Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics

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CHAPTER 5

ZINC (II), IRIDIUM (III) AND TIN (IV) COMPLEXES FOR

NANOSCALE OLED DEVICES

Trinh Dac Hoanh and Burm-Jong Lee

Department of Chemistry and Institute of Functional Materials,

Inje University, Gimhae 621-749, Korea

E-mail: chemlbj@inje.ac.kr

In this chapter we present a review on the organometallic complexes

which are employed for the organic light-emitting diodes (OLED)

having nanoscale multilayer structures. The device fabrication and

operation mechanism of OLED are followed by the ligands and

metal complexes of Ir(III), Zn(II) and Sn(IV). The discussions have

emphasized on their chemical structures and the electronic properties at

the multilayer interfaces. The organometallic complexes are classified

based on their colors and luminescent efficiencies.

1. Introduction

Organic and organometallic compounds applied in electroluminescent

devices are of considerable interests because of their attractive

characteristics and potential applications to flat panel displays.

1,2

The

devices take advantages over their inorganic counterparts such as

high luminous efficiency and fine-pixel formation.

Due to low driving

voltage, high contrast, ease of fabrication, low cost, and wide viewing

angle, OLED is believed to be the next-generation flat panel display.

The phenomenon of organic electroluminescence (EL) is the electrically

driven emission of light from non-crystalline organic materials.

2,4

1987, a research group of Kodak introduced a double layer device, which

was used aluminum (III) tris(8-hydroxyquinoline) (Alq3) as the emitting

layer by Tang and Van Slyke. It combined modern thin film deposition

T. D. Hoanh and B.-J. Lee 70

techniques with suitable materials and structure to give moderately low

bias voltages and attractive luminance efficiency.

Since then, there have

been increasing interests and research activities in this new field, and

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. As a consequence, various OLED displays have been

demonstrated.

The efficiency of an OLED is determined by charge balance,

radiative decay of excitons, and light extraction. Significant progress has

been made recently in developing phosphorescent emitters. In spite of

the remarkable advance of OLED, it still remains some drawbacks.

Charge injection and transport are the limiting factors in determining

operating voltage and luminance efficiency. In OLEDs, the hole current

is limited by injection, and the electron current is strongly influenced by

the presence of traps owing to metal-organic interactions. In order to

enhance carrier injection the selection of efficiently electron-injecting

cathode materials and the use of appropriate surface treatments of anodes

are of great importance. The most critical performance characteristic for

OLEDs is the operational lifetime. Continuous operation of OLEDs

generally leads to a steady loss of efficiency and a gradual rise in bias

voltages. Despite OLEDs have achieved long operational stability, the

material issues underlying the EL degradation are not fully understood.

1.1. Structure and Operation of OLED Device

An OLED has an organic EL medium consisting of extremely thin layers

(< 0.2 mm in combined thickness) 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 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.

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

Metal Complexes for Nanoscale OLED Devices 71

Fig. 1. Device structure and operation. Multilayer structure including g

lass substrate,

anode and cathode, hole injection layer (HIL), hole transfer layer (HTL), emitting la

yer

(EML), electron transfer layer (ETL), and electron injection layer (EIL).

anode into the hole transfer layer (HTL), while electrons are injected

from the cathode into the electron transfer layer (ETL). The injected

holes and electrons each migrate toward the oppositely charged electrode,

and the recombination of electrons and holes occurs near the junction in

the luminescent ETL. Upon recombination, energy is released as light,

which is emitted from the light-transmissive anode and substrate.

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. 1, the highest occupied molecular orbital (HOMO) of the HTL is

slightly above that of the ETL, so that holes can readily enter into the

ETL, while the lowest unoccupied molecular orbital (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 build up of hole

density, and thus enhance the collision capture process. Furthermore, by

spacing this interface at a sufficient distance from the contact, the

probability of quenching near the metallic surface is greatly reduced.

T. D. Hoanh and B.-J. Lee 72

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 primarily optimized 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.

1.2. Emission Mechanism of OLED Device

As presenting in the above section, the light is produced after the

recombination of hole-electron in the emitting layer. Light consists of

particles called photons and light emission occurs in a material when it is

excited by energy sources. If the materials are excited by the heat source

the emitted radiation is called thermal light, and analogue, by electron

beam is cathodoluminescence, by mechanical mean is triboluminescence,

by chemical reaction is chemiluminescence, by bio-organism is

bioluminescence, and by light source is photoluminescence. In OLED

the external source is electricity to emitting material, so it is

electroluminescence (EL). Luminescence can be achieved by several

physical or chemical mechanism which move the atoms and molecules of

material to an excited state with energy higher than at their ground state,

the lowest energy level of the atoms or molecules, in the luminance

process, the decay from an excited state to the ground state will rise the

emission of a photon, based on the principle of conservation of energy.

In a molecular orbital, there are two electrons of opposite spins per

orbital. But when an electron is excited to a higher energy level, its spins

Metal Complexes for Nanoscale OLED Devices 73

can point either in opposite or in the same direction. In the case of

opposite direction, we call singlet state or S, other case is triplet states or

T. The transitions between these states with the ground state will be

governed by the selection rule, which allows only those occurring

between states of identical spin multiplicity. Jablonski diagram (Fig. 2(b))

summarized the different kinds of transitions. We can distinguish two

main photoemission mechanisms, fluorescence and phosphorescence.

When the transition occurs from the singlet state S

to the ground

state S

(arrow b in Fig. 2(b)),

it is called fluorescence, this emission is

spontaneous and the decay rate depends on the temperature. In the case

of the triplet state T

to the ground state S

(arrow f in Fig. 2(b)), it is

called phosphorescence. The decay rate is much slower than that of the

fluorescence process.

To operate the OLED, the voltage must be applied efficiently high

bias to the device. Because this bias enables a current moving within the

diode and allows the charge carriers of electrons and holes to drain from

the electrode to the emitting layer to recombine there, then the photons

produced can go out of the diode. The process can be simply understood

as a conversion of electrical energy to the light energy. The conversion

rate can be defined for the electroluminescence. In general, the overall

(a) (b)

Fig. 2.

Recombination of an electron and hole resulting in the formation of excited states

(singlet or triplet) in organic material (Fig. 2(a)). Figure 2(b) illustrates

different

transitions states based on Jablonski diagram. (a) S

inglet absorption; (b) fluorescence

emission; (c) single quenching transition; (d) inter-

recombination; (e) triple absorption;

(f) phosphorescence emission; and (g) triplet quenching transition. Reproduced from Ref. 2.

T. D. Hoanh and B.-J. Lee 74

mechanisms of light emission in the device can be described in five-step

sequence such as charge injection, transport of carrier through the

emitting layer, recombination of charge carrier, emission of photons, and

transport of photons out of the device.

Charge injection: When we consider the injection of charge carriers

from the electrodes, it is required that charges overcome or tunnel

through a potential barrier at the interface of emitter and electrode. The

height of the barrier depends on the nature of the metal and that of the

semiconductor (organic or polymer compounds). According to the Mott-

Schottky model,

the vacuum and Fermi levels of the electrode and the

semiconductor are continuous to across the interface, leading the three

types of contact: neutral, ohmic, and blocking. If we call Ψ

and Ψ

the work function of the metal and the semiconductor, when Ψ

= Ψ

, it

is neutral. There is no charge transfer between the materials and the

conduction band remains flat up to the interface. When Ψ

> Ψ

, this type

is ohmic. Under the applied voltage the electrode can supply charges

to compensate for those flowing to the semiconductor. At thermal

equilibrium, electrons are injected from the electrode into the conduction

band of conducting material, creating a negative space charge region of

width λ

. This type of contact can supply as many electrons to the

semiconductor as are required by bias conditions.

When Ψ

< Ψ

, this type of contact is blocking, electrons flow from

the conduction band of the semiconductor to the metal, forming the

negative space charge region. The depletion region has a low electron

density, therefore an applied voltage to the contact will drop almost

entirely across it. Figure 3 shows the electrons injected into the

semiconductor via different mechanism.

The field emission mechanism is described by direct tunneling

through the base of the barrier from Fermi level E

at low temperature,

provide that the barrier is thin enough to allow the electrons to cross

through it.

This mechanism was first treated by Fowler and Nordheim

and

the model was modified by many researchers for adaptation to

semiconductor. Assuming a triangular shape for the barrier, the current

density can be written as follow:

Metal Complexes for Nanoscale OLED Devices 75

* 2 3/2

exp

B B

A T eF

k T eF

αφ

φ α

 

 

 

(1)

where

* * 2 3

(4 )/

A em k h

is the potential barrier and

* 1/2

4 (2 )

= .

The thermionic field emission is described by tunneling from an

energy level E

> E

from the metal to the semiconductor at a relatively

high temperature range. In this case, the barrier is thinner for the carriers

because of their higher energy. The mechanism is understood that when a

semiconductor-metal contact is brought to a sufficiently high temperature,

electrons are thermally excited over the potential barrier into the

conduction band of the semiconductor. The current density is given by

the Richardson thermionic emission equation.

* 2

exp

J A T

k T

 

= −

 

 

(2)

where

takes the value 120 when J is expressed in Acm

-2

Thermionic or Schottky emission: By emission over the potential

barrier at a high temperature range. The energy of the electrons is higher

than the barrier height

, allowing them to flow to the conduction band

of the semiconductor.

Fig. 3.

Different injection mechanism of electrons from the cathode into the

semiconductor under the applied voltage: (a) thermionic emission; (b) thermionic field

emission; and (c) direct tunneling.

T. D. Hoanh and B.-J. Lee 76

Charge transport: After being injected into the emitter, electrons and

holes are transported inside the material under the applied electric field.

They can contact each other and recombine, but they can also go through

the emitting layer and be extracted at the opposite electrode. The moving

of the carrier inside the organic layer depends on some parameters

inherent to the nature of the material. It can occur in the conduction band

or the band gap via the localized states. The former mechanism is

possible when the charge carrier have efficiently high energy to enter the

conduction band while the latter is involved when the carriers are

confined in the band gap on localized states but can jump along these

with less energy because of the proximity of these states. When the

number of injected charges to a semiconductor is higher than that the

material has in thermal equilibrium without carrier injection, the excess

of injected charges will form a space charge inducing an electric field

inside the semiconductor, which, in turn, will reduce the charge injection

from the electrode. Therefore the current flow is no longer limited by the

charge injection from the electrode but by the bulk of the semiconductor.

The condition for the mechanism to occur is that one of the injecting

contacts should be able to provide as many carriers as needed.

Charge recombination: After the charge carriers are injected to the

semiconductor, they may decay by recombination to produce a photon,

but they may also move along several different paths to reach the

opposite electrode. In conventional semiconductors, which contain

both the donor and acceptor, the recombination of the electrons and

holes can occur through several mechanisms like direct band-to-band

recombination, indirect recombination through a recombination center-

example, donor to valence band, conduction band to acceptor, and donor

to acceptor. Recombination can lead either to an electron-hole pair

that decays to the ground state by the emission of photon (radiative

recombination) or to exclusively photon emission (non-radiative

recombination). Band-to-band transitions occur at energies less than

equal to or greater than the band gap and free electrons and holes may

become bound via the Coulomb interaction to form free excitons. In

contrast, indirect transitions occur at energies less than the band gap.

OLED fabrication: There are many methods to fabricate OLED such

as vacuum deposition, Langmuir-Blodgett, spin-coating, inkjet-printing,

Metal Complexes for Nanoscale OLED Devices 77

dye-diffusion and silk-screen. Depending on the material we can choose

a proper method. In this section we introduce about vacuum deposition

method, which widely used for fabrication of OLED of the small

molecular materials.

Vacuum deposition: Organic materials are evaporated on to an

indium-tin-oxide (ITO)-coated glass substrate. Organic materials are

previously purified by vacuum sublimation. The EL cells are generally

constructed by a conventional thermal evaporation method at a chamber

pressure of about 1

× 10

-5

Torr. This technique consists of heating the

materials under reduce pressure. The material is put into a metal boat

or crucible, which is heated by Joule effect, or with an electron gun.

Emitting layers are sometimes deposited each one of two metal sources.

The cathode is deposited onto the organic layer. The cathode is generally

selected from low work function metals (Table 1). For example, Mg:Al

(10:1) alloy as the cathode is also deposited from each one of two metal

sources.

Table 1. The work function of metal.

Metal eV

ITO 4.7

Ga 2.87-3

MgAg 3.7

Al 4.06-4.41

Ag 4.26-4.74

Au 5.1-5.46

In 4.12-4.2

Yb 2.6

2. Ligands Coordinated to Ir(III), Zn(II) and Sn(IV)

Small molecules as the metal complexes with ligands are very important

to fabrication for OLED devices due to their luminance efficiency and

other advance properties. The main characteristic of those is that they can

be evaporated under vacuum to form thin films. It enables their use in the

field of small area devices, especially to making colored pixels for

T. D. Hoanh and B.-J. Lee 78

display application. This deposition technique allows precise control of

the film thickness, so for this reason, multilayer diodes for multicolor

emission are usually made using these materials. Therefore the ligands

are received much attention to make complexes with metal for the

application of this area. We introduce three main types of ligands which

have popular complexes with Ir(III), Zn(II), and Sn(IV) presented in this

review.

8-Hydroxyquinone and its derivatives: 8-Hydroxyquinone (1) has

become the well-known ligand since it was successfully synthesized with

aluminum to form Alq3 at the first time in 1955 by Freeman and White.

Then Tang

et al. utilized Alq3 for the fabrication of the first OLED

operating at low voltage (< 20 V), green photoluminescence (PL) with a

quantum efficiency around 32%.

And also the material is subjected to an

electrical field.

On the other hand, it is easily sublimated under vacuum

to form a smooth thin layers.

The 8-hydroxyquinone was first prepared from o-aminophenol, glycerol

and H

in 1882 by Skraup.

The structure of 8-hydorxyquinone (1)

shows in Fig. 4. It is the basic ligand due to the capacity to produce the

chelate compounds with metals. Researchers have been tried to modify

the chemical structure of 8-hydroxyquinon to investigate the EL and PL

of the complexes by adding the donor or withdraw functional group to

the structure of the 8-hydroxyquinone.

Fig. 4. The basic ligands for the coordination with Ir(III), Zn(II), Sn(IV). 1: 8-

hydroxyquilone; 2: 2-phenyl pyridine; 3: benzo-compounds.