organic light-emitting diodes (oleds) · organic light-emitting diodes (oleds) are based on...

Organic Light-Emitting Diodes (OLEDS)

Ruiqing Ma

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Organic Electroluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Charge Transport in Organic Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Charge Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Luminescence of Organic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

OLED Device and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Bilayer Heterojunction Small Molecule OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Polymer OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Advanced OLED Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Recent Research Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14OLED Fabrication and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Future Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

AbstractOrganic light-emitting diode (OLED) technology has experienced a substantialgrowth in the last three decades. It has emerged as a strong flat panel displaytechnology with many new products ranging from mobile displays to TVsintroduced into the marketplace every year. After a brief review of the historyof OLED development, this chapter will explain how OLEDs work by diving intothe key electroluminescent process steps: charge injection, charge transport, andcharge recombination and light emission. Then the focus will shift to detaileddiscussion on OLED device structure and materials. This is followed by asummary of recent research topics and latest results. After a quick discussion

R. Ma (*)Universal Display Corporation, Ewing, NJ, USAe-mail: [email protected]

# Springer-Verlag Berlin Heidelberg 2016J. Chen et al. (eds.), Handbook of Visual Display Technology,DOI 10.1007/978-3-642-35947-7_79-2

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mailto:[email protected]

on device fabrication, degradation, and encapsulation, this chapter will end with abrief discussion on the future directions of OLED research and development.

List of AbbreviationsETL Electron transport layerHOMO Highest occupied molecular orbitalHTL Hole transport layerIQE Internal quantum efficiencyLUMO Lowest unoccupied molecular orbitalOLED Organic light-emitting diodeOVPD Organic vapor phase depositionPHOLED Phosphorescent OLEDRISC Reverse intersystem crossingSMOLED Small molecule OLEDTADF Thermally activated delayed fluorescenceTTA Triplet-triplet annihilationVTE Vacuum thermal evaporation

Introduction

Organic light-emitting diodes (OLEDs) are based on electroluminescence in organicmaterials, whose discovery can be traced back to the 1950s when light emission wasobserved by applying high voltage to thin films of acridine orange and quinacrine(Bernanose et al. 1953). In 1963, Pope et al. observed bright blue electrolumines-cence in single crystal anthracene by applying a high DC voltage (�400 V) acrossthe crystal (Pope et al. 1963). Later, Vincett et al. used evaporated thin film ofanthracene and was able to reduce the drive voltage to below 30 V. However, thequantum efficiency of this device was only about 0.05 % (Vincett et al. 1982). Thefirst report of an organic LED was in the seminal work by Tang and Van Slyke in the1980s when they demonstrated a heterojunction device and developed a dopingmethod by introducing a small amount of fluorescent materials into a host matrix(Tang and VanSlyke 1987; Tang et al. 1989). The electroluminescence in conjugatedpolymer poly( p-phenylene-vinylene) (PPV) was reported in 1990 (Burrougheset al. 1990). This work was followed by a breakthrough in the late 1990s whenForrest and Thompson discovered phosphorescent light-emitting materials whichdramatically improved the quantum efficiency of the OLED devices (Baldoet al. 1998). Today, phosphorescent OLED (PHOLED) is a cornerstone technologyfor low-power flat panel displays and energy-efficient light sources.

As a flat panel display technology, OLED stands out with its ultrathin profile,vivid color, ultrafast switching speed, wide viewing angles, pitch-black off state andthus extremely high contrast ratio, and low power consumption. In addition, becauseof its simple, elegant structure, OLED can be easily made into transparent andflexible displays, which enable novel device architectures and applications.

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In the early stage of its commercialization, OLED had difficulty finding its placein the market. Simple passive matrix driven OLEDs were first used in cars and cellphones. Later, active matrix OLEDs (AMOLEDs) were adopted for digital cameras,mp3 players, and digital picture frames. It is very surprising that the excellent imagequality of OLEDs couldn’t land them with a killer application. Ten years after thefirst commercial OLED display, Samsung started featuring AMOLED in its Galaxysmart phone products. Just like laptop computers made the AMLCD industry, smartphones kicked off the commercialization of AMOLEDs. AMOLEDs are super thinbecause no backlight is needed. This is a virtue that smart phone designers andconsumers really appreciate. In addition, AMOLEDs provide a road map of contin-uous innovation in low power, transparent, and flexible displays. In recent years,AMOLEDs have also been commercialized by LGD for TV applications, where thesuperb image quality is highlighted. Even with the rapid progress in AMLCDtechnology, AMOLED still possess significant advantages in its super-fast switchingspeed, pitch-black off state, and novel form factors.

Organic Electroluminescence

In an OLED device, electrons and holes are injected into and transported throughelectron and hole injection and transport materials, respectively. They recombine inthe light emission layer, forming a neutral excited state, or an exciton. When theexcited state relaxes to the ground state radiatively, light is emitted. The operation ofan OLED requires conductive or semiconductive materials for transporting chargesand luminescent materials for light emission.

Charge Transport in Organic Semiconductors

The key ingredient in organic semiconductors is carbon-carbon double bond. In asingle bond structure, each carbon atom is bonded to four other atoms and theelectronic orbitals are fully saturated. While in a carbon-carbon double bond struc-ture, the electrons from the 2 s and 2p orbitals of each carbon atom form three sp2

hybridized orbitals and one pz orbital, as shown in Fig. 1. The sp2 orbitals fromadjacent carbon atoms overlap forming a strong molecular σ orbital. The remainingpz orbitals overlap, leading to energy level splitting and the formation of bonding andantibonding molecular π and π* orbitals. In the ground state, all the bonding orbitalsare filled with pairs of electrons with antiparallel spin while the antibonding orbitalsare empty. The highest bonding orbital is referred to as the highest occupiedmolecular orbital (HOMO), while the lowest antibonding orbital is referred to asthe lowest unoccupied molecular orbital (LUMO). Roughly speaking, HOMO andLUMO levels are equivalent to the valence band maximum (EV) and conductionband minimum (EC) of conventional inorganic semiconductors.

Organic Light-Emitting Diodes (OLEDS) 3

A charged excited state is either an additional electron in an antibonding orbital ora missing electron in a bonding orbital, i.e., a hole. In the carbon-carbon double bondstructure, the electrons in the π orbital can move freely between the two carbonatoms. Once electrons or holes are injected into the organic semiconductors, theyperturb the π orbitals and cause local distortions to trap the charges, which arepolarons (Kafafi 2005). As rings or double bond containing systems are linkedtogether, the electrons can further delocalize, giving the material semiconductingproperties. In OLED devices, polarons migrate through the organic transportinglayers to reach the recombination zone under the influence of applied potential.

Unlike inorganic semiconductors in which the long-range interactions lead to theformation of a continuous conduction band, organic materials used in OLEDs do notpossess long-range order. Even though some of recent research works have beenfocused on promoting certain degree of in-plane alignment of organic molecules,most of the charge transport films in OLEDs are amorphous in nature. The individualmolecules interact with each other via a weak van der Waals force. As a result of thedisorder, charges are localized and charge transport is dominated by a hoppingprocess where charges jump between interacting molecules (Gartstein and Conwell1995).

An effective way to improve charge transport is to dope the organic semiconduc-tors with electron donors or acceptors to introduce additional charge carriers. Inn-type doping, the dopants donate electrons to the LUMO states of the organicswhile in p-type doping the dopants extract electrons from the HOMO states of theorganics (Lussem et al. 2013). Examples of p-type dopants are F4-TCNQ, NDP2,and NDP9 (Olthof et al. 2009). For organic semiconductors, n-type doping is muchmore challenging because the dopant has to have sufficiently high energy level – itsHOMO needs to lie above the LUMO of the organic semiconductors. This causesserious stability issue and most times the dopants need to be handled in inertconditions. Typical n-type dopants for organic semiconductors include alkali metals(Li, Cs), molecular compounds with very high HOMO levels, and air-stable precur-sor molecules which can be activated later.

Charge Injection

As discussed earlier, charged excited states are formed by addition or removal ofelectrons. One simple way to generate charges is to introduce dopants into theorganic materials. Another way to introduce charges is by optical excitation. Upon

C CCH3CH3

H3C

H3C

Fig. 1 The molecular orbitalsin carbon-carbon double bondand benzene

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absorbing a photon, an electron can be excited into higher energy orbitals, yielding aneutral excited state. A free positive and a free negative charge can be generatedwhen this electron–hole pair dissociates, as in an organic solar cell.

In electroluminescent devices such as OLEDs, charges are injected from elec-trodes into the organic materials. The work function of metal, defined as the energydifference between its Fermi level and the vacuum level, is different from theelectron affinity of the organic material, which is the energy difference between itsLUMO level and the vacuum level. This energy difference is the barrier for thecarrier injection at the metal-organic interface.

For conventional crystalline inorganic semiconductors which possess widevalence and conduction bands, the carrier injection mechanism is either the Schottkythermally excited emission or the Fowler-Nordheim tunneling with the help of alocal high electric field. However, as explained earlier, the charge transport inorganic semiconductors is incoherent and the intermolecular transport is of thehopping type. As a result, the above two theories are not suitable to explain thecharge injection at the metal-organic interface. For organic materials, although thedisorder limits charge mobility, it does facilitate charge injection by loweringinjection barrier. The disorder in the organics, especially at the surface, couldproduce a population of energy states in the forbidden energy gap. A thermallyactivated electron can jump to one of these states and, subsequently, undergoes ahopping transport process under the influence of an external field.

Luminescence of Organic Materials

When an electron and a hole reach the recombination zone, they recombine to forman exciton which is a charge-less electron–hole pair that can transport energy. Unliketypical inorganic semiconductors that have delocalized excitons over many latticesites, typical organic materials have excitons confined to local sites or molecules.This is due to a strong exciton binding energy (0.5 � 1.0 eV) as a result of strongCoulomb attraction between the electron–hole pair in organic semiconductors withsmall relative dielectric constant (�3). These highly localized excitons, or Frenkelexcitons, can migrate by hopping to the neighboring molecules. Their energy can bereleased through radiative or nonradiative decay processes or transferred to theneighboring molecules.

The weak charge delocalization of organic semiconductors has many conse-quences, one of which is the existence of well-defined spin states as in isolatedmolecules. In the EL devices, the electrons and holes are injected from separateelectrodes without any correlation between them. The spins of the two polarons(an electron and a hole) to be recombined have random orientations with respect toeach other. Only when they recombine, their spins become correlated (or coupled)leading to either singlet or triplet states. The two coupled spin vectors in a singlet areantiparallel and collinear resulting in zero spin angular momentum (S = 0). In tripletstates, the coupled spins add to each other resulting in a total spin of S = 1 (Turro1978). Because of the independent nature of the initial spin states of the polaron pair,


quantum mechanical spin statistics dictates that there are four ways for the spin wavefunctions of the individual polarons (α,β) to combine – one singlet state (αβ � βα)and three triplet states (αα, ββ, αβ + βα). As a result, excitons generated byrecombination are generally considered to be a statistical mixture of 25 % singletand 75 % triplet states. The singlet excited states can readily relax to singlet groundstates through a fast (�1 ns), efficient fluorescent process. Phosphorescence is thetransition from triplet excited states to singlet ground states with the emission ofphotons. It is considered as a spin-forbidden process. Phosphorescence in conven-tional organic aromatic compounds usually occurs at very low temperature so is notuseful for practical applications.

In most SMOLEDs, the EML layer is composed of at least one charge transportmaterial as a host and at least one light-emitting material as a dopant. During OLEDoperation, the singlet and triplet excited states are typically first formed in the hostmaterials and then transferred to the guest (dopant) molecules, as shown in Fig. 2.There are two types of energy transfer process. In a Forster energy transfer, theenergy released from an excited donor going back to ground state could simulta-neously excite the acceptor from the ground state based on dipole-dipole interaction.It is a long-range process (�30–100 Å) and only singlet to singlet transfer is allowed.Forster triplet to triplet transfer is spin-forbidden. Dexter energy transfer is the directexchange of electrons and requires the overlap of the wavefunctions of the involvedelectrons. It typically occurs within 10 Å. With Dexter energy transfer, both singletto singlet and triplet to triplet transfers are spin-allowed. Triplet-triplet annihilation(TTA) is a special case of exchange energy transfer and will be discussed further.

In a fluorescent OLED, light emission occurs when singlet excited states relax tothe ground states, which limits the internal quantum efficiency (IQE) to approxi-mately 25 %. In the phosphorescent system, all singlet excitons may be convertedinto triplet states through intersystem crossing (ISC) around a heavy metal atom. Thetriplet states may emit radiatively, thus enabling up to 100 % internal quantumefficiency (Baldo et al. 1998). In this phosphorescence process, the lifetime of theexcited states is in the order of microseconds. Since phosphorescent OLEDs utilizetriplet states and only short-range Dexter energy transfer is spin-allowed for triplet to

Host Guest

S1

Transfer

ISC

PF

T1

So So

S1

T1

Fig. 2 Light generation in aguest-host system. S0, S1, andT1 are singlet ground state,first excited singlet state, andfirst excited triplet state,respectively. ISC, F, and Pstand for intersystem crossing,fluorescence, andphosphorescence, respectively

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triplet transfer, the dopant concentration in phosphorescent OLEDs (�10 %) istypically higher than fluorescent OLEDs (�1 %).

Because of the very high IQE, phosphorescent OLED (PHOLED) technologyenables displays to have lower power consumption than a backlit AMLCD, signif-icantly extending battery life for mobile devices. The higher efficiencies also lead toreduction in display temperature rise, and thus extending the display lifetime.

OLED Device and Materials

Bilayer Heterojunction Small Molecule OLEDs

In a simple bilayer OLED device, two different organic materials are sandwichedbetween an anode and a cathode, as shown in Fig. 3. The material next to anode hasgood conductivity for positive charges (holes) and is used as the hole transport layer(HTL), while the material next to cathode transports electrons better and is used asthe electron transport layer (ETL). The holes and electrons injected from anode andcathode migrate through the HTL and ETL, respectively, and recombine at theHTL-ETL interface to form excitons. When excitons radiatively relax to the lowerenergy ground state, light is generated.

Figure 3 also shows the typical materials used for constructing a bilayer OLEDdevice: aromatic diamine as HTL and tris (8-hydroxy-quinolinato) aluminum (Alq3)as ETL. Alq3 also serves as the light-emitting material in this device.

A typical energy diagram of the bilayer OLED structure under forward biascondition is shown in Fig. 4. From the energy diagram, we can see this bilayerOLED is basically a heterojunction diode. With forward bias, holes are injected fromthe indium tin oxide (ITO) anode to diamine, overcoming a small energy barrier. Thehole mobility of organic materials is reasonably high and there is little voltage dropat the HTL layer. Once the holes reach the HTL/ETL interface, they encounter anenergy barrier ΔEV, which is the HOMO energy level difference between the twoorganic materials. For electrons, they are injected from the cathode into Alq3, andonce they migrate to the HTL/ETL interface, they experience ΔEC, which is theLUMO energy level difference between the two materials. Because of the existence

N

Glass substrate

HTL: Diamine

Anode: ITO

ETL: Alq3

Cathode: Mg:Ag Diamine Alq3

NAl

N

O NO

O

N

Fig. 3 The structure and materials used in a bilayer OLED device (Reproduced from Tang andVanSlyke (1987) with permission by American Institute of Physics)


of the barriers ΔEC for electrons and ΔEV for holes, carriers (electrons and holes)will accumulate at the HTL/ETL interface and have a better chance to recombine andthereby emit photons.

In a different design,ΔEV is small so the holes can overcome the barrier and moveto the ETL region whileΔEC is large enough so electrons will not be able to cross theinterface. This way the recombination sites will be increased significantly becausethe depth of ETL material is utilized.

Figure 5 shows the current density-voltage-luminance (J-V-L) characteristics of adevice with the following structure: ITO/NPD(40 nm)/Alq3(40 nm)/Mg:Ag. Brightgreen emission can be observed from this simple device. With a small forward bias,there is a very small current and little exciton formation. As the voltage is raised, thecurrent quickly increases typically obeying a power law. When the current density islarger than 0.01 mA/cm2, the luminance output is directly proportional to the currentdensity, and efficiencies of a few lumens per Watt (lm/W) are easily attainable.

Polymer OLEDs

Since the early demonstration of electroluminescence in conjugated polymer PPV in1990, there has been intensive research and development effort in the field ofpolymer OLEDs. The structure of a simple one layer polymer OLED is shown inFig. 6, where conjugated polymer PPV is sandwiched between ITO anode and a lowwork function metal Calcium cathode. The polymer layer can be deposited byprinting or spin-coating, while the low function metal is deposited using vacuumthermal evaporation process. During the OLED operation, positive charges areinjected from ITO anode and electrons are injected from cathode into the PPVlayer. The electrons and holes move along and “hop” between the polymer chains,and when they meet, excitons are formed. These excited states can radiatively relaxto the ground state, giving out light. The color of the light is determined by theenergy gap of the polymer and can be modified by changing the chemical structure ofthe polymer.

-

++Anode

Cathode

HTL ETL

DEV

DEC -Fig. 4 Energy level diagramof a bilayer smallmolecule OLED

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Conjugated polymers typically transport holes much better than electrons. As aresult, some of the holes will reach the cathode without recombination because oflimited availability of electrons, resulting in a low-efficiency device. To solve thisproblem, an electron transport layer can be inserted between the PPV and cathode(Brown et al. 1992), similar to the bilayer structure shown in Fig. 3. This ETL layerwill improve electron injection, block holes from reaching the cathode withoutrecombination, and increase the charge density at the interface and promote recom-bination. Notice that the light-emitting material is a hole transport material (PPV) inthis case, while it is an electron transport material (Alq3) in the SMOLED shown inFig. 3.

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0 2 4 6 8 10Voltage [V]

Cur

rent

den

sity

J[m

A/c

m2 ]

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

Lum

inan

ce [c

d/m

2 ]

Fig. 5 J-V-L characteristics of an OLED with the structure of ITO/NPD (40 nm)/Alq3(40 nm)/Mg:Ag

Glass substrate

ITO anode

Conjugatedpolymer PPV

Cathode (Calcium)

n

Fig. 6 The structure of a single-layer polymer OLED


This chapter puts more emphasis on small molecule OLED which is the technol-ogy used today in mass production. For readers interested in learning more aboutpolymer OLEDs, some excellent references have been provided (Greenham andFriend 1995; Friend 2001; Akcelrud 2003).

Advanced OLED Structure

Amore sophisticated SMOLED structure is demonstrated in Fig. 7, where HIL, EIL,HBL, EBL, and EML stand for hole injection layer, electron injection layer, holeblocking layer, electron blocking layer, and emissive layer, respectively. The EMLlayer of a typical high-efficiency OLED device consists of a phosphorescent dopantin an organic host matrix. When a transparent anode such as ITO and an opaquecathode are used, the light will be emitted through the substrate that the OLED isbuilt upon and this configuration is called a bottom-emitting OLED. With an opaqueanode and a transparent or a semitransparent cathode, the light will be emittedthrough the cathode and the device is called a top-emitting OLED. When bothelectrodes can let light go through, the device is a transparent OLED.

The energy level diagram of an archetype phosphorescent OLED device with ITOanode and Mg:Ag cathode is shown in Fig. 8. The organic layers are α-NPD as HTL,BCP as HBL, Alq3 as ETL, and Ir(ppy)3 doped in CBP as EML. The electrolumines-cence process of OLED can be broken down into the following steps: (1) chargeinjection to organics by anode and cathode, (2) charge transport within the organics –injected charges referred as polarons predominantly migrate through the transport layersthrough a “hopping” mechanism, and (3) carrier recombination and light emission.

Glass substrate

HIL

Anode

HTL

Cathode

EML (Phosphorescent OLED)

HBL

ETL

EIL

EBL

Fig. 7 A more sophisticated high-efficiency OLED device structure

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The external quantum efficiency (EQE) of an OLED device can be calculated by

ηext ¼ ηintηoc ¼ γηsηPLηoc

where ηint is the internal quantum efficiency (IQE), γ is the charge balance factor, ηsis the percentage of excitons that contribute to radiative process dictated by their spinstates, ηPL is the photoluminescence quantum efficiency, and ηoc is the outcouplingefficiency. In fluorescent OLEDs, ηs has an upper limit of 25 %. While in phospho-rescent OLEDs, this value can be increased to 100 % because of intersystem crossingachieved by integrating transition metals such as iridium or platinum. One parametermissing from the above formula is the drive voltage, which needs to be consideredwhen calculating power efficiency.

The key considerations in designing a high-efficiency OLED device include:(a) equal number of electrons and holes, (b) high transport rates at low operating voltage,and (c) efficient charge recombination and efficient emission from excited states.

CathodeAs explained earlier, the key to a good carrier injection is to reduce the barrier at themetal-organic interface. This requires low work function metals or metal alloys suchas Ca (φ = 2.9 eV), Yb (φ = 2.4 eV), and Mg (φ = 3.7 eV). However, low workfunction also implies high chemical reactivity. The ease of oxidation of low workfunction metals makes them very sensitive to the presence of oxygen and waterduring the OLED fabrication and operation. Typically, the reacting site becomesopen circuit and a dark spot can be observed in a light-emitting background when thedevice is turned on. This is the main driving force for the hermetic sealing of theOLED devices.

3.2 eV

2.6 eV

4.7 eVITO

3.2 eV

EML

Ir(ppy)3in CBP20 nm

HTL

a-NPD 40 nm

5.7 eV

6.3 eV

HBL

BCP6 nm

6.7 eV

ETL

Alq320 nm

2.7 eV

6.0 eV

3.7 eVMgAg

Fig. 8 Energy level diagram of a phosphorescent OLED device with ITO anode; Mg:Ag cathode;and HTL, EML, HBL, and ETL organic layers


Two of the popular cathode choices are LiF/Al and Mg:Ag. Studies show that aLiF/Al cathode, when brought into contact with organic materials, may generateanionic species that facilitate electron injection (Mason et al. 2001). In the case of aMg:Ag cathode, 10 % of Ag helps to stabilize Mg and improve the adhesion of Mgto the organic materials while maintaining a low work function.

AnodeThe most used anode material is ITO for several very important reasons: (1) as ahighly degenerate n-type semiconductor, ITO has high conductivity for displayapplications; (2) ITO is transparent so light can be emitted through; (3) ITO hasreasonable work functions (�4.7 eV) to match with the HTL; and (4) it has beenused as electrodes for liquid crystal displays (LCDs) for many years so the technol-ogy and infrastructure are readily available.

The work function of ITO, and thus the performance of the OLED, can bemodified by surface treatment such as solvent or plasma treatment or the applicationof a thin interfacial layer. Besides ITO, some examples of the materials investigatedfor anode application include aluminum, gold, silver, and other conductive oxidematerials.

Carrier Injection LayersAs shown in the energy level diagram in Fig. 8, there exists a large barrier for holeinjection at the ITO-HTL interface. To address this issue, an interfacial HIL materialcan be inserted between ITO and HTL to facilitate the injection. For example, byintroducing a thin layer of copper phthalocyanine (CuPc) between ITO and HTLlayers, a highly stable OLED was achieved (Vanslyke et al. 1996). This is due to thebetter matched HOMO levels between CuPC (�5.1 eV) and ITO (�4.7 eV). Incomparison, the HOMO level of the HTL is much larger (�5.7 eV).

Besides CuPc, some examples of the HIL materials include PEDOT/PSS (poly-3,4-ethylenedioxythiophene/polystyrene sulfonic acid), fluorohydrocarbon (CFx),and thin inorganic oxide films.

Similar to HIL, a thin layer of interfacial EIL material can be introduced betweencathode and ETL materials. The LiF layer in the LiF/Al cathode is a good example.Other materials that have been studied as EIL materials include CsF, M2O, BCPdoped with Li/Cs, and organic polymer surfactants.

Charge Transport LayersThe key requirements of charge transport materials include high carrier mobility,good energy level alignment for charge injection and migration between layers, highelectrochemical stability, high glass transition temperature (>100 �C) to ensurethermal stability, and long thermal evaporation stability.

The functions of the HTL layer are to deliver the holes to the EML efficiently, andat the same time, to prevent electrons from entering the HTL and reaching the anode.Conjugated polymers are typically good conductors for holes so the need for a HTLlayer is much less in a polymer OLED device. For a SMOLED, the introduction of aHTL layer is one of the important milestones in its development.

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Two commonly used HTL materials are N,N0-(3-methylphenyl) � 1,10-biphenyl� 4,40-diamine (TPD: Tg= 60 �C), and 4,40-bis[N-(1-naphthyl-1)-N-phenyl-amino]-biphenyl (α-NPD: Tg= 95 �C). Some other materials have been investigated as HTLincludes starburst 4,40,400-tri(N-carbazolyl)triphenylamine (TCTA: Tg = 151 �C),Spiro-NPB (Tg = 147 �C), indolocarbazole (Tg � 164 �C), triphenylmethanes, andphenylazomethines.

The functions of the ETL materials are to deliver electrons to the emissive layer,and at the same time, block holes from reaching cathodes without recombination. Anideal ETL material should possess: (1) a LUMO level closely matched with cathodeto facilitate electron injection; (2) a high electron mobility for electron transport;(3) the capability of forming a stable, amorphous thin film; and (4) an optimizedband gap so not to absorb emitted light.

Since its first appearance as emissive and electron transport material in the 1980s,Alq3 has been one of the most studied and commonly used ETL materials. Itselectron mobility is about two orders of magnitude higher than the hole mobility(Kepler et al. 1995). Some other examples of the ETL materials studied include1,3,5-tris(N-phenylbenzimidizol-2 � yl)benzene (TPBi), bis(10-hydro-xybenzo-quinolinato) beryllium (Be(bq)2), oxadiazole, triazole, and silole.

Light-Emitting LayersThe most important consideration in designing a host material is to facilitate theenergy transfer to the guest. Studies on guest-host phosphorescent systems show thatthe band gap of the host material should be larger and enclose that of the guest(Thomas et al. 2003). Other considerations include high charge mobility, compati-bility with the guest molecule in forming homogeneous thin films, and good thermaland electrochemical stability.

It is very common to choose host materials from the charge transport materials.Several good electron transport hosts include Alq3 (LUMO: �3.0 eV; HOMO:�5.7 eV; T1 = 2.0 eV), TPBI (LUMO: �2.7 eV; HOMO: �6.2 eV), and aluminumbis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq: LUMO: �3.0 eV; HOMO:�5.9 eV; T1 = 2.2 eV), while two examples of hole transport hosts are 4,40-bis(9-carbazolyl)-biphenyl (CBP, LUMO: �3.0 eV; HOMO: �6.3 eV; T1 = 2.7 eV)and N,N0-dicarbazolyl-3,5-benzene (mCP, LUMO: �2.4 eV; HOMO:�5.9 eV;T1 =3.0 eV).

For fluorescent systems, a band gap of 2.7 eV makes Alq3 suitable as hostmaterial for red and green emitters. For fluorescent blue host, a larger band gap isneeded. One commonly used material is ADN with a band gap of 3.2 eV. However,for phosphorescent systems where the transition is from triplet excited states,materials with appropriate triplet energies should be selected. For example, Balqand CBP, with triplet energies of about 2.2 eV and 2.7 eV, can only be used as hostmaterials for phosphorescent red and green emitters, respectively. For a material tobe used as a deep blue phosphorescent host, its triplet energy needs to be larger than3 eV. For example, arysilane compounds have been investigated for this purpose.

Examples of classic fluorescent dopants for red, green, and blue colors are redarylidene laser dyes such as 4-(Dicyanomethylene)-2-methyl-6-[p-(dimethylamino)


styryl]-4H-pyran (DCM), Alq3 or coumarin laser dyes such as C-545 T, and 9,10-(2-naphthyl)anthracene (ADN) and its derivatives, respectively.

Examples of early phosphorescent dopants for red, green, and blue colors arephenylisoquinoline iridium complexes, phenyl pyridine iridium complexes such asIr(ppy)3, and Firpic, respectively.

Recent Research Areas

High efficiency and long lifetime has been the consistent theme of the research anddevelopment of OLED materials and devices. Most mobile AMOLED displays useRGB side-by-side configuration – individual colors are patterned next to each other.For large size AMOLED TVs, the popular pixel structure is white OLED plus colorfilters. In this configuration, color is generated by placing color filters on top ofindividually driven white OLED pixels. White OLEDs can be built by using atandem structure where multiple colors, either yellow/blue or R/G/B, are stackedtogether. In most commercial AMOLED products, phosphorescent red, yellow, andgreen and fluorescent blue are being used. Researchers are working hard to develophigh efficiency, long lifetime blue materials and device systems. Deep blue color canbe generated in two ways: one by intrinsic emission mainly at deep blue wavelengthand the other by optically enhancing deeper part of the spectrum through cavitystructure. The down side of a cavity structure is that the color and intensity stronglydepend on the viewing angle. As a result, cavity structure can only be used inapplications where viewing angle performance is not important. One example isthe displays used in personal, mobile devices. For TVapplication, intrinsic deep bluelight-emitting materials have to be used in order to meet the viewing anglerequirements.

Fluorescent OLEDsThere is limited publication on the structure of fluorescent blue dopants, but it is wellknown that they are typically based on pyrene, stilbene, perylene, fluoranthene, andantracene. In recent years, blue fluorescent OLEDs with deep color and long lifetimehave been reported. Kawamura et al. reported a deep blue device with CIEx =0.143,CIEy= 0.078, efficiency of 6.5 cd/A and voltage of 3.8 Vat 10 mA/cm2, and LT50>9000 h at 500 cd/m2 (Kawamura et al. 2011). Heil et al. reported a deep blue devicewith CIEx =0.148, CIEy = 0.062, efficiency of 4.7 cd/A and voltage of 3.8 V at1000 cd/m2, and LT50 > 10,000 h at 500 cd/m2 (Heil et al. 2014).

Most research on fluorescent OLED materials has been around breaking the 25 %singlet spin statistics limitation. Assuming a perfect charge balance and a 20 %outcoupling efficiency, the external quantum efficiency of a fluorescent OLED hasan upper limit of only �5 %. To overcome the singlet population limitation, one ofthe most important approaches is to take advantage of the 75 % triplet excitons andconvert them into singlet excitons through reverse intersystem crossing (RISC). Twoexamples are triplet-triplet annihilation and thermally activated delayed fluorescence(TADF).

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When two triplet excitons interact, they can produce a singlet exciton throughTTA process, providing a theoretical IQE upper limit of 62.5 % (Monkman 2013).Spindler et al. reported high efficiency fluorescent red OLEDs with 11.5 % EQE(15 cd/A) at 3 Vand yellow OLEDs with 8.6 % EQE (28 cd/A) at 2.9 V. In additionto a nonemitting assist layer adjacent to the emission layers and low-voltageelectron-transporting and electron-injecting layers, time-resolved electrolumines-cence measurements indicated a significant efficiency contribution (up to 50 %)from triplet-triplet annihilation processes (Spindler et al. 2009).

In recent years, thermally activated delay fluorescence has generated significantinterest in the OLED research community. In typical organic molecules, the energydifference between S1 and T1 is in the range of 0.5 � 1.0 eV. In TADF approach, thelight-emitting material has a small singlet-triplet energy splitting (ΔEST < 0.1 eV).This can facilitate the T1 to S1 reverse intersystem crossing by thermal activationleading to delayed fluorescence from S1 to S0 states, as shown in Fig. 9 (Adachi2013).

The key to achieving a very small ΔEST is to introduce spatially separated donorand acceptor moieties. Adachi et al. reported a series of highly efficient TADFemitters with carbazole as a donor and dicyanobenzene as an electron acceptor.Because of the steric hindrance, HOMO and LUMO are localized on the donor andacceptor moieties. This separation suppresses the overlap between HOMO andLUMO leading to the small ΔEST. OLEDs made with green emitting 4CzIPNshowed a very high EQE of 19.3 � 1.5 %, which is equivalent to an IQE of64.3–96.5 % depending on the outcoupling factor (20–30 %) (Uoyama et al. 2012).

RISC from high-lying energy triplets was first reported in 1969 (Keller 1969).Recently Hu et al. reported their work using this approach to improve the fluorescentefficiency. Figure 10 shows the structure of the molecule used in the study and its

4CzlPN

NN

N

N

CNNC

ElectricalExcitation

e

25%

TAD

F

Fluo

resc

ence

75%

Phosphorescence

T1S1

S0

h

Fig. 9 Energy diagram to explain the concept of TADF, and molecular structure of TADF emitter4CzIPN (Reproduced from Adachi (2013) with permission by SID)


RISC process. The key to the molecular design is to introduce donor-acceptorstructure to generate charge transfer (CT) excited states. In TPA-NZP, there existsa large energy gap between the S1 and T1 excited states. In addition, the energy gapbetween T2 and T1 is very large (1.63 eV). However, the energy splitting between S2(SCT) and T2 (TCT), which belong to CT states, is very small (ΔEST(CT) = 0.29 eV).As a result, RISC process kRISC (T2 ! S2) could be more competitive than theinternal conversion process kIC (T2 ! T1). Experimental results showed that 93 %exciton utilization was achieved with TPA-NZP (Hu et al. 2015).

In addition to RISC, there is also research effort in improving efficiency throughmolecular alignment. In an electromagnetic wave, the vibration directions of electricand magnetic fields are orthogonal to the wave propagation direction. A horizontal(parallel to substrate) emitting dipole will lead to normal (to substrate) emission,resulting in minimum loss due to waveguide effect. Ogiwara et al. conducted asystematic study of orientation effects of fluorescent blue dopants and found that thehigher the ratio of horizontal emitting dipoles, the larger the EQE value. By utilizingthis orientation effect, a 11.5 % EQE blue dopant was developed with CIE color of(0.138, 0.092) and LT70 lifetime of 5500 h at 500 cd/m2(Ogiwara et al. 2014).

In addition to fluorescent blue dopant development, there is also effort on othermaterials in the device to improve the performance. One of such materials is the holeblocking layer. Placed between the EML and ETL layers, the purpose of HBLmaterial is to block holes from leaking into ETL layers without recombining withelectrons and while at the same time to transport electrons into the EML layer. TheHBL material needs to have a large HOMO level (>5.5 eV) and good electronmobility. Introducing electron withdrawing group with high electron affinity is an

Fig. 10 Molecular structure of TPA-NZP and its RISC process (Reproduced from Hu et al. (2015)with permission by SID)

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effective way to improve electron mobility. However, it should not reduce the bandgap and thus the HOMO level of the material. Kim et al. reported that a bluefluorescent OLED with a newly developed HBL material achieved an efficiency of9.5 cd/A at 500 cd/m2 with a color coordinate of (0.135, 0.108) (Kim et al. 2015).

Phosphorescent OLEDsBecause of their high efficiency and long operational device lifetime, red, green,yellow, and light blue phosphorescent OLEDmaterials are being used in commercialdisplay and lighting products (Weaver et al. 2009; Levermore et al. 2012; http://www.udcoled.com). A deep blue phosphorescent material, which is not commer-cially available yet, has been under intensive development. Most reported phospho-rescent metal complexes are those containing Iridium (Ir) and Platinum (Pt), whichare capable of harvesting up to 100 % excitons. Recently Fleetham et al. reportedblue phosphorescent dopants employing tetradentate platinum complexes. In onesuch material, the triplet state is localized on the phenyl-azole (CN) portion which isoxygen bridged to a carbazolyl-pyridine ligand with a t-butyl group attached to thepyridine ring. OLED devices with this dopant showed EQE exceeding 25 %, deepblue color with CIE (0.15, 0.08), and potential for long operational lifetimes(Fleetham et al. 2015). Yam reported a class of luminescent cyclometalated gold(III) complexes. One example is the 2,6-diphenyl-4-(2,5-difluorophenyl)pyridinederivative. OLED devices built with this material showed a broad spectrum peakedat �528 nm with full width at half maxima of 90 nm (Yam 2013).

One critical material for blue phosphorescent OLEDs is the host material. Recentresearch work has been focused on developing a host material or a host system thathas appropriate triplet energies and bipolar charge transport property. As discussedearlier, the host needs to have a high triplet energy in order to “contain” the excitonson the dopant – the excitons should not transfer back to the host material. Wide band-gap materials such as carbazole, dibenzothiophene, or phenylsilane are frequentlyused as host materials for blue phosphorescent OLEDs.

The importance of bipolar charge transport host has been well studied. Organiccharge transport materials are predominantly for single charge carriers. When suchmaterials are used as hosts, charges with the favorable polarity move quickly throughthe EML layer before being stopped by the blocking layer (BL), while charges withopposite polarity move slowly in the EML layer. As a result, a large concentration ofcharges is accumulated at EML-BL interface resulting in a high local field whichadversely affects device reliability. To address this issue, host materials with bipolarcharge transport properties can be used. This can be done by either using a mixture ofhole transporting and electron transporting materials or integrating appropriateelectron and hole transporting moieties in a single molecule.

In the mixed host system, each of the materials retains its original HOMO andLUMO energy levels because of the weak van der Waals coupling between theorganic molecules. Electrons will hop through the electron transport material andholes through the hole transport material leading to a much wider recombination zone(Choong et al. 1999). Another advantage of a mixed system is that the ratio betweenthe two types of materials can be continuously adjusted through the EML layer.


http://www.udcoled.com/


To make a single molecule with bipolar charge transport property for phospho-rescent OLEDs, a hole transporting and an electron transporting moieties with hightriplet energies need to be used. However, if there is π conjugation between the two,the triplet energy could be significantly reduced. Macdonald et al. proposed to useC-N bond-forming reaction to unite electron-transporting diimidazole moiety andhole-transporting carbazole moiety. A conformational twist due to the steric conges-tion about the aryl-imidazole moieties would disrupt the conjugation (Macdonaldet al. 2012). Chiu et al. synthesized bipolar host materials by conjugating hole-transporting carbazole and electron-transporting triazole with improvement in deviceperformance (Chiu et al. 2015). Byeon et al. synthesized bipolar host materials usingcarbazole and carboline moieties as core and benzonitrile, phenyl, and carbazolesubstituent groups to fine-tune the charge transport property (Byeon et al. 2015).

OLED Fabrication and Operation

OLED FabricationTo fabricate a bottom-emitting OLED device, a transparent conductive oxide such asITO is first patterned using standard photolithographic techniques to form the anodewith predetermined dimensions. The ITO is then cleaned to give a smooth, particle-free surface, and treated by plasma or other surface treatment to improve chargeinjection. Then the substrate is transferred into a vacuum deposition chamber.Organic layers are deposited sequentially by thermal evaporation onto the substrate.During this process, organic materials are sublimed from metallic or ceramic cruci-bles at a base pressure of �10�7 torr or lower. The deposition rate on the substrate istypically in the range of 0.1–0.5 nm/s, monitored by a quartz oscillator. In the case ofguest-host doping layer, the host and dopant are deposited simultaneously and thedoping concentration is controlled by adjusting the relative deposition rate of thedopant with respect to that of the host material. The footprint of the organic layer isdefined by a shadow mask and the thickness is controlled by deposition time at agiven rate. After all the organic layers have been deposited, a cathode, typically alow work function metal, is then evaporated onto the organic layers through a secondshadow mask. The organic and metal depositions are preferred to be done in separatechambers to eliminate cross-contamination.

Although many techniques have been investigated for organic deposition, ther-mal evaporation and solution processing (coating, printing) are the two dominantprocesses. Vacuum thermal evaporation (VTE) offers pristine amorphous thin filmsand thus excellent device performance. However, patterning materials requires ashadow mask to be used, which results in lower material utilization and is difficultto implement for large size substrate. Since no shadow mask is needed, solutionprocessing can have high scalability and high material utilization and has thepotential of developing into a high-throughput, low-cost manufacture process.While polymer materials are excellent candidates for solution process, smallmolecule OLED materials have also been developed for this purpose (Xiaet al. 2009).

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One exciting new technique for organic deposition is organic vapor phasedeposition (OVPD). In this process, organic materials are vaporized and an inertcarrier gas such as nitrogen is used to dilute and carry the materials to the cooledsubstrate where organic materials condense to form thin films. Organic vapor jetprinting (OVJP) uses similar concept, but the deposition is through a nozzle. Bycontrolling the size and arrangement of the nozzles, a high-resolution, dry patterningprocess can be realized (Sun et al. 2005).

Degradation and EncapsulationThe organic semiconductor materials used in OLEDs are prone to degradation due totheir intrinsic properties. These include crystallization or other morphologicalchanges over time, intrinsic photo/electrochemistry, migration, and reaction ofimpurities within the organic and electrodes (Yamamoto et al. 2011). In addition,because of the high reactivity of low work function metals, the cathodes in OLEDsare easily oxidized and delaminate from organics when exposed to atmosphericoxygen and water (Liew et al. 2000). During this process, water and oxygen firstdiffuse to the cathode-organic interface through pinholes and other defects in thecathode. Then cathode and organic react with water and oxygen to form insulatinglayers (e.g., oxide) between the cathode and organic, which results in local opencircuits, seen by an observer as dark spots in the bright emissive background at on-state.

To protect the devices from external water and oxygen, OLEDs need to behermetically sealed. This can be achieved by placing a metal can or a piece ofcover glass on top of the OLED device followed by sealing the edges with epoxy orother sealant materials with good barrier properties. This work should be done in thecontrolled, dry, inert environment such as a nitrogen or argon glove box. Inside thechamber, desiccant materials in the forms of powder, getter, or gel are used to furtherprotect the OLED devices.

Another way to encapsulate OLEDs is to apply thin films of dense materials ontop of the OLEDs. For example, silicon oxide and silicon nitride films have beenused to encapsulate OLEDs. The challenge for this approach is that the film cannotbe grown thick enough because it would crack. A very thin film is not effective incovering particles and topographical features. These defects and also pinholes in thevery thin film leave paths for moisture and oxygen to penetrate. In a multilayerapproach, the barrier consists of alternating layers of polymer films and inorganicoxide (Hack et al. 2005). Oxide layers act as permeation barriers to the diffusion ofwater and oxygen, while the polymer layers prevent propagation of defects throughthe multilayer structure and planarize any particles or rough surfaces.

Future Development

In recent years, AMOLED and AMLCD have strongly influenced each other in theirresearch and development efforts. While AMLCD continuously improves its view-ing angle with multidomain structures, switching speed with new driving schemes,


and color performance with LED backlights, AMOLED development has beenfocused on improving organic material performance in efficiency and lifetime,low-power-consumption device architectures, high-resolution patterning, 3D andflexible displays, and high-yield low-cost manufacturing process.

The development of better materials and device structures will continue, withmost intensive research being done in high-efficiency, long lifetime blue OLEDs.Another active research area is material and architecture for white OLEDs whichhave applications in both displays and lighting.

For small size mobile applications, AMOLED has established itself as a strongtechnology with an attractive roadmap that enables next generation mobile deviceswith transparent and flexible displays. Integrating these novel form factors into a realproduct involves heavy development in areas such as thin-film encapsulation andsystem integration. For large TV applications, developing high-yield, low-costmanufacturing process is critical. Specific challenges include the scaling up ofbackplanes, OLED patterning, and encapsulation technologies.

Further Reading

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High efficiency phosphorescent emission from organic electroluminescent devices. Nature395:151

Bernanose A, Comte M, Vouaux P (1953) A new method of emission of light by certain organiccompounds. J Chem Phys 50:64–68

Brown AR, Bradley DDC, Burroughes JH, Friend RH, Greenham NC, Burn PL, Holmes AB, KraftA (1992) Poly( p-phenylenevinylene) lightemitting diodes: enhanced electroluminescent effi-ciency through charge carrier confinement. Appl Phys Lett 61:2793–2795

Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, Burn PL, HolmesAB (1990) Light-emitting diodes based on conjugated polymers. Nature 347:539–541

Byeon S, Hwang S, Kim O, Lee J (2015) Synthesis of host materials for blue phosphorescentorganic light emitting diodes with high efficiency and low drive voltage. SID 15 Dig 46:323

Chiu T, Chen S, Hsieh Y, Leung M,Wang P, Lee J, Ho H (2015) CbzTAZ hosts in blue organic lightemitting devices perform a high current efficiency more than 50 cd/A. SID 15 Dig 46:320

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Fleetham T, Li G, Zhu Z, Li J (2015) Development of tetradentate Pt complexes for efficient, stable,and high color purity blue OLEDs. SID 15 Dig 46:411

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Hack M, Chwang A, Tung Y-J, Hewitt R, Brown J, Lu JP, Shih C, Ho J, Street RA, Moro L, Chu X,Krajewski T, Rutherford N, Visser R (2005) Status and opportunities for high efficiency OLEDdisplays on flexible substrates. Mater Res Soc Symp Proc 870E:H3.1.1

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Heil H, Rodriguez L, Burkhart B, Meyer S, Riedmueller S, Darsy A, Pflumm C, Buchholz H,Boehm E (2014) High-performance OLED materials. SID 14 Dig 45:495

http://www.udcoled.comHu D, Yao L, Yang B, Ma Y (2015) Reverse intersystem crossing from high-lying triplet energy

levels to excited singlet: a “hot exciton” path for OLED. SID 15 Dig 46:404Kafafi Z (2005) Organic electroluminescence. Marcel Dekker, New YorkKawamura Y, Kuma H, Funahashi M, Kawamura M, Mizuki Y, Saito H, Naraoka R, Nishimura K,

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