsic (open) hole-only

single carrier devices with different host materials, reprinted from Qiao et al.[53] .................... 18

Figure 2-12: Conductivity and effective work function of the doped film versus doping

concentration of MoO3 clusters, reprinted from Ref. 47 .............................................................. 19

Figure 2-13: Electric field at charge injecting contact versus injection barrier for devices with

varying thickness, average field in device is 0.5 MV/cm, reprinted from Ref. 42 ....................... 20

Figure 2-14: Current Voltage relations with respect to injection barrier height, with barrier at

0.25 eV, ohmic injection and SCLC behaviour is observed, with injection of 0.55 eV, injection

limited (ILC) behaviour is observed, reprint from Ref. 42 ........................................................... 20

Figure 2-15: Thickness dependence of work function of MoO3 doped CBP based on with doping

concentration of 2.2 mol%, the depletion width is shown to be 20 nm, reprinted from Ref. 52 .. 21

viii

Figure 3-1: CBP molecular structure, mobility and energy level. Injection barrier is 0.69 eV in

the ITO/CBP interface, note discrepancy in energy level and barrier height is due to interfacial

dipole, energy level from Ref. 40, mobility from Ref. 55 ............................................................ 23

Figure 3-2: MoO3 structure, energy level as injection layer, CT complex and doped layer,

effective Fermi level and conductivity reprinted from Ref. 47 .................................................... 24

Figure 3-3: OLED spectra of CBP based C545T doped Alq3 emitters(on right), no major changes

to spectra was shown during the different devices used in this thesis .......................................... 25

Figure 3-4: Schematic of substrate structure (reprinted from Ref 59)[59] ................................... 26

Figure 3-5: Substrate preparation area (left), UV-Ozone processor (center), Cluster tool load lock

for loading sample and mask change (right) ................................................................................. 26

Figure 3-6: Kurt J Lesker Luminos (C) cluster tool (on left), metallization and organic chamber

(on right) ....................................................................................................................................... 27

Figure 4-1: Schematic of MoO3 doped CBP devices for study of doping concentration ............. 30

Figure 4-2: IV Plot of doped samples with different doping concentration (5, 10, 15%) compared

to standard CBP HTL.................................................................................................................... 31

Figure 4-3: Current and power efficiency of doped and standard devices ................................... 32

Figure 4-4: Spectra of various doped samples .............................................................................. 32

Figure 4-5: Device structure for doping thickness study, a) is the device structure of the control

device with fixed HTL thickness of 50 and varying intrinsic CBP thickness (x nm) and doped

layer (15%, 50-xnm); b) is variation 1 with MoO3 as HIL and varying intrinsic CBP thickness; c)

is variation 2 with CBP:MoO3 (5nm) injection to have similar injection..................................... 34

Figure 4-6: Devices with varying coping layer thickness with 5% doping concentration, due to

the instability of 5% doped CBP:MoO3, first and second round of doping is inconsistent .......... 34

ix

Figure 4-7: a) Current-voltage characteristics, b) operating voltage at 100mA/cm2 for control set

of devices ...................................................................................................................................... 36

Figure 4-8: a) power efficiency, b) power & current efficiency at 100 mA/cm2 of controlled

devices........................................................................................................................................... 37

Figure 4-9: a) driving voltage, b) current efficiency and c) power efficiency of variation 1 and

control set at 100 mA/cm2............................................................................................................. 39

Figure 4-10: a) driving voltage, b) current efficiency and c) power efficiency of control set (red),

variation 1 (black), variation 2 ( blue) devices at 100 mA/cm2 .................................................... 40

Figure 5-1: Improved injection with doped layer thickness dependence, schematic of hole-only

single carrier devices are shown with a) Intrinsic device with MoO3 HIL b) short MoO3 doped

CBP length where depletion zone not fully developed and c) long doped length where the

depletion zone is fully developed, injection barrier shifts correspondingly ................................. 44

Figure 5-2: Current-voltage characteristics for single carrier devices with HIL of 1nm MoO3, and

15% MoO3 doped CBP of 1, 5, 60 nm with 250 nm intrinsic CBP layer ..................................... 45

Figure 5-3: Current-voltage characteristic of hole-only single carrier device with 15% MoO3

doped CBP HIL of varying thickness (5, 20, 40, 60nm) for thickness dependent injection study,

schematic device structure in inset ............................................................................................... 46

Figure 5-4: Current efficiency of doped devices with varying concentration from section 4.2,

current efficiency decrease with increase in doping concentration .............................................. 48

Figure 5-5: Driving voltage and current efficiency of doped set and injection set for devices .... 49

Figure 5-6: Schematic of increased electron imbalance with decreased HTL thickness .............. 51

Figure 5-7: Current Efficiency at 100 mA/cm2 for doped set of device, and schematic to illustrate

charge balance in the doped set, three square representing doped length scenarios ..................... 52

x

List of Abbreviations

CBP 4,4’-bis-carbozyl-biphenyl

CT Charge-transfer

DOS Density of states

EIL Electron injection layer

EL Electroluminescence

EML Emission layer

EQE External quantum efficiency

ET Energy transfer

ETL Electron transport layer

HIL Hole injection layer

HOMO Highest occupied molecular orbital

HTL Hole transport layer

IQE Internal Quantum Efficiency

ISC Intersystem crossing

ITO Indium Tin Oxide

LUMO Lowest unoccupied molecular orbital

MoO3 Molybdenum Trioxide

OLED Organic light-emitting diode

UPS Ultraviolet photoelectron spectroscopy

XPS X-ray photoelectron spectroscopy

1

1. Introduction and motivation

Since the discovery of organic electroluminescence in anthracene crystals by researchers at

the Nation Research Council of Canada, 45 years ago,[1] the pursuit to bring forth a light source

based on organic semiconductors has never ceased. The field of Organic Light Emitting Diodes

(OLEDs) has garnered significant interest since the first OLED demonstration by Tang et al[2]

as an effective means to convert electrical power into light. In recent years, the field of OLEDs

has seen explosive growth; OLED technology is generally viewed as a rising star in the field of

mobile display as well as general lighting, and many companies are currently vying to

commercialize OLED technology. OLEDs are widely viewed as the next generation flat screen

display platform replacing LCD technology with hopes of achieving 40% market share of the

mobile display market with an annual market value of 10 billion dollars by 2015, according to

research firm Display Search.[3] More critical, is the emerging lighting market for Solid State

Lighting (SSL), SSL includes Light Emitting Diodes (LEDs) and OLEDs, and allows for

efficiencies near the theoretical limit for the conversion of electrical power into light.[4] This

high efficiency is critical, as 22% of the US electricity generation is used for lighting.[5] A

switch from current light source to efficient SSL can therefore lead to significantly reduced

power consumption, and hence to an equivalent reduction in the consumption of fossil fuels.

Figure 1-1: NASA - Earth at Night

2

OLEDs are superior to LEDs in some aspects. For example, an OLED is a planar source,

does not have the problem of glare and overheating associated with LEDs, and have the

potential to be manufactured inexpensively.[6] Moreover, OLEDs have the unique properties of

lightweight, flexible, transparent and color tuneability, which makes them an ideal modern light

source.[7-8]

However, in its current state of development, OLEDs are not able to challenge incumbent

lighting technologies such as florescent lights. OLED technology also lags significantly behind

in development compared to LED technology. In the key areas that influence the adoption of

lighting technologies: cost, efficiency and lifetime; OLEDs are inferior to fluorescent lights in

terms of cost and inferior to LEDs in terms efficiency and lifetime.[9] Due to the inexpensive

material and possibility for roll-to-roll processing,[6] the ultimate cost of OLEDs can be

significantly reduced by economics of scale. However, at the current time, the efficiency and

lifetime of OLEDs are low compared to other lighting technologies as shown in Figure 1-2.

Intensive investigation is required to develop efficient and long lasting OLEDs that would be

competitive to LED technology.

Figure 1-2: OLED power efficiency (in red) relative to competing lighting technology,

DOE target for SSL the line in green (reprinted from DOE-SSL Multi Year Plan FY’09-

FY’15)

The pursuit for efficient and long lasting OLEDs has seen significant effort in four fields of

development: Materials, Device Structure, Optical Engineering and Electrical Engineering.[10]

3

Materials are a critical factor for both efficiency and lifetime, the utilization of new materials

has allowed revolutionary improvements in OLED efficiency. From the first generation

fluorescent materials by Tang et al at Kodak,[11] to the utilization of phosphorescent materials

developed by the Forrest group and UDC at Princeton,[12] to the novel transport and emission

layer host materials,[13] the efficiency of OLEDs have grown more than tenfold and can now

challenge and defeat LEDs in terms of efficiency at wavelengths close to 550 nm.[14-15]

Moreover, continued development of OLED materials have allowed for devices with hundreds

of thousands of hours of operating lifetime.[16] The improvements to OLED by advances in

materials are shown in Figure 1-3.

Figure 1-3: Development of OLED power efficiency (lm/W) 1987-2005,(reprinted from Ref

17) a major shift occurred in 1999 when phosphorescent materials were first used as

dopants by the Forrest Group in Princeton, significant progress has since been made in

efficiency and lifetime[17]

A diverse set of device structures have also been developed since the invention of the simple

bi-layer structure and host-guest doping systems pioneered by Kodak[11]. For example, various

architectures have been developed to increase stability (introduction of LiF interlayer),[18] to

enhance color stability in White OLEDs (introduction of exciton blocking layers)[19], to enable

transparent and top-emitting devices[20-21], and to significantly reduce driving voltage by p-i-n

doping.[22-23] Novel architectures are constantly developed to adapt to the changing OLED

materials and manufacturing practices.

4

More recently, there is a shift in focus towards optical engineering, to allow for light

generated in the OLED to transmit in the desired direction, have shown promise to increase

OLED efficiency by more than 100%.[23] The two key components of optical engineering in

OLEDs are out-coupling treatment of the substrate and optical design in the device structure.

The pioneering work was done by the Forrest group, having shown that patterning of the

substrate with outcoupling schemes can produce improvements in efficiency up to 90%,[24] as

shown in Figure 1-4. Whereas, it is traditionally believed by linear ray-optics [25] that without

outcoupling schemes the External Quantum Efficiency (EQE), which is the overall conversion

efficiency from electrons to photons, is limited to 20%.[26] Current research strives to provide

new means to break such limits. Through optimizations in the device structure, OLEDs with a

record EQE of 29% without out-coupling layers have been demonstrated in the Organic

Optoelectronics Research Group at the University of Toronto.[15]

Figure 1-4: Light out-coupling scheme (reprinted from the pioneering work by the Forrest

group[24]) in an standard OLED device only 17.5% of light is out-coupled

The electrical engineering of OLED is in essence the design for efficient injection and

transport of charge carriers and the design of the electron-hole balance in OLED devices.

Though transport and injection of charges is well understood in organic semiconductors,[27-29]

interactions of carriers inside a bi-polar OLED device are not well understood.[30] Currently,

little effort has been made to understand the electron-hole balance into OLED devices. The

principle tool for the electrical engineering of OLEDs is to chemically dope the transport layers.

This allows for an increase in mobility within the transport layers, which in turn allows for

reduced operating voltage and improved efficiency.[31] The pioneering work for doped

5

transport layers in organic semiconductors was done by the Leo group and the Kido group, with

the introduction of 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped

Hole Transport Layer (HTL) by Leo et al [32] and metal doped Electron Transport Layer (ETL)

by Kido et al. [33] These works have shown significant promise with the development of the p-

i-n device architecture, with p-type doped HTL and n-type doped ETL, led by the Leo group and

Novaled. They have recently demonstrated White OLEDs with power efficiency comparable to

inorganic white LEDs, as shown in Figure 1-5. [23] However, the need to achieve electron-hole

balance is an issue that the Leo group has often raised[32, 34-35], but never fully addressed.

Thus an investigation into the properties of the doped transport layer and its effect on the

electron-hole balance is needed to elucidate a key concept in electrical engineering of OLEDs.

Figure 1-5: Efficiency of WOLED (reprinted from work of Leo group[23]) efficiency at

luminance of 1000 cd/m2 is superior to fluorescent lights and comparable to inorganic

LED light source, this allows OLEDs to be competitive to traditional technology

This thesis focuses on the study of varying doping conditions in the HTL of OLED

devices and its effect on the driving voltage and efficiency of these doped OLED devices. Based

on these results, concepts of charge balance and electrical engineering of OLEDs are developed.

This thesis is organized in the following fashion: In Chapter 2, the general device physics of

OLEDs (i.e. injection, transport and emission), is discussed, particularly the theory and device

6

physics of chemical doping of the HTL in OLEDs. In Chapter 3, the materials, fabrication and

characterization procedures used in this study are described. In Chapter 4, the detailed

experimental work of the MoO3-doped HTL are laid out, with focus on the study of

Molybdenum trioxide doped 4,4’-bis-9-carbozyl biphenyl (CBP:MoO3) as a p-type doped HTL.

In Chapter 5, a study of injection in the CBP:MoO3 HTL is discussed and a deeper insight into

charge balance and electrical engineering of OLEDs is formed. In Chapter 6, the results will be

summarized and an outlook for future works shall is provided.

7

2. Physics and theory of p-type doped OLEDs

2.1 Organic Semiconductors

The concept of an organic semiconductor can be separated into two components, organic

and semiconductor. Such materials are semiconducting in nature and are based upon interactions

between organic molecules.

According to the Bloch-Wilson theory of conduction,[36] energy levels of a material

form bands, which are filled with electrons from the material. A semiconductor material has a

completely filled valence band (VB), and a relatively small band gap (Eg) that allows for

thermal excitation of electrons from the filled valence band into the empty conduction band

(CB). In addition, impurities can be intentionally added into the semiconductor material with

energy levels inside the band gap to allow for excess carriers in the semiconducting material.

This process is known as doping and is the central focus of this thesis. A schematic of doping in

a semiconductor is shown below in Figure 2-1.

Figure 2-1: Schematic of semiconductor and effect of doping on energy levels

8

Organic semiconductors are different from conventional crystalline semiconductors

(such as Si or GaAs) in many ways; this leads to significant differences in physical and

electrical characteristics.

The interactions between organic semiconductors are based upon organic molecules as

fundamental units and the interacting force is the weak Van der Waals force, as compared to the

atomic units and covalent force for traditional crystalline semiconductors.[37] In addition, the

small molecules (as opposed to polymers) of organic semiconductor materials, which iare the

primary focus of this thesis, are amorphous thin-films. Thus the individual molecular units are

isolated and lack long range order. As a result, the properties of organic semiconductors (i.e.,

dynamic of charge carriers and carrier excitation) is largely dependent on the structure and

characteristics of the individual molecules, as opposed to the crystalline structure of the lattice

found in traditional semiconductors.

The energy level of an organic semiconductor is determined by the π-orbital overlap

between organic molecules, forming π-bonding and π*-antibonding states. In addition to the

vibronic and rotational modes, these form the energy levels of the organic semiconductor. [38]

The most important levels are the Highest Occupied Molecular Orbital (HOMO) and the Lowest

Unoccupied Molecular Orbital (LUMO), which form the corresponding valence band and

conduction band of the organic semiconductor.

2.2 Organic Light Emitting Diode (OLED)

2.2.1 A simplistic picture

An OLED device is an organic semiconductor based device which emits light when a

forwards bias is applied. Structurally, an OLED consists of stacked organic thin films with a

total thickness of about 100 nm sandwiched between a reflective and a transparent electrode.

Three important processes govern the effectiveness of the OLED device, which are: charge

injection, charge transport and emission. Various layers in the organic stack are dedicated to one

of the three processes above, such as surface modification in Hole Injection Layer (HIL) and

Electron Injection Layer (EIL), high mobility materials for transport with Hole Transport Layer

(HTL) and Electron Transport Layer (ETL), as well as Emission Layers (EL) with high

efficiency emitter dopants. Ultimately, charge carriers are injected into the OLED device at the

9

electrodes, electrons are transported through the LUMO and holes through the HOMO, these

electron-hole pairs travel to the emission zone and form excitons, which then emit light through

recombination. A simple schematic is shown below in Figure 2-2.

Figure 2-2: A simple schematic of light emission from injected charge carrier

2.2.2 Charge injection

Charge injection is the process of promoting electrons or holes from an electrode into the

corresponding states (HOMO and LUMO) in the organic layers of the OLED. Charge injection

can be determined by two key parameters, the injection barrier between the electrode and the

organic material, and the interfacial dipole formed when placing the organic semiconductor

adjacent to a metal with a difference in vacuum level. Ideally, the anode should have a higher

work function than the HOMO of the HIL and the cathode should have a lower work function

than the LUMO of the EIL. This guarantees an Ohmic contact, which allows carriers to freely

enter the organic material. In reality however, an injection barrier always exists; charge carriers

can be injected by thermionic excitation from the band like states in the electrodes into the

localized states in the organic semiconductor. This has been shown by Scott et al, as shown in

the simple schematic below, electrons that are thermally excited can tunnel into the localized

states of the ETL organic material.[39]

10

Figure 2-3: Energetic charge injection process of electron from cathode to ETL (reprinted

from Scott et al.)[39]

Most recently, novel concepts have been introduced into OLED design with the use of

wide bandgap organic semiconductors such as 4,4'-N,N'-dicarbazole-biphenyl (CBP) as a single

layer HTL has the immense advantage of simplicity and high efficiency.[40] However, it

becomes a significant challenge to find an anode with a sufficiently high work function to allow

for Ohmic injection, as CBP has a HOMO that is significantly deeper than any known

conductive anode. Thus the enhancement of injection from the anode into CBP is a key focus of

this thesis.

2.2.3 Charge transport

The amorphous nature of the organic semiconductor material dictates that states in the

organic material are highly localized, has a Gaussian distribution of energy disorder and has a

low intrinsic mobility.[39] Charge localization through molecular polarization (and the

formation of polarons) would mean carrier transport occurs through a thermally activated

hopping mechanism in an incoherent fashion from one molecule to another. The key

determining factor in charge transport is mobility. Mobility determines the speed at which

carriers can transverse through an organic semiconductor layer. Typical mobility of amorphous

organic materials used in OLEDs is between 10-3

to 10-7

cm2/V▪S, which is low compared to

traditional semiconductors. The low mobility is likely due to the large amount of traps in the

amorphous organic layers.

11

Due to the low mobility, the current of an OLED device can be limited by charge

transport in the organic layers. This current limit is called the Space Charge Limited Current

(SCLC). In the SCLC regime, more charge carriers are injected than the amount of charge, that

the organic layer can transport away, which creates a region of excess charge, known as space

charge. As organic semiconductors lack intrinsic states inside the band gap for carriers to reside,

the depletion width can be very large and thus limits the current.[27] Under SCLC conditions,

the localized hopping of charges result in Poole-Frenkel type field dependence of the mobility of

the organic semiconductor, and the SCLC follows the Murgatroyd law:[41]

Equation 2-1: Poole-Frenkel form of field dependent mobility[42]

Equation 2-2: Murgatroyd's law of trap modified SCLC[41]

Where ε is the material permittivity, εo is the permittivity of free space, μo is the zero

field mobility, β is the disorder coefficient, V is the applied voltage, and d is the film thickness.

In order to achieve high device efficiency, the amount of electrons and holes arriving at

the recombination and emission zone must be balanced. Engineering the mobility of the

transport layers has a significant role in achieving electron-hole balance, and thus is a focus of

this thesis.

2.2.4 Recombination and emission

Recombination of charges, formation of excitons and the emission of light is the final process in

the OLED. Excitons are electron-hole pairs that are formed when these two species meet in the

recombination zone of an OLED. Light emission results from these excitons when they

radiatively relax from their excited state to ground state. Electrically injected charges will form

excitons in a 1:3 mix of singlet excitons (anti-symmetrical spin) and triplet excitons

(symmetrical spin). Singlets radiates through the fluorescence process while triplets radiate

through the phosphorescence process. A schematic of the recombination process is shown below.

12

S0

S1

S2

Singlet Exciton

hν

Radiative Relaxation

T2

T1

Triplet Exciton Non- Radiative

Relaxation

ISC

Figure 2-4: Recombination and emission process for fluorescent emitter

In this thesis, only the singlet excitons are utilized using fluorescent dopants. Since

recombination and emission is not the focus of this thesis, all emission layer structures are kept

the same.

2.3 Doped transport layer in OLEDs

2.3.1 Doping Fundamentals

The basic principle of doping in organic semiconductors is that mobile carriers from an

electron donor (n-type) or acceptor (p-type) will be injected into the host semiconductor.

Electrons are injected into LUMO for n-type doping, holes (or equivalently the removal of

electrons) are injected into HOMO for p-type doping. As mentioned previously, organic

semiconductors have no intrinsic charge carrier, and the mobility for intrinsic amorphous

organic semiconductor is low. Thus a significant amount of energy must be used to transport the

charges into the emission zone, which is manifested as a field drop in organic devices.[30] For

effective p-type doping, a dopant material with electron accepting characteristics is needed. In

order to allow for effective transfer and transport, electrons from the filled HOMO must be able

to charge transfer into the unfilled dopant levels, thus forming mobile holes in the HOMO of the

host matrix. This would dictate that the dopant material would have a Conduction Band (CB)

energy level lower than the HOMO level of the organic semiconductor.[34] In essence, the

13

transfer of electrons from the HOMO of the host matrix into the dopant CB forms a Charge

Transfer (CT) complex. With the aid of the CT complex, charges have a faster alternate

transport path compared to the slow intrinsic hopping process.[43] The doped transport layer

exhibits significantly higher mobility and provides an effective means to transport charge

carriers without a large field drop across the transport layer.

Figure 2-5: Charge Transfer (CT) complex formation in p-type doping system

The primary features of doping in an organic semiconductor are increased conductivity, and a

shift in the effective work function of the HTL. These corresponds to improvement in transport

and injection for the doped HTL.

2.3.2 Increased conductivity

The pioneering work by the Leo group has adopted the idea of doping from inorganic

semiconductors into the field of small molecule OLEDs.[32, 43] The material system used by

Pfeiffer et al (Ref. 43) is a strong small molecule based electron acceptor, 2,3,5,6-tetrafluoro-

7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as dopant, and vanadyl-phthalocyanine (VOPc)

as host, deposited in controlled co-sublimation fashion. Such a system was chosen due to the

superior conductivity of VOPc in the phthalocyanine family as well as high doping efficiency of

the F4-TCNQ dopant.[43] As a result of F4-TCNQ doping, for the highest doped sample (2%

doping), conductivity in room temperature showed many orders of magnitude increase

compared to intrinsic VOPc (σ2%doped=5×10-4

S/cm, σintrinsic=1×10-10

S/cm). The method of study

by Pfeiffer et al. was the Seebeck effect, which can provide information about the energy

difference in the dopant transport state and the Fermi level of the matrix. Using Seebeck effect

measurements, hole concentration and transport layer mobility can be derived, as shown below:

14

Equation 2-3: Hole concentration based on Seebeck coefficient

Equation 2-4: Transport Layer mobility based on conductivity and hole concentration

Where p is the hole concentration, Nμ is the density of states at the dopant transport level,

e is the electron charge, kB is the Boltzman constant, S is the Seebeck effect coefficient, σ is the

conductivity, and μ is the hole mobility.

Conductivity and Seebeck coefficient can vary greatly with doping concentration, as

shown in the figure below. At a low doping concentration of 0.2%, the conductivity is 3 orders

below that of the 2% doped VOPc, yet still 3 orders above the conductivity of the intrinsic

VOPc.

Figure 2-6: Conductivity (a) with conductivity versus doping concentration in inest, and

Seebeck coefficient (b) with VOPc and F4-TCNQ structure temperature dependence plot

for various doping ratio, (reprinted from Pfeiffer et al [43])

15

The increase in conductivity would be a key contributing factor to reduced field drop in

the OLED devices, which is manifested as a significantly decreased driving voltage. Blochwitz

et al[32] used the F4-TCNQ doped VOPc system to show that operating voltage of OLEDs can

be reduced from 15 V to 6V, which can significantly improve power efficiency.

Figure 2-7: Luminance - Voltage and current - voltage plot for OLED devices with p-type

doped HTL based on F4-TCNQ:VOPc system with multiple doping ratios (reprinted from

Blochwitz et al[32])

2.3.3 Shift in effective work function of the HTL

As shown in the previous section, the energy difference between the dopant states and

the Fermi level of the host is the cause of improved conductivity. Fundamentally, the

introduction of dopants will shift the effective work function of the HTL closer to the HOMO

level, as the dopant introduces states and mobile carriers close to the HOMO level. The energy

difference between the Fermi level and the HOMO decreases with increasing dopant

concentration, as more excess holes shift the Fermi level downwards. Blochwitz et al (Ref. 44)

used both X-ray Photoelectron Spectroscopy (XPS) and its lower photon energy sister technique

Ultra-violet Photoelectron Spectroscopy (UPS), to determine the location of the Fermi level

inside the organic layer.[44] XPS/UPS data for ZnPc and F4-TCNQ doped ZnPc system is

shown below.

16

Figure 2-8: UPS spectra of intrinsic (left) and F4-TCNQ doped (right) ZnPc organic layer,

p-type doped layer has a significantly reduced EF-HOMO difference, reprinted from Ref

44

Also from the above data, the effective Fermi level of the organic films shows a clear

thickness dependent trend. This would suggest the existence of band bending, due to the

development of a space charge region. This spatially resolved bending of the HOMO level

accounts for the shift in work function. As the UPS spectra above suggest, band bending occurs

over a 20 nm range, as the measured Fermi level continues to shift for the intrinsic ZnPc;

whereas for the F4-TCNQ doped layer, band bending stops at 5 nm, which would indicate a very

thin depletion width. This would suggest that injection is improved due to the reduced width of

space charge region. A summary of the results is shown below:

Figure 2-9: Fermi energy and HOMO level of intrinsic and F4-TCNQ doped ZnPc,

significant band bending and low EF-HOMO energy difference can be seen for the doped

layer, reprinted from Ref 44

17

Figure 2-10: Energy level schematic of intrinsic and F4-TCNQ doped ZnPc, reprinted

from Ref 44

As shown in the energy level schematic above, the space charge length is significantly

reduced for the doped layer, and the Fermi level of the ITO is much closer to the HOMO level

of the F4-TCNQ doped ZnPc. This would suggest a reduced injection barrier and easier hopping

conditions for the holes in the ITO substrate to be injected into the doped HTL.

2.4 Molybdenum Trioxide (MoO3) as p-type dopant for CBP

2.4.1 MoO3, p-type dopant for deep HOMO level organic semiconductors

Recently, a novel device structure has been proposed to eliminate hole accumulation in

the OLED, this leads to reduced quenching and world record device performances.[45] The

critical material for this system is the wide band gap HTL material 4,4’-N,N’-dicarbazole-

biphenyl (CBP), which has a very deep HOMO level of 6.1 eV. [46

] As shown in the previous

sections, OLEDs with doped transport layers can have significantly reduced operating voltage

and high power efficiency. Thus a good p-type dopant for CBP is sought as the operating

voltage for this novel device structure still needs to be improved. The challenge is to find a

dopant with a lower CB energy level compared to the deep HOMO of CBP, as the archetypical

dopant F4-TCNQ does not have a shallow enough electron affinity to form an effective CT

complex.[47

] Alternatively, one class of material has shown remarkably deep CB level; oxides

of transitional metals (i.e., V2O5, MoO3 and WO3) serve as both surface modifying injection

layers[48] as well as forming CT complexes with organics.[49] MoO3 was chosen as the

dopant out of the various oxides due to the ease of processing compared to other oxides, as well

as a wide array of fundamental supporting literature available for the MoO3 doped CBP

18

system.[47, 50-53] MoO3 was also found to be an excellent p-type dopant for longstanding

academic standard HTL of N,N’-diphenyl-N,N’-bis- (1-naphthyl)-1-1’-biphenyl-4,4’-diamine

(α-NPD), which showed improved efficiency and lifetime for OLEDs with MoO3 doped α-NPD

as transport layer.[53-54]

In additional to the above advantages, MoO3 doped CBP is shown to have improved

conductivity,[47] forms a CT complex,[47] and more critically, excellent injection from ITO

without the need for additional surface modification or interlayers.[47, 52] This demonstrates

the significant value of integrating MoO3 as a p-type dopant into the novel CBP architecture.

2.4.2 Improved conductivity for MoO3 doped CBP

MoO3 is a p-type dopant material that can be incorporated into a wide variety of HTL

systems, due to its very deep electron affinity level. This was shown by Qiao et al,[53] who

showed significant improvement in conductivity for MoO3 doped CBP compared to intrinsic

CBP, using hole-only single carrier devices, as shown below in Figure 2-11. The single carrier

devices once again confirm that ITO has excellent injection into MoO3 doped CBP. In addition,

MoO3 is different from the archetypical F4-TCNQ, as a significantly higher doping

concentration is required for effective doping compared to the optimal 2% for F4-TCNQ.

Figure 2-11: Current - Voltage cureve of MoO3 doped (filled) and intrinsic (open) hole-

only single carrier devices with different host materials, reprinted from Qiao et al.[53]

Qiao suggests the reason behind the higher required doping is that only a partial charge

transfer occurs between MoO3 clusters and the matrix organic. Instead, holes directly hos

between MoO3 sites inside the host matrix, as conductivity in the MoO3 doped organics is not

19

dependant on the energy difference between the HOMO and the effective work function of the

organic, but dependant on the hole mobility of the host. This would require a significantly

higher doping concentration of MoO3 compared to F4-TCNQ. The relation between doping

concentration and conductivity for the MoO3:CBP system has been studied by Kroger et al

using UPS.[47] The MoO3 CB level was found to be 6.7 eV while CBP’s HOMO found to be

6.23 eV; this confirms that a CT complex can be formed between the MoO3 dopant and CBP.

Doping concentration of MoO3 has a critical concentration of 2%, below which a rapid decrease

in effective Fermi level and conductivity is observed, above the 2% doping concentration the

effective Fermi level plateaus and conductivity continues to slowly increase. Corresponding to

the hopping site theory presented by Qiao et al, below a critical concentration the connectivity

between sites is not established. This high critical concentration is due to the ineffectiveness of

the dopant, Kroger et al found that dopant activation in the MoO3 doped CBP system is less than

0.5%, which is vastly lower compared to the archetype dopant F4-TCNQ.

Figure 2-12: Conductivity and effective work function of the doped film versus doping

concentration of MoO3 clusters, reprinted from Ref. 47

2.4.3 Energy level alignment in MoO3 doped CBP

Close alignment of the Fermi level of the anode and the HOMO of the HTL allows for

good injection, which has a significant effect on device performance and operating voltage.

Wang et al have shown that an injection barrier of 0.5 eV or higher is sufficient to have all field

drop occur at the injection interface (i.e., charge injection is the dominant constraint in

devices).[42] Thus due to the large difference between the Fermi level of the ITO anode and the

HOMO of the CBP, typically a 1 nm of MoO3 surface modification layer is required to modify

20

the anode surface level to 5.8 eV.[40] However, there is still a 0.3-0.4 eV difference, which

means the injection can still be improved. The relation between injection and energy level

difference is shown below, when the field at the injecting contact is equal to the average field,

the device current is injection limited. [42]

Figure 2-13: Electric field at charge injecting contact versus injection barrier for devices

with varying thickness, average field in device is 0.5 MV/cm, reprinted from Ref. 42

Figure 2-14: Current Voltage relations with respect to injection barrier height, with

barrier at 0.25 eV, ohmic injection and SCLC behaviour is observed, with injection of 0.55

eV, injection limited (ILC) behaviour is observed, reprint from Ref. 42

The above provides an alternate explanation to the trend observed in Figure 2-12. As the

sharp rise in conductivity prior to 2% doping concentration is accompanied by an almost 1 eV

increase in effective work function of the doped layer. The measured conductivity may be

limited by the injection of the charge carriers. After 2% the energy levels are sufficiently aligned

21

to allow for good injection. Thus the increase in conductivity is less pronounced; the further

increase in conductivity is probably due to improved transport. This means there are two aspects

to be considered for any doping scenarios, injection and transport.

In addition to doping concentration dependant work function, work function is also

dependant on the thickness of the depletion zone. Hamwi et al has shown that the depletion zone

for a MoO3 doped CBP system can be as wide as 20 nm, and work function of a MoO3 doped

CBP based HTL is also doping thickness dependant.[52] Therefore in devices with different

doped HTL thickness, the reduction of operating voltage can be sourced from both increased

conductivity of the HTL as well as improved injection at the anode interface. The thickness

dependence of work function on MoO3 doped CBP is shown below:

Figure 2-15: Thickness dependence of work function of MoO3 doped CBP based on with

doping concentration of 2.2 mol%, the depletion width is shown to be 20 nm, reprinted

from Ref. 52

22

3. Materials and Experimental Setup

3.1 Materials

3.1.1 CBP (4,4'-N,N'-dicarbazole-biphenyl)

CBP is the principle material for the novel direct injection device design for high

efficiency OLED, as CBP has a deeper HOMO level compared to other OLED materials. The

role of CBP is the Hole Transport Layer (HTL), which facilitates the conduction of holes into

the Emissive Layer (EL), such material typically has a relatively high hole mobility. The CBP

used in this thesis is supplied by Lumetc Corp, evaporated in a Knudsen cell. The glass

transitional temperature is 78°C,[55] which is relatively low, this is indicative of the ease of

crystallization of CBP; this is confirmed by low temperature cryo-stat studies done previously in

the Organic Optoelectronics Research Group. The hole mobility of CBP is found to be ~10-3

cm2/V▪s and the electron mobility of CBP is found to be ~10

-4 cm

2/V▪s by time of flight

technique.[55] The hole mobility of CBP is high relative to other conventional HTLs such as α-

NPD (~10-4

cm2/V▪s )[56] and spiro-TAD(~10

-4 cm

2/V▪s )[57]. The most remarkable feature for

CBP is the deep HOMO energy level and the wide band gap; the HOMO level was found by

UPS as 6.1 eV and the band gap as 3.1 eV.[40] The deep HOMO level of CBP, 6.1 eV

compared to 5.4 eV for NPB, allows holes to freely flow into the EL and prevents charge

accumulation at the CBP/EL interface. Such accumulation which is prevalent in traditional

designs has been found to significantly reduce device performance.[45] The injection barrier for

ITO/CBP interface is high at 0.69 eV, which means the injection is poor and the device is

injection limited. [40] In addition, the large band gap prevents the CBP from absorbing the

emitted light from the OLED. The material characteristics above are summarized in Fig 3.1

below.

23

CBP (4,4’-N,N’-dicarbazolylbiphenyl)

ITO

CBP

4.7 eV

6.1 eV

Energy Level

µh ~ 10-3 cm2/V▪s

Structure

Mobility

ΦB=0.69 eV

Figure 3-1: CBP molecular structure, mobility and energy level. Injection barrier is 0.69

eV in the ITO/CBP interface, note discrepancy in energy level and barrier height is due to

interfacial dipole, energy level from Ref. 40, mobility from Ref. 55

3.1.2 MoO3 (Molybdenum Trioxide) and MoO3 doped CBP

MoO3 is an oxide semiconductor widely used as a surface modification layer of the injecting

ITO anode. A thin layer of MoO3 is sufficient to increase the effective work function of the

anode. The MoO3 used in this thesis is supplied by Sigma-Aldrich, evaporated from a BN

crucible. The MoO3 exists as clusters of (MoO3)n (n= 3 – 5), mostly as Mo3O9 clusters; the

clustering of dopants is a key cause of the low doping efficiency. [58] Due the band structure of

the MoO3 and the lack of oxygen deficiencies, the Conduction Band (CB) level is very deep,

Kroger et al. reports the CB band level is 6.7 eV.[47] The use of MoO3 as an injection layer can

reduce the injection barrier from 0.69 eV for the ITO/CBP interface to 0.5 eV for the

ITO/MoO3/CBP interface, this allows for good charge injection as the barrier value fall within

the quasi-Ohmic region.[42] A deep CB level allows for the formation of the charge transfer

complex with CBP which has a HOMO of 6.1 eV, the archetypical dopant F4-TCNQ which has

a LUMO of 5.4 eV cannot form a charge transfer complex and thus is completely ineffective.

For MoO3 doped CBP, the p-type dopant MoO3 increase hole concentration and shifts the Fermi

level towards the HOMO level, which reduces the injection barrier. Kroger et al have shown

24

that with a ~15wt% doping concentration of MoO3 doped CBP, the injection barrier at

ITO/CBP:MoO3 interface is 0.45 eV and the hole concentration is equivalent to 2% doping of

F4-TCNQ.[47] Thus 15wt% doping concentration can guarantee better injection than

ITO/MoO3/CBP and improved transport from higher carrier concentration.

MoO3 (Molybdenum Trioxide)

ITO

CBP

4.7 eV

6.1 eV

Energy Level (HIL)

Structure

ΦB=0.50 eV

ITO/MoO3

5.6 eV

Mo3O9Clusters

Energy Level (Doped)

ITO

CBP:MoO3

4.7 eVΦB=0.45 eV

MoO3

6.7 eV↑

↓

CT complex

Effective Fermi level and conductivity

Figure 3-2: MoO3 structure, energy level as injection layer, CT complex and doped layer, effective

Fermi level and conductivity reprinted from Ref. 47

3.1.3 C545T doped Alq3 green florescent emitter system

The C545T (2,3,6,7-tetrahydro - 1,1,7,7,-tetramethyl - 1H,5H,11H-10-(2-benzothiazolyl)

quinolizino-[9,9a,1gh] coumarin) doped Alq3 (tris- (8-hydroxy-quniolinato) aluminum) emitter

is one of the most commonly used systems for high efficiency green fluorescent emission, with

the Alq3 as host and C545T as dopant. The peak location of the emission from this system is 525

nm and Full Width Half Max (FWHM) of the peak is 50 nm. The maximum non-outcoupled

efficiency attained for the C545T dopant is 30 cd/A, however that uses a unique host material

and proprietary transport materials. Using the Alq3 as host, and C545T as dopant, the highest

achieved efficiency in undoped systems is report by Wang et al, at 22.5 cd/A, using the direct

25

hole injection architecture and CBP as HTL, compared to the archetypical NPB HTL, which has

an relatively low efficiency of ~15 cd/A. As the emission layer is not the focus of this thesis, the

composition of this layer is kept constant throughout all OLEDs in this thesis.

500 525 550 575 600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Rela

tive Inte

nsity (

a.u

.)Wavelength (nm)

C545T doped Alq3 (EL)

Figure 3-3: OLED spectra of CBP based C545T doped Alq3 emitters(on right), no major

changes to spectra was shown during the different devices used in this thesis

3.2 Fabrication Procedures

3.2.1 Substrate preparation

All substrate in this thesis are 2” x 2” glass substrates with pre-patterned ITO with resistivity

of 15Ω/□ manufactured by Kintec Inc. Eight possible variations in organic composition can be

achieved through a series of shadow mask systems, each individual organic variation has four

devices, a total of 32 ITO lines is present on each Kintec substrate. A schematic of the device

structure can be seen below.

26

Figure 3-4: Schematic of substrate structure (reprinted from Ref 59)[59]

Substrates undergo a cleaning procedure to remove environmental defects such as dust. The

standard cleaning procedure consists of the following:

1. Substrates are scrubbed intensively by hand on both side with Alconox© solutions for 1 min

2. Substrate and Alconox© solution is sonicated in ultrasonic bath

3. Substrate is sonicated in acetone

4. Substrate is sonicated in methanol

5. Substrate is placed in a Sen Light Photo Surface Processor with ITO side up and undergo

UV-ozone treatment for 15 minutes

6. Substrate is placed in a substrate holder and load into the cluster tool with mask

Figure 3-5: Substrate preparation area (left), UV-Ozone processor (center), Cluster tool

load lock for loading sample and mask change (right)

3.2.2 OLED Fabrication

The OLED is fabricated by successive thermal evaporation of layers of organic/inorganic

molecules and caped with an Al cathode. The fabrication of OLEDs is done with the Kurt J.

Lesker LUMINOS© cluster tool. All fabrication steps are done inside the cluster tool without

27

breaking vacuum. All steps of fabrication other than the Al cathode is done in the organic

chamber in high vacuum at a pressure under 1x10-7

Torr. The Al is done in the metallization

chamber due to high temperature required. Typical device structure is MoO3 doped CBP / CBP /

C545T doped Alq3 / Alq3 / LiF /Al, each of the above represent a layer. This typical device

structure has 6 layers and would require 5 organic deposition and 1 Al deposition. Thickness is

monitored by quartz microbalance, which is cross calibrated with ellipsometry measurements to

ensure accuracy. For the doped layers of the OLED, two sources are heated and molecules are

co-evaporated onto the substrate. Thus all doping percentages presented in this thesis are

specified by ratio of dopant weight to total layer weight (weight %). Specifically for MoO3

doped CBP, a top covering layer is immediately deposited on top to ensure the interfacial

characteristics are unchanged in vacuum. Up to eight variations in the organic layers are

fabricated on the same substrate to ensure run-to-run variance is minimized in comparative

experiments. Experimental fabrication tools are shown below.

Figure 3-6: Kurt J Lesker Luminos (C) cluster tool (on left), metallization and organic

chamber (on right)

3.3 Characterization Procedure

Luminance of OLED devices is measured by a Minolta LS-110 luminance meter, voltage and

current characteristics are measured by an HP4140 meter, OLED spectra is measured with an

Ocean Optics USB 2000 spectrometer with optical fibre. OLED devices are measured

immediately after fabrication. OLED samples remain in the substrate holder and are placed on

the testing stand. Gold tipped probes connected to the HP4140 meter is placed on the ITO

28

contacts of the substrate; one on the ITO line of device to be measured, one on the ITO line of

the cathode. Luminance, current and voltage measurements are done simultaneously, with the

HP 4140 meter providing driving current to the OLED device and measures current and voltage.

The luminance meter is synchronized to take luminance measurements per current step

(typically 0.2 V), 3 second cycles occur per step. IV measurements for single carrier devices are

done with an HP4140 meter without luminance meter synchronization. Spectra are measured

with driving current provided by an HP4140 at constant current, and acquired by optical fibre.

29

4. MoO3 doped CBP as p-type HTL in OLEDs

4.1 Introduction

MoO3 doped CBP has been introduced to lower the driving voltage compared to the standard

HTL of CBP. As shown in Chapter 2, the MoO3 doped CBP can have improved conductivity

and better energy level alignment. In this chapter, the specifics on the variation of doping

parameters and its effect on OLED performance shall be discussed. Finally, an optimized design

scheme shall be formulated based upon these results and an 18% improvement in power

efficiency compared to the highly efficient CBP device architecture is demonstrated.

To achieve this, a general survey of the suitable doping concentration is first conducted to

find the effect of doping concentration on device performance. Using this information,

subsequent sets of control devices are formed to maximize device performance, as well as study

the fundamental cause of the improved performance. Three sets of control devices are formed to

study the effect of doping length. Using the knowledge of doping concentration and doping

length an optimized design is formed.

4.2 Doping concentration

As shown by Kroger et al,[47] doping concentrations lower than 2 molar % (molar doping

ratio, mol%) is ineffective for doping CBP as the effective Fermi level experiences a quick rise

under 2 mol%. It is well known that MoO3 vacuum deposited through a Knudsen cell or a

thermal heater exists in the form of Mo3O9 clusters.[58] The doping in the devices studied in

this thesis is achieved by co-deposition of two molecules through thermal heating in the organic

chamber of the cluster tool system, and the doping ratio is monitored by a quartz crystal

microbalance. Thus the doping ratio is the ratio of the deposited weight on the substrate, or

weight % (wt%). The conversion factor of wt% to mol% for Mo3O9 in CBP is given by the

following equations:

30

Equation 4-1: Molar doping ratio and weight doping ratio conversion factor

Thus the equivalent wt% doping ratio for the 2 mol% doping concentration is 1.8%. Thus it

would be effective to study doping concentrations at higher than 1.8% in the general survey.

To elucidate the effect of doping concentration, a controlled set of devices with four different

doping concentrations (5%, 10%, and 15%) is fabricated to compare the differences. As dopant

molecules can form CT complexes with the host matrix, the dopant sites can act as traps for

excitons formed in an OLED, and is likely to lead to exciton-polaron quenching.[31] Thus an

intrinsic layer of CBP must be inserted between the MoO3 doped CBP HTL and the emission

zone. For all control devices, the device structure for the ETL and EL is kept constant to ensure

the effects that are observed is solely due to the variation of doping conditions in the HTL. Also

to ensure the devices are comparable, the thickness of the HTL is fixed at 50 nm for the doped

devices; the 40 nm MoO3 doped CBP (5%, 10% and 15%) HTL is capped with 10 nm of

intrinsic CBP. In addition, a standard device with a 1 nm MoO3 HIL and 50 nm of intrinsic CBP

HTL is selected for comparison. Device structures are shown in the schematic below:

ITO / Glass Substrate ITO / Glass Substrate

CBP:MoO3 (x%) (40 nm) MoO3 (1 nm)

CBP (Intrinsic) (10 nm)CBP (Intrinsic) (50 nm)

Alq3 : C545T (EL) /Alq3 (ETL)

Alq3 : C545T (EL) /Alq3 (ETL)

Cathode Cathode

Doped Standard

Doping Concentration

X= 5%, 10%, 15%

Structure: ITO/CBP:MoO3(x%) (40) / CBP (10) / Alq3:C545T (30,1%) / Alq3 (15) / LiF (1) / Al (100)

HTL

EL, ETL

Figure 4-1: Schematic of MoO3 doped CBP devices for study of doping concentration

The principle feature of the MoO3 doped CBP (doped) devices is the significantly reduced

driving voltage; this is clearly observed in the Current-Voltage (IV) characteristics

31

measurements. IV characteristics of the 4 doped samples with different concentration are

compared to that of the standard device.

0 2 4 6 8 10 12

0

100

200

300

400

MoO3 (1) / CBP (50)

CBP:MoO3(5%) (40) / CBP (10)

CBP:MoO3(10%) (40) / CBP (10)

CBP:MoO3(15%) (40) / CBP (10)

Cu

rre

nt D

ensity (

mA

/cm

2)

Voltage (V)

Figure 4-2: IV Plot of doped samples with different doping concentration (5, 10, 15%)

compared to standard CBP HTL

The IV plot shows that all doped devices have significantly reduced driving voltage. The

driving voltage of all doped devices is at least 2V lower compared to the standard CBP device at

100 mA/cm2. This reference current is chosen to eliminate the effect of contact resistance and

injection at low current. This level of current corresponds to around 20,000 cd/m2 for which

fluorescent dopants such as C545T are advantageous compared to phosphorescent dopants. As

for the 3 doped devices, the effects of increased dopant concentration of MoO3 are not

significant. No further increase in current density is observable beyond the concentration of 15%.

This is evident that at a doping length of 40 nm, 5% doping concentration is slightly higher in

driving voltage, while 10%, and 15% is almost equivalent, this suggests the effect of doping is

saturated at 10%. With this conclusion, the effect of doping concentration on device efficiency

is further investigated below.

32

10 100 1000 100000

5

10

15

20

MoO3 (1) / CBP (50)

CBP:MoO3(5%) (40) / CBP (10)

CBP:MoO3(10%) (40) / CBP (10)

CBP:MoO3(15%) (40) / CBP (10)

Cu

rre

nt E

ffic

ine

cy (

cd

/A)

Luminance (cd/m2)

0

5

10

15

20

Po

we

r E

ffic

ien

cy (

lm/W

)

Figure 4-3: Current and power efficiency of doped and standard devices

The current and power efficiency display two different trends. For power efficiency, the

doped samples generally provide at least a 15% improvement over the standard sample at a

luminance of 1000 cd/m2. Specifically, the 5% sample generated the best performance, while at

higher doping concentrations; the power efficiency is slightly lower. While for current

efficiency, surprisingly, the standard device has the best performance, and with increasing

doping concentration current efficiency decreases. As both power efficiency and current

efficiency simultaneously decrease for concentrations above 5% doping ratio, this would

suggest the cause of this decrease is intrinsic; the reason for this decrease shall be discussed in

chapter 5.

480 500 520 540 560 580 6000.0

0.2

0.4

0.6

0.8

1.0

MoO3 (1) / CBP (50)

CBP:MoO3(5%) (40) / CBP (10)

CBP:MoO3(10%) (40) / CBP (10)

CBP:MoO3(15%) (40) / CBP (10)

No

rma

lize

d In

ten

sity (

a.u

.)

Wavelength (nm)

Figure 4-4: Spectra of various doped samples

33

Spectra of the doped and standard sample show no difference in peak location or peak width

between any doped sample and the standard sample. This is indicative that the doping

concentration up to 15% is insufficient to have major impact on the optical characteristics of the

CBP layer. As the emission layers is kept the same and thus not a contributing factor, and optics

is not a contributing factor to the disparity in device performance, the difference in electrical

properties must be the cause of the difference in performance.

4.3 Doping Length

Doping length is an important parameter in the optimization of device performance and more

importantly this will provide useful information required for designs that would improve the

charge balance in an OLED. In the previous section, the maximum doping length (i.e., 40 nm

doped CBP and 10 nm of intrinsic CBP) has been shown for concentrations of 5%, 10%, and

15%. Also for the first set of control devices, doping length will be varied from 5 nm to 40 nm,

while total HTL thickness (i.e., doped length + intrinsic length) will be kept the same to ensure

any optical effect is not a contributing cause for the change in device behaviour. Then two

variations of the first set of control device shall be fabricated. The first variation is one with

ideal transport condition, as it is well known that doped CBP has a far superior transport

characteristics compared to intrinsic CBP. Ideally the doped CBP layer should exhibit no field

drop and thus should be equivalent to a device with reduced total CBP thickness. Thus the

doped CBP is compared to a device with reduced intrinsic CBP thickness. The second variation

is with a 5 nm injection layer of 15% MoO3 doped CBP, so as to ensure the energy level

alignment of this variation is a match to that of the first set of control devices. Similar to the first

variation, the intrinsic CBP thickness is equal in this set of control devices. This set of device is

used to see the effect of injection compared to first variation. Using these two variations as well

as the first control devices, the effect on doping length in terms of transport and injection shall

be studied. The device structures used to study doping length and variations is shown in the

schematic below.

4.3.1 Doping length study with 5% CBP:MoO3 doped HTL

34

CBP (x nm)

Glass substrate

ITO

LiF/Al (100 nm)

Alq3:C545T(30 nm)

CBP:MoO3 (50 - x nm)

Alq3(15 nm)

CBP (x nm)

Glass substrate

ITO

LiF/Al (100 nm)

Alq3:C545T(30 nm)

CBP:MoO3 (5 nm)

Alq3(15 nm)

Glass SubstrateGlass Substrate

ITOITO

MoO3 (1nm)CBP (x nm)

Alq3:C545T (30nm)

Alq3 (15 nm)

LiF/Al (100nm)

a) b) c)

Figure 4-5: Device structure for doping thickness study, a) is the device structure of the

control device with fixed HTL thickness of 50 and varying intrinsic CBP thickness (x nm)

and doped layer (15%, 50-xnm); b) is variation 1 with MoO3 as HIL and varying intrinsic

CBP thickness; c) is variation 2 with CBP:MoO3 (5nm) injection to have similar injection

From the study of doping concentration, this shows that doping concentration from 5% to

15% is acceptable for the study of doping length. Also from the doping concentration study, 5%

was shown to have the best performance, as well as the reduced requirement of dopant use and

reduced influence of dopant on optical properties as well as quenching centers. Thus the initial

investigation of doping length is done on 5% doped sets of control devices (i.e., on the same

substrate). The current-voltage characteristics are shown below:

4 5 6 7 8 90

20

40

60

80

100

Cu

rre

nt D

ensity (

mA

/cm

2)

Votlage (V)

CBP:MoO3 (45) / CBP (5)

CBP:MoO3 (40) / CBP (10)

CBP:MoO3 (30) / CBP (20) Second

CBP:MoO3 (30) / CBP (20)

CBP:MoO3 (20) / CBP (30)

CBP:MoO3 (10) / CBP (40)

CBP:MoO3 (5) / CBP (45)

Figure 4-6: Devices with varying coping layer thickness with 5% doping concentration,

due to the instability of 5% doped CBP:MoO3, first and second round of doping is

inconsistent

35

As shown in the current voltage characteristics, the IV characteristics from two different runs

of 5% doped CBP HTL are inconsistent. This would suggest that this low doping concentration

is not suitable for in-depth doping thickness study, as repeatability across devices is critical to

the observation and formulation of the trend related to doping thickness. There are multiple

possible causes for this inconsistency, but the most likely reasons are that the doping

concentration is close to the threshold doping concentration of 2 mol%, this is exacerbated by

the doping thickness dependence of injection demonstrated by Blochwitz et al.[44] As MoO3 is

an inefficient dopant, not all MoO3 clusters can successfully become dopant sites. Thus the

quoted 5 wt% may not have reached a threshold 2mol% doping concentration required for a

clear hopping path. In addition, as doping occurs by co-evaporation and the evaporation rate of

MoO3 cannot be independently verified, it is plausible that the rate fluctuates during the

deposition could also lower the doping ratio. Thus for the doping length study, 15% doping

concentration is chosen to eliminate the possibility of the doping ratio falling below the 2mol%

threshold.

4.3.2 Control devices with varying doping length and fixed HTL thickness

A set of control devices with varying doping length is fabricated to study the effect of the

variation of doped layer on the hole transport and hole injection. These devices will also

determine the optimal thickness of doped layer thickness. Special care is taken to ensure the

consistency of result: i) all devices in the control set are done on one substrate to eliminate run

to run differences in the emission layer and electron side; ii) each CBP:MoO3 layer is

individually deposited and immediately after an intrinsic CBP layer is deposited to cover the

doped layer to eliminate the effect of vacuum on the interfacial sites; iii), the HTL layer is

maintained at a fixed thickness to eliminate any optical effects.

The critical data is the current-voltage characteristics and the operating voltage curve, as the

main effect of improved power efficiency is in the reduced driving voltage.

36

b)

10 15 20 25 30 35 40 45

7.2

7.4

7.6

7.8

8.0

8.2

8.4

8.6

Drivin

g V

oltage (

V)

x (nm)

4 6 8

0

20

40

60

80

100

Curr

ent D

ensity (

mA

/cm

2)

Voltage (V)

45

40

35

30

25

20

15

10

a)

Figure 4-7: a) Current-voltage characteristics, b) operating voltage at 100mA/cm2 for

control set of devices

The operating voltage plot summarizes the current voltage characteristics. Two regions of

operating voltage behaviour can be seen in the above figure, a region of rapid change in

operating voltage at 100mA/cm2 (x=45 to x= 30) and a region with little change (x=30 to x=10).

This is indicative of two effects caused by doping, injection and transport. The flat region can be

seen as an increase in device thickness without additional field drop, characteristic of the

improved transport of the doped CBP HTL. While the region with rapid drop in driving voltage

is where doped CBP thickness is small (i.e., 50-x=5 to 50-x=20), Hawmi et al[52] has found

that the depletion region in a doped CBP HTL can be up to 20 nm thick, which matches the

observed thickness dependant behaviour in the control set of devices. A more detailed study on

this interesting phenomenon is included in the next chapter.

37

b)

10 20 30 40 5017

18

19

20

21

Pow

er

Effic

iency (

lm/W

)

Curr

ent E

ffic

iency (

cd/A

)

Thicnkess (x nm)

7

8

9

100 200 300 4005.5

6.0

6.5

7.0

7.5

8.0

8.5

Pow

er E

ffic

iency

(lm

/W)

Current Density (mA/cm2)

45

40

35

30

25

20

15

10

a)

Figure 4-8: a) power efficiency, b) power & current efficiency at 100 mA/cm2 of controlled

devices

Interestingly, the current efficiencies of all devices are almost equivalent and the value is

~20.2 cd/A which is in good agreement with results reported in the doping concentration study

in the previous section. As current efficiency is defined by the amount of light outputted over

the amount of current used, with devices of fixed optical spectra as in the controlled devices,

current efficiency is equivalent to the external quantum efficiency (i.e., amount of photons

outputted over the amount of electrons used). The external quantum efficiency is determined by

four factors:

Equation 4-2: Factors of External Quantum Efficiency (EQE)

In this case, the EQE is equivalent for devices in the control set with doped HTL ranging

from 5 nm to 40 nm. As shown by the spectra of the previous devices, spectra do not change as

a result of doping, as well as the same thickness and same substrate is used, thus outcoupling is

equivalent in the control set. Also as the same emitter is used for all devices and deposition of

the emitter for all devices occurred simultaneously, thus the internal is equivalent in the control

set. For singlet excitons used by fluorescent emitters such as C545T, the exciton diffusion length

is short, ~ 1nm, thus the factors that affect the formation of exciton and recombination is limited

38

to a short distance from the CBP/Alq3:C545T heterojunction. As all devices in the control set

have at least 10 nm of intrinsic CBP as a buffer layer, which is significantly larger than the

singlet exciton diffusion length, thus recombination is equivalent in the control set.[60] This would

suggest that the remaining charge balance factor must be equivalent in all devices in the control

set, and thus the improved injection and transport of holes which would increase the amount of

holes reaching the exciton formation zone is counterbalanced by an increase in electrons as well.

The power efficiency curve closely mirrors the inverse of the operating voltage, which shows

the same two regions in power efficiency, as in the driving voltage plot. The close correlation is

due to the derivation of power efficiency as shown below:

Equation 4-3: Power efficiency from current efficiency integrated over emission angle and

driving voltage, in lambertian emitters, emission angle integration factor = π

As this structure has been shown to have a lambertian emission pattern,[25] the integration

factor is fixed for all devices of the control set; the current efficiency is very similar. Thus

power efficiency is inversely proportional to the driving voltage, and for a doping thickness of

40 nm has the lowest driving voltage, which means highest power efficiency. Realistically, as

the doping region increases above 20 nm, the drop in operating voltage is minimal. Thus the

ideal design is for doping thickness to be above 20 nm for high power efficiency.

In addition to this control set of experiments, parallel experiments have been done for low

doping thickness for confirmation of the energy levels as a function of doping layer thickness.

However, devices with doping thickness lower than 5 nm became unstable and have generally

poor injection, which significantly affects the uniformity and emission of the device. This makes

the measurement unreliable and thus not comparable to that of the control set.

4.3.3 Variations of control set with varying intrinsic CBP thickness and injection

Two variations in the doping thickness experiment have been formed to measure the

principle cause of improved performance in the MoO3 doped CBP HTL. The two effects are

improved transport and improved injection. It is commonly believed that the principle cause in

39

reduction of driving voltage is due to higher mobility of the doped layer and the reduced field

drop in the transport layer due to higher carrier concentration. Thus variation 1 is formed to

mimic ideal transport conditions in the HTL; the thickness of the intrinsic CBP (x nm) is kept

the same, while the doped CBP layer is eliminated, thus eliminating the field drop in the doped

CBP layer. If indeed the contribution mainly stems from improvement in transport, then the

driving voltage of the devices in variation 1 will be lower than that of the control set.

10 20 30 40

6.5

7.0

7.5

8.0

8.5

Intrinsic

Doped

Po

we

r E

ffic

ien

cy (

lm/W

)

Intrinsic CBP Thickness (x nm)

10 20 30 40

14

16

18

20

22

Intrinsic CBP Thickness (x nm)

Cu

rre

nt

Eff

icie

ncy (

cd

/A)

10 20 30 40 507.0

7.5

8.0

8.5

9.0

9.5

10.0

Dri

vin

g V

olta

ge

(V

)

Intrinsic CBP Thickness (x nm)

Figure 4-9: a) driving voltage, b) current efficiency and c) power efficiency of variation 1

and control set at 100 mA/cm2

Above is the result of the variation 1, compared to the control set of devices; in black is

variation 1 and in red is the control set. To ensure good comparison, data between two sets is

compared based on the intrinsic CBP thickness in the device.

Two interesting trends can be observed for this set of devices, which is indicated by the linear

trend in the driving voltage and current efficiency. The linear trend in the driving voltage can be

explained as a uniform field drop in the HTL, as energy is dissipated to transport carriers in the

low mobility CBP HTL, to achieve similar field required to transport carriers with higher

intrinsic HTL thickness means higher driving voltage. The field drop for intrinsic CBP can be

calculated as F= 0.417 MV/cm at 100 mA/cm for the variation 1 set with MoO3 injection layer.

The second linear trend is that of the current efficiency, which is surprising as current efficiency

is convoluted by multiple factors such as device structure, dopants, charge balance and optical

effects. Out of the above factors, the use of the barrier free CBP based device structure and the

care taken to ensure uniformity of dopant / fabrication conditions eliminates the first two

conditions as possible contributing factors. Thus only electron-hole balance and optical effects

40

could lead to the unique shape of the current efficiency. More in-depth analysis shall be

discussed in Chapter 5, as well as discussion of the higher current efficiency for x>35 nm.

The above result in driving voltage shows that transport is not the main factor leading to

lower driving voltage, as the elimination of the of the doped CBP layer to return to the standard